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A. de la Macorra

Publications and source records attributed to A. de la Macorra.

At least 271 records · Page 15Linked to original sources
arXiv2005

Inflation at the maxima of the potentials

We construct a two-stage inflationary model which can accommodate early inflation at a scale $Λ_1$ as well as a second stage of inflation at $Λ_2$ with a single scalar field $ϕ$. We use a symmetric potential, valid in a frictionless world, in which the two inflationary periods have exactly the same scale, i.e. $Λ_1=Λ_2$. However, we see today $Λ_1 \gg Λ_2$ due to the friction terms (expansion of the universe and interaction with matter). These type of models can be motivated from supergravity. Inflation occurs close to the maxima of the potential. As a consequence both inflations are necessarily finite. This opens the interesting possibility that the second inflation has already or is about to end. A first inflation is produced when fluctuations displace the inflaton field from its higher maximum rolling down the potential as in new inflation. Instead of rolling towards a global minimum the inflaton approaches a lower maximum where a second inflation takes place.

astro-ph↗
arXiv2004

A model of inflation independent of the initial conditions, with bounded number of e-folds and n_s larger orsmaller than one

We study a supergravity model of inflation essentially depending on one parameter which can be identified with the slope of the potential at the origin. In this type of models the inflaton rolls at high energy from negative values and a positive curvature potential. At some point defined by the equation $η=1$ inflation starts. The potential curvature eventually changes to negative values and inflation ends when $η=-1$. No spontaneous symmetry breaking type mechanism for inflation of the new type to occur is here required. The model naturally gives a bounded $\it{total}$ number of e-folds which is typically close to the required number for observable inflation and it is independent of the initial conditions for the inflaton. The energy scale introduced is fixed by the amplitude of the anisotropies and is of the order of the supersymmetry breaking scale. The model can also accommodate an spectral index bigger or smaller than one without extreme fine tuning. We show that it is possible to obtain reasonable numbers for cosmological parameters and, as an example, we reproduce values obtained recently by Tegmark $\it{et. al.}$, from WMAP and SDSS data alone.

astro-ph↗
arXiv2004

A Realistic Particle Physics Dark Energy Model

We present a realistic dark energy model derived from particle physics. Our model has essentially no free parameters and has an equivalent fit to the observational data (CMB, SN1a and LSS) as LCDM and a better fit than the best effective $w(z)$ model. With the lack of a clear determination of the cosmological parameters theoretical considerations should be taken seriously to distinguish between dark energy models.

astro-ph↗
arXiv2004

Cosmology for Scalar Fields with Negative Potentials and w <-1

We study the cosmology of canonically normalized scalar fields that lead to an equation of state parameter of w_ϕ=p_ϕ/ρ_ϕ<-1 without violating the weak energy condition: rho=Σ_iρ_i \geq 0 and ρ_i+p_i\geq 0. This kind of behavior requires a negative scalar potential V, widely predicted in particle physics. We show that the energy density ρ_ϕ=E_k+V takes negative values with an equation of state with w_ϕ< -1. However, the net effect of the $ϕ$ field on the scale factor is to decelerate it giving a total equation of state parameter w=p/ρ> w_b=p_b/ρ_b, where ρ_b stands for any kind of energy density with -1\leq w_b \leq 1. The fact that ρ_ϕ<0 allows, at least in principle, to have a small cosmological constant or quintessence today as the cancellation of high energy scales such as the electroweak or susy breaking scale. While V is negative |ρ_ϕ| is smaller than the sum of all other energy densities regardless of the functional form of the potential V. We show that the existence of a negative potential leads, inevitable, to a collapsing universe, i.e. to a would be "big crunch". In this picture we would still be living in the expanding universe.

astro-ph↗
arXiv2002

Acceptable Inverse Power Law Potential Quintessence with n=18/7

We present a particle physics quintessence model which agrees well with existing cosmological data, including the position of the acoustic peaks. This model has an inverse power law potential (IPL) with $n=18/7\sim 2.57$ and it gives $\weff=-0.75$, an acoustic scale $l_A=307$ and a density contrast $\s8=0.95$. Models with $n>1$ have been said to be disfavored by the analysis of the acoustic peaks. However, the results are not correct. The main reason is that the tracker approximation has been used in deriving the IPL constrains and for $n<5$ the scalar field has not reached its tracker value by present day. The model can be derived from particle physics, using Affleck-Dine-Seiberg "ADS" superpotential, for a non-abelian gauge group with $N_c=8, N_f=1$. The advantage of having $N_f=1$ is that there is only one degree of freedom below the condensation scale given by the condensate (quintessence) $ϕ^2= $ field. The condensation scale is at $1 GeV$ a very interesting scale since it connects the quintessence "Q" with the standard model "SM" scale. The similarity in energy scales between $Q$ and $SM$ scale gives an "explanation" to the coincidence problem. The fact that only recently the universe is accelerating is a natural consequence of the Q scale and the evolution of $ϕ$.

