© 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 www.astro.ucla.edu

COsmic Background Explorer(COBE)

Source: Ned Wright www.astro.ucla.edu

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 < 10-2 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

θ180

=l

Source: Wayne Hu: background.chicago.edu

Fundamental Mode

• Many oscillations– We only see those caught at extrema

• Peaks on power spectrum

• 1st peak – only time to compress once

• Sound horizon– Distance that a wave can travel before

recombination

Overtones

• Other peaks:

– 2nd peak: compression then rarefaction

– 3rd 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 1st 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, 2001.

• Harrison, Edward; Cosmology – The Science of the Universe, 2001.

• Wayne Hu’s CMB tutorial:

– background.chicago.edu

• Ned Wright’s Cosmology tutorial:

– www.astro.ucla.edu

Questions?

© Gary Larson – The Far Side