Cosmic poser

The fate of our universe depends on one question: open or closed?

Published: Thursday 15 June 2000

imageOne of the most tantalising questions in cosmology concerns the curvature of the universe. Is our universe ‘flat’ or is it curved? If it is curved, is it ‘open’ or ‘closed’? These questions are of paramount importance for the ultimate fate of the universe we live in. There have been a spate of experiments in the recent past, which provide a clue to this fundamental query. Now a group of researchers have claimed that the universe is flat. This result if true will have a profound impact on the standard model of cosmology.

Curvature in the universe is not like what we are used to in our everyday world. When we say that the universe is curved, we are talking about the curvature of space itself. This has bizarre, non-commonsense implications for instance, if we travel long enough we come back to the same point. A flat universe is more like what we are used to in our lives.

Another strange thing about cosmology, as we understand it today, is that the curvature is intimately linked with the fate of the universe. If the universe is curved positively, that is, in some senses like a sphere (in 4 dimensions) then present theories imply that it contains enough mass to halt the expansion of the universe and force it to contract. Such a universe will ultimately implode to a crunch. A flat universe on the other hand has no such end. Of course, the crunch, even if it comes will occur in billions of years!

To study the curvature of space is very difficult. The fundamental reason being that to really explore the large-scale structure of our universe we need accurate measurements of regions very distant from the earth. This is in general very difficult. Nevertheless, there have been some attempts by astronomers to get information regarding the curvature by using various tools like supernovae explosions and gravitational lenses. The current measurements are of the microwave radiation that bathes our universe and is supposed to be a remnant of very early times.

The early universe was very hot and contained protons and electrons which were densely packed and had too much energy to form any stable structures like hydrogen. The radiation energy was trapped inside such a sea, bouncing off the charged particles. As the universe expanded, it cooled. When it was about 300,000 years old, the protons and electrons cooled down enough to form neutral hydrogen atoms. Once this happened, the radiation energy escaped and has been cooling since. This remnant energy is in the microwave part of the spectrum and was first discovered in the mid-sixties by accident at the Bell Laboratories by Penzias and Wilson. The temperature of this radiation is 3° above absolute zero, that is, about –270° C . The radiation is known as the Cosmic Microwave Background Radiation ( CMBR) . The COBE satellite provided a detailed map of the CMBR in 1992 and it has been extensively studied since then.

Now a new, more accurate map of the radiation has been provided by an experiment known as ‘Balloon Observations Of Millimetric Extragalactic Radiation and Geomagnetics’ (Boomerang). The experiment is more accurate than the COBE satellite-based experiment and consists of balloon measurements over the Antarctica.

A Lange of Caltech and P de Bernardis of the University of Rome led a 36-member international team of scientists in the experiment. Their experiment has measured the radiation with a greater degree of accuracy, some 10-30 times more accurate than COBE . Since the early universe is relatively simple, scientists think they have a pretty good idea of the mechanisms at work at that time. With these measurements of the CMBR , they can in a sense study the conditions prevailing when the universe was only 300,000 years old (compared to today when it is about 12-15 billion years old). This can provide us with a clue to its curvature and density at that time.

The group has analysed the data obtained from the balloon flights and has concluded that the data best fits a flat universe. There is just enough matter in the universe to make it flat and avoid the big crunch! Another group that has independently analysed the data and come up with the same conclusions also supports their findings. Though promising, several other astronomers are hedging their bets as to whether this can be taken as the final word on the curvature and hence the fate of our universe. They feel that a lot more data and theoretical study is required before we can pronounce a judgement on this matter of life and death! ( Nature , Vol 404, p955; and Physical Review Letters , Vol 84, p3523.

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