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 CartwrightUniversity of

Sheffield

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!

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

Steps towards precision

abundances of light elements: measuring Ωbh2

type Ia supernovae:

measuring ΩΛ − Ωm

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

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 by

Penzias and Wilson in 1965 predicted years earlier by

Gamow et al. as consequence of Big Bang

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

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

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

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

Measuring a map

►Need to quantify anisotropies express as sum of increasingly high-frequency

components (similar to sythesiser) plot amplitudes of successive components

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CMB Power Spectrum

Cosmological parameter dependence

Movies from Martin White’s website

Hubble parameter

Cosmological constant

Baryon density

Spectral index

Making a map

COBE satellite:

discovered the fluctuationsBOOMERanG balloon:

first of the new generation

More Mappers

the Cosmic Background Imager

the Very Small Array

WMAP

the Wilkinson Microwave Anisotropy Probe

orbiting the Sun/Earth L2 pointbetter view, less background

WMAP results

►Map covers whole sky resolution ~0.2°

• good power spectrum to 3rd peak

also measuring polarisation

WMAP Cosmology

►h = 0.72 0.05

►Ωbh2 = 0.02260.0008

►Ωmh2 = 0.133 0.006

►Ωtoth2 = 1.02 0.02

►Ωh2 < 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

Conclusion

►The Universe is dominated by dark energy why? how? what?

►About 85% of the matter in the universe is non-baryonic cold 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!