MOND or Matter

When it comes to simple MOND, I just think 'silk damping'. We are here, galaxies with mass ~10^12 Msun exist, and it is hard to come by that if there is no dark matter (or I suppose some unknown scalar field - perhaps - http://arxiv.org/abs/astro-ph/0505519 ).

Or gravity is different on those scales.

When it comes to simple MOND I think of it not as a theory, but as a parametrization of where the naive baryonic structure formation theory goes wrong - it identifies the scale where the problem occurs.

From that perspective it is not surprising the scale is a0 ~ c/T_H

So taking out the theology and the zealotry on both sides, the fundamental issue is that we need new physics at this particular scale, and since the physics is poorly constrained it could be new particle physics, which fits nicely into other speculation, or it could be further modifications to gravity, which is ugly, but then maybe elegance is over rated.

Fortunately, this is at least one are where things are falsifiable and we have and will test the theories.

@Steinn #2: It may be worth visiting Ethan Siegel's Starts With a Bang from 18 Jan, for a discussion of the various lines of evidence, beyond cluster-level rotation curves, for which MOND is inadequate to explain the existing data. Baryon acoustic oscillations in the CMB are the most significant of these.

I am quite familiar with Ethan's blog, thanks.

As I noted, I think CDM is more likely than any modified gravity theory, and I am also acutely aware of the problems of the LambdaCDM models, and interested in what the underlying cause is.

Stein, I agree we should not take elegance too seriously. I was only stating simple mond, which gives flat rotation curves, needs its own extensions and fine tuning to fit other observables. CDM fits a lot of the astrophysics data (plus it is simple to simulate and understand and it conforms with a linear theory) but is mostly untested on the particle level.

I worry that a_o ~ c H can be a red herring. Indeed a spherical collapse model with something like CDM gives a similar result:

Given a few assumptions from the spherical collapse model and the inferred perturbation amplitude of densities:
1) After a halo virializes, it has an overdensity ~ 200 rho_c, where rho_c is the critical density of the universe at formation.
2) \delta rho/rho is ~ 10^-3 (which we will just call delta)

We are interested in calculating a_0 = G M_h / r_vir^2.

First we note, G M_h/r_vir ~ (delta c)^2 - this is just the statement that potential wells are set by the density perturbations.

We also have M / (4/3 pi R^3) ~ 200 rho_c ~ 200 3 H^2/(8piG) .

Combining these two relations gives a_0^2 ~ 10^2 delta^2 c^2 H^2 ~ 10^-2 c H.

So this is 100 times smaller than cH. But is actually comparable to the inferred acceleration at 200 kpc of the MW. Taking something like an NFW profile (or anything that posits a steep density profile at large radii) the acceleration at 20 kpc is something like 100x larger ((r / Rvir)^-2 ~ 10^2) - right about where we expect it to be.

This is all a little bit tautological - I need to assume CDM to get delta ~ 10^-3. The CMB anisotropies, the direct observable, are smaller than this. I also need to assume a dark matter profile that goes ~ r^-3 at large radii. This wouldn't work for a true top hat collapse model, but numerical simulations do get numbers like this.

Finally, it doesn't explain flat rotation curves - this appears to be nature's way of making complex processes appear simple.

This is really just to say either I) a_0 ~ c H isn't a very clear reason to start changing physics on that scale or II ) Theorists can find a way to make their favorite model fit all the data.

Option II is why we have experimentalists.

Ok, Devil's Advocate time - but I should note that while I have done non-Newtonian gravity sims in a different context, I have never done MOND motivated structure of galaxy dynamics simulations.

CMD works really well - we agree on that - after you add a free parameter, Lambda, to the models. That takes care of the large scale structure, which, as you know, was discrepant.
But, the density profiles, shape and luminosity function of galaxies are not correct in detail on small scales - which may just be messy physics, or may be CDM physics, but either way they are discrepant still.

MOND parametrises this - it is a measure of where things are. It is also easy to kill (ok, we could have CDM and modified gravity but don't want to go there) - direct detection of CDM nails things down, and we are interestingly close to being at that point.

@Ryano - that way of looking at things actually highlights the CDM/MOND discrepancy - going from 20 kpc to 200 kpc steep halo profiles imply pseudo-Keplerian rotation curves and the acceleration decreasing - but MOND implies isothermal rotation curves and constant acceleration across the 20-200 kpc scale.

The rotation curve data implies that the halo density profiles are too flat - the really look isothermal from few kpc out to as far as we can probe, still.

btw if CDM is right, detection of cold dark mater particle is overdue. When I was a grad student I was told that the LHC would discover would. Well first phase of LHC is over and
no sign of WIMP or supersymmetry. There are rumors that AMS
is coming out with a big announcement soon, but again it might be like the Pamela result(another mirage).
It would be nice if there is some evidence from astrophysical observations alone that DM has weak scale interactions with ordinary matter. I think the WIMP miracle argument is way over-rated.

btw if CDM is right, detection of cold dark mater particle is overdue. When I was a grad student I was told that the LHC would discover the dark matter particle. Well first phase of LHC is over and no sign of WIMP or supersymmetry. There are rumors that AMS is coming out with a big announcement soon, but again it might be like the Pamela result(another mirage).
It would be nice if there is some evidence from astrophysical observations alone that DM has weak scale interactions with ordinary matter. I think the WIMP miracle argument is way over-rated.
Having said that there is also no independent (laboratory based) evidence that gravity becomes like MOND at low accelerations.

i know this is off the topic, but i was wondering if you can gveme a deffinition in your own word, ' whayt science means' to you. if you could respond i would gladly appreciate it. thank you.

@Destiny: "doing your own homework" is what science means to me.