Dark Matter — "Alternative Gravity" Team Responds

An anonymous reader writes, "Following previous results, an international team of astronomers answers, defending the case for a modification of the theory of gravity. This article presents an alternative to dark matter and states constraints on the neutrino mass. In short, dark matter is still not a necessity, provided that neutrinos weigh 2eV. This is allowed by what we currently know and should be tested in the KATRIN experiment in 2009."

Comments

Re:why would matter be dark

[SigILL]

I am not sure that photons 'have' to have mass. I would indeed suspect that they are 'forbidden' from having mass, due to the fact that they are traveling at the speed of light.

Photons lack mass but (since they move at 1,0 c) they do have momentum. This is wat makes solar sails work.

Hey, maybe that's the answer: substitute momentum for mass in all gravity calculations and see if that makes it all work.

Use the chain rule Luke

[sweetser]

Here's Newton's law of gravity:

        d mV/dt = - G M m/R^2 R_hat

It doesn't work for galaxies, it doesn't work for the big bang, it is broken for almost anything BIG. It also has a tiny bit of error that GR corrects, but that is minor. The problems with this law are HUGE. So we have two schools of thought. One wants to stuff the big M box with dark matter:

        d mV/dt = - G (M + Dark_M) m/R^2 R_hat

These folks get to put Dark_M wherever it needs to go to get the answer right. Then there the MOND folks who want to mess with the R:

        d mV/dt = - G (M + Dark_M) m/R^2 or if dV/dt is small, d m V/dt = - a_0 sqrt(G M/R^2) m R_hat

where a_0 is a new constant in nature that changes the form of gravity's law if tiny. I got my own proposal. Remember the chain rule from calculus?

        d mV/dt = m dV/dt + V dm/dt

That V dm/dt is the stuff of rocket science. We know it is not relevant for stars cause those big star things and galaxies don't change. But we could, just for the fun of it, do a relativistic swap-out, and consider:

        d mV/dt = m dV/dt + V dm/dt + V c dm/dR

Force is a change in momentum, which can be seen either as the usual acceleration, the rocket-ship effect, or as where stuff is distributed in space. That sounds like what is going on. So my proposed modification is this one:

        d mV/dt = m dV/dt + V dm/dt + V c dm/dR = - G M m/R^2 (R_hat + V_hat)

Too bad I suck at numerical integration or I'd try and see if it could match real data sets. I like it because it uses stuff we know is true (the chain rule) with a fun twist to make an old law point in a new direction.

doug

Re:Use the chain rule Luke

[sweetser]

The relativistic 4-force law is d mV^u/dtau, where V^u has four places, and tau is the spacetime interval, sqrt(t^2 - R^2). Nothing is going fast, so we get classical laws. In the case of gravity, the road from d/dtau goes to d/dt. Simple, and standard enough.

d/dtau is asking about changes with respect to spacetime intervals. We know the changes with respect to time work for our little solar system. What I am suggesting is that a change in spacetime may in the classical limit also be seen as a change in space. That would require c*d/dR to have the same units.

There are limits to what can be done in ASCII, so they appear as derivatives.

There is nothing radical about the V dm/dt, which people sometimes mention does not amount to squat. There is nothing radical about saying the "truer" force law must be a 4-force law. There is nothing wrong with the units in the switch from d/dt to c*d/dR. Don't worry, I do think it is a darn strange thing to do, but the data is forcing us in an odd direction, and at least the math here is far more constrained, as there are NO new factors or mass distributions, just relativistic rocket science.

From someone who just finished *reading* the paper

[Anonymous Coward]

The real result of Clowe et al's fascinating work was to show that the missing mass in the bullet cluster must be COLLISIONLESS, whatever gravity looks like (a purely baryonic bullet cluster has been *falsified*). However, a big misconception about it was to think it was a direct *confirmation* of the Lambda-CDM concordance model that everybody is supposed to believe (may I recall that real science is about *falsifying* things, not "proving" them right), or that it was falsifying MOND. Actually, it is known for years that MOND is UNABLE to fit the temperature profiles of X-ray emitting clusters from their pure baryonic content. The fix, for MOND to stay in the game, was to propose that neutrinos have a 2eV mass and can then make up for the missing mass, in clusters ONLY, because they are too light to cluster on the galaxy scales (incidentally they are also too light to form structure in GR, but this is not a problem for structure formation in MOND). However, if dark matter is indeed cold as the lambda-CDM guys tend to take for granted, and even more since Clowe's work, why does the 2eV neutrino combined with MOND seem to work in ALL clusters??? The bullet cluster being a totally new kind of constraint for MOND on the galaxy cluster scale (constraint coming from gravitational lensing instead of temperature profiles), it was mandatory to check if 2eV neutrinos were excluded even in MOND, which would have *falsified* MOND indeed! This is what those guys wanted to do, to *falsify* MOND once and for all, but the surprising result is that they didn't manage to do so, because the SAME neutrino mass as the one needed to fit temperature profiles of other clusters ACTUALLY WORKS in the bullet cluster too. Their conclusion is thus just that MOND is *not excluded* by Clowe's data. One will thus have to wait for particle physics experiments to rule out massive neutrinos to rule out MOND. Until then, place your bets...