According to a new study of the frequency of the ‘cry’ gravitational waves emitted when two black holes collide and merge, black holes have a preference for forming around two ‘universal’ masses equivalent to about 9 and 16 times the mass of our sun. These discoveries could eventually pave the way for an independent measurement of the expansion of the Universe. Since 2015, 90 gravitational wave events have been identified with detectors at sites specially built to look for these information-rich ripples in spacetime. This includes laboratories such as the Laser Interferometer Gravitational Wave Observatory (LIGO) in the United States, its sister site, Virgo, in Italy, and the Kamioka Gravitational Wave Detector (KAGRA) in Japan . Each merger produces a so-called chirping, which is a gravitational wave that rapidly increases in frequency as the two black holes move closer and closer together before colliding and merging. The frequency and amplitude of this chirping are related to the masses of the merged black holes; Their combined masses are sometimes referred to as “chirp masses”. Eva Laplace, an astrophysicist at Germany’s Heidelberg Institute for Theoretical Research and study author, told Space.com: “When two black holes merge, they produce gravitational waves that can be ‘heard’ ‘ on the earth. “By listening to these hums and analyzing them, it is possible to measure the aggregate mass of distant merging black holes.” Related: The universe is rocking with gravitational waves. Here’s why scientists are so excited about this discovery A stellar-mass black hole forms when a massive star dies. In some cases, a massive star will explode into a supernova and leave behind a compact neutron star, in other cases there will be no explosion. Instead, the star’s core shrank under such severe gravity that it formed a black hole and eventually caused the rest of the star around it to collapse. The mass of these forming black holes determines the chirp frequency of the gravitational waves emitted during their merger and is also related to the mass of the stars that form them. Thus, a variety of stellar-mass black holes would exist in the universe, reflecting the different masses of their progenitor stars, and in fact this is the case most often. However, astronomers have had difficulty finding more black holes associated with gravitational wave events of about 8-9 solar masses and 14-16 solar masses, but because for some reason there is almost no intermediate mass. Now, new research by Laplace with colleagues astrophysicists Fabian Schneider and Philip Podsiadlowski, also from the Heidelberg Institute for Theoretical Research in Germany, addresses this apparent preference for hole mergers. black to converge on certain blocks rather than others. “What our study shows is that there is still a gap in the mass of a black hole between 9 and 16 times the mass of the Sun,” Schneider told Space.com. What Happens Inside Massive Stars The existence of a mass hole is determined by what happens inside a massive star as it nears the end of its life. Young stars “burn” hydrogen in their cores through their intrinsic nuclear fusion processes; In high-mass stars, the dominant version of this process is known as the carbon-nitrogen-oxygen (CNO) cycle. It refers to a long chain of reactions involving hydrogen, as well as these elements, that eventually produce helium and release a lot of energy to power the star. However, once a star’s core runs out of hydrogen, its energy output gradually decreases. Without enough energy to hold the star together, the core begins to contract under the influence of gravity. This raises the core’s temperature by millions of degrees Celsius, until it is hot and dense enough to start burning helium and temporarily stop shrinking. At this stage the star is like an onion, with many different layers. Its core ignites helium. Surrounding the core is a layer of unburnt helium, and often around this layer is a shell that still burns off the remaining hydrogen to produce more helium that flows towards the core. This excess helium further increases the mass and temperature of the core, accelerating the nuclear reactions that control the star’s evolution. This eventually leads to a supernova and often an isolated neutron star or black hole, depending on the density of the stellar core (in the case of stars with masses between 130 and 250 times the mass of the Sun and stars). their chemical composition is quite primitive, they can sometimes explode and completely self-destruct in so-called pair-unstable supernovas, leaving nothing behind).
Illustration of gravitational waves emitted by binary black holes spiraling toward merger (Image credit: LIGO/T. Pyle) In contrast, black hole mergers are the product of star systems big double. While they still exist as stars, the close companions can steal each other’s matter, stripping each other of each other’s hydrogen-burning shells. Without this shell, a star’s core doesn’t receive this extra helium, which changes the star’s evolutionary trajectory. The conditions inside the core of a star that has lost its hydrogen shell are that thermal neutrinos – small ghost particles that form spontaneously – exit the star, carrying some of the core’s thermal energy. This lowers the temperature of the nucleus and slows down the nuclear reaction. The result is a reduction in energy production that allows the nucleus to contract slightly more gravitationally. This results in a very dense core, where when the star runs out of all its nuclear fuel and dies, it can collapse to form a black hole. In a binary system, this could lead to two black holes eventually merging with a cry of gravitational waves. “Due to the complex interplay between neutrino loss, nuclear combustion, and core contraction, we found that stars with a specific core mass are more likely to collapse into black holes than explode. supernova and leave behind a neutron star,” said Schneider. This interaction results in black holes of common mass, as calculated by Schneider, Laplace and Podsiadlowski. In their models, the mass of the black hole tends to converge to two values, 9 and 16 times the mass of our Sun. These values are very close to the peaks that have been observed in the gravitational wave data, at between 8 and 14 solar masses, so they do not exactly match, but are still within the observed uncertainty. Okay. Measuring the expansion of the universe The prevalence of certain black hole masses not only tells us about the physics of massive stars, but it also provides astronomers with a way to measure the expansion of the universe. to measure the expansion rate of the universe, known as the Hubble constant. This has attracted attention in recent years because different methods give conflicting values for the Hubble constant. The frequency of the gravitational rumble depends mainly on the combined masses of the black holes involved, but part of it is also related to their redshifts, which tells us how far apart they are. distance, because the further they are apart, the farther the expansion is. of the universe converted them to longer wavelengths. Until now, it was not possible to separate the black hole’s mass from the redshift in the chirping. However, knowing that a large proportion of black holes have these universal masses gives scientists an advantage.
source: https://iopscience.iop.org/article/10.3847/2041-8213/acd77a