After the discovery of the theory of relativity, Einstein introduced a mathematical term called `cosmological constant' (which represents an imaginary force to counter gravity) in his theory, so that equations can fit into a static universe.

But when the universe was found to be expanding, Einstein said the ``constant'' was his ``biggest blunder'' as it prevented him from predicting the expansion.

After almost eighty years, Varun Sahni at the Inter University Centre for Astronomy and Astrophysics (IUCAA) here is resurrecting this long-forgotten idea in a modern guise to answer a fundamental question -- what is the fate of the universe?

The answer lies with a hypothetical parameter called omega that is related to the total mass of the universe. If the value of omega is one,the universe is considered ``flat''.

If it exceeds the critical value of one, the universe will collapse after about 100 billion years but if the value is less than one, the universe will go on expanding forever.

However, studies showed the total amount of matter present in the universe could not account for a omega value greater than 0.2, which indicates an ever expanding universe.

In order to ``balance the books'' in favour of a flat universe, Sahni reintroduced the concept of cosmological constant with a ``non-zero value'' which helps maintain an exactly critical value of omega.

According to the theory, cosmological constant is not an imaginary force but exists in reality.

``It is an unusual form of energy that produces particles out of vacuum,'' Sahni said.

These numerous particles created out of nothing account for the remaining value of the omega, thereby making the universe flat, he said.

The theory was published recently in the journal Physical Review Letters.

Experimentalevidences also support the theory. By observing about 30 supernovae -- stars that suddenly burst into brilliance -- an international team of astronomers led by S Perlmutter from the Lawrence Berkley University in USA has found that the cosmological constant has a non-zero value, as predicted by Sahni.

The findings could help fix the age of universe too, Sahni added.