Alternative theory of gravity explains large structure formation without dark matter

[SF]

When i read "Alternative" i immediately thought "crackpots!", but then I read on and found out they're on to interesting stuff.

While altering the theory of gravity may seem like pulling the rug out from under a century of observations and pain-staking calculations, an alternative theory may simply be “more correct” than today’s standard theory. Just as Einstein’s theory was “more correct” than Newton’s because it improved upon the older one by noticing more specific details (e.g. extraordinary masses and speeds), a new alternative theory may only drastically change gravity at certain scales.

“Perhaps a fundamental theory of gravity which differs from general relativity on large scales can explain the observations without recourse to new, unobserved particles,” wrote Dodelson and Liguori in their study published in Physical Review Letters. “Now more than ever before, there are very good reasons to explore this idea of modifying gravity. For, the case of dark energy also hinges on the assumption that general relativity describes gravity on larges scales. Dark energy is even more difficult to explain than dark matter, so it seems almost natural to look at gravity as the culprit in both cases.”

The new theory (or groundwork for it) under investigation would be Jacob Bekenstein’s relativistic covariant theory of gravity (TeVeS), published in 2004. Bekenstein based his theory on a modified version of Newtonian theory from the early ‘80s, dependent on gravitational acceleration and called modified Newtonian dynamics (MOND) by its founder, Mordecai Milgrom.

“MOND, the original theory on which TeVeS is based, was already quite successful at explaining galactic dynamics (even better, in some cases, than the dark matter paradigm), but it failed completely at explaining other observations—gravitational lensing in particular,” explained Liguori. “For this reason, it couldn't be considered a real alternative to dark matter. Bekenstein’s theory, by generalizing MOND, retains its good features while overcoming its main problems at the same time. This makes TeVeS a much more interesting theory than MOND. It is then worthwhile (and necessary) to test TeVeS’ predictions in detail and compare them to the standard dark matter paradigm to see if TeVeS can be a viable alternative.”

Dodelson and Liguori find Bekenstein’s theory intriguing in this context because, for one, the gravitational acceleration scale in the theory is very close to that required for the observed acceleration of the Universe. The scale is also very similar to that proposed in “post hoc” theories such as dark energy. Even more interesting is the fact that the origins of Bekenstein’s theory had nothing to do with cosmic acceleration.

But the feature of Bekenstein’s theory that Dodelson and Liguori focus on most is that the theory—unlike standard general relativity—allows for fast growth of density perturbations arising from small inhomogeneities during recombination. Building on this finding from scientists Skordis et al. earlier this year, Dodelson and Liguori have found which aspect of the theory actually causes the enhanced growth—the part that may solve the cosmological structure problem.

The pair has discovered that, while Bekenstein’s theory has three functions which characterize space-time—a tensor, vector and scalar (TeVeS)—it’s the perturbations in the vector field that are key to the enhanced growth. General relativity describes space-time with only a tensor (the metric), so it does not include these vector perturbations.

“The vector field solves only the enhanced growth problem,” said Dodelson. “It does so by exploiting a little-known fact about gravity. In our solar system or galaxy, when we attack the problem of gravity, we solve the equation for the Newtonian potential. Actually, there are two potentials that characterize gravity: the one usually called the Newtonian potential and the perturbation to the curvature of space. These two potentials are almost always very nearly equal to one another, so it is not usually necessary to distinguish them.

http://www.physorg.com/news85310822.html

 

[Mike2]

When i read "Alternative" i immediately thought "crackpots!", but then I read on and found out they're on to interesting stuff.

http://www.physorg.com/news85310822.html

There's also Conformal Gravity theory promoted by Philip D. Mannheim. This theory is suppose to explain dark energy and dark matter by suggesting that gravity gets weaker at great distances. It sounds something like MOND but based more on first principles than on curve fitting.
See:

http://arxiv.org/abs/gr-qc/9407010

He also has a video at the Perimeter Institute. I can provide a pointer to the video if there is interest. Lee Smolin appears in the lecture which tends to lend the idea some credibility.

I have to wonder if the recent extented supernova data rules out any of these alternative theories.

