UNDERSTANDING TEN BILLION YEARS OF COSMIC EVOLUTION
A team of astronomers from the University of Durham is now able to explain, for the first, time why galaxies are clustered together in space in just the way they are. Simulations of the evolution of the universe, carried out using the largest supercomputers, show that great clusters of galaxies populating the universe now are the descendants of primitive superclusters already in existence when the universe was only one tenth its present age. These results will be presented by Professor Carlos Frenk of the University of Durham on Tuesday 10th August at the UK's National Astronomy Meeting in Guernsey, Channel Islands (10 - 13 August).
In an important breakthrough, American astronomers recently discovered an enormous supercluster of many tens of galaxies already in place when the universe was no more than about one tenth of its current age. Now, the Durham team have shown that these incipient superclusters are the progenitors of today's great clusters of galaxies. Their simulations follow the evolution of the clustering pattern of galaxies, from their infancy to the present, revealing the complex processes by which galaxies like our own Milky Way have grown over 10 billion years of cosmic evolution.
The American researchers used the largest telescope in the world, the 10-metre Keck telescope located in the island of Hawaii, to measure how fast these galaxies are receding because of the expansion of the universe. From their velocity, or redshift, astronomers can determine how far away these galaxies are and for how long their light has been travelling before it reaches telescopes on Earth. These galaxies are so distant that their velocities are a significant fraction of the speed of light and we see them as they were when the universe was a small fraction of its current age. They appear very young and are undergoing their very first episode of rapid star formation. Astronomers were puzzled to find that these galaxies were not distributed at random in the early universe, but seem instead to have congregated in groups, clusters and superclusters, much as their descendants do, 10 billion years later.
For the past twenty years, theorists have speculated that the main agent responsible for the formation of galaxies is a kind of dark matter known as cold dark matter, which is composed of exotic, yet to be discovered, elementary particles, much smaller than individual atoms. If these particles exist, they would dominate the evolution of the universe and cause ripples in the early universe to grow into ever larger structures. Ripples in the early universe were discovered earlier this decade by NASA's COBE satellite. They are the fossil records of the embryos from which galaxies later grew.
The first models of a hypothetical cold dark matter universe were calculated in the mid 1980s using computers that, by today's standards, were feeble. Yet, these first calculations indicated that, if the universe were indeed dominated by cold dark matter, then early galaxies would be born already congregated in the kind of superclusters that have now been observed with the Keck telescope.
Today, computer simulations can take awesome proportions. For example, the Virgo consortium, a multinational team of astronomers from the UK, Germany, the USA and Canada, recently carried out a simulation of virtually the entire visible universe, the "Hubble-volume". This employed 1 billion particles, compared to only 30,000 used in the simulations of the 1980s. This huge calculation, in conjunction with other smaller-scale ones, confirmed the important prediction of 15 years ago that bright galaxies should be born in superclusters. However, modern techniques allow astronomers to go beyond this and ask, "What are the descendants of these huge complexes of galaxies in the early universe?" The supercomputer reveals that these primitive structures are destined to become the majestic great clusters of galaxies, like the nearby Coma cluster, that dominate the appearance of our universe today.
The observed pattern of galaxy clustering has encoded within it crucial information about the conditions prevailing in the universe at very early times, close to the initial Big Bang singularity. The supercomputer simulations make it possible to decode this information and learn about the fundamental processes that control cosmic evolution.
To explain why galaxies are clustered the way they are, the Durham team found that they needed to invoke, in addition to cold dark matter, a mysterious form of energy known as the "cosmological constant". This is precisely the form of energy that observers claimed last year to have discovered from studying very distant supernovae.
The cosmological constant, first proposed by Einstein in the 1930s, plays a key role in regulating the rate at which our universe expands. Cosmologists and supernova experts seem to be converging on the same explanation for their findings. Not only are distant supernovae dimmer than anticipated due to the existence of the cosmological constant, but the entire pattern of galaxy clustering reflects the influence of this energy acting over the lifetime of the universe. The energy associated with the cosmological constant will ensure that our universe continues to expand forever at an accelerating rate.
The work described here was carried out by a team of Durham astronomers consisting of Andrew Benson, Carlton Baugh, Shaun Cole, Carlos Frenk and Cedric Lacey. The supercomputer simulations by the Virgo consortium were carried out at the Max-Planck supercomputing centre in Garching, Germany and at the Edinburgh Parallel Supercomputer Centre.
Further details may be found at the following web site: http://star-www.dur.ac.uk/cosmology/theory/tgweb.html
Prof. Carlos S. Frenk
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