Colliding neutron stars provide a new way to measure the expansion of the Universe | Science & Technology

In recent years, astronomy has seen itself in a bit of crisis: Although we know that the Universe expands, and although we know approximately how fast, the two primary ways to measure this expansion do not agree. Now astrophysicists from the Niels Bohr Institute suggest a novel method which may help resolve this tension.

The Universe expands

We’ve known this ever since Edwin Hubble and other astronomers, some 100 years ago, measured the velocities of a number of surrounding galaxies. The galaxies in the Universe are “carried” away from each other by this expansion, and therefore recedes from each other.

The greater the distance between two galaxies, the faster they move apart, and the precise rate of this movement is one of the most fundamental quantities in modern cosmology. The number that describes the expansion goes by the name “the Hubble constant,” appearing in multitude of different equations and models of the Universe and its constituents.

Hubble Trouble

To understand the Universe we must therefore know the Hubble constant as precisely as possible. Several methods exist to measure it; methods that are mutually independent but luckily give almost the same result.

That is, almost…

The intuitively easiest method to understand is, in principle, the same that Edwin Hubble and his colleagues used a century ago: Locate a bunch of galaxies, and measure their distances and speeds. In practise this is done by looking for galaxies with exploding stars, so-called supernovae. This method is complemented by another method that analyzes irregularities in the so-called cosmic background radiation; an ancient form of light dating back to shortly after the Big Bang.

The two methods — the supernova method and the background radiation method — always gave slightly different results. But any measurement comes with uncertainties, and a few years back the uncertainties were substantial enough that we could blame those for the…

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