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Siren call

Now that gravitational waves have been discovered, it is time to put them to use.

The Universe is big, and getting bigger all the time. A little-known fact about gravitational waves, the latest cosmological discovery to get physicists excited is that they could help to measure this expansion. And they could show why the expansion has been accelerating, rather than slowing down as expected, under the push of a mysterious force dubbed dark energy.

The way in which astronomers conventionally measure distances has ancient roots. With ingenuity and a dash of basic trigonometry, the ancient Greek astronomer Aristarchus of Samos was able to measure the Moon’s distance from Earth with surprising accuracy — in the third century bc.

A similar method to Aristarchus’, using a concept known as stellar parallax, was first applied to measure a star’s distance from Earth in 1838, and is still used today. The European Space Agency’s Gaia probe is currently using it to compile a state-of-the-art catalogue of one billion stars in the Milky Way, extending the reach of parallax to unprecedented distances and cutting errors down to less than 1%.

Stellar parallax is good, but it can go only so far. It entails measuring a star’s apparent position in the sky at different times of the year, as Earth (or a space probe such as Gaia) orbits the Sun. The distance between the two observing points, measured to high accuracy, provides the base of a triangle. The distant star is at the opposite vertex. The smaller the angle at that vertex, the farther away the star is.

But because the size of Earth’s orbit is fixed, as the vertex moves farther away the angle becomes smaller and smaller, and ultimately impossible to measure with any accuracy. (The basic unit of measurement of astronomical distance, the parsec, is short for ‘parallax of one arcsecond’, which refers to the size of that angle. One arcsecond is 1/3,600th of a degree, and in typical parallax measurements the angles are much smaller.)

For objects in more distant galaxies, astronomers have devised steps that build on the parallax method. Each step is a ‘rung’ on what they call the cosmic distance ladder. For example, the distance from Earth of the Andromeda Galaxy, the closest large galaxy to the Milky Way, is estimated by measuring the brightnesses of various types of star in it and comparing them to the brightnesses of similar stars closer to Earth whose parallax is known. Such estimates exploit the fact that similar stars look fainter the farther away they are.

Andromeda is roughly 780 kiloparsecs (2.54 million light years) away. Telescopes cannot resolve individual stars in galaxies that are hundreds of millions of parsecs away — except when those stars happen to blow up as supernovae. Astronomers use some supernovae as signposts of cosmic distances, or ‘standard candles’, meaning that their measured brightness is an indicator of their distance.

A major complicating factor is that the observed brightness of distant objects can be affected by foreground matter such as dust. Wouldn’t it be wonderful to have a more direct and reliable way of measuring distances — one that were as precise as Gaia and worked at scales from the galactic to the cosmic?

Stellar parallax is good, but it can go only so far.

Beginning with a paper in this journal 30 years ago (B. F. Schutz Nature 323, 310–311; 1986), physicists have suggested that gravitational waves could provide such a tool. The ripples, predicted by Albert Einstein in 1916 as a consequence of his general theory of relativity, travel across the Universe without being dimmed significantly by dust or gas.

The gravitational waves that struck Earth in September and were recorded by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) carried information that revealed their strength at the source. In theory, this information can be used to work out the source’s distance.

In the next few years, other interferometers are scheduled to join LIGO to form a global network of gravitational-wave observatories. Together, these instruments could calculate the positions and distances of merger events. Neutron-star mergers are especially interesting to cosmologists because they should also produce bursts of short, high-energy γ-rays, which would help to pinpoint their galaxies of origin.

Researchers hope that they will be able to use information from mergers as a way to calculate the distances of known galaxies. Because gravitational waves are more similar to sound than they are to light, physicists have dubbed these potential signposts ‘standard sirens’.

One of the main uses of supernova standard candles has been to measure the current rate of cosmic expansion. Standard sirens could provide an independent way to do this. And, if space-based interferometers are added to the network, they could be used to track dark energy. Hear the call.

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Siren call. Nature 531, 413–414 (2016). https://doi.org/10.1038/531413b

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