Seekers of gravitational waves are on a cosmic scavenger hunt.
Since the Advanced Laser Interferometer Gravitational-wave Observatory turned on in 2015, physicists have caught these ripples in spacetime from several exotic gravitational beasts — and scientists want more.
This week, LIGO and its partner observatory Virgo announced five new possible gravitational wave detections in a single month, making what was once a decades-long goal almost commonplace (SN Online: 5/2/19).
“Were just beginning to see the field of gravitational wave astronomy open,” LIGO spokesperson Patrick Brady from the University of Wisconsin–Milwaukee said May 2 in a news conference. “Opening up a new window on the universe like this will hopefully bring us a whole new perspective on whats out there.”
The speed and pitch of gravitational wave signals allow astronomers to make out whats stirring up the waves. Here are the sources of gravitational waves that scientists that already have in their nets, and what theyre still hoping to find.
1. Pairs of colliding black holes
LIGOs first catch was a pair of colliding black holes, each around 30 times the mass of the sun (SN: 3/5/16, p. 6). The experiment detected vibrations from the merging black holes on September 14, 2015, four days before the official start of observations for the freshly upgraded LIGO.
That first discovery proved that massive, moving objects do in fact shake spacetime to produce gravitational waves, as Einstein predicted a century earlier. Not only that, the find proved that experiments on Earth could detect those waves, something of which Einstein was skeptical. Three of LIGOs founders were awarded the Nobel Prize in physics in 2017 for the detection (SN: 10/28/17, p. 6).
In total, LIGO and Virgo have detected gravitational waves from 10 confirmed pairs of colliding black holes, plus another three candidates netted in the last month.
2. Pairs of colliding neutron stars
It was thought that a pair of merging neutron stars, the dense stellar corpses of massive stars that died in a supernova, could also set off gravitational waves. Then, in August 2017, the LIGO-Virgo team caught the first known instance of such an event, and a second on April 25 (SN: 11/11/17, p. 6).
COSMIC CHIRP Gravitational waves are analogous to sound waves. If human ears could hear the waves created by a pair of colliding neutron stars, they would sound like a low rumble as the neutron stars circled each other. Then, the sound would increase in pitch as the stars drew closer and closer, culminating in a faint “chirp” as the objects merged (around 29 seconds).
Max Planck Institute for Gravitational Physics
Follow-up observations with telescopes that are sensitive to light across the electromagnetic spectrum revealed hidden details of that first neutron star crash, including that the collision forged precious elements like gold, silver and platinum.
3. A neutron star crashing into a black hole
Another type of merger that could spawn ripples in spacetime is like the chocolate-vanilla swirl at an ice cream stand: one black hole and one neutron star merging into a single object. The observatories saw a possible signature of this kind of merger on April 26, but the signal was too weak for scientists to be sure.
If the team confirms that that signal really represents a black hole and neutron star swirl, it would prove that the two kinds of objects can live side by side. Before merging, the black hole and neutron star would have had to orbit each other in a close binary system.
“Wed be surprised if they didnt exist, but we havent seen one” of these combinations, says LIGO team member Christopher Berry of Northwestern University in Evanston, Ill.
Studying such a system could help illuminate the mysterious material called nuclear pasta that makes up neutron stars (SN: 10/27/18, p. 8). “Neutron stars are kind of like giant atomic nuclei. Theyre nothing like what we can create on Earth,” Berry says. The neutron star merger spotted in 2017 gave some details of the stars makeup (SN: 12/23/17, p. 7), including their maximum mass and squishiness. Spying a black hole-neutron star merger could show how a neutron star deforms near the extreme gravity of a black hole, another piece in the puzzle of how nuclear pasta behaves.
4. A collision involving an intermediate-mass black hole
Status: Not yet
All the black holes that LIGO and Virgo have detected so far have been stellar mass, which means that they typically weigh less than 100 times the mass of the sun. Physicists also know of supermassive black holes that weigh millions or billions times the mass of the sun (SN: 4/27/19, p. 6). But its not clear if there are black holes with masses in between.
Such intermediate-mass black holes “could be the link between stellar mass black holes and supermassive black holes in the centers of galaxies,” Virgo team member Giovanni Andrea Prodi of the University of Trento in Italy said May 2 at a news conference.
Previous research has seen hints of such middleweight black holes, but a collision detected with gravitational waves would be more definitive proof. If they dont exist, “thats really interesting,” Berry says, because it would mean supermassive black holes must have been born bigger than physicists can explain (SN Online: 3/16/18).
5. A bumpy neutron star
Status: Not yet
Another way to steal the secrets of neutron stars mysterious nuclear pasta is to detect miniature “mountains” on their surfaces. All massive objects that accelerate generate gravitational waves, but most of them are too faint to detect. Physicists think that a lone neutron star with a slight imperfection on it, like a bump about a millimeter high, would emit detectable gravitational waves as it spins. Such waves could help tell how stiff the neutron star material can be, in order to support the bumps.
Unlike most other sources on this list, bumpy neutron stars would produce continuous gravitational waves, detected as a constant “hum” by the observatories.
6. Supernova explosions
Status: Not yet
LIGO and Virgo might also be able to pick up gravitational waves from supernova explosions, the bright cataclysms at the end of massive stars lives.