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 is not the primary time that humanity has detected gravitational waves. Scientists have been detecting these ripples in the material of space using facilities just like the Laser Interferometer Gravitational-Wave Observatory (LIGO) since 2015.
So, with that in mind, why is not this just one other — inarguably impressive — detection of gravitational waves? The reply is all about three connected qualities: The frequency, the wavelength and the period of gravitational waves, and what these tell scientists concerning the objects and events that first sent them rippling through space.
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 speed up, they need to generate ripples in spacetime, a sort 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, are available 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 even have long periods, the time it takes between one peak of the wave passing a set point to the following peak passing that time.
Not all gravitational waves are created equal
The invention announced on June 28 marks the primary detection of low-frequency gravitational waves. The source of those low-frequency gravitational waves is believed to be supermassive black hole binaries within the very early universe. Consider this when it comes to an orchestra. LIGO can hear the dramatic single “crash” of symbols 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.
The strength of this signal indicates that a gravitational wave orchestra of tons of of 1000’s and even hundreds of thousands of supermassive black hole binaries existed within the early universe.
“This finding opens up a brand new low-frequency window on the gravitational universe which can allow us to study how galaxies and their central black holes merge and grow with time,” National Radio Astronomy Observatory (NRAO) astronomer Scott Ransom, one in all the around 190 scientists working with NANOGrav, told Space.com.
As black holes and neutron stars swirl around one another, they generate a continuous regular stream of low-frequency gravitational waves, effectively causing spacetime to ring like a gently struck bell. As they’re emitted, the gravitational waves carry away angular momentum (spin), and this causes the black holes to attract together.
The closer orbiting objects are, the faster they emit gravitational waves and the upper the frequency of this gravity radiation becomes; moreover, the closer they’re, 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.
Moreover, there are also more exotic possible explanations for these faint ripples in space-time. A fraction of this signal might 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.
Why NANOGrav can do what LIGO and LISA cannot (and vice versa)
Similar to it takes different telescopes to see different frequencies of sunshine within 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 attributable to collisions between black holes, neutron stars, and even mixed mergers between the 2, but lower-frequency gravitational waves have been evasive.
It’s 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!
Whilst sensitive because it is, LIGO and its fellow ground-based gravitational wave observatories cannot pick up low-frequency gravitational waves. Even the forthcoming space-based gravitational wave detector, the Laser Interferometer Space Antenna (LISA) is not going to find a way to choose up should signals.
The gravitational waves that LIGO and other ground-based detectors can hear gravitational waves with wavelengths of around 1000’s of miles, concerning the size of Earth, with periods starting from milliseconds to seconds. LISA will cover wavelengths the scale of hundreds of thousands to billions of miles; think the space from Earth to the sun or the space of Earth or Pluto. The periods of those gravitational waves last from seconds to hours.
The gravitational waves that NANOGrav is designed to listen to are at nanoHertz frequencies and have wavelengths on the scale of trillions of miles, making them light-years in length. And in accordance with NANOGrav, these nanoHertz gravitational waves can have periods of months, years, and even many years.
For this detection to happen, astronomers needed a gravitational wave antenna the scale of your complete galaxy and an incredibly precise way of measuring time consisting of a network of “cosmic clocks.” That is where NANOGrav is available in.
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 Latest Mexico, NanoGrav turned 68 pulsars inside the Milky Way right into a huge gravitational wave antenna the scale of your complete galaxy. This unique and sensitive gravitational wave detector known as 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 on this process ceases. This ends in the core of those stars collapsing under their very 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 as much as twice our star’s mass crammed right into a body no wider than that of the common city here on Earth. Because of 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! Consider this as being like a figure skater drawing of their arms to extend their spin, just on an entire different scale!
The collapse of stellar cores has one other consequence; the magnetic field of the unique star can also be squashed down. When magnetic field lines are crammed closer together, this increases the strength of the magnetic field they comprise.
Consequently, neutron stars have among the most powerful magnetic fields within the known universe. These magnetic fields act to channel particles to the poles of pulsars, where they’re 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 that is the results of the sunshine these jets create turning towards us at incredibly precise regular intervals. This implies pulsars may be used as an excellent timing device.
The compression and stretching of spacetime as gravitational waves wash through it must have a discernible on the timing of pulsars, either slowing them down or speeding them up as they pass. This causes a really slight difference within the arrival time of sunshine from these pulsars. Since the effect is small, pulsar timing arrays must consist of many widely dispersed pulsars that need to be monitored for years.
For NANOGrav, patience has now paid off with this effect on pulsars now revealing an indication from low-frequency signal gravitational waves.
“Mainly, the Earth is bobbing around — a tiny bit — on gravitational waves which might be light-years in length,” Ransom said. “And we have now seen this using an array of virtually 70-millisecond pulsars scattered around our a part of the Milky Way.”
The rationale this discovery is essential is we have now detected gravitational waves from sources we hadn’t investigated before. It has revealed that the early universe was full of 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 don’t seem to be yet sure how these cosmic titans grow. One suggested mechanism is a series of mergers between subsequently larger and bigger black hole binary pairs.
This low-frequency gravitational wave signal hints at a option to understand how this might have proceeded within the early universe resulting in some supermassive black holes which have masses hundreds of thousands and even billions of times that of the sun.
Moreover, because these black holes are likely delivered into the spiral dance of death that ends in their merger by the collisions of galaxies, a greater understanding of this black hole binary merger process means a greater understanding of how galaxies grow and the way the universe as an entire has evolved.
There may be also the small likelihood that a tiny fragment of the gravitational wave signal this pulsar timing array the scale of the Milky Way has picked up comes from gravitational waves created initially of time through the Big Bang, which might have wavelengths starting from around the scale of the Milky Way — around 100,000 light-years — to the scale of the Virgo Supercluster of galaxies — around 100 million light-years.
“That is exciting. The evidence reported by NANOGrav shows once more that gravitational wave observations are opening up an entire latest window onto the universe,” KU Leuven, cosmologist and long-time Spethen Hawking collaborator Thomas Hertog, who was not involved within the study, told Space”In the approaching years and many years, we’ll be patching together your complete history of the universe in great detail by listening to the hum of gravitational waves passing through our planet. Exciting times indeed!”
Almost about the long run, Ransom explained how NANOGrav will now search for a sensitive radio telescope within the northern hemisphere to substitute for the Arecibo telescope, which collapsed in Dec. 2020. Until that’s 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 must always start seeing individual sources as pure tones above this gravitational wave background. Those sources might be pinpointed and studied with electromagnetic waves as well — a brand new style of extragalactic multi-messenger astronomy,” Ransom concluded. “I’m very enthusiastic about this development! We have been working on this for over 15 years, and I’m not a really patient person!”