Primordial black holes
Dark matter is the invisible material, that holds together the galaxies.
The nature of dark matter, is a deep cosmic enigma.
Some scientist believe that dark matter comprises of weakly interacting massive particles, or WIMPs.
However, as of now, no WIMPs have been found.
Primordial black holes, may have formed, shortly after the big bang.
They are an alternative candidate for dark matter.
These also have not been discovered yet.
More evidence for primordial black holes may emerge from findings in gravitational wave detector,
an other observatories.
If confirmed to exist, these objects could solve the mystery of dark matter,
and several other cosmic conundrum.
About a billion years ago, two black holes in the distant universe, collided with each other.
The spiralling collision was so violent that it shook the fabric of space time.
It sent perturbations, or gravitational waves, which repelled outward,
through the cosmos, at the speed of light.
In September 2015, after travelling for more than a billion light years,
these ripples were detected by,
advanced Laser Interferometer Gravitational wave Observatory, or LIGO.
This was the first direct detection of gravitational waves.
This confirmed Einstein's prediction of the existence of gravitational waves.
The signals revealed an interesting fact.
Each of the merger's progenitor black holes, was 30 times heavier than the sun.
There masses were 2 to 3 times larger than ordinary black holes,
which are born from explosion of massive stars.
These black holes were so heavy, it is hard to explain, how they formed from stars.
It is possible to suspect, that these massive black holes,
formed via, some other more exotic pathway, which does not involve stars.
May be these black holes predate the formation of stars.
Some scientists speculate, that such primordial black holes could have formed,
in astronomical numbers, from the hot dense plasma that filled the cosmos,
less than one second after the big bang.
May be primordial black holes, constitute some if not all of dark matter.
Dark matter is the invisible 85% of the matter in the universe, that holds galaxies together.
Black holes seem to be the ideal candidates for dark matter, since they emit no light.
'MACHO' stands for MAssive Compact Halo Objects'.
MACHOs are formed in spherical halo surrounding each galaxy.
It is also found near each galaxies luminous centre.
Galaxies seem to be rotating too fast, to be held together, by visible mass of stars.
Dark matter provides the extra pull to prevent galaxies, from flinging of their stars.
What ever dark matter is, it shakes the universe's largest structures.
It determines the origin and growth of galaxies, galaxy clusters, and superclusters of galaxies.
These objects coalesce from the gravitational collapse of gas inside dark matter halo.
The big bang left an afterglow called Cosmic Microwave Background or CMB.
There are tiny temperature differences in the CMB.
The diffuse mass of dark matter in galaxies,
bends space to distort the light from distant background objects.
This phenomenon known as gravitational lensing.
Scientist did extensive research, and found that MACHOs do not make up the entire galactic halo.
They have ruled out MACHOs up to about 10 solar masses,
as the primary constitute of dark matter.
Scientists proposed an alternative hypothesis.
It is called as weakly interacting massive particles.
Till date no evidence has been found, of their existence.
Scientists are reconsidering the concept of primordial black holes.
The proposed possibility is that massive primordial black holes,
could actually be most or even all the dark matter.
This hypothesis hinges on an idea called cosmic inflation.
Cosmic inflation is a hypothetical phase of prodigies expansion,
immediately after the big bang.
In 10 to the power of minus 35 seconds, two points separated by less than a atomic radius,
it would become separated by four light years.
This would be comparable to the distance of the closest stars.
During inflation, tiny quantum fluctuations, are magnified to macroscopic scales.
This seeds the growing universe, with under dense and over dense regions of matter and energy.
It is from these regions that all cosmic structures subsequently emerge.
The theory of inflation is supported by observations of such density fluctuations,
in the cosmic microwave background.
In one analysis, scientists showed how quantum fluctuations enormously magnified by inflation,
would naturally produce, particularly dense regions,
that would collapse to form black holes.
Such black holes would behave as dark matter, and would dominate,
the matter content of present day universe.
In this model, clusters of black holes of different masses,
from hundred to ten thousand times the mass of the sun, was generated.
With in half a million years of the big bang, each growing evolving cluster,
could contain millions of primordial black holes,
in a volume just hundred of light years across.
From time to time, the trajectories of two primordial black holes could cross.
The objects become gravitational bound to each other.
They would then spiral closer together for millions of years.
They would radiate gravitational waves until they merge.
LIGO is already detected merger events.
If LIGO, and other labs, detect many more merger events,
it would be possible to determine the range of masses and spin,
for all the progenitor black holes.
There is a possibility that dark matter is mostly made up of primordial black holes.
There are other theories relating to dark matter.
As of now, we are in the dark, about dark matter.
Further research will hopefully throw some light, on this matter.