Cosmological constant

The cosmological constant ( usually abbreviated by the great Greek lambda) is a physical constant in Einstein's equations of general relativity, which describes the force of gravity as a geometric curvature of space- time. In SI units the dimension 1/m2. Its value can a priori be positive, negative or zero.

Definition

While the prevailing view in physics was for a long time that the value of the cosmological constant is zero, recent observations come at a very small positive value. The cosmological constant is no longer considered parameters of the general theory of relativity ( as introduced by Einstein ) is interpreted, rather than the time-constant energy density of the vacuum:

Wherein the circular number Pi, the gravitational constant and the velocity of light in vacuum.

In modern cosmology, the dimensionless density parameter ΩΛ is usually used instead of Λ:

With the critical mass density

It is

The Hubble constant.

The assumption that the vacuum energy density is constant even when the expansion of the universe, resulting in the state equation

That is a positive vacuum energy density leads to a negative pressure, which drives the accelerated expansion of the universe. This effect, each form of energy ( photons in gases, however ), but the energy density is not constant over time in the general case. The generalization of the cosmological constant on time-varying energy densities of this type is referred to as dark energy.

For a number of different observations of the value of the cosmological constant is to estimate precisely, that is, about 70 % of the energy density in the universe is in the form of the cosmological constant or dark energy before.

History

The Einstein's field equations of general relativity can be formulated with or without a cosmological constant. However, a material fulfilling the universe, the development is described by equations without the constant not be static, but must necessarily expanding or collapsing. When Einstein set up his equations, the universe, however, was considered to be static. Thus, the equations describe a static universe with matter ( and not a collapsing due to the gravitational attraction), Einstein introduced the constant a in an ad hoc hypothesis. It works (if it is positive ) as one of the gravitational force opposite " expansion power ". The adjective " cosmological " expresses here is that this constant is important for cosmology.

However, this static solution is unstable, and the slightest deviation from the ideal distribution of matter allow the universe to collapse or expand yet again depending on the sign of the disorder. When Edwin Hubble discovered then the expansion of the universe through the galaxy alignment and also Alexander Alexandrovich Friedmann (1922, 1924) and Georges Lemaître (1927 ) discovered cosmological expanding solutions of the field equations, Einstein rejected the idea of the cosmological constant, which he described reputed as the "greatest blunder of my life ". However, the task of the cosmological constant did not happen immediately, but sat up until the early 1930s by.

Modern relationships

After the cosmological constant had lost by the discovery of the expansion of the universe in importance, it was more of academic interest. She won again in importance by attempts to develop a unified theory of all forces of nature. These are described by quantum field theories, and the vacuum fluctuations of the fields of these quantum field theories would provide many orders of magnitude too large contribution to the cosmological constant. This is called the problem of the cosmological constant. The problem remains unresolved. For example, today, have often favored theories with supersymmetry the advantage that indeed cancel the contributions of fermions and bosons in the vacuum fluctuations to the cosmological constant with exact supersymmetry, the symmetry is broken but in nature.

Another approach to understanding the cosmological constant is the theory of the inflationary universe. This can be well explained by a positive cosmological constant.

From 1998, the cosmological constant has experienced a renaissance: With the brightness and redshift of distant supernovae, the so-called type 1a may find that the universe is expanding faster. This accelerated expansion can be very good with a cosmological constant describing and is part of the successful Lambda - CDM model, the standard model of cosmology.