Dark Matter or a New Force?

This paper presents a brief review of the evidence for dark matter in the Universe on the scales of galaxies. In the interests of critically and objectively testing the dark matter paradigm on these scales, this evidence is weighed against that from the only other game in town, modified Newtonian dynamics. The verdict is not as clear cut as one might have hoped.

Flat galaxy rotation curves and the accelerating Universe both imply the existence of a critical acceleration, which is of the same order of magnitude in both the cases, in spite of the galactic and cosmic length scales being vastly different. Yet, it is customary to explain galactic acceleration by invoking gravitationally bound dark matter, and cosmic acceleration by invoking a `repulsive` dark energy. Instead, might it not be the case that the flatness of rotation curves a...

Any connection between dark matter and extra dimensions can be cognizably evinced from the associated effective energy-momentum tensor. In order to investigate and test such relationship, a higher dimensional spacetime endowed with a factorizable general metric is regarded to derive a general expression for the stress tensor -- from the Einstein-Hilbert action -- and to elicit the effective gravitational potential. A particular construction for the case of six dimensions is p...

A solution to Einstein's field equations via the Friedman equations is shown to produce a cosmological model that is in exact agreement with the measurements made by the dark energy astronomers. All the essential physical parameters are obtained as epoch dependent functions all in closed form. The equations of state are obtained for total density, non-dark energy density and dark energy density. An interpretation of the structure involving a dark energy mass distribution that...

It is still a challenging problem to the theoretical physicists to know the exact nature of the galactic dark matter which causes the galactic rotational velocity to be more or less a constant. We have proposed that the dark matter as an effect of f(T) gravity. Assuming the flat rotation curves as input we have shown that f(T) gravity can explain galactic dynamics. Here, we don' have to introduce dark matter. Spacetime metric inspired by f(T) gravity describes the region up t...

Spiral galaxies are considered as static, spherically symmetric Dark Matter Configurations (DMC)in which non-zero rest-mass particles (NZRPs) move along appropriate trajectories. Using general relativity (GR), we show that a mass of dark matter about 127 - 212 $\times 10^{11} M_{\odot}$ is required inside the sphere of size $a \sim 10 $ kpc for agreement with the observed typical orbital velocity ranging from 150 to 250 km sec$^{-1}$. In our model, it is possible to obtain fl...

Without observational or theoretical modifications, Newtonian and general relativity seem to be unable to explain gravitational behavior of large structure of the universe. The assumption of dark matter solves this problem without modifying theories. But it implies that most of the matter in the universe must be unobserved matter. Another solution is to modify gravitation laws. In this article, we study a third way that doesn't modify gravitation neither matter's distribution...

Rotation curves of spiral galaxies are known with reasonable precision for a large number of galaxies with similar morphologies. The data implies that non-Keplerian fall--off is seen. This implies that (i) large amounts of dark matter must exist at galactic scales or (ii) that Newtonian gravity must somehow be corrected. We present a method for inverting the integral relation between an elemental law of gravity (such as Newton's) and the gravitational field generated by a thi...

To the astrophysicist faced with the puzzle of dark matter, this one appears under two different aspects: on the one hand in cosmology, i.e. at very large scales, where it seems to be made of a bath of particles; on the other hand at the scale of galaxies, where it is described by a set of specific phenomena, looking incompatible with a description in terms of particles, and suggesting that we are seeing a modification of the law of gravity. Reconciling these two distinct asp...

The existence of "dark matter", inferred from the observed rotation curves of galaxies, is a hypothesis which is widely regarded as problematic. This paper proposes an alternative hypothesis based on the space-time geometry near a rotating body and formulated in terms of the dragging of inertial frames. This hypothesis is true in a certain linear approximation to General Relativity (D Sciama, Mon. Not. Roy. Astron. Soc. 113 (1953) 34--42) and is justified in general by Mach's...