Cosmology from the Cosmic Microwave Background

Katy Lancaster

Astrophysics Group

Who am I?

• Postdoc in the Physics department working with Professor Mark Birkinshaw

• Previously: – PhD at the Cavendish Laboratory,

Cambridge, working on similar topics– MSci at Bristol 1995-1999!

• Astrophysics: ‘That branch of astronomy which treats of the physical or chemical properties of the celestial bodies. Hence astrophysicist, a student of astronomical physics.’

• OBSERVATIONAL

• Observe celestial bodies (stars, galaxies etc) at various wavelengths

• Fit theoretical models to data to choose the most appropriate

• THEORETICAL

• Simulate celestial bodies (stellar evolution, galaxy formation etc)

• Create models of possible physical processes

• OBSERVATIONAL

• Observe celestial bodies (stars, galaxies etc) at various wavelengths

• Fit theoretical models to data to choose the most appropriate

• THEORETICAL

• Simulate celestial bodies (stellar evolution, galaxy formation etc)

• Create models of possible physical processes

Stars

CMB

AGN

Planets

GalaxiesGalaxies

ClustersClusters Clusters

RE

DS

HIF

T

Stars

CMB

AGN

Planets

GalaxiesGalaxies

ClustersClusters Clusters

RE

DS

HIF

T• Aside…..

• Redshift: The Doppler shift observed due to the expansion of the Universe

• The light from an object moving away from us is shifted to longer wavelengths

• ie towards the red end of the visible spectrum.

• Multi-wavelength information is essential in all branches of Astronomy

• In my work: mainly radio frequencies, but still spanning the range 30-300GHz (requires different technology)

• Can also combine with X-ray data and lensing data (optical)

• More about this later!

• Now onto the specifics

Topics in Astrophysics…..• Solar System: planets, the sun• Stars: stellar composition, stellar evolution, star

formation, supernovae, extra-solar planets• Galaxies: structure, properties, stellar velocities (dark

matter), formation, evolution, clustering…• Active galaxies: mechanisms, power sources (black

holes)• High-energy phenomena: Gamma ray bursts• Galaxy clusters: galaxy properties, gas properties,

lensing (dark matter), super clustering….• Large scale structure, structure formation theories• Cosmology: properties of the Universe as a whole,

formation (the Big Bang), fate??

• Cosmology: ‘The science or theory of the universe as an ordered whole, and of the general laws which govern it. Also, a particular account or system of the universe and its laws.’

Big questions in cosmology

• Is the Universe open or closed?– Depends on the mean density – We can constrain this using the CMB

• What is the Universe made from?– ‘Normal’ stuff plus Dark Matter– What is Dark Matter? Particle physicists working on it!

• Why does it appear to be accelerating?– It is being ‘pushed’ by Dark Energy– We can constrain this using the CMB

Critical density: Universe expands foreverLess dense: Expansion rate increasesMore dense: Universe will collapseAccelerating: Dark energy???

My Work:

• COSMOLOGY from:– The ‘Cosmic Microwave Background Radiation

(CMB)’– The interaction of the CMB with ‘Galaxy Clusters’

via the ‘Sunyaev Zel’dovich Effect’

• OBSERVATIONAL - ie obtaining data, data processing, extracting science

• Tenerife, Poland, Hawaii, Taiwan…..

Very hot topics in Astrophysics

at the moment!

Talk Structure:

• The Big Bang and production of the Cosmic Microwave Background

• Galaxy Clusters and the Sunyaev Zel’dovich Effect

• The Science we can hope to learn via observations of the above

• Current Research

The Big Bang

IN THE BEGINNING…….

EVERYTHING!

BOOM!

The Big Bang

• Not really an ‘explosion’ • Universe expanded rapidly as a whole• Universe is still expanding today as a

result of the Big Bang • Matter was created in the form of tiny

particles (protons, neutrons, electrons)• Too hot for normal ‘stuff’ to form (eg

atoms, molecules)

PROTONNEUTRON

ELECTRON

COSMIC ‘SOUP’

Charged particles - photons scatter (like ‘fog’)

300,000 years later

Much cooled, atoms form, photons escape

Formation of the CMB

• The Universe is initially hot, dense and ionised • Photons continually scatter from charged

particles until….• ….temperature decreases and atoms form

(neutral particles)• Photons ‘escape’ and stream freely through the

Universe. • Observe the same photons today, much cooled,

as the Cosmic Microwave Background

The CMB today

•Can observe the CMB photons today, 13.7Gyr after the Big Bang•Radiation has been highly redshifted by the Hubble Expansion•Much cooled: 2.73 K (compare this with 3000K at recombination)•Conclusive evidence for the Big Bang theory - proves Universe was once in thermal equilibrium•So..... what does it look like?

•Observe ‘blank’ sky with a radio telescope.

