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."

religeon of dark matter

[pizpot]

For me, dark matter is like religion. Made up to explain what we can't understand, and wrong.

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.

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[account_deleted]

Comment removed based on user account deletion

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:No mass for photons

[Fordiman]

"If objects lose mass when they emit photons, where does it go?"

Well that's not always true.

In a star, it loses mass which is converted to the energy of the photon. In a lightbulb, the photon's energy comes from the covering of a potential difference in voltage.

The thing is, while E=mc^2 and E=hw (E is Energy; m is mass; c is speed of light; h is planck's constant; w is angular frequency [similar to frequency, but in radians per time unit]) state that more energetic photons have more apparent mass than non-energetic photons (m=hw/c^2), the fact is that they have no rest mass (at rest, w=0, so m=0).

The truth is, however, that a photon's apparent mass is only really useful for momentum calculations. Higher frequency light takes more energy to redirect than lower frequency light.

Though, it gives me a question about the idea behind solar sails:
picture two perfectly paralell low-mass perfect reflectors (ie: no loss in the reflected light). They are in vacuum, and there is no friction. According to the theory that predicts the way a light sail would work, you should be able to shine a light perpendicular to one of the sails from between them, and they would slowly accelerate apart. When you shut the light off, the light bouncing back and forth would keep pushing.

Would the light decrease in frequency until it is 'at rest', and thus nonexistant? If not, where does the energy come from? Would the light, instead, decrease in speed, becoming normal matter? Is this even possible?

Re:No mass for photons

[roemcke]

When you shut the light off, the light bouncing back and forth would keep pushing.

After each "push" the photons will not be reflectet with the same frequency (the sails are moving away from the light. hint: doppler effect)

Occam's Razor != Science

[pavon]

I am fine with the possibility that there is a lot of normal matter which is not detectable from earth. I am also fine with the idea that more exotic forms of matter and energy might exist. The current dark energy models are the best matches for astronomical observations thus far. And when it is all said and done, if dark energy continues to be the best description, it will prevail, but that doesn't mean we shouldn't stop testing it.

At one time, all the scientists thought there was this stuff called ether, and it was the best explanation for the observations we had. Then people did more tests, and discovered incongruities. In the end it was proven an incorrect idea, and was supplanted with a better model.

Einstein spent many years trying to find a deterministic alternative to quantum mechanics. There were many respectable scientists that felt that QM was merely a useful approximation, but after years of testing, a the consensus finally turned, and the community accepted that the non-deterministic aspects of QM were real.

Should we have blindly accepted Ether or QM, just because preliminary results showed promise with the ideas? No - we continued to question them and test them until they were disproved or time had shown them to be solid ideas. Dark Matter is in the same place as these theories once were. I don't know whether it will turn out to be correct or not, but I do know we should continue to challenge it, to think of new ways to test it, and to think of alternative explanations, because that is what science is about and that is how we take good ideas and turn them into a rigorous and well-established understanding of the universe.

You would call these people pseudo-scientists, and yet your only argument an application of Occam's Razor (and as others pointed out, faulty understanding of principles). But that's the funny thing about Occam's Razor - it is dependant on one's personal opinion of what is the most likely, or most simple explanation. Some would consider making up new particles that we have never observed a real stretch, others consider tweaking the existing rules a hack. That someone has a different view of what is elegant than you, does not make their ideas pseudo-science. What matters is if they are predictive and falsifiable, which these are.

Honestly, if you can't tell the difference between people that present testable alternative hypothesis, and people whose best "theory" that they could present amounts to "does this not appear irreducible", then you are the one that needs a refresher on what is and is not 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...

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.

Re:why would matter be dark

[eonlabs]

The real question is "Is the converse true" As light propagates through the universe, does it warp space/time as well, allowing it to attract other bodies. Would high intensity light warp more than low intensity? Would high frequency light warp more than low frequency?