Hello and welcome back! I hope you, must have read my last post on ‘gravitational lensing‘ and would be curious to know what contributed to the extra mass? In today’s post, I will explore the 95.1% of the invisible energy density of our universe and eventually answer the question.

All the matter which you see on earth, in the cosmos constitutes only 4.9% of the energy density of our universe. The remaining energy density is in the form of what scientists say, dark matter and dark energy. Even today, research is on finding what it is and whether it truly exists or not.

PREDICTION AND EVIDENCE OF DARK MATTER:

In 1920s astronomers were taking pictures of distant galaxies with help of Hubble telescope. From those pictures, astronomers predicted that, since the luminosity decreases as we go away from the center of the galaxy, the velocity of the stars orbiting at the periphery should be less. Luminosity here also means the density of matter i.e. more the luminosity, greater has to be the mass and density. 

On the basis of this, they predicted a graph and tried to compare it with the observations. And to everyone’s surprise, the graphs didn’t match! In fact, it was observed that the velocities of the stars orbiting at the periphery were same as of the stars orbiting midway from the center.

You can see from the observation that, the line goes parallel instead of moving downwards. After being baffled, scientists predicted that the missing mass near the spirals of the galaxy must be coming from something which is invisible or not interacting with electromagnetic radiations; they gave the term dark matter to this weakly interacting massive particle (WIMP).

Again in 1933, when Fritz Zwicky was studying a cluster of the galaxy called ‘coma cluster’ observed that the galaxy which was rotating around each other had missing mass. He came to this conclusion by finding the mass of the galaxies from their luminosity and then comparing it with the mass needed for the gravitational force to hold them together. 

While comparing these two observations and calculations he found that the mass was missing. So he concluded that it’s some mysterious matter, the ‘dark matter’ which might be giving additional mass to the galaxies. So we can conclude that if this WIMP was absent then, the stars or galaxies moving with such velocities couldn’t have been able to hold together and galaxies and universe wouldn’t have formed as the way it is now.

Another observation which showed the possibility of the existence of dark matter is gravitational lensing (talked in the previous article). In that article, I asked you that, why the calculated mass of the galaxy did not match with observation? 

The reason is dark matter; this is holding the galaxy together and is adding the extra mass to the galaxy. Even though we have never detected or created or practically observed dark matter, but by all these observations we have a good possibility that it exists.  

Presently there are detectors set underground to filter cosmic rays and to detect a rare collision of dark matter with an extremely cooled ideal gas. India has one such lab in Jharkhand called Subterranean Lab which is located at the depth of 905m. Since years LHC is also being operated to make and detect dark matter. 

PREDICTION AND EVIDENCE OF DARK ENERGY:

For a long time, scientists thought that gravity is always attractive, but in the twentieth century, the observation made by Hubble telescope baffled everyone’s mind. 

The observation of redshifts in distant galaxies made a conclusion that space is expanding, and the expansion is speeding up, whereas scientists earlier thought that our universe might be slowing down due to gravity. And indeed they were right, gravity can never be repulsive, it’s always an attractive force. 

So they predicted and introduced negative pressure in their mathematical theories and concluded that it’s the negative pressure which is causing the expansion. 

This negative pressure is given by an unknown form of energy called dark energy, which pervades almost all of space and contributes to 68.3% of the energy density of the universe.

In 1915, Einstein came up with his theory of general relativity. In his theory, when he tried to apply his field equations on the entire universe, he was unhappy with its results. This was because the equations predicted that the universe is not static, it might be expanding or contracting. 

So he went back to his equations and added a cosmological member i.e. a cosmological constant. This cosmological constant showed a kind of outward force which balances the inward force of gravity, resulting in a static universe. 

But in 1929, Hubble’s observation showed that the universe is expanding, and what’s making it expand is this cosmological constant. After getting Hubble’s result, Einstein said that it was his biggest blunder of adding the cosmological constant and making the universe static. Even his biggest blunder gave us the solution of what’s making the universe expand.

But wait a minute; the rate at which space is expanding is much slower than the rate which was during the inflation phase of the universe. That means dark energy cannot be constant since it’s dynamic in nature. 

Quintessence is thus a hypothesized form of dark energy which is more precisely a scalar field which is dynamic in nature. Some physicists often consider quintessence as the fifth force of nature. But we don’t need to worry about any kind of fifth force because many string physicists strongly believe that there is no fifth force. 

However, research is still going on to understand dark matter and dark energy in a better way as they both constitute 95.1% of the total energy density of our universe.  

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