Lecture 19 Big Bang Nucleosynthesis As with all course material (including homework, exams), these lecture notes are not be reproduced, redistributed, or sold in any form.
The CMB as seen by the WMAP satellite.!2
Observing the Cosmos d3, t3, z3 d2, t2, z2 d1, t1, z1 the Milky Way due to the finite speed of light and the vast time and physical scales of the Universe, distance and time are connected such that we can only observe each point in space at ~one time.*
Observing the CMB d3 > 10 23 km t3 ~ 400,000 years after the big bang z3 ~ 1000
prior to 400,000 years, photons would scatter off of electrons so as to be unable to travel to us (today) from that time. t ~ 400,000 years e - after big bang
History of the Universe For the first 300,000 yrs the Universe was so hot that light could not propagate freely. Electrons were not bound to atoms (atoms were ionized) and since light tends to scatter off of free electrons, the Universe was like a thick fog. microwave background light nuclei & free electrons Quarks & primordial particles e e p n n p Big bang n e e p - e - + Observable Universe, Galaxies 300,000 years 3 min 10-20 s 10-35 s
4 Pillars of the Big Bang theory I. Expansion of the Universe II. Cosmic Microwave Background III. Primordial Nucleosynthesis IV. Evolution of galaxies and large scale structure over the last ~14 billion yrs.
The Cosmological Principle When viewed on sufficiently large scales, the properties of the Universe are the same for all observers....or the Universe looks the same whoever and wherever you are. This implies that the Universe is homogenous and isotropic on large scales (i.e. over large distances). Observations of the CMB (among other things) support this idea!
4 Pillars of the Big Bang theory I. Expansion of the Universe II. Cosmic Microwave Background III. Primordial Nucleosynthesis IV. Evolution of galaxies and large scale structure over the last ~14 billion yrs.
Alpher, Bethe, & Gamow (1948) George Gamow predicted background blackbody radiation with T~5K!11
History of the Universe At 300,000 yrs after the big bang, the Universe became transparent to light. This is the era we see from cosmic microwave background radiation. microwave background We can only determine what was happening at this time using theoretical estimates and residual tracers. We cannot see this epoch directly. p n n p n p Big Bang p n - + - 300,000 years 3 min 10-35 s
<1 sec after the big bang the Universe would have been >1 Billion degrees. At these temperatures, the protons and neutrons are maintained in an equilibrium by the following electronneutrino weak interactions... Timescale for these interactions depends on the energy of the neutrinos and the number density of particles, such that... So at T ~ 1 x 10 10 K (or t ~ 0.7s), the timescale exceeds the age of the Universe. What happens then?
So at T ~ 1 x 10 10 K (or t ~ 0.7s), the timescale exceeds the age of the Universe. What happens then? Freeze-out! At this point, the number of neutrons to protons will be... [ n ] = 0.21 n + p But neutrons are not stable, so they begin to decay...all the while, the Universe continues to expand and cool.
So neutrons begin to decay, as the Universe continues to expand and cool...at some point, the Universe is cool enough that photons can no longer dissociate light nuclei. And so then protons and neutrons begin to form the light elements at t ~ 300s (or ~5 min later)... Nearly every neutron that survives ends up in a He nucleus, such that for every 2 neutrons surviving we get 1 4 He.
So, based on our knowledge of neutron decay and the theory of the Big Bang, we would expect that the resulting neutron density at t ~ 300s is... [ n ] = 0.123 n + p And the corresponding Helium abundance would be... This is a strong prediction from the Big Bang theory. Observations of 4 He, 3 He, D, support the theory.
100 sec after the big bang 100 sec after the big bang the Universe would have been ~1 Billion degrees. This is the relevant temperature and conditions for protons and neutrons to combine to form light nuclei like He. The ratio of Helium to Hydrogen that would be made is predicted very robustly. Observations indicate great agreement with the Primordial Nucleosynthesis predictions. That is, the He/H ratio in old stars agrees with what is expected from the Big Bang.
Why didn t the light elements (i.e. Hydrogen & Helium) form earlier than 3 minutes after the big bang? It was too hot!
When did the heavier elements (C, N, O, Fe, Au, etc.) form? > 400,000 years into the history of the Universe! Stars must form first!
main points from BBN discussion The big bang model makes a strong prediction for the relative abundance of hydrogen and helium in the early Universe. Observations confirm this relative abundance. The Universe started out as primarily H with some Helium (and trace amounts of Li, D) Heavier elements (C, N, O, etc.) form as part of stellar evolution.
4 Pillars of the Big Bang theory I. Expansion of the Universe II. Cosmic Microwave Background III. Primordial Nucleosynthesis IV. Evolution of galaxies and large scale structure over the last ~14 billion yrs.