<p>Observations of the cosmic microwave background suggests that the universe is geometrically flat, with the total energy density of the universe equalling the critical energy density (to within very small error bars). Dark energy is essential to explain how an accelerating universe can be flat.</p>
<p>However, inventories of the amount of matter (and hence rest mass energy), taken from the dynamics of galaxies, clusters of galaxies and mapping with gravitational lensing, suggest that only about 30% of the critical density can be in the form of normal or dark matter.</p>
<p>Dark energy accounts for the rest and in an amount that agrees well with what is required to explain the accelerating expansion.</p>

CC BY-SA 4.0
2023-02-16T07:59:57.017Z
2023-02-16T09:57:33.660Z
2023-02-16T09:57:33.660Z
ProfRob
<p>In addition to its effect on the expansion history, dark energy also slows the growth of structure. The idea here is that regions with above-average density gradually become even more overdense by accreting surrounding material, and the opposite happens with regions of below-average density. When matter dominates the energy density of the universe, <em>contrasts</em> in the density grow rapidly; the fractional contrast <span class="math-container">$\delta\equiv(\rho-\bar\rho)/\bar\rho$</span> grows proportionally with the expansion factor, <span class="math-container">$\delta\propto a$</span> (in the perturbative <span class="math-container">$\delta\ll 1$</span> regime). However, dark energy slows the growth of structure. When dark energy dominates, fractional density contrasts <span class="math-container">$\delta$</span> are constant.</p>
<p>In this way, general relativity (with dark energy) predicts a tight connection between the cosmic expansion history and the growth history. Modified gravity theories that attempt to explain accelerated expansion can break this connection, which is why growth vs expansion is one of the classic tests of modified gravity (e.g. <a href="https://arxiv.org/abs/astro-ph/0507263" rel="nofollow noreferrer">Linder 2005</a>).</p>

CC BY-SA 4.0
2023-02-16T08:40:24.583Z
2023-02-16T10:02:18.563Z
2023-02-16T10:02:18.563Z
Sten
<p>Dark energy is a very interesting concept , even though it mainly explains the acceleration of expansion of spacetime fabric , it also explains the very less mass distribution of ordinary matter and dark matter because dark energy is thought to have roughly 68% of mass of the whole universe.</p>
<p>Dark energy also probably explains the (indirectly) configuration of all stars if big bang really happened , and slowing expansion at the beginning and faster expansion now (as it is accelerating). Hence it might still be a useful concept</p>

CC BY-SA 4.0
2023-02-16T03:40:11.647Z
2023-02-16T03:40:11.647Z
Naveen V
<p>This question is really hard to answer, because it is very vague.</p>
<p>Before going further, take a look at <a href="https://en.wikipedia.org/wiki/Dark_energy#Evidence_of_existence" rel="nofollow noreferrer">the observational evidence for dark energy</a>. There are five lines of evidence, which are (copy/pasting from a <a href="https://physics.stackexchange.com/questions/530639/what-is-the-evidence-that-dark-energy-exists-as-of-2020">previous question of mine on the Physics SE</a>):</p>
<ol>
<li><p><strong>Supernovae</strong>. A specific type of supernovae (type 1A) are supposed to be standard candles - that is, their luminosity is known. In turn this lets us measure distance to faraway galaxies, check how fast those galaxies are receding from us, and check if that recession speed is increasing over time.</p>
</li>
<li><p><strong>CMB</strong>. Data from the Cosmic Microwave Background indicates the universe is approximately flat. Visible matter + dark matter can account for ~30% of the energy content required to make the universe flat, leaving ~70% for something else - dark energy.</p>
</li>
<li><p><strong>BAO</strong>. Baryon acoustic oscillations act as a &quot;standard ruler&quot; that lets us measure how the Hubble constant varies with redshift (i.e. time), and see if the recession speed is increasing with time.</p>
</li>
<li><p><strong>Late-time ISW</strong>. ISW stands for integrated Sachs-Wolfe effect. The idea is that, usually, a photon that enters a potential well gains energy as it falls in and loses energy as it emerges, and gain/loss exactly cancel. If a universe is expanding in accelerated fashion, then this is untrue; the potential wells / hills are smoothed out and there is a permanent shift in the photon's energy and therefore temperature. If we see a correlation between hot and cold spots on the CMB and the locations of superclusters and voids, then it's a sign of accelerated expansion.</p>
</li>
<li><p><strong>Galaxy evolution</strong>. This uses (known) evolution of early-type galaxies as a standard clock. Once we know how long it takes for a galaxy to evolve from one state to another, as well as their redshifts, we can reconstruct how the Hubble constant varies over time, and see if the recession speed is increasing with time.</p>
</li>
</ol>
<p>This list sounds impressive, and it is, but note that <em>all</em> of the evidence here hinges on General Relativity as a frame assumption. In other words, if GR is incorrect, none of these lines of evidence hold up. Dark energy could very well not exist. This shouldn't be surprising if you think about it, since GR is the theory underpinning cosmology.</p>
<p>Then consider your question:</p>
<blockquote>
<p>Or put into other words: If a different theory (e.g. modified gravity) was able to explain the accelerated expansion and was consistent with all other fully understood aspects of cosmology, would dark energy still be a useful concept?</p>
</blockquote>
<p>If a modified theory of gravity is assumed, then dark energy might not exist. Or it still might - this is because any modified theory of gravity needs to reduce to GR in some limit (in the same way GR must reduce to Newtonian gravity in some limit). All five lines of evidence for dark energy would need to be re-examined. Some of them might hold up in the new theory, some might not. If some line of evidence holds up, then dark energy remains useful since it can (potentially) explain that line of evidence. If all of them don't hold up, then dark energy might be useless.</p>
<p>But all this depends on what the new theory is. Without details of that theory, we can't begin to answer the question. So the question &quot;would dark energy still be a useful concept?&quot; is too vague to answer.</p>

