Did we answer any questions this term?

1. What is the age of the universe as determined from the observed expansion rate and cosmological distance scale? Is there a need to invoke the cosmological constant to reconcile the ages of the oldest stars with the value of the Hubble constant?

• H_o 50 vs 100 battle has collapsed to 60 vs 85

• The major source of uncertainty in the distance scale continues to be disagreement over distances to nearby galaxies, particularly the LMC.

• Probably the most encouraging development in the last few years is the convergence on the distance to the Virgo cluster. Three somewhat independent methods, Cepheids, Surface Brightness Fluctuations, and the Planetary Nebula Luminosity Function all give a consistent distance to Virgo.

  But, HST Cepheid-distances aren't producing a consistent TF relation!

  

• The distance to Virgo is one step significantly removed from the determination of H_o to the tremendous difficulty of determining the true cosmic velocity of Virgo. This difficulty is caused both by the structure of the Virgo Cluster itself, and the unknown infall velocity of the Local Group.

• Evidence for non-zero Lambda now comes from five distinct areas:
  1. If globular clusters are 16 billion years old (or older); Lambda must be > 0.76 given the value of H_o - this has yet to be unambiguously sorted out.

  2. The fits to the power spectrum are the best in either open CDM universes or universes in which the spatial flatness is dominated by Lambda (this is consistent with 1 above)

  3. The existence of elliptical galaxies at high redshift with with apparent ages of several billion years (see Dey \etal 1997), acts like one above and requires the universe to be older than H_o^-1

  4. Standard CDM can not put enough power on small scales to account for the observations of forming elliptical galaxies at z ~4. More time is required, hence non-zero Lambda

  5. Pen (1997) shows that its not possible to reconcile CDM with both standard Big Bang nucleosynthesis and the luminosity-temperature relation for clusters of galaxies. Pen concludes that the luminosity-temperature relation, in conjunction with the measurment of a possibly low deuterium abundance by Tytler \etal (1996), greatly favors the Lambda-dominated Universe

What is the nature of the large-scale distribution of matter in the universe as traced by the three-dimensional galaxy distribution?

Its a MESS!

And it is "filled" with Voids

What is the evidence for the existence of dark matter and what is its overall contribution to the total mass density of the Universe?

• Evidence for Dark Matter is abundant on galactic and cluster scales.

• Most data indicate that OMEGA is in the range 0.1--0.3 --> there is rather little evidence for OMEGA = 1 but this may be a reflection of not obtaining the right sample to do the experiment correctly (see Strauss and Willick 1995).

• If OMEGA = 1 then it is clear that the great majority of the DM must be non-baryonic. The existence of non-baryonic gravitating DM may be key to the formation of galaxies in the Universe as such material will be much less affected by radiation drag in the early Universe and hence density perturbations can begin to grow at very early times.

How did structure form in the Universe and what formation scenarios are consistent with the current observational data?

A quick summary is trivial: we don't have a structure formation scenario that works. More definitively, no single structure formation scenario can satisfy the simultaneous large and small scale constraints.
• Traditional CDM model over-produces small scale structure when normalized to the COBE data.

• Hence to retain this model once must think of ways that small scale structure has been surpressed. Lmabda dominated models can do this as they allow for along time for radiation and matter density to equilibrate thus increasing the probability that small scale fluctuations will be erased.

Where do the baryons reside? Are they predominately inside or outside of galaxies? How efficient was the process of galaxy formation? What is the true nature of the galaxy population? Do we have a representative survey of galaxies in the nearby universe from which coherent arguments about galaxy formation and evolution can be made?

If we can make this diagram 100 years after the first catalog of galaxies was made, then it means we still have a long way to go in determining the true space density of galaxies.

So it looks like most baryons are in potentials and these potentials are either defined by easy to detect galaxies, mostly invisible galaxies, or purely gaseous systems like QSO absorption line clouds.

Summary remark:

Cosmology in the mid 90's is substantially different than it was a mere 10 years ago. Hopefully the same rate of progress will be seen in the next 10 years.

My personal bias:

• H is 85--90
• Neutrinos have mass in the range 1--3 eV
• Lambda is 0.8--0.9
• Inflation is correct
• The CDM seed particle will remain mysterious for the next few decades. SUSY will be ruled out when the Higgs field is discovered at energies in excess of 135 Gev.
• We will never understand galaxy formation

But hey, I could be wrong ...

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The Electronic Universe Project

e-mail: nuts@moo.uoregon.edu