Gravitational waves could help us find out how fast the expansion of the universe is accelerating

    an artist's illustration of gravitational waves, showing spiral ripples on a gridded surface emerging from two merging spheres

an artist’s illustration of gravitational waves, showing spiral ripples on a gridded surface emerging from two merging spheres

Gravitational waves emitted when distant black holes collide and merge, ringing the very fabric of space-time like bells, could be used to help measure how fast the universe is expanding, according to a new study.

Since the late 1990s, astronomers have known that not only is the universe expanding, but it’s doing so at an accelerating rate. The cause of this so-called late acceleration has remained mysterious, earning it the placeholder name “dark energy.”

And the researchers were puzzled by the fact that the two main ways of measuring cosmic expansion give different values ​​for the rate, which is called Hubble constant. This the discrepancy remainedalthough both methods have gotten more accurate over the years.

Related: “Hubble’s problems” could get worse with a new measurement of the expansion of the universe

These two techniques are the “late-time” method, which considers the speed of the galaxies and their distance from us, and the “early time” method, which studies the “fossil light” immediately after the big Bang called the cosmic microwave background.

Late measurements currently give an expansion rate of about 73 1 km/s per megaparsec, while early measurements give a value of 67.5 0.5 km/s per megaparsec.

This has led scientists to look for a corroborating method to measure the Hubble constant. And this is where the new study comes into play.

Illustration showing gravitational waves from a merger of black holes being reflected off a galaxy as they travel towards Earth.

Illustration showing gravitational waves from a merger of black holes being reflected off a galaxy as they travel towards Earth.

Gravitational lensing and gravitational waves

The new study suggests the use of a phenomenon predicted by Albert Einstein and usually associated with the distortion of light called gravitational lensing to measure the Hubble constant.

Gravitational lens it is an effect that derives from that of Einstein general theory of relativity. The great physicist’s 1915 theory of gravity predicts that mass has a warping effect on space and time, united as a four-dimensional entity called space-time.

This deformation means that when light from a background source passes through a massive object such as a galaxy, its path is deflected by this “gravitational lens”. This effect can cause the background source to magnify and is used to great effect for seeing early galaxies from observatories such as NASA’s James Webb Space Telescope (JWST).

Gravitational lensing is usually associated with light. But gravitational-wave ripples in spacetime created by the acceleration of massive objects, such as two black holes spiraling toward each other, should be similarly affected. This means gravitational waves from these violent merger events they should also exhibit gravitational lensing, as does light.

Light can take different paths past a lensing object because the amount it deflects depends on how close it gets to the gravitational lens. This means that light arrives at Earth at different times, and this delay can cause the same object to appear in multiple places in a single image. Since gravitational waves can also follow different paths beyond a gravitational lens, they should also exhibit a similar arrival time delay, meaning that gravitational wave detectors could, in theory, detect gravitational waves from the same event at different times.

This can be used as a measure of the Hubble constant, study team members said. This is because the rate of expansion of the universe affects the distance between the sources of gravitational waves black hole mergers, for example, and the galaxy that is warping space-time and acting as a gravitational lens and distance from Earth.

The team said the amount of gravitational lensing should depend on the expansion rate of the universe and, therefore, the Hubble constant. They suggest that a larger Hubble constant would result in a higher fraction of lensed black hole mergers and also smaller time delay values ​​than would be seen in the case of a smaller Hubble constant.

Related: The background gravitational wave of the universe has been heard for the first time

You wait years for one gravitational wave, and then two come at once

The advantage of measuring the expansion rate of the universe with gravitational waves rather than light is that these ripples are not affected as they pass through huge clouds of gas and dust, while light can be absorbed or change its frequency. This means that the technique could allow astronomers to “see” further back into the history of the universe than even strongly lensed light allows.

Scientists have not yet detected strong gravitational lensing on gravitational waves from merging black holes, and the team’s suggested technique will depend on a catalog of thousands of gravitational wave events, which is not yet available. The first gravitational waves were detected for the first time only in 2015, so this is still a new area of ​​\u200b\u200bscience. Yet important developments are underway.

The sensitivity of ground-based gravitational wave detectors has improved with recent significant upgrades to the Laser Interferometer Gravitational Wave Observatory (LIGO), Virgo and the Kamioka gravitational wave detector (KAGRA). In addition, the first space-based gravitational wave detector, the Europa Laser Interferometer Space Antenna (LISA), will be launched in 2037.


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With these improved tools, scientists could begin building a database that allows observation of gravitational lensing in gravitational waves. In these data, the team expects to find a small fraction of repeating signals from the same black hole merger events, just as the same distant light sources appear multiple times in JWST images due to gravitational lensing.

“An important scientific goal of future detectors is to provide a comprehensive catalog of gravitational-wave events, and this will be an entirely new use of the remarkable dataset,” said study co-author Tejaswi Venumadhav Nerella, a theoretical astrophysicist at the University of California. California. Santa Barbara, said in a statement.

The research was published June 30 in the journal Physical Review Letters.

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