When i read "Alternative" i immediately thought "crackpots!", but then I read on and found out they're on to interesting stuff.(adsbygoogle = window.adsbygoogle || []).push({});

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

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# Alternative theory of gravity explains large structure formation without dark matter

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