In the past, scientists believed that gravity caused slowing down the expansion of the universe. No one had observed this process, but according to theories, it was supposed to be so. However, in 1998, the Hubble Space Telescope observations proved this theory wrong.
They showed that a long time ago, the universe had been expanding much more slowly than nowadays, which means that gravity does not cause slowing down the expansion of the world. Just the opposite something made this process to accelerate. No one found out the reason at that time.
So scientists grouped around three different theories for the cause of the acceleration of the universe:
- It could be a result of Einstein’s theory of gravity, related to the ‘cosmological constant’;
- There could be some energy fluid filling up space;
- There could be something wrong with Einstein’s theory of gravity, so a new method was needed to show what caused that acceleration;
None of these prospects appeared to be plausible enough, so scientists still didn’t know the real explanation, but they gave a name to the solution; they called it ‘dark energy.’
What is it exactly?
There are a lot of mysteries about dark energy. We only know that it exists, and we can estimate how much dark energy there is because we know how it is related to the expansion of the universe.
Scientists calculated that around 68% of the universe is dark energy. Then the dark matter is 27%, and everything else that we have ever observed with all kinds of telescopes, observatories, and spacecraft is just 5%.
Einstein's discoveries brought us closer to the explanation of dark energy
Einstein was the first one to realize that empty space is not “nothing”. He began understanding the different properties of space, the first one being the fact that there could be more space coming to exist.
According to one of his theories related to the cosmological constant, it becomes clear that it is possible for ‘empty space’ to have its energy. So if more space comes into existence, more of this energy will emerge. This process will cause the universe to expand faster.
Quantum physics also gives us some explanations of dark energy
The quantum theory provides another proof of how we can acquire energy from empty space. According to it, empty space consists of small particles that consistently form and then disappear. However, scientists couldn’t calculate accurately how much energy we could get from space according to this theory.
Is there any need for a new theory of gravity?
Yet another theory tries to prove that Einstein's theory of gravity is wrong. This fact would affect not only the expansion of the universe but also the way we see how galaxies behave. According to these assumptions, we need a new theory of gravity. There are some candidate theories to replace the one of Einstein, but none of them is good enough to explain the motion of the bodies in the Solar System.
So dark energy remains a mystery for future generations to reveal.