Dark Energy and its distance

Many things about the nature of dark energy remain matters of speculation. The evidence for dark energy (see below) is indirect. However, it comes from three independent sources. These are

*. Distance measurements and their relation to redshift, which suggest the universe has expanded more in the last half of its life. [ 5 ]

*. The theoretical need for a type of additional energy that is not matter or dark matter to form our observationally flat universe (absence of any detectable global curvature), and

*. It can be inferred from measures of large scale wave-patterns of mass density in the universe.

Dark energy is thought to be very homogeneous , not very dense and is not known to interact through any ofthe fundamental forces other than gravity . Since it is quite rarefied—roughly 10 −29 g/cm 3 —it is unlikelyto be detectable in laboratory experiments. Dark energy can only have such a profound effect on the universe, making up 74% of universaldensity, because it uniformly fills otherwise empty space. The two leading models are a cosmological constant and quintessence . Both models include the common characteristic that dark energy musthave negative pressure.Effect of dark energy: a smallconstant negative pressure of vacuum

Independently from its actual nature, dark energy would need to have a strong negative pressure (acting repulsively) in order to explain the observed acceleration in the expansion rate of the universe .

According to General Relativity, the pressure within a substance contributes to its gravitational attraction for other things just as its mass density does. This happens because the physical quantity that causes matter to generate gravitational effects is the Stress-energy tensor , which contains both the energy (or matter) density of a substance and its pressure and viscosity.