Immediately after the birth of the universe, a very small mass of “primordial black holeThere is a theory that it was created, but none of the actual objects have been discovered to date. So, what would happen if a star captured a primordial black hole and held it at its center?
A research team led by Earl P. Bellinger of the Max Planck Institute for Astrophysics simulated the effects of assuming a primordial black hole at the center of the sun. the result,If a primordial black hole is small, it can exist without any observable changes to the Sun.I understand that. It has also been found that certain conditions can cause changes in stars, so it may become possible to indirectly estimate the number of primordial black holes through observations of stars.
[▲Figure 1: Schematic diagram of a Hawking star with a primordial black hole at its center (Credit: Max-Planck-Institut für Astrophysik)]
■What is a primordial black hole?
「primordial black hole” is an extremely small black hole that is thought to have been created in a locally dense space in the universe shortly after its birth. A normal black hole has a mass several times that of the sun, but primordial black holes are much smaller than the mass of a star, and the smallest one is thought to have the mass of a small mountain
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*Theoretically, a primordial black hole is thought to have any mass greater than approximately 0.02 mg (Planck mass). However, as time passes from the birth of the universe to the present, the mass of the primordial black holes that have survived to this day is thought to be approximately 10 million tons or more, as they lose mass due to Hawking radiation.The primordial black hole isOne of the candidates for the mysterious gravitational source “dark matter” that cannot be observed by any method other than gravity.
is. Unlike many other dark matter candidates, primordial black holes have the advantage that their existence can be postulated without rewriting modern physics theory. On the other hand, we have not yet succeeded in observing primordial black holes, and it is unknown whether they really exist, and if so, how many exist.
■Even if there was a primordial black hole in the sun, would we not notice it?
Bellinger’s research team simulated whether the existence of a primordial black hole within a star would affect the evolution of the star. The possibility that stars harbor primordial black holes was proposed by Stephen Hawking in 1971, and Bellinger and his colleagues proposed calling these types of stars “Hawking stars.” Masu.
It is thought that the situation in which a star absorbs a primordial black hole is rare, but if it does occur, the primordial black hole will gradually fall into the center of the star, where it will gradually swallow the star and grow. . However, the radiation at the center of a star is extremely strong, and most of the material moves through the gravity of the primordial black hole, so it is thought that the star is consumed only extremely slowly.
Bellinger and his colleagues examined how the properties and evolution of the Sun would change if it absorbed a primordial black hole. By performing simulations with primordial black holes of various masses, we can verify the possibility that the Sun’s behavior deviates from the standard situation.As a result of the simulation,If the mass of a black hole inside a star is less than 1/1 millionth that of the Sun (approximately 1/3 that of the Earth), there will be no observable effect on the brightness of the Sun or the amount of neutrinos produced.
It turns out that. The possibility of the birth of a primordial black hole with this mass is also being considered theoretically.
[▲Figure2:Time-serieschangesinthebrightnessoftheSunbetweennormal(left)andwhenthereisaprimordialblackholeatitscenter(right)withamass1/100billiontimesthatoftheSunNormalstarsgraduallyincreaseinbrightnessevenbeforebecomingaredgiantbutinthecaseofaprimordialblackholethebrightnessincreasesmorerapidlyafternuclearfusionstops(Credit:EarlPBellingeretal)】
However, even if there are no observable effects now, the future of the Sun will be significantly altered depending on the weight of the primordial black hole.
According to standard stellar evolution theory, the Sun will become a red giant star approximately 7 billion years from now (approximately 12 billion years after its birth), and after its outer layer expands to the point that it swallows the Earth, it will gradually move away, and finally It is thought that it will leave behind a white dwarf.
