Cosmologists need gravitational wave measurements from 50 binary neutron star mergers to work out just how fast our universe is really expanding, according to new research.
The cosmos has been ballooning ever since it was birthed from the Big Bang some 13.8 billion years ago. Edwin Hubble, an American astronomer, discovered that galaxies further away from our own Milky Way were moving away at a faster rate, a sign that the universe was expanding.
The rate of expansion, known as Hubble’s constant, is seemingly anything but constant. When scientists try to calculate it by analyzing the cosmic background radiation or by studying stars and supernovae, they get different answers. Now, an international team of physicists believe the conflict can be solved by probing a new data source: gravitational waves from neutron stars smashing into one another.
“We’ve calculated that by observing 50 binary neutron stars over the next decade, we will have sufficient gravitational wave data to independently determine the best measurement of the Hubble constant,” said Stephen Feeney, lead author of the paper published in Physical Review Letters and a research fellow at the Flatiron Institute, United States.
“We should be able to detect enough mergers to answer this question within five to 10 years.”
Although gravitational waves were predicted more than a century ago by Albert Einstein, they were first observed in 2016. That confirmed sighting was produced by two giant black holes, 29 and 36 times the mass of our Sun, spiraling into one another.
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A few other events have been spotted since, and labs like LIGO (Laser Interferometer Gravitational Observatory) and the Virgo Observatory have been upgraded to be more sensitive to detect smaller gravitational waves. The first neutron star collision was detected in 2017, though there haven’t been enough for cosmologists to begin crunching numbers to work out Hubble’s constant.
As two neutron stars crash into one another, some of the resulting energy is converted into gravitational waves that ripple throughout spacetime. They provide telltale signs of the stars’ mass and distance, and allow scientists to work out how fast they were both travelling in space in order to work out Hubble’s constant.
“This in turn will lead to the most accurate picture of how the universe is expanding and help us improve the standard cosmological model,” said Hiranya Peiris, co-author of the paper and a physics and astronomy professor at the University College London. ®