> [!infobox] <s class="aside-in"><em>mentioned in 2 topics, 1 source</em></s> #### <s class="topic-title">[[cosmological constant]]</s> > [!wikipedia] [cosmological constant](https://en.wikipedia.org/wiki/Cosmological%20constant) > > In [[cosmology]], the cosmological constant (usually denoted by the Greek capital letter lambda: Λ), alternatively called [[Einstein]]'s cosmological constant, is the constant coefficient of a term Albert Einstein temporarily added to his field equations of [[general relativity]]. He later removed it. Much later it was revived and reinterpreted as the energy density of space, or vacuum energy, that arises in [[quantum mechanics]]. It is closely associated with the concept of [[dark energy]]. > > Einstein originally introduced the constant in 1917 to counterbalance the effect of gravity and achieve a static universe, a notion which was the accepted view at the time. Einstein abandoned the constant in 1931 after [[Hubble]]'s confirmation of the [[expanding universe]]. From the 1930s until the late 1990s, most physicists agreed with Einstein's retraction, assuming the cosmological constant to be equal to zero. That changed with the surprising discovery in 1998 that the expansion of the universe is accelerating, implying the possibility of a positive value for the cosmological constant. > > Since the 1990s, studies have shown that around 68% of the mass–energy density of the [[universe]] can be attributed to so-called [[dark energy]]. The cosmological constant Λ is the simplest possible explanation for dark energy, and is used in the current standard model of cosmology known as the [[ΛCDM model]]. > > According to [[quantum field theory]] (QFT) which underlies modern particle [[physics]], empty space is defined by the vacuum state which is a collection of quantum fields. All these quantum fields exhibit fluctuations in their ground state (lowest energy density) arising from the zero-point energy present everywhere in space. These zero-point fluctuations should act as a contribution to the cosmological constant Λ, but when calculations are performed these fluctuations give rise to an enormous [[vacuum]] energy. 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". 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". ##### ^dataviews > [!dataview]+ Related unlinked notes > > No results to show for list query. > [!dataview]- Other unlinked mentions > > - [[video - The Sixth Dimension]]