NASA image of galaxy IC 10
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Galaxy IC 10, which NASA scientists think could be swarming with gravitational waves. Credit: NASA

Could something missing in the universe be revealed by ripples in spacetime?

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Jan 2, 2021, 7:09 PM EST

For all its planets and stars and black holes and mind-blowing phenomena, the universe seems to be missing something, but that something might just be hiding.

Every strange and fascinating thing out there is supposed to belong in the universe. So what has gone unseen? New research suggests that gravitational waves could help figure out more about the mysterious dark energy thought to be lurking in the void. It is possible that gravitational waves—ripples in spacetime—could illuminate dark energy. These ripples encounter supermassive black holes or enormous galaxies as they traverse space.

Because it has been proven that gravitational waves (which are possibly everywhere in galaxy IC 10, above) are are bent when they pass through or near these objects, dark energy might also have an effect on them.

“Gravitational waves can be used probe the nature of dark energy,” Jose Maria Ezquiaga, who coauthored a paper recently published in Physical Review Letters, told SYFY WIRE. “If the dark energy is in its essence a modification of gravity, this will affect the way in which the gravitational waves propagate. This is in some sense similar to the use of light to probe the nature of some material. In other words, gravitational waves can be used as probes of the components of the universe.

Dark energy is allegedly behind the universe’s expansion, but the problem is that its origin remains unknown. There are scientists who do not even think it exists. If it really is dark energy that is causing the accelerated expansion of the universe, gravitational waves, which emerge from black holes and neutron stars colliding, may tell us something as they trek through the darkness. If, as Ezquiaga said, dark energy is a strange way that gravity can be modified, it should affect gravitational waves.

The galaxies and black holes that ripples in spacetime run into have a tremendous amount of gravity. That level of gravity will bend the trajectory of a gravitational wave. When enormous globs of mass distort surrounding space, as described in Einstein’s general theory of relativity, they create a gravitational field that magnifies light behind them and makes them more observable. This is gravitational lensing. It is often taken advantage of by telescopes like Hubble to study faraway galaxies that are otherwise beyond what our technology can see. However, light is not the only thing that gravitational lensing can bend.

“If gravity is modified, then these modifications are a good place to look,” Ezquiaga said. “If a gravitational wave crosses these mediums, it can generate waves associated with the additional components of gravity. In many theories these are scalar waves, which differ from the gravitational waves in their polarization properties.”

When gravitational waves venture close to an object massive enough to be capable of lensing, they are supposed to either release an “echo” or get scrambled. This is where scalar waves come in. Scalar waves, which may or may not exist in the realm of physics depending on who you ask, are electromagnetic waves believed to run lengthwise. When gravitational waves come close to an object with intense gravity, the difference in speed between them and the scalar waves that are generated is what determines whether the gravitational waves echo or end up emitting a scrambled signal.

If there is enough difference in speed between the two types of waves, it will cause the gravitational wave to split in two, sending out an echo. This can also occur if scalar waves are generated in an expanse of space that is large enough. If there is not enough difference in speed and the delay is shorter than the time it takes for the gravitational wave to pass by a massive object, things get scrambled. Searching for these echoes in gravitational wave data might tell us what it is encountering, and whether it does come face to face with dark energy.

Ezquiaga believes that how we look for dark energy in the future depends on what evidence we find of gravity being modified.

“If some of these modifications are found, then the properties of the signal can serve to constrain the possible modifications of gravity,” he said. “For example, information about the time delay between the echoes or the polarization content of the signal will be very important. If no such modification is found, we can discard some theories. These constraints will become stronger as more gravitational waves are detected.”

Even though neither we nor our most powerful telescopes can see it, dark energy may not stay in the dark forever.