astro-ph↗
arXiv2002

Quintessence Unification Models from Non-Abelian Gauge Dynamics

We show that the condensates of a non-abelian gauge group, unified with the standard model gauge groups, can parameterize the present day cosmological constant and play the role of quintessence. The models agree with SN1a and recent CMB analysis. These models have {\it no free} parameters. Even the initial energy density at the unification scale and at the condensation scale are fixed by the number of degrees of freedom of the gauge group (i.e. by $N_c, N_f$). The values of $N_c, N_f$ are determined by imposing gauge coupling unification and the number of models is quite limited. Using Affleck-Dine-Seiberg superpotential one obtains a scalar potential $V=\Lm_c^{4+n}ϕ^{-n}$. Models with $2<n<4.27$ or equivalently $2\times 10^{-2} GeV < \Lm_c < 6 \times 10^3 GeV$ do not satisfy the unification constrain. In fact, there are only three models and they have an inverse power potential with $6/11 \leq n \leq 2/3$. Imposing primordial nucleosynthesis bounds the preferred model has $N_c=3, N_f=6$, with $n=2/3$, a condensation scale $\Lm_c=4.2\times 10^{-8} GeV$ and $\wpo=-0.90$ with an average value $\weff=-0.93$. Notice that the tracker solution is not a good approximation since it has $w_{tr}=-\fr{2}{n+2}=-0.75$ for $n=2/3$. We study the evolution of all fields from the unification scale and we calculate the relevant cosmological quantities. We also discuss the supersymmetry breaking mechanism which is relevant for these models.

hep-ph↗
arXiv2002

Quintessence Restrictions on Negative Power and Condensate Potentials

We study the cosmological evolution of scalar fields that arise from a phase transition at some energy scale $\Lm_c$. We focus on negative power potentials given by $V=c\Lm_c^{4+n}ϕ^{-n}$ and restrict the cosmological viable values of $\Lm_c$ and $n$. We make a complete analysis of $V$ and impose $SN1a$ conditions on the different cosmological parameters. The cosmological observations ruled out models where the scalar field has reached its attractor solution. For models where this is not the case, the analytic approximated solutions are not good enough to determine whether a specific model is phenomenologically viable or not and the full differential equations must be numerically solved. The results are not fine tuned since a change of 45% on the initial conditions does not spoil the final results. We also determine the values of $N_c, N_f$ that give a condensation scale $\Lm_c$ consistent with gauge coupling unification, leaving only four models that satisfy unification and SN1a constraints.

astro-ph↗
arXiv2001

Natural Quintessence with Gauge Coupling Unification

We show that a positive accelerating universe can be obtained simply by the dynamics of a non-abelian gauge group. It is the condensates of the chiral fields that obtain a negative power potential, below the condensation scale, and allow for a quintessence interpretation of these fields. The only free parameters in this model are $N_c$ and $N_f$ and the number of dynamically gauge singlet bilinear fields $ϕ$ generated below the condensation scale. We show that it is possible to have unification of all coupling constants, including the standard and non standard model couplings, while having an acceptable phenomenology of $ϕ$ as the cosmological constant. This is done without any fine tuning of the initial conditions. The problem of coincidence (why the universe has only recently started an accelerating period) is not solved but it is put at the same level as what the particle content of the standard model is.

astro-ph↗
arXiv2000

Low-scale supergravity inflation with R-symmetry

We study a supergravity model of inflation with R-symmetry and a single scalar field, the inflaton, slowly rolling away from the origin. The scales of inflation can be as low as the supersymmetry breaking scale of 10^10 GeV or even the electroweak scale of 10^3 GeV which could be relevant in the context of theories with submillimiter dimensions. Exact analytical solutions are presented and a comparison with related models is given.

hep-th↗
arXiv1999

Model Independent Accelerating Universe and the Cosmological Coinc idence Problem

We show that the evolution of the quintessence energy density $Ω_Q$ is model independent in an accelerating universe. The accelerating behaviour has lasted at most 0.5 e-folds of expansion assuming a present day value of $Ω_Q=2/3$. The generic evolution differs from the exact solution by a percentage given by $r=2\gm_Q$, with $\gm_Q=(ρ_Q+p_Q)/ρ_Q$. For a small $\gm_Q$ the evolution remains a good approximation for a long period. Nucleosynthesis bounds on $Ω_Q$ suggest that the model independent solution is valid for at least 12 e-folds of expansion which includes then the scale of radiation and matter equality. We can, therefore, establish model independent conditions on the cosmological parameters at some factor scale $a_i \ll a_o$. Finally, we discuss the relevance of this result to the cosmological coincidence problem.

astro-ph↗
arXiv1999

Can Moduli Fields parametrize the Cosmological Constant?

