• The silhouette of a scientist against a visualisation of gravitational waves pictured during a press conference by the Max Planck Institute for GravitationalPhysics (Albert Einstein Institute) at the Leibniz University in Hanover, Germany, 11 February 2016. (AAP)Source: AAP
Sure, we now have direct evidence of gravitational waves - but what are they good for?
Lisa Grossman, Jacob Aron, Joshua Sokol

New Scientist
19 Feb 2016 - 3:36 PM  UPDATED 19 Feb 2016 - 3:44 PM

On 11 February, the Laser Interferometer Gravitational-Wave observatory, or LIGO, announced it had spotted gravitational waves, the stretching and squeezing of space-time created by the movement of massive objects.

The announcement caused a sensation among physicists and astronomers across the world, and they are now gearing up to exploit this new window on the universe.

In detail
Gravitational waves detected by scientists in a historical first
For the first time ever physicists have detected evidence of gravitational waves, the final prediction of Einstein’s general relativity.

This particular signal, picked up by LIGO’s two observatories on 14 September 2015, was made by two black holes, each about 30 times the mass of the sun, colliding with each other. This immediately resolved one open question for astronomers: before the signal came in, the very existence of such black hole binaries was contested. Further observations could tell us more about exotic objects like neutron stars and supernovas.

But that’s just the beginning. Gravitational waves will allow us to explore fundamental physics and possibly even peer back to the universe’s earliest moments. Here are four mysteries of cosmology that may finally be solved in the era of gravitational wave astronomy.

1. Dark energy

Put several detections together and we should gain insights into the history and composition of the universe as a whole, says Avi Loeb of Harvard University. Combining the signals from a number of black hole mergers, for example, could help understand the nature of dark energy, which is causing the universe’s expansion to accelerate.

From the “shape” of the signal – how the waves’ frequency and amplitude rise and fall – we can discern the sizes of the black holes involved, and determine how strong the event was at its source. Comparing how powerful it really was to the faint vibrations LIGO detected tells us how far away it occurred. Combined with observations from standard telescopes, this can reveal how space has expanded during the time the waves took to reach us, providing a measure of dark energy’s effect on space.

This measure should be stronger and more reliable than anything we have used so far. Spotting just a few black hole mergers would change everything, Loeb says. “If you have tens of them, it will be a new branch in cosmology.”

2. Equivalence principle

Other researchers are hoping to use gravitational wave signals to put Einstein’s general theory of relativity through even more stringent tests. One way is to investigate the equivalence principle, an assumption that gravity affects all masses in the same way.

“In the age of GPS and space travel, where even minute deviations from the assumed theory of gravity would have major consequences, it is of enormous importance,” says XueFeng Wu of Purple Mountain Observatory in Nanjing, China.

Erminia Calabrese, an astronomer at the University of Oxford, sees gravitational waves as a way to check whether gravity behaves as relativity predicts it should over large distances.

“If their strength fell off with distance in a surprising way, we could detect this with the upcoming LIGO data,” she says.

3. Cosmic inflation

LIGO’s success could see more gravitational wave detectors being built around the world. More sensitive detectors, working at shorter wavelengths than LIGO, may allow us to sense primordial gravitational waves from the very young universe. These waves should have been produced in the period of inflation – the tremendous cosmic growth spurt in the first instants after the big bang.

Unlike photons and other electromagnetic radiation, they would have travelled freely through the newborn universe. At the moment we can only see as far back as 380,000 years after the big bang, when the universe became transparent to light.

“We can potentially see almost all the way to the big bang,” says Dejan Stojkovic of the State University of New York in Buffalo. LIGO itself won’t be able to sense such vibrations, but the detector’s success raises hopes that future experiments will get off the ground. “Now that we know gravitational waves exist, it will be much easier to convince people to invest money and make all kinds of gravitational wave detectors,” Stojkovic says.

4. Grand unified theory

Gravitational waves may even point the way toward a grand unified theory of the universe. Models predict that at some point in the universe’s history, all four fundamental forces were united into a single force. As the universe expanded and cooled, the forces split off from one another in a series of as yet poorly understood events.

“Gravitational wave observatories that can detect much shorter wavelengths could probe those,” Stojkovic says.

LIGO team member Daniel Holz at the University of Chicago thinks this is just the beginning. “Every time we’ve opened a window to the universe, we’ve found all sorts of unexpected things,” Holz says. “I’d be surprised if I wasn’t surprised.”

More on this topic
Explainer: gravitational waves and why their discovery is such a big deal
There is no doubt that the finding is one of the most groundbreaking physics discoveries of the past 100 years, writes physicist Gren Ireson.
What happens when LIGO texts you to say it's detected one of Einstein's predicted gravitational waves
The morning of Monday, September 14, 2015 caught many LIGO scientists completely off guard, writes physicist Chad Hanna.
Hunting for gravitational waves: how does an experiment at LIGO actually work?
Delve into the background of how experiments are run at LIGO, the ground-breaking Laser Interferometric Gravitational-Wave Observatory in the US.

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