Cosmology from the Cosmic Microwave Background

Katy Lancaster

08/02/08

About Me…..

• ‘Postdoc’ in the Astrophysics group at Bristol working with Professor Mark Birkinshaw, world expert in our field

• Various projects, OCRA, AMiBA

• Previously – PhD in Cambridge, working on the VSA

• MSci in Bristol (many moons ago!)

About Me…..

Talk Structure:

• The point of all this – what are we trying to achieve in the field of Cosmology?

• The Cosmic Microwave Background (relic radiation from the Big Bang)

• Galaxy clusters and the Sunyaev Zel’dovich effect

• Two new SZ experiments– OCRA– AMiBA

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!

• 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

Astronomy Research: How it Works

Onto the specifics:

What are we trying to achieve in Cosmology today?

Hubble 1929: The Universe is expanding

Zwicky 1933: Galaxy clusters contain Dark Matter

1998: Supernovae suggest Universe is accelerating

Big questions in cosmology

• Will the Universe expand forever?– 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???

But what on earth is it??

The Cosmic Microwave Background is central to our cosmological

understanding

• Observing the galaxy, detected ‘annoying level of static’ in all directions

• Pigeon poo? Aliens??

• No!

• At the same time, Dicke at Princeton predicted the existence of ‘relic radiation from the big bang’, ie the CMB

• Nobel Prize, 1978

Penzias and Wilson, 1965

The sky is BRIGHT at radio frequencies.

If we observe the sky with a radio telescope, in between the stars and

galaxies, it is NOT DARK.

Visualising the CMB…..

But where does it come from?

It all started with:

The Big Bang

IN THE BEGINNING…….

EVERYTHING!

BOOM!

COSMIC ‘SOUP’

PROTONNEUTRON

ELECTRON

COSMIC ‘SOUP’

The Big Bang• Not really an ‘explosion’ • Universe expanded rapidly as a whole and is

still expanding today as a result of the Big Bang (Hubble)

• Matter was created in the form of tiny particles (protons, neutrons, electrons)

• Too hot for normal ‘stuff’ to form (eg atoms, molecules)

• Photons scatter off charged particles – like a ‘fog’ (Thomson scattering)

• Universe much cooler, atoms start to form…..

• Hydrogen, Helium, normal ‘stuff’

300,000 years later……

Much cooled, atoms form, photons released

Universe now neutral, Photons escape

These photons, viewed today, form the

Cosmic Microwave Background Radiation

Summary: Formation of the CMB

• The Universe started with the Big Bang• It was initially hot, dense and ionised • Photons were continually scattered from charged

particles until….• ….temperature decreased and atoms formed

(neutral particles)• Photons (light) ‘escaped’ and became able to

stream freely through the Universe. • Observe the same photons today, much cooled, as

the Cosmic Microwave Background

An important aside – formation of structures

At the same time as all this was going on, structures were starting to form out of the

cosmic ‘soup’

GRAVITY!

Back to the CMB…..

The CMB today

•Can observe the CMB today, 13.7 billion years after the Big Bang

•Radiation is much cooled: 2.73 K (-270.42°C) •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

•Turn up the resolution......

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

• Tiny temperature differences (microK)• 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 COBE 1992:

A plethora of other experiments followed this up….until….

What does the CMB tell us?

• Measure the strength of the temperature differences on different scales, eg WMAP 2003:

What does the CMB tell us?• In practice, we need information from a wide range of

‘resolutions’, or scales• Measure the strength of the temperature differences on

different scales– Low resolution (eg COBE)– Higher resolution (eg WMAP)

• Theorists: come up with a model (function, like straight line y=mx +c but more complex!) including all of the physics of CMB/structure formation

• Observers: fit the model to real observations of the CMB (like drawing a line of best fit), tweaking the values of each parameter

What does this tell us?

• The function on the previous slide is complex and involves many terms including:– Density of Universe in ORDINARY MATTER– Density of Universe in DARK MATTER– Density of Universe in DARK ENERGY– (The sum 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 (critical)– Will just expand forever

• 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?– Set to solve all the mysteries…..allegedly!

• 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)

Before we move on:Quick CMB revision….

