Scientists agree that the universe is expanding at an increasing rate, but by exactly how much is still an enigma.
To measure the pace of expansion, slightly confusingly known as Hubble’s Constant because it's not actually constant, boffins need two bits of information. One is the speed at which an object is receding from Earth due to the expansion of space time, and the other is its current distance.
Hubble’s Constant can be measured in different ways. Scientists have done this by inspecting tiny fluctuations in the temperature of the cosmic microwave background, the energy leftover from the Big Bang.
They also measure the redshift and distance of classes of objects known as “standard candles”. These can be supernovae or stars that have a well established luminosity and so can be tracked easily. Both methods, however, give different values for the Hubble’s constant.
“The cosmic microwave background gets a value of 67 kilometers per second per megaparsec, and the standard candle method gets a value of 72. Both groups are very confident in their values, so the discrepancy is troubling,” Daniel Holz, a cosmology professor at the University of Chicago, told The Register.
Holz and his colleagues at the University of Chicago and Harvard University believe the problem can be solved by gravitational waves. Previous measurements rely on numerous assumptions and multiple steps to deduce the value for Hubble’s Constant. Gravitational waves provide a more direct way to calculate the distance of faraway objects, like supermassive black holes or neutron stars over billions of light years away.
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The energy from merging black holes or neutron stars sends ripples through the fabric of spacetime. Scientists detecting the gravitational waves can work out the mass and energy of the colliding black holes. Together with the strength of the signals, they can work out how far away the event was from Earth and then calculate Hubble’s Constant from there.
The only problem is that there aren’t enough confirmed gravitational wave detections to get an accurate reading of the universe’s expansion. The researchers reckon that with 25 observations they can measure Hubble’s Constant within an accuracy of 3 per cent, according to a paper published in Nature this month. As the number of detections increases to 200, however, they think they can achieve an accuracy to within 1 per cent within a decade.
There are two sites dedicated to hunting gravitational waves: The Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and VIRGO in Italy. LIGO is currently undergoing renovation to upgrade the sensitivity of its detectors and is expected to reopen next year.
Scientists hope that this upgrade will help them find more gravitational waves as they will be able to detect smaller, less energetic merger events. Measuring a more accurate Hubble’s Constant will unveil more cosmic mysteries, Holz told El Reg.
“Measuring the expansion provides a precise estimate of the age of the Universe," he explained. "It is also a crucial quantity for studying properties of the dark matter and, especially, dark energy.” ®