BigBang - Chapter21 Introduction...

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4/5/10 1 Chapter 21 Introduction Lemaitre was the frst to point out that iF the Universe is expanding, it must have been much hotter and denser in the past This era was frst called the big bang by steady‐state cosmologist ±red Hoyle as a term oF ridicule Now all cosmological models with an evolving Universe are called “big‐bang cosmologies” Cosmic Background Radiation (or CBR/CMB) George Gamow, Ralph Alpher and Robert Herman (1948) suggested that the Universe should have been flled with radiation shortly aFter the Big Bang. A remnant oF this radiation should still be detectable today as low intensity background microwaves. In an expanding Universe, the wavelength oF radiation From the Big Bang would also expand, becoming redshiFted and cooled. ThereFore, we should be able to detect the redshiFted cooled‐o² remnant oF the Big Bang explosion all around us (pervading the entire Universe)! ±urthermore, the remnant oF the Big Bang explosion should have a blackbody spectrum .
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4/5/10 2 • t = 0 Universe is all radiation – no matter – very hot (~billion K) • t ~ 1 minute Photons have enough energy to transform themselves into elementary particles (protons, neutrons, electrons, etc). • t = 2 minutes (900 million K) Primordial nucleosynthesis occurs - protons and neutrons fuse to form deuterium. As the universe continues to cool and expand, the heavier helium-4 nuclei are formed. • t =15 minutes The universe is 25% Helium and 75% Hydrogen • t = few thousand years Matter begins to dominate over radiation • t = 400,000 years (3000 K) Atoms form (nuclei and electrons come together) and the Universe becomes transparent to light Where/when exactly (in the history of the Universe) did the CMB originate? Decoupling or Recombination This is the point at which we observe the microwave background radiation . The decoupling of photons from matter form a “surface of last scattering” around the Earth. Surface of Last Scattering Remnant of the Big Bang •At the time of the Big Bang, the Universe was very HOT – emitting gamma‐rays •This radiation has been redshifted by the expansion of the Universe so that the peak of the radiation is now at radio/microwave wavelengths. •Current temperature is is 2.7 K Energy density in photons for a blackbody at temperatureT is At the current time, the frequency is ν o and ν o = Rν where R is scale factor Thus, radiation has a blackbody spectrum cooled by a factor R sinceT o =TR
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4/5/10 3 Matter Dominated Universe Just before the surface of last scattering, the Universe changed from radiation to matter dominated. Energy density (matter) ~ ρ m c 2 ρ m ~ 1/R 3 # density (photons) ~ 1/R 3 But, the redshifting means that energy per photon ~ 1/R. Thus, energy density (radiation) ~ 1/R 4 Energy density of radiation drops more quickly than matter as R increases! 30,000 K
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BigBang - Chapter21 Introduction...

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