A Big bang caused the universe to begin and cosmological vacuum energy caused it to expand, and since then the universe is expanding at a rate of about 70 Km/s/Mpc (for every megaparsec). A megaparsec is a measurement of distance equal to one million parsecs or 3.26 million light years.
This expansion is the spreading of space-time structure and not the actual moving of heavenly bodies. But galaxies are vast structures, the largest being 5.5 million light in diameter, one can ask why not galaxies are expanding and thus getting withered away after some time.
Is there some force binding these vast galaxies together? If there is such force, why is it not visible and perceivable?
Fritz Zwicky’s Calculations
There are many types of galaxies, most common being the spiral one like our own Milky way Galaxy. Earliest evidence of some invisible force binding the galaxy came from the study of spiral galaxies. In 1933, astrophysicist Fritz Zwicky studied a group of galaxies called the Coma clusters and he calculated the mass of the galaxy based on the motion of galaxies near the edge.
How do we calculate mass of space bodies
Let’s first understand some basics of astronomical calculations. In astronomical measurements, we often find relation between various parameters, such as Luminosity of star is directly proportional to the mass of star and also to temperature of star raised to the power of 4 according to Stefan-Boltzmann law.
Brightness can be measured by calculating all the light emitting from a particular region and is dependent upon distance and scale factor. Stars revolve around the galactic center and their velocity is dependent on the distance of the star from the center. As the distance increases, the orbital velocity decreases according to classical mechanics.
From equating kinetic energy to the gravitational potential energy, we can calculate the velocity of the star. Thus, measuring the brightness, luminosity and velocity of stars and galaxies, we derive a relation to find out the mass.
Zwicky performed both calculations, he calculated the mass of the galaxy cluster primarily on the basis of motion of bodies on the outer edge and secondarily on calculating the brightness of stars and galaxies visible, but the numbers don’t match. Mass calculated measuring the values of brightness was much lower than the mass calculated on the basis of velocity.
Galaxy Rotation Curve
These velocities can be plotted on a graph called Galaxy Rotation curve. On the x-axis we take the distance of the star from the galactic center and y-axis takes the value of orbital speed. From the visible analysis, the velocity curve should show decreasing trend as the velocity will decrease with distance from center but from velocity analysis, the curve shows a constant value and this shows discrepancy in the calculation process. What could be its explanation?
Zwicky proposed that there is presence of some invisible matter that adds up the mass of galaxies in the outer region. Flattening of the rotation curve suggests that the total mass of galaxies is increasing with increase in the distance with respect to galactic center.
From classical physics we know that the mass of the galaxy is centered around the center, so speed will decrease linearly but from flattening of the curve, we know that speed is constant at the edge of the galaxy, thus there is presence of matter also around the edge of galaxy and in between.
He found that the individual galaxies within the cluster are moving so fast that they would escape if the cluster were held together only by the gravity of its visible mass. Since the cluster shows no signs of flying apart, it must contain a preponderance of “dark matter”—about ten times more than the visible matter —to bind it together. Zwicky’s conclusion was correct, but his colleagues had been skeptical.
Vera Rubin’s discovery
Vera Rubin, an American astronomer realized that she had discovered compelling evidence for Zwicky’s dark matter. Most of the mass of the universe is indeed hidden from our view. She teamed up with Kent Ford, an astronomer who had developed an extremely sensitive spectrometer and went on to study some sixty spiral galaxies and always found the same thing.
“What you see in a spiral galaxy, is not what you get”, she concluded. Her calculations showed that galaxies must contain about ten times as much “dark” mass as can be accounted for by the visible stars. In short, at least ninety percent of the mass in galaxies, and therefore in the observable universe, is invisible and unidentified. Since this matter is invisible, a new term was coined called “Dark Matter”.
Composition of the Universe
This dark matter interacts with the surrounding matter strongly and holds the galaxies together. Dark matter accounts for 27% of total matter present in the universe. 67% of total energy is represented by another name called Dark energy that acts as a cosmological constant which is a repulsive force that pushes the universe outward.
Rest 5% is total visible matter we see around today. This fact alone is enough to show the vastness and unpredictable nature of the universe. Among hundreds of unsolved problems in physics, the theory of dark matter is one of the most important problems to be solved to further understand the working of the universe.
There are various proposals to solve the mystery of dark matter like self interacting dark matter, axions, WIMPs etc.
Weak Interacting Massive particles (WIMPs)
WIMP stands for Weak Interacting Massive particles and this proposal is most widely studied in the field of Dark Matter cosmology. This theory proposes that dark matter is composed of particles whose mass is close to the value of 1 TeV (Tera Electron Volts in terms of energy).
These WIMPs interact with normal matter particles only through weak Nuclear forces and gravitational forces. It does not interact with matter electromagnetically and thus we can not see them in visible range.
Gravitational attraction bound them together with matter present in the galaxies. Due to the large mass of WIMPs, they have low velocity and hence their normal temperature is very close to the ambient temperature of the universe.
Cold Dark Matter
Thus they are called Cold Dark Matter. Due to low velocity, they exist around each other and can clump together smoothly, forming a large structure.
WIMPs have all the requirements to be announced as the dark matter particle but there’s a twist, as they are still not discovered or observed. The mass range of WIMP is so high that it can only be observed only during the collision in the Large hadron Collider.
Unfortunately, apart from Higgs Boson no significant particles had yet been observed in LHC. There has been planning to increase the capacity of LHC in near future and we can only hope that something pops up again just like Higgs emerged from that giant collision.