Scalar Field Dark Matter: behavior around black holes

We present the numerical evolution of a massive test scalar fields around a Schwarzschild space-time. We proceed by using hyperboloidal slices that approach future null infinity, which is the boundary of scalar fields, and also demand the slices to penetrate the event horizon of the black hole. This approach allows the scalar field to be accreted by the black hole and to escape toward future null infinity. We track the evolution of the energy density of the scalar field, which determines the rate at which the scalar field is being diluted. We find polynomial decay of the energy density of the scalar field, and use it to estimate the rate of dilution of the field in time. Our findings imply that the energy density of the scalar field decreases even five orders of magnitude in time scales smaller than a year. This implies that if a supermassive black hole is the Schwarzschild solution, then scalar field dark matter would be diluted extremely fastComment: 15 pages, 21 eps figures. Appendix added, accepted for publication in JCA

Confusing the extragalactic neutrino flux limit with a neutrino propagation limit

We study the possible suppression of the extragalactic neutrino flux due to a nonstandard interaction during its propagation. In particular, we study neutrino interaction with an ultra-light scalar field dark matter. It is shown that the extragalactic neutrino flux may be suppressed by such an interaction, leading to a new mechanism to reduce the ultra-high energy neutrino flux. We study both the cases of non-self-conjugate as well as self-conjugate dark matter. In the first case, the suppression is independent of the neutrino and dark matter masses. We conclude that care must be taken when explaining limits on the neutrino flux through source acceleration mechanisms only, since there could be other mechanisms for the reduction of the neutrino flux.Comment: 15 pages, 4 figures. Important changes implemented. Abstract modified. Conclusions remain. To be published in JCA

Constraining scalar fields with stellar kinematics and collisional dark matter

The existence and detection of scalar fields could provide solutions to long-standing puzzles about the nature of dark matter, the dark compact objects at the centre of most galaxies, and other phenomena. Yet, self-interacting scalar fields are very poorly constrained by astronomical observations, leading to great uncertainties in estimates of the mass $m_\phi$ and the self-interacting coupling constant $\lambda$ of these fields. To counter this, we have systematically employed available astronomical observations to develop new constraints, considerably restricting this parameter space. In particular, by exploiting precise observations of stellar dynamics at the centre of our Galaxy and assuming that these dynamics can be explained by a single boson star, we determine an upper limit for the boson star compactness and impose significant limits on the values of the properties of possible scalar fields. Requiring the scalar field particle to follow a collisional dark matter model further narrows these constraints. Most importantly, we find that if a scalar dark matter particle does exist, then it cannot account for both the dark-matter halos and the existence of dark compact objects in galactic nucleiComment: 23 pages, 8 figures; accepted for publication by JCAP after minor change

Modeling galactic halos with predominantly quintessential matter

This paper discusses a new model for galactic dark matter by combining an anisotropic pressure field corresponding to normal matter and a quintessence dark energy field having a characteristic parameter $\omega_q$ such that $-1<\omega_q< -\frac{1}{3}$. Stable stellar orbits together with an attractive gravity exist only if $\omega_q$ is extremely close to $-\frac{1}{3}$, a result consistent with the special case studied by Guzman et al. (2003). Less exceptional forms of quintessence dark energy do not yield the desired stable orbits and are therefore unsuitable for modeling dark matter.Comment: 12 pages, 1 figur

Minimum mass of galaxies from BEC or scalar field dark matter

Many problems of cold dark matter models such as the cusp problem and the missing satellite problem can be alleviated, if galactic halo dark matter particles are ultra-light scalar particles and in Bose-Einstein condensate (BEC), thanks to a characteristic length scale of the particles. We show that this finite length scale of the dark matter can also explain the recently observed common central mass of the Milky Way satellites ($\sim 10^7 M_\odot$) independent of their luminosity, if the mass of the dark matter particle is about $10^{-22} eV$.Comment: 10 pages, 1 figure, accepted in JCA

Galactic Halos of Fluid Dark Matter

Dwarf spiral galaxies - and in particular the prototypical DDO 154 - are known to be completely dominated by an unseen component. The putative neutralinos - so far the favored explanation for the astronomical dark matter - fail to reproduce the well measured rotation curves of those systems because these species tend to form a central cusp whose presence is not supported by observation. We have considered here a self-coupled charged scalar field as an alternative to neutralinos and investigated whether a Bose condensate of that field could account for the dark matter inside DDO 154 and more generally inside dwarf spirals. The size of the condensate turns out to be precisely determined by the scalar mass m and self-coupling lambda of the field. We find actually that for m^4 / lambda = 50 - 75 eV^4, the agreement with the measurements of the circular speed of DDO 154 is impressive whereas it lessens for larger systems. The cosmological behavior of the field is also found to be consistent - yet marginally - with the limits set by BBN on the effective number of neutrino families. We conclude that classical configurations of a scalar and self-coupled field provide a possible solution to the astronomical dark matter problem and we suggest further directions of research.Comment: 20 pages, 7 figures; one reference added, version to be published in PR

