<p>Dark energy's influence on cosmic expansion isn't a new force. It's just gravity. Calling dark energy a force in the context of cosmology would be like calling the Sun a force in the context of the Solar System.</p>
<p>That said, it's possible that accelerated expansion could be caused by a fifth force <em>instead of dark energy</em>. See for example <a href="https://doi.org/10.1146/annurev-nucl-102115-044553" rel="noreferrer">this review article</a>.</p>

CC BY-SA 4.0
2023-09-11T00:22:19.380Z
2023-09-11T00:22:19.380Z
Sten
<h1>Dark Energy is a scalar field, whereas forces are vector interactions</h1>
<p>As far as we know, we are sure that dark energy, or more accurately, the expansion of space-time, is a scalar quantity. That is, dark energy has no direction or anything, just magnitude, it just causes space-time to expand. Meaning that, there isn't actually any direction in which dark energy acts. You can't say, for e.g. that &quot;space-time is expanding away from us&quot; or that &quot;dark energy pushes galaxies away from us&quot;. It is simply <strong>&quot;Spacetime is expanding&quot; and there is no actual direction of the expansion.</strong></p>
<p>If dark energy were indeed directional, then we wouldn't be observing a uniform redshift all around us. Instead we would be looking at both a <strong>cosmological blueshift</strong> in the direction of where our galaxy is moving towards, and a redshift in the direction where our galaxy is moving away from. Our sky would look dark red in half, whereas in the other direction, a bright blue would dominate the other.</p>
<p>But that's not the case in our universe. We observe a uniform redshift all around where we see. And there are only a handful of blueshifted galaxies and even that is not due to dark energy, but in the rare case of a galaxy actually moving towards us. Except for a handful of the galaxies, the rest of the galaxies seem to be &quot;moving&quot; away from us. So no, dark energy is not a force.</p>
<p>A better way to visualise this is to imagine the observable universe, as a &quot;sphere&quot; made up of a countless number of &quot;cubes&quot;, each representing a pocket of spacetime with uniform dimensions. Now, expand this. What happens?</p>
<p>When the sphere expanded, you did not get any new cubes on the surface of the sphere. Instead, the individual &quot;cubes&quot; themselves expanded in increments, albeit a miniscule amount. The reason why we cannot perceive this expansion in spacetime, is because of the fact that each of these &quot;cubes&quot; themselves, expanded by a miniscule amount. So on a local scale, the observer perceives nothing at all.</p>
<p>However, the &quot;sphere&quot; that is representing the observable universe, contains a gargantuan amount of these &quot;cubes&quot;. And when you calculate the total expansion of all the &quot;cubes&quot;, i.e. adding the increments of all the &quot;cubes&quot; present in the &quot;sphere&quot;, you end up with a gargantuan volume. This volume is the amount that the entire &quot;sphere&quot; expanded by, or the overall expansion that occured.</p>
<p>You can also use the raisin-bread model to visualise the expansion of spacetime:<a href="https://i.sstatic.net/mDreG.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/mDreG.gif" alt="enter image description here" /></a></p>
<p>As you can see, after baking the bread, you haven't actually added any bread to the existing bread already. The bread itself has expanded, and the raisins in it, which symbolise galaxies, haven't budged an inch at all. The raisins haven't moved at all, the expansion of the bread has simply increased the spacing between each raisin, creating the apparent &quot;movement&quot; of the raisins from each other.</p>
<p>Similarly, as space-time expands, dark energy doesn't &quot;push&quot; the galaxies or something like that, the spacing between each galaxy has increased. An observer perceives the galaxies as in &quot;motion&quot;, while in reality, the galaxies themselves haven't budged an inch at all, the spacing has just increased between them.</p>
<p>Also, we do know by the existence of cosmological redshift, that some galaxies far away from us are &quot;moving&quot; away from us faster than the speed of light. (Again, they aren't really moving, it is the expansion of spacetime causing this)</p>
<p><a href="https://physics.stackexchange.com/questions/438781/what-happens-if-we-exert-more-force-on-an-object-which-would-be-travelling-at-ne#:%7E:text=If%20you%20apply%20a%20force,exceed%2C%20the%20speed%20of%20light.">No force</a> is sufficient to move any object beyond the speed of light, which means that if dark energy, which is the key driver of the space-time expansion, were indeed a 5th fundamental force, then, the galaxies which would be &quot;moving&quot; would violate causality! Thankfully, this is not the case. We now know that dark energy is not a force, but instead a scalar field that is accelerating the expansion of space. General relativity doesn't prohibit space-time itself from expanding faster than the speed of light, which means, the apparently FTL expansion of space-time isn't exactly violating the laws of physics, since the galaxies aren't moving themselves, it's merely the spacing between the galaxies that's increasing, and that is not prohibited by relativity, either special or general.</p>
