Cosmological constant
In 1917, Albert Einstein published a paper titled Cosmological considerations in the General Theory of Relativity. He added a specific term to his field equations that he called the cosmological constant. This addition was not an afterthought but a deliberate move to force gravity into a static universe model. At that time, scientists assumed the cosmos did not expand or contract over time. Without this new term, Einstein's original equations predicted that gravity would cause any non-expanding universe to collapse inward. Einstein felt uneasy about introducing this mathematical fix. He later stated that he had always carried a bad conscience regarding the term. He described the concept as an ugly thing that he could not believe existed in nature. The resulting model became known as the Einstein static universe. It remained stable only under very specific conditions. Any slight perturbation in matter density would cause the equilibrium to fail. If the universe expanded slightly, vacuum energy release would drive further expansion. If it contracted slightly, gravity would pull it down faster.
By 1929, Edwin Hubble confirmed through observation that the universe is expanding. This discovery led Einstein to abandon the cosmological constant and accept a dynamic universe. For decades, most physicists believed the value of the constant was zero. That view changed dramatically in 1998 when two teams began measuring distant supernovae. Saul Perlmutter worked at Lawrence Berkeley National Laboratory while Brian Schmidt led efforts at the Australian National University. Adam Riess operated out of the Space Telescope Science Institute. They expected to find evidence of deceleration caused by gravitational attraction. Instead, their data showed that supernovae were accelerating away from Earth. Both groups announced these surprising results in July 1998. The findings implied that the speed of galaxies' recession increases over time. A positive cosmological constant was required to explain this acceleration. Following this discovery, the term returned to general relativity equations. In 2011, Perlmutter, Schmidt, and Riess jointly received the Nobel Prize in Physics for this work. Their measurements proved that dark energy produces a repulsive force counterbalancing matter's gravitational braking.
Quantum field theory defines empty space as a vacuum state composed of fluctuating quantum fields. These fluctuations create zero-point energy existing everywhere in space. Calculations suggest this energy should contribute significantly to the cosmological constant. Actual predictions yield an enormous vacuum energy value compared to observations. The discrepancy between theorized vacuum energy and observed values exceeds some 120 orders of magnitude. This gap has been called the worst theoretical prediction in the history of physics. It represents one of the greatest mysteries in modern science. Many physicists believe the vacuum holds the key to understanding nature itself. No known natural way exists to derive the tiny cosmological constant used in cosmology from particle physics. Some supersymmetric theories require a cosmological constant exactly equal to zero. String theory offers no known vacuum supporting a metastable positive value. In 2018, four physicists advanced a controversial conjecture implying such universes do not exist. The problem remains unsolved despite decades of intense research efforts.
Current measurements indicate around 68% of the universe's mass-energy density comes from dark energy. The Lambda-CDM model serves as the standard framework for cosmology today. This model includes the cosmological constant as its simplest explanation for dark energy. Data from the Planck Collaboration published in 2018 estimated the dark energy density parameter at 0.714. WMAP results also support this figure clearly. The dimensionless ratio Omega-Lambda describes the fraction of the universe made up of dark energy. Critical density changes with cosmological time while energy density due to the constant remains unchanged. As the universe grows, the amount of dark energy increases but matter does not. Recent proposals suggest the Hubble tension and CMB dipole might challenge the cosmological principle. Some researchers argue the FLRW metric breaks down in the late universe. If true, observations attributed to accelerating expansion could simply reflect errors in applying the cosmological principle. Despite these debates, the measured value stays on the order of 10^-52 per square meter.
Steven Weinberg noted a possible explanation for the small non-zero value in 1987 using the anthropic principle. He argued that if vacuum energy took different values across domains, observers would necessarily measure values similar to what is observed. Life-supporting structures form only where vacuum energy allows galaxy formation. A negative vacuum energy substantially larger than observed would make the universe closed and short-lived. A large positive constant would expand too fast for intelligent life to emerge. Weinberg predicted the cosmological constant would be less than a hundred times the currently accepted value. In 1992 he refined this prediction to five to ten times the matter density. Alexander Vilenkin improved the argument further in 1995 to predict about three times the current value. Multiverse theories predict parallel universes with different laws or constants. The anthropic principle states we can live only in one compatible with intelligent life. Critics claim these explanations commit the inverse gambler's fallacy. No evidence exists showing vacuum energy varies despite theoretical possibilities involving scalar fields like quintessence.
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Common questions
When did Albert Einstein publish the paper Cosmological considerations in the General Theory of Relativity?
Albert Einstein published the paper titled Cosmological considerations in the General Theory of Relativity in 1917. He added a specific term to his field equations that he called the cosmological constant to force gravity into a static universe model.
What year did Edwin Hubble confirm that the universe is expanding and cause Einstein to abandon the cosmological constant?
Edwin Hubble confirmed through observation by 1929 that the universe is expanding. This discovery led Einstein to abandon the cosmological constant and accept a dynamic universe.
Who won the Nobel Prize in Physics in 2011 for work on the cosmological constant and what was their role?
Saul Perlmutter, Brian Schmidt, and Adam Riess jointly received the Nobel Prize in Physics in 2011 for this work. Saul Perlmutter worked at Lawrence Berkeley National Laboratory while Brian Schmidt led efforts at the Australian National University and Adam Riess operated out of the Space Telescope Science Institute.
How many orders of magnitude does the discrepancy between theorized vacuum energy and observed values exceed?
The discrepancy between theorized vacuum energy and observed values exceeds some 120 orders of magnitude. This gap has been called the worst theoretical prediction in the history of physics.
What percentage of the universe's mass-energy density comes from dark energy according to current measurements?
Current measurements indicate around 68% of the universe's mass-energy density comes from dark energy. Data from the Planck Collaboration published in 2018 estimated the dark energy density parameter at 0.714.