Dark energy Definition and 559 Threads

In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovae, which showed that the universe does not expand at a constant rate; rather, the expansion of the universe is accelerating. Understanding the evolution of the universe requires knowledge of its starting conditions and its composition. Prior to these observations, it was thought that all forms of matter and energy in the universe would only cause the expansion to slow down over time. Measurements of the cosmic microwave background suggest the universe began in a hot Big Bang, from which general relativity explains its evolution and the subsequent large-scale motion. Without introducing a new form of energy, there was no way to explain how an accelerating universe could be measured. Since the 1990s, dark energy has been the most accepted premise to account for the accelerated expansion. As of 2021, there are active areas of cosmology research aimed at understanding the fundamental nature of dark energy.Assuming that the lambda-CDM model of cosmology is correct, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable universe. The mass–energy of dark matter and ordinary (baryonic) matter contributes 26% and 5%, respectively, and other components such as neutrinos and photons contribute a very small amount. The density of dark energy is very low (~ 7 × 10−30 g/cm3), much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the mass–energy of the universe because it is uniform across space.Two proposed forms of dark energy are the cosmological constant, representing a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities having energy densities that can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to the zero-point radiation of space i.e. the vacuum energy. Scalar fields that change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.
Due to the toy model nature of concordance cosmology, some experts believe that a more accurate general relativistic treatment of the structures that exist on all scales in the real universe may do away with the need to invoke dark energy. Inhomogeneous cosmologies, which attempt to account for the back-reaction of structure formation on the metric, generally do not acknowledge any dark energy contribution to the energy density of the Universe.

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  1. Ivan Seeking

    Hubble chases exploding stars: re dark energy

    http://news.bbc.co.uk/2/hi/science/nature/3155200.stm
  2. wolram

    What is Dark Energy? - Exploring Its Profound Discovery

    what is dark energy? i think that when dark energy was discovered science didnt know how profound the discovery was. i think that dark energy is the primal source of everything, i imagine an instability in the dark energy built to such an extent that it could no longer be contained, this...
  3. T

    What Are the Properties of Dark Energy?

    Properties of "Dark Energy" ? Has their been any progress in discovering what properties this Dark Energy might have?
  4. B

    What is the Evidence for Dark Energy on Small Scales?

    Well, apparently we have now found new and more direct evidence for the existence of the so called 'dark energy.' I find it quite remarkable. http://www.space.com/scienceastronomy/dark_energy_030805.html
  5. M

    What is Dark Energy: The Most Common Type of Energy in the Universe

    What's dark energy? It's important to know what is the most common type of energy of the universe
  6. marcus

    Lee Smolin predicted dark energy back in 1995

    I was just reading a 1995 paper by Lee Smolin arXiv:gr-qc/9505028 "Linking topological quantum field theory and non-perturbative quantum gravity" not such a catchy title, but contains some surprising things We all know the story about how Einstein put Lambda (cosmological constant) in...
  7. Ivan Seeking

    Dark energy and the speed of light

    What implications if any does the potential existence of dark energy have on the speed of light passing through empty spaces between galactic structures [remote space where dark energy is thought by some to dominate]? How might the permittivity and permeability constants be effected?
  8. G

    Exploring Dark Energy: The Driving Force of the Universe's Expansion

    Dark Energy is now thought by some to contain 65% of the total mass of the universe. It will accelerate the expansion of the universe for some 20 million years. Then all normal matter/energy will have its constituents torn asunder resulting in a universe consisting entirely of Dark Energy...
  9. marcus

    Density of dark energy in Planck units

    this is not to argue or persuade but just to share a calculation. no answer needed unless you particularly like something about it. several websites, eg. NedWright's Cosmology tutorial have formulas for things like the critical density and the estimate that real density is very close to...
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