<h3>Now another reason, if you still aren't convinced, why dark energy is not a 5th force:</h3>
<p>The more general and scientific definition of a fundamental force, or more accurately, a &quot;fundamental interaction&quot; is a &quot;interaction mediated by a gauge/vector boson&quot;, excluding gravity, which doesn't have its own boson carrier.</p>
<blockquote class="spoiler">
<p> (Gravity is not a proper force, it is caused by the distortion of
 spacetime by a massive body, but for all purposes, it is regarded as a
 force, because it has a direction as well, which is, &quot;Towards the body&quot;)</p>
</blockquote>
<p>The reason why they are &quot;vector bosons&quot;, because they have a direction in which they go. Simply, taking for example a photon (A massless vector boson, that mediates the electromagnetic force, more accurately, the stuff that composes light). If it is moving in the direction of an observer, you cannot just say, &quot;The photon is moving&quot;. You'll have to specify it's direction in which it's moving, which in this case is &quot;towards the observer&quot;. Similarly, gluons also hold together quarks to form hadrons and baryons, which in this case, the direction is &quot;towards the quarks&quot;. Also gravity, which doesn't have any force carrier though (ignoring the hypothetical graviton), has a direction. Since objects with mass exert the force towards themselves, gravity also has a direction, which is &quot;towards the objects&quot;. For the weak force, which is responsible for radioactive decay via beta radiation, and neutrino flavor changing, also has a direction, and it varies in each case. For example, an electron caused by beta decay of an isotope, via the weak force, moves away from the decaying nucleus. Similarly, W and Z bosons, which are the weak force mediators, are &quot;exchanged&quot; between particles, to create an &quot;attractive/repulsive weak force&quot;, and they are also responsible for beta decay of particles like neutrons into a proton, electron and an electron antineutrino, quark transformations and so on.</p>
<p>Dark energy, sadly, has no such force carrier boson of its own. Gravitons, hypothetical bosons that mediate gravity, might be likely to exist (in the unlikely case that gravity is not basically spacetime curvature around a massive body), but alas, dark energy has no such boson which mediates it. We do know that dark energy is indeed a scalar field, but if it indeed is a force, then it must be a force that works like gravity (not the graviton type, the spacetime curvature type). But yet, we don't have a good understanding of what dark energy exactly is, aside from the fact that we know that it is a scalar field that is responsible for the expansion of space-time.</p>
<p>There are some speculations about a scalar boson, similar to the Higgs boson, called the <a href="https://en.wikipedia.org/wiki/Chameleon_particle" rel="nofollow noreferrer">chameleon particle</a>, which could be a carrier of dark energy, but then the theory is extremely foggy. So, I guess we won't be looking at dark energy chameleons soon.</p>
<p>Another point of fact to look at, most crucially: Dark energy is pathetically weak at even municipal scales. In fact, the <a href="https://en.wikipedia.org/wiki/Dark_energy#:%7E:text=Dark%20energy%27s%20density%20is%20very%20low%3A%206%C3%9710%E2%88%9210%C2%A0J/m3%20(%E2%89%887%C3%9710%E2%88%9230%C2%A0g/cm3)%2C%20much%20less%20than%20the%20density%20of%20ordinary%20matter%20or%20dark%20matter%20within%20galaxies.%20However%2C%20it%20dominates%20the%20universe%27s%20mass%E2%80%93energy%20content%20because%20it%20is%20uniform%20across%20space.%5B9%5D%5B10%5D%5B11%5D" rel="nofollow noreferrer">energy density</a> of dark energy is just <span class="math-container">$6×10^{10}J/m^3$</span>, which if converted to matter density via <span class="math-container">$E=mc^2$</span>, works out to be a pathetic <span class="math-container">$7×10^{−30} g/cm^3$</span>. If you want to measure the gravitational constant deep in a mine, forget it. You wouldn't even detect a 0.000001% deviation in value. The 2-3% value deviation would most likely be a bungle-up of the instruments used to measure the constant.</p>
<p>And even if there is a fifth fundamental force, then it most likely won't be linked to dark energy.</p>
<p>So, TLDR; No, Dark energy is not a 5th fundamental force due to the reasons above.</p>
<blockquote class="spoiler">
<p> I have read the rules of the Stack Exchange website, and there is nothing wrong with answering my own question.</p>
</blockquote>