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2023-02-17T07:06:41.447Z
2023-02-17T07:06:41.447Z
Allure
<p>Dark energy is a vital part of standard cosmology and allows for an explanation of the accelerated expansion of the universe. Does it explain anything else?</p>
<p>Or put into other words:
If a different theory (e.g. modified gravity) was able to explain the accelerated expansion and was consistent with all other fully understood aspects of cosmology, would dark energy still be a useful concept?</p>

CC BY-SA 4.0
2023-02-16T17:07:47.087Z
Actually one of the leading explanations for Dark Energy *IS* a modification of one of the constant parameters of gravity under General Relativity (the "Cosmological Constant").  However this "modified gravity" has nothing to do with the "modified gravity"  being proposed as an alternative to Dark *Matter*, and we usually call it the Cosmological Constant, to distinguish it from that other (more) Modified Gravity.
RBarryYoung
<cosmology><dark-energy>
What does Dark Energy explain other than the accelerated expansion of the universe?
Liberty
# What does Dark Energy explain other than the accelerated expansion of the universe?

(Asked by: username_0 on 2023-02-15)

Dark energy is a vital part of standard cosmology and allows for an explanation of the accelerated expansion of the universe. Does it explain anything else?

Or put into other words:
If a different theory (e.g. modified gravity) was able to explain the accelerated expansion and was consistent with all other fully understood aspects of cosmology, would dark energy still be a useful concept?

**Tags:** cosmology, dark-energy

> **Comments:**
> **username_1**: Actually one of the leading explanations for Dark Energy *IS* a modification of one of the constant parameters of gravity under General Relativity (the "Cosmological Constant").  However this "modified gravity" has nothing to do with the "modified gravity"  being proposed as an alternative to Dark *Matter*, and we usually call it the Cosmological Constant, to distinguish it from that other (more) Modified Gravity.  

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## 4 answers

### Answer by username_2 on 2023-02-16. Score: 3 | Accepted answer

Observations of the cosmic microwave background suggests that the universe is geometrically flat, with the total energy density of the universe equalling the critical energy density (to within very small error bars). Dark energy is essential to explain how an accelerating universe can be flat.

However, inventories of the amount of matter (and hence rest mass energy), taken from the dynamics of galaxies, clusters of galaxies and mapping with gravitational lensing, suggest that only about 30% of the critical density can be in the form of normal or dark matter.

Dark energy accounts for the rest and in an amount that agrees well with what is required to explain the accelerating expansion.


