Two heavy objects – like black holes – that rotate quickly around each other can make spacetime vibrate like a kind of drill pudding. These so-called gravitational waves were predicted by Albert Einstein in 1915 and measured for the first time in 2015.
Spacetime shivered last summer. That only takes a few seconds and was completely unnoticeable to people on Earth, but the ultra-sensitive gravity wave detectors from LIGO and Virgo were able to measure this on August 14, 2019. Researchers knew almost immediately that a collision had occurred between two extremely heavy objects some 800 million light-years away.
After dozens of such measurements since the very first in 2015, astronomers are no longer surprised. But what does lead to excitement is the nature of one of these two objects. The researchers infer from the signal that on the one hand they are dealing with a probably not very exceptional black hole of between 22 and 24 times the mass of our own sun. The other object is more interesting: it appears to consist of 2.6 solar masses. On paper it is therefore too heavy for a neutron star (a burnt-out core of a star that just did not collapse into a black hole) and too light for a black hole.
The scientists involved write that it is not clear what they saw. It was either the heaviest known neutron star or the lightest known black hole. Others speculate about objects that go beyond many booklets, such as one made of exotic dark matter particles. The study was published in the scientific journal last week The Astrophysical Journal Letters published.
This diagram shows the neutron stars (yellow) and black holes (purple and blue) hitherto found at their mass. There seems to be a gap between the weight of neutron stars and black holes. The latest discovery is the amalgamation shown in the center (pink).
Chris Van Den Broeck calls the result ‘very surprising’. He is a professor at Utrecht University and a research leader in the department of gravitational physics at the Nikhef research institute. As a researcher, he is also on the long authors list of the article. “No neutron star of more than two solar masses has ever been seen, nor has it ever seen a black hole lighter than five times the mass of the sun,” he says. “The reason for this is not entirely clear. Computer models are inconclusive, but the observations show that the formation of objects with this mass is difficult. ”
Both the lighter neutron stars and the heavier black holes arise when heavy stars have no fuel and their core collapses. The theory says that a neutron star heavier than two solar masses collapses into a black hole under its own gravity. Yet black holes only seem to originate ‘from’ about five solar masses. Van Den Broeck says that astronomers therefore speculate that the formation of these two objects is very different.
Incidentally, it is not impossible to have a (compact) object of between two and five solar masses. Take two neutron stars of one and a half solar masses, fuse them and you have an object of three solar masses. What it is difficult to explain, according to Van Den Broeck, is how this object subsequently ended up in a small orbit around a much heavier black hole. “The models show that this is quite rare. On average, such a scenario requires a lot of time, ”he says.
Star cluster Terzan 5 contains hundreds of thousands of stars and is located about 20 thousand light years from Earth in our own Milky Way. These ‘busy’ environments may offer opportunities for creating duos of heavy compact objects.
“This object is in the middle of the previously observed mass gap,” says Van Den Broeck. “In the models you can play with buttons that stretch the possibilities for creating special combinations. But an object in the mass gap and the other 23 solar masses? The buttons don’t turn that far. I think the theorists should return to the drawing board for this. ”
Some scientists hint at more exotic explanations. Van Den Broeck says that there are possible alternatives to neutron stars or black holes from string theory, for example. Fuzzballs for example, which consist of a large amount of tiny vibrating strings, or largely transparent forest stars. You can also think of aggregations of mysterious dark matter that do not form a black hole. Yet all of these statements are speculative. In any case, there is no conclusive evidence for the existence of such objects.
That is why Van Den Broeck puts his money on a neutron star or black hole. “At least we know that they exist. On the other hand, if we demonstrate the existence of a more exotic object, it has far-reaching consequences for physics. ”
The chance is gone to find out what the object was. Not only has it been destroyed, but the resulting object (a new, slightly heavier black hole) no longer emits measurable gravitational waves or radiation. Several telescopes on Earth have searched for a counterpart in the electromagnetic spectrum, but it has not (as expected) been found. According to theory, the black hole absorbs every measurable radiation from the collision. In 2017, scientists did manage to capture the radiation from the collision of two neutron stars with a large number of telescopes.
We have to wait for a new measurement of a comparable collision. Van Den Broeck is hopeful. “The fact that we have already measured such a collision within a few years probably means it is more common. I think this heralds a whole population of these objects, ”he says. “A new measurement is absolutely necessary to learn more about this.”
Such a new detection may have to wait for a while due to more earthly causes. Currently, LIGO and Virgo detectors are on hold for upgrades to make them even more sensitive. They will likely be able to look even deeper into the universe afterward, along with the new Japanese KAGRA gravity wave detector. The plan is to take new joint measurements from 2022 at the earliest.
Impression of two black holes in a ring of matter around an even bigger black hole (top).
Flaming black holes
There was more news from the world of black holes last week. Astronomers write in the scientific journal Physical Review Letters that they may have detected light from two colliding black holes orbiting around an even heavier black hole. On May 21, 2019, they observed a gravitational wave signal that exactly coincided with a light signal from a bright core of a galaxy, the so-called quasar J1249 + 3449.
The researchers suspect that these are two merging black holes in a hot ring of matter around a large central black hole of the galaxy. The collision supposedly created a new black hole that launched in a random direction after the merger. It draws a trail through the material ring, as it were. Precisely this absorption of matter from the ring can lead to the emission of radiation.
- Abbott R. et al., GW190814: Gravitational Waves from the Coalescence of a 23 Solar Mass Black Hole with a 2.6 Solar Mass Compact Object, The Astrophysical Journal Letters (23 juni 2020), DOI:10.3847 / 2041-8213 / ab960f