Dark Energy

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Dark energy

In physical cosmology and astronomy, dark energy is a proposed form of energy that affects the universe on its largest scales. Its primary effect is to drive the accelerating expansion of the universe. It also slows the rate of structure formation. Assuming that the lambda-CDM model of cosmology is correct, dark energy dominates the universe, contributing 68% of the total mass-energy in the present-day observable universe while dark matter and ordinary (baryonic) matter contribute 27% and 5%, respectively, and other components such as neutrinos and photons are nearly negligible. Dark energy's density is very low: 7×10−30 g/cm3 (6×10−10 J/m3 in mass-energy), much lower than the density of ordinary matter or dark matter within galaxies. However, it dominates the universe's mass–energy content because it is uniform across space. The first observational evidence for dark energy's existence came from measurements of supernovae. Type Ia supernovae have constant luminosity, which means they can be used to accurately measure distances. Comparing this distance to the redshift (which measures the speed at which the supernova is receding) shows that the universe's expansion is accelerating. Prior to this observation, scientists thought that the gravitational attraction of matter and energy in the universe would cause the universe's expansion to slow over time. Since the discovery of accelerating expansion, several independent lines of evidence have been discovered that support the existence of dark energy. The exact nature of dark energy remains a mystery, and many possible explanations have been theorized. The main candidates are a cosmological constant (representing a constant energy density filling space homogeneously) and scalar fields (dynamic quantities having energy densities that vary in time and space) such as quintessence or moduli. A cosmological constant would remain constant across time and space, while scalar fields can vary. Yet other possibilities are interacting dark energy (see the section Dark energy § Theories of dark energy), an observational effect, cosmological coupling, and shockwave cosmology (see the section § Alternatives to dark energy).

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• Dark energy - Wikipedia

  In physical cosmology and astronomy, dark energy is a proposed form of energy that affects the universe on its largest scales. Its primary effect is to drive the accelerating expansion of the universe. It also slows the rate of structure formation. Assuming that the lambda-CDM model of cosmology is correct, dark energy dominates the universe, contributing 68% of the total mass-energy in the present-day observable universe while dark matter and ordinary (baryonic) matter contribute 27% and 5%, respectively, and other components such as neutrinos and photons are nearly negligible. Dark energy's density is very low: 7×10−30 g/cm3 (6×10−10 J/m3 in mass-energy), much lower than the density of ordinary matter or dark matter within galaxies. However, it dominates the universe's mass–energy [...] The vacuum energy, that is, the particle–antiparticle pairs generated and mutually annihilated within a time frame in accord with Heisenberg's uncertainty principle in the energy–time formulation, has been often invoked as the main contribution to dark energy. The mass–energy equivalence postulated by general relativity implies that the vacuum energy should exert a gravitational force. Hence, the vacuum energy is expected to contribute to the cosmological constant, which in turn impinges on the accelerated expansion of the universe. However, the cosmological constant problem asserts that there is a huge disagreement between the observed values of vacuum energy density and the theoretical large value of zero-point energy obtained by quantum field theory; the problem remains unresolved. [...] The term "dark energy" was coined by cosmologist Michael S. Turner in 1998 for a paper written with Saul Perlmutter and Martin White. ## Nature [edit] The nature of dark energy is more hypothetical than that of dark matter, and many things about it remain in the realm of speculation. Dark energy is thought to be very homogeneous and not dense, and is not known to interact through any of the fundamental forces other than gravity. Since it is rarefied and un-massive—roughly 10−27 kg/m3—it is unlikely to be detectable in laboratory experiments. The reason dark energy can have such a profound effect on the universe, making up 68% of universal density in spite of being so dilute, is that it is believed to uniformly fill otherwise empty space.

• What is dark energy : r/AskPhysics - Reddit

  Dark energy is a placeholder for whatever is causing the acceleration in expansion observed by Adam Reiss and others in the 1990s. Theoretically

• What is dark energy? | University of Chicago News

  UChicago Explainer Series # Dark energy, explained “Dark energy” is a term scientists use to refer to whatever is causing the universe to expand faster over time. We don’t know exactly what dark energy is—no one has ever directly seen or measured it—but some force seems to be currently pushing the universe to expand faster than it should, according to everything else we know about the cosmos. Whatever it is, there’s a lot of it: It appears to make up about 70% of the universe today. Coined by University of Chicago astrophysicist Michael Turner in 1998, dark energy remains one of the great mysteries of physics. ### Jump to a section: [...] ### Jump to a section: What is dark energy? How do we know dark energy is there? What does it tell us about the universe? What do we know and not know about dark energy? What are the possible explanations for dark energy? Why is it called dark energy? What’s the difference between dark energy and dark matter? How do we figure out what dark energy is? What experiments are studying dark energy? Why is dark energy so difficult to figure out? ### What is dark energy? “Dark energy” is a term scientists use to refer to whatever is causing the universe to expand faster over time. It’s a catchall term because we don’t know exactly what dark energy is—no one has ever directly seen or measured it. [...] The cosmological constant or vacuum energy theory. According to this theory, dark energy is energy created by the vacuum of space. It’s named after a term Einstein invented to solve a problem in his calculations. In the early 1900s, everyone thought the universe stayed the same size, but Einstein’s theory said space is dynamic, so he invented a number called the cosmological constant to balance the equation. (He later supposedly called it “his biggest blunder” after evidence came out indicating the universe is expanding…but the concept was useful and so was reinterpreted for modern theory).

• Our Expanding Universe: Delving into Dark Energy | Department of Energy

  So far, the evidence has spurred two competing theories. The leading theory is that dark energy is the "cosmological constant," a concept Albert Einstein created in 1917 to balance his equations to describe a universe in equilibrium. Without this cosmological constant to offset gravity, a finite universe would collapse into itself. Today, scientists think the constant may represent the energy of the vacuum of space. Instead of being "empty," this would mean space is actually exerting pressure on cosmic objects. If this idea is correct, the distribution of dark energy should be the same everywhere.

• What is Dark Energy? Inside Our Accelerating, Expanding Universe - NASA Science

  ### Vacuum Energy Some scientists think that dark energy is a fundamental, ever-present background energy in space known as vacuum energy, which could be equal to the cosmological constant, a mathematical term in the equations of Einstein's theory of general relativity. Originally, the constant existed to counterbalance gravity, resulting in a static universe. But when Hubble confirmed that the universe was actually expanding, Einstein removed the constant, calling it “my biggest blunder,” according to physicist George Gamow. [...] Dark energy has been described by some as having the effect of a negative pressure that is pushing space outward. However, we don't know if dark energy has the effect of any type of force at all. There are many ideas floating around about what dark energy could possibly be. Here are four leading explanations for dark energy. Keep in mind that it's possible it's something else entirely. ### Vacuum Energy [...] While other possible explanations for these observations have been explored, astronomers studying even more distant supernovae or other cosmic phenomena in more recent years continued to gather evidence and build support for the idea that the universe is expanding faster over time, a phenomenon now called cosmic acceleration. But, as scientists built up a case for cosmic acceleration, they also asked: Why? What could be driving the universe to stretch out faster over time? Enter dark energy. ## What Exactly is Dark Energy? Right now, dark energy is just the name that astronomers gave to the mysterious "something" that is causing the universe to expand at an accelerated rate.

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