We are in the final phase of our Basics of Astrophysics series. We have come a long way. We started by answering the question, what is astrophysics? Then, we learned some basics such as the importance of EM spectrum, types of redshifts, distances in astronomy, celestial coordinates, the classification of stars, and the Hertzsprung Russell diagram. Later, using these tools, we studied the structure of the sun, the evolution of stars, nuclear reactions in stars, supernovas and their types, galaxies and their interactions, black holes, quasars, nebulae, and the gravitational waves. Today, we will cover another crucial topic in astrophysics: the dark matter.
What Is Dark Matter?
Jacobus Kapteyn, a Dutch astronomer was the first one to suggest the existence of dark matter in 1922. Dark Matter is basically a hypothetical form of matter that probably accounts for approximately 21% of the matter in the observable Universe. According to cosmological models, the mass-energy of the universe contains 5% ordinary matter and energy, 21% dark matter, and 74% of an unknown form of energy known as dark energy.
Thus the ‘dark universe’ comprises 95% of the mass-energy of the universe.
No doubt, that even the most sensitive detectors have failed to find convincing evidence to prove the existence of dark matter, still there are many unusual phenomenons happening in the cosmos that cannot be explained without considering the presence of this untraceable form of matter, and these observations have always kept the hope of directly detecting the dark matter in near future alive. Here, I have compiled a list of the five most important phenomenons that truly support the fact that dark matter should exist!
You can also watch our short video on dark matter below:
Evidence Of Dark Matter
Galaxy Rotation Curves
For Spiral galaxies like the Milky Way, we derive the gravitational mass by observing the motions of stars and gas clouds in the disk as they orbit the center. The rotation curve of a galaxy shows how the velocity of stars around the center varies as the distance from the center increases.
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From Kepler’s Second Law, it is expected that the rotation velocities will decrease with distance from the center, similar to the solar system. However, this is not observed. Instead, most spiral galaxies show flat rotation curves out as far as we can trace them, even where no more stars are visible.
Therefore, to support the Kepler’s laws and to resolve this discrepancy, we conclude that the gravitational mass is more than 10 times more massive than the luminous mass, hence there should exist a non-luminous matter, i.e. dark matter to compensate for this extra gravitational mass.
Velocity dispersion, in astrophysics, is a parameter that describes the statistical dispersion of velocities about the mean velocity for a group of astronomical objects. The Swiss astronomer Fritz Zwicky used the velocity dispersion of galaxies in clusters as determined from Doppler shifts to estimate their dynamical mass.
However, with some exceptions, velocity dispersion estimates of elliptical galaxies do not match with the predicted velocity dispersion from the observed mass distribution, even by assuming complicated distributions of stellar orbits. Again, the most obvious way to resolve this discrepancy comes by postulating the existence of the non-luminous matter.
Structure formation refers to the period after the big bang when density perturbations collapsed to form stars, galaxies, and clusters. Prior to the structure formation, the Friedmann solutions to general relativity described a homogeneous universe.
Later, small anisotropies gradually grew and condensed the homogeneous universe into stars, galaxies, and larger structures. Since ordinary matter is affected by radiation, its density perturbations are washed out and unable to condense into a structure.
If there were only ordinary matter in the universe, then there would not have been enough time for density perturbations to grow into the galaxies and clusters currently seen.
Hence, here dark matter comes to the rescue and provides a solution to this problem as it is unaffected by radiation.
The Bullet Cluster
Consisting of two colliding clusters of galaxies, the bullet cluster is claimed to provide the best current evidence for the nature of dark matter. The bullet cluster provides a challenge for modified gravity theories because its apparent center of mass is far displaced from the baryonic center of mass.
However, this observation can be easily explained by the existing standard dark matter theory. Hence, if dark matter does not exist, then it points out that the prevailing theory of gravity, i.e. general relativity is incorrect, which is not the case (at least so far).
The Cosmic Microwave Background Radiation
We have a dedicated article on the CMB radiation coming up in the series so it’s okay if you don’t know about in detail.
Dark matter does not interact directly with radiation, but it does affect the CMB by its gravitational potential (mainly on large scales), and by its effects on the density and velocity of ordinary matter. The cosmic microwave background contains very small temperature anisotropies of a few parts in 100,000.
A sky map of anisotropies can be decomposed into an angular power spectrum, which is observed to contain a series of acoustic peaks at near-equal spacing but different heights. These peaks have indicated that the universe contained about five times as much dark matter as normal matter when the neutral hydrogen formed.
Combined with measurements of supernovae and the clustering of galaxies, this indicates that dark energy comprises 73 percent of the universe, dark matter 23 percent, and normal matter just 4 percent.
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The Nature Of Dark Matter
The majority of dark matter is thought to be non-baryonic in nature, which is possibly composed of some as-yet-undiscovered subatomic particles. These unknown particles are expected to be probably some weakly-interacting massive particles (WIMPs), or gravitationally-interacting massive particles (GIMPs). Other potential candidates include dim brown dwarfs, white dwarfs, and neutrino stars.
The WIMPs are expected to have ten to a hundred times the mass of a proton, but their weak interactions with “normal” matter make it difficult to detect them. Dark matter has also been classified into hot and cold categories, depending upon the velocities of their particles which further determine their thermodynamic properties. The candidates for hot dark matter travel with relativistic velocities like neutrinos. However, the ones moving slowly like hypothetical axions, WIMPs, etc come under the category of cold dark matter.
The Search For Dark Matter
Unlike normal matter, dark matter does not interact with the electromagnetic force, hence, it does not absorb, reflect, or emit light, making it extremely hard to spot. This is the reason why it had been named ‘dark‘. The only inference of its existence has been drawn from its gravitational effects on the visible matter.
Several ultra-sensitive instruments have been constructed to detect these bizarre subatomic particles. These include vats of liquid xenon stored miles underground, and telescopes looking for dark matter particles decaying into things we can see and measure, like gamma rays. It also includes the Large Hadron Collider, one of the most expensive science experiments ever built.
The Hunt Continues
Sadly, as these particles are highly non-interacting, so, even with these advanced instruments, we haven’t been able to find anything in favor of their presence to date. We haven’t found convincing evidence that they exist, except for the persistent evidence we can’t ignore the fact that the universe is heavier than what we can see.
This fact is the sole motivation behind scientists all over the world putting forward various theories, day by day, in this context. Consequently, a lot of advancements are taking place in the hope to discover the unknown one fine day! Hence, the hunt continues…
You are entitled to say if you are so smart, why don’t you tell me what that dark matter is? And, I’ll have to confess that I don’t know! –Jim Pebbles ( A Nobel Laureate making major contributions to the field of dark matter since 1970)