History of the Early Universe

We will not (at this time), consider the detailed evolution of the Universe before ~ 0.000001 seconds when the Temperature was greater than 10,000,000,000,000 Kelvin. This early epoch contains the Planck era, the GUTs era, and Electro-Weak era. Here, we will concentrate on the times when we believe we have a good understanding of what is going on.

A. Creation < t < 10**(-6) s

• before about 0.000001 seconds, T was greater than 10,000,000,000,000 Kelvin and matter and radiation were in thermal equilibrium, that is, the creation and annihilation of material

  

  was percolating along at a rate defined solely the temperature of the Universe. Nothing else mattered. The creation of matter via the annihilation of photons and the liberation of virtual pairs and the annihilation of matter and the production of photons were balanced, however ...

  The density and temperature were also high enough so that the (weak) interactions between neutrinos and other particles were fast enough to maintain a balance between the neutrinos and the Universe:

  

  In addition, there were also processes like:

  • electron + positron <--> nu + anti-nu

    nu + electron <--> nu + electron (scattering)

  The upshot of the above (weak) interactions is that the neutrinos were able to talk to all of the other kinds of matter in the Universe at this time. As a result, (1) all particles were able to share energy and so were able to homogenize as the Universe evolved; and (2) neutrinos were not able to travel very far from their birth sites before they interacted with matter (the Universe was opaque to neutrinos as well as to photons).

B. 10**(-6) s < t < 0.01 s, 10**11 K < T < 10**13 K

Above T = 10**13 K, protons and anti-protons and neutrons and anti-neutrons can be copiously produced. At T ~ 10**13 such pair production stops while annihilation continues. This causes most of the matter/anti-matter to convert into photons.

C. 0.01 s < t < 0.1 s, 3 x 10**10 K < T < 10**11 K

Density and T no longer high enough to maintain strong coupling between neutrinos and other forms of matter ===> neutrinos de-couple from the Universe (this de-coupling is in the same sense as the photons de-couple at the Epoch of Recombination). At this point the Universe becomes transparent to neutrinos! If we ever become technologically advanced enough to detect low energy neutrinos, then we, in principle, would be able to probe directly the Universe when it was less than ~ 1 second old!

D. Nucleosynthesis Era

The Big Picture

The era of Nucleosynthesis starts when the temperature drops below ~ 10**9 K (and density ~ 1 gram per cubic centimeter) -- at higher T, one cannot make complex nuclei. The photons are so hot that simple nuclei would be destroyed rapidly. In particular, any deuterium (1 proton + 1 neutron) which was produced would be rapidly destroyed.

An amusing note is that our understanding of nuclear reactions is much better for the conditions found in the Big Bang than for the conditions found in the interiors of stars!! The Big Bang is hotter than the interiors of stars. Terrestrial experimenters actually perform their experiments at temperatures closer to Big Bang temperatures than to the low temperatures found in say the core of the Sun.

Nucleosynthesis roadmap

Nucleosynthesis (yields)

As is apparent, the Universe does not not make massive elements. This due to the fact that elements with A = N(neutrons) + N(protons) = 5 and 8 are unstable. In particular, the intermediate step for the 3 alpha process ([1] Helium + Helium ---> Berylium, [2] Berylium + helium ---> Carbon), Berylium has A = 8 and decays into 2 helium nuclei with a half-life on the order of 10**(-16) seconds! In the early Universe, the density is too low for the Berylium to capture a helium nucleus before it decays. In stars, the densities are much higher and the capture rates are high enough to allow carbon to form.

• Tests and Implications of Big Bang Nucleosynthesis

  • The Helium Test
    • Hydrogen and Helium abundance in the Universe
  • Two Implications
    • Constraints on Omega from Nucleosynthesis
    • Constraints on Types of Neutrinos

E.Recombination -- t ~ 300,000 yr, T ~ 3,000 K