 

[Wallace]

Modified Gravity vs Dark Matter has been a long running debate. However recent observations of an interesting system of colliding clusters has strongly tipped the balance towards dark matter. http://cosmicvariance.com/2006/08/21/dark-matter-exists/" explains it far better than I could.

In general though, the problem with modified gravity in general is that the parameters describing the magnitude of the modification to GR/Newtonian gravity come out to be different every time you try and measure them. So you can could explain one galaxy rotation curve by a modified gravity model, but if you apply that model to a different galaxy the parameters need to be tweaked. The same goes for clusters and large scale cosmology. Of course Dark Matter dosn't have this problem as you would expect the amount of dark matter present in a galaxy halo to vary from system to system.

The reason the dark matter is favored at present over modified gravity is that it fits a wide range of data with the same parameters. There are no modified gravity theories at present that can do better than dark matter with a single parameter set. This of course may change in the future!

 

[Nereid]

There's also an apparent (to me) selectivity in the galaxies the MONDians seem to have chosen to model the rotation curves of (if that makes sense).

On McGaugh's 'scorecard' webpage (don't have a link with me just now), some very well known spirals are missing - M31 and M81, for example. These are among the first to have had rotation curves measured, and among the best studied of spirals, period. How come no MONDian has chosen to analyse them?

 

[SF]

Just found on PhysOrg: http://www.physorg.com/news85326859.html

 

[SF]

Experiment sets the ultimate test for Newton's laws

A physicist in Australia has come up with an experiment that could potentially reveal a flaw in Newton's law of gravitation. If the flaw exists, it would be the first evidence in support of theories that explain the movement of galaxies without having to introduce "dark matter" (Phys. Rev. Lett. 98 101101).

For the past 70 years or so, physicists have been bothered by a nagging question: why do the centres of galaxies rotate too fast for the amount of mass we can see through telescopes? The most popular answer is that most of the mass is hidden in large bands of "dark matter", a substance that is invisible because it doesn't interact strongly with light. If it exists, dark matter could account for 95% of the mass in galaxies, and would explain many other aspects of the universe.
X marks the spot


However, a lack of evidence for dark matter has led a small camp of physicists to promote an alternative answer: the gravitational force that holds galaxies together decays more gently with distance than presently estimated, meaning that Newton's law of gravitation is not quite as simple as an inverse-square relationship. The theory, which is known as modified Newtonian dynamics (MOND), proposes adding extra factors to Newton's 300-year-old equations so that the gravitational behaviour only alters at very low accelerations. Unfortunately the turmoil of gravitational forces produced in the galaxy means that such accelerations are hard to come by, leaving proponents of MOND with no easy way to test their theory.

However, Alex Ignatiev from the Theoretical Physics Research Institute in Melbourne claims to have predicted instances on the Earth where most of these forces will cancel out. Ignatiev first considered how an object at rest in the centre-of-mass of our galaxy would appear to be accelerating when viewed from a laboratory on Earth. This involved listing all the major accelerations such as the Earth's rotation around the Sun and the Sun's orbit in our galaxy. He then looked for solutions where all of the accelerations add up to zero.

The solutions indicated that, on either of the two annual equinoxes, there will be two places on the Earth's surface where the force cancellation occurs. For example, on the equinox of 22 September 2008, one will be in the far north of Greenland and the other will be on the opposite side of the world in Antarctica (see figure: "X marks the spot"). Ignatiev says that if a gravitational wave detector is set-up to monitor a static test object at one of these times and places, it might just be able to glimpse a tiny, 0.2 × 10-16 m deflection over a period of 0.5 ms – what he calls "SHLEM" (static high-latitude equinox modified inertia). If SHLEM is observed, it would be the first evidence in support of MOND.

"Even if the result were negative it would be a very significant step forward, because an interesting theory would be ruled out," Ignatiev told Physics Web. "But if the predicted SHLEM effect were observed – well, we'd have to rewrite our most basic theories."

http://physicsweb.org/articles/news/11/3/12/1

 

[emc2cracker]

Modified Gravity vs Dark Matter has been a long running debate. However recent observations of an interesting system of colliding clusters has strongly tipped the balance towards dark matter. http://cosmicvariance.com/2006/08/21/dark-matter-exists/" explains it far better than I could.

