Vacuum Energy as a candidate for Dark Energy

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Contrary to what the name Dark Energy may suggest, it is not associated with dark mystical force possessed by some Dark Lord. It is not as deadly as Malekith’s Aether which means it doesn’t want to destroy the galaxies, its just making the communication between the galaxies harder by pulling them apart.

Then the question arises, what exactly is the Dark energy? And the answer is we don’t know yet. Analogous to finding earth-like planets astronomers search for planets first and then narrow down the possibilities later, in the same way there has been some candidates for Dark energy as well and hopefully one of them will turn out to be the right answer.

As present knowledge suggests that Dark energy is fairly uniform in space and time which leads to considering vacuum energy as one of the possible candidates for Dark energy. The concept of vacuum energy has its origin in dazzling world of Quantum Mechanics. Quantum mechanics says even the smallest of volumes(greater than the cutoff volume allowed by Quantum Mechanics) in vacuum not containing any form of matter or field has some energy associated with it. So, vacuum energy is inbuilt in space.

Before 1929, when astronomers were oblivious to the fact of expanding universe and believed the static universe, Einstein had to modify his equations(which previously suggested that universe must be either expanding or contracting) to explain the static nature of the universe by introducing cosmological constant having an accelerating impulse which cancels the decelerating impulse of the matter thus leading to static universe. But the modified model was unstable, thus even a tiny perturbation would lead to universe collapse or expand. But as Hubble found out that universe is expanding, Einstein said cosmological constant as his ‘greatest blunder’.

The cosmological constant did not come out of the blue, it has contributions from vacuum fluctuations. According to Quantum Mechanics no perfect vacuum can be created, there will be virtual particles popping in and out which will lead to energy associated with vacuum, and it has been experimentally observed as well. But calculations tell that the value of the vacuum energy is approximately equal to 10^112 ergs/cm^3 which is insanely higher than density of dark energy having the value of 10^-8 ergs/cm^3. If the value of dark energy density was zero the problem would have been much simpler. In that case, physicists would have used their best friend symmetry for exact cancellation of vacuum energy. But, unfortunately that is not the case. These troubling results lead to many possibilities, one of them is that may be vacuum energy is cancelled after all by some unknown symmetry and dark energy is something else. But again, there might be the possibility that vacuum energy is not cancelled exactly.

So, for above reasons vacuum energy neither can be definitely said to be responsible for dark energy nor can be completely thrown out of the window. So we have to explore different possibilities for definitive answers.

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