In 1917, Einstein introduced the cosmological constant in order to explain how our universe is not falling into itself because of gravity and stays static. The force presented by cosmological constant supposedly counteracts the gravity forces of the bodies inside the universe. Therefore, it prevents the collapse and stabilizes the universe. According to its value, the constant would allow for an expanding, contracting or static universe. Since at the beginning of twentieth century universe was considered stationary, he chose a particular value for it, which would permit a static universe. The nature of this constant was not known or explained completely at the time.

When in 1929 Hubble's observation showed that universe is expanding and is not static, Einstein himself regarded the cosmological constant unnecessary. It was considered a non- contributory factor and everybody accepted a zero value for it.

 

The introduction of Big Bang Theory introduced the force, which is supposed to be responsible for expansion of universe despite the presence of the gravity. This would be the residual force that is left from the Big Bang explosion. It was anticipated though, that gravity would slow down and ultimately stop the expansion sometime in the future. That would be when the density of matter in the universe exceeds the critical density.

Critical density is that density of overall matter and energy in the universe that if we exceed it, the gravity has the strength to contract the universe. At critical density, the shape of universe will be flat. Above critical density, the shape of it will be convex and spherical. If the density falls bellow critical density, the shape of the universe will be concave and it will expand forever. All of this happens if we take the cosmological constant as zero. Which means if there is no other factor at work to expand the universe?

In January 1998, Alex Pilippenko in Astrophysist Conference in Southern California announced that cosmological constant could no longer be taken as zero. Different cosmologists reached to this point after long-term observation of supernovae IA in longer red shifts (far distances more than 3 billion light years away).

 

The Wilkinson Microwave Anisotropy Probe (WMAP) the newest probe launched by NASA in 2002 has determined, within the limits of instrument error, that the universe is almost flat.   This is an explanation for the cosmologic constant effect released by NASA.