Cosmology for Beginners -- John Gribbin

7/25/2019 Cosmology for Beginners -- John Gribbin

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Cosmology for Beginners

by John Gribbin

Inflation for Beginners

INFLATION has become a cosmological buzzword in the1990s. No selfres!ecting theor" of the #ni$erse iscom!lete without a reference to inflation and at thesame time there is now a bewildering $ariet" ofdifferent $ersions of inflation to choose from. %learl"&what's needed is a beginner's guide to inflation& wherenewcomers to cosmolog" can find out (ust what thise)citing de$elo!ment is all about. This is it newreaders start here.

The reason wh" something li*e inflation was needed incosmolog" was highlighted b" discussions of two *e"!roblems in the 19+0s. The first of these is the horizon!roblem the !uzzle that the #ni$erse loo*s the same

on o!!osite sides of the s*" ,o!!osite horizons- e$enthough there has not been time since the ig ang forlight ,or an"thing else- to tra$el across the #ni$erse andbac*. /o how do the o!!osite horizons *now how to*ee! in ste! with each other The second !uzzle iscalled the flatness !roblem This is the !uzzle that thes!acetime of the #ni$erse is $er" nearl" flat& whichmeans that the #ni$erse sits (ust on the di$iding linebetween eternal e)!ansion and e$entual recolla!se.

The flatness !roblem can be understood in terms of thedensit" of the #ni$erse. The densit" !arameter is ameasure of the amount of gra$itating material in the

#ni$erse& usuall" denoted b" the 2ree* letter omega,O-& and also *nown as the flatness !arameter. It isdefined in such a wa" that if s!acetime is e)actl" flatthen O 3 1. efore the de$elo!ment of the idea ofinflation& one of the great !uzzles in cosmolog" was thefact that the actual densit" of the #ni$erse toda" is$er" close to this critical $alue certainl" within afactor of 10. This is curious because as the #ni$ersee)!ands awa" from the ig ang the e)!ansion will !ushthe densit" !arameter awa" from the critical $alue.

If the #ni$erse starts out with the !arameter less thanone& O gets smaller as the #ni$erse ages& while if it

starts out bigger than one O gets bigger as the #ni$erseages. The fact that O is between 0.1 and 1 toda" meansthat in the first second of the ig ang it was !recisel"

1 to within 1 !art in 1040-. This ma*es the $alue of thedensit" !arameter in the beginning one of the most!recisel" determined numbers in all of science& and thenatural inference is that the $alue is& and alwa"s hasbeen& e)actl" 1. One im!ortant im!lication of this isthat there must be a large amount of dar* matter in the#ni$erse. Another is that the #ni$erse was made flat b"inflation.

Inflation is a general term for models of the $er" earl"#ni$erse which in$ol$e a short !eriod of e)tremel"ra!id ,e)!onential- e)!ansion& blowing the size of whatis now the obser$able #ni$erse u! from a region farsmaller than a !roton to about the size of a gra!efruit

,or e$en bigger- in a small fraction of a second. This!rocess would smooth out s!acetime to ma*e the#ni$erse flat& and would also resol$e the horizon!roblem b" ta*ing regions of s!ace that were once closeenough to ha$e got to *now each other well ands!reading them far a!art& on o!!osite sides of the$isible #ni$erse toda".

Inflation became established as the standard model ofthe $er" earl" #ni$erse in the 1950s. It achie$ed thissuccess not onl" because it resol$es man" !uzzles aboutthe nature of the #ni$erse& but because it did so usingthe grand unified theories ,2#Ts- and understanding of6uantum theor" de$elo!ed b" !article !h"sicistscom!letel" inde!endentl" of an" cosmological studies.These theories of the !article world had beende$elo!ed with no thought that the" might be a!!liedin cosmolog" ,the" were in no sense designed totac*le all the !roblems the" turned out to sol$e-& andtheir success in this area suggested to man" !eo!le thatthe" must be telling us something of fundamental

im!ortance about the #ni$erse.

The marriage of !article !h"sics ,the stud" of the $er"small- and cosmolog" ,the stud" of the $er" large-seems to !ro$ide an e)!lanation of how the #ni$ersebegan& and how it got to be the wa" it is. Inflation istherefore regarded as the most im!ortant de$elo!mentin cosmological thin*ing since the disco$er" that the#ni$erse is e)!anding first suggested that it began in aig ang.

Ta*en at face $alue& the obser$ed e)!ansion of the#ni$erse im!lies that it was born out of a singularit"& a!oint of infinite densit"& some 17 billion "ears ago,cosmologists still disagree about e)actl" how old the#ni$erse is& but the e)act age doesn't affect theargument-. 8uantum !h"sics tells us that it ismeaningless to tal* in 6uite such e)treme terms& andthat instead we should consider the e)!ansion as ha$ingstarted from a region no bigger across than the so

called lanc* length ,10:7m-& when the densit" was notinfinite but onl" some 109; grams !er cubiccentimetre. These are the absolute limits on size anddensit" allowed b" 6uantum !h"sics.

On that !icture& the first !uzzle is how an"thing thatdense could e$er e)!and it would ha$e an enormousl"strong gra$itational field& turning it into a blac* holeand snuffing it out of e)istence ,bac* into thesingularit"- as soon as it was born. ut it turns out thatinflation can !re$ent this ha!!ening& while 6uantum!h"sics allows the entire #ni$erse to a!!ear& in thissu!ercom!act form& out of nothing at all& as a cosmicfree lunch. The idea that the #ni$erse ma" ha$ea!!eared out of nothing at all& and contains zero energ"o$erall& was de$elo!ed b"


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electron!ositron !airs- out of nothing& !ro$ided thatthe" disa!!ear in a short time. The less energ" isin$ol$ed& the longer the bubble can e)ist. %uriousl"& theenerg" in a gra$itational field is negati$e& while theenerg" loc*ed u! in matter is !ositi$e. If the #ni$erse ise)actl" flat & then as Tr"on !ointed out the twonumbers cancel out& and the o$erall energ" of the#ni$erse is !recisel" zero. In that case& the 6uantum

rules allow it to last fore$er. If "ou find this mindblowing& "ou are in good com!an". 2eorge 2amow toldin his boo* >" ?orld Line ,@i*ing& New =or*& re!rinted19+0- how he was ha$ing a con$ersation with Albert


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two ste!s after two clic*s& three ste!s after threeclic*s& and so on. In the 1950s& howe$er& when thetheor" of inflation suggested that the #ni$erse reall"did undergo a stage of e)!onential e)!ansion during thefirst s!litsecond after its birth& this inflationar"e)!onential e)!ansion turned out to be e)actl"described b" the de /itter model& the first successfulcosmological solution to


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>odern !ro!onents of the inflationar" scenario arri$edat these e6uations entirel" inde!endentl" of Eo"le'sa!!roach& and are reluctant to acce!t this analog"&ha$ing cut their cosmological teeth on the ig angmodel. Indeed& when 2uth was as*ed& in 1950& how thethen new idea of inflation related to the /tead" /tatetheor"& he is re!orted as re!l"ing what is the /tead"/tate theor" ut although inflation is generall"

regarded as a de$elo!ment of ig ang cosmolog"& it isbetter seen as marr"ing the best features of both theig ang and the /tead" /tate scenarios.

This might all seem li*e a !hiloso!hical debate as futileas the argument about how man" angels can dance onthe head of a !in& e)ce!t for the fact that obser$ationsof the bac*ground radiation b" %O< showed e)actl"the !attern of tin" irregularities that the inflationar"scenario !redicts. One of the first worries about theidea of inflation ,long ago in 1951- was that it might betoo good to be true. In !articular& if the !rocess was soefficient at smoothing out the #ni$erse& how could

irregularities as large as gala)ies& clusters of gala)iesand so on e$er ha$e arisen ut when the researchersloo*ed more closel" at the e6uations the" realised that6uantum fluctuations should still ha$e been !roducingtin" ri!!les in the structure of the #ni$erse e$en whenour #ni$erse was onl" something li*e 10,e)!7- of acentimetre across a hundred million times biggerthan the lanc* length.

The theor" said that inflation should ha$e left behindan e)!anded $ersion of these fluctuations& in the formof irregularities in the distribution of matter and energ"in the #ni$erse. These densit" !erturbations would ha$e

left an im!rint on the bac*ground radiation at the timematter and radiation decou!led ,about :00&000 "earsafter the ig ang-& !roducing e)actl" the *ind ofnonuniformit" in the bac*ground radiation that has nowbeen seen& initiall" b" %O< and later b" otherinstruments. After decou!ling& the densit" fluctuationsgrew to become the large scale structure of the#ni$erse re$ealed toda" b" the distribution of gala)ies.This means that the %O< obser$ations are actuall"gi$ing us information about what was ha!!ening in the

#ni$erse when it was less than 100of a second old.

No other theor" can e)!lain both wh" the #ni$erse is souniform o$erall& and "et contains e)actl" the *ind ofri!!les re!resented b" the distribution of gala)iesthrough s!ace and b" the $ariations in the bac*groundradiation. This does not !ro$e that the inflationar"scenario is correct& but it is worth remembering thathad %O< found a different !attern of fluctuations ,orno fluctuations at allthat would ha$e !ro$ed theinflationar" scenario wrong. In the best scientifictradition& the theor" made a ma(or and unambiguous!rediction which did come true. Inflation also !redictsthat the !rimordial !erturbations ma" ha$e left a tracein the form of gra$itational radiation with !articular

characteristics& and it is ho!ed that detectors sensiti$eenough to identif" this characteristic radiation ma" bede$elo!ed within the ne)t ten or twent" "ears.

