Dark energy

Updated Thursday, 7th February 2013
Matter tells space how to curve, and space tells matter how to move. But what does it mean to equate the curvature of the Universe with the amount of stuff in the Universe?

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Euclid telescope Artist's impression of the future Euclid space telescope. Watch this animated video on dark energy

Einstein’s theory of gravity has a very strange equation at the root of it. Einstein had a beautiful way of measuring the curvature of space and time, which he set equal to something that measured the matter and energy inside space. As the physicist John Wheeler later put it: matter tells space how to curve, and space tells matter how to move. 

But what does it MEAN to equate the curvature of the Universe with the amount of stuff in the Universe? Is matter somehow generating the curvature in some strange way? Or does it work the other way – is the curvature of space and time somehow connected to the origin of mass? These are deep mysteries in physics, and there may well be Nobel prizes in wait for people who make crucial insights on these deep problems.

Einstein also found that he had the freedom to put an extra term into his equation: he could give space an in-built tendency to fling itself apart. This is the cosmological constant mentioned in the animation linked to above.

But it’s an equation, so if you subtract the same thing from both sides, it stays a true statement. You can subtract the cosmological constant term from both sides. Subtracting it from the space-curvature side removes it entirely from that side of the equation, so the only place the cosmological constant is left is on the other side.

Mathematically this is just a bit of arithmetic, but it leads to a very different way of thinking about the cosmological constant. Physically, what this means is that you’re regarding the cosmological constant as some stuff INSIDE space, rather than as a property of space itself. 

So, what is this stuff? Einstein’s theory says it doesn’t change with time or from place to place – but suppose the Universe is stranger than Einstein imagined. Then this cosmological ‘constant’ wouldn’t really be constant, but could change. In this situation astronomers call this mysterious cosmological stuff ‘dark energy’. 

At the moment there’s no evidence that the Universe is any stranger than Einstein imagined – it’s just a cosmological constant like he proposed. But if we discovered Einstein was wrong, it could be a wonderful, deep clue to how space and time work. The European Space Agency will launch a new space telescope, Euclid, to try to detect indirect effects of dark energy, via the effect that dark energy has on how the Universe expands. This could lead to the most important breakthrough in fundamental physics this century.

Question: Euclid’s scanning half the sky and making images almost as sharp as the Hubble Space Telescope - what else do you think Euclid will discover? Share your answer using our Comments facility.

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