The pull against the push

Cosmologists take a closer look at how gravity is strangling an expanding universe

Published: Tuesday 31 October 1995

COSMOLOGY is somewhat of an oddball among the physical sciences. Experimental facts about the origin and the ultimate fate of the universe are usually scarce, making cosmology a concoction of theory, some facts and much speculation. However, the scenario has undergone a change in the recent past because of the availability of better equipment. The accurate measurement of microwave background radiation by the Cosmic Background Explorer (com) satellite in 1993 has proved that cosmologists can have access to good and reliable experimental data (Science, Vol 269, August 11, 1995).

One such cosmological aspect which has proved difficult to measure has been the deceleration parameter q0. This is an extremely important cosmological quantity, being a measure of how fast the pull of gravity is slowing down the expansion of the universe.

Applying different techniques, some groups of astronomers working at Caltech in Pasadena, California, Lawrence Berkeley Labs in Berkeley, California and Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts are now trying to mea- sure the value of the elusive deceleration parameter. The Berkeley group, for instance, is using type IA supernovas, stars which are exploding with immense energies and having roughly the same actual brightness. This point is crucial because to measure q0, "standard candles" - sources whose apparent brightness can be taken as a measure of their distance - are needed. Comparing the distances of these objects with their redshifts (the change in the colour of the light from them because of their velocity away from the earth) to see how much the relation is distorted will give hints of the presence of cosmic deceleration.

Other methods of measuring the deceleration parameter include using radio galaxies as standard candles and looking at giant voids in the distribution of galaxies. For the more ambitious, plans are afoot to Use the LIGO gravity wave detector (a series of wave detectors around the world) when it will appear in 1999 to study pairs of neutron stars for measuring the q0. Astronomers, however, feel that conclusive data will be unavailable for quite some time. Till then, the fate of our universe will remain open to speculation.

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