We study the cosmological evolution of string/M moduli fields T. We use T-duality to fix the potential and show that the superpotential W is a function of the duality invariant function j(T) only. If W is given as a finite polynomial of j then the moduli fields {\it do not} give an accelerating universe, i.e. they {\it cannot} be used as quintessence. Furthermore, at T >>1 the potential is given by a double exponential potential V \simeq e^{-a e^{\sqrt{2} T}} leading to a fast decaying behaviour at large times. For moduli potentials with a finite v.e.v. of T the energy density redshift is model dependent but if T has a finite mass, m < \infty, then the moduli energy density redshifts faster or equal to matter. Only if the moduli mass is infinite can the moduli energy density dominate the universe independently of the initial conditions.

hep-ph↗
arXiv1999

General Scalar Fields as Quintessence

We study the cosmological evolution of scalar fields with arbitrary potentials in the presence of a barotropic fluid (matter or radiation) without making any assumption on which term dominates. We determine what kind of potentials V(phi) permits a quintessence interpretation of the scalar field phi and to obtain interesting cosmological results. We show that all model dependence is given in terms of lambda= - V'/V only and we study all possible asymptotic limits: lambda approaching zero, a finite constant or infinity. We determine the equation of state dynamically for each case. For the first class of potentials, the scalar field quickly dominates the universe behaviour, with an inflationary equation of state allowing for a quintessence interpretation. The second case gives the extensively studied exponential potential. While in the last case, when lambda approaches infinity, if it does not oscillate then the energy density redshifts faster than the barotropic fluid but if lambda oscillates then the energy density redshift depends on the specific potential.

hep-ph↗
arXiv1998

Supersymmetric Models with Product Groups and Field Dependent Gauge Couplings

We study the dilaton-dependence of the effective action for N=1, SU(N1) x SU(N2) models with one generation of vectorlike matter transforming in the fundamental of both groups. We treat in detail the confining and Coulomb phases of these models writing explicit expressions in many cases for the effective superpotential. We can do so for the Wilson superpotentials of the Coulomb phase when N2=2, N1=2,4. In these cases the Coulomb phase involves a single U(1) gauge multiplet, for which we exhibit the gauge coupling in terms of the modulus of an elliptic curve. The SU(4) x SU(2) model reproduces the weak-coupling limits in a nontrivial way. In the confining phase of all of these models, the dilaton superpotential has a runaway form, but in the Coulomb phase the dilaton enjoys flat directions. Had we used the standard moduli-space variables: Tr M^k, k=1,..., N2, with M the quark condensate matrix, to parameterize the flat directions instead of the eigenvalues of M, we would find physically unacceptable behaviour, illustrating the importance to correctly identify the moduli.

hep-th↗
arXiv1998

Constant versus Field Dependent Gauge Couplings in Supersymmetric Theories

We briefly discuss the differences between considering the gauge coupling as a constant or as a field, the dilaton, in N=1 supersymmetric theories. We emphasize the differences regarding supersymmetry breaking. Recent developments on the nonperturbative dynamics of these theories provide new ideas on the induced dilaton potential and its stabilization.

hep-ph↗
arXiv1997

Fixing the Dilaton with Asymptotically Expensive Physics?

We propose a general mechanism for stabilizing the dilaton against runaway to weak coupling. The method is based on features of the effective superpotential which arise for supersymmetric gauge theories which are not asymptotically free. Consideration of the 2PI effective action for bilinear operators of matter and gauge superfields allows one to overcome the obstacles to constructing a nonvanishing superpotential.

hep-th↗
arXiv1995

Inflation in S-Dual Superstring Models

We study inflationary potentials in the framework of superstring theories. Successful inflation may occur due to chiral fields, but only after the dilaton and moduli are stabilized. This is achieved by demanding an S-duality invariant potential. Then, it is possible to have inflation at any scale and even to have more than one stages of inflation. This occurs for a limited class of scalar potentials, where certain conditions are obeyed. Density fluctuations of $10^{-5}$ require the inflationary potential to be at a scale of $V^{1/4}\simeq 5 \times 10^{15}$ GeV.

hep-ph↗
arXiv1994

Unification Scale in String Theory

We study the unification scale and gauge coupling constant in 4D string theory. We show that the fine structure constant is determined by the dimension of the hidden gauge group and only $SU(6)$ and $SO(9)$ are consistent with minimal string unification while the unification scale can be of order of $10^{16}\,GeV$.

hep-ph↗