# Dark energy Definition and 181 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. ### I How strong is dark energy relative to gravity in the voids?

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...
2. ### A Black hole mass coupled to expansion source of dark energy?

Observational evidence for cosmological coupling of black holes and its implications for an astrophysical source of dark energy Comments?
3. ### 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...
4. ### 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...
5. ### 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...
6. ### 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...
7. ### 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...
8. ### 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...
9. 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...
10. ### B The Relationship of Gravity and Dark Energy

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...
11. ### I Universe Expansion

So the universe is expanding, and galaxies are getting farther apart from one another on average. Does this motion count the same as ordinary motion, in that if a galaxy is being expanded away from us at 0.5c, that clocks in that galaxy would appear to tick slower at 0.866 the rate of clocks here?
12. ### B Question concerning dark energy

Was all dark energy "born" at the moment of the Big Bang? Or does it "spring" into existence continually?
13. ### B How can string theory be proved

The fundamental building blocks of the universe is thought of super strings, if proved can solve the mysteries of the universe but if proved than how? And how can it solve the mysteries of dark energy &dark matter and black holes?
14. ### I What will eventually happen to all EM waves / photons?

I'm not an expert in this matter, and at best only aware of some superficial facts and a layman's understanding of them. So please forgive me for any ignorant mistakes in my thoughts, and kindly point them out to me. Going by the Lambda-CDM model, the expansion of the Universe will eventually...
15. ### Question About Conservation of Energy, the Cosmological Constant and Dark Energy

I am confused about the cosmological constant and dark energy. In the most accepted theory, energy is created as the vacuum of space expands. This contravenes the conservation of energy. The law of conservation of energy does not hold in curved spacetime but isn't our universe flat spacetime ...
16. ### B Quantum entanglement mechanics

I should clarify, I am a linux administrator by trade and I have no physics or scientific background outside of working at a medical institute, So if my ideas are off and seem wild and misguided, please forgive my ignorance, and while you are at it be grateful for a fresh perspective. (joke) So...
17. ### B Questions regarding energy in the universe

As I am watching the current season of How the Universe Works, I am a bit confused when listening to physicists talking about energy as if it was a thing. The way I understand it, energy doesn't exist by itself, but is rather the result of interactions involving matter. Energy is released...
18. ### I Variable dark energy

If dark energy density were to vary with time, in the equation of state w = p/ρ, would p remain constant and only ρ vary?
19. ### A Unification of dark energy and dark matter

Dr James Farnes of Oxford's e-Research Centre recently proposed a new model which unifies dark energy and dark matter into a single phenomenon - a fluid which possesses "negative mass". https://news.sky.com/story/mystery-of-dark-matter-may-have-been-solved-by-oxford-scientists-11572089 The paper...
20. ### I Experiments on dark energy

I’m thinking about modifying some force related equations to include dark energy in them for an essay on physics. My reasoning is that if it can separate galaxies the objects in our day to day lives must also have a small tendency to move apart. To do so, I have to dispose of an already done...
21. ### B Galaxy Expansion or Attraction?

First of all, sorry for my naive question here which likely doesn't make sense. The universe is currently expanding with galaxies receding from each other at increasingly faster pace. Is it possible that the universe is a closed hypersphere system, where galaxies are actually not actively...
22. ### I Is the expansion actually accelerating? Or does it just appear so?

I pulled this quote from an article on the Hubble Constant: "...for example, if the Hubble Constant was determined to be 50 km/s/Mpc, a galaxy at 10 Mpc, would have a redshift corresponding to a radial velocity of 500 km/s." In this illustration provided from the article, if this was the...
23. ### A Interpreting hubble data

this graph: http://www.pnas.org/content/pnas/112/11/3173/F1.large.jpg?width=800&height=600&carousel=1 from: http://www.pnas.org/content/112/11/3173 can be found on many sites. the origin is D=0 and t=0. cosmology claims the universe is accelerating over time. the graph shows acceleration over...
24. ### I Why does the expanding universe redshift light?

How does the expansion of space cause light to lose energy?
25. ### A Generalized free fields as dark matter?

@vanhees71 reminds us that which suggests something I've wondered about for a while, whether dark matter might be adequately modeled by generalized free fields, which do not have asymptotic free states. Ray Streater, in Rep. Prog. Phys. 1975 38 771-846, "Outline of axiomatic relativistic...
26. ### I Black Holes and Dark Energy

Ordinarily a black hole’s Schwarzschild radius is linearly proportional to its mass. However, wouldn’t there be a deviation from this rule for extremely large black holes? Suppose we assume dark energy is due to a cosmological constant, whose value is the same everywhere (including inside the...
27. ### The Big Picture: From the Big Bang to the Meaning of Life - with Sean Carroll

The talk, given at the Royal Institution in October 2016, will take us on a breath-taking journey from the origin of the Universe, through the evolution of life and consciousness, to the eternal question of what it all really means.
28. ### Q&A The Big Picture - with Sean Carroll

Dr Sean Carroll is an astrophysicist at the California Institute of Technology. He has written a variety of popular science books along with textbooks and has long been interested in the biggest questions in astronomy: Where does probability come from? How does time work? What is dark matter?
29. ### I Stress–energy pseudotensor of gravitation field for DE

Suppose we have Einstein equation for *Universe free of matter* in form $$G_{ik} = \chi T_{ik},$$ where the cosmological constant $\Lambda$ is transferred to the RHS of equation and written in the form of stress–energy tensor of Dark Energy...
30. ### I If Dark Matter and Dark Energy exist, what produces them?

