A geometrical justification for spiral galaxies rotation curves
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ORIGINAL ARTICLE
A geometrical justification for spiral galaxies rotation curves Mohammad Rahim Bordbar1 · Leila Aghababaei1
Received: 7 September 2019 / Accepted: 16 March 2020 © Springer Nature B.V. 2020
Abstract In this paper, we construct dark halo models which are merely supported by geometry of space time. We concentrate on the inhomogeneous space time around some spiral galaxies whose have dark matter. After characterizing the dark halo models, the possibility of the energy density of such a field which could justify the rotation curves of stars in the dark halo is explored. Keywords Dark halo · Inhomogeneous model
1 Introduction In last few decades, the existence of dark matter in different scales of the universe from galaxies (Rubin and Ford 1970; Roberts and Rots 1973) to cluster of galaxies (Zwicky 1933) and ultimately to cosmic scales (Jorg et al. 2012) has been proven by astronomers. According to Newton’s law, we expect the rotation curves drop as √1r for stars that located far from the center of the spiral galaxies. But measuring the velocities show that the rotation curves does not decrease but remain constant with increasing radius (Sahni 2004). It should be emphasized that although the rotation curves of most galaxies tend to become flat (Mielke et al. 2009; Korsaga et al. 2019 and Erroz-Ferrer et al. 2016), there are some cases that do not follow from this rule. Indeed, these galaxies have no dark matter (Dokkum et al. 2019). For ones with flat curves, the gravitational field due to the baryonic matter is not sufficient to account for flat rotation curves and
B M.R. Bordbar
[email protected] L. Aghababaei [email protected]
1
Department of Physics, University of Qom, Qom, Iran
significant amounts of dark matter should be exist as well. The density of this matter should decrease as r12 for spherical halo, with distance to the center of galaxy (or cluster of galaxies). Nowadays, we know that dark matter is responsible for about 26% of the total cosmic energy density (Planck Collaboration et al. 2016) but real physical nature is still unknown. To date the quest to understand the real nature of dark matter remains as main question. However, some investigations have been carried out by invoking baryonic matter but they don’t have sufficient density to explain flat rotation curves (Alcock 2003). Indeed, luminous matter comprise 4% out of 26% of what try to explain the flat rotation curves in small scales (around galaxies cluster or some spiral galaxies) or in large scales for account for gravitational lensing. The other nonbaryonic candidates concerning dark matter have been proposed in the literature namely, scalar fields (Hees et al. 2016), wimps, heaving neutrino, etc. In this article, we focus on the possibility of existence of dark halo on around spiral galaxies (or cluster of galaxeis) by invoking an inhomogeneous space-time. A subclass of Szekeres-Szafron metric which are supported by an isotropic fluid are considered. The equations which govern to the unknown metric coefficients and
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