DARK MATTER
Introduction:
Dark Matter is a major component of our Universe that is invisible to our naked eyes. We call it “dark” because it does not emit light and does not seem to interact with anything, except through gravity. Dark matter doesn’t participate in the strong or electromagnetic forces as normal matter does. Whether or not dark matter participates in the weak force hasn’t been established yet. But gravitational force continues to be its friend.
Why dark “matter”? From the Cosmic Microwave Background Radiation data, it has been predicted that this mysterious dark entity is a form of matter because its relative velocity relative to its surroundings is infuriatingly less than the speed of light, therefore it cannot be any form of radiation. To emphasize that, it is often called Cold Dark Matter. Why cold? Because Temperature is associated with the energy of atomic motion. Hot matter has high velocity and high kinetic energy. Cold matter has low velocity, therefore low kinetic energy. The CMB tells us that dark matter is cold. The map of CMB clearly shows us the same.
Image source- nasa.gov google
The first hint of dark matter came 75 years ago, yet it remains mystery to this day. We know more about what it isn’t than what it is. While its composition and characteristics are unknown, we are quite sure that:
Some form of cold, invisible matter is exerting huge gravitational forces on the normal energy and matter that what we do see.
This mysterious substance is not made of normal atoms nor their constituents: proton, neutrons and electrons.
Are we sure it exists?
The saga of dark matter began in 1933 when Caltech Astrophysicist Fred Zwicky discovered that galaxies in clusters are moving much faster than expected. Newton’s laws, which are quite adequate for this purpose, provide a powerful relationship for the orbit of a smaller body around a larger body of mass M. In natural units, rv² = M, where v is the smaller body’s velocity and r is the distance from the centre M. Zwicky measured v and r for several galaxies in several clusters, and from these he calculated the required mass of each cluster. He found that the mass needed to account for the galaxies’ high speed was far greater than the total mass observed in the cluster---- about 10 times greater. He concluded that most of the mass in a cluster must be invisible.
Scientists really didn’t know what to do with this, and Zwicky’s discovery languished for four decades. Finally, in 1970s, American astronomer Vera Rubin discovered that galaxies are rotating too fast for the amount of normal matter they contain. Using the same logic as had Zwicky, Rubin measured the orbital speed of stars at various distances from galaxy centres and found that much more mass must exist on the outskirts of galaxies than is visible.
Following is the evidence of presence of Dark Matter
Recently, studies of gravitational lensing, the bending of light by massive bodies found more evidence for invisible matter. The observed bending of light indicates that far more matter is present in galaxies and clusters than is visible.
Also, the CMB data depicts that the universe contains 6 times more matter than exist in protons, neutrons and electrons.
More evidence for the existence of dark matter comes from computer stimulations that show the density variations in the early universe, evident in the CMB, are too small to have developed into galaxies without the additional gravity of dark matter.
Can it be normal matter that we can’t see?
The production of certain elements is very sensitive to the density of protons and neutrons. If dark matter were composed of protons and neutrons, the cosmic abundance of deuterium(isotope of Hydrogen, atomic mass = 2u) and other light elements would be rather different than what is actually observed. The observed abundances are consistent with the amount of matter we do see, but not with 6 times as much.
OK, it’s something new. What is it?
Some say dark matter is WIMPs (Weakly Interacting Massive Particles). Not to be undone, others say dark matter is MACHOs (Massive Compact Halo Objects). But there is quite interesting concept here, according to which proposes that for every existing fermion, there is an as-yet-undiscovered boson, and vice versa. This prolific proposal is called Supersymmetry. Supersymmetry predicts hundreds of new particles, none of which have been yet found. Supersymmetry promises a new particle for almost every experimental physicist, it’s the theory of different perspectives. These new particles could decay to whichever particle of their type has a lower mass, which is dubbed the LSP (Least-massive Supersymmetric Partner). Perhaps dark matter is made of LSPs that might be WIMPs. All these speculations are fine, they provide models for experimental searches, but our knowledge will not really be advanced until experimental physics actually capture some dark matter.
The colour plate below is a very interesting image of the Bullet Cluster. Here we see the aftermath of the collision of two galaxy clusters that are 3.4 billion light years away. The larger cluster on the left and the smaller cluster on the right passed through one another and are now moving apart. The diffuse blue areas represent calculated dark matter haloes. Because we cannot see dark matter, the amount of extent of each dark matter halo is computed from the observed amount of gravitational lensing (of light from more distant sources) caused by each cluster. The two red patches between the cluster are plasma clouds---- vast oceans of charged particles (normal electrons and nuclei). Before the collision, each cluster was embedded in its own plasma cloud, as is normally the case. Measurements show that the plasma cloud contain far more mass than all the galaxies in both clusters combined, and also the dark matter contains far more mass than the plasma.
Image source: Google
The Bullet Cluster Galaxy
The image shows that the galaxies passed through one another with very little interaction because there is so much empty space between the galaxies. Like two hails of bullet fired toward one another, most bullets sailed on by without hitting any on-coming bullets. While there may have been some near misses., there probably were no head-on collisions between the galaxies. However, the plasma clouds did not sail on by. Charged particles interact vigorously and these interactions slowed down each plasma cloud, causing them to lag behind their clusters. The plasma cloud on the right represent a Shock Front, common in such violent events.
That leaves the dark matter. Unlike the galaxies, each dark matter halo is diffused like the plasma, yet neither halo interacted with the plasma, the galaxies, or even the other dark matter halo. Dark matter seems quite inert, except for the gravitational attraction it everts.
The colour plate above shows that dark matter cannot be made of normal atoms and particles.
Dark matter can’t clump
Another important point is that dark matter does not clump as normal matter does. When a gas cloud collapses due to its self-gravity, the gas gets hot. The atoms move faster and exert pressure that can stop the collapse. Normal matter is able to radiate energy through the electromagnetic force. This allows normal matter to reduce its temperature and pressure, and thereby continue collapsing to form stars and planets. It seems dark matter cannot cool itself by releasing energy and is not able to collapse to nearly the same density as normal matter. Apparently, this is why dark matter is detected diffusely surrounding galaxies and galaxy clusters.
We believe that large scale structures---galaxies and galaxy clusters---began with the accumulation of dark matter. Because there is so much more mass in dark matter, the attraction of its gravity was the most important factor in bringing material together from large structures. Once dark matter accumulated, normal matter fell toward the centre of the dark matter’s strong gravitational field. Ultimately, with its ability to cool by radiation, normal matter “out-clumped” dark matter and collapsed to a much denser state. Dark matter created fertile sites where normal matter condensed to form clusters, galaxies, stars and planets.
Numerous experiments are currently underway attempting to catch and identify pieces of dark matter. We wait hopefully but for now, dark matter remains a dark mystery.
Hope you find this article useful,
Sending you an abundance of energy from the Universe,
Love and Light,
Regards and credit- SaptashawDas ( 11th class student)
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