What is Dark energy: Definition and 558 Discussions

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. K

    I Dark energy and conservation of energy

    According to this Sean Carroll article, “https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ “ And other threads in here, depending on your definition of energy, dark energy does not violate conservation of energy, my questions is if this is true regardless of the type...
  2. Filip Larsen

    B Dark energy might not be constant after all

    https://arstechnica.com/science/2024/04/dark-energy-might-not-be-constant-after-all/ https://www.desi.lbl.gov/2024/04/04/first-cosmology-results-from-desi-most-precise-measurement-of-the-expanding-universe/ Interesting preliminary indications from DESI (which I did not know about until now)...
  3. S

    I Voids, dark energy and tidal forces...?

    I would like to ask you some questions I have about some interesting work I was reading (https://arxiv.org/abs/1205.4238 & https://academic.oup.com/mnras/article/527/4/11962/7457744) where the authors analysed the effects of dark energy in the shape and evolution of voids Apparently, they...
  4. mister i

    B Could Dark Energy be an (unknown) dark force acting at great distances?

    I am not a physicist but if someone can answer this question: It seems that there is a dark energy in the universe. Could it be associated with an (unknown) dark force acting at great distances?: I don't quite understand that when we talk about the approach of galaxies we talk in terms of force...
  5. I

    B Questions about dark matter/energy

    Does the astrophysics community really think dark energy/matter is real, or that we just haven't figured out the actual physics of what is happening and this just makes it work out (to our current understanding).
  6. H

    I Does dark-energy require a source of energy?

    In the Lamda-CDM model of cosmology, dark-energy is explained by a Lamda like curvature of space-time. In this description, space-time is curved in such a way as to cause a gentle outward repulsive force on the large scale, expanding all of the universe over time. This is one cause of the...
  7. T

    I Is there any mechanism possible for the Big Brake?

    https://arxiv.org/abs/0705.1688 https://ui.adsabs.harvard.edu/abs/2014IJMPD..2350054P/abstract https://www.scientificamerican.com/article/time-s-end-will-be-the-end-of-endings/ According to these references they suggest that dark energy can suddenly change sign and causing the universe expansion...
  8. S

    I Avoiding structure collapse due to dark energy?

    I had a question about this paper (https://arxiv.org/abs/1401.3742) There, the authors indicate that dark energy competes against gravity in oversdensities and can slow down or even prevent their collapse. I have a simple question about this: Galaxies will in principle evaporate their outer...
  9. S

    I Dark energy and the kinematic Sunyaev-Zeldovich effect?

    I was reading this interesting article about possible effects of dark energy in the formation of large-scale structures which should have an impact on the Sunyaev-Zeldovich effect ("Dark energy imprints on the kinematic Sunyaev-Zel'dovich signal" (https://arxiv.org/abs/1309.1163))There, the...
  10. PhanthomJay

    I Hubble Constant, Dark Energy and the Expanding Universe

    Why was the Hubble constant assumed to be decreasing and slowing down (decelerating) the expansion rate of the Universe, while at the same time Dark Energy is presumably accelerating the expansion? And to thicken the plot. recent news from NASA indicates that the Hubble constant is now...
  11. S

    I More extended equilibrium configurations due to dark energy?

    Dark Energy puts a constrain on the size of overdensities (like clusters and superclusters of galaxies) and their growth. A higher Dark Energy density would reduce the radius of the zone where matter would be gravitationally bound, because more Dark Energy density would mean that objects would...
  12. L

    I Is Dark Matter the Key to Understanding the Pre-Visible Universe?

    Regarding the following statement: "It would appear Dark matter and dark energy are what existed BEFORE the visible universe as we know it came into being. When you look at galaxies what you're seeing could be described as "displacements" in dark matter where ordinary matter formed and took...
  13. S

    I Dark Energy contribution to plasma temperature in galaxy clusters?

    I have a question about this work called "Dark energy and key physical parameters of clusters of galaxies"*There, towards the end, the authors talk about the isothermal velocities and tempreature parameters of the gas and particles circulating between galaxies in clusters. In particular they...
  14. S

    I Tidal effects of galaxies orbiting one another with dark energy?

    I recently found a question in a physics discussion site [1] about whether there was a "distance" between two galaxies where both the gravitational force and the influence of dark energy would be balanced. The answers and comments seem to indicate that there is indeed such a "radius" around a...
  15. S

    I Dark energy contributing to, or modifying, mass estimates?

