Are Alternative Dark Energy Models Treated More Favorably Than MOND?

[BWV]

I’m trying to get a clearer sense of how the physics community evaluates alternative explanations for dark energy compared to alternative explanations for dark matter, such as MOND. My impression is that modified‑gravity approaches to dark matter (e.g., MOND and its relativistic extensions) tend to face strong skepticism, whereas alternative dark energy models—like quintessence, k‑essence, or modified gravity frameworks that mimic cosmic acceleration—seem to receive somewhat broader consideration in mainstream cosmology.

To be clear, I’m not asking whether these alternatives are correct, only whether the degree of openness differs between the two cases, and why. Is it because dark energy is already poorly understood, so theorists feel more freedom to explore alternatives? Or is it because MOND‑type theories run into more direct conflicts with cosmological data?

For context, here is a recent paper that surveys a range of dark energy alternatives, including dynamical fields and modified gravity models:

Survey Paper:
Dark matter and dark energy: Models, challenges, and future perspectives (A. Kumar, 2024).
DOI link: https://doi.org/10.33545/26648636.2024.v6.i1a.136

And here is a relevant discussion of the methodological issues surrounding MOND and why many physicists remain unconvinced:

Citation:
Duerr & Wolf, Methodological Reflections on the MOND/Dark Matter Debate (forthcoming in Studies in History and Philosophy of Science).

I’d be interested in hearing from those familiar with cosmology or gravitational theory:

• Do you think the community is more receptive to dark‑energy alternatives than to MOND‑like dark‑matter alternatives?
• If so, what drives that difference—empirical constraints, theoretical coherence, historical precedent, or something else?
• Are there particular alternative dark‑energy models that are currently taken most seriously?


Thanks in advance for any insight!

 

[Ibix]

IIRC, Vanadium 50 used to comment that MOND (or at least some variations thereof) works almost embarrassingly well at the scale of galaxies and galaxy clusters. But it is completely unnecessary at smaller scales, and doesn't work at all at larger scales. That means it could be right and we haven't figured out how to incorporate it into GR (or something GR-like), or it could just be describing something about galactic dark matter dynamics that we don't understand.

The big point in dark matter's favour, I think, is that it was proposed to address galactic rotation curves, but also explains a peak in the CMB fluctuation power spectrum. That is, it solves a problem that isn't the one it was created to solve. That's suggestive (although far from proof!) of it being the right answer.

Also, I suspect that "new type of matter or astronomical body we haven't found yet" is favoured by most scientists wielding Occam's razor.

Dark energy, on the other hand, is more or less a complete unknown beyond being "whatever it is we need to add to the FLRW mix to make the model match observation". And it already has to be weird stuff that violates energy conditions, so I think there's no clear favourite candidate and it's a much more open field.

Just my opinion, of course. I'm not a professional in either field.

 

[Jaime Rudas]

I’m trying to get a clearer sense of how the physics community evaluates alternative explanations for dark energy compared to alternative explanations for dark matter, such as MOND. My impression is that modified‑gravity approaches to dark matter (e.g., MOND and its relativistic extensions) tend to face strong skepticism, whereas alternative dark energy models—like quintessence, k‑essence, or modified gravity frameworks that mimic cosmic acceleration—seem to receive somewhat broader consideration in mainstream cosmology.

I think the issue has to do with the fact that General Relativity is one of the most successful theories in history, so any model that deviates from it is usually viewed with skepticism. Thus, for example, quintessence or k-essence are not seen as alternatives to dark energy, but rather as specific models of it, whereas MOND theories are indeed alternatives to General Relativity.

 

[PeterDonis]

MOND theories are indeed alternatives to General Relativity.

Note that when you try to construct relativistic "MOND" theories, the boundary gets somewhat blurred. For example, TeVeS adds a scalar and a vector field. But are those fields "changing how gravity works"? Or are they just...new types of "matter" or "energy" with particular couplings? At least in the eyes of some, AFAIK, that's more a matter of choice of terminology than physics.

 

[Jaime Rudas]

Note that when you try to construct relativistic "MOND" theories, the boundary gets somewhat blurred. For example, TeVeS adds a scalar and a vector field. But are those fields "changing how gravity works"? Or are they just...new types of "matter" or "energy" with particular couplings? At least in the eyes of some, AFAIK, that's more a matter of choice of terminology than physics.

Yes, what I wanted to emphasize is that the level of skepticism is more related to whether or not the model deviates from General Relativity than to whether the model is an alternative to dark matter or dark energy.

 

[PeterDonis]

whether or not the model deviates from General Relativity

Whether its proponents claim it does, yes, I'll agree with that.

I'm just pointing out that those claims, even by model proponents, often aren't quite as cut and dried as they're made to appear. For example, TeVeS adds a scalar field, and "quintessence" is...a scalar field. The two theories aren't quite identical, since quintessence by definition is minimally coupled, while the scalar field in TeVeS is not. But I'm not sure they're as different as MOND proponents claim.

Which would mean that, by pitching their theories, or at least the relativistic versions of them, as "deviating from GR", MOND proponents might in fact be shooting themselves in the foot...

 

[PeterDonis]

Dark energy, on the other hand, is more or less a complete unknown beyond being "whatever it is we need to add to the FLRW mix to make the model match observation". And it already has to be weird stuff that violates energy conditions, so I think there's no clear favourite candidate and it's a much more open field.

It's worth noting, though, that from the perspective of deriving the Einstein Field Equation from a Lagrangian, the presence of a cosmological constant term (which is the simplest type of "dark energy") is not an anomaly; it's expected to be there just by the general criteria that are used--that the Lagrangian be linear in the second derivatives of the metric and contain no higher derivatives. A constant obviously satisfies these requirements.

 

[Jaime Rudas]

Whether its proponents claim it does, yes, I'll agree with that.

I think it's more about how the scientific community perceives it. In other words, the question in the OP is whether or not the scientific community is more receptive to dark-energy alternatives than to MOND-like dark-matter alternatives. My point is that it is more receptive insofar as it considers that it fits better with General Relativity

 

[PeterDonis]

I think it's more about how the scientific community perceives it.

Agreed. I would say that how the proponents describe it is a very significant contribution to how the community perceives it.

 

[timmdeeg]

I’m trying to get a clearer sense of how the physics community evaluates alternative explanations for dark energy compared to alternative explanations for dark matter, such as MOND.

https://arxiv.org/pdf/2412.15143

We compare the standard homogeneous cosmological model, i.e., spatially flat ΛCDM, and the timescape cosmology which invokes backreaction of inhomogeneities. Timescape, while statistically homogeneous and isotropic, departs from average Friedmann-Lemaître-Robertson-Walker evolution, and replaces dark energy by kinetic gravitational energy and its gradients, in explaining independent cosmological observations.

If correct an alternative explanation for dark energy isn't required. It deviates from the standard model but not from General Relativity.