1. Cosmology from the Cosmic Microwave Background
Katy Lancaster

2. About Me…..

3. 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!)

4. 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

5. Before we go any further…. some things you need to know.

6. STAR / PLANET

7. GALAXY
You are here!

8. THESE OBJECTS ARE THE FOCUS OF
GALAXY CLUSTER
THESE OBJECTS ARE THE FOCUS OF
MY CURRENT WORK

9. The Cosmic Web

10. ‘Photon’ – a PARTICLE of light
‘Photon’ – a PARTICLE of light. Remember this, or please ask if you forget!

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

12. 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??

13. 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.’

14. What do you astronomers
actually DO?

16. Obtain data
Go to telescope
Download from archive
Process data
Work out what it tells us!
Publish in journal

17. In practice, need 2 approaches
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

19. 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!

20. Onto the specifics: What are we trying to achieve in Cosmology today?

21. Hubble 1929: The Universe is expanding

22. Zwicky 1933: Galaxy clusters contain Dark Matter

23. 1998: Supernovae suggest Universe is accelerating

24. 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
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

25. The Cosmic Microwave Background is central to our cosmological understanding
But what on earth is it??

26. Penzias and Wilson, 1965
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

27. Visualising the CMB…..
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.

28. But where does it come from? It all started with:
The Big Bang

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

31. COSMIC ‘SOUP’

32. COSMIC ‘SOUP’
PROTON
NEUTRON
ELECTRON

33. 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)

34. 300,000 years later…… Universe much cooler, atoms start to form…..
Hydrogen, Helium, normal ‘stuff’

35. Much cooled, atoms form, photons released

36. Universe now neutral, Photons escape
These photons, viewed today, form the Cosmic Microwave Background Radiation

37. 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

38. 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’

40. GRAVITY!

41. Back to the CMB…..

42. The CMB today
Can observe the CMB today, 13.7 billion years after the Big Bang
Radiation is much cooled: 2.73 K ( °C)
Conclusive evidence for the Big Bang theory - proves Universe was once in thermal equilibrium
So what does it look like?

43. Observe ‘blank’ sky with a radio telescope.
Rather than darkness, see Uniform, high-energy glow
Turn up the resolution......

45. 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!

46. What does the CMB tell us?
Measure the strength of the temperature differences on different scales, eg COBE 1992:

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

48. What does the CMB tell us?
Measure the strength of the temperature differences on different scales, eg WMAP 2003:

49. 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

51. 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

52. 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

53. 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)

54. 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

55. 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

56. 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......

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

58. 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’

59. Sunyaev and Zel’dovich, 1969
Postulated that the CMB could interact with the gas in galaxy clusters
The ‘Sunyaev Zel’dovich (SZ) Effect’

60. 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

61. What is it, simply?
Cluster makes partial ‘shadow’ in the CMB

62. 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

63. What does it look like?
VSA image
(from earlier!)

64. 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

65. To study Cosmology via clusters, we need lots of them A large, sensible sample of objects is usually called a ‘catalogue’

66. 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

67. 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

68. 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

69. 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

70. 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’!

71. 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

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

73. 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

74. ANY QUESTIONS?