Saturday, 28 January 2017

The accelerating universe - what will and won't happen

Since 1999 we have known that the universe isn't just expanding, but the expansion is speeding up. This sounds weird, and it has led to suggestions that, eventually, everything will fall apart, including galaxies, stars, and even our bodies. That's not what an accelerating universe means, fortunately (assuming we are concerned with what happens to bodies tens of billions of years in the future). The expansion of the universe, at least the part we can see, speeding up happens because of a constant force. That force is called 'dark energy'. We don't know what dark energy is, but we have some ideas. The thing that is significant is that dark energy has constant density throughout space. It doesn't dilute as the universe expands. This sounds like it breaks physical law - how can new energy come into existence? The law of conservation of energy doesn't apply if space itself is expanding, so there is no problem with this new dark energy appearing. Anyway, the dark energy generates a very, very small repulsive (yes, repulsive) gravitational force. That force is so tiny it has virtually no effect on things that are held together by other forces. This means galaxies, solar systems, planets and our bodies. These things aren't going to be ripped apart, because the expansion force is very, very small and constant. Parts of the universe that are too far apart to have significant gravitational interaction will be separated, and that separation will not just increase but accelerate, because a constant force produces a constant acceleration. In the very distant future a being within our galaxy would not see any other galaxies, as they will have accelerated away and now be moving faster than light with respect to our galaxy (the expansion force expands space, and there is no limit for the speed at which space can move. But, those beings will be in no danger from the expansion force. It will just make astronomy a lot more boring.

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