Cosmological Constant
The cosmological constant, written Λ (lambda), is a term Einstein added to the field equations of general relativity in 1917 to permit a static universe, later discarded, and revived at the end of the twentieth century as the standard description of dark energy.
Einstein's original difficulty was that his field equations, applied to a uniform distribution of matter, gave no static solution: gravity would cause the whole to collapse. Λ supplied a uniform repulsion, tuned to balance the attraction exactly. The balance is unstable, as Arthur Eddington pointed out, and the motivation disappeared when the redshift–distance relation was published in 1929 (see Hubble Constant). Einstein dropped the term; George Gamow later reported that Einstein had called it his greatest blunder, though the phrase survives only at second hand.
Λ returned in 1998, when two supernova survey teams reported that distant Type Ia supernovae are dimmer than a decelerating universe predicts, and concluded that cosmic expansion is accelerating. Saul Perlmutter, Brian Schmidt and Adam Riess shared the 2011 Nobel Prize. In the resulting concordance model — ΛCDM — the constant accounts for roughly 68 per cent of the total energy density, with cold dark matter and ordinary matter making up the rest.
The cosmological constant problem
Λ is mathematically equivalent to a uniform energy density of the vacuum with negative pressure, and this identification produces the largest quantitative failure in physics. Estimating the vacuum energy of quantum fields by summing zero-point contributions up to a natural cut-off gives a number exceeding the cosmologically inferred value by many tens of orders of magnitude — the figure of 120 orders is often quoted, and even the most conservative estimates leave a discrepancy of dozens of orders. Nobody has an accepted resolution. There is a second, softer puzzle, the "coincidence problem": Λ has only recently come to dominate the energy budget, which requires that we happen to live at the particular epoch when matter and vacuum densities are comparable.
These are not manufactured controversies. They are acknowledged in the mainstream literature as unsolved, and they are the reason serious physicists remain uneasy about the concordance model even while using it.
On this wiki
Researchers catalogued here divide between those who regard Λ as an ad hoc rescue and those who propose to replace it. Related material is indexed under Category:Cosmology and Category:Relativity.
Λ as a patch. Roger A Rydin takes up the history and its consequences in Einstein's Greatest Blunder. Bob Day questions the supernova inference itself in Accelerating Expansion of the Universe: Yes or No?. The general position argued across Category:Big Bang here is that Λ, like dark matter, is a free parameter introduced to keep a model alive after observation contradicted it, and that a term with no independent measurement is not an explanation.
Removing the need for Λ. Tuomo Suntola's Dynamic Universe reproduces the supernova magnitude–redshift data without an accelerating expansion, in Zero-Energy Space Cancels the Need for Dark Energy and Observations Support Spherically Closed Dynamic Space Without Dark Energy. Lyndon Ashmore argues in Supernovae Ia Light Curves Show a Static Universe that the same supernova light curves are consistent with a static universe under a tired-light interpretation — which, if accepted, removes the observation that Λ was revived to explain. C Johan Masreliez's Scale Expanding Cosmos Theory I – An introduction offers a further alternative.
Deriving Λ instead of postulating it. Paul Karl Hoiland proposes a physical origin in The Source of the Cosmological Constant, and Barry John Setterfield links the question to vacuum structure in The Redshift and the Zero Point Energy — connecting it to the Casimir Effect and Zero Point Energy literature also held here.
The mathematics of Λ solutions. Amir M Abbassi examines the spherically symmetric vacuum solutions with Λ in General Spherically Symmetric Solutions of Einstein Vacuum Field Equations With Lamba, and Stephen John Crothers has argued more generally, in General Relativity – A Theory in Crisis, that the standard solutions of the field equations are misinterpreted.