arXiv subjects

Axel de la Macorra

Publications and source records attributed to Axel de la Macorra.

189 records · Page 11Linked to original sources

arXiv2012

Galactic phase transition at Ec=0.11 eV from rotation curves of cored LSB galaxies and nonperturbative dark matter mass

We analyze the a set of seventeen rotation curves of Low Surface Brightness (LSB) galaxies from the The HI Nearby Galaxy Survey (THINGS) with different mass models to study the core structure and to determine a phase transition energy scale (E_c) between hot and cold dark matter, due to nonperturbative effects in the Bound Dark Matter (BDM) model. Our results agree with previous ones implying the cored profiles are preferred over the N-body motivated cuspy NFW profile. We find an average galactic core radius of r_c = 260 \times 10^{+/- 1.3} pc and a phase transition energy E_c = 0.11\times 10^{+/- 0.46} eV, that is of the same order of magnitude as the sum of the neutrino masses.

astro-ph.CO↗

arXiv2011

Scalar Field Dark Energy Parametrization

We propose a new Dark Energy parametrization based on the dynamics of a scalar field. We use an equation of state $w=(x-1)/(x+1)$, with $x=E_k/V$, the ratio of kinetic energy $E_k=\dotϕ^2/2$ and potential $V$. The eq. of motion gives $x=(L/6)(V/3H^2)$ and with a solution $x=([1+2 L/3(1+y)]^{1/2}-1)(1+y)/2$ where $y\equiv \rm/V$ and $L\equiv (V'/V)^2 (1+q)^2,\, q\equiv\ddot\p/V'$. Since the universe is accelerating at present time we use the slow roll approximation in which case we have $|q|\ll 1$ and $L\simeq (V'/V)^2$. However, the derivation of $L$ is exact and has no approximation. By choosing an appropriate ansatz for $L$ we obtain a wide class of behavior for the evolution of Dark Energy without the need to specify the potential $V$. In fact $w$ can either grow and later decrease, or other way around, as a function of redshift and it is constraint between $-1\leq w\leq 1$ as for any canonical scalar field with only gravitational interaction. Furthermore, we also calculate the perturbations of DE and since the evolution of DE is motivated by the dynamics of a scalar field the homogenous and its perturbations can be used to determine the form of the potential and the nature of Dark Energy. Since our parametrization is on $L$ we can easily connect it with the scalar potential $V(ϕ)$.

astro-ph.CO↗

arXiv2009

BDM Dark Matter: CDM with a core profile and a free streaming scale

We present a new dark matter model BDM which is an hybrid between hot dark matter HDM and cold dark matter CDM, in which the BDM particles behave as HDM above the energy scale E_c and as CDM below this scale. Evolution of structure formation is similar to that of CDM model but BDM predicts a nonvanishing free streaming l_fs scale and a inner galaxy core radius r_core, both quantities determined in terms of a single parameter E_c, which corresponds to the phase transition energy scale of the subjacent elementary particle model. For energies above E_c or for a scale factor a smaller then a_c, with a a_c then the BDM particles acquire a large mass through a non perturbative mechanism, as baryons do, and rho redshifts as matter with the particles having a vanishing velocity. Typical energies are E_c=O(10-100) eV giving a l_fs \propto E_c^{-4/3}\lesssim Mpc and m_fs\propto E_c^{-4}\lesssim 10^9 M\odot. A l_fs\neq 0, r_core\neq 0 help to resolve some of the shortcomings of CDM such as overabundance substructure in CDM halos and numerical fit to rotation curves in dwarf spheroidal and LSB galaxies. Finally, our BDM model and the phase transition scale E_c can be derived from particle physics.

astro-ph.CO↗

arXiv2007

Cosmological consequences of scalar mesons from gauge/gravity correspondence

We consider the spectrum of mesons for the gauge theory dual to a supergravity configuration of intersecting D3/D7 branes, and use the expression for the Lagrangian of the scalar mesons to compute explicitly the Lagrangian for the lightest states in the infrared limit. Assuming that the matter content of this gauge theory is part of a hidden sector, which interacts with the standard model only via gravity, we explore the cosmological consequences of these lightest scalar mesons for a FRW universe. We show that phantom fields may appear naturally in this kind of scenarios.

hep-th↗

arXiv2004

Causality and Sound Speed for General Scalar Field Models, including w < -1, tachyonic, phantom, k-essence and curvature corrections