In general though, the problem with modified gravity in general is that the parameters describing the magnitude of the modification to GR/Newtonian gravity come out to be different every time you try and measure them. So you can could explain one galaxy rotation curve by a modified gravity model, but if you apply that model to a different galaxy the parameters need to be tweaked. The same goes for clusters and large scale cosmology. Of course Dark Matter dosn't have this problem as you would expect the amount of dark matter present in a galaxy halo to vary from system to system.

The reason the dark matter is favored at present over modified gravity is that it fits a wide range of data with the same parameters. There are no modified gravity theories at present that can do better than dark matter with a single parameter set. This of course may change in the future!

YOu got to be kidding me, the same parameters eh? Can you show ANY consistent relation to dark matter and visible matter? No there is not, dark matter is simply adding up visible mass, then filling in the balance with dark matter. Every single time new objects are discovered it changes the so called dark matter. So MOG needs far less tweaking, there is no purpose in applying different theories to ever more distant and less known galaxies and clusters because until we detect EVERY object in one galaxy we cannot make any leap over the simple "theory" term. No theory is above proof, even relativity with dark matter. Actually I would make the opposite argument.

Whichever model works the best with the closest and more studied celestial groups with the least amount of variable integers would be the best. In other words we should put all our effort into explaining something we can see well enough to discover it all. I get so angry when people throw up, well this theory can't explain this cluster, or this galaxy, or this far flung corner of space we know next to nothing about. We are still discovering massive objects close to home that change our makeup of galaxies. Magnetars are my favorite example, here is a massive object we know very little to nothing about, new observations are placing more and more mass in our universe all the time. Once we have observed enough of these types of bodies, then we can possibly accurately approximate their distribution throughout our galaxy, or others.

I mean let's start close to home, relativity cannot even explain how far our most distant probes have traveled without shoveling in undetectable energy sources and mass. They have had plenty of time to discover dark matter, its not going to happen relativity and dark matter are toast. Time to at least stop pretending we can understand and predict all the movements in the universe, there will be no magic equation until maybe we have colonized the entire galaxy and have detected every last object in our own backyard.

 

[emc2cracker]

Experiment sets the ultimate test for Newton's laws

A physicist in Australia has come up with an experiment that could potentially reveal a flaw in Newton's law of gravitation. If the flaw exists, it would be the first evidence in support of theories that explain the movement of galaxies without having to introduce "dark matter" (Phys. Rev. Lett. 98 101101).

For the past 70 years or so, physicists have been bothered by a nagging question: why do the centres of galaxies rotate too fast for the amount of mass we can see through telescopes? The most popular answer is that most of the mass is hidden in large bands of "dark matter", a substance that is invisible because it doesn't interact strongly with light. If it exists, dark matter could account for 95% of the mass in galaxies, and would explain many other aspects of the universe.
X marks the spot


However, a lack of evidence for dark matter has led a small camp of physicists to promote an alternative answer: the gravitational force that holds galaxies together decays more gently with distance than presently estimated, meaning that Newton's law of gravitation is not quite as simple as an inverse-square relationship. The theory, which is known as modified Newtonian dynamics (MOND), proposes adding extra factors to Newton's 300-year-old equations so that the gravitational behaviour only alters at very low accelerations. Unfortunately the turmoil of gravitational forces produced in the galaxy means that such accelerations are hard to come by, leaving proponents of MOND with no easy way to test their theory.

However, Alex Ignatiev from the Theoretical Physics Research Institute in Melbourne claims to have predicted instances on the Earth where most of these forces will cancel out. Ignatiev first considered how an object at rest in the centre-of-mass of our galaxy would appear to be accelerating when viewed from a laboratory on Earth. This involved listing all the major accelerations such as the Earth's rotation around the Sun and the Sun's orbit in our galaxy. He then looked for solutions where all of the accelerations add up to zero.

