Tuesday, October 25, 2005

The universe, dark matter, and everything

It all fits together.

Over the years, there have been many skeptics of what is currently the consensus theory -- the big bang -- of how the universe evolved from a very hot, very dense state about 13.7 billion years ago to its current stage. In the 1950s and early 1960s, an alternative -- the "steady state theory" -- briefly flourished. But, in the view of most cosmologists, this alternative took a fatal hit in 1964 with the discovery of the cosmic microwave background (CMB).

Nevertheless, a handful of big bang skeptics have persisted until the present day. Some argue, for instance, that the spectral red shifts that are generally presumed to be due to a relatively simple relationship between the distance of an object and its velocity relative to the Earth could in fact be explained by some other means.

Unfortunately for the skeptics, there is more than one type of evidence for the big bang theory. For example, measurements of the relative abundance of a few light isotopes of hydrogen, helium, and lithium are very consistent with predictions of the big bang theory on the basis of the process of cosmic nucleosynthesis of these isotopes. The steady state model has no obvious explanation for this consistency, because the predictions rest on the universe being in a very hot, dense state in which the isotopes were created about 5 minutes after the big bang.

An even deeper problem for the skeptics is that just about everything we have observed about the distant universe -- things like the expansion rate at different times, the apparent existence of "exotic" dark matter, and various detailed properties of the CMB -- all fit very nicely and neatly in the big bang theory, like the pieces of a mechanical puzzle. There is no alternative theory that explains even one of these features well, let alone all of them.

Suppose you took all the observational evidence we have regarding dark matter, the CMB, and so forth and combined all that with the theoretical assumptions behind the big bang model in order to make a detailed computer simulation of the evolution of the universe. This would be much like the way the behavior of hurricanes, say, can be simulated from what we know about the circulation of the atmosphere, ocean currents, and basic gas dynamics. Would such a simulation of the universe on a computer predict various additional features of the universe we can actually observe? Most importantly, things like the way galaxies and clusters of galaxies are distributed in space and the kinds of objects we can observe at very great distances, such as quasars and very young galaxies.

A very detailed simulation of this kind has recently been constructed and run. The results of this Millennium Simulation were announced in June of this year. (News stories here, here, here.)

An image from the 3-dimensional visualization of the Millennium Simulation

Then in August a nice article by Ron Cowen appeared in Science News -- Cosmic Computing: Simulating the universe

Basically, what any simulation allows you to do is to derive consequences and predictions that follow from given theoretical assumptions and observational data. The computer allows making numerical predictions and (sometimes) presenting them in viusal form, even when exact solutions of the underlying equations aren't known. And when the observational data is only approximate, it is possible to figure out the consequences when the data is varied, in order to determine what produces the best fit with other observations.

Here are some of the most noteworthy findings:

  • Temperature and density fluctuations in the CMB are markers for the distribution of dark matter, and as this distribution participates in the expansion of the universe, it leads to a distribution of galaxies and clusters of galaxies which is very consistent with what has been determined by large-scale galaxy surveys, especially the Sloan Digital Sky Survey and the Two Micron All Sky Survey.
  • The model confirms that the expansion of the universe is accelerating, implying the existence of some sort of dark energy.
  • Supermassive black holes and quasars could have evolved in the universe very early. Some extremely bright quasars that existed about 870 million years after the big bang must have been powered by black holes with the mass a billion times the mass of our sun.
  • The simulation shows that such supermassive black holes could have formed that early, because they would have grown more quickly in unusually dense regions of the early universe. They would also have become the cores of supermassive galaxies which can now be observed at the centers of the largest galaxy clusters.
  • Galaxies form in a bottom-up rather than top-down manner -- they start out small and grow as they capture more matter, instead of by the splitting of larger aggregations of matter. Eventually larger elements of a structure hierarchy develop -- galaxy clusters and then clusters of clusters.
  • Clusters of galaxies form within dense regions of dark matter known as halos. Among halos having the same mass, the ones that formed earliest have the densest clusters of galaxies. Hence old galaxies cluster more stongly than newer ones. That is, clusters of galaxies are denser if they formed earlier, as well as if the halo in which they formed is itself denser. (See The age dependence of halo clustering.)
  • About 85% of gravitating matter in the universe must be in the form of non-baryonic "exotic" dark matter in order for the simulation to be consistent with observations.



The Virgo Consortium - research group responsible for the Millennium Simulation

The Big Bang


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