The clean sim!licit" of this sim!le !icture of inflationhas now& howe$er& begun to be obscured b"refinements& as inflationar" cosmologists add bells andwhistles to their models to ma*e them match moreclosel" the #ni$erse we see about us. /ome of the bellsand whistles& it has to be said& are studied (ust for fun.Linde himself has ta*en great delight in !ushinginflation to e)tremes& and offering entertaining new

insights into how the #ni$erse might be constructed.For e)am!le& could our #ni$erse e)ist on the inside of asingle magnetic mono!ole !roduced b" cosmic inflationAccording to Linde& it is at least !ossible& and ma" beli*el". And in a delicious touch of iron"& Linde& who nowwor*s at /tanford #ni$ersit"& made this outrageousclaim in a lecture at a wor*sho! on the irth of the#ni$erse held recentl" in Come& where the $iew of%reation is usuall" rather different. One of the reasonswh" theorists came u! with the idea of inflation in thefirst !lace was !recisel" to get rid of magneticmono!oles strange !articles carr"ing isolated north orsouth magnetic fields& !redicted b" man" 2rand #nified

Theories of !h"sics but ne$er found in nature. /tandardmodels of inflation sol$e the mono!ole !roblem b"arguing that the seed from which our entire $isible#ni$erse grew was a 6uantum fluctuation so small thatit onl" contained one mono!ole. That mono!ole is stillout there& somewhere in the #ni$erse& but it is highl"unli*el" that it will e$er !ass our wa".

ut Linde has disco$ered that& according to theor"& theconditions that create inflation !ersist inside amagnetic mono!ole e$en after inflation has halted inthe #ni$erse at large. /uch a mono!ole would loo* li*ea magneticall" charged blac* hole& connecting our

#ni$erse through a wormhole in s!acetime to anotherregion of inflating s!acetime. ?ithin this region ofinflation& 6uantum !rocesses can !roduce mono!oleantimono!ole !airs& which then se!arate e)!onentiall"ra!idl" as a result of the inflation. Inflation then sto!s&lea$ing an e)!anding #ni$erse rather li*e our ownwhich ma" contain one or two mono!oles& within eachof which there are more regions of inflating s!acetime.

The result is a ne$erending fractal structure& withinflating uni$erses embedded inside each other andconnected through the magnetic mono!ole wormholes.Our #ni$erse ma" be inside a mono!ole which is insideanother uni$erse which is inside another mono!ole& andso on indefinitel". ?hat Linde calls the continuouscreation of e)!onentiall" e)!anding s!ace means thatmono!oles b" themsel$es can sol$e the mono!ole!roblem. Although it seems bizarre& the idea is& hestresses& so sim!le that it certainl" deser$es furtherin$estigation.

That $ariation on the theme reall" is (ust for fun& and itis hard to see how it could e$er be com!ared withobser$ations of the real #ni$erse. ut most of themodifications to inflation now being made are in

res!onse to new obser$ations& and in !articular to thesuggestion that s!acetime ma" not be 6uite flat afterall. In the mid1990s& man" studies ,includingobser$ations made b" the refurbished Eubble /!aceTelesco!e- began to suggest that there might not be

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6uite enough matter in the #ni$erse to ma*e it!erfectl" flat most of the obser$ations suggest thatthere is onl" 0 !er cent or :0 !er cent as much matteraround as the sim!lest $ersions of inflation re6uire. Theshortfall is embarrassing& because one of the mostwidel" !ublicised !redictions of sim!le inflation was thefirm re6uirement of e)actl" 100 !er cent of this criticaldensit" of matter. ut there are wa"s around the

difficult"B and here are two of them to be going onwith.

The first suggestion is almost heretical& in the light ofthe wa" astronom" has de$elo!ed since the time of%o!ernicus. Is it !ossible that we are li$ing near thecentre of the #ni$erse For centuries& the histor" ofastronom" has seen human*ind dis!laced from an"s!ecial !osition. First the il*" ?a" 2ala)"& then the2ala)" was seen to be an ordinar" member of thecosmos. ut now comes the suggestion that the

ordinar" !lace to find obser$ers li*e us ma" be in themiddle of a bubble in a much greater $olume ofe)!anding s!ace.

The con$entional $ersion of inflation sa"s that ourentire $isible #ni$erse is (ust one of man" bubbles ofinflation& each doing their own thing somewhere out inan eternal sea of chaotic inflation& but that the !rocessof ra!id e)!ansion forces s!acetime in all the bubblesto be flat. A useful analog" is with the bubbles thatform in a bottle of fizz" cola when the to! is o!ened.ut that suggestion& along with other cherishedcosmological beliefs& has now been challenged b"

Linde& wor*ing with his son Dmitri Linde ,of %alTech-and Arthur >ezhlumian ,also of /tanford-.

Linde and his colleagues !oint out that the #ni$erse weli$e in is li*e a hole in a sea of su!erdense&e)!onentiall" e)!anding inflationar" cosmic material&within which there are other holes. All *inds of bubbleuni$erses will e)ist& and it is !ossible to wor* out thestatistical nature of their !ro!erties. In !articular& thetwo Lindes and >ezhlumian ha$e calculated the!robabilit" of finding "ourself in a region of this su!er#ni$erse with a !articular densit" for e)am!le& the

densit" of our #ni$erse.

ecause $er" dense regions blow u! e)!onentiall"6uic*l" ,doubling in size e$er" fraction of a second-& itturns out that the $olume of all regions of the su!er#ni$erse with twice an" chosen densit" is 10 to the!ower of 10 million times greater than the $olume ofthe su!er #ni$erse with the chosen densit". For an"chosen densit"& most of the matter at that densit" isnear the middle of an e)!anding bubble& with aconcentration of more dense material round the edge ofthe bubble. ut e$en though some of the higher densit"material is round the edges of low densit" bubbles&

there is e$en more ,$astl" more- higher densit"material in the middle of higher densit" bubbles& and soon fore$er. The disco$er" of this $ariation on the themeof fractal structure sur!rised the researchers so much

that the" confirmed it b" four inde!endent methodsbefore $enturing to announce it to their colleagues.ecause the densit" distribution is nonuniform on thea!!ro!riate distance scales& it means that not onl" ma"we be li$ing near the middle of a bubble uni$erse& butthat the densit" of the region of s!ace we can see ma"be less than the critical densit"& com!ensated for b"e)tra densit" be"ond our field of $iew.

This is con$enient& since those obser$ations b" theEubble /!ace Telesco!e ha$e suggested thatcosmological models which re6uire e)actl" the criticaldensit" of matter ma" be in trouble. ut there is more.Those Eubble obser$ations assume that the !arameterwhich measures the rate at which the #ni$erse ise)!anding& the Eubble %onstant& reall" is a constant&the same e$er"where in the obser$able #ni$erse. IfLinde's team is right& howe$er& the measured $alue ofthe constant ma" be different for gala)ies at differentdistances from us& trul" throwing the cat among thecosmological !igeons. ?e ma" seem to li$e in a low

densit" uni$erse in which both the measured densit"and the $alue of the Eubble %onstant will de!end onwhich $olume of the #ni$erse these !ro!erties aremeasured o$er

That would mean abandoning man" cherished ideasabout the #ni$erse& and ma" be too much for man"cosmologists to swallow. ut there is a sim!ler solutionto the densit" !uzzle& one which in$ol$es tin*ering onl"with the models of inflation& not with longheld andcherished cosmological beliefs. That ma" ma*e it moreacce!table to most cosmologists and it's so sim!lethat it falls into the wh" didn't I thin* of that

categor" of great ideas.

A double dose of inflation ma" be something to ma*ethe 2o$ernment's hair turn gre" but it could be (ustwhat cosmologists need to rescue their fa$ourite theor"of the origin of the #ni$erse. " turning inflation ontwice& the" ha$e found a wa" to ha$e all the benefits ofthe inflationar" scenario& while still lea$ing the#ni$erse in an o!en state& so that it will e)!andfore$er.

In those sim!lest inflation models& remember& the big

snag is that after inflation e$en the obser$able #ni$erseis left li*e a mass of bubbles& each e)!anding in its ownwa". ?e see no sign of this structure& which has led toall the refinements of the basic model. Now& howe$er&>artin ucher and Neil Turo*& of rinceton #ni$ersit"&wor*ing with Alfred 2oldhaber& of the /tate #ni$ersit"of New =or*& ha$e turned this difficult" to ad$antage.

The" suggest that after the #ni$erse had beenhomogenised b" the original bout of inflation& a secondburst of inflation could ha$e occurred within one of thebubbles. As inflation begins ,essentiall" at a !oint-& thedensit" is effecti$el" reset to zero& and rises towards

the critical densit" as inflation !roceeds and energ"from the inflation !rocess is turned into mass. utbecause the #ni$erse has alread" been homogenised&there is no need to re6uire this bout of inflation to last

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until the densit" reaches the critical $alue. It can sto! alittle sooner& lea$ing an o!en bubble ,what we see asour entire $isible #ni$erse- to carr" on e)!anding at amore sedate rate. The" actuall" use what loo*ed li*ethe fatal flaw in 2uth's model as the basis for theirscenario. According to ucher and his colleagues& anend !roduct loo*ing $er" much li*e the #ni$erse we li$ein can arise naturall" in this wa"& with no fine tuning

of the inflationar" !arameters. All the" ha$e done is touse the $er" sim!lest !ossible $ersion of inflation& goingbac* to Alan 2uth's wor*& but to a!!l" it twice. And "oudon't ha$e to sto! there. Once an" !ortion of e)!andings!acetime has been smoothed out b" inflation& newinflationar" bubbles arising inside that $olume ofs!acetime will all be !resmoothed and can end u! withan" amount of matter from zero to the critical densit",but no more-. This should be enough to ma*ee$er"bod" ha!!". Indeed& the biggest !roblem now isthat the $ocabular" of cosmolog" doesn't 6uite seemade6uate to the tas* of describing all this acti$it".

The term #ni$erse& with the ca!ital #& is usuall" usedfor e$er"thing that we can e$er ha$e *nowledge of& theentire s!an of s!ace and time accessible to ourinstruments& now and in the future. This ma" seem li*ea fairl" com!rehensi$e definition& and in the !ast it hastraditionall" been regarded as s"non"mous with theentiret" of e$er"thing that e)ists. ut the de$elo!mentof ideas such as inflation suggests that there ma" besomething else be"ond the boundaries of the obser$able#ni$erse regions of s!ace and time that areunobser$able in !rinci!le& not (ust because light fromthem has not "et had time to reach us& or because ourtelesco!es are not sensiti$e enough to detect their

light.

This has led to some ambiguit" in the use of the term#ni$erse. /ome !eo!le restrict it to the obser$able#ni$erse& while others argue that it should be used torefer to all of s!ace and time. If we use #ni$erse asthe name for our own e)!anding bubble of s!acetime&e$er"thing that is in !rinci!le $isible to our telesco!es&then ma"be the term %osmos can be used to refer tothe entiret" of s!ace and time& within which ,if theinflationar" scenario is correct- there ma" be anindefinitel" large number of other e)!anding bubbles ofs!acetime& other uni$erses with which we can ne$ercommunicate. %osmologists ought to be ha!!" with thesuggestion& since it ma*es their sub(ect infinitel" biggerand therefore infinitel" more im!ortant

Further reading Gohn 2ribbin& %om!anion to the%osmos& ?eidenfeld J Nicolson& London& 1994.

