What is the current status on MOND

Chalnoth

The problem is that when it comes to examining objects in the universe, we have to understand the history of the objects to understand what a specific theory predicts what structure it will take.

This is why most of the evidence which contrasts MOND and CDM isn't concerned with why galaxies (and other objects) take on a particular structure, but instead with just how the gravitational attraction in the current structure behaves. For example, star rotation curves don't mention anything about why a galaxy obtained a particular density profile: they are merely concerned with how gravity keeps it in (approximately) that shape.

When it comes to formation, well, galaxy formation in general is an unsolved problem, not because we don't understand gravity, but because it's fantastically difficult to take into account the full effect of the behavior of normal matter (e.g. what impact do supernovae have on galaxy shapes? What about the supermassive black hole at the center?). This isn't to say we know nothing about how galaxies form, but rather that there are lots of big, unsolved questions here.

So when we want to compare gravity theories, the thing to do is focus on observations where these other uncertainties, e.g. regarding galaxy formation and structure, simply do not play a role. This is one reason why the bullet cluster observation is so neat: it's a very clean observation of a pair of galaxy clusters that recently passed through one another, as can be clearly seen by the bow shock of the hot x-ray cluster gas. With about 10 times as much matter in this cluster gas than exists in the normal matter in the galaxies, a modified gravity theory would tend to predict that most of the mass should have been surrounding this hot cluster gas instead of the galaxies. Instead, the galaxies had most of the mass, which indicates that the dark matter (which, like the galaxies, was not slowed by the collision) is what contains most of the mass of these galaxies.

Some alternative gravity theory advocates claim that their theory can explain this without dark matter, but then they require a new species of heavy neutrino (i.e. another sort of dark matter). At that point, the whole enterprise becomes rather ridiculous.

Chalnoth

That is completely false. MOND most definitely has parameters whose value is not known a priori and must be determined from observation. And the idea that CDM doesn't predict anything is positively ludicrous. Consider the CMB: the relative height difference between the even and odd harmonic peaks is a direct prediction of the effect of having most of the matter in our universe, as measured by mass, being cold dark matter. Now, the exact ratio of the even-odd peak heights was not predicted, but that there would be such a difference was.

matt.o

I have to guess at which paper you are referring to here. Initial claims of the incompatibility of the Bullet cluster with LCDM inferred the velocity of the Bullet subcluster from the shock properties. The velocity of the Bullet has been reduced for two reasons: 1) The newest Chandra calibrations lower the shock temperature and, therefore, the Mach number and measured velocity for the shock (although the density jump at the shock may have been used to determine the Mach number). 2) The velocity measured from the shock is not the same as the velocity of the Bullet subcluster (see e.g., Springel and Farrar 2007). The speed of the bullet subcluster is more like 2300km/s (c.f. ~4700 km/s from the shock). This is compatible with LCDM.

If you are referring to work which compares initial velocities taken from the idealised simulation of the Bullet cluster and compares them to those seen in cosmological simulations, then this method assumes that the initial conditions in such idealised simulations reflect what happens in the universe. Also, these initial conditions are tweaked so that the end result of the simulations mattch the observed properties of the Bullet. This means that, if the initial conditions are to reflect the real universe, the physics of the collision must be well known and modeled. I think these two assumptions fail and, therefore, so any test comparing initial velocities from idealiased simulations to LCDM simulations is flawed.

Correction: Colliding *clusters* of galaxies.

The offsets in A520 are as expected. The issues was the dark core, which has since disappeared (see Clowe et al. 2012).