Cosmological constant doubts I have read about cosmological constant given by einstein in universe in nutshell as well as in general and special relativity. But still I am not able to understand the aim to use it or to introduce it.
Can someone explain in brief.
 A: The cosmological constant has an interesting history behind it. Originally, when Einstein introduced his theory of general relativity in the early 20th century, the Einstein Field Equation, which was the equation for the gravitational field, described gravity as the effect of the curvature of space-time due to the presence of matter and energy. Perhaps you have seen the equation already, for it is:
$R_{\mu\nu}$-$1/2Rg_{\mu\nu}$=$kT_{\mu\nu}$ 
However, Einstein's equation predicted that the influence of gravity would cause the universe to either collapse on itself or expand. Such a prediction contradicted Einstein's view, for he believed that the universe was infinite and static, or never changing. Thus, he introduced the cosmological constant as a small energy density contribution required to stop the collapsing or expanding force of gravity and to keep the universe in balance. The new equation became:
$R_{\mu\nu}$-$1/2Rg_{\mu\nu}+\Lambda g_{\mu\nu}$=$kT_{\mu\nu}$ 
Where the lambda is the cosmological constant. However, Hubble's discovery that the universe was indeed not static but expanding (about a decade later after the cosmological constant was introduced) made Einstein's constant unnecessary. Due to this, Einstein supposedly called his introduction of the constant the "biggest blunder of my life." The constant was dropped for many decades until recently scientists discovered that the universe is accelerating in its expansion. The constant was then reintroduced is some inflation theories to describe the small energy density that propels such an accelerated expansion of the universe. The other answers to this question pretty much discuss its current uses. So after all, Einstein may have been right about the constant! 
A: The cosmological constant is important for at least two reasons.


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*Our universe is currently asymptotically evolving towards a universe where a constant energy density dominates the total energy density. The cosmological constant can be interpreted as exactly this. Therefore, analysis of the current state of our universe relies heavily on the concept of a cosmological constant, even if we don't know how it arises.

*inflationary theory, which has recently received strong experimental support from the BICEP2 experiment, asserts that the early universe went through a de Sitter-like epoch during which it underwent exponential expansion, driven by a large (quasi-)constant energy density (i.e. effective cosmological constant). 


Thus, the cosmological constant is acutely relevant to both the past and future of our universe. This is an extremely condensed account and could be expanded immensely. It is also interesting for historical reason, and because 'why not?'.
