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Where the Equivalence Principle Breaks Down

Modified gravity theories violate the equivalence principle. If they are correct, we should see the effect in nearby galaxies.

One way to explain the puzzling acceleration of the universe is to modify Einstein’s theory of general relativity to create a fifth force that can account for the acceleration. These theories have to be carefully constructed so that this fifth force works on the cosmic scale but not on the scale of planets like ours, where we’d have spotted it by now.

Today, Christopher Stubbs from Harvard University in Cambridge and a couple of buddies from Columbia University in New York city examine the consequences of this idea.

Their main result is that on a galactic scale, these modified versions of gravity would cause a failure of the equivalence principle: so gravitational and inertial mass would no longer be the same for galactic objects. That means large objects like galaxies would not all fall at the same speed.

That should have some easily observable effects. For example, small galaxies should accelerate faster than large galaxies while stars and diffuse gas in small galaxies should have different velocities, even if they
are on the same orbits.

So with the right kind of gear it should be possible to confirm the predictions of these modified theories of gravity or put strong limits on the influence they must be having.

Better get those lens cloths out and start dusting off a few old plates.

Ref: arxiv.org/abs/0905.2966: Equivalence Principle Implications of Modified Gravity Models

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