The result from the SN1a projects suggest that the dark energy can be represented by a fluid with w<-1. However, it is commonly argued that a fluid with |w|>1 contradicts causality. Here, we will show that a fluid with |w|>1 does not contradict causality if $w$ is not constant. Scalar field are the most promising candidates for describing the dark energy and they do not have a constant equation of state parameter w. Scalar potentials may lead to regions where w is larger or smaller than one and even regions where the group velocity $d p/dρ$ diverges. We study the evolution of scalar field perturbations and we show that the "sound speed" is always smaller than the speed of light independently of the value of w=p/ρor d p/dρ. In general, it is neither the phase velocity nor the group velocity that gives the "sound speed". In the analysis we include the special cases of $|w|>1$, tachyonic, phantom, k-essence scalar fields and curvature corrections. Our results show that scalar fields do not contradict causality as long as there is dispersion.

astro-ph↗

arXiv2004

Dark Group: Dark Energy and Dark Matter

We study the possibility that a dark group, a gauge group with particles interacting with the standard model particles only via gravity, is responsible for containing the dark energy and dark matter required by present day observations. We show that it is indeed possible and we determine the constrains for the dark group. The non-perturbative effects generated by a strong gauge coupling constant can de determined and a inverse power law scalar potential IPL for the dark meson fields is generated parameterizing the dark energy. On the other hand it is the massive particles, e.g. dark baryons, of the dark gauge group that give the corresponding dark matter. The mass of the dark particles is of the order of the condensation scale $Λ_c$ and the temperature is smaller then the photon's temperature. The dark matter is of the warm matter type. The only parameters of the model are the number of particles of the dark group. The allowed values of the different parameters are severely restricted. The dark group energy density at $Λ_c$ must be $ \Omdgc \leq 0.17$ and the evolution and acceptable values of dark matter and dark energy leads to a constrain of $\Lmc$ and the IPL parameter $n$ giving $Λ_c=O(1-10^3) eV$ and $0.28 \leq n \leq 1.04$.

astro-ph↗

arXiv2002

Implications to CMB from Model Independent evolution of w and Late Time Phase Transition

We present model independent determination of the CMB from any kind of fluid that has an equation of state taking four different values. The first region has $w=1/3$, the second $w=1$, the third $w=-1$ while the last one has $-1<w=cte<-2/3$. This kind of dynamical $w$ contains as a limit the cosmological constant and tracker models. We derive the model independent evolution of $\wp$, for scalar fields, and we see that it remains most of the time in either of its three extremal values given by $\wp=1,-1,w_{tr}$. This "varying" $w$ is the generic behavior of scalar fields, quintessence, and we determine the size of the different regions by solving the dynamical equations in a model independent way. The dynamical $w$ models have a better fit to CMB data then the cosmological constant and the tracker models. We determine the effect of having the first two regions $w=1/3,1$ and depending on the size of these periods they can be observed in the CMB. In general, the CMB spectrum sets a lower limit to $\D N_T$ and to the phase transition scale $\Lm_c$. For smaller $\D N_T$ the CMB peaks are moved to the right of the spectrum and the hight increases considerably. For $\Ompi=0.1$ the CMB sets a lower limit to the phase transition scale $\Lm_c\geq 0.2 eV$.

astro-ph↗

arXiv1995

Phenomenology of Superstrings

We consider the low energy phenomenology of superstrings. In particular we analyse supersymmetry breaking via gaugino condensate and we compare the phenomenology of the two different approaches to stabilize the dilaton field. We study the cosmological constant problem and we show that it is possible to have supersymmetry broken and zero cosmological constant. Finally, we discuss the possibility of having an inflationary potential. Requiring that the potential does not destabilize the dilaton field imposes an upper limit to the density fluctuations which can be consistent with the COBE data.

hep-ph↗

arXiv1995

Vanishing of the Cosmological Constant, Stability of the Dilaton and Inflation

We study the possibility of canceling the cosmological constant in supergravity string models. We show that with a suitable choice of superpotential the vacuum energy may vanish with the dilaton field at its minimum and supersymmetry broken with a large hierarchy. We derive the condition for which the introduction of a chiral potential, e.g. the inflaton potential, does not destabilize the dilaton field even in the region where the scalar potential takes positive values. This allows for an inflationary potential with the dilaton frozen at its minimum.

hep-ph↗