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:
