Vacuum energy is an underlying background energy that exists in space throughout the entire Universe. Since there is so much space, this background energy is currently estimated to make up about 73% of the total mass-energy content of the Universe. (Dark matter apparently makes up 23%, with all the atoms we know about coming in at 4%.) Current theory has it that the vacuum energy of "empty" space not only contributes to the mass-energy content, it carries the added quality of negative pressure, which provides a mechanism for the accelerating expansion of the universe.
Associated with the vacuum energy are the virtual particles, which are known to be particle pairs that blink into existence and then annihilate in a timespan too short to measure. They do this everywhere, throughout the Universe. Their behavior is codified in Heisenberg's energy-time uncertainty principle. Still, the exact effect of such fleeting bits of energy is difficult to quantify. The observed "value" of the background vacuum energy is so small, it is involved in one of the largest discrepancies in the history of physics (which is often accurate to 10 decimal places). Particle physicists, in their calculations, somehow come up with an estimate that is 120 orders of magnitude too large. Understandably, this research area is one of the least understood and most active in physics, astronomy, astrophysics, and cosmology.
The effects of vacuum energy can be experimentally observed in various phenomena such as spontaneous emission, the Casimir effect, the van der Waals bonds and the Lamb shift, and are thought to influence the behavior of the Universe on cosmological scales. Using the upper limit of the cosmological constant, the vacuum energy in a cubic meter of free space has been estimated to be 10−9 Joules. However, in both Quantum Electrodynamics (QED) and Stochastic Electrodynamics (SED), consistency with the principle of Lorentz covariance and with the magnitude of the Planck Constant requires it to have a much larger value of 10113 Joules per cubic meter.
Read more about Vacuum Energy: Origin, Implications, History
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