My query is really to find out what everybody thinks about 'empty space'. Does dark matter/energy exist and if so, it fills all voids in the universe? Where does it come from? Is it produced by some force? Without DM/DE, does space even exist? Say the supernovas, the collision of neutron stars...
31. ### I Dark Matter and Dark Energy Conundrum

Question 1: Do the calculations/models for galactic structure, the basis for the missing 6x mass problem that created the notion of "Dark Matter," include any factor/variable or set of variables that account for space-time dilation due to relativistic effects caused by the super-massive black...
32. ### I Dark matter and energy may not exist?

https://www.sciencedaily.com/releases/2017/11/171122113013.htm How plausible does this seem?
33. ### B What is dark energy in the fabric of space-time?

I know that according to Einstein's theory of relativity, space-time is like a fabric which can be pliable. Gravity is the shape, or the warping of that fabric. In this analogy, what would dark energy (the unknown form of energy that is causing the universe to expand) be?
34. ### I Can gravity be negative?

Now, I know that it is theoretically possible for negative mass to exist and for negative energy to exist. But any gravity would act on all objects and all energy, no matter if that mass and energy is positive or negative. So negative gravity would for example, pull a person upwards. Now yes, I...
35. ### What should I learn?

Hello, I don't know if this is the right place to post this topic, I could not figure out the right one. I have recently finished my Masters in Condensed Matter. Now I want to follow a PhD where I can work/research on the dynamics of the Universe especially on dark energy, modified gravity...
36. ### I Expanding universe and quantum wavefunctions

In a dark energy dominated universe, it seems that all the particles get away from each other and that the final state will be one with one or zero particles per horizon. This sounds very intuitive, but it is based on classical physics and GR. Particles have wavefunctions and this is whar...
37. ### I Less gravity vs. dark energy

One way to get the universe to expand is with dark energy that pulls at the matter of the galaxy separating it or equivalently for space-time to not be perfectly flat. An alternative, in principle, would be for the gravitational pull between objects like galaxies and galactic clusters to be...
38. I

### B Can this be another reason for dark energy?

Suppose the initial force and energy bursting out during big bang was very huge . The matter just after big bang would have been very close , so the gravitational attraction would have been very high , which would have caused the acceleration produced by big bang to decrease . As time increased...
39. ### B Dark matter and energy creation

If space is expanding and at the same time speeding up, something must be driving it. It also appears to be self perpetuating, as something cannot continue to expand, speed up and backfill at the same time? Could the driving force be dark matter/energy?
40. ### B Are Black Holes linked to our Universe & Dark Energy/Matter?

After a quick and not fruitful Google Search I leave this question in better minds than my own. Q- Has there been any discussion linking the start of our Universe with a black hole? <Reason behind Question> I feel as if certain phenomenon seems quite similar to one another. A) star dies...
41. ### I Can a finite universe end in heat death?

Hello guys, I was reading some models about the topology and size of the universe (always a controversial topic), then a question came to my mind. It is predicted that our universe will expand until it reaches heat death. Can a closed, finite universe also reach heat death and be described by...
42. ### Stargazing Expanding universe

Hi everyone, I was thinking of a new theory in physics regarding dark energy, black holes and the accelerating expanding universe. What if most of the matter created by the Big Bang was pushed at the edge of the new created space/time like explosions, the matter is at the edge of the explosion...
43. ### I Where did the Dark Energy (or similar mechanism) come from?

Hi all! I'm new to this forum and, as a matter of fact I only recently rediscovered my interest for physics. I am starting to catch up, yet I find that Wikipedia sometimes falls sort for some basic questions, and I was hopping that this community could gently help me understand some concepts...
44. ### I Studying Dark Energy

Hey guys, I was wandering that, if dark matter and dark energy makes up 68 and 27 percentage of the the universe, then is it justifiable to study these in terms of models of the known parts of the universe which comprises of less than 5 percentage of the universe, also it is clearly evident...
45. ### I How can Andromeda collide with the Milky Way?

If everything in the universe is moving away from everything else in the universe, how can Andromeda collide in the distant future with the Milky Way?
46. ### I Is vacuum energy infinite? If it is, how and why?

https://en.m.wikipedia.org/wiki/Vacuum_energy And it is this type of energy dark energy or a form of it?
47. ### A Exact solutions of quintessence models of dark energy

Hi everyone, I got the basic ideas quintessence (minimally coupled) and derived the KG equation for scalar field: $$\ddot{\phi} + 3 H \dot{\phi} + \frac{\partial V(\phi)}{\partial \phi} = 0$$ where $$H=\frac{\dot{a}}{a}$$ and $\phi$ is the scalar field. There are various models depending...
48. ### I View from the Void

Imagine you could jump in space/time from Earth to a point in deep intergalactic space smack dab in the middle of a void, those regions where dark energies are at their strongest. What would the view be like? Of course it's not total dark. You still have the light of distant galaxies. Would the...
49. ### Drawing dark energy from space as a power source

Hi. I was wondering if someone could tell me whether there is any reason to believe that one might eventually (distant future) draw power from dark energy or dark matter as a power source for spaceships. Thanks.
50. ### A Dark Matter Energy Content

What is meant by the statement by Harvard scientist Lisa Randall that dark matter carries five times the energy of ordinary matter? Does E=mc2 not apply to dark matter? Does the statement refer to ordinary energy or to dark energy?