In order for his field equations to describe a static universe, the state in which it was believed to be at the time 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 which had the effect of countering or balancing out gravitational attraction and thus stabilizing the universe, avoiding that the solutions to his equations would describe a universe collapsing upon itself.
The cosmological constant is denoted by the uppercase Greek letter lambda (Λ) and 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).
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This temporary fix to his theory of general relativity would eventually prove to be faulty primarily due to the fact that Einstein solely based his conclusions upon astronomical phenomena occurring in our galaxy. For soon after, Friedmann derived a solution to Einstein’s field equations which seemed to describe an expanding universe and as such, disregarding the premise that our universe had to remain stable.
Hubble’s study of nearby galaxies
Interestingly, Friedmann’s hypothesis was, a few years later, confirmed by experimental evidence of the expansion of the universe causing Einstein to dub his addition of the cosmological constant his “biggest blunder”. Erwin Hubble, by showing that galaxies were moving further and further away from us, eliminated the need to account for this term and 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 not to be equal to zero due to numerous experiments which seemed to require it.
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The reviving of the cosmological constant
In recent years, as scientists were studying supernovae in an attempt 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 abundances of redshift (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.
Another piece of observational evidence in favour of a non-zero lambda is the readings of the Wilkinson Microwave Anisotropy Probe (WMAP) which is able to 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 now undeniable however, its nature has yet to be determined.
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