Black holes have a preference for forming around two “universal” masses akin to about nine and 16 times the mass of our sun, based on a brand new study of the frequency of the gravitational-wave ‘chirps’ released when two black holes collide and merge. These findings could ultimately pave the best way for an independent measure of the expansion of the Universe.
Since 2015, 90 gravitational-wave events have been identified by detectors at sites specifically built to seek out these information-rich ripples in spacetime. This includes laboratories equivalent to the Laser Interferometer Gravitational-wave Observatory (LIGO) within the US, its sister site, Virgo, in Italy and the Kamioka Gravitational-Wave Detector (KAGRA) in Japan. Each merger produces what’s often called a chirp, which is a blast of gravitational waves that rapidly increase in frequency as two black holes spiral closer and closer around each other before colliding and merging. The frequency and amplitude of this chirp is connected to the mass of the black holes which have merged; their combined mass is usually known as the “chirp mass.”
“When two black holes merge, they produce gravitational waves that might be ‘heard’ on Earth,” Eva Laplace, an astrophysicist on the Heidelberg Institute for Theoretical Studies in Germany and an writer on the study, told Space.com. “By listening to those chirps and analyzing them, it is feasible to measure the combined mass of distant merging black holes.”
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Stellar-mass black holes form through the death of a massive star. While in some cases a large star will explode as a supernova and leave behind a compact neutron star, in other cases there is no such thing as a explosion. As an alternative, the star’s core collapses under gravity so severely it forms a black hole that eventually causes the remaining of the star to cave in around it.
The masses of those resulting black holes determine the frequency of the gravitational-wave chirp that’s emitted once they merge, and are also related to the mass of the celebrities that formed them. One would subsequently expect a wide selection of stellar mass-black holes to exist within the universe, reflecting the varied masses of their progenitor stars, and indeed this is usually the case. Nevertheless, astronomers have been baffled to seek out more black holes related to gravitational-wave events which have masses around 8–9 solar masses and 14–16 solar masses, but for some reason, hardly any with masses in between.
Now, recent research conducted by Laplace alongside fellow astrophysicists Fabian Schneider, and Philip Podsiadlowski, also of the Heidelberg Institute for Theoretical Studies in Germany, addresses this apparent preference for merging black holes to converge on certain masses over others.
“What our study shows is that there’s at all times a spot in black-hole masses between 9 and 16 solar masses,” Schneider told Space.com.
What happens inside massive stars
The existence of the mass gap is dictated by what is going on inside a large star because it nears the top of its life.
Young stars “burn” hydrogen of their cores via their intrinsic nuclear fusion processes; in massive stars, the dominant version of this process is often called the carbon–nitrogen–oxygen (CNO) cycle. This refers to an extended chain of reactions involving hydrogen, plus those elements, that eventually produce helium and release a whole lot of energy to power the star. Nevertheless, once the star’s core runs out of hydrogen, its energy production falters. Without enough energy to carry the star up, the core begins to contract under gravity. This increases the core’s temperature by tens of millions of degrees Celsius, until it’s hot and dense enough to start burning helium and temporarily halt the contraction.
At this stage, the star is like an onion, with various layers. Its core is burning helium. Across the core is a non-burning layer of helium, and ordinarily, around that could be a shell that’s still burning some leftover hydrogen to provide much more helium that sinks to the stellar core. This extra helium further increases the core’s mass and temperature, speeding up the nuclear reactions which control the star’s evolution. Ultimately, this results in a supernova and typically either a neutron star or a lone black hole, depending upon the compactness of the star’s core (within the case of stars with 130-250 solar masses and fairly primitive chemical compositions, they will sometimes explode and utterly destroy themselves in a so-called pair-instability supernova, leaving nothing behind).
In contrast, black-hole mergers are the product of massive binary star systems. Throughout the time that they still exist as stars, the close companions are capable of steal matter from one another, stripping away one another’s hydrogen-burning shell. Without this shell, a star’s core doesn’t gain that extra helium, which changes the star’s evolutionary trajectory. Conditions contained in the core of a star that has lost its hydrogen shell are such that thermal neutrinos – tiny, ghost-like particles that spontaneously form – escape the star, carrying a few of the core’s thermal energy with them. This lowers the temperature of the core and slows down nuclear reactions. The result’s a decrease in energy production which allows the core to gravitationally contract some more. This results in a really dense core that, when the star exhausts all its nuclear fuel and dies, can collapse to form a black hole.
In a binary system, this could result in two black holes that eventually merge with a chirp of gravitational waves.
“Due to a posh interplay between neutrino losses, nuclear burning and core contraction, we discover that stars of specific core masses are more vulnerable to collapsing to black holes somewhat than exploding as a supernova and abandoning a neutron star,” said Schneider.
This interplay results in common black hole masses, based on the calculations of Schneider, Laplace and Podsiadlowski. Of their models, the black hole masses are likely to converge on two values, that are 9 and 16 times the mass of our Sun. These values are very near the peaks which have been observed within the gravitational-wave data, that are at about 8 and 14 solar masses, so that they don’t exactly match, but remain inside observational uncertainties.
Measuring the expansion of the universe
A prevalence of certain masses of black hole not only tells us concerning the physics of massive stars, but it surely also gives astronomers one other method to measure the expansion rate of the universe, often called the Hubble constant. This has come under the highlight in recent times because different methods give contradictory values for the Hubble constant.
The frequency of a gravitational-wave chirp depends totally on the combined masses of the black holes involved, but a portion of additionally it is connected to their redshift, which tells us their distance, since the farther away they’re, the more the expansion of the universe has shifted them to longer wavelengths.
Until now, it had been unattainable to disentangle the black hole masses from the redshift within the chirp. Nevertheless, knowing that a big proportion of black holes have these universal masses gives scientists a bonus.
“We will then take a statistical approach to decouple the masses from the redshift,” said Schneider. This method would require a bigger sample of gravitational-wave events than we currently have, but in principle, would supply a method of measuring the Hubble constant from the redshift that’s independent of methods involving standard candles equivalent to Type Ia supernovae.
A bigger sample of gravitational-wave events could also be coming soon. A brand new observing run involving LIGO, Virgo and KAGRA that can last 20 months has recently begun, and the aim is to find one other 300 events. We’ll know soon enough whether the brand new results enhance the peaks within the distribution across the universal masses and the gap between them.
The findings were published in The Astrophysical Journal Letters.