CC BY-SA 4.0
2023-09-10T10:39:03.497Z
What's this with asking your question and answering it in the same minute? Why don't you answer on the existing question you obviously link?
planetmaker
CC BY-SA 4.0
2023-09-10T10:53:27.143Z
So, since dark energy is just the absence of gravity, like gravity bends spacetime whereas cosmological constant is the hump of spacetime, would you consider it as a avatar of gravity itself, as both gravity and dark energy lie on spacetime itself?
Arjun
CC BY-SA 4.0
2023-09-10T13:35:07.103Z
@Arjun No, dark energy is not the absence of gravity. Although it might look like anti-gravity or something, it really is just a scalar field, that causes the expansion of spacetime.
Arcturus
CC BY-SA 4.0
2023-09-10T13:36:54.097Z
@planetmaker https://stackoverflow.com/help/self-answer#:~:text=Yes!%20Stack%20Exchange,Stack%20Exchange%20site.
Arcturus
CC BY-SA 4.0
2023-09-11T00:27:31.157Z
This answer is misconceived in several ways. For one, a scalar can mediate a force, e.g. the [Yukawa interaction](https://en.wikipedia.org/wiki/Yukawa_interaction).
Sten
CC BY-SA 4.0
2023-09-11T02:52:42.597Z
Some of the other reasons this answer makes no sense: (1) Scalar field is just one class of models of dark energy. We don't have any indication of what dark energy is below the effective fluid description. (2) A field being a vector, in the context of quantum field theory, has nothing to do with whether its particle moves in a direction. (3) Existence of gravitons isn't opposed to gravity being curvature of spacetime. (4) Expansion of space is not different from objects moving apart. (5) Dark energy does "push" galaxies apart (gravitationally).
Sten
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2023-09-11T12:40:02.337Z
Yer raisins haz moved!
Russell McMahon
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2023-09-11T16:11:44.530Z
@Sten Dark energy is a scalar field. The cosmological constant, which is regarded as the most probable candidate for dark energy (despite being off by a factor of $10^{120}$) is a scalar field, see https://www.google.com/search?q=cosmological+constant+scalar+or+not&oq=cosmological+constant+scalar+or+not&aqs=chrome..69i57j69i59l3.6338j0j7&sourceid=chrome&ie=UTF-8#:~:text=In%20cosmology%20we%20are%20already%20used%20to%20dealing%20with%20two%20examples%20of%20scalar%20fields%20%2D%20the%20cosmological%20constant%20and%20the%20inflaton%20field%20(or%20fields)%20that%20drive%20inflation.
Arcturus
CC BY-SA 4.0
2023-09-11T16:14:14.790Z
(cont'd) The question asked for a fundamental force. The Yukawa Interaction is actually the coupling of particle fields like the electron field and quark field, to the Higgs Field, which for all purposes, cannot be regarded as a fundamental force.
Arcturus
CC BY-SA 4.0
2023-09-11T16:23:17.093Z
@FuriousArcturus The cosmological constant is a [Lorentz scalar](https://en.wikipedia.org/wiki/Lorentz_scalar), not a scalar field in the quantum field theory sense. [Quintessence](https://en.wikipedia.org/wiki/Quintessence_(physics)) is an example of a dark energy candidate that is a scalar field.
Sten
CC BY-SA 4.0
2023-09-11T16:25:09.677Z
@FuriousArcturus The Yukawa interaction doesn't refer to a specific interaction in the Standard Model, it's just a theoretical interaction mediated by a scalar. The point is that a scalar field *can* mediate a force.
Sten
CC BY-SA 4.0
2023-12-30T15:19:04.507Z
Also, the CMB doesn't look isotropic relative to Earth. We have to subtract the proper motions of Earth, the Sun, and the Milky Way to see that in the CMB frame it's almost isotropic (to ~1 part per 100,000). See the final diagram here: https://physics.stackexchange.com/a/411082/123208
PM 2Ring
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2023-09-10T10:21:50.553Z
2023-12-30T12:51:56.843Z
2023-12-30T12:51:56.843Z
Arcturus
<p>So I have recently been researching for multiple articles about fundamental forces, currently there are just 4 forces:</p>
<ul>
<li>Gravity</li>
<li>Electromagnetism</li>
<li>Strong nuclear force</li>
<li>Weak nuclear force</li>
</ul>
<p>So far there have been no signs of a 5th fundamental force, as the results have been extremely obscure, with the exception of the <a href="https://en.m.wikipedia.org/wiki/X17_particle" rel="nofollow noreferrer">X17 boson</a> and the <a href="https://en.wikipedia.org/wiki/Anomalous_magnetic_dipole_moment#Muon:%7E:text=The%20anomalous%20magnetic%20moment%20of%20the%20muon,the%20electron%E2%80%93positron%20annihilation%20experiments.%5B15%5D" rel="nofollow noreferrer">muon magnetic dipole moment anomaly</a>. Most proposals vary from a force that either has an infinite or finite range. In the case of finite range, the range of the fifth could possibly vary from a few milimeters, municipal scales or vast cosmological scales spanning billions of light years. There indeed have been some observations that seem to support the existence of a repulsive fifth force:</p>
<blockquote>
<p>Australian researchers, attempting to measure the gravitational
constant deep in a mine shaft, found a discrepancy between the
predicted and measured value, with the measured value being two
percent too small. They concluded that the results may be explained by
a repulsive fifth force with a range from a few centimetres to a
kilometre. <a href="https://en.m.wikipedia.org/wiki/Fifth_force#:%7E:text=Australian%20researchers%2C%20attempting,to%20a%20kilometre." rel="nofollow noreferrer">(SOURCE)</a></p>
</blockquote>
<p>And also:</p>
<blockquote>
<p>An Airy-type geophysical experiment was conducted in a 2 km deep hole
in the Greenland ice cap at depths between 213 and 1673 m to test for
possible violations of Newton's inverse-square law. An anomalous
gravity gradient was observed. The anomaly cannot unambiguously be
attributed to a breakdown of Newtonian gravity because it might be due
to unexpected geological features in the rock below the ice.<a href="https://ui.adsabs.harvard.edu/abs/1989PhRvL..62..985A/abstract" rel="nofollow noreferrer">(SOURCE)</a></p>
</blockquote>
<p>The repulsive fifth force has not yet been discovered, as considering the fact that the force affected the value of the gravitational constant by just 2-3% [<span class="math-container">$6.6743×10^{-11}$</span> vs <span class="math-container">$6.5408×10^{-11}$</span>], this would mean that the fifth force would be an extremely weak force, that would have an even smaller constant than that of gravity, or that its range is pathetically limited to a small scale, like that of a few meters to some kilometers. However there does seem a likely candidate to this problem.</p>
<h1>Dark Energy</h1>
<p>From what I have read, it seems like dark energy, definitely doesn't operate on a small range. It in fact operates on a vast cosmological scales or even perhaps an infinite range. However, from what I have heard, dark energy has a pathetically weak effect on small scales, as it is pathetically weak at that scale.</p>
<ul>
<li><p>Dark energy already gets overpowered by the strong nuclear force and weak nuclear force at atomic scales, which means, forget about trying to make atoms bigger due to expanding space</p>
</li>
<li><p>Electromagnetism, which is responsible for holding molecules and atoms in the molecules together, and also the interactions between charged particles, also overpowers dark energy easily, which prevents molecules from expanding.</p>
</li>
<li><p>Even gravity, the weakest of all the 4 fundamental forces, overwhelms dark energy, and that's the reason why orbits do not expand, or that planets get bigger, just because space time is expanding.</p>
</li>
</ul>
<p>However, it might be possible that despite being overwhelmed, dark energy might still be able to produce negligible effects on municipal scales. And considering the fact that dark energy is repulsive in nature, as can be seen by the moving of galaxies away from us, it also seems to be responsible for the Australian mineshaft anomaly and the Greenland ice cap. So I think, but this is just a guess, that dark energy might be the 5th fundamental force, which could explain the anomalous variations of the gravitational constant, due to its repulsive nature. But scientists don't seem to be keen on this idea, and seem to think that dark energy is something else.</p>
<p><strong>Why can't dark energy be considered a 5th fundamental force? If it isn't a fundamental force, then what is it?</strong></p>

