“It feels like when my children were born,” exclaimed Brian P. Schmidt, co-winner of this year’s Nobel Prize winner for Physics, soon after the Royal Swedish Academy of Sciences made the announcement in Stockholm on Tuesday.

Prof. Schmidt, a U.S.-Australian citizen, shares the Nobel Prize in Physics for 2011 with American scientists Saul Perlmutter and Adam G. Riess for their simultaneous discovery in 1998 that the universe is expanding at an ever-accelerating rate.

“I feel weak at the knees,” Prof. Schmidt told journalists who had gathered at the Royal Swedish Academy of Sciences for the Nobel news conference. Speaking from his home in Canberra, Australia, Prof. Schmidt said that he did not expect the Prize but he was happy to receive it.

“It is something that people occasionally mention, but one thinks that it is probably not going to happen,” Prof. Schmidt said. Asked what he planned to do now, he joked that he was going to wander around a bit and then try to sleep. (It was 9 p.m. in Australia when the news broke.)

The 44-year-old American who grew up in Alaska and now lives in Canberra said that he would celebrate on Wednesday when he is due to teach a class in cosmology at the Australian National University.

The Nobel citation said that the trio were being awarded “for the discovery of the accelerating expansion of the universe through observations of distant supernovae”, or exploding stars.

One half of the total Prize amount of 10 million Swedish krona (Over 7.1 crore Rupees) would go to Perlmutter from the Lawrence Berkeley National Laboratory and University of California in the U.S., and the other half jointly to Prof. Schmidt and Prof. Riess, who is with the Johns Hopkins University and Space Telescope Science Institute in the U.S..

For almost a century, the universe has been known to be expanding as a consequence of the Big Bang about 14 billion years ago. However, the discovery that this expansion is accelerating has helped to unveil a universe that to a large extent is unknown to science.

If the expansion will continue to speed up, driven by an enigmatic “dark energy”, the universe will probably end in ice, the Nobel committee for Physics at the Royal Swedish Academy of Sciences said in a statement.

While Prof. Perlmutter began his research in 1988, a rival team headed by Prof. Schmidt, and comprising Prof. Riess, came together at the end of 1994.

Monday’s award of the Nobel Prize in Physiology/Medicine went to Canadian scientist Ralph Steinman, Bruce Beutler of the U.S., and Luxembourg-born Jules Hoffmann. Steinman passed away on September 30, but the Nobel committee said in a statement after the announcement of the award that its decision would remain unchanged in spite of a rule that the Nobel Prize cannot be awarded posthumously.

The Nobel Prize in Chemistry will be announced on Wednesday.