It all started with a bang!!. Big Bang- the widely accepted theory of the creation points to a singularity in spacetime, a point whose energy density and space-time curvature were both infinite. Big Bang has provided many insights into how things were in the past up to the point where three of four fundamental forces i.e. electromagnetic, strong nuclear, and weak forces were unified into a single force. Before that Quantum gravity dominated the physics of the early universe about which we do not know in detail.
But we can still ask the question about the fate of our universe.
In 1929, Edwin Hubble provided the first observational evidence for the expanding universe and discovered that the more distant a galaxy is from us, the faster it appears to be receding into space. This means that the universe is expanding uniformly in all directions. But the question is will it keep expanding? Or will it ever stop? What is responsible for this expansion?
The Future of Our Universe
Will the universe continue to expand forever? This depends on how much matter the universe contains and how fast it is expanding. There are three possibilities:
An icy death
If the average density of the universe is smaller than a certain critical density, that is a function of the expansion rate, the universe is open and expansion will never stop.
Eventually, new galaxies and stars will cease to form and existing ones will end up as black dwarfs, neutron stars, and black holes-An icy death.
The Big Crunch
If the average density is greater than a certain critical density, the universe is closed, and sooner or later the gravity will stop the expansion. The universe will begin to contract. The progression of events will be the reverse of those after the Big Bang. Big crunch- a fiery death. If that is so, then the universe is cyclic with no beginning and no end.
A flat Universe
If the average density is equal to a certain critical density, the expansion will continue at an ever-decreasing rate but the expansion will not stop. In this case, the universe has a flat geometry.
The role of dark matter
The critical density for a flat universe depends only on Hubble`s parameter which is not accurately known: critical density, p=3H^2/8*pi*G.But there isn’t enough ordinary matter that can account for the required value of critical density. Ordinary matter from which everything around us is made only makes up a small portion of the actual matter. A larger portion of matter is dark matter which is non-luminous and does not interact with light or any other force except gravity. There is no direct confirmation of the existence of dark matter but its gravitational effects on the rotation speeds of the outer stars in spiral galaxies are unexpectedly high, which suggests that a spherical halo of invisible matter must surround each galaxy.
In conclusion, fate hinges on the delicate balance between matter, dark matter, and the expansion rate. If the average density is below the critical density, the expansion will continue indefinitely, leading to an icy death. If the density exceeds the critical value, gravity will eventually halt the expansion, resulting in a fiery Big Crunch. However, if the density matches the critical value, the universe will persistently expand at a decreasing rate, forming a flat geometry. The existence of dark matter, though elusive, plays a crucial role in shaping the destiny of our universe.
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