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

So We Must Wait.

[Mikkeles]

Basically, then, until the mass of the neutrino has been tested, dark matter or alternate gravity are just speculations with the arguments being:

is too!
is not!
is too!

...

It's the Ether

[Fysiks Wurks]

Dark Matter is the 21st century's ether.

Re:It's the Ether

[dmatos]

To the scientific understanding at the time, there was evidence of the aether. It had been observed that light exhibited wave-like characteristics, and could, in fact, be understood as a wave. At that time, all waves were known to travel through a medium. There were no waves that could travel without one. There was no other medium in the vacuum of space, so it was decided that there must be an aether.

A perfectly valid scientific theory, as it was also falsifiable - as demonstrated by Michelson and Morley. When it was falsified, it required a major change in how the scientific communtiy thought about light. It is entirely possible that we'll see something similar with dark matter. Sure, an unobserved WIMP could explain things like the rotation of galaxies at their current rates. But, what happens when we get out there and don't find any? What then? Well, maybe it will require a major change in how we think about gravity. Maybe there's an entirely new force out there, that's weak enough that we can't see it on terrestrial or even solar scales. Who knows?

Re:Anti-dark-matter scientists are like ID scienti

[denominateur]

err, neutrinos do have mass, but not as much as stated in the paper. As far as current experiments go neutrinos come in three flavours and interchanging between them is only possible if they have mass. It has been shown in experiments that they change type and hence must have mass.

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.

Re:Anti-dark-matter scientists are like ID scienti

[FhnuZoag]

Enh.

Neutrinos *do* have mass, and this fact is accepted by pretty much all physicists. The argument for this comes from discovery that they change states over the course of their lives, which means that they experience time, which means that they cannot travel at the speed of light, which means they must have a small mass. (This explains the apparently deficiency of solar neutrinos which was a problem in the 70s) Pinning down the exact value of this mass is more troublesome, though - for now, we know only that it's small, but positive.

What more puzzles me about this statement is that neutrinos have generally been counted as *part* of dark matter - in particular, they are proposed to constitute some of those so-called Weakly Interacting Massive Particles (WIMPs) which is one of two possible models for dark matter. I don't see how changing the details of these particles would change how neccessary they are, unless these guys are trying a bait and switch by redefining dark matter to be unneccessary. (Which would be a very dirty trick.)

Re:why would matter be dark

[SixByNineUK]

I do not beleve that this is true.

Newtonian mechanics implies that for gravity to affect an object it must have a mass, however General Relitivity does not impose this restriction.
I am pretty sure that the gravity effect is caused by the distortion of space time such that the 'shortest' path (That which the light must follow) is curved.

I am not an expert in GR, but perhaps someone here can verfy my claims.

No mass for photons

[jpflip]

I'd like to clear this up because there are very common misconception that photons are massive or that something has to be massive to feel gravity, both of which are false.

THEORY: In our current understanding, photons are forbidden from having mass because of the way quantum electrodynamics (the most precisely tested theory in the history of science) works. It's an exercise in field theory to show it, but the gist is that electromagnetism (light, charge conservation, electric and magnetic forces...) are a consequence of a symmetry of nature, and that symmetry only works if the associated carrier particle (the photon) has exactly zero mass.

EXPERIMENT: If the photon had even a very tiny mass, it would also mean that the electromagnetic interactions would become short range (just like the weak interactions, which are mediated by a massive carrier). The usual inverse square law would become an exponential falloff. This has been tested for in laboratories (and in astronomy!) very precisely, so there are ridiculously strict upper limits on the photon mass.

This doesn't mean photons don't feel gravity!! Gravity interacts with all energy, not just mass, and so the energy of a photon is enough to cause it to bend around massive objects.

Re:why would matter be dark

[Claws Of Doom]

Their paths don't bend - it is the paths themselves that are distorted in space-time by the gravity well. This distortion appears to be bent in three dimensions - to the photon it is perfectly straight...

(ok, ok, simply *massive* oversimplification here - to the point of error, but I hope you understand my motives.)

Re:Anti-dark-matter scientists are like ID scienti

[vondo]

I think I'd regret responding to the complete misunderstanding of forces and neutrinos in the body of your post. That would take pages.

Let me just respond to your title. That is completely wrong as well. Now, I think the alternative gravity guys are probably wrong and at this point I think they are stretching their theories to their limits. Dark matter is the "easiest" explanation. But, what they are doing is science. They are coming up with an alternate theory that makes predictions and testing them. The are countering circumstantial evidence for DM with another theory. They are not picking just one small thing, saying "Well that can't be true because of [insert some non-science babble like you just posted] so clearly God created everything." in contradiction to vast bodies of scientific evidence. And the alternative gravity people are publishing in peer-reviewed journals.

ID can't say any of those things. While the motivations may be similar (not wanting to give up on old ways of thinking about things) the methodology is completely different.

Dark Matter Lite!

[Rob T Firefly]

This article presents an alternative to dark matter

Just as dark as your regular matter, but with only 1/3 the calories!

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.

Clarification: dark matter is STILL real!

[jpflip]

People should NOT take the impression from this article that there is doubt that dark matter exists. The only doubt being raised is over what form the dark matter takes. Let me clarify:

(Note: Baryons are protons and neutrons. "Non-baryonic" means not made up of the building blocks of ordinary atoms.)

