The universe is humming with gravitational waves. Here's why scientists are so excited about the discovery

The discovery of gravitational waves by mankind is not new. Why is this discovery so significant, then?

Researchers have now "heard" the cosmic symphony that gravitational waves have been playing since the beginning of the cosmos.

After 15 years of searching, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) announced on June 28 that it has discovered low-frequency gravitational waves. However, this is not the first instance of gravitational waves being discovered by humans. Since 2015, researchers have begun employing tools like the Laser Interferometer Gravitational-Wave Observatory (LIGO) to find these ripplings in the fabric of space.

In light of this, why isn't this just another gravitational wave discovery, despite its undeniably amazing nature? Three related attributes are the key to the solution: gravity waves' duration, wavelength, and frequency, as well as what these measurements reveal about the things and activities that initially caused them to ripple over space.

Gravitational waves: what are they?

General relativity, Albert Einstein's 1915 theory of gravity, states that gravity results from the warping of space and time, which is collectively referred to as "spacetime," and is caused by the mass of things. Additionally, general relativity predicts that when objects speed, ripples in spacetime—a type of gravitational radiation we refer to as gravitational waves—should be produced. When supermassive black holes and neutron stars are involved in the acceleration, the effect becomes considerable.

The frequency range of gravity waves is similar to that of electromagnetic radiation; high-frequency gravitational waves, like high-frequency light, have shorter wavelengths and are more energetic, whereas low-frequency gravitational waves have longer wavelengths and are less energetic. Long periods, or the interval between one peak of the wave crossing a certain place and the next peak passing that point, are another characteristic of low-frequency longwave gravitational waves.

Different gravitational waves have different properties.

The first low-frequency gravitational wave detection was made, as stated on June 28. Supermassive black hole binaries in the very early cosmos are thought to be the source of these low-frequency gravitational waves. Consider this like an orchestra. The stunning single "crash" of cymbals from cataclysmic occurrences like collisions and mergers may be heard by LIGO. What NANOGrav, a low-frequency gravitational wave detector, detected sounds like a soft background violin harmony.

This signal's brightness suggests that in the early cosmos, there was a gravitational wave symphony composed of hundreds of thousands or maybe millions of supermassive black hole pairs.

One of the roughly 190 scientists working on NANOGrav, National Radio Astronomy Observatory (NRAO) astronomer Scott Ransom, told that the discovery "opens up a new low-frequency window on the gravitational universe that will let us study how galaxies and their central black holes merge and grow with time."

Spacetime begins to ring like a softly struck bell as black holes and neutron stars revolve around one another, producing a constant stream of low-frequency gravitational waves. The gravitational waves carry away angular momentum (spin) as they are released, which causes the black holes to gravitationally attract one another.

The frequency of this gravitational radiation increases as orbiting objects grow closer together; also, as they approach closer together, they lose angular momentum more quickly and spiral together more quickly until they meet and merge. High-frequency gravitational waves are blasted into space by this cataclysmic collision.

There are certainly more bizarre hypotheses that may be used to explain these little ripples in space-time. A portion of this signal could represent gravitational waves from the Big Bang and the universe's creation that predate even these early black hole pairings.

Why LIGO and LISA can't perform what NANOGrav can (and vice versa)

Similar to how different telescopes are required to perceive different frequencies of electromagnetic light, various gravitational wave detectors are required to "hear" various frequencies of this gravity-based spectrum of radiation.

Higher-frequency gravity waves produced by collisions between black holes, neutron stars, and even mixed mergers between the two have been very successfully detected by instruments like LIGO, while lower-frequency gravitational waves have proven to be elusive.

This is due to the fact that gravitational waves' influence is already very small—NANOGrav calculates that it is just around one part in 1,000,000,000,000,000!

The ground-based gravitational wave observatories, including LIGO, are not sensitive enough to detect low-frequency gravitational waves. Even the Laser Interferometer Space Antenna (LISA), a future space-based gravitational wave detector, won't be able to detect should signals.

The gravitational waves with periods ranging from milliseconds to seconds that LIGO and other ground-based detectors can detect have wavelengths of around tens of thousands of miles, roughly the size of Earth. Think of the distance from the Earth to the Sun or the distance between the Earth and Pluto when considering the size of the wavelengths that LISA will cover. These gravitational waves have durations that range from seconds to hours.

The gravitational waves that NANOGrav is intended to detect have wavelengths that are trillions of miles long and oscillate at nanoHertz rates, making them light-years long. These nanoHertz gravitational waves may last for months, years, or even decades, according to NANOGrav.

