MAPping the Universe

1. MAPping the Universe • Introduction: the birth of a new cosmology • The cosmic microwave background • Measuring the CMB • Results from WMAP • The future of cosmology Susan Cartwright University of Sheffield

2. The Birth of a New Cosmology • Cosmology is the science of the whole universe • its origin • its structure and evolution • Cosmological data must apply to the whole universe • large distances • faint sources • large uncertainties “Cosmology in the 1950s was a science of 2½ facts.” 1980s: maybe 8 facts, but all with factor ~2 uncertainty!

3. Precision Cosmology • Aim: determine cosmological parameters to a few percent • H0: the expansion rate of the universe • and how it changes over time • k: its geometry • Ω: its density • Ωb: the density of ordinary matter • Ωm: the density of all matter • ΩΛ: the “dark energy” (or “cosmological constant”) • t0: its age

4. Steps towards precision type Ia supernovae: measuring ΩΛ − Ωm abundances of light elements: measuring Ωbh2

5. More steps… • HST Key Project on Extragalactic Distance Scale • H0 using variety of methods • result: 72 ± 4 ± 7 km/s/Mpc • 10% accuracy • dominated by systematics need an independent technique: the Cosmic Microwave Background

6. What is it? • Look at the sky at wavelengths of a few mm (microwaves) • very uniform faint glow • spectrum is thermal, temperature ~3 K • discovered accidentally byPenzias and Wilson in 1965 • predicted years earlier byGamow et al. as consequence of Big Bang

7. Where did it come from? • Early universe was hot, dense and ionised • photons repeatedly interacted with protons and electrons: universe opaque • result: thermal (blackbody) spectrum • Universe expands and cools • at ~3000 K neutral atoms form: universe transparent • photons no longer interact with matter • thermal spectrum cools as expansion continues

8. What does it tell us? • The Big Bang happened! • no other way to generate a uniform thermal spectrum • The universe was very uniform when it was emitted • about 300000 years after the Big Bang • So how did galaxies form then? • well…it’s not exactly uniform

9. Anisotropies • Our rest frame ≠ CMB rest frame • dipole anisotropy of ~0.1% • Foreground sources • most obviously our own Galaxy • Density fluctuations in early universe • anisotropies of ~10-5 • seeds of galaxy formation COBE data

10. Generation of anisotropies • Density fluctuations in early universe • series of potential wells • oscillations in and out of wells • characteristic size = horizon radius • present size of horizon radius depends on geometry of universe Pictures by Wayne Hu

11. Measuring a map • Need to quantify anisotropies • express as sum of increasingly high-frequency components (similar to sythesiser) • plot amplitudes of successive components

12. CMB Power Spectrum

13. Cosmological parameter dependence Hubble parameter Cosmological constant Baryon density Spectral index Movies from Martin White’s website

14. Making a map COBE satellite: discovered the fluctuations BOOMERanG balloon: first of the new generation

15. More Mappers the Very Small Array the Cosmic Background Imager

16. WMAP the Wilkinson Microwave Anisotropy Probe orbiting the Sun/Earth L2 point better view, less background

17. WMAP results • Map covers whole sky • resolution ~0.2° • good power spectrum to 3rd peak • also measuring polarisation

18. WMAP Cosmology • h = 0.72  0.05 • Ωbh2 = 0.02260.0008 • Ωmh2 = 0.133  0.006 • Ωtoth2 = 1.02  0.02 • Ωnh2 < 0.0076 (95%) • n = 0.99  0.04 • age of universe = 13.7  0.2 Gyr (WMAP only)(also uses 2dF and Ly α) first stars born 200 Myr after Big Bang

19. Conclusion • The Universe is dominated by dark energy • why? how? what? • About 85% of the matter in the universe is non-baryoniccold dark matter • not atoms • not neutrinos • The Universe is about 14 billion years old, and will expand forever • Cosmology is no longer a science of 2½ facts!