In a scenario in which a primordial black hole with a mass 1/100 billion times that of the Sun (comparable to the mass of large asteroids and satellites) exists at the center of the Sun immediately after its birth, approximately 1.4 billion years from now (the birth of the Sun Approximately 7 billion years from now), the mass of the black hole is thought to have grown to 1/1000th that of the Sun. The Sun, whose nuclear fusion has stopped due to the loss of matter in its core, turns into a black hole, while radiation is generated by the flow of matter sucked into the black hole (Bondi accretion flow), so the sun’s brightness is due to nuclear fusion. It will be even better than when it was shining.
Additionally, large amounts of helium will be detected at the surface, as partial fusion near the core will cause convection from the core to the surface. Due to this convection, the Sun expands to a much smaller size (approximately 4.5 million km) than the red giant star assumed by conventional stellar evolution theory. The Earth will avoid the fate of being swallowed up by the Sun, but it seems inevitable that increased radiation will heat the oceans to the point of boiling.
On the other hand, in the scenario where we start with a primordial black hole that is 10 times more massive than the previous one (1/10 billionth that of the Sun), the Sun will be replaced by a black hole about 2 billion years after its birth. The simulation results show that “if the mass of the absorbed black hole is less than 1/1 millionth of the sun’s mass, it will have no observable effect.” If the mass of the hole was 1/10 billionth that of the Sun, the Sun would become a black hole in 2 billion years. Therefore, even if there was a black hole with this mass at the center of the Sun, it would be absorbed immediately after its birth. It turns out that it wasn’t.
Conversely, if the scenario starts with a primordial black hole that is one-tenth the mass of the first scenario (one trillionth the mass of the Sun), the effects will only become apparent after it evolves into a red giant. Therefore, the effect is expected to be smaller for smaller primordial black holes. However, further research will be needed to assess the exact impact, as it will require a closer look at the later stages of a star’s evolution, which can change dramatically over a short period of time. .
■We may be able to discover primordial black holes at the center of stars in the future
[▲Figure3:Changesinradiationintensityandsurfacetemperaturewhenthereisaprimordialblackholeatitscenter(redblueyellowandgray)comparedtoanormalstar(black)Becauseitexistsasararetypeofstarthatdeviatesfromnormalstellarevolutionduringitsevolutionitmaybediscoveredthroughobservationinthefuture(Credit:EarlPBellingeretal)
This research shows that the change in the properties of a star that has become a Hawking star may lead to the discovery of a Hawking star, and may indirectly lead to the detection of a primordial black hole. Stars that have absorbed primordial black holes develop brightness and temperatures similar to rare types of stars called sub-subgiants and red stragglers in their later stages of evolution. This study revealed that. These stars have been studied in detail, and it is also known under what environmental conditions these types of stars appear. This means that if these types of stars are found in non-standard environments, they may be stars that have absorbed primordial black holes. The search for Hawking stars may indirectly become an important factor in estimating the number of primordial black holes.however,The probability of a star absorbing a primordial black hole is currently considered to be extremely low, so more detailed verification is required to determine whether a star actually incorporates a primordial black hole and becomes a Hawking star.
It will be. In the situation in which a star takes in a primordial black hole, we must assume that their relative velocities to each other are extremely slow. A typical primordial black hole has a considerable velocity, so the probability of this situation occurring is thought to be extremely low.
Furthermore, even if a primordial black hole were to be incorporated into a star, it is estimated that it would take a considerable amount of time for the primordial black hole to fall into the center of the star. This is because primordial black holes are extremely small and experience little drag that slows them down and causes them to fall into the core. Furthermore, stars have a low density of matter outside of their cores, and primordial black holes suck in very little material, so they have little effect on the star’s evolution. Because of this slow progression, it is possible that the star will reach the end of its life before the primordial black hole falls into its core.
On the other hand, Bellinger and his colleagues have also shown that it is possible to find Hawking stars using the method of “star seismology,” which is the unique vibration of stars. Since the effects of primordial black holes are not easily visible on the surface of stars, the asteroseismology method, which allows us to learn about the interior of stars, may be useful in the search for Hawking’s star. However, as this method was recently devised, further research will be needed to determine whether it can be used in practice.
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Written by Riri Aya
