The Contents of the Universe

HST press release

For the rest of the quarter, I will concentrate on the contents of the Universe. Over the next week or so, I will concentrate on the following two topics:

• Dark Matter in the Universe

  The Universe which we see around us actually only accounts for a small fraction of the mass of the Universe. Depending upon whom you believe, anywhere from 90 - 99 % of the Universe is composed of dark matter, i.e., matter which does not radiate or reflect a large amount of visible radiation. The dark matter is detected through its effects on the topology of the Universe (motions of nearby bodies). If Big Bang nucleosynthesis is correct, then Omega(baryon) < 0.1 (or so) and more exotic explanations explanations for the dark matter must be produced. If the constraints due to Big Bang nucleosynthesis can be gotten around then the dark matter simply may be baryons (normal matter). We now investigate these possibilities:

  • Baryonic dark matter
  • Non-Baryonic dark matter

• Large-Scale Structure in the Universe (picture)

Big Bang Nucleossynthesis Crisis?

Old Results:
• Nucleosynthesis
• Neutrino Families

  New Results (Hata et al. 1995, preprint, PRL [?]):

• abundance constraints
• number of neutrino families

Terrestrial collider results ===> N = 2.98 with an uncertainty of 0.06. The Terrestrial accelerators are sensitive to neutrinos with mass less than 45 GeV (note--an electron has a mass of 0.5 MeV and a proton has a mass of ~ 940 MeV ~ 1 GeV). Recall that the upper limit on the mass of an electron neutrino mass is something like 10 eV!

The Big Bang nucleosynthesis results are sensitive to neutrinos with mass less than around 10 MeV.

This leads to the interesting possibility that both experiments will see the electron and muon neutrinos but that only the Terrestrial accelerator experiments may be sensitive to the tau neutrino. The current upper limit on the tau neutrino mass is 24 MeV. Thus the two results could be compatible if the tau neutrino had a mass between 10 MeV and 24 MeV. However, there is one further constraint in that the tau neutrino must also be unstable, decaying on a time scale of seconds. It must decay so that it cannot contribute to the driving of the expansion of the Universe. If it decays, the effective number of neutrino families decreases.

Comment--All of the above assumes that the standard Big Bang nucleosynthesis models are correct and the measured values for the chemical abundances are correct. Further work may shed light on these questions.