Astronomers required a gravitational wave antenna the size of the entire galaxy as well as a network of extremely accurate "cosmic clocks" to measure time in order to make this detection. The solution is NANOGrav.

How did NanoGrav detect low-frequency waves?

The Arecibo Observatory in Puerto Rico, since demolished, the Green Bank Telescope in West Virginia, and the Very Large Array in New Mexico served as the three radio observatories via which NanoGrav converted 68 Milky Way pulsars into a massive gravitational wave antenna the size of the whole galaxy. A pulsar timing array is a distinctive and sensitive gravitational wave detector.

Pulsars are created, like other neutron stars, when huge stars run out of nuclear fusion fuel and the "push" of energy created by this process stops. As a result, these stars' cores collapse under the force of their own gravity, and their outer layers are destroyed in a supernova explosion.

The stellar core's breadth narrows to the point where neutron stars have bodies little bigger than the typical metropolis on Earth with masses ranging from close to that of the sun to up to twice that of our star. Since angular momentum is conserved, the shrinkage of the stellar remnant also causes its rotation to "spin up," with some neutron stars spinning as quickly as 700 times per second! Imagine this as being on a whole other scale from a figure skater drawing in their arms to intensify their spin.

Another effect of the collapse of stellar cores is the compression of the parent star's magnetic field. The intensity of the magnetic field that is made up of the magnetic field lines grows as they are packed closer together.

As a result, neutron stars have some of the universe's strongest magnetic fields. Particles are drawn to the poles of pulsars by these magnetic fields, where they are shot out as jets at close to lightspeed from each pole. Astronomers first thought pulsars were pulsating stars because of their apparent "on-and-off" behavior, but this is really caused by the light that these jets produce rotating towards us at highly exact regular intervals. This implies that pulsars make a great timing tool.

Pulsar timing should be affected by the compression and stretching of spacetime caused by gravity waves, either slowing or speeding them up as they pass. The light from these pulsars arrives at different times as a result, albeit only very little. Pulsar timing arrays must include a large number of widely separated pulsars that must be continuously observed for years because the effect is modest.

Patience has finally paid off for NANOGrav, as this gravitational wave effect on pulsars is now showing a sign.

According to Ransom, "the Earth is basically bobbing around — a little bit — on gravitational waves that are light-years in length." And we have observed this with a collection of almost 70-millisecond pulsars dispersed over our region of the Milky Way.

This discovery is significant since it is the first time gravitational waves have been found coming from unknown sources. It has been discovered that supermassive black hole binaries were abundant in the early cosmos.

This is important because, despite the fact that most galaxies, if not all of them, now have supermassive black holes at their centers, it is still unclear how these cosmic giants evolve. A succession of mergers between successively bigger and bigger black hole binary pairs is one theory put out as a mechanism.

This low-frequency gravitational wave signal provides a clue as to how the early cosmos may have developed in order to produce certain supermassive black holes with masses millions or even billions of times greater than the sun.

A better understanding of this black hole binary merger process also means a better understanding of how galaxies grow and how the universe as a whole has evolved, as these black holes are likely delivered into the spiral dance of death that results in their merger by the collisions of galaxies.

There is also a remote possibility that a tiny portion of the gravitational wave signal detected by this pulsar timing array, which is the size of the Milky Way, is due to gravitational waves produced during the Big Bang, which would have wavelengths ranging from roughly 100,000 light-years for the Milky Way to about 100 million light-years for the Virgo Supercluster of galaxies.

It's thrilling. KU Leuven cosmologist and long-time Spethen Hawking colleague Thomas Hertog, who was not involved in the work, told Space that the data provided by NANOGrav "shows once again that gravitational wave observations are opening up a whole new window onto the universe." "By listening to the buzz of gravitational waves traveling through our planet, we'll be piecing together the whole history of the cosmos in amazing detail in the years and decades to come. Yes, these are exciting times.

Ransom discussed how NANOGrav is now searching for a sensitive radio telescope in the northern hemisphere to take the place of the Arecibo telescope, which crashed in December 2020. The cooperation will compare data with information from other pulsar timing arrays in order to pinpoint the origin of low-frequency gravitational wave signals until that is discovered.

"With further measurements, we should be able to distinguish specific sources from the gravitational wave background as pure tones. The electromagnetic waves might also be used to identify and study those sources, creating a novel extragalactic multi-messenger astronomy approach, Ransom said. "This new development has me extremely thrilled! I'm not a very patient person, and we've been working on this for more than 15 years!"