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 Space.com 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!"
