Although the Nobel Prize last year went for the accelerated expansion of the Universe, in fact *acceleration* is not a many-sigma result. What *is* a many-sigma result is that the expansion is *not decelerating* by as much as it should be given the mass density. This begs the question: Could gravity be weaker than expected on cosmological scales? Models with, say, an exponential cutoff of the gravitational force law at long distances are theoretically ugly (they are like massive graviton theories and usually associated with various pathologies) but as *empirical objects* they are nice: A model with an exponentially suppressed force law at large distance is predictive and simple.

The idea is to compute the detailed expansion history and linear growth factor (for structure formation) for a homogeneous and isotropic universe and compare to existing data. By how much is this ruled out relative to a cosmological-constant model? The answer may be a lot

but if it is only by a few sigma, then I think it would be an interesting straw-man. For one, it has the same number of free parameters (one length scale instead of one cosmological constant). For two, it would sharpen up the empirical basis for acceleration. For three, it would exercise an idea I would like to promote: Let's choose models on the joint basis of theoretical reasonableness and computability, not theoretical reasonableness *alone*! If we had spent the history of physics with theoretical niceness as our top priority, we would never have got the Bohr atom or quantum mechanics!

One amusing note is that if gravity *does* cut off at large scales, then in the very distant future, the Universe will evolve into an inhomogeneous fractal. Fractal-like inhomogeneity is something I have argued against for the present-day Universe.

Although there is an explosion of papers on modified gravity theories,I think any modification to GR to explain dark energy or dark matter

ReplyDeletealways runs into some problem either in strong field limit or wrt stability.

However wanted to draw your attention to an extension(rather than modification) to GR called Einstein-Cartan-Kibble-Sciama gravity on which very few people are working on which has recently been shown

could solve the flatness and horizon problem without inflation, baryon asymmetry, cosmological constant problem, arrow of time, etc.

Would be interested to hear your opinion on this.

Although there are lot of papers on modified gravity , all such theories

ReplyDeletedesigned to explain dark energy or dark matter always run into some problems with instability, or clash with solar system tests, or problems in strong field. There is however one extension (not modification) to GR

known as Einstein-Cartan-Kibble-Sciama gravity, which has recently been shown to solve initial singularity, horizon and flatness problem(without inflation), baryon asymmetry as well cosmological constant problem. Unfortunately very few people are working on this and you won't get to hear much of this in cosmology conferences.

Would be interested to know your take on this.