We often heard about dark matter and dark energy. Let’s explore this topic in a more simple and understandable way. This all starts with an interesting concept of physicists nowadays. Sometimes if a physicist can’t find an answer about something or really have no idea about What it is? Or How it can exist? He would call it unknown, dark, black or so on…. as we use in mathematics to consider something x and then find different possible solutions of ‘’x”.
Similarly, physicist calls something like the presence of excessive gravity around galaxies dark matter and reason for accelerated expansion of universe ‘’Dark energy’’. Although this approach looks dumb actually it has been excellent in a lot of ways. It gives an open opportunity to everyone to find answers or to know something as this feeling is always I ourselves.
All of this came into play when the orbital speed of stars around the center of the galaxy (a supermassive black hole) was calculated. According to Kepler law, the orbital speed of stars at a far distance from the center of the galaxy should be less than those at the near center. But when the Graph was plotted it came out to be nearly the same.
Similarly, when the same is repeated in a cluster of galaxies it was found that the mass of a cluster of galaxies was just 1-3 % of the mass required to keep the galaxy into the cluster. So, eventually, in 1933 It was initially named‘’ missing mass’’ as it was indicating that there was more gravity around galaxies the there should be, so there must be more mass than we calculate or we see. Soon after it was named ‘Dark Matter’.
Since then scientists are trying to discover the properties of that matter. This matter only interacts with other masses through gravity and they are weakly interacting. So, some theories are put forward like
They might be neutrinos in excess. These particles are released in fusion reaction on stars. But dark matter particles are also named nowadays as ‘’WIMP’’ or ‘’GIMP’’
WIMP means ‘’Weakly interacting massive particles’’
GIMP means ‘’Gravitationally interacting massive particles’’
Even though a lot of great minds have been working on this, still we have only a little bit of progress on this topic. Through phenomena like gravitational lensing and using satellite-like Hubble space telescope we are able to map dark matter in the universe and its quantity came out to be 27% off the entire mass of the universe. Still, we are hoping to discover a lot about the nature and properties of these particles using advanced telescopes.
In 1915 Einstein included the cosmological constant ‘’lambda’’ as a term in his field equations for general relativity because the equations did not allow, apparently, for a static universe: gravity would cause a universe that was initially at dynamic equilibrium to contract. So he used lambda as an opposing quantity to gravity. In 1929 Edwin Hubble discovered that the universe was expanding. So cosmological constant was removed from Einstein equations. But in 1998 two teams working on supernova explosions found that the universe was not only expanding but expanding with acceleration. So again, the cosmological constant came into play as a force opposite to gravity called‘’ dark energy’’.
Since then a new era of science came into play. The greatest minds throughout the universe started thinking about that problem, as we did not have any idea about the origin or nature of this force it was named Dark Energy.
Scientists have found that this force is causing the universe to expand outward. Now there are only two results
- Either universe will continue to expand and dark energy will overcome all the fundamental forces of nature and everything including galaxies to quarks everything will break down to nothing called BIG RIP.
- Either universe will stop expanding and then start contracting on itself until it reaches a point from where it was started i.e. singularity. And this condition is called BIG CRUNCH.
- Either it may reach a point where equilibrium is established between dark energy and gravity and the universe becomes static.
This approach is similar to a ball thrown on the earth surface
It has three possibilities either it will escape earth gravity and move perpendicularly to the earth’s surface forever (BIG RIP), or it will come back to the earth’s surface (BIG CRUNCH) or it will reach earth orbit and maintain its height forever (STATIC MODEL).
There has been vast research ongoing in that area. Its quantity in-universe has been calculated as 68% of the entire mass of the universe.