CC BY-SA 4.0
2023-09-11T00:16:14.933Z
This question is ambiguous. Are you asking "is accelerated cosmic expansion caused by a fifth force instead of dark energy" or "is dark energy, the fluid with $p=-\rho$, a fifth force"? The answer to the latter question is no, and not for the reason provided in your answer. The answer to the former question is "maybe, it's an active research topic".
Sten
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2023-09-11T01:49:24.217Z
You would probably get more interest to this question on https://physics.stackexchange.com/
Paul T.
CC BY-SA 4.0
2023-09-11T10:22:53.127Z
Why should it be considered as anything, as long as we don't even know what it is? It is something we cannot directly measure, it's more like a placeholder name for effects we did not manage to identify yet.
vsz
CC BY-SA 4.0
2023-09-11T12:37:35.143Z
"Why can't dark energy be considered a 5th fundamental force?" --> Because it doesn't exist. [[  :-). ]]
Russell McMahon
CC BY-SA 4.0
2023-11-20T05:36:04.363Z
In your last paragraph, what "Australian mineshaft anomaly" are you referring to?
Fred
CC BY-SA 4.0
2023-11-20T10:35:05.970Z
@Fred the first "SOURCE", the one in the first quoted paragraph
Arcturus
<astrophysics><cosmology><expansion><space-time><dark-energy>
Why can't dark energy be considered a 5th fundamental force?
Arcturus
# Why can't dark energy be considered a 5th fundamental force?

