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Berkeley ASTRON 10 - Lecture Notes

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The Birth of Our UniverseToday’s Lecture:(Chapter 19, pages 454-481)• Cosmic Microwave Background (CMB)• The first few minutes of the Universe• Inflation!Cosmic Microwave Background (CMB)• A. Penzias and R. Wilson (1965) discoveredthat the whole Universe radiates like a thermalemitter (black body) having T ≈ 3 K! (Peak of thespectrum at λ ≈ 1 mm).• This was strong evidence that the Universebegan in a Big Bang, killing most competingmodels.• The Big Bang has been the favored theory eversince (although there have been importantmodifications, which we’ll discuss).Cosmic Background BackgroundRadiation SpectrumWhere did the CMB come from?• The early Universe was very hot and ionized (basicallyelectrons and protons running free).• These free electrons can emit and absorb any wavelengthof light, so it was very opaque.• The Universe cooled as it expanded.• After about 400,000 years, the T = 3000 K and the protonsand electrons combined to form hydrogen (“recombination”).• Since hydrogen has discrete energy levels, most photonsare no longer absorbed, so now the universe is transparent.• The photons are “decoupled” from the hydrogen, leavingthe Universe filled with radiation in the form of a 3000 Kblack body.Where did the CMB come from? (cont.)• As the Universe expanded, the wavelength of theCMB has increased (become redder).• The CMB is now corresponds to a black body withT ≈ 3 K.• Alternatively, you can think of the CMB as lightcoming from great distances in the past that hasbeen redshifted.• They come from an opaque “wall” (like a bank offog) very far away at a redshift of z = 1000.• Can’t see beyond it with photons, even in principle!(There is some hope that someday we can see toearly times using gravitational waves or neutrinos.)Earth’s motion through Universe• The CMB looksslightly hotter in onedirection, andslightly cooler in theopposite direction byroughly 1 part in1000.• This is due to the Solar System’s motion around theMilky Way Galaxy, and the Milky Way Galaxy’smotion relative to the expanding Universe (The MWGhas a “peculiar velocity” of about 600 km/s).COBE map of CMB temperature• Tiny variation in temperature (roughly 1 part in100,000) correspond to clumps.• The smallest ones (about 10º across) are the“seeds” from which giant superclusters of galaxies(and voids) formed!• These small variations are the imprints of tinyripples in the fabric of space-time put there duringthe Big Bang.• Angular size of smaller (typical) variations firstmeasured relatively accurately in year 2000: about1º diameter. This suggests that the Universe isFLAT! (Ω = 1).Wilkinson Microwave Anistropy Probe (WMAP)An even better measurement of the fluctuationsover the entire sky.• Ωtotal = 1.0 ± 0.02: The Universe is spatially flaton the largest scales that we can see.• ΩM = 0.27, so ΩΛ = 0.73: Consistent with the SNIa results: ΩM = 0.28 and ΩΛ = 0.72.• Hubble constant (combining WMAP and otherdata): H0 = 71 ± 4 km/s/Mpc• Age of the Universe (combining WMAP and otherdata): 13.7 ± 0.2 Gyr.Content of the Universe• Total ΩM = 0.27, but only 0.04 in normalparticles (protons, neutrons, electrons).• Thus ΩM = 0.23 in dark, exotic particles(WIMPs?).• ΩΛ = 0.73 is mysterious “dark energy”• So about 95% of the Universe consists ofdark energy and dark matter--things forwhich we have no freakin’ clue what theyare!The First Few Minutest < 10-43 sec: Planck time. ???? We need“quantum gravity” theory. T > 1032 K10-35 < t< 10-6 s: “Soup” of particles,antiparticles, quarks, antiquarks, andphotons in equilibrium.Slight excess (1 part per billion!) of quarksover antiquarks formed (since our Universeis mostly “regular” particles).The First Few Minutes (cont.)t = 10-6 sec:protons andneutrons formThey combinewith theirantiparticles,producingphotons andleaving a slightexcess of matter.T = 1013 K.Protons andneutrons stillconvert back andforth freely.Time ≈ 1 secondElectrons and positrons annihilate, producingphotons. Temperature ≈ 1010 KNeutrons begin to decay into protons, electron,and antineutrinosnpe−ν−Time ≈ 100 secondsPrimordialNucleosynthesis!T ≈ 109 K• The first elements in thehistory of the Universe.• Hydrogen was 75% bymass, helium 25%• Everything else in smallamounts, the heaviestwas lithium-7• Nothing heavier wasmade--have to wait forstars!• Stopped at ≈ 10 minutesBig Bang Theory: Successes• The Universe is expanding.• The Universe has a finite age.Measured to be 13.7 GyrNo objects have ages that exceed this.• Galaxies evolve with time.• There is a sea of photons left over from the hot densebeginning: cosmic microwave background.• The light elements (specifically helium) are seen inthe same ratios as predicted by the Big Bang.• The light elements are distributed evenly throughoutthe Universe (Heavy elements are CLUMPY).Problems with thestandard Big Bang theory1) Why is the Universe so uniform in temperature?There is no way for widely separated part of theUniverse to have come into equilibrium with each other(they should be at the same temperature).2) Why is the Universe so close to being flat (Ω = 1)?Ω = 1 must have been exceedingly close to 1 in thedistant past to be nearly 1 now.CMBCMB13.7 GyrT = 2.725º K13.7 GyrT = 2.725º KWhy is Universe so flat?In standard Big Bang theory, Ω divergesfrom 1 very rapidly, unless it was1.000000000000000 at t = 0.Ω110-5105time t0 (now)Radical idea: “Inflation”Solution (A. Guth; A. Linde; 1980): Supposethe early Universe expanded extremelyrapidly (in fact, exponentially) for a while (itstarted out very small).The early volume of the observable Universewas small enough to be causally connected(light could cross it), explaining why the CMBlooks homogeneous and isotropic today.How did the Universeget to be so flat?All curvature flattens out because ofexponential expansion. (The Universe couldhave any shape, but it is too large to measure.)A numerical example• At t = 10-37 s, radius of currently observable Universewas R = 0.01 m (standard Big Bang theory).• Light could have traveled onlyd = ct = (3 x 108 m/s)(10-37 s) = 3 x 10-29 m,Light could NOT have connected the Universe at thistime -- no equilibrium!• Suppose the Universe were actually 10-52 m in size att = 10-37 s. Small enough for the observable Universeto be causally connected.• If expanded by factor of 1051 by t=10-35 s, then thenew radius


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