•Rather than darkness, see Uniform, high-energy glow

•High sensitivity measurements reveal......

• Tiny temperature differences• When the CMB photons ‘escaped’,

structures were starting to form• These structures have now become

galaxies• The structure formation processes have

affected the CMB and we see the imprint as ‘hot’ and ‘cold’ spots

• Very difficult to measure!

What does the CMB tell us?

• Measure the strength of the temperature differences on different scales, eg:

What does the CMB tell us?

• Measure the strength of the temperature differences on different scales, eg:

What does the CMB tell us?

• Measure the strength of the temperature differences on different scales

• Theorists: come up with a model including all of the physics of CMB/structure formation

• Observers: fit the model to real observations of the CMB

• The model contains many parameters which describe the Universe

Parameters

• The function on the previous slide is complex and involves many parameters including:– Density of Universe in ORDINARY MATTER– Density of Universe in DARK MATTER– Density of Universe in DARK ENERGY– (The sum of which is the total density, and governs

the fate of the Universe as discussed earlier).

• We can constrain some of the big questions in cosmology by observing the CMB

Current ‘best model’

• The Universe appears to be flat– Ratio of total density to critical density =1

• But measurements suggest that only 30% of this density can come from matter– Contributions from ‘ordinary’ and ‘dark’ matter

• This points towards the existence of ‘something else’ which we call Dark Energy– Dark energy is believed to be pushing the Universe

outwards, i.e. accelerating the expansion

What next for CMB research?

• New satellite, Planck, launch date 2008?

• This, and some ground based experiments are trying to measure CMB polarisation (difficult!)

• Another route: look for ‘secondary’ features in the CMB (ie those that have occurred since the Big Bang)

Other imprints on the CMB

• Let’s forget the tiny temperature fluctuations for now!

• Majority of CMB photons have travelled through the Universe unimpeded

• Some have interacted with ionised material on the way

• Main contributor: Galaxy clusters

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

• Rich Clusters - congregations of hundreds or even thousands of galaxies

• See cluster galaxies and lensing arcs in the optical

• But only around 5% of a cluster’s mass is in galaxies (Most of the mass is in Dark Matter)

• But a sizeable fraction is found in hot gas......

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

X-rays - see hot gasvia Bremstrahlung emission

10-30% of total mass

Chandra Image of the Coma cluster

Cluster Gas

• Gas trapped in huge gravitational potential

• Hot, dense and energetic

• Ionised - may interact with incident radiation (such as the CMB)

• Believed to share the same characteristics as Universal matter

Sunyaev and Zel’dovich

• Postulated that the CMB could interact with the gas in galaxy clusters

• The ‘Sunyaev Zel’dovich (SZ) Effect’

The SZ Effect

• Low energy CMB photon collides with high energy cluster electron

• Photon receives energy boost

• Net effect: shift CMB to higher frequencies in the direction of a cluster

At low radio frequencies, observe decrement (shadow) towards a cluster.

Strength is proportional to the temperature and density of the cluster gas

No dependence on redshift!!!

• Very briefly:– SZ can be used to find how much gas

there is in a cluster compared with its total mass. This tells us about the matter density of the Universe

– SZ can also be used to constrain the distance scale - because we don’t understand the geometry of the Universe it is difficult to infer distances directly

SZ Science

• In most branches of Astronomy, it is difficult to observe very distant objects

• The SZ effect is redshift-independent, so in theory we can observe ALL clusters in existence

• Current hot topic: surveying the sky using radio telescopes to find new clusters via the SZ effect

Exciting new Science!

• Generate catalogues of ALL clusters

• Cluster evolution– Study how cluster properties change as a

function of distance (and hence cluster age)

• Evolution of the Universe– Study how the cluster number density

changes with redshift: cosmology

Cluster surveys

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

OCRA

• New detector on Torun telescope, Poland

• Good results from 4 well-known clusters

• Now observing larger sample

• Future: array receiver, blind surveys

AMiBA

• Taiwanese project, based in Hawaii

• Testing observations with 7 dishes

• Ultimately: 19 dishes?

• Very powerful survey instrument

What do we expect to see?

Problems…..• The SZ effect is TINY• Galaxy clusters contain galaxies, which may

emit radio waves and drown the SZ signal– Require further information, or observations at

multiple frequencies. – Radio galaxies are less bright at higher frequencies,

but higher frequency observations suffer from atmospheric contamination

• Remember the fluctuations in the CMB itself? They can also contaminate!– Go to higher resolution

Can overcome most problems but it’s not easy!

Summary

• The CMB is relic radiation from the Big Bang– Contains the imprint of early structure formation

• The CMB may interact with ionised structures along its path towards us– The dominant process is the ‘Sunyaev Zel’dovich

effect’ in galaxy clusters

• Observing the CMB can tell us about important cosmological parameters…

• ….but Sunyaev Zel’dovich studies are really the next step