The beauty of the Clowes work (the "proof that dark matter exists" from a couple of weeks ago) is that the colliding clusters they worked on give simple, clean evidence that galaxy clusters are really dominated by invisible, non-baryonic dark matter. At it's core, it's a very simple argument. Two clusters collided, and the baryonic clouds (hot gas, seen with X-rays) experienced drag and got a bit hung-up passing through one another. Most of the mass, however (seen with gravitational lensing), passed straight through with no drag. We see the X-rays and lensing in two different places on the sky - they really are two different kinds of stuff. This is VERY direct proof that most of the mass in galaxy clusters is not the ordinary matter we see on earth - it's something non-baryonic that does not interact with light and does not interact much with ordinary matter. In other words, dark matter is real, physical stuff!

This article argues only about what that dark matter might actually be. It's generally believed that it can't be neutrinos, because neutrinos are so light that they would mess up galaxy formation, and so must be some new, exotic kind of particle. The logic here is that very light particles move so fast that they don't clump together well under their own gravity, which would disrupt the formation of galaxies and smaller clusters of galaxies. All this paper argues is that the dark matter might not be a truly new particle - the combination of modified gravity and neutrinos can be made to work. They still conclude that the invisible neutrinos must outmass the baryons in the clusters by a factor of at least 2.5.

Many people (particularly those who do not understand the evidence) dislike the idea of dark matter, thinking it sounds too much like epicycles. That's understandable, and it's good to be very skeptical of such a weird idea (I know I was). The truth is that there is now enough evidence to say that it really does exist, no matter how strange it may seem to us. The future lies is figuring out what the dark matter is actually made of, not bland assertions that "that just can't be right...".

Re:Clarification: dark matter is STILL real!

[jpflip]

That's an excellent question! You've just described the MACHO model of dark matter (Massive Compact Halo Objects). The idea is that there could be cold, compact objects made of ordinary matter filling our dark matter halo and giving us all the extra mass. In this theory, the invisible mass exists but is still "ordinary" - no modification to gravity and no fancy new particles. It contrasts with the more exotic WIMP model of dark matter - Weakly Interacting Massive Particles.

This theory was extremely popular for many years, but has fallen out of favor for two main reasons:

(1) Using studies of the cosmic background radiation and light element abundances, you can conclude that the bulk of the matter in the hot early universe was not made up of baryons ("ordinary matter"). If it were, you would expect very different abundances of deuterium in the universe today and a very different spectrum of fluctuations in the microwave background. So we need most of the universe's matter to be non-baryonic anyway (e.g. WIMPs), and baryonic MACHOs cannot make up all of the missing mass

(2) Every now and then a MACHO should pass in front of a distant star (say, in the Large Magellanic Cloud), producing a "micro-lensing" event. Many collaborations around the world studied the skies for years looking for such events, and did find a few. The number and kind of lensing events they observed, however, was insufficient to account for all of the missing mass.

For these and other reasons, the cosmological community has rejected the MACHO hypothesis. There are objects like that out there, but the bulk of the dark matter must be something else.

simple definitions

[peter303]

"Dark matter": an invisible attractive force operating on galaxy-level distances (at million light years). Size: about 23% of the energy-mass of the observed universe. Evidence: Galaxies spinning faster than the number of visible stars justify. Gravitational lenses stronger than visible stars justify. Suspects: known low mass particles like neutrinos; unknown low or high mass particles like strings, wimps; a new phsyical force; non-r-squared term in Newton's equation of gravitation, observational error ...

"Dark energy": an invisible repulsive force operating on universe-size distances (at billion light years). Size: about 73% of the energy-mass of the observed universe. Evidence: Hubble expansion is accelerating over time when gravity would suggest eventual deceleration or collapse. Suspects: energy in fabric of space-time, unknown force, observational error ...

"Observed matter": stars, galaxies, gas clouds, neutrinos; Size: about 4% of the energy-mass of the universe.

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...

Re:Useful research

[Daniel Dvorkin]

Attention, parent poster and everyone who agrees with him: please immediately cease enjoying benefits of government-funded science. That means logging off /., getting rid of your computer -- in fact, not owning any personal electrical devices whatsoever -- refusing any medical diagnostic procedure developed after the invention of X-rays, and generally living life ca. 1900.

For those who say, "that's technology, not science!" I will note that the examples I gave were based largely on previously abstract, largely government-funded scientific research whose applications were not immediately obvious, but which have since transformed the way we live. If you don't understand the research, that's fine; you don't have to in order to take advantage of it. But just because you don't give a shit about the way things work doesn't mean that you get to stand in the way of people who do, and whose work will benefit you and your children's lives, no matter how little you deserve it.

Re:why would matter be dark

[jfengel]

No. The photon never has any "rest mass". It has momentum without mass. That falls out of the equations. You can think of the speed of light as infinity, because under special relativity the equations have a term 1/sqrt(1-v^2/c^2). When v=c, the term goes to 1/0, and numbers pop out of nowhere.

The term "c" is the speed of light in a vacuum. Experiments like these don't happen in a vacuum. Light isn't really being slowed down per se; it's still moving at c when it's away from the atoms. These experiments are a very clever way to keep the light pulses intact while keeping the light itself from actually going very far. The net effect is to slow down the light pulse without actually slowing the light itself, so the mass of the light is unchanged, as is its net momentum. Inside the system the momentum of the light is bounced around and interacting with electrons, but at that level the light is just behaving as light does with nothing to affect its speed beyond plain old quantum juju, so there's no change in mass or momentum there, either.