Can gravitational waves solve the crisis of cosmology?

The twentieth century gave the world a lot of amazingdiscoveries: in 1916, a world famous physicist named Albert Einstein published general theory of relativity (GR); then, in 1927, astronomer Edwin Hubble discovered that galaxies were moving away from Earth (and from each other) at an ever-increasing rate; in the following decades, such outstanding minds as Niels Bohr, Max Planck, Louis de Broglie, Werner Heisenberg and others worked on the creation of quantum theory. Today, their work is at the heart of our knowledge of the Universe - we know that it was born 13.8 billion years ago and has been expanding at an accelerating rate since then. But the reason why the universe is getting bigger and bigger remains a mystery and scientists cannot agree on why. This is largely due to different ways of measuring the Hubble constant (a fundamental parameter describing the expansion of the Universe), which show different results. But recently, scientists have proposed a new way that could potentially resolve the crisis in cosmology. We will talk about it in this article.

A team of scientists has proposed a new way to resolve the crisis in cosmology - using gravitational waves.

Black holes and gravitational waves

In the winter of 2016, scientists announced the discoverygravitational waves - ripples in spacetime caused by the collision of massive black holes. Their existence was first predicted by Einstein's theories of relativity in 1916, and in 2017 awarded the Nobel Prize in Physics. In fact, gravitational waves represent a traveling deformation of absolute emptiness - these are changes in the gravitational field that propagate like waves. When a gravitational wave passes between two bodies, the distance between them changes.

The discovery of gravitational waves also confirmsthe existence of black holes - massive objects, the gravitational attraction of which is so great that even quanta of the light itself cannot leave them. The boundary that separates the black hole from the rest of the cosmos is called the event horizon. Scientists managed to photograph it in 2019, read more about this discovery in the material of my colleague Ilya Khel.

The collision of two black holes is the cause of the appearance of gravitational waves (in the artist's view).

As wave detection confirms boldhypotheses about how our Universe works, many scientists called their discovery the beginning of a new era of astronomy. Now scientists believe that with their help it is possible to resolve the crisis of modern cosmology.

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New Approach to Measuring the Hubble Constant

In 1929, two years after hisdiscovery, Edwin Hubble calculated the rate at which our universe is expanding - the Hubble constant. Only in subsequent years, each new method of measuring it showed new, inconsistent results. It is interesting that today there are two main ways to measure it, with the only difference that one set of methods considers relatively close objects in the Universe, and the other - very distant ones. But no matter what method scientists use, the results are different.

The mismatch of the Hubble constant provokedcrisis of modern cosmology and formed the basis of disputes between scientists: either they are doing something wrong, or somewhere in the vastness of the Universe, something unknown is happening.

Recently, a team of researchers from the UniversityPennsylvania proposed using gravitational waves to resolve the Hubble constant. The fact is that when massive objects, such as black holes or neutron stars (which are not visible with optical telescopes), collide with each other, they deform the fabric of space-timecreating gravitational waves.

Since 2015, the American laserthe interferometric gravitational wave observatory (LIGO) and its European counterpart Virgo are listening to space for similar "accidents" that ring like bells in their detectors.

“Gravitational waves can give you a different idea of ​​the Hubble constant,” Ssohrab Borkhanyan, a physicist at the University of Pennsylvania, told Live Science.

Depending on the distance from the Earth collisionblack holes will sound louder or quieter for LIGO, allowing scientists to calculate how far away these objects are. In some cases, the collision of space monsters also results in a flash of light that astronomers can pick up with telescopes.

So far, researchers have only observed onesuch an event with gravitational waves and one with light signals - a pair of neutron stars that astronomers observed in 2017. Based on the data obtained, physicists calculated the value of the Hubble constant. Previous studies have shown that cosmologists would need to observe about 50 such events in order to obtain a more accurate calculation of the Hubble constant.

See also: Physicists have rethought the structure of the Universe. Is dark energy no longer needed?

But these space accidents don't happen that often.and, moreover, they are not associated with flashes of light, which contain the most important information about speed. These events, invisible except for gravitational waves, are the most common signals received by LIGO and other gravitational wave devices.

Way out of the crisis

Over the next five years, LIGO detectors willthey are expected to receive updates that will allow them to unpack much more detail of gravitational wave signals and capture a lot more events, including more collisions of black holes. US and European installations have recently been joined by the Kamioka Gravitational Wave Detector (KAGRA) in Japan, and the Indian detector is due to hit the network around 2024.

Ripples in spacetime caused by the collision of massive objects.

According to the authors of the new study,published in the Bulletin of the American Physical Society, detectors in the future will be able to determine where in space a collision has occurred 400 times better than today. With this information, astronomers hope to identify the exact location of the galaxy where the collision occurred, and then determine how quickly that galaxy is moving away from Earth. It will also not be necessary to look for an appropriate flash of light.

More on the topic: Is it possible to unravel the mystery of the expansion of the Universe?

In their work, scientists have shown that collisionsbetween massive objects will be especially information-rich, producing data that can be used to calculate the Hubble constant with high precision. The results also suggest that in the future gravitational detectors will better and more accurately capture incoming signals. And yet, the possibility that other dimensions will help resolve the Hubble constant crisis earlier should not be ruled out.