Dark Energy is an unknown form of energy that affects the universe at large scales. The evidence for it came through supernovae which showed that the universe was accelerating in its expansion. The energy driving this expansion was called Dark Energy. Since the 1990s, Dark Energy has been the preferred explanation for accelerating expansion.
History of Dark Energy
Einstein's Cosmological Constant
The cosmological constant was a constant that could be added to the equation of General Relativity. It could be considered as the mass of empty space, or vacuum energy.
This was first proposed by Einstein to arrive at a solution that proposed a static universe, effectively using this constant to balance gravity. Although Einstein was wrong in his assumption of a static universe, he may be right about energy like the cosmological constant accelerating the expansion.
Inflationary Dark Energy
In 1980, Alan Guth and Alex Starobinsky proposed a negative pressure field, that could drive cosmic inflation. Inflation postulates that some repulsive force, qualitatively similar to dark energy, resulted in an enormous and exponential expansion of the universe slightly after the Big Bang. Such expansion is an essential feature of most current models of the Big Bang.
Nature of Dark Energy
The nature of dark energy is more hypothetical than that of dark matter, and many things about it remain in the realm of speculation. Dark energy is thought to be very homogeneous and not very dense, and is not known to interact through any of the fundamental forces other than gravity. Since it is quite rarefied and un-massive—roughly 10−27 kg/m3—it is unlikely to be detectable in laboratory experiments. The reason dark energy can have such a profound effect on the universe, making up 68% of universal density in spite of being so dilute, is that it uniformly fills otherwise empty space.
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