Gravitational waves may have previously only existed in Einstein’s mind. As far as scientists could tell prior to their discovery, these spacetime ripples only appeared in the general theory of relativity.
Now, there are two approaches for researchers to find the waves. And they are looking for more. According to astrophysicist Karan Jani of Vanderbilt University in Nashville, the field of gravitational wave research is flourishing. “This is very amazing. No area of fundamental physics that I can think of has experienced such rapid development.
Gravitational waves have a spectrum, or a range of wavelengths, much like light does. Different wavelengths need various sorts of detectors and point to various kinds of cosmic origins.
The main sources of gravitational waves with wavelengths of a few thousand kilometers, such as those discovered by LIGO in the United States and its collaborators Virgo in Italy and KAGRA in Japan, are merging pairs of black holes with masses of about 10 times that of the sun or collisions of neutron stars, which are dense cosmic spheres (SN: 2/11/16). These detectors could also pick up waves from specific kinds of supernovas, or exploding stars, as well as from pulsars, which are neutron stars that rotate rapidly (SN: 5/6/19).
On the other hand, massive circling pairs of supermassive black holes with masses billions of times that of the sun are estimated to be responsible for the enormous waves that span light-years. By using the entire galaxy as a detector and seeing how the waves altered the timing of regular blinks from pulsars dispersed throughout the Milky Way, researchers published the first convincing evidence for these waves in June (SN: 6/28/23).
Physicists now expect to dive into a vast, cosmic ocean of gravitational waves of all sizes, with the equivalent of both minute ripples and large tsunamis in hand. These waves may shed new light on the hidden existence of strange objects like black holes and unexplored elements of the cosmos.
The gravitational wave spectrum is still largely unexplored, according to Stanford University physicist Jason Hogan. But he asserts that it makes sense to cover all the bases. Who knows what else we could uncover?
This mission to collect the whole complement of gravitational waves from the universe may send observatories to the moon or outer space, as well as to the atomic scale and other places. Here is a list of some of the frontiers researchers are looking at in their search for novel wave types.
Go to deep space
LISA, short for Laser Interferometer Space Antenna, initially seems unbelievable. Three spacecraft would be positioned in a triangle with sides of 2.5 million kilometers, beaming lasers to one another as they circled the sun. However, the European Space Agency mission, scheduled for the middle of the 2030s, is not only a pipe dream (SN: 6/20/17). It is a lot of scientists’ best chance of exploring gravitational wave frontiers.
Theoretical physicist Diego Blas Temio of the Universitat Autonoma de Barcelona and the Institut de Fsica d’Altes Energies calls LISA “a mind-blowing experiment.”
The Laser Interferometer Space Antenna, or LISA, will be made up of a trio of spacecraft orbiting the sun (illustrated in foreground). LISA will observe gravitational waves from orbiting supermassive black holes in distant galaxies (illustrated in the background).SIMON BARKE/UNIVERSITY OF FLORIDA (CC BY 4.0)
Based on how the laser beams interfere at the triangle’s corners, LISA would be able to detect the stretching and constriction of the triangle’s sides as a gravitational wave passed by. LISA Pathfinder, a proof-of-concept experiment with a single spacecraft, flew in 2015 and proved the viability of the method (SN: 6/7/16).
In general, a larger detector is required to detect gravitational waves with longer wavelengths. Scientists could view wavelengths that are millions of kilometers long thanks to LISA. Thus, LISA may find orbiting black holes that were modestly massive – millions rather than billions of times the mass of the sun.
Go to the moon
Scientists are taking cues from Earth’s neighbor as NASA’s Artemis program seeks to send astronauts back to the moon (SN: 11/16/22). A gravitational wave detector would be installed on the moon as part of an experiment dubbed LILA, or the Laser Interferometer Lunar Antenna.
The moon should make gravitational waves easier to detect as it is free of the vibrations caused by human activities and other factors related to the Earth. It almost has a spiritual stillness, claims Jani. There is no site in the solar system better than our moon if you want to hear the noises of the universe.
LILA would operate similarly to LISA, with three stations shooting lasers in a triangle, but its sides would be around 10 kilometers long. It could capture wavelengths that were tens of thousands or perhaps millions of kilometers long. That would close the discrepancy in wavelength measurements between the Earth-based LIGO and the space-based LISA.
The Laser Interferometer Lunar Antenna, LILA, is a proposed gravitational wave detector on the moon. Thanks to the moon’s paltry atmosphere, LILA’s triangle of lasers (illustrated) wouldn’t need to be enclosed in vacuum tubes, unlike similar observatories on Earth.VANDERBILT LUNAR LABS/VANDERBILT UNIVERSITY
As they approach a merger, orbiting objects like black holes accelerate, which causes them to generate gravitational waves with ever shorter wavelengths. Therefore, LILA might observe how closely two black holes approach one another in the weeks prior to their merging, alerting scientists to the impending collision. Once the wavelengths are sufficiently short, terrestrial observatories like LIGO would detect the signal and record the impact moment.
