F. Siddhartha Guzman

Scalar fields as dark matter in spiral galaxies

We present a model for the dark matter in spiral galaxies, which is a result of a static and axial symmetric exact solution of the Einstein-dilaton theory. We suppose that dark matter is a scalar field endowed with a scalar potential. We obtain that (a) the effective energy density goes like 1/(r 2 +rc 2 ) and (b) the resulting circular velocity profile of test particles is in good agreement with the observed one.

Spherical scalar field halo in galaxies

We study a spherically symmetric fluctuation of scalar dark matter in the cosmos and show that it could be the dark matter in galaxies, provided that the scalar field has an exponential potential whose overall sign is negative and whose exponent is constrained observationally by the rotation velocities of galaxies. The local space-time of the fluctuation contains a three-dimensional spacelike hypersurface with a surplus of angle.

Gravitational cooling of self-gravitating Bose-Condensates

Equilibrium configurations for a self-gravitating scalar field with self-interaction are constructed. The corresponding Schrodinger-Poisson (SP) system is solved using finite differences, assuming spherical symmetry. It is shown that equilibrium configurations of the SP system are late-time attractor solutions for initially quite arbitrary density profiles, which relax and virialize through the emission of scalar field bursts—a process dubbed gravitational cooling. Among other potential applications, these results indicate that scalar field dark matter models (in their different flavors) tolerate the introduction of a self-interaction term in the SP equations. This study can be useful in exploring models in which dark matter in galaxies is not pointlike.

Evolution of the Schrodinger-Newton system for a self-gravitating scalar field

Using numerical techniques, we study the collapse of a scalar field configuration in the Newtonian limit of the spherically symmetric Einstein-Klein-Gordon (EKG) system, which results in the so called Schrodinger-Newton (SN) set of equations. We present the numerical code developed to evolve the SN system and topics related, like equilibrium configurations and boundary conditions. Also, we analyze the evolution of different initial configurations and the physical quantities associated to them. In particular, we readdress the issue of the gravitational cooling mechanism for Newtonian systems and find that all systems settle down onto a 0-node equilibrium configuration. PACS numbers: 04.40.-b, 98.35.Jk, 98.62.Gq I. INTRODUCTION In a previous paper of ours(1), we studied the forma- tion of a gravitationally bounded object comprised of scalar particles, under the influence of Newtonian gravity. The dynamics of the system is described by the coupled Schrodinger-Newton (SN) system of equations, which is nothing but the weak field limit of its general relativistic counterpart, the Einstein-Klein-Gordon (EKG) system. As at the moment we have no hints to finding an an- alytic solution for the evolution, we found necessary to develop numerical techniques to study the formation pro- cess of the scalar objects. The study of the dynami- cal properties of the fully time-dependent SN system has been done before in the literature(2, 3, 4, 5), but more is needed in order to understand the gravitational collapse of a weakly gravitating scalar field. The main aim of this paper is to perform an exhaustive numerical study of the collapse and evolution of a spher- ically symmetric scalar object in the Newtonian regime. Here, we develop a numerical strategy to evolve the SN system, and study important issues like the stability and the formation process of gravitationally bound scalar sys- tems, a topic that has recently become attractive in Cos- mology (1, 2, 5, 6, 7, 8, 9). A summary of the paper is as follows. In Sec. II, we present the relativistic EKG and Newtonian SN equa- tions that describe the evolution of a self-gravitating scalar field in the spherically symmetric case. Corre- spondingly, it is described how the EKG and the SN

Newtonian collapse of scalar field dark matter

In this paper, we develop a Newtonian approach to the collapse of galaxy fluctuations of scalar field dark matter under initial conditions inferred from simple assumptions. The full relativistic system, the so-called ‘

Galactic collapse of scalar field dark matter

We present a scenario for core galaxy formation based on the hypothesis of scalar field dark matter. We interpret galaxy formation through the collapse of as calar field fluctuation. We find that a cosh potential for the self-interaction of the scalar field provides a reasonable scenario for the formation of a galactic core plus a remnant halo, which is in agreement with cosmological observations and phenomenological studies in galaxies. PACS numbers: 0425D, 9530S, 9535, 9862A, 9880 In the last few years, the quest concerning the nature of the dark matter in the universe has received much attention and has become of great importance for understanding the structure formation in the universe. Some candidates for dark matter have been discarded and some others have recently appeared. The standard candidates of the cold dark matter (CDM) model are axions and WIMP’S (weakly interacting massive particles), which are themselves not free of problems. Axions are massive scalar particles with no self interaction. In order for axions to be an essential component of the dark matter content of the universe, their mass should be m ∼ 10 −5 eV. With this axion mass, the scalar field collapses forming compact objects with masses of the order of Mcrit ∼ 0.6 m 2 m ∼ 10 −6 M� [1, 2], which corresponds to objects with the mass of a planet. Since the dark matte rm ass in galaxies is ten times higher than the luminous matter, we would need tenths of millions of such objects around the solar system, which is clearly not the case. On the other hand, there are many viable particles with nice features in super-symmetric theories, such a sW IMP’S, which behave just like standard CDM. However, a central debate nowadays is whether CDM can explain the observed scarcity of dwarf galaxies and the smoothness of the galactic-core matter densities, since high resolution numerical simulations with standard CDM predict an excess of dwarf galaxies and density