Bianchi {VI}0_{0} in Scalar and Scalar-Tensor Cosmologies

We study several cosmological models with Bianchi \textrm{VI}$_{0}$ symmetries under the self-similar approach. In order to study how the \textquotedblleft constants\textquotedblright\ $G$ and $\Lambda$ may vary, we propose three scenarios where such constants are considered as time functions. The first model is a perfect fluid. We find that the behavior of $G$ and $\Lambda$ are related. If $G$ behaves as a growing time function then $\Lambda$ is a positive decreasing time function but if $G$ is decreasing then $\Lambda$ is negative. For this model we have found a new solution. The second model is a scalar field, where in a phenomenological way, we consider a modification of the Klein-Gordon equation in order to take into account the variation of $G$. Our third scenario is a scalar-tensor model. We find three solutions for this models where $G$ is growing, constant or decreasing and $\Lambda$ is a positive decreasing function or vanishes. We put special emphasis on calculating the curvature invariants in order to see if the solutions isotropize.Comment: Typos corrected. References added, minor corrections. arXiv admin note: text overlap with arXiv:0905.247

Classical and Quantum Decay of Oscillatons: Oscillating Self-Gravitating Real Scalar Field Solitons

The oscillating gravitational field of an oscillaton of finite mass M causes it to lose energy by emitting classical scalar field waves, but at a rate that is non-perturbatively tiny for small GMm, where m is the scalar field mass: d(GM)/dt ~ -3797437.776333015 e^[-39.433795197160163/(GMm)]/(GMm)^2. Oscillatons also decay by the quantum process of the annihilation of scalarons into gravitons, which is only perturbatively small in GMm, giving by itself d(GM)/dt ~ - 0.008513223934732692 G m^2 (GMm)^5. Thus the quantum decay is faster than the classical one for Gmm < 39.4338/[ln(1/Gm^2)}-7ln(GMm)+19.9160]. The time for an oscillaton to decay away completely into free scalarons and gravitons is ~ 2/(G^5 m^11) ~ 10^324 yr (1 meV/m)^11. Oscillatons of more than one real scalar field of the same mass generically asymptotically approach a static-geometry U(1) boson star configuration with GMm = GM_0 m, at the rate d(GM/c^3)/dt ~ [(C/(GMm)^4)e^{-alpha/(GMm)}+Q(m/m_{Pl})^2(GMm)^3] [(GMm)^2-(GM_0 m)^2], with GM_0 m depending on the magnitudes and relative phases of the oscillating fields, and with the same constants C, alpha, and Q given numerically above for the single-field case that is equivalent to GM_0 m = 0.Comment: 75 pages, LaTe

On the dark energy clustering properties

We highlight a viable mechanism leading to the formation of dark energy structures on sub-horizon cosmological scales, starting from linear perturbations in scalar-tensor cosmologies. We show that the coupling of the dark energy scalar field, or Quintessence, to the Ricci scalar induces a "dragging" of its density perturbations through the general relativistic gravitational potentials. We discuss, in particular, how this process forces dark energy to behave as a pressureless component if the cosmic evolution is dominated by non-relativistic matter. This property is also analyzed in terms of the effective sound speed of the dark energy, which correspondingly approaches the behavior of the dominant cosmological component, being effectively vanishing after matter-radiation equality. To illustrate this effect, we consider Extended Quintessence scenarios involving a quadratic coupling between the field and the Ricci scalar. We show that Quintessence density perturbations reach non-linearity at scales and redshifts relevant for the structure formation process, respecting all the existing constraints on scalar-tensor theories of Gravity. This study opens new perspectives on the standard picture of structure formation in dark energy cosmologies, since the Quintessence field itself, if non-minimally coupled to Gravity, may undergo clustering processes, eventually forming density perturbations exiting from the linear regime. A non-linear approach is then required to further investigate the evolution of these structures, and in particular their role in the dark haloes surrounding galaxies and clusters.Comment: 15 pages including three figures, final version accepted for publication by Phys.Rev.

Scalar field "mini--MACHOs": a new explanation for galactic dark matter

We examine the possibility that galactic halos are collisionless ensembles of scalar field ``massive compact halo objects'' (MACHOs). Using mass constraints from MACHO microlensing and from theoretical arguments on halos made up of massive black holes, as well as demanding also that scalar MACHO ensambles of all scales do not exhibit gravothermal instability (as required by consistency with observations of LSB galaxies), we obtain the range: m\alt 10^{-7} M_\odot or 30 M_\odot\alt m\alt 100 M_\odot. The rather narrow mass range of large MACHOs seems to indicate that the ensambles we are suggesting should be probably made up of scalar MACHOs in the low mass range (``mini--MACHOs''). The proposed model allows one to consider a non--baryonic and non--thermal fundamental nature of dark matter, while at the same time keeping the same phenomenology of the CDM paradigm.Comment: 5 pages, 1 eps figure. RevTex 4 style. To appear in Physical Review