麻豆传媒

TRY to imagine living in a collapsing universe. Galaxies would hurtle towards each other before colliding, ending up in an unimaginably hot and dense crunch, like the big bang in reverse. Yet the chances of intelligent life evolving in this hostile place are higher than you might expect, says one physicist. The concept might also help scientists understand why our universe is so hospitable.

Whether a universe expands forever or collapses in a big crunch depends on the strength of the dark energy it contains, a number called lambda. A weak positive value for lambda is causing our universe鈥檚 expansion to accelerate, but if it were only slightly greater, the universe would expand too quickly for gravity to pull material together to form stars and planets, which host life. Physicists don鈥檛 know why our dark energy is so weak, but some have used the anthropic principle to explain it 鈥� the idea that the universe has to be the way it is in order for us to be around to observe it. Some have tried to make this more precise by calculating how the number of expected observers in a universe changes depending on the strength of its dark energy. These calculations tend to suggest the small, positive lambda of our universe is a probable value for an observer to see.

鈥淯nder the anthropic principle, observers in collapsing universes would represent 12 per cent of the total intelligent life鈥�

John Peacock, a physicist at the University of Edinburgh, UK, says in making these calculations scientists have been assuming that intelligent life only exists in expanding universes and therefore have been unfairly throwing out many other possible kinds of universe in which lambda is negative, making dark energy attractive. Peacock鈥檚 calculations suggest that star formation would be efficient in collapsing universes, since gravity doesn鈥檛 have to fight against the expansion of space to pull matter together, so solar systems could form to host life.

It鈥檚 not easy for intelligent life to arise in this kind of universe though: it may not have the time to evolve before a big crunch, and temperatures may become too hot as the universe shrinks. 鈥淚t鈥檚 the ultimate environmental catastrophe,鈥� Peacock says. However, if life emerges at the right time, there is a small chance it will survive, he says. For example, if life arose at least 9 billion years before a big crunch, it might have time to evolve into something intelligent before being cooked. If these observers were included when physicists use the anthropic principle to calculate the likelihood of our hospitable universe, they would represent 12 per cent of the total intelligent life in all universes.

Alexander Vilenkin of Tufts University in Cambridge, Massachusetts, who has published calculations based on the anthropic principle, cautions that it is hard to be sure how long it takes intelligent observers to evolve, but agrees that collapsing universes need to be counted. 鈥淚f lambda is very small and negative, the universe may have a very long lifetime,鈥� he says.