(Asked by: username_0 on 2023-09-10)

So I have recently been researching for multiple articles about fundamental forces, currently there are just 4 forces:

* Gravity
* Electromagnetism
* Strong nuclear force
* Weak nuclear force

So far there have been no signs of a 5th fundamental force, as the results have been extremely obscure, with the exception of the [X17 boson](https://en.m.wikipedia.org/wiki/X17_particle) and the [muon magnetic dipole moment anomaly](https://en.wikipedia.org/wiki/Anomalous_magnetic_dipole_moment#Muon:%7E:text=The%20anomalous%20magnetic%20moment%20of%20the%20muon,the%20electron%E2%80%93positron%20annihilation%20experiments.%5B15%5D). Most proposals vary from a force that either has an infinite or finite range. In the case of finite range, the range of the fifth could possibly vary from a few milimeters, municipal scales or vast cosmological scales spanning billions of light years. There indeed have been some observations that seem to support the existence of a repulsive fifth force:

> Australian researchers, attempting to measure the gravitational
> constant deep in a mine shaft, found a discrepancy between the
> predicted and measured value, with the measured value being two
> percent too small. They concluded that the results may be explained by
> a repulsive fifth force with a range from a few centimetres to a
> kilometre. [(SOURCE)](https://en.m.wikipedia.org/wiki/Fifth_force#:%7E:text=Australian%20researchers%2C%20attempting,to%20a%20kilometre.)

And also:

> An Airy-type geophysical experiment was conducted in a 2 km deep hole
> in the Greenland ice cap at depths between 213 and 1673 m to test for
> possible violations of Newton's inverse-square law. An anomalous
> gravity gradient was observed. The anomaly cannot unambiguously be
> attributed to a breakdown of Newtonian gravity because it might be due
> to unexpected geological features in the rock below the ice.[(SOURCE)](https://ui.adsabs.harvard.edu/abs/1989PhRvL..62..985A/abstract)

The repulsive fifth force has not yet been discovered, as considering the fact that the force affected the value of the gravitational constant by just 2-3% [$6.6743×10^{-11}$ vs $6.5408×10^{-11}$], this would mean that the fifth force would be an extremely weak force, that would have an even smaller constant than that of gravity, or that its range is pathetically limited to a small scale, like that of a few meters to some kilometers. However there does seem a likely candidate to this problem.

# Dark Energy

From what I have read, it seems like dark energy, definitely doesn't operate on a small range. It in fact operates on a vast cosmological scales or even perhaps an infinite range. However, from what I have heard, dark energy has a pathetically weak effect on small scales, as it is pathetically weak at that scale.