Cosmic bubblesolves cosmological conundrum

%osmologists baffled b" the a!!arent e$idence that the#ni$erse is "ounger than the stars it contains ma" ha$ebeen guilt" of reading too much into our immediate

surroundings in the #ni$erse. According to a grou! of%hinese researchers& the !roblem is that we li$e in alow densit" bubble which is not t"!ical of the #ni$erseat large. ?hen the a!!ro!riate measurements are made

on large enough scales& e$er"thing slots into !lace.

The *ind of scales cosmologists deal with are muchgreater than the distances between stars. The" areinterested in the distance between clusters of gala)ies&and regard a whole gala)" of se$eral hundred billionstars& li*e our >il*" ?a"& merel" as a test !article inthe #ni$erse at large. Their efforts to measure the

scale of the #ni$erse are rather li*e tr"ing to measurethe distribution of island archi!elagos across the acificOcean from a base on one of those islands with theadded com!lication that each archi!elago is mo$inga!art from e$er" other archi!elago as the #ni$ersee)!ands. ut stars are useful in one res!ect tocosmologists. The ages of the oldest stars in our 2ala)"are at least 1 billion "ears& and ob$iousl" the #ni$ersemust be older than the stars it contains. The !uzzle&highlighted b" recent obser$ations using the Eubble/!ace Telesco!e& is that the sim!lest inter!retation ofmeasurements of the distances to nearb" clusters ofgala)ies and the rate at which the" are mo$ing a!art

suggests that the #ni$erse started e)!anding from a!oint ,the ig ang- onl" 5 billion "ears ago.

ut this inter!retation de!ends& among other things& onthe assum!tion that the #ni$erse contains e)actl"enough matter& o$erall& for gra$it" to one da" bring thee)!ansion to a halt. This critical densit" is re6uired b"the detailed theor" of the ig ang& called inflation&which most cosmologists fa$our. If the densit" of the#ni$erse is less than the critical densit"& it alters thed"namics of the situation and e)tends estimates of theage of the #ni$erse. The *e" 6uestion& which has notreall" been considered much b" cosmologists until now&

is how t"!ical the region of the #ni$erse o$er which wecan ma*e these measurements is. Gust as theh"!othetical acific islander ma!!ing the *nownuni$erse ma" be unaware of the e)istence of thecontinents on either side of the ocean& so our localbubble of s!ace ma" not gi$e us enough information to!redict the beha$iour of the entire #ni$erse. Kianging?u& of the ei(ing Astronomical Obser$ator"& andse$eral colleagues& suggest in a !a!er to be !ublished inthe Astro!h"sical Gournal that this is indeed the case.The" !oint out that although this *ind of stud" of the#ni$erse e)tends out to distances of a few hundredmillion light "ears& if the measurements made forclusters at different distances are anal"sed se!aratel"&instead of all being lum!ed together to gi$e onea$erage figure& the" show that the densit" of matter inthe #ni$erse increases the further out we loo*. On ascale of about :0 million light "ears& the densit" os onl"10 !er cent of the critical $alue& while on a scale of :00million light "ears it ma" be as much as 90 !er cent ofthe critical $alue.

The direct im!lication of this is that on the scale o$erwhich recent measurements of the e)!ansion of the#ni$erse ha$e been made& the e)!ansion rate ,gi$en b"

the socalled Eubble constant- is bigger than the o$eralla$erage e)!ansion rate b" as much as ;0 !er cent. Thatmeans that the age of the #ni$erse has beenunderestimated b" ;0 !er cent& which is almost e)actl"the correction needed to boost the age from about 5

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billion "ears to about 1 billion "ears& matching theages of the oldest stars. In cosmological terms& it ma"be that our acific islanders ha$e (ust disco$eredAmerica.

Gropingin the dark

A/TCONO>il*" ?a" 2ala)" is made of ha$ehad to cancel the celebrations. Their obser$ations&suggesting that the bright stars of our 2ala)" areembedded in a halo of thousands of billions of dar*stars& are as good as e$er. ut unfortunatel"& acom!letel" different series of obser$ations im!lies thatthere sim!l" are not enough atoms a$ailable to ma*e allthose dar* stars. There is a conflict& and bothsuggestions cannot be right. ut the good news is thatthis *ind of conflict usuall" leads ,sooner or later- to anim!ro$ement in our understanding of the #ni$erse.

It was hard enough for astronomers to come to termswith the idea that there is more to the #ni$erse thanmeets the e"e. /ince 2alileo first turned a telesco!es*"ward almost ;00 "ears ago& astronomers ha$e&naturall" enough& concentrated their attention on whatthe" can see with their own e"es. s ?ea*l"Interacting >assi$e articles. A t"!ical ?I> wouldweigh about as much as a light atom !erha!s half asmuch as a carbon atom. If there are as man" as would

be re6uired to e)!lain the motions of gala)ies& largenumbers are whizzing through the room "ou are sittingin& and through "our own bod"& without "ou noticing.

The idea is not com!letel" off the wall& because thecherished theories of !article !h"sics ,the onesregarded as ste!s towards a final theor" of e$er"thing-actuall" !redict the e)istence of such !articles. roof

that the" e)ist would be !owerful e$idence that bothcosmologists and !article !h"sicists are bar*ing u! theright tree or at least& the same tree.

ut there is a ri$al theor". erha!s all of this dar*matter is ordinar" atomic stuff& the same sort of stuffthat stars and !lanets& and oursel$es& are made of. Atleast as far as our own 2ala)" is concerned& the dar*material in the halo could be in the form of large!lanets ,(u!iters- or small& faint stars ,browndwarfs-. /uch ob(ects would be much more massi$e&indi$iduall"& than a single ?I>& but 6uite com!act inastronomical terms. And the" li$e in the halo. ?hat else

could the" be called but >assi$e Astro!h"sical %om!actEalo Ob(ects& or >A%EOs

The great thing about >A%EOs is that it ought to be!ossible to detect them not directl"& but b" theirgra$itational influence on light from e$en more distantob(ects. This de!ends on the wa" an" gra$itating massbends light that !asses near it& a *e" !rediction ofA%EO to mo$e in front of& but ha!!il" that is

!ro$ided b" a com!anion gala)" to our >il*" ?a"s"stem& *nown as the Large >agellanic %loud ,L>%-.

For a t"!ical >A%EO with a mass 1 !er cent of that ofour /un& "ou would see one of these lensing e$entse$er" 70&000 "ears or so. ut modern astronomicaltechni6ues& using solidstate chargecou!led de$icesinstead of !hotogra!hic !lates& ma*e it !ossible tomonitor millions of stars in the L>%& with com!utersanal"sing light $ariations in real time& so that as soon asa flash is detected other telesco!es can be turned onthe star of interest.

O$er the !ast few months& three teams of researchersha$e detected flashes of this *ind& bearing all thehallmar*s of gra$itational lensing caused b" >A%EOs.Flash is not 6uite the right word& because in each case

+


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the star being studied brightens u! and then fades awa"o$er a cou!le of wee*s& as the !utati$e >A%EO mo$esslowl" in front of it. This e)actl" matches the!redictions& and details of the light cur$es& as the"are called& suggest that the halo is full of >A%EOswhich each ha$e a mass ma"be 10 !er cent of that ofour /un.

If the" account for all of the mass re6uired to e)!lainhow the 2ala)" rotates& that would mean a cool fi$ethousand billion of these ob(ects in the halo of our2ala)" alone& com!ared with (ust one or two hundredbillion bright stars.

ut the !o!!ing of cham!agne cor*s had to be !ut onhold when the first results from the new ec*telesco!e& in Eawaii& were announced. This is thelargest singlemirror telesco!e in the world& 10 metresacross& and able to stud" faint ,and therefore distant-ob(ects in more detail than e$er before. One of the firstdisco$eries made b" the ec* telesco!e is that

e)tremel" distant gala)ies& far awa" across the#ni$erse& contain much more deuterium than the starsof our 2ala)" do. It might see a rather e)otic disco$er"Bbut it ma" !ut !aid to >A%EOs. The !oint is thatdeuterium ,also *nown as hea$" h"drogen- was made inthe ig ang& but cannot be made inside stars. Indeed&it is destro"ed b" stellar !rocesses. ?e *now how muchdeuterium ,and other elements- there is in stars andgala)ies because it lea$es its characteristic finger!rintin the form of lines in the s!ectrum of light from theseob(ects. @er" distant gala)ies are seen as the" werelong ago& because light from them ta*es a long time ,inthis case& billions of "ears- to reach us. /o in effect

measurements ,using s!ectrosco!"- of the amount ofdeuterium in distant gala)ies is the same as measuringthe amount of deuterium around when the #ni$erse was"oung. The snag is that& according to the standard andhighl" successful calculations of what went on at thebirth of the #ni$erse& the amount of deuterium aroundis $er" closel" tied to the total amount of atomicmatter made in the ig ang. The more deuteriumthere is& the less atomic matter there can be o$erall.

#sing the deuterium abundances measured for stars inour 2ala)"& the ig ang could ha$e !roduced ten times

more atomic matter than we see in bright stars. utusing the new figures from the ec* obser$ations& thereis barel" enough sco!e to ma*e the stars themsel$es&and no room for >A%EOs.

The im!lication is clear an" dar* matter around mustbe in the form of ?I>s& after all. Onl"& something isma*ing the stars in the L>% flic*er as we watch them&and nobod" *nows how ?I>s could be made to clum!together to ma*e the *ind of massi$e& com!act ob(ectsneeded to do the gra$itational lensing tric*.

%onfused /o are the astronomersB but the" are also

intrigued b" the !ossibilit" that whate$er is out therema" be different from an"thing the theorists ha$e "etbeen able to imagine.

The people who put the geometry into relativity

or?ith a little hel! from his friends

orGeneral relativity the hard way

orThe roots of relativity

Gust how cle$er was Albert in*ows*i ga$e a lecture in %ologne

5


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in 1905. It was this lecture& !ublished in 1909 shortl"after >in*ows*i died& that first !resented the ideas ofthe s!ecial theor" in terms of s!acetime geometr". Eiso!ening words indicate the !ower of the new insight

The $iews of s!ace and time which I wish to la" before"ou ha$e s!rung from the soil of e)!erimental !h"sics&and therein lies their strength. The" are radical.

