Cosmological constant

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In cosmology, the cosmological constant (usually denoted by the Greek capital letter lambda: Λ), alternatively called Einstein's cosmological constant, is a coefficient that Albert Einstein initially added to his field equations of general relativity. He later removed it; however, much later it was revived to express the energy density of space, or vacuum energy, that arises in quantum mechanics. It is closely associated with the concept of dark energy.[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.[3] From the 1930s until the late 1990s, most physicists agreed with Einstein's choice of setting the cosmological constant to zero.[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.[6][7][8] The cosmological constant Λ is the simplest possible explanation for dark energy, and is used in the 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 composed of a collection of quantum fields. All these quantum fields exhibit fluctuations in their ground state (lowest energy density) arising from the zero-point energy existing everywhere in space. 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".[11]

  1. ^ Cite error: The named reference CC Definition was invoked but never defined (see the help page).
  2. ^ Einstein (1917).
  3. ^ Cite error: The named reference Rugh 2001 3 was invoked but never defined (see the help page).
  4. ^ Cite error: The named reference Λ = 0? was invoked but never defined (see the help page).
  5. ^ Cite error: The named reference 1998 Discovery was invoked but never defined (see the help page).
  6. ^ Ellis, G. F. R. (2009). "Dark energy and inhomogeneity". Journal of Physics: Conference Series. 189 (1): 012011. Bibcode:2009JPhCS.189a2011E. doi:10.1088/1742-6596/189/1/012011. S2CID 250670331.
  7. ^ Colin, Jacques; Mohayaee, Roya; Rameez, Mohamed; Sarkar, Subir (20 November 2019). "Evidence for anisotropy of cosmic acceleration". Astronomy and Astrophysics. 631: L13. arXiv:1808.04597. Bibcode:2019A&A...631L..13C. doi:10.1051/0004-6361/201936373. S2CID 208175643. Retrieved 25 March 2022.
  8. ^ Redd (2013)
  9. ^ Rugh & Zinkernagel (2001), p. 1
  10. ^ Cite error: The named reference CC Problem was invoked but never defined (see the help page).
  11. ^ Cite error: The named reference CC Problem 3 was invoked but never defined (see the help page).