Astronomy Photos ByAcian

2016年10月22日

How were the earliest black hole formed?

58 Wood Villager

Introduction

A black hole is a region in the universe where gravity pulls so much that even light cannot get out. Since light cannot get out of a black hole, we cannot observe and detect it easily. As a consequence, what’s inside a black hole is still a mystery in our universe. We are now knowing more about black holes, such as their formation process and development, but still there is an interesting field of study.

What types of black holes are there?

There are three types of black hole – primordial black holes, stellar black hole and supermassive black hole, which are classified depending on their mass and they are formed in different ways. Primordial black hole is the black hole born in early universe. It is formed by compressing matter with extremely high density, existing in the early expansion of the universe so it is the smallest in mass. Another type is stellar black hole. A stellar black hole is formed by the collapse of a massive star, which has a mass 8 times the mass of our Sun. The star then explodes and the mass remaining in the core is a stellar black hole. This process is called hypernova explosion. The third type of black hole is supermassive black hole. They possess the highest mass among the three types of black hole and they can be found in almost all currently known massive galaxies.

How does a black hole develop?

It is well-known that a black hole will go through the process of accretion to become more massive. For instance, a primordial black hole, which is just born with low density, enlarges by pulling in materials like gases from its surroundings. A black hole may be merged with another black hole nearby due to their gravitational attraction between them to form a more massive black hole.

Is it possible to have a black hole during the early stages of the universe?

Some scientists believe that black holes existed in the universe as early as about one billion years after the Big Bang, as they hold an idea that quasars, which are billions of light years away from us, with their brightness (radiation) 10-100000 times that of the Milky Way, should have been powered by a supermassive black hole in it. Discoveries of big black holes in tiny galaxies show us that big black holes should have formed in the early universe before galaxy collisions, and thus supporting the theory of the early formation of a black hole.

Now the question is, how can a “mature” black hole be formed by accretion given that there were inadequate gases during the early stages of the universe? If a black hole developed through the conventional development process, the formation of quasars would be too slow as the gases required were not abundant at that early time.

“Direct collapse” black hole

To explain the existence of black holes in the early stage of the universe, “direct collapse” black hole was made a hypothesis in the 2000s. “Direct collapse” black hole hypothesis suggests that the primordial gas, i.e. primordial cloud of hydrogen and helium, would undergo direct collapse to form a black hole. In the process, the primordial gas flows along the dark matter filaments, forming a cosmic web that could connect the structures in the early universe (dark matter is an unidentified type of matter that makes up considerable part of the universe). The gas was kept hot due to the presence of a sea of ultraviolet photons, thus any star formation is suppressed. There will be intersections between these dark matter filaments, leading to the formation of the first group of galaxies. The process of direct collapse repeated throughout the early stage of the universe, and more and more galaxies were formed. Thus, even though the density of gas was not high at the time, black holes can still develop quickly, producing a large amount of quasars.

Here is a simulation image of the cosmic web structure based on a supercomputer:

(Credit: Aaron Smith/TACC/UT-Austin.)

Study of the early universe

In order to understand more about the early universe, astronomers have been studying old stars. Through these studies, not only can they know more about the structure of the young universe, but also prove the existence of direct collapse black holes.

CR7 (Cosmos Redshift 7), one of the brightest existing stars from the early universe, was identified in a Hubble Space Telescope investigation called COSMOS. The star is formed soon after the Big Bang. Astronomers deemed this particular star as an ideal sample for studying early stars and investigating early black holes. From the studies of CR7’s electromagnetic spectrum, it was discovered that certain hydrogen line known as “Lyman-alpha”, was several times brighter than expected in the spectrum. Furthermore, its electromagnetic spectrum also revealed several unusual features, including an unusual bright helium line, and absence of lines from other element heavier than helium. These findings give evidence to the existence of direct collapse black hole, providing us with a more comprehensive view of early black holes.

Besides CR7, NASA recently announces the discovery of two other “direct collapse” black hole candidates based on its observation with the Chandra X-ray Observatory. This further supports the hypothesis of direct-collapse black holes.

As scientists successfully identify the existence of early black holes and obtain more information of their formation, we are indeed getting closer to solving the mysteries of black holes, as well as understanding the origin of our universe.

Reference

1. Johnson, R. (2016). Astronomers find evidence for ‘direct collapse’ black hole. Retrieved from:
2. Smith, A., Bromm, V. and Loeb, A. (2016). “Evidence for a direct collapse black hole in the Lyman α source CR7”. Monthly Notices. Volume 460. Issue 3.
3. May, S. (2008). “What is Black Hole?”. NASA Knows. (Grades K-4). Retrieved from:


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