Eenceforth s!ace b" itself& and time b" itself& aredoomed to fade into mere shadows& and onl" a *ind ofunion of the two will !reser$e an inde!endent realit".

>in*ows*i's enormous sim!lification of the s!ecialtheor" had a huge im!act. It is no coincidence thatin*ows*i had& infact& been one of a)well's theor" ofelectromagnetism.

The acce!ted wa" for a "oung academic li*e Ciemannto ma*e his wa" in a 2erman uni$ersit" in those da"swas to see* an a!!ointment as a *ind of lecturer *nownas a ri$atdozent& whose income would come from thefees !aid b" students who $oluntaril" chose to ta*e hiscourse ,an idea which it might be interesting to re$i$etoda"-. In order to demonstrate his suitabilit" for suchan a!!ointment& the a!!licant had to !resent a lectureto the facult" of the uni$ersit"& and the rules re6uiredthe a!!licant to offer three !ossible to!ics for thelecture& from which the !rofessors would choose theone the" would li*e to hear. It was also a tradition&though& that although three to!ics had to be offered&the !rofessors alwa"s chose one of the first two on thelist. The stor" is that when Ciemann !resented his listfor a!!ro$al& it was headed b" two to!ics which he hadalread" thoroughl" !re!ared& while the third& almost an

afterthought& concerned the conce!ts that under!ingeometr".

Ciemann was certainl" interested in geometr"& buta!!arentl" he had not !re!ared an"thing along theselines at all& ne$er e)!ecting the to!ic to be chosen. ut2auss& still a dominating force in the #ni$ersit" of2ttingen e$en in his se$enties& found the third item onCiemann's list irresistible& whate$er con$ention mightdictate& and the + "ear old wouldbe ri$atdozentlearned to his sur!rise that that was what he wouldha$e to lecture on to win his s!urs.

erha!s !artl" under the strain of ha$ing to gi$e a tal*he had not !re!ared and on which his career de!ended&Ciemann fell ill& missed the date set for the tal*& anddid not reco$er until after


10/31

!re!ared the lecture o$er a !eriod of se$en wee*s& onl"for 2auss to call a !ost!onement on the grounds of illhealth. At last& the tal* was deli$ered& on 10 Gune 157;.The title& which had so intrigued 2auss& was On theh"!otheses which lie at the foundations of geometr".

In that lecture which was not !ublished until 154+&the "ear after Ciemann died he co$ered an enormous

$ariet" of to!ics& including a wor*able definition ofwhat is meant b" the cur$ature of s!ace and how itcould be measured& the first descri!tion of s!hericalgeometr" ,and e$en the s!eculation that the s!ace inwhich we li$e might be gentl" cur$ed& so that the entire#ni$erse is closed u!& li*e the surface of a s!here& butin three dimensions& not two-& and& most im!ortant ofall& the e)tension of geometr" into man" dimensionswith the aid of algebra.

Although Ciemann's e)tension of geometr" into man"dimensions was the most im!ortant feature of hislecture& the most astonishing& with hindsight& was his

suggestion that s!ace might be cur$ed into a closedball. >ore than half a centur" before a)well's e6uations was $er" a!!arent at the beginningof the twentieth centur"& it is often said that thes!ecial theor" is $er" much a child of its time& and thatif


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a stro*e of uni6ue genius& decades ahead of its time&that s!rang from


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force does its wor*. /o 2amow's tunneling !rocesse)!lains how the /un generates heat.

In 19:1& 2amow was called bac* to the #//C& where hewas a!!ointed >aster of Cesearch at the Academ" of/ciences in Leningrad& and rofessor of h"sics atLeningrad #ni$ersit"& at the tender age of +. ut hisebullient nature and inde!endence of mind hardl"

suited him to a ha!!" life under /talin's regime& andwhen he was allowed to attend a scientific conferencein russels in 19:: he seized the o!!ortunit" to sta"awa"& mo$ing to 2eorge ?ashington #ni$ersit" in?ashington D%& where he was rofessor of h"sics from19:; to 1974& and then to the #ni$ersit" of %olorado inoulder& where he sta"ed until his death in 1945.

The idea of stic*ing !rotons together to ma*e heliumnuclei led 2amow to !uzzle o$er the wa" !articles mustha$e interacted under the conditions of e)treme heatand !ressure in the ig ang in which the #ni$erse wasborn. In the 19:0s& it became clear from obser$ations

of gala)ies be"ond the >il*" ?a" that the #ni$erse ise)!anding& with em!t" s!ace between the gala)iesstretching in a wa" !redicted b" the e6uations of


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from the ig ang& cooled all the wa" down to about 7. The radiation would be in the form of microwa$es&(ust li*e those used in radar or in a microwa$e o$en. Ineffect& the #ni$erse is an o$en with a tem!erature ofa few . >icrowa$es are in the radio !art of thes!ectrum& and could be detected b" radio telesco!es.ut radio astronom" was onl" (ust getting into its stridein the earl" 1970s& and 2amow didn't realise that it

might actuall" be !ossible to measure this microwa$ebac*ground.

Eis own career soon too* a new !ath& or he might ha$elearned how much !rogress the radio astronomers werema*ing and urged them to loo* for this bac*groundradiation.

In 197:& Francis %ric* and Games ?atson& wor*ing in%ambridge& re!orted that the" had disco$ered thestructure of the molecule of life& the nowfamousdouble heli) of DNA. It soon became clear that theinformation carried b" DNA the information which

tells a fertilised egg how to grow to become a humanbeing& and which tells each cell in that human beinghow to function is in the form of a genetic code&s!elled out on chemical units along the DNA doubleheli). ut nobod" *new how the code wor*ed.

At the time& 2amow was $isiting the er*ele" cam!usof the #ni$ersit" of %alifornia& and& as he laterrecalled I was wal*ing through the corridor at theCadiation Lab& and there was Luis Al$arez going withNature in his hand . . . he said Loo*& what a wonderfularticle ?atson and %ric* ha$e written. This was thefirst time that I saw it. And then I returned to?ashington and started thin*ing about it.PQ 2amowwas hoo*ed. /cientific codebrea*ing was (ust the *indof thing to intrigue him& and he soon wrote to ?atsonand %ric*& introducing himself and !resenting someideas about how the DNA code might be translated intoaction inside the cell. Eis first !a!er on the sub(ect was!ublished in 197;& and !resented the *e" idea thathereditar" !ro!erties could be characterised b" a longnumber in digital form. This is e)actl" the wa"com!uters wor*& e)!ressing e$er"thing in terms ofbinar" numbers& long strings of 0s and 1s& and it wase$entuall" confirmed that the DNA code does indeed

wor* li*e this& but with four digits ,li*e ha$ing thenumbers 0& 1& & :- instead of two. ut it too* a longtime for the code to be crac*ed and read. /ome of the*e" wor* was carried out in aris& b" Gac6ues >onod&Francois Gacob and their colleaguesB but 2amow *e!t intouch with all of the researchers in$ol$ed& contributingstimulating ideas to the debate. The code was finall"crac*ed in 1941& and it is no coincidence that %ric* and?atson recei$ed their Nobel rize the following "ear.

" then& 2amow had almost forgotten his team's!ioneering in$estigation of the tem!erature of the#ni$erse. ut in 194: two "oung radio astronomers&

Arno enzias and Cobert ?ilson& began to !uzzle o$ersome strange interference the" were getting withtheir telesco!e& a microwa$e detector built on%rawford Eill in New Gerse". The !uzzle was that

e$er"where the" !ointed the telesco!e the" found a!ersistent hiss of radio noise& corres!onding tomicrowa$es with a tem!erature of about : . The"tried e$er"thing to locate the source of theinterference& e$en ta*ing the whole antenna a!art andcleaning off the !igeon dro!!ings that had accumulatedon it& then !utting it bac* together. Nothing made an"difference. It seemed that the #ni$erse was filled with

a bac*ground of microwa$e radiation. News of thedisco$er" was !ublished in 194;& and the radio noisewas 6uic*l" e)!lained b" other researchers as thelefto$er radiation from the fireball of the ig ang. "then& the wor* of 2amow and his team had been soneglected for so long that the first accounts failed tomention them& and the fact that the" had !redicted thee)istence of this radiation& at all. #nderstandabl"& thisu!set 2amow& Al!her and Eerman greatl". ut theomission was later rectified& and there is now no doubtin the mind of an" astro!h"sicist that the radiationdisco$ered b" enzias and ?ilson accidentall" in theearl" 1940s is the radiation !redicted b" 2amow's team

in the 19;0s.

The im!ortance of the disco$er" cannot be o$erem!hasised. efore it was made& e$en the cosmologistsdid not reall" belie$e in the ig ang and therewere $er" few !eo!le who e$en called themsel$escosmologists. The" regarded cosmolog" rather li*e agreat game of chess& in which the" could wor* outtheories and construct mathematical models of the#ni$erse& with no e)!ectation that the e6uations the"scribbled on their blac*boards actuall" described thereal world.

The disco$er" of the bac*ground radiation changed allthat. After 194;& those e6uations had to be ta*enseriousl". ?ith the realization that cosmolog" wasindeed a real science& man" !h"sicists turned to itsin$estigation& leading to the situation toda"& thirt""ears later& where the stud" of the ig ang is !ossibl"the most im!ortant branch of theoretical !h"sics. As/te$en ?einberg& one of those !h"sicists who turned tocosmolog" after 194;& has summed u! the situation2amow& Al!her and Eerman deser$e tremendous creditabo$e all for being able to ta*e the earl" uni$erseseriousl"& for wor*ing out what *nown !h"sical lawsha$e to sa" about the first three minutes.P:Q

2amow died in 1945& ten "ears before the Nobel%ommittee ga$e their award for !h"sics to enzias and?ilson. Nobel rizes are ne$er awarded !osthumousl"&but it would surel" be right& on this occasion& to includethe name of Dr 2eorge 2amow in absentia. Ee hadshown how the stars shine& almost singlehandedl"in$ented the ig ang theor"& and contributed toe)!laining the secret of life itself. The ri!!lesdisco$ered b" %O


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wrote describing the m"thical ad$entures of >rTom!*ins& a mildmannered ban* cler* who has $i$iddreams in which he $isits the world of the $er" small&inside the atom& and the world of the $er" large& the#ni$erse itself. Although the" first a!!eared in the19;0s& the" still !ro$ide an e)cellent and entertainingguide to basic !h"sics. >an" readers seem to agree the collected edition was re!rinted in aril"n >onroe orAlbert onroe& il*" ?a" mo$ing a!art from one another as thes!ace between them stretches.

