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Comment by dnautics | Hacker News Reader

MOND explains 1.

2 is "who can say" because nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND).

3 is subject to questions like "is the CMB really what we think it is" -- if it's early thermalized dust, then that ALSO resolves hubble tension, e.g.

MOND explains several things LCDM cannot:

- why most elliptical galaxies seem to "not have dark matter" (effectively a prediction)

- external field effect (predicted and confirmed)

- renzo's rule

- DM halos that are way too big

- early galaxies (this was a prediction)

HM. people have been downvoting. Anyone care to post a substantive rebuttal?

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2 needs a more substantive rebuttal. LCDM correctly predicts where the dark matter is located after a galaxy collision, such as in the bullet cluster. There is no reasonable interpretation of MOND that has the center of mass of the galaxy shifted away from where it's visible matter lies, precisely how LCDM says it should be.

There is a reason why LCDM used to be a lot more disputed before the work of Clowe, Gonzales and others on the bullet cluster, and is now generally treated as settled science by practitioners. We might still be surprised by something, the universe is more wondrous and complex than we can possibly understand, but Occam's razor massively supports LCDM now. If you want to propose any alternative, you need to start by showing how it explains bullet cluster as well or better than LCDM. (And the bullet cluster specifically is not the only place where this is visible, there are others like MACS J0025.4-1222.)

> LCDM correctly predicts where the dark matter is located after a galaxy collision, such as in the bullet cluster. There is no reasonable interpretation of MOND that has the center of mass of the galaxy shifted away from where it's visible matter lies, precisely how LCDM says it should be.

It does not really make that "prediction", its a post hoc assignment of dark matter density based on weak lensing for which you can make a plausible "this is how it started" explanation.

you can counter with lcdm cant explain tons of stuff that MOND can, from tully fisher relation through barred spiral galaxies (n >> thousands) etc.

> nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND).

My take on this is a bit different. The obvious way to get a relativistic theory that looks like MOND is to add a scalar field. (And possibly a vector field, as in TeVeS.) But from a relativistic, quantum field theory perspective, adding more fields is just...adding a new type of "matter". I.e., dark matter!

In other words, when you take relativity into account, "MOND" and "dark matter" don't really look like two different possibilities any more. They just look like two different ways of describing the same math in imprecise ordinary language.

2. It's called relativistic MOND. That exists as a category. RelMOND(for short) includes TeVeS, Einstein–Aether variants, BIMOND, and others.

3. I would wager that what we call the CMB is more of a local effect than a big bang remnant.

I lack the karma to downvote, don't blame me. :)

2. not a physicist but iiuc the relativistic MONDs have been disappointing? but not enough to rule out

They have varying degrees of 'inelegance' or 'complexity.'

But generally they closely conform to Einstein gravity/standard model, as they incorporate relativity.

But that's still the thing. No theory can explain gravity across the spectrum of variables (from micro to macro scales, apparent lack of dark matter in some galaxies, black holes, why galaxies don't fly apart, etc.) We have a dark gravity problem regardless of which theory you like best.

"HM. people have been downvoting. Anyone care to post a substantive rebuttal?"

Just that asking this might get you more downvotes. I upvoted as I found your input interesting. I would suggest to edit that out (then I will delete my comment)

I don't really care (i'm doing just fine fake-internet-points-wise), just if I made a mistake it would be nice to know. Otherwise worth it to know that there are people out there who find "alternative explanations by professional scientists" threatening. for some reason.

Well, I rather meant there are guidelines to keep this space meta debate free and I do think they make sense.

(And apart from that, HN is not free of dogmatic ideologists of all sorts who feel triggered by anything straying off from mainstream theory. That is my guess here and my advice to ignore it, but I would be also curious if there is something clearly wrong with your comment deserving downvotes.)

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