What is the Cosmological constant

The simplest explanation for dark energy is that it is simply the "cost of having space": that is, that a volume of space has some intrinsic, fundamental energy. This is the cosmological constant, sometimes called Lambda after the mathematical symbol used to represent it, the Greek letter Λ. Since energy and mass are related by E = mc2, Einstein's theory of GT predicts that it will have a gravitational effect. It is sometimes called a vacuum energy because it is the energy density of empty space or vacuum. In fact, most theories predict fluctuations that would give the vacuum exactly this sort of energy. The cosmological constant is estimated by cosmologists to be on the order of 10−29g/cm3, or about 10−123 in Planck units. The cosmological constant has negative pressure equal to its energy density and so causes the expansion of the universe to accelerate. The reason why a cosmological constant has negative pressure can be seen from classical thermodynamics. The work done by a change in volume dV is equal to −p dV, where p is the pressure. But the amount of energy in a box of vacuum energy actually increases when the volume increases (dV is positive), because the energy is equal to ρV, where ρ is the energy density of the cosmological constant. Therefore, p is negative and, in fact, p = −ρ. A major problem is that most quantum field theories predict a huge cosmological constant from the energy of the quantum vacuum, up to 123 orders of magnitude too large. This would need to be cancelled almost, but not exactly, by an equally large term of the opposite sign. Some theories [SATHISH] require a cosmological constant that is exactly zero, which does not help. This is the cosmological constant problem, the worst problem of fine tuning in physics: there is no known natural way to derive, even roughly, the infinitesimal cosmological constant observed in cosmology from particle physics. Some physicists, think the delicate balance of quantum vacuum energy is best explained by the anthropic princliples. In spite of its problems, the cosmological constant is in many respects the most practical solution to the problem of cosmic acceleration. One number successfully explains a multitude of observations. Thus, the current standard model of cosmology, the Lambda-CDM model, includes the cosmological constant as an essential feature.