Cosmologists can鈥檛 agree on how fast the universe is expanding because the two different methods they use to find out give distinctly different results. Now a third method involving gravitational waves is looking more promising, and it could help break the deadlock.
Gravitational waves are the minuscule ripples in space-time that were discovered in 2015 by the Laser Interferometer Gravitational-Wave Observatory, better known as LIGO.听They are produced when gigantic objects like black holes or neutron stars smash together.
To calculate the Hubble Constant, which quantifies the expansion rate of the universe, astronomers usually look at objects far away in the universe and find out two pieces of information about them. First they need to know how far away they are. Then they need to know their redshift, which is the degree to which light coming from them has been stretched as it passes through expanding space.
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The events that produce gravitational waves don鈥檛 always produce light, and even if they do it can be hard to spot. But in 2017, LIGO researchers showed that if you got lucky and caught some light from the explosive merger that produced the gravitational waves, you could measure the redshift. The characteristics of the gravitational wave detection gives the distance, so you could calculate the Hubble Constant.
Now they have extended their work to black hole mergers. No light is emitted from these events, but it is still possible to use galaxy catalogues to identify the most likely place that the gravitational waves are coming from. You then use the galaxy鈥檚 redshift to compute the Hubble constant.
The two more standard methods for computing the Hubble constant involve looking at supernovae or the cosmic microwave background, often called the afterglow of the big bang. These give values of or 82 or 64 kilometres per second per megaparsec respectively.
It is possible that this discrepancy is caused by unrecognised errors in the two methods, but some researchers believe that it is evidence of unknown physics.
The new gravitational wave method is important because it is completely independent of the other two. But it can鈥檛 provide the answer yet. With just ten detections to work with, the answer it provides is only an estimate that spans a range that includes both of those from the traditional methods.
鈥淎t the moment our method is like Switzerland, completely neutral,鈥� says Patricia Schmidt at the University of Birmingham and a member of the LIGO consortium.
But as more detections are made, the method will yield an ever more accurate result.
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