Towards standard testbeds for numerical relativity

In recent years, many different numerical evolution schemes for Einsteins equations have been proposed to address stability and accuracy problems that have plagued the numerical relativity community for decades. Some of these approaches have been tested on different spacetimes, and conclusions have been drawn based on these tests. However, differences in results originate from many sources, including not only formulations of the equations, but also gauges, boundary conditions, numerical methods and so on. We propose to build up a suite of standardized testbeds for comparing approaches to the numerical evolution of Einsteins equations that are designed to both probe their strengths and weaknesses and to separate out different effects, and their causes, seen in the results. We discuss general design principles of suitable testbeds, and we present an initial round of simple tests with periodic boundary conditions. This is a pivotal first step towards building a suite of testbeds to serve the numerical relativists and researchers from related fields who wish to assess the capabilities of numerical relativity codes. We present some examples of how these tests can be quite effective in revealing various limitations of different approaches, and illustrating their differences. The tests are presently limited to vacuum spacetimes, can be run on modest computational resources and can be used with many different approaches used in the relativity community.

Accretion disk onto boson stars: A way to supplant black hole candidates

The emission spectrum from a simple accretion disk model around a compact object is compared for the cases of a black hole (BH) and a boson star (BS) playing the role of the central object. It was found in the past that such a spectrum presents a hardening at high frequencies; however, here it is shown that the self-interaction and compactness of the BS have the effect of softening the spectrum; the less compact the star is, the softer the emission spectrum at high frequencies. Because the mass of the boson fixes the mass of the star and the self-interaction the compactness of the star, we find that, for certain values of the BS parameters, it is possible to produce similar spectra to those generated when the central object is a BH. This result presents two important implications: (i) Using this simple accretion model, a BS can supplant a BH in the role of compact object accreting matter, and (ii) within the assumptions of the present accretion disk model, we do not find a prediction that could help distinguish a BH from a BS with appropriate parameters of mass and self-interaction.

Scalar field dark matter : Nonspherical collapse and late-time behavior

Instituto de F´isica y Matem´aticas, Universidad Michoacana de San Nicol´as de Hidalgo. Edificio C-3,Cd. Universitaria, A. P. 2-82, 58040 Morelia, Michoac´an, M´exico.(Dated: February 5, 2008)We show the evolution of non-spherically symmetric balls of a self-gravitating scalar field in theNewtonian regime or equivalently an ideal self-gravitating condensed Bose gas. In order to do so,we use a finite differencing approximation of the Shcr¨odinger-Poisson (SP) system of equationswith axial symmetry in cylindrical coordinates. Our results indicate: 1) that spherically symmetricground state equilibrium configurations are stable against non-spherical perturbations and 2) thatsuch configurations of the SP system are late-time attractors for non-spherically symmetric initialprofiles of the scalar field, which is a generalization of such behavior for spherically symmetric initialprofiles. Our system and the boundary conditions used, work as a model of scalar field dark mattercollapse after the turnaround point. In such case, we have found that the scalar field overdensitiestolerate non-spherical contributions to the profile of the initial fluctuation.

Scalar field dark matter: Head-on interaction between two structures

Instituto de F´isica y Matem´aticas, Universidad Michoacana de San Nicol´as de Hidalgo. Edificio C-3,Cd. Universitaria, C. P. 58040 Morelia, Michoac´an, M´exico.(Dated: February 5, 2008)In this manuscript we track the evolution of a system consisting of two self-gravitating virializedobjects made of a scalar field in the newtonian limit. The Schr¨odinger-Poisson system containsa potential with self-interaction of the Gross-Pitaevskii type for Bose Condensates. Our resultsindicate that solitonic behavior is allowed in the scalar field dark matter model when the totalenergy of the system is positive, that is, the two blobs pass through each other as should happenfor solitons; on the other hand, there is a true collision of the two blobs when the total energy isnegative.