* Dark energy already gets overpowered by the strong nuclear force and weak nuclear force at atomic scales, which means, forget about trying to make atoms bigger due to expanding space
* Electromagnetism, which is responsible for holding molecules and atoms in the molecules together, and also the interactions between charged particles, also overpowers dark energy easily, which prevents molecules from expanding.
* Even gravity, the weakest of all the 4 fundamental forces, overwhelms dark energy, and that's the reason why orbits do not expand, or that planets get bigger, just because space time is expanding.

However, it might be possible that despite being overwhelmed, dark energy might still be able to produce negligible effects on municipal scales. And considering the fact that dark energy is repulsive in nature, as can be seen by the moving of galaxies away from us, it also seems to be responsible for the Australian mineshaft anomaly and the Greenland ice cap. So I think, but this is just a guess, that dark energy might be the 5th fundamental force, which could explain the anomalous variations of the gravitational constant, due to its repulsive nature. But scientists don't seem to be keen on this idea, and seem to think that dark energy is something else.

**Why can't dark energy be considered a 5th fundamental force? If it isn't a fundamental force, then what is it?**

**Tags:** astrophysics, cosmology, expansion, space-time, dark-energy
> **Comments:**
> **username_1**: This question is ambiguous. Are you asking "is accelerated cosmic expansion caused by a fifth force instead of dark energy" or "is dark energy, the fluid with $p=-\rho$, a fifth force"? The answer to the latter question is no, and not for the reason provided in your answer. The answer to the former question is "maybe, it's an active research topic".  
> **username_2**: You would probably get more interest to this question on https://physics.stackexchange.com/  
> **username_3**: Why should it be considered as anything, as long as we don't even know what it is? It is something we cannot directly measure, it's more like a placeholder name for effects we did not manage to identify yet.  
> **username_4**: "Why can't dark energy be considered a 5th fundamental force?" --> Because it doesn't exist. [[  :-). ]]  
> **username_5**: In your last paragraph, what "Australian mineshaft anomaly" are you referring to?  
> **username_0**: @Fred the first "SOURCE", the one in the first quoted paragraph  

---

## 2 answers

### Answer by username_0 on 2023-09-10. Score: 8 | Accepted answer

# Dark Energy is a scalar field, whereas forces are vector interactions

As far as we know, we are sure that dark energy, or more accurately, the expansion of space-time, is a scalar quantity. That is, dark energy has no direction or anything, just magnitude, it just causes space-time to expand. Meaning that, there isn't actually any direction in which dark energy acts. You can't say, for e.g. that "space-time is expanding away from us" or that "dark energy pushes galaxies away from us". It is simply **"Spacetime is expanding" and there is no actual direction of the expansion.**

If dark energy were indeed directional, then we wouldn't be observing a uniform redshift all around us. Instead we would be looking at both a **cosmological blueshift** in the direction of where our galaxy is moving towards, and a redshift in the direction where our galaxy is moving away from. Our sky would look dark red in half, whereas in the other direction, a bright blue would dominate the other.

But that's not the case in our universe. We observe a uniform redshift all around where we see. And there are only a handful of blueshifted galaxies and even that is not due to dark energy, but in the rare case of a galaxy actually moving towards us. Except for a handful of the galaxies, the rest of the galaxies seem to be "moving" away from us. So no, dark energy is not a force.

A better way to visualise this is to imagine the observable universe, as a "sphere" made up of a countless number of "cubes", each representing a pocket of spacetime with uniform dimensions. Now, expand this. What happens?

When the sphere expanded, you did not get any new cubes on the surface of the sphere. Instead, the individual "cubes" themselves expanded in increments, albeit a miniscule amount. The reason why we cannot perceive this expansion in spacetime, is because of the fact that each of these "cubes" themselves, expanded by a miniscule amount. So on a local scale, the observer perceives nothing at all.

However, the "sphere" that is representing the observable universe, contains a gargantuan amount of these "cubes". And when you calculate the total expansion of all the "cubes", i.e. adding the increments of all the "cubes" present in the "sphere", you end up with a gargantuan volume. This volume is the amount that the entire "sphere" expanded by, or the overall expansion that occured.

You can also use the raisin-bread model to visualise the expansion of spacetime:[![enter image description here](https://i.sstatic.net/mDreG.gif)](https://i.sstatic.net/mDreG.gif)

As you can see, after baking the bread, you haven't actually added any bread to the existing bread already. The bread itself has expanded, and the raisins in it, which symbolise galaxies, haven't budged an inch at all. The raisins haven't moved at all, the expansion of the bread has simply increased the spacing between each raisin, creating the apparent "movement" of the raisins from each other.