    I have found some papers (like this one: https://www.aanda.org/articles/aa/full_html/2009/45/aa12762-09/aa12762-09.html) which say that dark energy increases the potential energy in a system of a quasi-stationary gravitationally bound many-body system. It also says that because of this, the...
  16. M

    A Vavryčuk's conformal FLRW metric as an alternative to dark energy and dark matter

    Václav Vavryčuk has written two articles on replacing the standard FLRW metric in cosmology with what he calls a "conformal FLRW metric", which he claims explains astrophysical and cosmological phenomena traditionally attributed to dark energy and dark matter/MOND, such as the dimming of type 1a...
  17. M

    I Testing for dark energy using the Local Group

    A group of astrophysicists have developed a method to measure the value of the cosmological constant using the dynamics of the Local Group: Constraining Dark Energy from Local Group dynamics
  18. Ranku

    I Dark energy density proportion

    Is there a way to independently determine the proportion of dark energy density to total energy density of the universe apart from using 1 -(Ωmatter+Ωdark matter )?
  19. H

    I Dark Energy Strength in Great Voids of Galaxies

    Assuming dark energy is fairly, uniformly distributed through out the cosmos, how strong is it, or how much energy is associate with it, out in the deepest, emptiest voids in space? I'm specificlaly refering to the great voids in between the great walls of galaxy clusters. I'm making the...
  20. Bandersnatch

    A Black hole mass coupled to expansion -- astrophysical source of dark energy?

    Observational evidence for cosmological coupling of black holes and its implications for an astrophysical source of dark energy Comments?
  21. ForTheLoveOfPhysics

    Currently spending some time outside observable universe…

    It asked for a snappy title so don’t judge! I’m here for the physics not the BS. And I’ve checked my Dunning-Krugerness & Confirmation Bias at the door! In a way I’ve always been fascinated by physics and the biggest of questions but recently I thought I’d jump in and actually participate as...
  22. mpross

    B Dark Energy, Dark Matter and the Speed of Light

    Quote from NASA: My understanding of dark energy is based on NASA's report: https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy; were NASA state as follows: "It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest - everything on...
  23. S

    I Sachs-Wolfe effect in a Dark Energy universe?

    The integrated Sachs-Wolfe effect occurs when a photon goes through a gravitational potential that changes due to spacetime expansion (presumably caused by dark energy). For that reason, a photon going through a gravitational well would gain energy (blueshift) when entering and it would lose...
  24. S

    I Mining the Universe's expansion?

    There is an article written by astrophysicist Edward Harrison [1] which defends that energy could be extracted from attaching an imaginary cosmologically long string to a receding object from us in an expanding universe. He says that the energy extracted is potentially limited (in decelerating...
  25. S

    I Receding movement of a stopped object in an accelerating universe?

    Imagine we attach an imaginary cosmological scale rope to an object that is very far away from us. Before attaching the string, the object would be receding from us due to spacetime expansion. After attaching it, tension would form in the string and we would eventually stop the object. After...
  26. Bob Walance

    B Could the source of dark energy be our parent black hole feeding?

    About a year ago, I heard Leonard Susskind discussing how entangled black holes could create spacetime. As I was listening to Prof. Susskind describe the mechanisms for creating entangled black holes, and how these black holes might create their own spacetime, it occurred to me that if we were...
  27. D

    I Questions about Negative Pressure and Vacuum Energy

    Currently, dark energy is described as a being that exerts a negative pressure while having a positive energy density. {\rho _\Lambda } + 3{P_\Lambda } = {\rho _\Lambda } + 3( - {\rho _\Lambda }) = - 2{\rho _\Lambda } However, there seems to be a problem with the negative pressure assertion...
  28. Adwit

    From General Relativity to Dark Energy

    If we insert the values from (2.9), (2.10), (2.11) into (2.5) & (2.6) how can we get (2.13) & (2.14) ?? I need to see the calculations step by step.
  29. K

    I Gravitational Field with Dark Energy: Observable Effects?

    since the cosmological constant observed is that there is a small amount of energy in empty space, and in general relativity anytime there is energy there is curvature and therefore gravity, how to calculate gravitational field with dark energy and does it have any observable effects on matter...
  30. RogerWaters

    B How do the candidates for Dark Energy 'work'?

    When it comes to explaining some phenomena, I think in terms of mechanisms whereas physics explains through mathematics. With this in mind, I'm trying to compare and contrast the differences between the candidate explanations for dark energy, as nicely summarised on the NASA website...
  31. RogerWaters

    B Does dark energy or cosmic inflation explain flatness?

    As I understand, the main theoretical virtue of Guth's inflation hypothesis is that it explains a bunch of otherwise hard-to-account-for phenomena under the standard big bang model without inflation: the Horizon Problem, the Flatness problem, the Monopole problem, and also the problem of how...
  32. jeffinbath

    B Can the "Dark Energy" concept be wrong?

