© Gary Larson – The Far Side The Cosmic Microwave Background (CMB)

The 3 pillars of Big Bang Cosmology The expansion of the Universe The abundances of the light elements (esp. H, He) The Cosmic Microwave Background (CMB)

What is the CMB? Photons everywhere in the universe About 400 per cubic centimeter Microwaves – about 150 GHz or wavelength of 2mm

So what? Blackbody Prediction: Source: Ned Wright

COsmic Background Explorer (COBE) Source: Ned Wright

What created the CMB? If the CMB was created by objects in the universe: –They must be very evenly spread –In very good thermal equilibrium Steady State Theory –Eternal, infinite universe –Expands, with new matter being constantly created

The Big Bang Theory Very early ( t < s ) –Antiparticle / particle annihilation creates CMB photons –A billion photons for every particle.

Plasma Fluid Before 300,000yr – photons trapped in plasma – acts like a fluid.

Recombination After 300,000yr – neutral atoms form - recombination Universe becomes transparent to photons

Last Scattering Surface We see the plasma clear in a sphere centred at us Snapshot of the early universe

COBE - Anisotropies Source: Wayne Hu: background.chicago.edu

COBE – looking down Source: Wayne Hu: background.chicago.edu

COBE - Resolution Source: Wayne Hu: background.chicago.edu

COBE - Anisotropies

Horizon The distance from us to where light has had just enough time to reach us In the CMB, we can see two points that: –Are both in our horizon –Are not in each other’s Horizon on CMB ≈ 1°

COBE - Anisotropies COBE – resolution ≈ 7° –Super – horizon scale Slight over and under-densities –but not caused by gravity; too big

Inflation Quantum fluctuations –Stretched by rapid expansion Seeds of cosmic structure –Initial conditions of the universe

Hot and Cold Spots Some CMB photons must “climb” out of areas of high density –Lose energy – lower frequency –Appear colder than average Sachs-Wolfe effect –Dominates on super-horizon scales

Sub - Horizon Structure formation already at work Before recombination: –Plasma is a fluid with pressure Compression by gravity –Pressure causes fluid to rarefy Acoustic oscillations –Sakharov oscillations

Hills and Springs Hills and valleys caused by gravity Springs represent fluid pressure Source: Wayne Hu: background.chicago.edu

Oscillations on many scales Source: Wayne Hu: background.chicago.edu

Frozen Oscillations At recombination, the oscillations are frozen Photons caught at extremes form anisotropies Spatial inhomogeneity becomes angular anisotropy

Power Spectrum Source: Wayne Hu: background.chicago.edu

Fundamental Mode Many oscillations –We only see those caught at extrema Peaks on power spectrum 1 st peak – only time to compress once Sound horizon –Distance that a wave can travel before recombination

Overtones Other peaks: –2 nd peak: compression then rarefaction –3 rd peak: compression then rarefaction then compression Pure harmonic series

The Power of the Spectrum Nature of the peaks sensitive to: –Density of the universe –Proportions of baryonic matter and dark matter –The effect of gravitational waves –Etc...

Density of the Universe Determines the fate of the universe: –Too much – big crunch –Not enough – eternal expansion –Borderline: “Critical density”

Sound Horizon Length of sound horizon depends on: –speed of sound in the plasma,which depends on: – the density of the fluid Angular size of the sound horizon i.e. position of first peak, indicates the density of the universe.

The Data Data for the 1 st peak – density within a few percent of critical Source: Wayne Hu: background.chicago.edu

Baryonic Matter Remember –odd peaks are compressions –Even peaks are rarefactions Adding baryonic matter enhances odd peaks over even peaks

The Data Source: Wayne Hu: background.chicago.edu

Microwave Anisotropy Probe Planck Explorer

References and Further Reading Coles, Peter (Ed.); The Routledge Companion to the New Cosmology, Harrison, Edward; Cosmology – The Science of the Universe, Wayne Hu’s CMB tutorial: –background.chicago.edu Ned Wright’s Cosmology tutorial: –

Questions? © Gary Larson – The Far Side