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

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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)

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

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So what? Blackbody Prediction: Source: Ned Wright

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COsmic Background Explorer (COBE) Source: Ned Wright

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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

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

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

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

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

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

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

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

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COBE - Anisotropies

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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°

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

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

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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

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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

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

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

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

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

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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

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

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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...

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

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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.

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

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

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

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Microwave Anisotropy Probe Planck Explorer

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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: –www.astro.ucla.edu

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Questions? © Gary Larson – The Far Side

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