Cosmology from the Cosmic Microwave Background

Katy Lancaster

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:

• Some key concepts in astrophysics, and what we spend our time doing!

• 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

Before we go any further….

some things you need to know.

STAR / PLANET

You are here!

GALAXY

GALAXY CLUSTER

THESE OBJECTS ARE THE FOCUS OF MY CURRENT WORK

The Cosmic Web

‘Photon’ – a PARTICLE of light.

Remember this, or please ask if you forget!

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

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.’

What do you astronomers actually DO?

•Obtain data•Go to telescope•Download from archive

•Process data•Work out what it tells us!•Publish in journal

• 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

In practice, need 2 approaches

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!

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, inbetween 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......

• 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 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......

Chandra 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

OCRA

• Prototype detector on Torun telescope, Poland

• 32m dish• Various receivers, ours

works at 30GHz• Suffers from atmospheric

contamination• Most useful observations

are made in the winter

OCRA

• We recently published results from 4 well-known clusters

• Now observing larger sample, should be able to derive more science from this

• Future: array receiver, blind surveys

• Excellent imaging instrument

AMiBA• Taiwanese project, based

in Hawaii• 90 GHz Interferometer• Hexapod mount• Testing observations with

7 60cm dishes• High significance

detections of well-know clusters, will be published ‘soon’!

AMiBA• Ultimately: 19 dishes,

1.2m diameter?• Potential problems with

the platform flexing…• Also problems with

ground emission• Very powerful survey

instrument• Also – polarisation in the

CMB

Challenges….• 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 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

ANY QUESTIONS?