In the earl" 1940s& mathematical !h"sicists also to"edwith another great idea that the" could not !ro$e hadan" direct rele$ance to the #ni$erse we li$e in. This

was the conce!t of what are now *nown as blac* holes the" were onl" gi$en the name in 1949. According tothe best laws of !h"sics we *now ,ind- !ro$ed that all thematter inside a blac* hole must colla!se into thesingular !oint& a !oint of infinite densit" at which the

laws of !h"sics brea* down.

Eaw*ing's first ma(or scientific contribution was to (oinforces with enrose to turn this idea on its head.%olla!sing things must form singularities& according toenrose's wor*B now& the" found that e)!anding thingsmust come from singularities. In !articular& the" !ro$edthat the e)!ansion of the #ni$erse must ha$e startedfrom a !oint of infinite densit" where the laws of!h"sics brea* down. At the same time& the ig angtheor" was being reinforced b" the disco$er" of thenowfamous cosmic bac*ground radiation& inter!retedas the echo of the ig ang itself. In the 19+0s and

since& su!!orted b" both theor" ,!artl" Eaw*ing'stheor"- and obser$ations& the ig ang became thedefiniti$e descri!tion of the #ni$erse.

For a time& Eaw*ing left the uni$ersal studies to others&and concentrated on blac* holes. Ee found a curiousfeature of their beha$iour& in which ,according totheor" nobod" has "et seen a blac* hole- !articlesought to bubble off from the e$ent horizon. In effect&the energ" of the intense gra$itational field at thesurface of the blac* hole is con$erted into mass ,in linewith eanwhile& as the" used to sa" at the /aturda" morning!ictures& the cosmologists' $iew of the #ni$erse waschanging. At first& the" had thought that the ig angwas a uni6ue e$ent& and that the #ni$erse was destinedto e)!and fore$er. ut o$er the !ast twent" "ears therehas been increasing e$idence& culminating with theri!!les in the bac*ground radiation disco$ered last "ear

1;


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b" the %O< satellite& that this is not the case.

The onl" thing that can sto! the e)!ansion is gra$it"&and there is not enough matter in all the bright starsand gala)ies to do the tric*. ut it now seems that the#ni$erse actuall" contains ten to a hundred times morematter than we can see& in the form of dar* ob(ects.This is enough to ensure that the e)!ansion will one da"

halt& and then re$erse& crushing e$er"thing togetheragain in a singularit" the Omega oint.

/e$eral cosmologists& including Eawing& ha$e de$elo!edthe idea that this means that the #ni$erse is a blac*hole we are li$ing inside an e)tremel" large blac*hole& and will one da" suffer the fate of an" matterinside a blac* hole& as s!elled out b" enrose thirt" odd"ears ago. /o cosmologists ha$e recentl" !uzzled o$erwhat ha!!ens at the singularit" at the end of time& theOmega oint. The ob$ious guess is that the singularit"that mar*s the death of our #ni$erse mar*s the birth ofanother uni$ersal c"cle& and this is born out b" the

mathematics.

ut if one singularit" can gi$e birth to a #ni$erse& wh"can't others /!ecificall"& what ha!!ens at thesingularities that form inside blac* holes in our own#ni$erse According to some inter!retations of thee6uations ,and here I ha$e to admit that not e$er"oneagrees on this-& the singularities could form their ownbab" uni$erses. On this !icture& stuff that falls into ablac* hole singularit" is shunted sidewa"s into anotherset of dimensions& its own s!acetime. It sounds li*escience fiction& but it isn't reall" science fictionwriters are ne$er as imaginati$e as mathematical!h"sicists.

The best wa" to !icture what is going on is to imaginethe e)!anding #ni$erse as li*e the s*in of an e)!andingballoon. ?hen I was a child& m" friends and I used toindulge in the rather disgusting habit of suc*ing the s*inof a balloon to ma*e a little bubble& that could betwisted at its base to retain its sha!e. Eaw*ing's bab"uni$erses are rather li*e that& little bubbles on thesurface of the e)!anding balloon& each e)!anding intheir own right& still connected to the mother #ni$erseb" a wormhole.

And& of course& the bab" uni$erses can ha$e babies oftheir own& while our #ni$erse ma" be the offs!ring of ablac* hole that formed in another s!acetime. @er"6uic*l"& the !icture in "our mind comes to resemble amass of e)!anding frogs!awn& or the froth of a bubblebath being whi!!ed u! e$er higher.

And that's wh" /te!hen Eaw*ing is regarded as a to!ran* mathematical !h"sicist. ecause he hel!ed to!ro$e that the #ni$erse was born in a ig ang& becausehe found a wa" of combining relati$it" theor"& 6uantumtheor" and thermod"namics to describe what goes on atthe surface of a blac* hole& and because he has somee)tremel" interesting ideas about how the #ni$erse wasborn& and how it will end. ?hate$er an"one ma" tell"ou& though& he is not the greatest scientific thin*er

since Albert


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o!tical light is. The shorter wa$elength radiation has&the more easil" it is absorbed and scattered. This is wh"sunsets on il*"?a" 2ala)"& a dis* of billions of stars& mi)ed with gasand dust& swirling around in a s!iral !attern& li*e creamstirred into blac* coffee. Cegions of star formationwithin the >il*" ?a" show u! as bright dots to infrareddetectors& regions where grains of dust mainl" made ofcarbon are radiating awa" heat from colla!sing stars"stems.

Infrared studies of the $er" heart of the >il*" ?a"& thecentre of our 2ala)"& show a scene of $iolent acti$it"&hidden from the $iew of o!tical telesco!es b" the

inter$ening dust. This ma" be the location of a blac*hole millions of times more massi$e than our /un. /omeof the most astonishing disco$eries made b" infraredastronom"& though& come from far be"ond our >il*"

?a". In the de!ths of the #ni$erse& whole gala)ies&each containing billions of stars& can be seen as singleob(ects& e$ol$ing& interacting and interfering with oneanother. ?hen two gala)ies !ass close enough to oneanother to be distorted b" mutual tidal forces& thistriggers a wa$e of star formation& heating the cloudswithin the gala)ies and ma*ing them radiate in theinfrared.

One famous e)am!le is the gala)" >5& which seems ino!tical !hotogra!hs to be undergoing an enormouse)!losion. Infrared studies show that what is actuall"ha!!ening is a wa$e of star formation triggered b" tidesfrom its neighbour& >51. In other cases& strangel"distorted blobs of light in ordinar" !hotogra!hs arere$ealed in the infrared as colliding gala)ies& with twose!arate cores in the !rocess of merging.

The infrared s*" shows the #ni$erse to be a more$iolent !lace than it a!!ears in o!tical light. Infrareddetectors !robe dee!er into the heart of the >il*" ?a"

and stellar maternit" wards& and show whole gala)iestearing each other a!art and swallowing each other u!.And this is (ust the beginning of a new era ofin$estigation that reall" began (ust ten "ears ago& withthe launch of ICA/.

#ar" matter and the $niverse

Gohn 2ribbin There is more to the #ni$erse than meetsthe e"e. /tudies of the wa" stars and gala)ies mo$ethrough s!ace show that the" are being tugged b" largeamounts of unseen dar* matterB in addition& the

fa$oured cosmological theories of the origin of the#ni$erse ,ADD C


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s!read between the stars& but that still ma*es u!&together with the stars themsel$es& onl" two thirds ofthe gra$itational mass re6uired to e)!lain the locald"namics of the 2ala)".

The unseen dar* matter can be measured in terms of anumber called the masstolight ratio& >RL. This isdefined to be 1 for our /un one solar mass of matter&

in the form of a star& !roduces one solar luminosit" oflight. Oort's figures tell us that in our neighbourhood>RL is about :. ut the number is bigger for the 2ala)"as a whole. In the 1950s& s!ectrosco!ic techni6uesbecame good enough to measure details of the rotationof s!iral gala)ies& s"stems li*e our own >il*" ?a"2ala)". The $isible !art of a s!iral gala)" consists of acentral bulge of stars surrounded b" a thin dis* ofstellar material the !ro!ortions are roughl" those ofthe "ol* and white of a fried egg. If such a gala)"ha!!ens to be oriented edge on to us on the s*"& it is!ossible to measure the s!eed with which different!arts of the disc are rotating b" !lacing a narrow slit

across the image of the disc at different !laces& andmeasuring the Do!!ler shift in the s!ectrum at differentdistances out from the central bulge. >ore recentl"& thetechni6ue has been e)tended further out from thecentres of some gala)ies& using radio astronom"techni6ues to measure the $elocities of clouds ofh"drogen gas& still !art of the dis*.

?hen astronomers !lot the $elocities of the stars andclouds orbiting the dis* of a distant gala)" at differentdistances from its nucleus& the" obtain a rotation cur$ethat is usuall" $er" s"mmetrical. The stars at a certaindistance from the centre on one side of the gala)" are

mo$ing towards us at the same s!eed that stars on theother side of the gala)"& at the same distance from thecentre& are mo$ing awa" from us ,these measurementsha$e the o$erall red shift caused b" the e)!ansion ofthe #ni$erse subtracted out-. This was no sur!rise. utastronomers were sur!rised to find that outside theinnermost regions of a s!iral gala)"& on either side ofthe nucleus& the s!eed with which the stars are mo$ingis the same all the wa" across the disc. In astronomers'(argon& the rotation cur$es are e)tremel" flat.

