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> Dark energy is really the fudge for all this. The idea is that creating new space takes energy. That has to come from somewhere and dark energy is our fudge factor for that. No one really knows what dark energy is or looks like just like we don't know what creating new space looks like. It just solves a conservation problem.

That's simply not true. You don't need energy to create new space (and you wouldn't magically get energy from nowhere if the universe were contracting). General relativity gives you expanding (or contracting) space just fine if dark energy doesn't exist -- in fact, that was the most common cosmological model before the late 1990s discovery that the expansion was accelerating.

The fact that the simplest version of dark energy is the cosmological constant tells us your basic idea is wrong: if that energy was going into creating new space, then it would decrease over time, not remain constant.

In fact, it's worse than that, because the cosmological constant is a constant energy density. So if expansion doubles some volume of intergalactic space, then the total amount of dark energy also doubles (while the total energy in the form of ordinary or dark matter stays the same, and the energy in the form of photons decreases due to redshifting).

Conservation of energy in general relativity is messy and sometimes counter-intuitive; e.g. https://math.ucr.edu/home/baez/physics/Relativity/GR/energy_...

> It just solves a conservation problem.

This is news to me. The story I was taught was that Einstein's equations predicted a universe that wasn't static, i.e. an expanding universe. Einstein didn't like that and introduced the cosmological constant to leave the possibility of a static universe. Then, it was found that it was indeed expanding. Einstein promptly regretted his decision ("biggest blunder") to introduce the cosmological constant. Then, it was found that the universe was not only expanding, but expanding in an accelerated fashion. That is at odds with Einstein's equations without the cosmological constant, so it was re-introduced, but with the opposite sign. If you interpret the cosmological constant as part of the already present energy-stress tensor, it could represent the energy density of vacuum itself. That is only one interpretation of "dark energy" though, as the term is an umbrella term for whatever is responsible for the acceleration.

Energy conservation never entered the discussion. In fact, energy conservation generally isn't even expected to hold in cosmology. Energy is not even well defined, as it is directly associated with time translation invariance as per Noether's theorem, which doesn't exist in an expanding universe. See: https://www.preposterousuniverse.com/blog/2010/02/22/energy-...

> The calculation of vacuum energy by QFT isn't infinite. It's just really, really large. This is one of the largest errors between theory and observation in all of physics because the measured energy of a vacuum is ~120 orders of magnitude less.

They are infinite if we sum up all frequencies naively. But just like with renormalization, we kinda say handwaveingly that we can probably cut off frequencies at the Planck scale, and then we arrive at this number 120.

There were two problems with Einstein's original (1917) cosmological model. The first was what you mention: Einstein assumed the universe was static, but later observations showed this wasn't true. The second was that his static model turned out to be unstable to either expansion or contraction, something he had forgotten to check.

> This isn't gravitational repulsion. It's why the universe is ~13.8 billion years old but we can see the light of objects that we now estimate to be >40 billion light years away. Space itself has expanded. It's not gravitational repulsion.

To add:

Masses repelling and space expanding would have the same effect on the observable universe boundary: linear models break down.

However, light redshifting, where more distant objects are more redshifted, fits with expansion. That cannot be explained by gravity. (Whether it's expansion or some other mechanism is still unknown, AFAIU.)

Nucleons are made up three quarks and those quarks are tiny compared to the nucleon's size.

Does it really make sense to even refer to the "size" of a point particle? In physics there's really no such thing as a "solid" object that has a "volume". It's just fields of a certain radius and the centers of those fields we refer to as the "location" of a zero-volume entity.

Quarks have a non-zero collision cross-section, so I'm not sure it's reasonable to think of them as zero-volume.

How does one measure that?

Depends. For instance in the case of the Lennard-Jones potential, which is basically the easiest but still good enough approximation of intermolecular interactions that we have, you have two fields between ~two molecules : one short-range repulsive, another long-range attractive. This results in a minimum potential at a distance r_min (see graph) that you can arguably think of as the "size" of the (spherical) molecule (at low temperatures, molecules will form "clumps" with that distance between them) :

https://en.wikipedia.org/wiki/Lennard-Jones_potential

How exactly is that space empty when it’s being exclusively occupied by a probability cloud?