A different moon-based option would employ lunar laser ranging, a method that uses reflectors set up on the moon’s surface during previous moon landings to allow researchers to estimate the distance from Earth to the moon with lasers.
Blas Temio and a colleague announced the method in Physical Review D in 2022. The method could detect waves jiggling the Earth and the moon, with wavelengths between those measured by pulsar timing methods and LISA. However, that method would call for upgraded lunar reflectors, which is another justification for returning.
Go atomic
By observing how laser beams interact after traveling via their detectors’ extended arms, LISA, LIGO, and other laser observatories quantify the stretching and squeezing of gravitational waves. However, a suggested method takes a different path.
This novel method monitors the separation between two clouds of atoms when gravitational waves pass, as opposed to looking for minute variations in the lengths of detector arms. A consequence of atoms’ quantum characteristics is that they behave like self-interfering waves. The separation between the atom clouds changes if a gravitational wave passes through. Based on that quantum interference, scientists can deduce that change in distance.
According to Hogan, the method might detect gravitational waves with wavelengths between those of LIGO and LISA. He is working on the MAGIS-100 detector prototype at Fermilab in Batavia, Illinois.
Atom interferometers can detect Earth’s gravity and test the laws of basic physics, but they have never been used to monitor gravitational waves (SN: 2/28/22; SN: 10/28/20). The concept is “totally futuristic,” according to Blas Temio.
Go back in time
A different endeavor seeks to locate gravitational waves from the very beginning of the universe. These waves would have been created during inflation when the universe grew in size right after the Big Bang. The wavelengths of these waves would be 1 sextillion kilometers, or 1021 kilometers, longer than anything ever observed.
The search, however, got off to a false start in 2014 when researchers with the BICEP2 experiment announced the discovery of gravitational waves imprinted in swirling patterns on the cosmic microwave background, or CMB, the earliest light in the universe. Later, the assertion was refuted (SN: 1/30/15).
The hunt will continue under CMB-Stage 4, with plans for numerous more telescopes to examine the earliest light of the universe for evidence of the waves – perhaps successfully this time.
Measurements of the cosmic microwave background (data from the Planck satellite shown) could reveal gravitational waves stirred up just after the Big Bang.PLANCK COLLABORATION/ESA
Go for the unknown
Scientists have a general idea of what to anticipate for the majority of gravitational wave types that they have focused on. These waves can be produced by well-known phenomena, such as neutron stars or black holes.
The “story is different” for gravitational waves with the shortest wavelengths, which may just be a few centimeters, according to theoretical physicist Valerie Domcke of CERN near Geneva. There is no known source that could possibly produce gravitational waves with an amplitude that would allow for their detection.
However, physicists want to confirm the existence of the minute waves. Phase transitions, in which the cosmos changes from one condition to another, similar to how water condenses from steam to liquid, may have caused these waves early in the history of the universe. Another idea is that the early cosmos gave birth to tiny, primordial black holes that were too small to create via conventional processes. Since the physics in these regimes is so little known, “even looking for [gravitational waves] and not finding them would tell us something,” claims Domcke.
Given how enigmatic these gravitational waves are, it is unclear how to detect them. However, instead of using massive detectors, the wavelengths might be observed using high-precision, laboratory-scale investigations.
Even data from studies with different objectives could be useful to scientists. The ripples produced when gravitational waves interact with electromagnetic fields can exhibit characteristics resembling those of hypothesized subatomic particles known as axions (SN: 3/17/22). Therefore, experiments looking for such particles may also find small gravitational waves.
A new view
Similar to paddling against the tide, catching gravitational waves is difficult but worthwhile for the beautiful scenery. Hogan claims that it is extremely difficult to detect gravitational waves. Before LIGO detected its initial swells, decades of investigation were required; the same is true for the pulsar timing method. However, astronomers started to benefit right away. It’s a completely new perspective on the universe, says Hogan.
Already, gravity waves have revealed the spectacular events that result from the collision of two extremely compact objects known as neutron stars, discovered a new class of black holes with moderately sized masses, and helped to prove Einstein’s general theory of relativity (SN: 2/11/16; SN: 9/2/20; SN: 10/16/17).
Additionally, gravitational wave detection is still in its infancy. What will be exposed by future detectors is simply a hunch for scientists. There’s a lot more to learn, says Hogan. It will undoubtedly be fascinating.
