Modified models of gravity

The existence of anomalous astrophysical and cosmological phenomena like the cosmic acceleration and the dynamics of clusters of galaxies and gas in galaxies themselves recently boosted the growth of several long-range modifications of the usual laws of gravitation. Indeed, the aforementioned phenomena did not found satisfactorily explanations in terms of the standard Newton–Einstein gravitational physics, unless exotic and still undetected forms of matter-energy are postulated. Examples of such entities are, e.g, dark matter and dark energy, identified with the Cosmological Constant. At galactic scales, the most famous modified model of gravity is, perhaps, the MOdified Newtonian Dynamics (MOND) proposed in 1983 by Mordehai Milgrom ^[1]. More recently, also the MOdified Gravity (MOG) by John Moffat ^[2] took place.

MOND and MOG

While MOND modifies Newton's inverse square law at a characteristic acceleration scale $a_0\approx 1\times 10^{-10} {\rm m\ s}^{-2}$, MOG's modifications occur at large distances, i.e. MOG is a length scale-dependent model.

DGP model

Another proposal to accommodate the cosmic acceleration without invoking the concept of dark energy is the multidimensional braneworld model by Gia Dvali, Gregory Gabadadze and Massimo Porrati (DGP model). In it our ordinary space-time is a 3+1 dimensional brane embedded in a five-dimensional bulk space.

References

1. ^ Milgrom, M. (July 1983). "A modification of the Newtonian dynamics as a possible alternative to the hidden mass hypothesis". The Astrophysical Journal 270: 365–370. Bibcode 1983ApJ...270..365M. doi:10.1086/161130. 
2. ^ Moffat, J.W. (2006.). "Scalar–tensor–vector gravity theory". Journal of Cosmology and Astroparticle Physics 03. arXiv:gr-qc/0506021. Bibcode 2006JCAP...03..004M. doi:10.1088/1475-7516/2006/03/004. 004.