MAPping the Universe

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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
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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!
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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
• O its density
• Ob the density of ordinary matter
• Om the density of all matter
• O? the dark energy (or cosmological
  constant)
• t0 its age

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Steps towards precision
type Ia supernovae measuring O? - Om
abundances of light elements measuring Obh2
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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
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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

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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

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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?
• wellits not exactly uniform

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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
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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
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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
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Cosmological parameter dependence
Hubble parameter Cosmological constant Baryon
density Spectral index
Movies from Martin Whites website
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Making a map
COBE satellite discovered the fluctuations
BOOMERanG balloon first of the new generation
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More Mappers
the Very Small Array
the Cosmic Background Imager
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WMAP
the Wilkinson Microwave Anisotropy
Probe orbiting the Sun/Earth L2 point better
view, less background
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WMAP results

• Map covers whole sky
• resolution 0.2
• good power spectrum to 3rd peak
• also measuring polarisation

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WMAP Cosmology

• h 0.72 ? 0.05
• Obh2 0.0226?0.0008
• Omh2 0.133 ? 0.006
• Ototh2 1.02 ? 0.02
• Onh2 lt 0.0076 (95)
• n 0.99 ? 0.04
• age of universe 13.7 ? 0.2 Gyr
• (WMAP only)(also uses 2dF and Ly a)

first stars born 200 Myr after Big Bang
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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!