Alexandre Le Tiec — Page 3

arXiv2008

Phenomenology of the Modified Newtonian Dynamics and the Concordance Cosmological Scenario

After reviewing the modified Newtonian dynamics (MOND) proposal, we advocate that the associated phenomenology may actually not result from a modification of Newtonian gravity, but from a mechanism of "gravitational polarization" of some dipolar medium playing the role of dark matter. We then build a relativistic model within standard general relativity to describe (at some phenomenological level) the dipolar dark matter polarizable in a gravitational field. The model naturally involves a cosmological constant, and is shown to reduce to the concordance cosmological scenario (Lambda-CDM) at early cosmological times. From the mechanism of gravitational polarization, we recover the phenomenology of MOND in a typical galaxy at low redshift. Furthermore, we show that the cosmological constant Lambda scales like a0^2, where a0 is the constant MOND acceleration scale, in good agreement with observations.

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arXiv2008

Model of Dark Matter and Dark Energy Based on Gravitational Polarization

A model of dark matter and dark energy based on the concept of gravitational polarization is investigated. We propose an action in standard general relativity for describing, at some effective or phenomenological level, the dynamics of a dipolar medium, i.e. one endowed with a dipole moment vector, and polarizable in a gravitational field. Using first-order cosmological perturbations, we show that the dipolar fluid is undistinguishable from standard dark energy (a cosmological constant Lambda) plus standard dark matter (a pressureless perfect fluid), and therefore benefits from the successes of the Lambda-CDM (Lambda-cold dark matter) scenario at cosmological scales. Invoking an argument of "weak clusterisation" of the mass distribution of dipole moments, we find that the dipolar dark matter reproduces the phenomenology of the modified Newtonian dynamics (MOND) at galactic scales. The dipolar medium action naturally contains a cosmological constant, and we show that if the model is to come from some fundamental underlying physics, the cosmological constant Lambda should be of the order of a0^2/c^4, where a0 denotes the MOND constant acceleration scale, in good agreement with observations.

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