The universe is humming with gravitational waves. Here's why scientists are so excited about the discovery (2024)

The universe is humming with gravitational waves. Here's why scientists are so excited about the discovery (1)

The very fabric of the universe is ringing with gravitational waves from its earliest epoch, and researchers have finally "heard" this cosmic symphony.

On Thursday, June 28, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) revealed the detection of low-frequency gravitational waves, a historic breakthrough that represents 15 years of searching. Yet, this isn't the first time that humanity has detected gravitational waves. Scientists have been detecting these ripples in the fabric of space using facilities like the Laser Interferometer Gravitational-Wave Observatory (LIGO) since 2015.

So, with that in mind, why isn't this just another  —  inarguably impressive  —  detection of gravitational waves? The answer is all about three connected qualities: The frequency, the wavelength and the period of gravitational waves, and what these tell scientists about the objects and events that first sent them rippling through space.

Related: The mystery of how Mars meteorites reach Earth may finally be solved

What are gravitational waves?

Albert Einstein's 1915 theory of gravity, general relativity, predicts that objects with mass have a warping effect on the very fabric of space and time  —  unified as "spacetime" —  and gravity arises from this warping. General relativity also suggests that when objects accelerate, they should generate ripples in spacetime, a kind of gravity radiation we call gravitational waves. The effect becomes significant when the acceleration involves massive objects like supermassive black holes and neutron stars.

Gravitational waves, like electromagnetic radiation, come in a range of frequencies with high-frequency gravitational waves, like high-frequency light, having shorter wavelengths and being more energetic while low-frequency gravitational waves have longer wavelengths and are less energetic. Low-frequency longwave gravitational waves also have long periods, the time it takes between one peak of the wave passing a set point to the next peak passing that point.

Not all gravitational waves are created equal

The discovery announced on June 28 marks the first detection of low-frequency gravitational waves. The source of these low-frequency gravitational waves is believed to be supermassive black hole binaries in the very early universe.Think of this in terms of an orchestra. LIGO can hear the dramatic single "crash" of cymbals from violent events like collisions and mergers. What the low-frequency gravitational wave signal NANOGrav heard is akin to the gentle background harmony of violins.

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The strength of this signal indicates that a gravitational wave orchestra of hundreds of thousands or even millions of supermassive black hole binaries existed in the early universe.

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

As black holes and neutron stars swirl around each other, they generate a continuous steady stream of low-frequency gravitational waves, effectively causing spacetime to ring like a gently struck bell. As they are emitted, the gravitational waves carry away angular momentum (spin), and this causes the black holes to draw together.

The closer orbiting objects are, the faster they emit gravitational waves and the higher the frequency of this gravity radiation becomes; additionally, the closer they are, the more rapidly they lose angular momentum and the quicker they spiral together until they collide and merge. This violent collision sends a blast of high-frequency gravitational waves barreling through space.

Additionally, there are also more exotic possible explanations for these faint ripples in space-time. A fraction of this signal could be a gravitational wave background predating even these early black hole pairs and originating from the Big Bang and the origin of the universe itself.

The universe is humming with gravitational waves. Here's why scientists are so excited about the discovery (2)

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

Just like it takes different telescopes to see different frequencies of light in the electromagnetic spectrum, it takes different gravitational wave detectors to "hear" different frequencies of this gravity-based spectrum of radiation.

Facilities like LIGO have been very successful in detecting higher-frequency gravitational waves caused by collisions between black holes, neutron stars, and even mixed mergers between the two, but lower-frequency gravitational waves have been evasive.

This is because the influence of gravitational waves is already tiny, with NANOGrav estimating the effect on spacetime as being as small as around one part in 1,000,000,000,000,000!

Even as sensitive as it is, LIGO and its fellow ground-based gravitational wave observatories can't pick up low-frequency gravitational waves. Even the forthcoming space-based gravitational wave detector, the Laser Interferometer Space Antenna (LISA) will not be able to pick up should signals.

The gravitational waves that LIGO and other ground-based detectors can hear gravitational waves with wavelengths of around thousands of miles, about the size of Earth, with periods ranging from milliseconds to seconds. LISA will cover wavelengths the size of millions to billions of miles; think the distance from Earth to the sun or the distance of Earth or Pluto. The periods of these gravitational waves last from seconds to hours.

The gravitational waves that NANOGrav is designed to hear are at nanoHertz frequencies and have wavelengths on the scale of trillions of miles, making them light-years in length. And according to NANOGrav, these nanoHertz gravitational waves can have periods of months, years, or even decades.

