Existing in the universe means that you are regularly rocked by ripples in the fabric of spacetime, which imperceptibly warp your corporeal form as they pass through you.
These ripples, known as gravitational waves, are caused by catastrophic cosmic events, such as the explosions of stars or the unions of black holes. They are extremely subtle, oscillating at a fraction of the width of a proton, so detecting them is a challenge, but it’s not impossible.
Scientists snagged the first gravitational wave detection at the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015, and have confirmed more than 20 others since then.
“We’ve only been doing this for five years,” said Ryan Foley, an astronomer at UC Santa Cruz who studies gravitational waves, in a call. “The field is incredibly young.”
The sheer novelty of gravitational wave astronomy has resulted in “non-stop surprises,” Foley said, and that has been especially true in recent weeks. Two new studies about separate gravitational wave events, both published in late June, have revealed strange new phenomena that could help solve some of the most fundamental mysteries of the universe.
One of the studies, published in the Astrophysical Journal Letters, zeroes in on a gravitational wave created by the asymmetric union of a black hole and a “mystery object,” according to LIGO. Meanwhile, the first plausible glimpse of a radiant flare produced by a black hole merger—a major discovery, if confirmed, in part because visible traces of black holes are extremely rare—was reported in Physical Review Letters.
Every gravitational wave has, by definition, an epic backstory. But these new studies suggest that their origins are trippier than we ever expected.
Light from black holes
For years, a team of scientists has been developing a model that suggests that active galactic nuclei (AGN), which are energetic galactic cores surrounded by gas-rich disks, might reveal the hidden presence of some black holes.
Black holes swallow up anything that comes too close to them, including light, so they are typically difficult to detect because they are not luminous. However, if black holes merge within AGN, the energy of their union could catapult them through the disk at high speeds, creating a shockwave that would emit a visible blast of light.
K.E. Saavik Ford, an astronomer at the American Museum of Natural History and the City University of New York, and an author on the Physical Review Letters study, is part of the group that has been searching for these speculative signatures.
“We have this whole model that predicts that you should have relatively high rates of black hole binary formation and mergers in the centers of galaxies with a gas disk, which is to say inside active galactic nuclei,” said Ford in a call.
“If you have black holes that merge, and they are spinning before they merge, then general relativity predicts that there will be a recoil kick from the emission of gravitational waves due to the balance of angular momentum at the moment of merger,” she added.
The detection of gravitational waves is incredible on its own, but an optical counterpart (the pairing of two measurement techniques is called “multi-messenger astronomy”) to a black hole merger would be an especially mind-blowing discovery. So, when LIGO announced a candidate gravitational wave in May 2019, Ford and her colleagues started looking for any flashes of light that could potentially be associated with it.
“Honestly, when we went through the process, our expectation was that we weren’t going to turn up anything,” Ford said. “Then we saw this one event that looked really interesting.”
A few weeks after the wave, a flare lit up a “quasar” galaxy with an AGN located four billion light years away. After eliminating multiple different explanations, from supernovae to gravitational distortions, Ford and her colleagues think that the flash could plausibly be the luminous trail of a black hole merger hurtling through the gas of the AGN.
“In the very worst case scenario from a scientific perspective, this is a super-wacky flare that’s going to also tell us something interesting about the disks of gas that are swirling around supermassive black holes,” Ford said. “In a best case scenario, it is extremely suspicious that this is such a coincidence with the timing and location of LIGO’s observed event.”
It will take many follow-up observations to confirm if the two events are actually linked, and Ford cautioned that the general moodiness of AGN could scuttle total certainty about a connection.
“AGN disks do weird things,” she noted. “Every astronomer learns that in astronomy kindergarten. They vary, they change, they do all kinds of stuff. So, could it have been a variation just in the disk? That is almost impossible to exclude.”
Still, she said it is a “really weird” event, even in the context of general AGN quirkiness. Fortunately, if the light really was caused by a merged black hole, the object may create another flare when it re-enters the disk in the coming months or years. Ford and her colleagues are monitoring the quasar to hopefully catch a glimpse of its return, should it happen.
In addition to potentially revealing a new source of multi-messenger astronomy, the ongoing saga of this distant black hole merger could help to unveil the identity of the “mystery object” described by the other recent study that appeared in the Astrophysical Journal Letters. The paper is co-authored by hundreds of scientists involved in the LIGO Collaboration and its Italian sister detector, Virgo.
For nearly a year, the LIGO/Virgo collaboration has been examining the bizarre signature of a gravitational wave detected in August 2019. The detection initially made waves in the science community because it looked like it could be a neutron star merging with a black hole—an event that has never been observed before.
The new study confirms that there is definitely one black hole involved in this merger, which is about 23 times more massive than the Sun. However, the other object is somewhere between 2.5 and 3 times more massive than the Sun. That means it is either the heaviest neutron star or the lightest black hole ever discovered, and lands squarely in a zone of assumed impossibility known as the “mass gap.”
When a colossal star explodes into a supernova, its mass determines whether it will collapse into an afterlife as a neutron star or as black hole. However, it’s not clear exactly where the mass limit is between those two posthumous choices. It’s possible that a dying star might even temporarily condense into a neutron star, only to permanently crumple into a black hole if it gains a bit more mass—for instance, if its own gassy entrails from its supernova fall back on to it.
“If you keep making a neutron star bigger and bigger, eventually gravity wins, and then you make a black hole,” said Foley, who was not involved in the study. “There’s some maximum mass of a neutron star and it depends on the details of nuclear matter—the same stuff that’s creating the nucleus of an atom. It turns out that we don’t actually know physics well enough to know exactly what that is.”
The estimated upper mass of neutron stars has hovered around 2.5 times the mass of the Sun for years. At the same time, scientists have been unable to detect any black holes that are smaller than five times the mass of the Sun. This has led to an assumption that it might be impossible for a star to leave behind a corpse with a mass anywhere between, roughly, 2.5 and five solar masses. This is the mass gap.
The newly detected mystery object flauts the hypothetical gap. LIGO/Virgo researchers predict that the object is far more likely to be the lightest known black hole than the heaviest neutron star, but either way, it challenges current models about the pyrotechnic deaths of stars.
“That dividing line, that big gap, was built into a bunch of theoretical models,” Foley said. “People have to reevaluate how you explode a star now, which is pretty incredible.”
The August 2019 event is exceptional for another reason—the extreme difference between the masses of the two objects involved in the merger. Typically, these unions involve black holes that are comparable in mass, but this one has an asymmetric mass ratio of 9:1.
“This is the equivalent of an NBA star dancing with a toddler,” Foley said. “It’s like Shaq and a three-year-old. It just isn’t something that happens very naturally.”
As rare as it is to capture an observation of such an event, it has been modeled in past studies. Earlier this year, for instance, Ford and her colleagues published a paper that predicts that this type of off-kilter mass ratio may occur in AGN disks.
“If you have a low-mass object, it doesn’t move very much in the disk,” Ford said. “The higher-mass objects move around and sneak up on those lower-mass objects, so you can have these binaries form in the disk with a pretty high mass ratio.”
In other words, both of these studies may offer tantalizing glimpses of the strange unions that occur in energetic galactic cores across the universe. With the caveat that it will take a lot more research to interpret the findings, there’s no question that these recent detections have made the already-weird world of gravitational wave astronomy that much more mind-boggling.
“The thing about astronomy is that every single time we’ve opened up a new window on the universe, we have had incredible surprises,” Foley said. “We are starting to see what’s out there, but we haven’t gotten there completely.”
“There are all sorts of things that we know exist in gravitational waves that we haven’t seen yet.”