How many black holes are there in the universe?
According to a recent study, about 1% of the ordinary (baryonic) matter of the universe as a whole is locked in a stellar-mass black hole. Surprisingly, the researchers also estimated that the number of black holes in the observable universe is currently around 40,000,000,000,000,000,000.
In the past few years, there have been a series of major breakthroughs in the field of black holes, not only directly detecting the gravitational waves generated by the merger of black holes for the first time, but also taking real photos of supermassive black holes located at the center of galaxies.
The first picture of a black hole. | Image source: ETH
But there are still too many things that are unclear about black holes. One of the biggest mysteries is exactly how they were formed in the first place.
Recently, a team of researchers believes that they have observed this process. The researchers observed a distinct class of cosmic eruptions that could provide clues about what happened when a black hole formed. The findings have been published in two papers in Nature and the Astrophysical Journal.
The theory of black hole formation
For most of a star's lifetime, the outward pressure generated by its core fusion reactions competes with inward gravity, keeping the star stable. However, once the star runs out of fuel and the fusion reaction stops, gravity takes over. The inner layer of the star collapses inward under gravity and squeezes to an extraordinary density.
Most of the time, this catastrophic collapse stops when the core of a star condenses into a solid ball rich in neutrons. This led to a powerful rebound eruption that destroyed the star, the so-called supernova explosion, eventually leaving behind a strange object called a neutron star.
But if the initial star is massive enough, the collapse will continue until the star is pressed into a gravitational singularity and a black hole forms.
While the process of star collapse to form neutron stars is now often observed throughout the universe, astronomers aren't entirely sure what will happen when they collapse into black holes.
Some pessimistic models suggest that the entire star will be swallowed up, leaving almost no trace. But others have suggested that the process of collapsing into a black hole produces some other type of burst. For example, if a star is spinning as it collapses, some inward-falling material may be concentrated in a jet that escapes the star at high speed. Although these jets do not contain a lot of mass, they can bring a lot of impact. If they hit something, the impact could be enormous in terms of the energy released.
If we can find some clues from the observed similar explosions, we may have a chance to better understand the process of black hole formation. This is also the goal of astronomers.
Best candidate phenomenon
By far the best candidate for an explosion that occurs when a black hole is born is a strange phenomenon known as a prolonged gamma-ray burst.
Such events were first detected by military satellites in the 1960s. Scientists speculate that this phenomenon is caused by the newly formed black hole in the collapsing star accelerating the jet to an incredible speed.
However, the problem with this scenario has always been that gamma-ray bursts will also discharge large amounts of radioactive debris that will continue to shine for months. This suggests that most of the star appears to erupt outward into space (like a normal supernova) rather than collapse inward into a black hole. While that doesn't mean black holes can't form in such an outburst, some researchers believe there should be a more natural explanation. For example, a super-magnetized neutron star could form in such an explosion and produce powerful jets.
Now, new research has recently uncovered a new candidate event that (in their view) is better at creating black holes. In 2019 and 2021, the team observed an unusually rapid, fleeting type of burst in two galaxies about 1 billion light-years away, resulting from a small amount of extremely fast-moving matter violently hitting the gas in its neighboring environment.
These types of eruptions are very difficult to study because they occur for so short periods of time that they are often referred to as temporary sources and are very difficult to find throughout the universe.
Over the past few years, scientists have had to use several observatories in a row to characterize these eruptions, including discovering them with the help of the Zwicky Transient Facility, using the Liverpool Telescope and the Nordic Optical Telescope to confirm their properties, and large high-resolution observatories (hubble space telescope, Gemini observatory, and very large telescope) to analyze their composition.
Using spectroscopy, a technique that breaks down light into different wavelengths, the team extrapolated the composition of the stars that erupted at each event.
It turned out that these spectra were very similar to Wolf-Raye stars (WR-type stars). It is a class of extremely massive, highly evolved stars named after the two astronomers who first discovered them, Charles Wolf and Georges Rayet.
Excitingly, these results could even help researchers rule out the possibility of "normal" supernova explosions. The results showed that once the collision between the fast-moving material and the environment stopped, the light source actually disappeared, rather than continuously glowing. And this is precisely the phenomenon that occurs during the collapse of the core of the star, when the star only ejects a small amount of material, and the rest of the material collapses inward into a huge black hole.
Two of the more common types of outbreaks, including normal supernovae and gamma-ray bursts, as well as a class of newly discovered, fleeting bursts. | Image credit: Bill Saxton, NRAO/AUI/NSF
It's not the only possibility
While the research team currently tends to think that this may be a phenomenon that occurs when a black hole forms, it's not the only possibility.
One of the most "tedious" possibilities, for example, is that this was an ordinary supernova explosion that formed a huge shell of dust in the collision, hiding radioactive debris out of view. There's also the possibility that this outburst is a new type we're not familiar with, derived from stars we're not familiar with.
To answer these questions, scientists also need to look for more such objects, and more data from similar events may soon help validate or falsify this hypothesis. They can also help determine the link between these events and other types of unusual rapid outbreaks.
But in any case, scientists believe that we seem to have ushered in an era when we have begun to solve the mystery of black holes.
#创作团队:
Daniel Perley (Lecturer in Astrophysics, Liverpool John Moores University)
Compile: M ka
Typography: Wenwen
#参考来源:
https://theconversation.com/black-holes-we-think-weve-spotted-the-mysterious-birth-of-one-174726
https://www.eurekalert.org/news-releases/940381
#图片来源:
Cover/Header: Principle
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