Hacker News Reader

Comment by amai | Hacker News Reader

MOND might explain rotation curves of galaxies. But it doesn't explain all the other places in astrophysics, where one needs dark matter. Especially MOND (and any modification to gravity) cannot explain the Bullet Cluster: https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to...

The Bullet Cluster is especially significant, because it shows gravitational lensing by dark matter spatially separated from visible matter. No modification to gravity can explain that without giving up the idea that gravity is a https://en.wikipedia.org/wiki/Central_force .

View on HN

> Especially MOND (and any modification to gravity) cannot explain the Bullet Cluster

The creator of MOND addressed that:

http://www.astro.umd.edu/~ssm/mond/moti_bullet.html

Your link made me very grateful for Firefox's Reader View.

Is that part of the Pocket acquisition? I was comparing Safari and Firefox a while ago and Safari's reader view was one of the deciding factors in my choice.

It might be; looks like they were both added in 38.0.5:

https://www.mozilla.org/en-US/firefox/38.0.5/releasenotes/

It seems like the takeaway is not that galactic cluster anomalies support or break MOND, but that MOND is a way to reduce the general astroöbject mass anomaly problem to a modified gravity plus the cluster mass anomaly problem. This makes sense if the cluster anomaly is tractable (?) and there are no other problems created by MOND (the real question IMO).

The problem then is that we want to have enough examples where MOND "works" that we don't think we'll run into another cluster anomaly. However, there aren't too many types of objects in the sky. So we either need very good numerical evidence for MOND in all other systems (unlikely) or a solution to the cluster anomaly which might not be too hard, unless MOND is actually wrong of course, and that's a risk you often face as a scientist.

1) "But MOND (or other modified gravity theories) predict that the "DM" should be found around the visible matter not elsewhere"; so they say. However, this last statement is incorrect. For example, in galaxies the baryons are concentrated at small radii but the putative DM according to MOND is far beyond that. "

It is important to stress the word "around". In Milgroms example the DM is still around the baryons, just far out at a different radius. This is the modification you can get, as long as gravity is still a central force in your theory. However in case of the bullet cluster the DM simply doesn't follow the center of the baryons. It is completely separated at a different place. I think he doesn't get this important point.

2) "As I said we do know that there is some (still) dark matter in clusters and that's what they found."

So Milgrom is admitting that even with MOND one needs some dark matter to explain the results. But with only dark matter we don't need to use MOND. So according to Occams razor the Dark Matter theory wins, because it explains more with less.

This, whereas there's every possibility that dark matter isn't what we think it is, and holds some serious surprises, the evidence that there's a whole extra physical phenomenon (dark matter) that we've never observed seems pretty incontrovertible.

Dark energy, though, currently appears to be a "stuff we really don't understand" factor.

Concerning dark energy, it might be explainable by quantum vacuum fluctuations:

https://journals.aps.org/prd/abstract/10.1103/PhysRevD.95.10...

(the last author of that paper is rather well respected in that field)

Reply on news.ycombinator.com