User:MajoranaF/sandbox/Cosmological Constant
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In cosmology, the cosmological constant (usually denoted by the Greek capital letter lambda: Λ) is the energy density of space, or vacuum energy, that arrises in Albert Einstein's field equations of general relativity. It is closely associated to the concepts of dark energy and quintessence.[1]
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Einstein originally introduced the concept in 1917[2] to counterbalance the effects of gravity and achieve a static universe, a notion which was the accepted view at the time. Einstein abandoned the concept in 1931 after Hubble's discovery of the expanding universe.[3] From the 1930s until the late 1990s, most physicists assumed the cosmological constant to be equal to zero.[4] That changed with the surprising discovery in 1998 that the expansion of the universe is accelerating, implying the possibility of a positive nonzero value for the cosmological constant.[5]
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.[6] 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. While dark energy is poorly understood at a fundamental level, the main required properties of dark energy are that it functions as a type of anti-gravity, it dilutes much more slowly than matter as the universe expands, and it clusters much more weakly than matter, or perhaps not at all.[citation needed]
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.[7] The discrepancy between theorized vacuum energy from QFT 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!".[8] This issue is called the cosmological constant problem and it is one of the greatest unsolved mysteries in physics with many physicists believing that "the vacuum holds the key to a full understanding of nature".[9]