Related: How does astronomy use the electromagnetic spectrum?

For this detection to take place, astronomers needed a gravitational wave antenna the size of the entire galaxy and an incredibly precise way of measuring time consisting of a network of "cosmic clocks." That's where NANOGrav comes in.

The universe is humming with gravitational waves. Here's why scientists are so excited about the discovery (3)

How were low-frequency waves picked up by NanoGrav?

Via three radio observatories, the now destroyed Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia, and the Very Large Array in New Mexico, NanoGrav turned 68 pulsars within the Milky Way into a huge gravitational wave antenna the size of the entire galaxy. This unique and sensitive gravitational wave detector is called a pulsar timing array.

Like all neutron stars, pulsars form when massive stars exhaust their fuel for nuclear fusion, and the outward "push" of the energy produced in this process ceases. This results in the core of these stars collapsing under their own gravity and the outer layers being blasted away in a supernova explosion.

The width of the stellar core shrinks to such an extent that neutron stars have a mass from around that of the sun and up to twice our star's mass crammed into a body no wider than that of the average city here on Earth. Due to the conservation of angular momentum, the reduction in diameter also causes the rotation of the stellar remnant to "spin up," with some neutron stars spinning as fast as 700 times per second! Think of this as being like a figure skater drawing in their arms to increase their spin, just on a whole different scale!

The universe is humming with gravitational waves. Here's why scientists are so excited about the discovery (4)

The collapse of stellar cores has another consequence; the magnetic field of the original star is also squashed down. When magnetic field lines are crammed closer together, this increases the strength of the magnetic field they comprise.

As a result, neutron stars have some of the most powerful magnetic fields in the known universe. These magnetic fields act to channel particles to the poles of pulsars, where they are blasted out as jets at near-lightspeed from each pole. Pulsars appear to blink "on and off"  —  hence why astronomers initially believed they were pulsing stars —  but this is the result of the light these jets create turning towards us at incredibly precise regular intervals. This means pulsars can be used as an excellent timing device.

The compression and stretching of spacetime as gravitational waves wash through it should have a discernible on the timing of pulsars, either slowing them down or speeding them up as they pass. This causes a very slight difference in the arrival time of light from these pulsars. Because the effect is small, pulsar timing arrays need to consist of many widely dispersed pulsars that have to be monitored for years.

For NANOGrav, patience has now paid off with this effect on pulsars now revealing a sign from low-frequency signal gravitational waves.

"Basically, the Earth is bobbing around  —  a tiny bit  —  on gravitational waves that are light-years in length," Ransom said. "And we have seen this using an array of almost 70-millisecond pulsars scattered around our part of the Milky Way."

The reason this discovery is important is we've now detected gravitational waves from sources we hadn't investigated before. It has revealed that the early universe was packed with supermassive black hole binaries.

This matter because though scientists now know most, if not all, galaxies have at their heart a supermassive black hole, they aren't yet sure how these cosmic titans grow. One suggested mechanism is a series of mergers between subsequently larger and larger black hole binary pairs.

This low-frequency gravitational wave signal hints at a way to understand how this could have proceeded in the early universe leading to some supermassive black holes that have masses millions or even billions of times that of the sun.

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

There is also the small chance that a tiny fragment of the gravitational wave signal this pulsar timing array the size of the Milky Way has picked up comes from gravitational waves created at the beginning of time during the Big Bang, which would have wavelengths ranging from around the size of the Milky Way — around 100,000 light-years — to the size of the Virgo Supercluster of galaxies — around 100 million light-years.

"This is exciting. The evidence reported by NANOGrav shows once again that gravitational wave observations are opening up a whole new window onto the universe," KU Leuven, cosmologist and long-time Spethen Hawking collaborator Thomas Hertog, who was not involved in the study, told Space. "In the coming years and decades, we'll be patching together the entire history of the universe in great detail by listening to the hum of gravitational waves passing through our planet. Exciting times indeed!"

RELATED STORIES:

— Colliding black holes 'ring' across space-time with gravitational wave ripples

 — Faint gravitational waves may be from primordial fractures in space-time

 — Astronomers poised to hunt new kind of gravitational wave

With regards to the future, Ransom explained how NANOGrav will now look for a sensitive radio telescope in the northern hemisphere to substitute for the Arecibo telescope, which collapsed in Dec. 2020. Until that is found, the collaboration will compare data with other pulsar timing arrays to hone in on the source of low-frequency gravitational wave signals.

