Discovery of dark energy
Back in the 1990s after the discovery by Edwin Hubble that the universe was expanding, many astrophysicists were conducting experiments to know the nature of the expansion. Is the universe expanding with a constant rate of expansion or is the rate of expansion is slowing down? and will this expansion stop and can we have a static universe which was predicted by Albert Einstein.
As part of this investigation, two teams named the high redshift (high z) team lead by Australian astronomer Brian Schmidt and the supernova cosmology project team lead by American astronomer Saul Perlmutter started observations on distant objects. They were observing distant galaxies and stars to know the current rate of expansion.
In 1998 they started observing type 1a supernovae (which are exploding giant stars, one of the stars in a binary system dies and becomes a white dwarf and consumes the material of the neighbourhood star(white dwarf) when a mass of the star goes on increasing and at certain mass(1.38 times the mass of the sun ) star becomes unstable and explodes into a supernova instead of forming a neutron star or black hole( black hole is formed when the mass increases more than Chandrashekar limit(1.4 times the mass of the sun)). These types of explosions are called type 1a supernovae.
Type 1a supernovae are 5 billion times brighter than our sun. therefore, these are a very good target for the observations. All these supernovae have the same luminosity and brightness hence these are termed standard candles.
so, they looked at distant supernovae (why distant?) when we are looking at distant objects we are actually looking into the past because light can only travel at a finite speed (speed of light is very less in view of the entire universe!). So, if a galaxy is 10 billion light-years away from us then a light ray that is emitted from that galaxy would take 10 billion years to reach us. When we see that light ray, we are actually seeing how the galaxy was 10 billion years ago. Therefore, the present situation of the galaxy can be known only after 10 billion years! (light acts as a time machine)
So, they made observations of some distant type 1a supernovae, surprisingly the results were contradicting the purpose of the observation. The results were that the supernovae were much fainter and farther than calculated results, which were done by assuming that the rate of expansion is slowing down.
This was the first evidence that the universe is actually expanding with accelerating speed. The name given to this mysterious force responsible for the acceleration of the universe was dark energy.
Many astrophysicists say that dark energy is similar to one of the terms used by Albert Einstein in his general theory of relativity. That was a cosmological constant, he introduced this term in his theory to make a force that is anti-gravity to make the universe static. He believed in a static universe (steady-state model)
Dark energy is like anti-gravity, gravity tries to attract whereas dark energy tries to repel. Dark energy has negative pressure. astrophysicists say that dark energy started accelerating the universe 5 billion years ago (not at the time of the big bang). Currently, the universe comprises 70% dark energy and 25% of dark matter, and 5% of baryonic matter (normal matter) with which all galaxies and stars are formed (the matter which we can see)
Still, we don’t know the nature of dark energy it is hypothetical. If dark energy goes on increasing the rate of expansion of the universe. In some billion years, galaxies move very far away from each other (we cannot see any other galaxies from our milky way galaxy) and the rate of formation of the galaxies decreases and dark energy may stretch and rip off all the matter even the galaxies and stars too.