This was a sur!rise because astronomers had assumed

that the greatest amount of mass in a s!iral gala)" wasconcentrated in the bright central nucleus& where thereare man" stars. If that were the case& then stars furtherout from the nucleus should be mo$ing more slowl" intheir orbits& in e)actl" he same wa" that the outer!lanets of our /olar /"stem ,where most mass isconcentrated in the /un& at the centre- mo$e moreslowl" in their orbits than the inner !lanets do. Themost sim!le wa" to e)!lain the flatness of the rotationcur$es is if there is a great deal of dar* matter s!readaround each s!iral gala)" in a huge unseen halo. If thishalo is roughl" s!herical& then as it rotates it will dragthe $isible& bright stars around with it in (ust the wa"

we see. In other words& most of the mass of a s!iralgala)" li*e our own is not associated with the brightstars of the nucleus ,or e$en those of the disc-& and themasstolight ratio is about 7. >o$ing u! from the scaleof indi$idual gala)ies& the ne)t le$el of structure in the

#ni$erse is !ro$ided b" clusters of gala)ies& an"thingfrom a few gala)ies to man" hundreds of gala)ies heldtogether in a swarm through mutual gra$itationalattraction. The s!eed with which each gala)" in acluster is mo$ing can be inferred from the Do!!lereffect ,once again& the o$erall cosmological redshift& inthis case for the cluster& has to be subtracted out-& andthe amount of mass in each gala)" can be estimated

from its brightness& if we assume that the masstolightratio is about 1.

The first !erson to ma*e these *inds of studies was the/wiss astronomer Fritz Mwic*". At about the same timethat Oort was disco$ering e$idence of dar* matter closeto home in the #ni$erse& Mwic*" began to find e$idenceof dar* matter on a much more im!ressi$e scale. If thegala)ies in a cluster reall" are held together in anassociation b" gra$it"& while the cluster as a wholemo$es through s!ace li*e a swarm of bees& then the$elocities of indi$idual gala)ies in the cluster must beless than the esca!e $elocit" from the cluster. ut when

Mwic*" used the Do!!ler techni6ue to measure the$elocities of gala)ies in one grou!& the %oma cluster& hefound that the" were mo$ing much too ra!idl"& relati$eto one another& to be held together b" the gra$itational!ull of all the stars in all the gala)ies of the cluster.

It loo*ed as if the fl"ing gala)ies ought to ha$e mo$eda!art& dissol$ing the cluster& long ago when the#ni$erse was "oung. And he found the same thing whenhe loo*ed at other clusters the" were all mo$inga!art much too fast to be held together b" the gra$it"of the matter we can see.

Although the e$idence that clusters of gala)ies containlarge amounts of dar* matter& with masstolight ratiosas high as :00& continued to mount& for decades fewastronomers worried much about this !roblem. In the19:0s& the conce!t of the e)!anding #ni$erse& and e$enthe fact that the #ni$erse e)tended far be"ond our>il*" ?a" 2ala)"& were new ideas& and the !ossiblee)istence of dar* matter seemed a minor !uzzlecom!ared with de$elo!ing an o$erall !icture of theorigin and e$olution of the #ni$erse& and& indeed& ofthe gala)ies themsel$es. It was onl" in the 1940s thatthe ig ang model began to become established as the

standard model of the #ni$erse& and onl" after the igang model was established that !ro!er attentionbegan to be !aid to the detail of finding an e)!lanationfor the d"namic beha$iour of gala)ies in clusters.

One of the earl" trium!hs of the ig ang theor" wasthat it seemed to e)!lain how much matter there oughtto be in the #ni$erse& and the !redictions seemed tomatch the amount of matter we can see. The famouscosmic microwa$e bac*ground radiation& disco$ered in194;& was inter!reted as a lefto$er relic from thefireball in which the #ni$erse was born& and used tocalibrate conditions in the fireball. ?ith this

calibration& the standard ig ang model !redicted thatin the ig ang !rimordial h"drogen should ha$e been!rocessed into helium in (ust the right amount toe)!lain wh" the oldest stars are made u! of about 7

1+


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!er cent helium and +7 !er cent h"drogen.

ut the same calculation also limits the o$erall amountof matter there can be in the form of h"drogen& helium&and the rest of th familiar chemical elements ,socalledbar"onic matter-. In order to match the conditionsunder which helium was manufactured in the ig angfireball to the abundances of h"drogen and helium seen

in the #ni$erse toda"& cosmologists also had to s!ecif"the o$erall densit" of bar"ons in the #ni$erse. Assumingthat all the matter in the #ni$erse is made of bar"ons,the same sort of stuff that we and e$er"thing else onRL is at least :00. ut a nowwellestablished and highl" successful ig ang theor"sa"s that >RL for all the bar"ons in the #ni$erse mustbe comfortabl" less than 100. A great deal of e)tramass seemed to be needed and it also seemed clearthat it could not be in the form of bar"ons.

Then& the ig ang theor" itself underwent a dramatictransformation with the ad$ent of an idea dubbedinflation& which describes the $er" earliest era of the#ni$erse& which !roduced the fireball in whichh"drogen was !rocessed into helium. Inflation resol$ed

some remaining !uzzles about the ig ang model& andis now !art of the standard model itself. ut whileremo$ing some old cosmological !uzzles& it ma*es onefirm !rediction& concerning the densit" of the #ni$erse.It sa"s that there should be enough matter in the#ni$erse to *ee! all the gala)ies and clusters ofgala)ies in a gra$itational gri!& in much the same wa"that clusters of gala)ies are held together b" gra$it". Inthat case& there must be so much matter in the#ni$erse that the o$erall masstolight ratio is about1000& at least three times greater e$en than the figurefor clusters of gala)ies. There must be at least tentimes& !erha!s a hundred times& more matter in the#ni$erse than can be e)!lained b" all the bar"onicmatter !roduced in the ig ang fireball. ?hat is itAnd where is it h"sicists alread" *new of one other*ind of !article that might fit the bill. For the !ur!osesof these calculations& electrons are included withbar"onic matter the mass of an electron is onl" halfof one thousandth that of a !roton and there are thesame number of electrons in the #ni$erse as there are!rotons& so the" ma*e onl" a minor contribution to thedensit". ut there is another *ind of !article whiche)ists in $ast 6uantities. These are the neutrinos& which!artici!ate in nuclear reactions that in$ol$e the wea*force ,ADD C


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filaments& which bro*e u! to ma*e gala)ies and stars a to! down scenario.

In a uni$erse dominated bo cold dar* matter& howe$er&structure begins to form on smaller scales& $er" soonafter the ig ang. %lum!s of dar* matter attractbar"onic matter& li*e water flowing into a !othole& andstructure builds from the bottom u!& with stars and

gala)ies clum!ing together to ma*e su!erclusters andfilaments. oth theoretical calculations and com!utersimulations hel! to indicate what *ind of clum!inesswould be seen in a uni$erse dominated b" hot dar*matter& and what *ind of clum!iness we would e)!ectin a #ni$erse dominated b" cold dar* matter. Auni$erse dominated b" hot neutrinos is !redicted toha$e a rather sim!le structure& li*e the cells of ahone"comb ,though not so regular-& in which brightgala)ies form onl" in welldefined sheets and not at allin the $oids. The %D> uni$erse is more mess" andcom!licated& with a richer structure that loo*s moreli*e the real #ni$erse. /heets and filaments do form&

but the" intertwine in a com!licated wa"& and the $oidsare not com!letel" em!t".

ut the sim!lest $ersions of the %D> model cannotaccount for all of the details of the distribution ofgala)ies across the s*". /ome additional influence isneeded to account for the structure of the real#ni$erse. Arguments about what this additionalinfluence might be ha$e included the suggestion thatthe nature of gra$it" might ha$e to be modified& or thatri!!les !roduced b" gra$itational radiation ,ADD Car"and ?estfield %ollege& has gi$en $er" !recise figures forthe mi)ture of materials. The" suggest that the#ni$erse is made of 49 !er cent %D>& :0 !er cent ED>and 1 !er cent bar"onic matter. And the" e$encalculate the re6uired mass for the ED> neutrinos +.7e@. This is 0.001; !er cent of the mass of an electron&

and comfortabl" below the u!!er limit of 0 e@ so farset b" e)!eriments.

An e)!eriment which confirmed that the mass of theneutrino is around +5 e@ could& therefore& be ta*en ase$idence from the laborator" that we reall" do *nowwhat the #ni$erse is made of. It would mean that themass of the lightest !article ,a!art from those with zeromass- had been !redicted from measurements of theentire #ni$erse.

artin Cees& %osmic

%oincidences& lac* /wan& 1991.

The Bright tuff

The !attern of the distribution of $isible stuff acrossthe #ni$erse shows bright gala)ies distributed infilamentar" chains and sheets& with $oids containing$er" little bright stuff. In sim!le terms& this !attern canbe e)!lained if the a$erage distribution of matteracross the #ni$erse ,bar"onic matter and dar* matter-$aries slightl" from !lace to !lace& li*e the long smoothswell of an ocean. If gala)ies then form onl" frome)ce!tionall" high !ea*s in the initial densit"

distribution ,shortwa$elength ri!!les on to! of theswell-& the" will be strongl" concentrated in the crestsrather than the troughs of the longwa$e !erturbations.The distribution of bright stuff in gala)ies will be more

19


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clum!" than the o$erall distribution of mass.

>A%EOs are no match for ?I>s /ome of the dar*matter needed to e)!lain the d"namics of s!iralgala)ies ma" be in the form of massi$e astronomicalcom!act halo ob(ects& or >A%EOs ,the name wasdeliberatel" chosen as a counter to the term ?I>-.These could be either $er" dim stars ,brown dwarfs-

each with about the same mass as Gu!iter& or blac*holes each with a mass u! to a million times that of our/un. ut although >A%EOs could account for thein$isible halos needed to e)!lain how gala)ies li*e the>il*" ?a" rotate& the" are themsel$es made frombar"ons !roduced in the ig ang& and so cannot!ro$ide the much larger amount of dar* matter neededto e)!lain the o$erall structure of the #ni$erse.Although the" are dar*& in the conte)t of thecosmological discussions the" are !art of the 1 !er centof the #ni$erse made of ordinar" atomic stuff.

In a spin over fictitious forces

ach'srinci!le& that the fi)ed stars ,we would now refer tothe distant gala)ies- actuall" define the standard ofrest. The resistance to motion ,inertia- we feel whenwe tr" to !ush an ob(ect is a result& on this !icture& oftr"ing to ma*e it accelerate relati$e to the a$eragedistribution of all the matter in the #ni$erse. /imilarl"&centrifugal forces are caused b" rotation relati$e to thedistant gala)ies& and in !rinci!le "ou could !roducecentrifugal forces b" standing still and ma*ing the#ni$erse rotate about "ou.