    With the latest view that free space has virtual particles constantly popping in and out of existence, is it now rational to argue that light can electromagnetically travel through free space for ever and ever without the slightest energy loss? Edwin Hubble found that the general red shift of...
  33. R

    A Hubble Tension and Cosmic Acceleration: A measurement artifact?

    By analyzing 91,742 reported extra-galactic distances and their one sigma uncertainties for 14,560 galaxies, it was found that pairs of reported extra-galactic distances of the same galaxy differ from each other by 2.07 the reported uncertainties on average. In my opinion, this indicates that...
  34. Melbourne Guy

    B XENON1T and possible dark energy detection

    I can't see that this recent news has been discussed on PF, but the XENON1T dark matter detection collaboration is now suggesting that it has instead detected dark energy. They openly note speculation about what may have caused these results in the paper, but I'm more wondering if it adds weight...
  35. S

    A New Dark Energy Map Challenges Einstein's Theory

    A few days ago a new dark energy map was published, which some claim "strays from Einstein's theory of relativity". To quote from the BBC News website: "Dr Niall Jeffrey, of École Normale Supérieure, in Paris, who pieced the map together, said that the result posed a "real problem" for...
  36. PainterGuy

    I Dark energy is intrinsic to the space itself

    Hi, I was reading the following article. Source: https://www.forbes.com/sites/startswithabang/2019/01/31/dark-energy-may-not-be-a-constant-which-would-lead-to-a-revolution-in-physics How come the density of dark energy remains constant? Yes, if it's intrinsic to the space itself and more...
  37. R

    I Dark Energy: Exploring Negative Pressure's Role in Universe Expansion

    In an article published in Scientific American, authors Adam Riess and Mario Livio wrote: Why negative pressure? I would think the pressure would be positive, thus making gravity repulsive.
  38. Y

    Physical Valentine's Day -- quotes from particle physics magazine

    Here are the quotes https://www.symmetrymagazine.org/article/show-your-affection-with-physics-valentines , I find them funny except for the last one is hurtful, I shouldn't say such a thing for someone on a valentine's day because they would care about the valentine's day, personally I find it...
  39. Firehanger

    I Is dark energy pushing or is it a mirror concept of gravity?

    Hey peeps, Ok, so I need some help here. I've just been watching some videos explaining the existence of dark energy and dark matter - something which has always fascinated me. These videos pose the theory that for the first 9 billion years of the universe dark matter ruled but, for the last...
  40. Quotidian

    B How can dark energy comprise 74% of the Universe?

    I have often read that dark matter and dark energy are now thought to comprise 94% of the total mass of the Universe. Just now I was reading an article which says that dark energy comprises 74% of the total. So, I'm confused about how energy can comprise mass. Dark matter, I can understand -...
  41. Alain De Vos

    I Change law of gravitation to remove dark energy and dark matter

    The speed of star on the outer of are solar system is not according to the visible mass. Change the law of gravitation F = 1/r^2 to fix this. Galaxies far away are moving away with increasing speed. Change the law of gravitation F = 1/r^2 to fix this, include a repulsive force to fix. Would this...
  42. Dennis Plews

    B Gravity & Dark Energy: Newton, Einstein & Beyond

    What gravity is, as pointed out by Newton and Einstein, is still a mystery. Adding to that conundrum is dark energy which continues to inflate the cosmos. Both Newton and Einstein, and every one else as far as I am able to discern, assume that gravity works by drawing massive bodies together...
  43. WhatIsGravity

    I Exploring the Variability of Big G & Dark Energy

    I'm wondering if any of the physics Jedi out there might know of any credible papers that suggest big G and/or the cosmological (dark energy) aren't constant through time?
  44. Staticboson

    I Dark Energy as repulsive gravity

    Mass has an effect on the surrounding space which causes two massive objects within the extent of this effect to fall towards each other by crossing the space between them. There is a point source and a direction for the field. The effect of Dark energy causes an expansion of space itself and...
  45. Cerenkov

    B Other lines of evidence for Dark Energy? (Besides supernova data)

    Hello. My current understanding (please correct, if wrong) is that the expansion of the universe is observed to be accelerating, rather than coasting or slowing down. The tentative cause of this acceleration has been given the placeholder name of 'Dark Energy'. One line of evidence for this...
  46. N

    B Is Dark Energy a Continuous Creation Theory?

    Moderator's note: Spun off from another thread due to topic change. Isn't Dark Energy a continuous creation theory?
  47. .Scott

    B Debunking Dark Energy Theory: New Evidence Challenges Estimated Age of Universe

    Here is a link to the Phys.org article: New evidence shows that the key assumption made in the discovery of dark energy is in error Is this going to affect the estimated age of the universe?