That formulation strikes me as being a Rutherford model tier of wrong.

Particles have measurable sizes. Now you can argue this is a collapsing wavefunction, which I guess it is, but what size does a wavefunction have? I think the general interpretation is that the wavefunction is probability distribution and in that case the electron "cloud" is really the uncertainty about where the electron is not a larger object.

Is it really sensible to call a space “empty” when you know full well that it’s exclusively occupied?

> This isn't gravitational repulsion.

My layman’s understanding matches up with what you said, and my curiosity about this is what brought me to the comments. What I don’t get is that the author appears to be an astrophysicist who must surely understand the difference. Can someone shed more light on this? E.g. is there a version of the theory where these formulations are equivalent?

Yeah, I found this strange too. I too am a layman and that prompted me to comment: why is an astrophysics professor making statements that dark energy is gravity repulsion?

The most charitable interpretation is that the author has dumbed down the theory to make it digestible by the layman and in doing so has used a bad analogy. But I think you can correctly describe the core concepts in simpler terms that are a lot closer to taht theory.

Because it's true. When I got my PhD in cosmology, this is what everyone kept saying, including my thesis advisor, who is an accomplished professor in the field. So it's not just the author of this article.

Gravity works a bit different at the scales we are talking about, but generally we would expect an expanding universe filled with only matter and radiation to slow down and eventually collapse back into itself. Or at least it would slow down and approach some constant velocity. Not unlike a ball that you kick into the air with below or above the escape velocity. But what we actually observed in the late 90ies is that is indeed not slowing down, but accelerating, as if some force was pushing it apart. The cause of this - whatever it may be - has been dubbed "dark energy". At these scales we don't think of gravity as a "force" like we did in Newtonian physics, so I don't think it's wrong at all to say that dark energy has the opposite effect of matter and is pushing the universe apart, i.e. has an repelling effect.

Could energy decay (by fusion or chemical reaction) of glowing matter towards iron create space as byproduct? I find the idea quite funny, of dark matter being a sort of invisible smoke bubbling from light reactions and this "byproducts" bubbling along gravity wells towards "flat" space were they surface creating new space.

But that idea could be tested, the more flat the space becomes, the more "positional" push-back by the bubbling space a space faring object would experience.

I see a couple of issues with this idea (IANAP):

1. There is variance in matter distribution. If fusion created or expanded space I would assume there'd be a measurable variance in the wavelength of the cosmic microwave background;

2. The initial phase of the Universe was a massive period of inflation. This was before atoms could exist (and thus fuse) so to be consistent this would have to be a different mechanism;

3. The energy released by fusing light nuclei comes the nuclear binding energy (ie the strong interaction). So the nuclear binding energy of a helium atom is less than twice the nuclear binding energy of two hydrogen atoms. As you note, this goes as far as iron because this pattern is no longer true (ie the heavier fused element has more binding energy than the two lighter elements combined). I believe this particular conservation is "solved" or at least consistent with established theory, specifically QCD.

4. Fusion in stars is highly localized. Think of the volume of the Sun compared to the volume of every point in space to which the Sun was the closest star (ie approximately 2 light years in radius). It is astronomically small. To may layman's intuition it seems like this would be measurable in some way.

We know spacetime curves (based on the energy and mass present). We have detected the compression of space (ie gravity waves). We also know there is a minimum distance between objects (ie the Planck length) but (AFAIK) space isn't discrete as that might suggest.

There are huge gaps in our understanding of any of this.

Maybe the annhilation of anti-hydrogen and hydrogen produces massive amounts of space. Which would then pearl away from the mass, towards "flat" foamable space, aka the center, driving the sphere of annhilation apart, moving at almost lightspeed (relative to the created space), leaving behind only structures, who created lagrange points relative to the sphere, at which space could bubble up.

I have no idea what im doing, i just like to take a idea and run with it.