No wonder Arden M"lberszta(n& who discusses theim!lications in the assachusetts. This form of dar* matter would becom!osed of bar"onic material ,!rotons and neutrons-(ust li*e that of e$er"da" matter& but with a re$ersal ofleftright as"mmetr". ?ith one set of bar"ons lefthanded and the other right handed& the laws of !h"sicssa" that the" would ne$er interact with one anothere)ce!t through gra$it".

It is& of course& those laws of !h"sics themsel$es thatare reflected in the mirror world& not the !h"sicalsha!e of the !articles. The idea dates bac* to the

1970s& when the e)istence of a leftright as"mmetr" inthose laws was first recognised. It was later !ro$ed thatre$ersing the s"mmetr" in a magic mirror would indeedeliminate all contact between the !articles e)ce!tthrough gra$it".

/cience fiction writers lo$ed the idea& and de$elo!edstories in which two !lanets& one com!osed of righthanded matter and the other of left handed matter& coe)isted in the same s!ace& inter!enetrating one anotherwithout either being affected. article !h"sicists wereno less imaginati$e& coming u! with the notion of an

Alice string& a linear defect in s!ace with the curious!ro!ert" that a left handed !article circling the stringwould be turned into a right handed !article& and $ice$ersa.

0


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ut these earl" ideas onl" suggested that the mirror!articles might e)ist& not that the" must e)ist. In the1950s& interest in the mirror world reached a !ea*when it was disco$ered that one !romising $ersion of aunified theor" of e$er"thing& called su!erstringtheor"& automaticall" re6uired that e$er" t"!e of!article in our world should be mirrored b" ane6ui$alent t"!e of !article with o!!osite !arit". In this

$ersion of the stor"& the mirror !articles were dubbedshadow matter& !artl" because the name mirrormatter is sometimes used for antimatter& which is6uite a different !henomenon. %osmologists wereintrigued b" all these suggestions& because theirobser$ations show that there is a lot more matter in the#ni$erse than we can see in the form of bright stars andgala)ies. At least ten times as much dar* stuff is also!resent in the #ni$erse& holding e$er"thing togethergra$itationall" but otherwise not re$ealing its !resenceat all. /hadow matter would be ideal for the (ob if"ou could ma*e enough of it in the ig ang.

ut there was the rub.


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h"sical Ce$iew. This means that the solution to thesolar neutrino !roblem must lie in a betterunderstanding of the wa" neutrinos wor* that is& onnew !h"sics.

The !roblem us that a combination of astro!h"sics&telling us about the tem!erature& !ressure and so oninside the /un& and !article !h"sics& telling us how

man" neutrinos should be !roduced b" reactions goingon under those conditions& !redicts a higher flu) ofneutrinos at the artin ?hite& Lawrence rauss and


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middle of a bubble uni$erse& but that the densit" of theregion of s!ace we can see ma" be less than the criticaldensit"& com!ensated for b" e)tra densit" be"ond ourfield of $iew.

This is con$enient& since obser$ations b" the Eubble/!ace Telesco!e suggested recentl" that cosmologicalmodels which re6uire the critical densit" of matter ma"

be in trouble. ut there is more Those Eubbleobser$ations assume that the !arameter whichmeasures the rate at which the #ni$erse is e)!anding&the Eubble %onstant& reall" is a constant& the samee$er"where in the obser$able #ni$erse. If Linde's teamis right& howe$er& the measured $alue of the constantma" be different for gala)ies at different distancesfrom us& trul" throwing the cat among the cosmological!igeons. ?e ma" seem to li$e in a lowdensit" uni$ersein which both the measured densit" and the $alue ofthe Eubble %onstant will de!end on which $olume ofthe #ni$erse these !ro!erties are measured o$er

That would mean abandoning man" cherished ideasabout the #ni$erse& but with the bonus that b"measuring these strange !ro!erties we could ho!e tolearn something im!ortant about 6uantum cosmolog"and !article !h"sics near the lanc* densit".

The birth of the living $niverse

Gohn 2ribbin


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onewa" (ourne" to nowhere& man" researchers nowbelie$e that it is a onewa" (ourne" to somewhere toa new e)!anding uni$erse in its own set of dimensions.Instead of a blac*hole singularit" bouncing to becomean e)!loding out!ouring of energ" blasting bac* intoour #ni$erse& it is shunted sidewa"s in s!acetime.

The dramatic im!lication is that man" !erha!s all

of the blac* holes that form in our #ni$erse ma" be theseeds of new uni$erses. And& of course& our own#ni$erse ma" ha$e been born in this wa" out of a blac*hole in another uni$erse. ?hile the fact that the lawsof !h"sics in our #ni$erse seem to be rather !recisel"fine tuned to encourage the formation of blac* holesmeans that the" are actuall" finetuned for the!roduction of more uni$erses.

This is a s!ectacular shift of $iew!oint& which mostcosmologists are still struggling to come to gri!s with. Ifone #ni$erse e)ists& then it seems that there must beman" $er" man"& !erha!s e$en an infinite number of

uni$erses. Our #ni$erse has to be seen as (ust onecom!onent of a $ast arra" of uni$erses& a selfre!roducing s"stem connected onl" b" the tunnelsthrough s!acetime ,!erha!s better regarded as cosmicumbilical cords- that (oin a bab" uni$erse to its!arent.

It is relati$el" eas" to see how such a famil" ofuni$erses can continue to e)ist& and to re!roduce& oncesomething li*e our own #ni$erse e)ists. ut how did thewhole thing get started ?here did the first uni$erse& oruni$erses& come from

The *e" conce!t is 6uantum uncertaint". This sa"s thatthere is alwa"s an intrinsic uncertaint" in man" !h"sical!ro!erties of the #ni$erse and things in the #ni$erse.The most commonl" 6uoted e)am!le is the uncertaint"that relates the !osition of a !article to its motion.>omentum is a measure of where a !article is going&and 6uantum uncertaint" ma*es it im!ossible tomeasure the !osition of& sa"& an electron and itsmomentum at the same time. This is not a result of theinade6uacies of our measuring e6ui!ment& but afundamental law of nature& which has been thoroughl"tested and !ro$ed in man" e)!eriments. An ob(ect li*e

an electron sim!l" does not ha$e both a !recisemomentum and a !recise !osition.

Another !air of uncertain $ariables lin*ed in this wa" isenerg" and time. Again& the uncertaint" onl" a!!lies ona subatomic scale& as far as an" !ractical conse6uencesare concerned. ut what 6uantum !h"sics tells us isthat an" tin" region of the $acuum& which we thin* ofas em!t" s!ace& might actuall" contain a smallamount of energ" for a short time. In a sense& it isallowed to !ossess this energ" if the #ni$erse doesn'tha$e time to notice the discre!anc". The more energ"there is in$ol$ed& the shorter the time allowed. ut

because !articles are made of energ" ,< 3 mc-& thismeans that !articles are allowed to !o! into e)istencein the $acuum of em!t" s!ace. The" are made out ofnothing at all& and can onl" e)ist !ro$ided that the"

!o! bac* out of e)istence again $er" 6uic*l".

On this !icture& the 6uantum $acuum is a seething frothof !articles& constantl" a!!earing and disa!!earing& andgi$ing nothing at all a rich 6uantum structure. Thera!idl" a!!earing and disa!!earing !articles are *nownas $irtual !articles& and are said to be !roduced b"6uantum fluctuations of the $acuum.

It ma" seem that 6uantum theor" has run wild when!ushed to such e)tremes& and common sense might tell"ou that the idea is too craz" to be true. #nfortunatel"for common sense& these 6uantum fluctuations ha$e ameasurable influence on the wa" real !articlesbeha$e. The nature of the electric force betweencharged !articles& for e)am!le& is altered b" the!resence of $irtual !articles& and measurements of thenature of the electric force show that it matches the!redictions of 6uantum theor"& rather than matching u!to the common sense wa" it would beha$e in a bare$acuum.

?hat has all this got to do with the creation ofuni$erses It all hinges u!on the fact that we li$e insidea blac* hole. In 19+:&


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ro$ided that the #ni$erse is indeed closed& li*e theinside of a blac* hole& the energ" in$ol$ed in ma*ing auni$erse from a singularit" is indeed zero It is& in thewords of Alan 2uth& the ultimate free lunch. 8uantumuncertaint" allows bubbles of energ" to a!!ear in the$acuum& and energ" is e6ui$alent to mass. According tothe rules of 6uantum uncertaint"& the less massenerg"such a bubble has& the longer it can e)ist. /o wh"

couldn't a bubble with no o$erall mass energ" lastfore$er

The snag with all this& and the reason wh" Tr"on's ideadidn't cause much of a stir in 19+:& is that whate$er the6uantum rules ma" allow& as soon as a uni$ersecontaining as much matter as ours does start to e)!andawa" from a singularit"& its enormous gra$itationalforce ,imagine the !ull of gra$it" associated with anob(ect containing the entire mass of the #ni$erse in a$olume smaller than an atomic nucleus- would !ull itbac* together and ma*e it colla!se bac* into a newsingularit" in far less than the blin* of an e"e.

?hat Tr"on was sa"ing& in effect& was that not (ust$irtual !articles but $irtual uni$erses might be !o!!ingin and out of e)istence in the $acuum. In 19+:& he hadno idea how such a $irtual uni$erse might be made real.ut inflation !ro$ides a mechanism which can catchhold of a tin"& embr"onic uni$erse during that s!litsecond of its $irtual e)istence& and whoosh it u! to ares!ectable size before gra$it" can do its wor*. Then& itwill ta*e billions ,or hundreds of billions- of "ears forgra$it" to slow the e)!ansion& bring it to a halt& andma*e the uni$erse contract bac* into a singularit".

