Cosmological constant
[1] Einstein introduced the constant in 1917[2] to counterbalance the effect of gravity and achieve a static universe, which was then assumed.
Einstein's cosmological constant was abandoned after Edwin Hubble confirmed that the universe was expanding.
[4] That changed with the discovery in 1998 that the expansion of the universe is accelerating, implying that the cosmological constant may have a positive value.
[5] Since the 1990s, studies have shown that, assuming the cosmological principle, around 68% of the mass–energy density of the universe can be attributed to dark energy.
These zero-point fluctuations should contribute to the cosmological constant Λ, but actual calculations give rise to an enormous vacuum energy.
[9] The discrepancy between theorized vacuum energy from quantum field theory and observed vacuum energy from cosmology is a source of major contention, with the values predicted exceeding observation by some 120 orders of magnitude, a discrepancy that has been called "the worst theoretical prediction in the history of physics!".
[10] This issue is called the cosmological constant problem and it is one of the greatest mysteries in science with many physicists believing that "the vacuum holds the key to a full understanding of nature".
[14] In 1929, not long after Einstein developed his static theory, observations by Edwin Hubble[14] indicated that the universe appears to be expanding; this was consistent with a cosmological solution to the original general relativity equations that had been found by the mathematician Alexander Friedmann, working on the Einstein equations of general relativity.
Einstein reportedly referred to his failure to accept the validation of his equations—when they had predicted the expansion of the universe in theory, before it was demonstrated in observation of the cosmological redshift—as his "biggest blunder" (according to George Gamow).
[5] The explanation of this small but positive value is a remaining theoretical challenge, the so-called cosmological constant problem.
For example, Arthur Eddington claimed that the cosmological constant version of the vacuum field equation expressed the "epistemological" property that the universe is "self-gauging", and Erwin Schrödinger's pure-affine theory using a simple variational principle produced the field equation with a cosmological term.
is the Planck length.A positive vacuum energy density resulting from a cosmological constant implies a negative pressure, and vice versa.
If the energy density is positive, the associated negative pressure will drive an accelerated expansion of the universe, as observed.
Note that this value changes over time: The critical density changes with cosmological time but the energy density due to the cosmological constant remains unchanged throughout the history of the universe, because the amount of dark energy increases as the universe grows but the amount of matter does not.
[23] This ratio is w = −1 for the cosmological constant used in the Einstein equations; alternative time-varying forms of vacuum energy such as quintessence generally use a different value.
Observations announced in 1998 of distance–redshift relation for Type Ia supernovae[5] indicated that the expansion of the universe is accelerating, if one assumes the cosmological principle.
[6][7] As was only recently seen, by works of 't Hooft, Susskind and others, a positive cosmological constant has surprising consequences, such as a finite maximum entropy of the observable universe (see Holographic principle).
If the universe is described by an effective local quantum field theory down to the Planck scale, then we would expect a cosmological constant of the order of
No vacuum in the string theory landscape is known to support a metastable, positive cosmological constant, and in 2018 a group of four physicists advanced a controversial conjecture which would imply that no such universe exists.
[36] One possible explanation for the small but non-zero value was noted by Steven Weinberg in 1987 following the anthropic principle.
[37] Weinberg explains that if the vacuum energy took different values in different domains of the universe, then observers would necessarily measure values similar to that which is observed: the formation of life-supporting structures would be suppressed in domains where the vacuum energy is much larger.
On the other hand, a universe with a large positive cosmological constant would expand too fast, preventing galaxy formation.
Another theoretical approach that deals with the issue is that of multiverse theories, which predict a large number of "parallel" universes with different laws of physics and/or values of fundamental constants.
Again, the anthropic principle states that we can only live in one of the universes that is compatible with some form of intelligent life.
Critics claim that these theories, when used as an explanation for fine-tuning, commit the inverse gambler's fallacy.
An attempt to directly observe and relate quanta or fields like the chameleon particle or the symmetron theory to dark energy, in a laboratory setting, failed to detect a new force.
[41] Inferring the presence of dark energy through its interaction with baryons in the cosmic microwave background has also led to a negative result,[42] although the current analyses have been derived only at the linear perturbation regime.
It is also possible that the difficulty in detecting dark energy is due to the fact that the cosmological constant describes an existing, known interaction (e.g. electromagnetic field).
