Lecture 32: Evidence for the Big Bang Astronomy 101
The Three Pillars of the Big Bang Threefundamental pieces of evidence: Expansion of the Universe: Explains Hubble s Law Primordial Nucleosynthesis: Formation of Original 2 H, 4 He, and (Li,Be,B). Cosmic Background Radiation: Relic blackbody radiation from hot phases.
The Hot Big Bang Now: the Universe is cold & low density. as it expands, it cools matter (galaxies) gets further apart. In the past: Universe was smaller, hotter, & denser Is there any evidence of this early hot, dense phase in the past?
Where Did Helium come from? Pop I Stars (and the Sun): 70% H, 28% He, and ~2% metals Pop I metals frompop IIstar supernovae. Metal poor Pop II Stars: 75% H, 25% He, and <0.01% metals Where did the He in Pop II stars come from? If from the first stars, where are all the metals?
Primordial Nucleosynthesis When the Universe was only ~1 second old: Temperature: ~10 Billion K Too hot for atomic nuclei: Only protons, neutrons, electrons, & photons General hot, dense soup of subatomic particles & photons. As it expanded, it cooled off.
Primordial Deuterium Formation When the Universe was ~2 minutes old: Temperature: ~1 Billion K Neutrons & Protons fused into Deuterium ( 2 H) All freeneutrons go into Deuterium Leftover protons stay free as Hydrogen nuclei
Primordial Helium Formation Some Deuterium fused to form 4 He nuclei Other reactions made Li, Be, and B in very tiny quantities. By the time the Universe was ~4 minutes old: Much of the Deuterium turned into 4 He Universe cooled so much that fusion stops and no heavy elements get formed.
Aftermath After Primordial Nucleosynthesis stops: Theory Predicts: 4 H/H He/H = 20 26% 26% D/H = 0.0001 0.1% Observations: 4 He/H = 22 25% D/H = 0.001 0.02%
Current Status Predictions of Primordial Nucleosynthesis look good compared to observations: Observations: Need refinement of the primordial abundances Very difficult observations to make Theory: Need to know average density of p & n light element reactions need some refinement
Hot Early Universe After Nucleosynthesis, the Universe stays hotter than 3000 K for a long time: electrons & nuclei cannot combine to form neutral atoms Universe remains fully ionized. Free electrons easily scatter all photons. Universe is opaque to light during this time.
Blackbody Radiation Universe is filled at this time by a hot, dense, opaque ionized gas. Has a perfect blackbody spectrum. Characteristic temperature, T As the Universe expands & cools: photons redshift peak of the spectrum shifts redward d Blackbody temperature drops
Epoch of Recombination When the Universe is ~300,000 000 years old: Temperature drops below 3000 K: electrons & nuclei licombine to form atoms not enough free electrons to scatter photons Universe suddenly becomes transparent: Photons stream out through space Photon Spectrum: 3000 K Blackbody
Cosmic Background Radiation After Recombination, the Universe is filled with diffuse, relic blackbody radiation. As the Universe expands further: Blackbody photons redshift. Spectrum peak shifts to redder wavelengths, hence cooler temperatures. B d i dhif db f f By today, spectrum is redshifted by a factor of ~1000 down to ~3K
Discovery 1965: Penzias & Wilson (Bell Labs) Mapping sky at microwave wavelengths. Found a faint microwave background noise. First thought it was equipment problems (noisy amplifiers, pigeon poop in the antenna). Finally determined it was cosmic in origin. Won the Nobel Pri e in 1978 for disco ering Won the Nobel Prize in 1978 for discovering the Cosmic Background Radiation.
But, is it Blackbody Radiation? The Big Bang model makes very specific predictions: the spectrum is a perfect blackbody characterized by a single temperature. 1965 1990: 1990 Experiments with balloons, rockets, & radio antennas showed a rough blackbody spectrum Temperature ~2.7 K
COBE: Cosmic Background Explorer Satellite Launched in Nov 1989: Mapped the entire sky at Near IR to Microwave wavelengths. Searched for fluctuations in the background as evidence of early large scale structure. Far IR spectrometer mapped the spectrum in detail from 0.1 to 10 mm. Spectacular Blackbody with T=2.726 K
COBE Cosmic Background Spectrum
Spectacular Confirmation The COBE results confirm and greatly strengthen the Big Bang Model: Perfect blackbody spectrum as predicted Characterized by a single temperature as predicted Uniformly fills the Universe as predicted Details: Fine structure at part in 10 5 level is related to the largescale structure we see in the galaxies.
Evidence for the Big Bang Expansion of the Universe: CONFIRMED Hubble s Law Age is consistent with the oldest stars Primordial Nucleosynthesis: CONFIRMED Deuterium & Helium in about right amounts Cosmic Background drditi Radiation: CONFIRMED Perfect blackbody with a single temperature
Summary: Fundamental Tests of the Big Bang Primordial Nucleosynthesis Pi Primordial ldeuterium & Helium Hli Primordial light elements (Li, B, Be) Cosmic Background Radiation Relic blackbody radiation from Big Bang Temperature: T = 2.726 K