"With continued observations, we should start seeing individual sources as pure tones above this gravitational wave background. Those sources could be pinpointed and studied with electromagnetic waves as well — a new type of extragalactic multi-messenger astronomy," Ransom concluded. "I'm very excited about this development! We've been working on this for over 15 years, and I'm not a very patient person!"

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community@space.com.

The universe is humming with gravitational waves. Here's why scientists are so excited about the discovery (5)

Robert Lea

Senior Writer

RobertLeais a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.

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7 CommentsComment from the forums

  • MeS

    Great stuff. But this isn't high school newsletter writing. You can get rid of all the exclamations, IMO:

    "... the effect on spacetime as being as small as around one part in 1,000,000,000,000,000!"
    "... some neutron stars spinning as fast as 700 times per second!"
    " ... just on a whole different scale!"

    If it reads fine without exclamation marks, then you don't need them.

    Reply

  • kimbrock

    MeS said:

    Great stuff. But this isn't high school newsletter writing. You can get rid of all the exclamations, IMO:

    "... the effect on spacetime as being as small as around one part in 1,000,000,000,000,000!"
    "... some neutron stars spinning as fast as 700 times per second!"
    " ... just on a whole different scale!"

    If it reads fine without exclamation marks, then you don't need them.

    crash of the symbols? should be cymbals.

    Reply

  • enteralterego

    Overstating things a little to say GW were detected; the team is reporting the detection at a 3.5- to 4-sigma level. Promising but not the full monty.

    Reply

  • Atlan0001

    https://www.independent.co.uk/space/universe-speed-quasars-big-bang-b2368572.html
    Decelerator just a little bit slower and it would be an eternally fixed constant of Horizon. Oh, it is a constant of Horizon, as I have it! All it has to do is have the background still farther back become even slower, and slower, and slower, and slower.... toward a frozen, or virtually frozen, photo Horizon constant of picture.

    The comedy act, well almost a comedy act, continues.

    Reply

  • einsteinium99

    /\
    /\ / \
    /\ / \ / \
    /\ / \ / \ / \
    /\ / \ / \ / \ / \
    /\/ \/ \/ \/ \/ \

    \/\ /\ /\ /\ /\ /
    \/ \ / \ / \ / \ /
    \ / \ / \ / \ /
    \/ \ / \ /
    \/ \ /
    \/
    Instead of thinking of the singularity as a discrete "point" in spacetime, think of the singularity as a gravitational wave in spacetime with no upwards limit to its frequency.
    The expansion of the Universe is a natural consequence of a shared origin in a common space, a space which is not finite, but only appears to be due to the natural limits of human observation.
    In accordance with the limits of observation, a wave in spacetime of sufficiently high frequency would be virtually indistinguishable from a "point", and consequently, we measure the "age" of the Universe to be approximately 13.8 billion years relative to a perceived "zero point".
    However, because this point is not actually a point, but rather a gravitational wave with no upwards limit to its frequency, we can conclude that the Universe originates and expands from an infinite progression in spacetime. There is no "beginning of time", there only appears to be because observation is limited relative to the totality of the Universe, which is infinite by nature.

    Reply

  • einsteinium99

    Atlan0001 said:

    https://www.independent.co.uk/space/universe-speed-quasars-big-bang-b2368572.html

    Isn't this just a basic logical consequence of time dilation? Furthermore, does this not explain the accelerating expansion of the Universe?
    That's what I don't get. So-called "Dark Energy" is obviously just a logical consequence of a shared origin that appears to emerge from (or converge on, depending on how you look at it) a "point". That should never have been difficult for anyone who understands Relativity to wrap their brain around, and yet for some reason it is.

    Reply

  • Atlan0001

    einsteinium99 said:

    Isn't this just a basic logical consequence of time dilation? Furthermore, does this not explain the accelerating expansion of the Universe?
    That's what I don't get. So-called "Dark Energy" is obviously just a logical consequence of a shared origin that appears to emerge from (or converge on, depending on how you look at it) a "point". That should never have been difficult for anyone who understands Relativity to wrap their brain around, and yet for some reason it is.

    Look to "Does Time Exist...." in Cosmology, for my response regarding an identical 'Guardian' article on the same subject.

