What is Dark Energy?
In physical cosmology, dark energy is an invisible form of energy that exerts a negative pressure, causing an acceleration in the expansion of the universe. It makes up 68% of the total content of the universe. While its effects are clearly observable, its physical nature is unknown. The simplest and most popular model that explains dark energy is called the Lambda-CDM model, which invokes a non-zero cosmological constant lambda (Λ). In this context, lambda represents the vacuum energy density, also known as dark energy.
The name “dark energy” was first coined in 1994 by Michael Turner and Martin White to describe observations that suggested that approximately two-thirds of the mass–energy density of the universe consisted of a new kind of non-luminous matter distinct from known forms such as cold dark matter or baryonic matter. In 1998 two independent groups studying Type Ia supernovae discovered that distant supernovae were fainter than expected based on measurements of their redshifts—an effect now attributed to cosmic expansion accelerated by a repulsive force arising from some form of dark energy filling otherwise empty space. Subsequent observations have revealed more evidence for this accelerated expansion: for example, studies conducted between 2003 and 2008 found no evidence for time dilation due to gravitational lensing by intervening massive structures such as galaxy clusters; this would be expected if light passed through regions where spacetime was being stretched out faster than average due to cosmic acceleration caused by extra mass or extra radiation (which dilute with distance), but not if spacetime were instead being distorted less than average (as would occur with negative pressure). Several other pieces of evidence have also been put forth, including observations indicating that galaxy clusters do not have enough visible mass to account for their observed gravity under Newtonian dynamics unless they contain substantial amounts of some form of unseen “dark” matter; again this could arise either from additional normal mass/radiation which becomes diluted over large distances or from some sort of negative pressure stuff filling otherwise empty space. If indeed nearly 70% of the Universe’s mass–energy density consists of this new kindergentler repulsive force pushing everything apart then it has profound implications for our understanding bothof how structure Formation took place in the early cosmosand what will happen to our Universe in future billions Of years when it continues to expand forever