Dario Nunez

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.

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

Exact solution for scalar field collapse.

We give a class of exact spherically symmetric solutions for the Einstein-scalar field system. The solutions may be interpreted as inhomogeneous dynamical scalar field cosmologies. The spacetimes have a timelike conformal Killing vector field and are asymptotically conformally flat. They also have black- or white-hole-like regions containing trapped surfaces. The properties of the apparent horizons are described in detail.

Rotating scalar field wormhole

We derive an exact solution of Einstein’s equations with a scalar field stress– energy tensor with the opposite sign, and show that such a solution describes the inner region of a rotating wormhole. We also show that the non-rotating case of such a solution represents a static, asymptotically flat wormhole solution. We match the radial part of the rotating solution to the static one at both mouths, thus obtaining an analytic description for the asymptotic radial region of spacetime. We explore some of the features of these solutions.

Schwarzschild black holes can wear scalar wigs

We study the evolution of a massive scalar field surrounding a Schwarzschild black hole and find configurations that can survive for arbitrarily long times, provided the black hole or the scalar field mass is small enough. In particular, both ultralight scalar field dark matter around supermassive black holes and axionlike scalar fields around primordial black holes can survive for cosmological times. Moreover, these results are quite generic in the sense that fairly arbitrary initial data evolve, at late times, as a combination of those long-lived configurations.

Numerical studies of Φ2-oscillatons

We present an exhaustive analysis of the numerical evolution of the Einstein-Klein-Gordon equations for the case of a real scalar field endowed with a quadratic self-interaction potential. The self-gravitating equilibrium configurations are called oscillatons and are close relatives of boson stars, their complex counterparts. Unlike boson stars, for which the oscillations of the two components of the complex scalar field are such that the spacetime geometry remains static, oscillatons give rise to a geometry that is time-dependent and oscillatory in nature. However, they can still be classified into stable (S-branch) and unstable (U-branch) cases. We have found that S-oscillatons are indeed stable configurations under small perturbations and typically migrate to other S-profiles when perturbed strongly. On the other hand, U-oscillatons are intrinsically unstable: they migrate to the S-branch if their mass is decreased and collapse to black holes if their mass is increased even by a small amount. The S-oscillatons can also be made to collapse to black holes if enough mass is added to them, but such collapse can be efficiently prevented by the gravitational cooling mechanism in the case of diluted oscillatons.

Hydrodynamics of galactic dark matter

We consider simple hydrodynamical models of galactic dark matter in which the galactic halo is a self-gravitating and self-interacting gas that dominates the dynamics of the galaxy. Modelling this halo as a spherically symmetric and static perfect fluid satisfying the field equations of general relativity, visible baryonic matter can be treated as ‘test particles’ in the geometry of this field. We show that the assumption of an empirical ‘universal rotation curve’ that fits aw id ev ariety of galaxies is compatible, under suitable approximations, with state variables characteristic of a non-relativistic Maxwell–Boltzmann gas that becomes an isothermal sphere in the Newtonian limit. Consistency criteria lead to a minimal bound for particle masses in the range 30 eV <m< 60 eV and to a constraint between the central temperature and the particle mass. The allowed mass range includes popular supersymmetric particle candidates, such as the neutralino, axino and gravitino, as well as lighter particles (m ≈ keV) proposed by numerical n-body simulations associated with self-interactive CDM and WDM structure formation theories.

Dynamics and collision of massive shells in curved backgrounds

The authors analyse the dynamics of the collision of two spherical massive shells in a generally spherically symmetric background, obtaining an expression from the conservation law that imposes a constraint between the different parameters involved. They study the light-like limit and make some comparisons of the predictions of the master equation with the results obtained in the case of collision of light-like shells, like the short life of white holes or the mass-inflation phenomena. They present some particular cases of the constraint equation.

Scalar Field Dark Matter

The main goal of this work is to put the last results of the Scalar Field Dark Matter model of the Universe at cosmological and at galactic level in a work together. We present the complete solution to the 95% scalar field cosmological model in which the dark matter is modeled by a scalar field Φ with the scalar potential \( V(\Phi ) = V_o [\cosh (\lambda \sqrt {k_o } \Phi ) - 1] \) and the dark energy is modeled by a scalar field Ψ, endowed with the scalar potential \( \tilde V(\Psi ) = \tilde V_o [\sinh (\alpha \sqrt {k_o } \Psi )]^\beta\). This model has only two free parameters, λ and the equation of state ωψ. The results of the model are: 1) the fine tuning and the cosmic coincidence problems are ameliorated for both dark matter and dark energy and the models agrees with astronomical observations. 2) The model predicts a suppression of the Mass Power Spectrum for small scales having a wave number k>k min,φ, where k min,φ ≃4.5h Mpc −1 for λ≃20.3. This last fact could help to explain the dearth of dwarf galaxies and the smoothness of galaxy core halos. 3) From this, all parameters of the scalar dark matter potential are completely determined. 4) The dark matter consists of an ultra-light particle, whose mass is m φ ≃1.1×10 −23 eV and all the success of the standard cold dark matter model is recovered. 5) If the scale of renormalization of the model is of order of the Planck Mass, then the scalar field Φ can be a reliable model for dark matter in galaxies. 6) The predicted scattering cross section fits the value required for self-interacting dark matter. 7) Studying a spherically symmetric fluctuation of the scalar field Φ in cosmos we show that it could be the halo dark matter in galaxies. 8) The local space-time of the fluctuation of the scalar field Φ contains a three dimensional space-like hypersurface with surplus of angle. 9) We also present a model for the dark matter in the halos of spiral galaxies, we obtain that 10) the effective energy density goes like 1/(r 2 +K 2) and 11) the resulting circular velocity profile of tests particles is in good agreement with the observed one in spiral galaxies. This implies that a scalar field could also be a good candidate as the dark matter of the Universe

Schwarzschild scalar wigs: spectral analysis and late time behavior

Using the Greens function representation technique, the late time behavior of localized scalar field distributions on Schwarzschild spacetimes is studied. Assuming arbitrary initial data we perform a spectral analysis, computing the amplitude of each excited quasi-bound mode without the necessity of performing dynamical evolutions. The resulting superposition of modes is compared with a traditional numerical evolution with excellent agreement; therefore, we have an efficient way to determine final black hole wigs. The astrophysical relevance of the quasi-bound modes is discussed in the context of scalar field dark matter models and the axiverse.