The Biggest Things in The Biggest Things in the Universethe Universe

Thursday, February 21Thursday, February 21

Large scale structure of the universeLarge scale structure of the universe = how galaxies and clusters of galaxies

are distributed through space.

To visualize large scale structure, we need a mapmap of the universe.

Making maps isn’t easy.

Early maps of the Earth were faulty because of lack of information.

Making maps isn’t easy.

You must select what information goes on your map.

Making maps isn’t easy.

If the information you want isn’t available, you must use an

imperfect substitute.

Ideally, we would like to know how all all the energy & mass is

distributed through space.

This is difficult (and perhaps useless).

“…the Cartographers Guild drew a Map of the Empire whose size was that of the

Empire, coinciding point for point with it. The following Generations, who were not so fond of the Study of Cartography, saw the vast Map to be Useless and permitted it to

decay and fray under the Sun and winters.” – Jorge Luis Borges

Although dark energy & dark matter make up most of the universe,

they’re invisible.

Thus, astronomers map the large scale structure of the universe by mapping the positions of galaxies.

Although stars in galaxies provide only 0.4% of the universe, they’re good “markers” for large scale structure.

Dark energy: smoothly distributed. Dark matter: clumped in “halos” around starry galaxies.

An advantage of mapping the positions of galaxies:

Galaxies are easy to identify.

Sometimes galaxies are caught in the act of merging (is it one galaxy,

or two?), but that’s fairly rare.

A disadvantagedisadvantage of mapping the positions of galaxies:

When we simply map their position on the sky, information is lost.

Galaxies are spread through 33 dimensions.

When we make a 2-d map of the sky, information about the 3rd dimension (distance from us) is suppressed.

To make a full 3-d map of the positions of galaxies, we need to

know the distance to each galaxy.

We can “mass produce” measurements of galaxy distances.

Why it’s usefuluseful to know H0:

Measure redshift of galaxy: (λ-λ0)/λ0

Compute radial velocity: v = c (λ-λ0)/λ0

Compute distance: d = v / H0

Cheap, fast way to find distance!

Flashback slide!

Astronomers have done large “redshift surveys” of galaxies.

One such survey was the “2dF” galaxy survey, with >200,000 galaxy redshifts.

The 2dF “slice of the universe” shows a bubbly or spongy structure.

One way to visualize the 3-d structure of the universe is to do a simulated “fly-through”.

Big Dipper

Southern Cross

To map the whole universe, you need a redshift survey that covers the whole sky.

This is hard to do.

1) From the northern hemisphere, you can’t see the southern sky.

2) Dust in the Milky Way blocks our view of distant galaxies.

Dust blocks our view in these directions -

but not these directions.

Best attempt (IMHO) at an all-sky redshift survey (full view of the universe):

Sloan Digital Sky Sloan Digital Sky Survey (SDSS)Survey (SDSS)Telescope in New

Mexico: has measured redshifts for 800,000

galaxies.

The Sloan Digital Sky Survey has covered about ¼ of the sky.

Green = portion of sky surveyed.Green = portion of sky surveyed.

↓Polaris

Galaxies ≈ 0.1 Mpc across. Clusters of galaxies ≈ 1 Mpc across.

The biggest things we can see are superclusters superclusters (clusters of clusters): elongated filaments ≈ 30 Mpc long.

Superclusters are separated by voidsvoids, ≈ 100 Mpc across.

We live in the Virgo SuperclusterVirgo Supercluster.

10 Mpc

Why aren’t there superduperclusters?

Why are the largest structures < 100 Mpc across?

If there were superduperclusters as long as the Hubble distance (4300 Mpc), we could see them.

Superclusters grew from slightly overdense regions. Voids grew from

slightly underdense regions.

These density irregularities began as quantum fluctuations that were

then expanded by inflation.

Inflation must have produced density fluctuations on all scales…

…including the superdupercluster scale!

Superclusters are the largest structures in the universe because…

…larger objects have not yet had time to collapse.

A dense region takes time to collapse. For the largest dense regions, collapse

time is longer than Hubble time.

It’s possible (with a big computer) to simulate the formation of

large scale structure.

- Make a large (imaginary) box. - Fill it with (simulated) massive objects. - Make sure the density fluctuations are those predicted by inflation.

-Let ’er rip.

then now

redshift 29

redshift 0

(The size of the box grows from 1.5 Mpc to 43 Mpc.)

What do we see? Formation of structure is “bottom-up” -

smallest structure forms first.

First: dwarf galaxies Next: ordinary galaxies

Next: clusters of galaxies Now:Now:

superclusters

““The Cosmic Web”The Cosmic Web”

cluster

Rate at which structures grow depends on 2 competing factors:

1) Gravity compresses dense regions.2) Expansion of the universe pulls

them apart.

If the expansion of the universe continues to speed up, expansion will

win! No superduperclusters…

Tuesday’s Lecture:

Reading: Chapter 12

Formation and Evolution of Galaxies