February 12, 1998
PITTSBURGH--"When will it all end"? A Carnegie Mellon University astrophysics professor is weighing in on the ultimate fate of the universe, with a new analysis that shows the universe may eventually stop expanding.
"Our work does favor a high value for Omega. This has interesting consequences for cosmology, meaning the universe may eventually stop expanding and may even re-collapse. However, people shouldn't be worried, it will take an infinitely long time to happen," cosmologist Robert Nichol said. His work puts scientists one step closer to answering the question of how the universe will end.
Using new samples of X-ray emitting clusters of galaxies, Nichol and Daniel Reichart, a Ph.D. candidate at the University of Chicago, said they have been able to obtain a measurement that leads them to believe the value of Omega may be one. Omega, one of the two main parameters that explain the evolution of the whole universe - the other is Hubble's Constant - is the ratio of the observed average density of the universe compared to a critical value. This critical value is equivalent to only 11 hydrogen atoms per cubic centimeter and is the density of mass needed to reverse the expansion of the universe, by gravity, and make the universe turn around and retract, ending in the so-called "Big Crunch." Therefore, if the value of Omega is measured to be exactly one, the universe will eventually stop expanding. If Omega is below one, then the universe will expand forever with no end.
The data used by Nichol and his collaborators was obtained from the ROSAT, the ROentgen SATellite, and Einstein X-ray Observatories developed through cooperative programs between NASA, Germany and the United Kingdom. The data set was developed for analysis by an international group of astrophysicists including Kathy Romer of Carnegie Mellon, Mel Ulmer of Northwestern University, Francisco Castander and Brad Holden at the University of Chicago, and Chris Collins and Doug Burke of Liverpool John Moores University.
A measurement of Omega is central to all theories of cosmology, especially a theory called inflation that was introduced in the 1980s to explain, in part, why many cosmologists thought the value of Omega should be one. Nichol said that if Omega is observed not to be one, then many theorists will be looking for more exotic theories for the origin of the universe.
In the hot Big Bang Theory, the observable universe began from a rapidly expanding point, roughly fifteen billion years ago. Since then, the universe has continued to expand, gradually increasing the space (and time) between our Galaxy and external galaxies. This expansion of the universe "stretches" the wavelength of that light that is traveling to us from these distant galaxies, thus making their light appear redder. This measured "stretching" is called redshift and is an important measure of distances in the universe because of Hubble's famous law.
Nichol said this new data probes some of the "highest redshifts offered by present cluster surveys." By comparing the number density of clusters seen at these great distances to that seen in the near universe, Nichol explained that one can immediately obtain an insight into the value of Omega.
"The signal is very strong. For example, our standard cosmological theories tell us that if Omega is exactly equal to one, then the number of massive clusters in the universe at half it's present age should be 100 times fewer than that predicted for a universe with a low Omega. This is a big signal to measure and can easily differentiate between low and high values of Omega. That is basically what we have done with this new data set," he said.
In addition to stopping the expansion of the universe, a high value for Omega implies the existence of 'dark matter' made from exotic elementary particles. Nichol explained, "Omega measures the gravitational influence of all the mass in the universe, however, we can only see a fraction of this mass with our telescopes. About 90 percent of the mass could be invisible to our traditional observatories; we only see the tip of the iceberg."
During the next decade, the pursuit of Omega will heat up, culminating with the launch of two microwave satellites -- NASA's Microwave Anisotropy Probe (MAP) and the European Space Agency's Planck. These satellites will study the Cosmic Microwave Background in exquisite detail and should be able to measure Omega to about 99 percent accuracy.
"Cosmologists are in pursuit of one of the remaining Holy Grails of physics; the exact value of Omega," said Nichol. "Now, with many new measurements on the horizon, I believe we are at the beginning of the end of our pursuit of Omega."
The race for the answer is also on from ground-based efforts like the search for distant supernovae and Carnegie Mellon's millimeter telescope, Viper, which is beginning its operations at the Pomerantz Observatory at the South Pole. Nichol is planning to extend his work using NASA's Advanced X--ray Astrophysics Facility (AXAF) satellite due to be launched later this year.
This material has been posted on the World Wide Web at: http://www.lanl.gov
Professor Nichol can be reached at 412-268-8068 (work) or by e-mail at: nichol+@andrew.cmu.edu