For his field equations to describe a static universe, the state in which it was believed to be due to the observation of the extremely slow velocities of stars, Einstein had to add a term to his equation, which was dubbed the “cosmological constant.” The latter accounted for an unknown force that had the effect of countering or balancing out gravitational attraction and stabilizing the universe, avoiding that the solutions to his equations would describe a universe collapsing.

The cosmological constant is denoted by the uppercase Greek letter lambda (Λ). It represents the energy contained by the vacuum of space, which would imply that empty space would, unintuitively, possess a density, possibly caused by quantum fluctuations (the temporary, spontaneous, and random appearance of pair of matter-antimatter particles which would immediately annihilate).

This temporary fix to his theory of general relativity would eventually prove to be faulty, primarily because Einstein solely based his conclusions upon astronomical phenomena occurring in our galaxy. For soon after, Friedmann derived a solution to Einstein’s field equations that seemed to describe an expanding universe and disregard the premise that our universe had to remain stable.

**Hubble’s study of nearby galaxies**

Interestingly, a few years later, Friedmann’s hypothesis confirmed by experimental evidence of the universe’s expansion, causing Einstein to dub his addition of the cosmological constant his “biggest blunder.” By showing that galaxies were moving further and further away from us, Edwin Hubble eliminated the need to account for this term. It was set to equal zero in most future models of the universe described by Einstein’s field equations.

However, this change was merely temporary. Although Einstein’s premise was incorrect, the cosmological constant is now believed to be equal to zero due to numerous experiments that seemed to require it.

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**The reviving of the cosmological constant**

In recent years, as scientists were studying supernovae to quantify the expansion of the universe, which was thought to be decelerating, they discovered that it was actually accelerating. Galaxies within set solid angles were found in greater redshift abundances (an indication that an object is moving away from us) than expected.

This introduced the possibility of a gravitationally repulsive form of energy, similar to the interaction described by the cosmological constant and, as such, if fine-tuned to the right magnitude, could explain the difference between theory and observation concerning the rate of expansion of the universe. This unknown cause of our universe’s acceleration is now more commonly known as dark energy.

**WMAP**

Another piece of observational evidence in favor of a non-zero lambda is the readings of the Wilkinson Microwave Anisotropy Probe (WMAP), which can determine the basic cosmological parameters required for a spatially flat universe (zero curvature, where the total energy of the system is equal to 0). WMAP calculated the cosmological constant, finding that another type of energy was necessary to counteract the effects of baryonic matter.

As such, the need for dark energy, possibly quantified by the cosmological constant, is undeniable; however, its nature has yet to be determined.

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