Similarly, as space-time expands, dark energy doesn't "push" the galaxies or something like that, the spacing between each galaxy has increased. An observer perceives the galaxies as in "motion", while in reality, the galaxies themselves haven't budged an inch at all, the spacing has just increased between them.

Also, we do know by the existence of cosmological redshift, that some galaxies far away from us are "moving" away from us faster than the speed of light. (Again, they aren't really moving, it is the expansion of spacetime causing this)

[No force](https://physics.stackexchange.com/questions/438781/what-happens-if-we-exert-more-force-on-an-object-which-would-be-travelling-at-ne#:%7E:text=If%20you%20apply%20a%20force,exceed%2C%20the%20speed%20of%20light.) is sufficient to move any object beyond the speed of light, which means that if dark energy, which is the key driver of the space-time expansion, were indeed a 5th fundamental force, then, the galaxies which would be "moving" would violate causality! Thankfully, this is not the case. We now know that dark energy is not a force, but instead a scalar field that is accelerating the expansion of space. General relativity doesn't prohibit space-time itself from expanding faster than the speed of light, which means, the apparently FTL expansion of space-time isn't exactly violating the laws of physics, since the galaxies aren't moving themselves, it's merely the spacing between the galaxies that's increasing, and that is not prohibited by relativity, either special or general.

### Now another reason, if you still aren't convinced, why dark energy is not a 5th force:

The more general and scientific definition of a fundamental force, or more accurately, a "fundamental interaction" is a "interaction mediated by a gauge/vector boson", excluding gravity, which doesn't have its own boson carrier.

> (Gravity is not a proper force, it is caused by the distortion of
> spacetime by a massive body, but for all purposes, it is regarded as a
> force, because it has a direction as well, which is, "Towards the body")

The reason why they are "vector bosons", because they have a direction in which they go. Simply, taking for example a photon (A massless vector boson, that mediates the electromagnetic force, more accurately, the stuff that composes light). If it is moving in the direction of an observer, you cannot just say, "The photon is moving". You'll have to specify it's direction in which it's moving, which in this case is "towards the observer". Similarly, gluons also hold together quarks to form hadrons and baryons, which in this case, the direction is "towards the quarks". Also gravity, which doesn't have any force carrier though (ignoring the hypothetical graviton), has a direction. Since objects with mass exert the force towards themselves, gravity also has a direction, which is "towards the objects". For the weak force, which is responsible for radioactive decay via beta radiation, and neutrino flavor changing, also has a direction, and it varies in each case. For example, an electron caused by beta decay of an isotope, via the weak force, moves away from the decaying nucleus. Similarly, W and Z bosons, which are the weak force mediators, are "exchanged" between particles, to create an "attractive/repulsive weak force", and they are also responsible for beta decay of particles like neutrons into a proton, electron and an electron antineutrino, quark transformations and so on.

Dark energy, sadly, has no such force carrier boson of its own. Gravitons, hypothetical bosons that mediate gravity, might be likely to exist (in the unlikely case that gravity is not basically spacetime curvature around a massive body), but alas, dark energy has no such boson which mediates it. We do know that dark energy is indeed a scalar field, but if it indeed is a force, then it must be a force that works like gravity (not the graviton type, the spacetime curvature type). But yet, we don't have a good understanding of what dark energy exactly is, aside from the fact that we know that it is a scalar field that is responsible for the expansion of space-time.

There are some speculations about a scalar boson, similar to the Higgs boson, called the [chameleon particle](https://en.wikipedia.org/wiki/Chameleon_particle), which could be a carrier of dark energy, but then the theory is extremely foggy. So, I guess we won't be looking at dark energy chameleons soon.

Another point of fact to look at, most crucially: Dark energy is pathetically weak at even municipal scales. In fact, the [energy density](https://en.wikipedia.org/wiki/Dark_energy#:%7E:text=Dark%20energy%27s%20density%20is%20very%20low%3A%206%C3%9710%E2%88%9210%C2%A0J/m3%20(%E2%89%887%C3%9710%E2%88%9230%C2%A0g/cm3)%2C%20much%20less%20than%20the%20density%20of%20ordinary%20matter%20or%20dark%20matter%20within%20galaxies.%20However%2C%20it%20dominates%20the%20universe%27s%20mass%E2%80%93energy%20content%20because%20it%20is%20uniform%20across%20space.%5B9%5D%5B10%5D%5B11%5D) of dark energy is just $6×10^{10}J/m^3$, which if converted to matter density via $E=mc^2$, works out to be a pathetic $7×10^{−30} g/cm^3$. If you want to measure the gravitational constant deep in a mine, forget it. You wouldn't even detect a 0.000001% deviation in value. The 2-3% value deviation would most likely be a bungle-up of the instruments used to measure the constant.

