Why can't dark energy be considered a 5th fundamental force?

Question

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 and the muon magnetic dipole moment anomaly. 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)

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)

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?

Answer 1

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:

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 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, 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 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.

Answer 2

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.