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### Answer by username_3 on 2023-02-16. Score: 3

In addition to its effect on the expansion history, dark energy also slows the growth of structure. The idea here is that regions with above-average density gradually become even more overdense by accreting surrounding material, and the opposite happens with regions of below-average density. When matter dominates the energy density of the universe, *contrasts* in the density grow rapidly; the fractional contrast $\delta\equiv(\rho-\bar\rho)/\bar\rho$ grows proportionally with the expansion factor, $\delta\propto a$ (in the perturbative $\delta\ll 1$ regime). However, dark energy slows the growth of structure. When dark energy dominates, fractional density contrasts $\delta$ are constant.

In this way, general relativity (with dark energy) predicts a tight connection between the cosmic expansion history and the growth history. Modified gravity theories that attempt to explain accelerated expansion can break this connection, which is why growth vs expansion is one of the classic tests of modified gravity (e.g. [Linder 2005](https://arxiv.org/abs/astro-ph/0507263)).


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### Answer by username_4 on 2023-02-16. Score: 0

Dark energy is a very interesting concept , even though it mainly explains the acceleration of expansion of spacetime fabric , it also explains the very less mass distribution of ordinary matter and dark matter because dark energy is thought to have roughly 68% of mass of the whole universe.

Dark energy also probably explains the (indirectly) configuration of all stars if big bang really happened , and slowing expansion at the beginning and faster expansion now (as it is accelerating). Hence it might still be a useful concept


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### Answer by username_5 on 2023-02-17. Score: 0

This question is really hard to answer, because it is very vague.

Before going further, take a look at [the observational evidence for dark energy](https://en.wikipedia.org/wiki/Dark_energy#Evidence_of_existence). There are five lines of evidence, which are (copy/pasting from a [previous question of mine on the Physics SE](https://physics.stackexchange.com/questions/530639/what-is-the-evidence-that-dark-energy-exists-as-of-2020)):

1. **Supernovae**. A specific type of supernovae (type 1A) are supposed to be standard candles - that is, their luminosity is known. In turn this lets us measure distance to faraway galaxies, check how fast those galaxies are receding from us, and check if that recession speed is increasing over time.
2. **CMB**. Data from the Cosmic Microwave Background indicates the universe is approximately flat. Visible matter + dark matter can account for ~30% of the energy content required to make the universe flat, leaving ~70% for something else - dark energy.
3. **BAO**. Baryon acoustic oscillations act as a "standard ruler" that lets us measure how the Hubble constant varies with redshift (i.e. time), and see if the recession speed is increasing with time.
4. **Late-time ISW**. ISW stands for integrated Sachs-Wolfe effect. The idea is that, usually, a photon that enters a potential well gains energy as it falls in and loses energy as it emerges, and gain/loss exactly cancel. If a universe is expanding in accelerated fashion, then this is untrue; the potential wells / hills are smoothed out and there is a permanent shift in the photon's energy and therefore temperature. If we see a correlation between hot and cold spots on the CMB and the locations of superclusters and voids, then it's a sign of accelerated expansion.
5. **Galaxy evolution**. This uses (known) evolution of early-type galaxies as a standard clock. Once we know how long it takes for a galaxy to evolve from one state to another, as well as their redshifts, we can reconstruct how the Hubble constant varies over time, and see if the recession speed is increasing with time.

This list sounds impressive, and it is, but note that *all* of the evidence here hinges on General Relativity as a frame assumption. In other words, if GR is incorrect, none of these lines of evidence hold up. Dark energy could very well not exist. This shouldn't be surprising if you think about it, since GR is the theory underpinning cosmology.

Then consider your question:

> Or put into other words: If a different theory (e.g. modified gravity) was able to explain the accelerated expansion and was consistent with all other fully understood aspects of cosmology, would dark energy still be a useful concept?

If a modified theory of gravity is assumed, then dark energy might not exist. Or it still might - this is because any modified theory of gravity needs to reduce to GR in some limit (in the same way GR must reduce to Newtonian gravity in some limit). All five lines of evidence for dark energy would need to be re-examined. Some of them might hold up in the new theory, some might not. If some line of evidence holds up, then dark energy remains useful since it can (potentially) explain that line of evidence. If all of them don't hold up, then dark energy might be useless.

But all this depends on what the new theory is. Without details of that theory, we can't begin to answer the question. So the question "would dark energy still be a useful concept?" is too vague to answer.


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