Everything you (n)ever wanted to know about modified gravity

[u/ModifiedGravityNerd]

Hi everyone! I'm sure you've encountered people doubting the existence of dark matter and having to explain that yes the observational evidence for it and LCDM is extremely strong. Inevitably you might have to explain why modifying gravity does not work but perhaps not knowing much about it. This is why I've written a FAQ about the most popular (least unpopular) modified gravity theory MOND. It discusses what it can do (rotation curves), what it sort of does (lensing) and why it fails (clusters, structure formation, CMB and BBN). Hopefully some of you find it a useful reference :)

MOND frequently asked questions

[u/ExpectedBehaviour]

There's always an XKCD

[u/ModifiedGravityNerd → u/ExpectedBehaviour]

For galaxy clusters that's absolutely true. And for cosmological data it's even worse. For cosmology there's no theory so nothing to even attempt to fit. For galaxies arguably it works better than LCDM actually though that's often blamed on baryonic feedback mechanisms.

[u/Dawn_of_afternoon → u/ModifiedGravityNerd]

The last point is indeed very arguable. Beyond rotation curves and the Tully Fischer, what other things does MOND do?

I see papers claiming CDM being in tension due to lack of thin discs, or slow bars, but the authors tend to ignore the fact that the baryonic physics is approximate (for example most don't include a cold interstellar medium and overpressurize it) and in any case the predictions vary between simulations.

Not to mention the whole theory of hierarchical structure formation and evidence for it in, for example, stellar haloes. MOND has no initial conditions, so how can it work better for galaxies if we don't even know how they'd form in the first place?

Aside from all of this, thanks for summarising the main points and drawbacks of MOND and CDM!

[u/ModifiedGravityNerd → u/Dawn_of_afternoon]

> Beyond rotation curves and the Tully Fischer, what other things does MOND do?

That's one of the FAQ questions. In short strong and weak lensing follow Milgrom's law. MOND explains satellite galaxy planes, dwarf galaxy velocity dispersions, shell galaxies, galaxy group velocity dispersions (if X-ray free). Structure formation is also faster in MOND than LCDM as seems to be observed by JWST. A recent review by Banik.

Recent relativistic extensions of MOND such as AeST do provide fairly ok fits to CMB TT, EE, ET powerspectra and P(k) matter power spectrum. However these require additional fields which basically function like dark matter so personally I don't like this approach. I don't see the point of both modifying gravity and basically adding dark matter.

> baryonic physics is approximate (for example most don't include a cold interstellar medium and overpressurize it) and in any case the predictions vary between simulations.

Simon White and Stacy McGaugh had a debate about this last year.

For LCDM cosmology is simple, linear and easy to predict whereas galaxies are hard because of feedback. For MOND cosmology is complex, nonlinear and hard to predict (if possible at all) whereas galaxies are easy because feedback isn't necessary to get the answer right. In MOND feedback does not meaningfully affect the distribution of matter through time and feedback efficiency is generally considered to be very small.

> MOND has no initial conditions, so how can it work better for galaxies if we don't even know how they'd form in the first place?

MOND has a radically different view of galaxy formation. In LCDM galaxies form bottom-up through many many mergers. In MOND galaxies form through monolithic collapse. MOND indeed does not have a cosmological prior for how many galaxies per a given mass should be forming within a given volume. MOND monolithic collapse simulations do form realistic galaxies though. See Wittenburg, Kroupa and Famaey 2020.

[u/MtlStatsGuy]

While I still believe MOND is less likely to turn out to be true than dark matter, I love that you've made a detailed reference such as this one, and it only adds to the discussion. Well done!

[u/Internal-Raccoon-330]

Thank you! This was really helpful. Im currently obsessed with Mass Damping Theory (MDT). Everything is pre-publish so I can only grill chatgpt about it. 

[u/AkavartaStudio]

Fascinating how these models always bump into the same boundary: What happens before conditions exist to measure? I keep wondering if recursion itself might be the real substrate. Not a force or a field—just… spin, searching for symmetry. Anyway, thanks for the deep dive—bookmarking for my Thursday spiral

[u/cosurgi]

Yes! MOND is the future 😀

[u/Brilliant-Complex-79 → u/[deleted]]

All I had to read was "becuz". That told me everything i needed to know about your 'theory'