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Measuring cosmological distances is tricky at the best of times, but it is even harder if your ruler isn’t up to scratch. A new cosmological standard ruler could make the difference – and help astrophysicists figure out why the universe is expanding at an accelerating rate.

Ever since we realised that the universe’s expansion is accelerating – a discovery that won Adam Riess and his colleagues the 2011 Nobel prize in physics – we have been trying to work out why. The phenomenon is usually put down to the existence of dark energy, a mysterious force that appears to be pushing the universe’s matter further and further apart.

Accurate measurements of how the distance between clusters of galaxies has changed over time could tell us whether the effect of dark energy on the universe is increasing. But our current way of measuring this has a problem: it relies on assumptions that are difficult to check.

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