Look out into space and the signs are plain to see. The universe began in a Big Bang event some 13 billion years ago and has been expanding ever since. And the best evidence from the distance reaches of the cosmos is that this expansion is accelerating.
That has an important but unavoidable consequence: it means the universe will expand forever. And a universe that expands forever is infinite and eternal.
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Today, a group of physicists rebel against this idea. They say an infinitely expanding universe cannot be so because the laws of physics do not work in an infinite cosmos. For these laws to make any sense, the universe must end, say Raphael Bousso at the University of California, Berkeley and few pals. And they have calculated when that is most likely to happen.
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Their argument is deceptively simple and surprisingly powerful. Here’s how it goes. If the universe lasts forever, then any event that can happen, will happen, no matter how unlikely. In fact, this event will happen an infinite number of times.
This leads to a problem. When there are an infinite number of instances of every possible observation, it becomes impossible to determine the probabilities of any of these events occurring. And when that happens, the laws of physics simply don’t apply. They just break down. “This is known as the “measure problem” of eternal inflation,” say Bousso and buddies.
In effect, these guys are saying that the laws of physics abhor an eternal universe.
The only way out of this conundrum is to hypothesise some kind of catastrophe that brings an end to the universe. Then all the probabilities make sense again and the laws of physics regain their power.
When might his be? Bousso and co have crunched the numbers. “Time is unlikely to end in our lifetime, but there is a 50% chance that time will end within the next 3.7 billion years,” they say.
That’s not so long! It means that the end of the time is likely to happen within the lifetime of the Earth and the Sun.
But Buosso and co have some comforting news too. They don’t know what kind of catastrophe will cause the end of time but they do say that we won’t see it coming. They point out that if we were to observe the end of time in any other part of the universe we would have to be causally ahead of it, which is unlikely.
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In other words we’ll run headlong into this catastrophe before we can observe its effects on anything else.
The imminent end of time is a little unsettling but the argument is by no means water tight. Among other things it depends crucially on an important assumption about the laws of physics: that we ought to be able to understand why they work, not just observe that they do work. And that’s a philosophical point of view rather than a physical argument.
So Buosso and buddies raise some interesting questions but nothing to lose any sleep over. At least, not for another 3.7 billion years.
