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Dark Energy, Dark Matter

In the early 1990s, one thing was fairly certain about the expansion of the universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. Granted, the slowing had not been observed, but, theoretically, the universe had to slow. The universe is full of matter and the attractive force of gravity pulls all matter together. Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the universe was actually expanding more slowly than it is today. So the expansion of the universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it.

Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant." Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called dark energy

Explanation:

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Dark Energy is a hypothetical form of energy that exerts a negative, repulsive pressure, behaving like the opposite of gravity.

The existence of dark energy is supported by a number of observations. This includes

(i) the age of the Universe compared to the oldest stars,

(ii) supernovae observations,

(iii) Cosmic Microwave Background (CMB),

(iv) baryon acoustic oscillations (BAO), and

(v) large-scale structure (LSS).

• Dark energy can account for this discrepancy because its presence can make the cosmic age longer.
• The first strong evidence for the acceleration of the Universe today came however by measuring the luminosity distance of the type Ia supernovae (SN Ia).
• The CMB observations are also consistent with the presence of dark energy, although the constraint coming from the CMB alone is not so strong.
• The measurements of BAO have provided another independent test for the existence of dark energy.
• The power spectrum of matter distributions also favors a Universe with dark energy rather than the CDM Universe.

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