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Cosmology and Dark Matter II: The inflationary Universe. Jerry Sellwood. Matter-radiation equality. Next milestone Very little happens Expansion rate changes. Formation of the CMB. For the first 300,000 years, the intense radiation kept hydrogen ionized H + + e H +
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Cosmology and Dark Matter II: The inflationary Universe Jerry Sellwood
Matter-radiation equality • Next milestone • Very little happens • Expansion rate changes
Formation of the CMB • For the first 300,000 years, the intense radiation kept hydrogen ionized H+ + e H + • Constant scattering of photons • maintains thermal equilibrium with matter • makes the universe opaque • But as photons are redshifted, atoms are quite suddenly able to survive • The universe quickly becomes neutral and transparent
Re-ionization • Diffuse gas in the universe did not stay neutral for long • First stars and/or quasars emitted enough UV radiation to ionize all the diffuse gas • Density is far too low by that time for the ionized gas to be opaque
Density of Matter • BBN tells us that normal (baryonic) matter b = b / crit = 8Gb / 3H02 0.04 • In stars in galaxies, gas in galaxies, gas between galaxies in clusters of galaxies • yet most remains undetected – it is believed to fill the universe as very low density ionized gas • But galaxies and clusters of galaxies contain much more mass that emits no light
Galaxy rotation curves • Measure speed of gas using Doppler shift • Constant circular speed observed, whereas prediction from visible matter decreases • Large mass discrepancy → “dark matter halo” • Mass in dark matter (at 30kpc) 4 mass in stars & gas
Coma cluster of galaxies • Originally studied by Zwicky in 1930s • Galaxies moving much faster than expected • More than 50 times as much mass as we would have guessed from the brightness of the galaxies
Hot gas in galaxy clusters • Chandra data (Grego et al) • 0.3 – 10 keV from hot gas (+ 3 point sources) • Coincident with a distant galaxy cluster • Hot gas is gravitationally confined
Dark Matter • Some 80% – 90% of the matter in the universe is “dark”. What could it be? • Not regular protons, neutrons & electrons • Does not emit any detectable radiation • Does not feel electromagnetic forces • Does not feel the strong nuclear force • Exerts gravitational forces • Most popular guess is that it is made of WIMPs – fundamental particles that we have not yet detected in any other manner
An open universe? • Our best estimate: DM 0.250.05 • BBN gave us b 0.04 • Total matter density is therefore M 0.3crit • No other matter known • Seems to imply that we live in anopen universe, that will expand forever
The Hot Big Bang • Successfully accounts for: • Uniform expansion – Hubble’s law • Relic radiation • now cosmic microwave background • fills all space • almost perfectly isotropic • 76% hydrogen + 24% helium • set up in the first 3 minutes • But 4 serious problems with this beautiful picture were apparent by the late 1970s
Horizon Problem • Uniform temperature of CMB • At the time the radiation was emitted, the past light cone of matter in region A did not overlap with that from region B • Why are the temperatures the same to 1 part in 105? • Why are they chemically homogeneous • Apparently violates causality
Flatness problem • Recall Friedmann’s equation above • 1st term on RHS decreases as a-, where =3 for dust, =4 for radiation • 2nd term on RHS decreases as a-2 • a has increased by 1010 since BBN • Why is first term not negligible now? 1– was no more than 10-20, but not zero! • How did it start out so finely balanced?
Structure problem • What caused galaxies and clusters of galaxies to form? • Surveys reveal a clustering hierarchy • on scales that were larger than the horizon at much earlier times • How were the seeds of this frothy structure sowed?
Monopole problem • Unified gauge theories may predict the existence of one or more stable, superheavy relic particles – e.g. magnetic monopoles • They should have been formed in abundance and survived • Why is the universe today not dominated by such heavy relics?
Solution is Inflation • If the term dominates, then the scale factor a(t) grows exponentially (recall aH = da/dt) • A non-zero implies a constant energy density = c2/8G, generally interpreted as “false vacuum” or a scalar field • Postulate a large for a short period in the early universe • Long enough for a to increase by e100
Solves all 4 problems Horizon: Regions of the universe that were close enough to have become homogeneous are now far apart causality is not violated Note that this does not imply superluminal motion
Solves all 4 problems Flatness: radius of curvature increases by e100, making the universe flat to a very good approximation Monopole: If inflation occurs after the heavy relics have formed, their density is diluted by e300
Structure Problem • Quantum fluctuations during inflation cause tiny variations in the energy density • Normally they are short-lived • But during inflation, fluctuations on scales close to that of the local horizon get carried outside the horizon and become “frozen in” • Random variations about the mean energy density of a flat universe • Therefore known as “curvature fluctuations” • Re-enter the horizon later