In the 1950s& this idea of a uni$erse being created outof nothing at all was de$elo!ed b" man" researchers&including Tr"on himself& 2uth& and Ale) @ilen*in& ofTufts #ni$ersit". The consensus is that "es& indeed&uni$erses can be born out of nothing at all as a result of6uantum fluctuations. And the same !owerful influenceof inflation can transform an" bab" uni$erse in thesame wa" it doesn't matter how much& or how little&matter goes into a blac* hole to ma*e the singularit"Bonce the new singularit" starts e)!anding into its ownset of dimensions to ma*e a new uni$erse& the balancebetween mass energ" and gra$itational energ" means

that the new uni$erse can be an" size at all.

ut there is still a !uzzle of finetuning& because thereis no ob$ious reason wh" inflation itself should ha$e(ust the right strength to ma*e a uni$erse li*e our ownout of a tin" 6uantum fluctuation of the $acuum. Thenatural size for a uni$erse is still down in thesubatomic region where 6uantum effects rule& on thescale of the lanc* length& 10:7 of a metre. This iswhere e$olution comes in. Nobod" would argue& theseda"s& that human beings a!!eared out of nothing at allon the face of the


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The end !roduct of this !rocess should be not one butman" uni$erses which are all about as big as it is!ossible to get while still being inside a blac* hole ,asnearl" flat as !ossible-& and in which the !arameters of!h"sics are such that the formation of stars and blac*holes is fa$oured. Our #ni$erse e)actl" matches thatdescri!tion. This e)!lains the otherwise bafflingm"ster" of wh" the #ni$erse we li$e in should be set

u! in what seems& at first sight& such an unusual wa".Gust as "ou would not e)!ect a random collection ofchemicals to suddenl" organise themsel$es into ahuman being& so "ou would not e)!ect a randomcollection of !h"sical laws emerging from a singularit"to gi$e rise to a #ni$erse li*e the one we li$e in.

efore %harles Darwin and Alfred ?allace came u! withthe idea of e$olution& man" !eo!le belie$ed that theonl" wa" to e)!lain the e)istence of so unli*el" anorganism as a human being was b" su!ernaturalinter$entionB recentl"& the a!!arent unli*elihood of the#ni$erse has led some !eo!le to suggest that the ig

ang itself ma" ha$e resulted from su!ernaturalinter$ention. ut there is no longer an" basis forin$o*ing the su!ernatural. ?e li$e in a #ni$erse whichis e)actl" the most li*el" *ind of uni$erse to e)ist& ifthere are man" li$ing uni$erses that ha$e e$ol$ed in thesame wa" that li$ing things on ercur"& @enus and the


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*u++ling pulsars

On ; Februar" 1945& Nature carried the announcementof the disco$er" of !ulsars. Then& (ust three of theseob(ects were *nown& and their nature was a !uzzle.Toda"& more than 770 are *nown& and the" are amongthe most im!ortant !henomena in astro!h"sics Gohn2ribbin

ALTEO#2E !ulsars were first identified b" chance in194+& that disco$er" had its roots in a scientificde$elo!ment carried out during ?orld ?ar Two b"scientists who had been di$erted from more abstractresearch. That de$elo!ment was radar. efore the war&astronomers onl" had obser$ations of the #ni$ersemade at $isible wa$elengths& using o!tical telesco!es.Although the fact that radio wa$es from s!ace could bedetected on


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will show fainter stars than "ou can e$er see with "ourunaided e"es& but it will ne$er re$eal twin*ling. In thesame wa"& a radio telesco!e that integrates the signalfrom a distant ob(ect for a long time might be useful inlocating the ob(ect& but it will ne$er re$ealscintillation. The new scintillation telesco!e designedb" Eewish would o!erate in real time& with a $er" ra!idres!onse to fluctuating signals.

It was more li*e an orchard than the e$er"da" image ofa telesco!e. A field co$ering four and a half acres wasfilled with an arra" of &0;5 regularl" s!aced di!oleantennas.


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star can onl" res!ond in ste! to a disturbance if it issmall enough for the a!!ro!riate message to reache$er" !art of it before the message changes. Otherwise&some !arts will be getting brighter and others dimmer&in a confused mess of $ariations. A !recise !ulse 0.014seconds long& re!eating !recisel" e$er" 1.::+:011:seconds& could onl" come from something $er" smallindeed about the size of a !lanet& or e$en less.

Eewish and his team had to face the $er" real!ossibilit"& as of No$ember 194+& that what the" haddetected was indeed a signal coming from a !lanet abeacon radiated b" another intelligent ci$ilization.Tongues onl" slightl" in their chee*s& the" s!eculatedamong themsel$es that the" might ha$e made contactwith little green men& and dubbed the source L2> 1.And Eewish decided to *ee! the lid on news of thedisco$er" until the" had carried out more obser$ations.It was (ust as well that he did.

?or*ing with another research grou! in %ambridge at

that time& I *new& as did all the astronomers& that theradio !eo!le at the %a$endish were u! to something.ut (ust what it was the" were u! to& nobod" could!rise out of them. ?ell& we thought& no doubt the"would tell us in their own good time. I wasn't reall" $er"interested& an"wa"B I was too dee!l" embroiled in thefirst real tas* that I had been set as a research student&de$elo!ing a com!uter !rogramme that would describethe wa" in which stars oscillate& or $ibrate. At the endof 194+& this seemed about as useful as s!ending "ourda"s wiring u! a field full of antennas& and I still had noclear idea of how I might turn this wor* into an"thinguseful enough to earn m" hD. " the end of Februar"

1945& howe$er& e$er"thing had changed.

Gust before %hristmas& ell found another !iece ofscruff& coming from another !art of the s*". This oneturned out to be a similar source& !ulsing withcom!arable !recision to L2> 1& but with a !eriod of1.+:+9 secondsB soon& there were two more to add tothe list& with !eriods of 1.1550 seconds and 0.7:0+1seconds& res!ecti$el". The more sources weredisco$ered& the less li*el" the little green mane)!lanation seemed. And& in an" case& carefulobser$ations of the first of these ob(ects had shown& b"

the beginning of 1945& no trace of the $ariations that"ou would e)!ect if the" were actuall" coming from a!lanet in orbit around a star. The" must& after all& benatural. The L2> tag was 6uietl" dro!!ed& and Eewishdecided it would be safe to go !ublic first with aseminar in %ambridge& to let the rest of theastronomers there in on the act& and then& almostimmediatel"& with a !a!er in Nature ,the issue dated ;Februar" 1945- announcing the disco$er" to the world.

The radio astronomers had indeed disco$ered a new*ind of ra!idl" $ar"ing radio source. The title of thedisco$er" !a!er was Obser$ation of a Ca!idl" ulsating

Cadio /ource& and the term !ulsating radio sourcesoon ga$e rise to the name !ulsar& which stuc*. utwhat were these !ulsars that ell had disco$ered

?ith the announcement of the disco$er" of !ulsars& allhell bro*e loose among the theorists. A whole new *indof !re$iousl" unsus!ected astronomical ob(ects hadbeen disco$ered& and somebod" was going to ma*etheir name b" finding an e)!lanation for the!henomenon. In the disco$er" !a!er& Eewish& ell andtheir colleagues !ointed towards what seemed theob$ious !ossibilities. If the radio !ulses were being

!roduced b" a natural !rocess& not b" an alienci$ilization& the" had to be coming from a com!actstar. Nothing else could su!!l" the energ" re6uired to!ower the !ulses. A star the size of a !lanet li*e thean" stars were *nown to oscillate& or $ibrate&breathing in and out as a result of regular $ariations inthe !rocesses !roducing energ" inside them& and$ar"ing in brightness as a result. >a"be this could alsoha!!en in com!act radio stars& The e)treme ra!idit" ofthe !ulses& said the %ambridge team& suggests anorigin in terms of the !ulsation of an entire star.P;QAnd the" !ointed out that the ra!id s!eed of thefluctuation meant that the star doing the !ulsating hadto be either a white dwarf or a neutron star. There wasone snag. Although calculations of the !ulsation !eriodsof white dwarfs had been carried out b" theorists in

1944& the basic !eriods the" came u! with were nolower than 5 seconds& a little too big to e)!lain the!ulsars. On the other hand& e$en a sim!le calculationshowed that neutron stars would $ibrate with !eriodsmuch shorter than those of the first !ulsars disco$ered&around a few thousandths of a second. ?hite dwarfsloo*ed the better bet& if some wa" could be found toallow them to $ibrate a little more ra!idl" than theearlier calculations had suggested. Further calculationsshowed that b" allowing for the effects of rotation&white dwarfs might $ibrate as ra!idl" as ten times asecond. ut as more obser$ations of more !ulsars weremade b" radio astronomers around the world ,a cou!leof dozen b" the end of 1945B scores more b" now-& itbecame clear that !ulsars could not !ossibl" be whitedwarfs& after all.

The !roblem was that the fastest !ossible $ibration!eriod& using unrealistic amounts of rotation& was stillgreater than the !eriods of some of the new !ulsarsbeing disco$ered. One disco$er" was !articularl"significant. It was made b" astronomers using the :00foot dish antenna at 2reen an*& ?est @irginia (ustabout an" *ind of radio telesco!e can obser$e !ulsars&once "ou *now what to loo* for. The" found a !ulsar

flic*ing on and off thirt" times a second& near thecentre of a glowing cloud of gas *nown as the %rabNebula. The high s!eed of the %rab !ulsar& as it became*nown& was alread" enough to !ut the white dwarfmodel in trouble ,and e$en faster !ulsars ha$e been

9


30/31

found since-. Its location& howe$er& was e$en moresignificant than its s!eed.

The %rab Nebula is actuall" the debris from a su!erno$ae)!losion one which was obser$ed from


31/31

man" gala)ies ,!erha!s most- go through a 6uasar!hase in their lifetimes. A 6uasar li$es for about ahundred million "ears& and the" are seen o$er a rangeof redshifts corres!onding to a s!an of ten billion "ears&so less than one gala)" in a hundred need be acti$e inthis wa" at an" one time. The model that best fits all ofthese obser$ations is that gala)ies are centred aroundsu!ermassi$e blac* holes& and that 6uasar acti$it" is

!roduced in "oung gala)ies b" gas and other materialfalling in to the holes. After all the debris has beenconsumed& the gala)" settles down to a 6uiet life.

Tunnelling out of inflation

INFLATION theor"& although widel" fa$oured b"cosmologists& is not the onl" wa" to e)!lain wh" thetem!erature of the #ni$erse is the same in alldirections. According to Da$id Eochberg and Thomase!hart& of @anderbilt #ni$ersit"& in Nash$ille&Tennessee& the same effect could ha$e been achie$edb" allowing energ" to lea* through 6uantum wormholes

when the #ni$erse was "oung.

The uniformit" of the #ni$erse& and in !articular theremar*able smoothness of the bac*ground radiationmeasured b" the %O< satellite& !oses a !uzzle *nownas the horizon !roblem. If the #ni$erse was born in aig ang some 17 billion "ears ago& as the weight ofe$idence suggests& and has been e)!anding e$er since&then regions on o!!osite sides of the s*" toda" couldnot ha$e been touching one

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Cosmology for Beginners -- John Gribbin

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