And even if there is a fifth fundamental force, then it most likely won't be linked to dark energy.

So, TLDR; No, Dark energy is not a 5th fundamental force due to the reasons above.

> I have read the rules of the Stack Exchange website, and there is nothing wrong with answering my own question.

> **Comments:**
> **username_6**: What's this with asking your question and answering it in the same minute? Why don't you answer on the existing question you obviously link?  
> **username_7**: So, since dark energy is just the absence of gravity, like gravity bends spacetime whereas cosmological constant is the hump of spacetime, would you consider it as a avatar of gravity itself, as both gravity and dark energy lie on spacetime itself?  
> **username_0**: @Arjun No, dark energy is not the absence of gravity. Although it might look like anti-gravity or something, it really is just a scalar field, that causes the expansion of spacetime.  
> **username_0**: @planetmaker https://stackoverflow.com/help/self-answer#:~:text=Yes!%20Stack%20Exchange,Stack%20Exchange%20site.  
> **username_1**: This answer is misconceived in several ways. For one, a scalar can mediate a force, e.g. the [Yukawa interaction](https://en.wikipedia.org/wiki/Yukawa_interaction).  
> **username_1**: Some of the other reasons this answer makes no sense: (1) Scalar field is just one class of models of dark energy. We don't have any indication of what dark energy is below the effective fluid description. (2) A field being a vector, in the context of quantum field theory, has nothing to do with whether its particle moves in a direction. (3) Existence of gravitons isn't opposed to gravity being curvature of spacetime. (4) Expansion of space is not different from objects moving apart. (5) Dark energy does "push" galaxies apart (gravitationally).  
> **username_4**: Yer raisins haz moved!  
> **username_0**: @Sten Dark energy is a scalar field. The cosmological constant, which is regarded as the most probable candidate for dark energy (despite being off by a factor of $10^{120}$) is a scalar field, see https://www.google.com/search?q=cosmological+constant+scalar+or+not&oq=cosmological+constant+scalar+or+not&aqs=chrome..69i57j69i59l3.6338j0j7&sourceid=chrome&ie=UTF-8#:~:text=In%20cosmology%20we%20are%20already%20used%20to%20dealing%20with%20two%20examples%20of%20scalar%20fields%20%2D%20the%20cosmological%20constant%20and%20the%20inflaton%20field%20(or%20fields)%20that%20drive%20inflation.  
> **username_0**: (cont'd) The question asked for a fundamental force. The Yukawa Interaction is actually the coupling of particle fields like the electron field and quark field, to the Higgs Field, which for all purposes, cannot be regarded as a fundamental force.  
> **username_1**: @FuriousArcturus The cosmological constant is a [Lorentz scalar](https://en.wikipedia.org/wiki/Lorentz_scalar), not a scalar field in the quantum field theory sense. [Quintessence](https://en.wikipedia.org/wiki/Quintessence_(physics)) is an example of a dark energy candidate that is a scalar field.  
> **username_1**: @FuriousArcturus The Yukawa interaction doesn't refer to a specific interaction in the Standard Model, it's just a theoretical interaction mediated by a scalar. The point is that a scalar field *can* mediate a force.  
> **username_8**: Also, the CMB doesn't look isotropic relative to Earth. We have to subtract the proper motions of Earth, the Sun, and the Milky Way to see that in the CMB frame it's almost isotropic (to ~1 part per 100,000). See the final diagram here: https://physics.stackexchange.com/a/411082/123208  

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### Answer by username_1 on 2023-09-11. Score: 7

Dark energy's influence on cosmic expansion isn't a new force. It's just gravity. Calling dark energy a force in the context of cosmology would be like calling the Sun a force in the context of the Solar System.

That said, it's possible that accelerated expansion could be caused by a fifth force *instead of dark energy*. See for example [this review article](https://doi.org/10.1146/annurev-nucl-102115-044553).


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