The CMB is light originating from the Big Bang

We can see it coming from all directions

The sky ‘glows’ at radio frequencies

More recent imprints on the CMB

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

• Majority of CMB photons have travelled through the Universe unimpeded

• But some have interacted with ionised material on the way

• Main contributor: Galaxy clusters

• 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......

ROSAT image of the Coma cluster

•X-rays - see hot gas via Bremstrahlung•10-30% of total mass

Cluster Gas

• Gas stripped from galaxies and sucked in from outside

• Trapped in huge gravitational potential• Hot, dense and energetic• Ionised (charged) - may interact with incident

radiation (such as the CMB)• Accurately represents the characteristics of

the whole Universe Clusters are ‘Cosmic Laboratories’

Sunyaev and Zel’dovich, 1969

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

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

What is it, exactly?

• 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

What is it, simply?

• Cluster makes partial ‘shadow’ in the CMB

What is so interesting?

• It’s INDEPENDENT of the DISTANCE of the cluster responsible

• The strength of the shadow tells us about the characteristics of the CLUSTER GAS

Mirrors UNIVERSAL CHARACTERISTICS

What does it look like?

VSA image(from earlier!)

Exciting new Science!

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

• The SZ effect is distance-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

To study Cosmology via clusters, we need lots of them

A large, sensible sample of objects is usually called a ‘catalogue’

SZ Cluster surveys• Cluster catalogues to date have been derived from X-

ray observations– Severe limitations since the X-ray signal falls off quickly with

increasing distance

• SZ surveys will enable us to generate catalogues of ALL clusters in existence (with a few caveats!)

• Cluster evolution– Study how cluster properties change as a function of

distance (and hence cluster age)

• Evolution of the Universe– Study how the number of clusters per unit volume changes

with distance: cosmology

My Work

• I previously worked with the Very Small Array, looking at both the CMB and the SZ effect

• I am now involved with two new SZ experiments, OCRA and AMiBA

• We are:– Studying known clusters

– Performing surveys to find new ones

One Centimetre Receiver Array

The other members of the OCRA team:

Mark Birkinshaw, Aziz Mohammed, Peter Wilkinson, Ian Browne, Stuart Lowe, Michael Peel, Richard Davis,Richard Battye, Andrzej Kus, Marcin Gawronski, Roman Feiler, Eugeniusz Pazderski, Bogna Pazderska

OCRA: The One Centimetre Receiver Array

• Planned 100 ‘pixel’ 30-GHz array receiver• Excellent imaging and surveying capabilities• Ideal for performing surveys to study

populations of radio sources and SZ clusters• Prototype in operation to test concept• Two elements identical to those for full OCRA• Science observations more than feasible • All observations to date made with OCRA-p

Radio Sources• Always a problem! • Can completely drown out

the cluster signal• Need accurate 30GHz

measurements of sources • OVRO/BIMA @ 30GHz for

some clusters• Recently visited the Green

Bank Telescope to observe all sources

• Much higher resolution than the Torun telescope

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

What next? OCRA-F

• 8 beams (4 x OCRA-p)• Eventually 16 horns• Under construction, will be on

the telescope early next year• Imaging capabilities

– Resolve substructure in SZ

• Trial blind surveys

Ultimate goal: 100 beams

Array for Microwave Anisotropy

Other team members:Paul T.P. Ho, Ming-Tang Chen, J.H. Proty Wu, Keiichi Umetsu, Mark Birkinshaw, Chao-Te Li, Guo-Chin Liu, Kai-Yang Lin,Patrick Koch, Yo-Wei Liao, Hiroaki Nishioka

Current status

• Located on Mauna Loa, Big Island, Hawaii

• 90GHz ‘interferometer’– Radio sources less problematic

• 9m platform• 7 60cm dishes

– CMB fluctuations contaminate!

• ‘Hexapod’ mount system• Observing nearby clusters• Upgrade imminent

Cluster maps

A2142 A2163

A1995 A2261

Summary• The Big Bang left behind radiation which we can

observe at radio frequencies today– The Cosmic Microwave Background

• The CMB has imprints upon it caused by the formation of the structures we see today (eg galaxies)

• The CMB tells us much about the Universe as a whole• Galaxy clusters may create ‘shadows’ in the CMB

– The Sunyaev Zel’dovich Effect• The SZ effect is distance-independent so very useful

for cluster physics and also Cosmology– OCRA and AMiBA are working well!

ANY QUESTIONS?