1. Cosmology from the Cosmic Microwave Background
Katy Lancaster
Astrophysics Group

2. 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 !

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

4. 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

5. 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

7. ie towards the red end of the visible spectrum.
Stars
CMB
AGN
Planets
Galaxies
Clusters
REDSHIFT
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.
Stars
Planets
REDSHIFT
AGN
Galaxies
Galaxies
Clusters
Clusters
Clusters
CMB

9. Multi-wavelength information is essential in all branches of Astronomy
In my work: mainly radio frequencies, but still spanning the range GHz (requires different technology)
Can also combine with X-ray data and lensing data (optical)
More about this later!
Now onto the specifics

10. 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??

11. 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.’

12. Big questions in cosmology
Critical density: Universe expands forever
Less dense: Expansion rate increases
More dense: Universe will collapse
Accelerating: Dark energy???
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 ac
celerating?
It is being ‘pushed’ by Dark Energy

13. My Work: Very hot topics in Astrophysics at the moment!
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!

14. 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

15. The Big Bang

16. BOOM!
IN THE BEGINNING…….
EVERYTHING!

17. 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)

18. Charged particles - photons scatter (like ‘fog’)
COSMIC ‘SOUP’
PROTON
NEUTRON
ELECTRON
Charged particles - photons scatter (like ‘fog’)

19. Much cooled, atoms form, photons escape
300,000 years later
Much cooled, atoms form, photons escape

20. 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

21. 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?

22. Observe ‘blank’ sky with a radio telescope.
Rather than darkness, see Uniform, high-energy glow
High sensitivity measurements reveal......

24. 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!

25. What does the CMB tell us?
Measure the strength of the temperature differences on different scales, eg:

26. What does the CMB tell us?
Measure the strength of the temperature differences on different scales, eg:

27. 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

29. 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

30. 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

32. 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)

33. 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

34. 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......

35. X-rays - see hot gas
via Bremstrahlung emission
10-30% of total mass
Chandra Image of the Coma cluster

36. 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

37. Sunyaev and Zel’dovich
Postulated that the CMB could interact with the gas in galaxy clusters
The ‘Sunyaev Zel’dovich (SZ) Effect’

38. 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

39. No dependence on redshift!!!
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!!!

41. SZ Science 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

42. Exciting new 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

43. Cluster surveys 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

45. OCRA New detector on Torun telescope, Poland
Good results from 4 well-known clusters
Now observing larger sample
Future: array receiver, blind surveys

46. AMiBA Taiwanese project, based in Hawaii
Testing observations with 7 dishes
Ultimately: 19 dishes?
Very powerful survey instrument

47. What do we expect to see?

48. Can overcome most problems but it’s not easy!
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!

49. 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