Out of the total mass-energy content of the universe, regular matter- the matter that makes up stars, planets and people, adds up to less than 5%. The remaining 95% is made up of stuff that we can’t see or detect. Popularly known as dark matter and dark energy, they make up 26% and 69% of the universe respectively.
Dark matter, as the name suggests, does not interact with light making it difficult to detect. It acts as a cosmic “glue” that holds galaxies together. Dark energy, on the other hand, is believed to be responsible for the observed accelerating expansion of the universe.
There are four fundamental forces known to exist in the universe namely the electromagnetic force, strong nuclear force, weak nuclear force and the gravitational force. Each fundamental force has a fundamental particle as its carrier.
The electromagnetic force is carried by photons. The strong nuclear force, which holds the constituents of a nucleus together, is carried by gluons. The weak force, which is responsible for radioactivity, is carried by W and Z bosons. The last and the weakest of them all, the gravitational force is believed to be mediated by gravitons although presently there is no evidence for their existence. Photons, gluons, gravitons and W and Z bosons, all belong to the class of fundamental particles known as bosons-the force carrying particles.
In 2015, researchers at the Hungarian Academy of Science fired high-energy beams of protons at lithium-7. The experiment provided an evidence for the existence of a new sub-atomic particle which was 30 times heavier than an electron. Further analysis showed that the particle might actually be a boson which carries the fifth fundamental force.
The particle has been named as protophobic X boson. It interacts only with electrons and neutrons and has a very small range. The existence of the fifth fundamental force could explain dark matter and many other unexplained things in the universe. Physicists are on a quest for finding the fifth fundamental force. Most physicists believe that a fifth unknown force could explain mysterious substances like dark matter and dark energy that make up majority of the universe.
In 2018, a research group tested for a fifth force by studying a binary pulsar PSR J1713+0747, 3800 light years from Earth. They found out that if there is a fifth force that acts between dark matter and regular matter, it would be very very weak- less than 1% of the strength of gravitational force.
There have been several attempts to search for the fifth force experimentally. Perhaps the fifth force is very weak and difficult to detect or it is totally beyond our comprehension! Who knows?
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