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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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
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