Expansion of the Universe

            

In the last few decades, the expansion of the Universe has been a topic to debate for astronomers. When we talk about expanding the universe many questions arises in our mind like why it is expanding, evidence of its expansion, what makes it expand, how fast it is expanding. Many theories have been proposed to determine the actual reason behind expanding universe. 

           Theoretically, the expansion of the universe was proven by Hubble's Law which says that galaxies are moving away from the Earth with velocities proportional to their distance. Mathematically, the expansion was proven by the Friedmann-Lemaitre-Robertson-Walker metric. The FLRW metric is an exact solution of the field equation of general relativity proposed by Albert Einstein. It says that the universe, which is isotropic, homogenous and expanding is path-connected. Another way to prove the expansion of the universe is the Doppler Effect (redshift and blueshift). When distant galaxies are observed, they show redshift, which means the object is moving away from the observer or say galaxies are moving away from the Earth. 

            Also, the collision between two galaxies (like Milky Way and Andromeda) does not mean that universe is not expanding. The collision happens due to the gravitational attractions between two neighbouring galaxies. The gravitational influence of galaxies on each other is the reason for the collision.

            The bigger question for astronomers was not about proving the expansion, but it was to find out why the universe is expanding. Considering the universe as a whole, it must have enough energy density to stop the expansion. Also, the universe is made up of matter which has the tendency to pull or attract other matters (gravity), then why it is expanding instead of collapsing? Also, it was expected that the expansion must have to slow down. But the universe has shown no sign of slowing. Observations of the Hubble Space Telescope by distant supernovae and galaxies in 1998 says that the universe is expanding at a much higher rate than it was before. It is expanding 9% faster than it was before. This means that the expansion is accelerating instead of retarding. 

            Astronomers came to the conclusion that there is something wrong or missing in Einstein's theory of gravity. The theory was unable to prove or explain the cosmic acceleration. Astronomers are still in search of the correct solution, hence they named the solution 'the Dark Energy'. There is still no answer for this Dark Energy. One better explanation for dark energy is that 'it is a property of space'. The Dark Energy cannot be seen and is still not detected, it is just assumed. 

            The Hubble Deep Field and the Hubble Ultra Deep Field were also important resources to prove the expansion of the universe. 

            The galaxies are not moving through space, but instead, they are moving in space. Because space itself is also moving. People usually consider space and universe as the same words. But in reality, space is the 3D orientation of the objects in a given plane while the universe is everything we touch, feel, see etc. The universe consists of all matter and objects.

            There are three possible types of expanding universe. There can be flat, open and closed universes. If the universe is open, it will continue to expand forever. If the universe is flat, it will continue to expand, but the expansion will be slowed down and eventually stop after an infinite amount of time. If the universe is closed then it will eventually stop expanding and will recollapse leading to another big bang. 

            The easier way to understand it comparing the three cases with a spaceship. If the space shuttle does not have enough energy to cross the Earth's gravitational pull, then at some height it will stop and eventually collapse (closed universe). If it has energy just enough to cross the Earth's gravitational pull, it will enter the Earth's orbit but will stop after some distance (flat universe). But if the spaceship has more than enough energy or say it has infinite energy, then it will never stop even after crossing the Earth's gravity, it will always keep moving with some velocity (open universe). 

            For more than 90 years, astronomers are working on the expansion of the universe. Though understanding the complete theories of expansion needs higher studies, the basic idea and concepts are presented above. Hope you enjoy this article.