Cosmology from the Cosmic Microwave Background

1. Cosmology from the Cosmic Microwave Background Katy Lancaster 08/02/08

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: • 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

5. 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!

6. OBSERVATIONAL Observe celestial bodies (stars, galaxies etc) at various wavelengths Fit theoretical models to data to choose the most appropriate Astronomy Research: How it Works • THEORETICAL • Simulate celestial bodies (stellar evolution, galaxy formation etc) • Create models of possible physical processes

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

8. Hubble 1929: The Universe is expanding

9. Zwicky 1933: Galaxy clusters contain Dark Matter

10. 1998: Supernovae suggest Universe is accelerating

11. 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 • We can constrain this using the CMB

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

13. 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

14. 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…..

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

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

18. COSMIC ‘SOUP’

19. COSMIC ‘SOUP’ PROTON NEUTRON ELECTRON

20. 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)

21. 300,000 years later…… • Universe much cooler, atoms start to form….. • Hydrogen, Helium, normal ‘stuff’

22. Much cooled, atoms form, photons released

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

24. 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

25. 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’

27. GRAVITY!

28. Back to the CMB…..

29. 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?

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

32. 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!

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

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

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

36. 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

38. 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

39. 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

40. 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)

41. Before we move on:Quick CMB revision…. The CMB is light originating from the Big BangWe can see it coming from all directionsThe sky ‘glows’ at radio frequencies

42. 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

43. 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......

44. X-rays - see hot gas • via Bremstrahlung • 10-30% of total mass ROSAT image of the Coma cluster

45. 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’

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

47. 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

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

49. 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 • MirrorsUNIVERSAL CHARACTERISTICS

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