The solutions indicated that, on either of the two annual equinoxes, there will be two places on the Earth's surface where the force cancellation occurs. For example, on the equinox of 22 September 2008, one will be in the far north of Greenland and the other will be on the opposite side of the world in Antarctica (see figure: "X marks the spot"). Ignatiev says that if a gravitational wave detector is set-up to monitor a static test object at one of these times and places, it might just be able to glimpse a tiny, 0.2 × 10-16 m deflection over a period of 0.5 ms – what he calls "SHLEM" (static high-latitude equinox modified inertia). If SHLEM is observed, it would be the first evidence in support of MOND.

"Even if the result were negative it would be a very significant step forward, because an interesting theory would be ruled out," Ignatiev told Physics Web. "But if the predicted SHLEM effect were observed – well, we'd have to rewrite our most basic theories."

http://physicsweb.org/articles/news/11/3/12/1

Mond isn't close either, you have to use some kind of vector field to get close. Apply MOND to gravitational lensing and its dead in the water. MOND cannot account for gravitational lensing unless you add vector fields.

 

[twofish-quant]

No there is not, dark matter is simply adding up visible mass, then filling in the balance with dark matter.

No it's not. There are specific properties that dark matter has to have for this to work. Just to name one, the dark matter has to be cold.

Every single time new objects are discovered it changes the so called dark matter.

No it doesn't. All of the new objects that have been discovered are baryonic (i.e. made of ordinary matter) so we find something that isn't baryonic, it's totally irrelevant to dark matter.

Whichever model works the best with the closest and more studied celestial groups with the least amount of variable integers would be the best.

Just because it's closer doesn't mean it's better studied.

Magnetars are my favorite example, here is a massive object we know very little to nothing about, new observations are placing more and more mass in our universe all the time.

I don't think you quite understand the nature of the problem. Suppose you have lots of hidden ordinary matter in the universe, you then have a big problem because this increases the density of the early universe, and the denser the universe is, the more deuterium gets burned off. Also, you calculate the distribution of galaxies, and if the dark matter was baryonic, then the galaxy distributions would be less "fluffy."

If it turns out that there is a lot more ordinary matter than think is there, then we really have some major problems. So finding lots of unknown objects made of ordinary matter actually causes the dark matter problem to get worse, not better.

Also, what you are saying in general makes no sense. I know more about the moon and the sun than I do about Rio de Janeiro, since I can see the moon and the sun, and I've never been to Rio. Similarly, we have a *lot* of information from the early universe, but we don't have that much information about the Oort cloud or planets around Alpha Centauri.

 

[Vanadium 50]

Apply MOND to gravitational lensing and its dead in the water. MOND cannot account for gravitational lensing unless you add vector fields.

There are plenty of reasons to dislike MOND, but this is not one of them. The N in MOND stands for "Newtonian", and MOND does little to constrain the family of relativistic extensions. So the proper question to ask is whether the observed lensing is consistent with a relativistic extension of MOND or not, and this is precisely the question asked by Mortlock & Turner (2001, PASA, 18, 189), who concluded that it was.

 

[emc2cracker]

There are plenty of reasons to dislike MOND, but this is not one of them. The N in MOND stands for "Newtonian", and MOND does little to constrain the family of relativistic extensions. So the proper question to ask is whether the observed lensing is consistent with a relativistic extension of MOND or not, and this is precisely the question asked by Mortlock & Turner (2001, PASA, 18, 189), who concluded that it was.

I'm sorry but there has been many papers published since 2001 completely discrediting MOND's lensing explanations. NOt unless your talking of the TeVeS brand. Just Google MOND and lensing and look at the current info. I feel like I"m the only person here that watches the cosmic news lol. The evidence is completely overwhelming, the only way MOND is going to have any progress is under the relativistic generalization of TeVeS. Or by publishing some papers explaining the lensing data held against it. TeVeS has been able to account for lensing effects, however hasn't done the CMB yet.

You know I want to say that there is no complete theory. One day we may be able to predict all movements and everything in existence but that will never mean we can understand what forces do the moving, nor does it really matter because we will always continue searching for a better universe. We may one day find out the universe is in a locker, but you know what we would do then? Start making theories on what's down the hall...

 

[heldervelez]

Without a model (observational data,then model with some principle, then check) otherwise is pure data fit.
but at the extent of available data only a new model can solve the puzzle.
open your mind, find it, discuss it, be prepared to put our actual convictions to test.