    Reply

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The universe is humming with gravitational waves. Here's why scientists are so excited about the discovery (2024)

FAQs

Is the universe humming with gravitational waves? ›

The universe is filled with a constant hum of gravitational waves, according to new observations from separate teams of scientists around the world. Gravitational waves are ripples in the fabric of the universe, caused by the motion of giant, accelerating celestial bodies.

What did Einstein say about gravitational waves? ›

But gravity is by far the weakest of the forces, and gravitational waves are just tiny wrinkles on top of that. Einstein ultimately concluded that although they existed, they were unlikely to ever be detected.

What is the new discovery about gravitational waves? ›

On Thursday, June 28, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) revealed the detection of low-frequency gravitational waves, a historic breakthrough that represents 15 years of searching. Yet, this isn't the first time that humanity has detected gravitational waves.

Why is the discovery of gravitational waves important? ›

The gravitational waves as we understand them are going to bring us very different kinds of information about the universe that you could never see with electromagnetic waves. This timeline of the universe over 13.8 billion years, with stars emerging about 180 million years after the Big Bang.

Do gravitational waves affect us? ›

It's unlikely to end well. Gravitational waves spread out from any violent event involving matter – such as, say, the collision of two black holes. Like gravity, however, they're incredibly weak, so you'd have to be extremely close to their source in order to feel their effects.

Does the Earth emit a hum? ›

The planet Earth produces a constant rumble - a hum - far below the limits of human hearing. This sound, first discovered a decade ago, was recorded by seismometers. Researchers call it Earth's hum.

Have gravitational waves been proven? ›

How do we know that gravitational waves exist? In 2015, scientists detected gravitational waves for the very first time. They used a very sensitive instrument called LIGO (Laser Interferometer Gravitational-Wave Observatory). These first gravitational waves happened when two black holes crashed into one another.

What happens when gravitational waves hit Earth? ›

While the processes that generate measurable gravitational waves are extremely violent, by the time the waves reach Earth they are thousands of billions of times smaller (due to the 'stiffness' of space), and much smaller when they reach us, much as sounds are weaker when you move further from the source.

What does NASA say about gravity? ›

Gravity is an attractive force that all objects have for one another. The amount of the gravitational force between two objects is directly proportional to the product of their two masses and inversely proportional to the square of the distance between their centers of mass. The force of gravity is not constant.

Do scientists say the universe has a background hum? ›

These gravitational waves, detected at low frequencies, create a cosmic background hum that permeates the universe. The research indicates that space is filled with these waves, which oscillate over extended periods, primarily originating from pairs of supermassive black holes spiraling and merging together.

Is space time affected by gravity? ›

Gravity as Curved Spacetime

Gravity feels strongest where spacetime is most curved, and it vanishes where spacetime is flat. This is the core of Einstein's theory of general relativity, which is often summed up in words as follows: "matter tells spacetime how to curve, and curved spacetime tells matter how to move".

What is the new black hole found? ›

Astronomers spot a massive 'sleeping giant' black hole less than 2,000 light-years from Earth. Scientists found the most massive stellar black hole in our galaxy due to the wobbly motions of its companion star. An artist's illustration shows the orbits of the star and black hole, dubbed Gaia BH3.

Why is the discovery of gravity important? ›

Gravity helped prove the fact that the Earth revolves around the sun. It also paved the way for the rethinking of multiple scientific phenomena that were previously unexplained. Discovery of Calculus: Newton attempted to quantify the speed of a falling object.

What's so special about gravitational waves? ›

More importantly, since gravitational waves interact very weakly with matter (unlike EM radiation, which can be absorbed, reflected, refracted, or bent by gravity itself), they travel through the Universe virtually unimpeded, carrying information about their origins that is free of distortion.

What do gravitational waves tell us about the universe? ›

By tapping into this new source of information about the universe, gravitational wave astronomy might be able to solve some of the biggest puzzles in physics, including: how black holes form, how matter acts in extreme conditions, illuminating the nature of dark matter, and looking at the beginning of the universe long ...

Does the universe have a humming sound? ›

Gravitational waves sound like hum

Scientists liken the universe's gravitational wave background to the hum of a large gathering, where individual voices cannot be distinguished.

Do gravitational waves make noise? ›

The gravitational wave background, a kind of cosmic noise that has long been theorized but never detected, is made up of ultra-low-frequency gravitational waves. As black holes collide across the universe, these waves all hum and resonate together in the background.

Does the universe have a gravitational pull? ›

Every object in the universe — stars, planets, moons, even you—has gravity. Gravity is a force of attraction between all objects.

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