Dynamics Of Scalar Field Research Articles

Cosmological evolution of scalar fields and gravitino dark matter in gauge mediation at low reheating temperatures

We consider the dynamics of the supersymmetry-breaking scalar field and the production of dark matter gravitinos via its decay in a gauge-mediated supersymmetry breaking model with metastable vacuum. We find that the scalar field amplitude and gravitino density are extremely sensitive to the parameters of the hidden sector. For the case of an O'Raifeartaigh sector, we show that the observed dark matter density can be explained by gravitinos even for low reheating temperatures TR≲10GeV. Such low reheating temperatures may be implied by detection of the NLSP at the LHC if its thermal freeze-out density is in conflict with BBN.

Unified Dark Matter Scalar Field Models

We analyze and review cosmological models in which the dynamics of a single scalar field accounts for a unified description of the Dark Matter and Dark Energy sectors, dubbed Unified Dark Matter (UDM) models. In this framework, we consider the general Lagrangian of k‐essence, which allows to find solutions around which the scalar field describes the desired mixture of Dark Matter and Dark Energy. We also discuss static and spherically symmetric solutions of Einstein′s equations for a scalar field with noncanonical kinetic term, in connection with galactic halo rotation curves.

New agegraphic dark energy as a rolling tachyon

Combining the general relativity and the uncertainty relation in quantum mechanics, the energy density of quantum fluctuations of space-time can be viewed as dark energy. The so-called agegraphic dark energy model is just based on this viewpoint, in which the age of the universe is introduced as the length measure. Recently, the new agegraphic dark energy model was proposed, where the dynamical dark energy is measured by the conformal age of the universe. On the other hand, scalar-field dark energy models like tachyons are often regarded as an effective description of some underlying theory of dark energy. In this paper, we show that the new agegraphic dark energy can be described completely by a tachyon scalar-field. We thus reconstruct the potential and the dynamics of the tachyon scalar-field, according to the evolution of the new agegraphic dark energy.

Dirac-born-infeld and k-inflation: The CMB anisotropies from string theory

Inflationary models within string theory exhibit unusual scalar field dynamics involving non-minimal kinetic terms and generically referred to as k-inflation. In this situation, the standard slow-roll approach used to determine the behavior of the primordial cosmological perturbations cannot longer be used. We present a generic method, based on the uniform approximation, to analytically derive the primordial power spectra of scalar and tensor perturbations. At leading order, the scalar spectral index, its running and the tensor-to-scalar ratio are modified by the new dynamics. We provide their new expression, correct previous results at next-to-leading order and clarify the definition of what is the tensor-to-scalar ratio when the sound horizon and Hubble radius are not the same. Finally, we discuss the constraints the parameters encoding the non-minimal kinetic terms have to satisfy, such as the sound speed and the energy scale of k-inflation, in view of the fifth year Wilkinson Microwave Anisotropy Probe (WMAP5) data.

Cosmic steps in modeling dark energy

Past and recent data analyses gave some hints of steps in dark energy. Considering dark energy as a dynamical scalar field, we investigate several models with various steps: a step in the scalar potential, a step in the kinetic term, a step in the energy density, and a step in the equation-of-state parameter $w$. These toy models provide a workable mechanism to generate steps and features of dark energy. Remarkably, a single real scalar can cross $w=\ensuremath{-}1$ dynamically with a step in the kinetic term.

FROMWMAPTOPLANCK: EXACT RECONSTRUCTION OF FOUR- AND FIVE-DIMENSIONAL INFLATIONARY POTENTIAL FROM HIGH-PRECISION COSMIC MICROWAVE BACKGROUND MEASUREMENTS

We make a more general determination of the inflationary observables in the standard four-dimensional (4D) and five-dimensional (5D) single-field inflationary scenarios by the exact reconstruction of the dynamics of the inflation potential during the observable inflation with a minimal number of assumptions: the computation does not assume the slow-roll approximation and is valid in all regimes if the field is monotonically rolling down its potential. We address higher order effects in the standard and braneworld single-field inflation scenarios by fitting the Hubbble expansion rate and subsequently the inflationary potential directly to WMAP5+SN+BAO and Planck-like simulated data sets. Making use of the Hamilton-Jacobi formalism developed for the 5D single-field inflation model, we compute the scale dependence of the amplitudes of the scalar and tensor perturbations by integrating the exact mode equation. The solutions in 4D and 5D inflation scenarios differ through the dynamics of the background scalar field and the number of e-folds assumed to be compatible with the observational window of inflation. We analyze the implications of the theoretical uncertainty in the determination of the reheating temperature after inflation on the observable predictions of inflation and evaluate its impact on the degeneracy of the standard inflation consistency relation. We find that the detection of tensor perturbations and the theoretical uncertainties in the inflationary observable represents a significant challenge for the future Planck cosmic microwave background measurements: distinguishing between the observational signatures of the standard and braneworld single-field inflation scenarios. This work has been done in the frame of Planck Core Team activities.

Thermodynamics in Variable Speed of Light Theories

Variable speed of light theories (VSL) are interesting because they could solve several cosmological puzzles. In this work we study the thermodynamics and Newtonian limit of the varying speed of light theory developed by J. Magueijo (Magueijo 2000). In the covariant and locally Lorentz invariant VSL proposed by Magueijo c is a dimensionless dynamical scalar field c=c0eψ, where c0 is a constant. The matter and gravitational lagrangians are multiplied by the factors ebψ and eaψ respectively.

Freezing into stripe states in two-dimensional ferromagnets and crossing probabilities in critical percolation

When a two-dimensional Ising ferromagnet is quenched from above the critical temperature to zero temperature, the system eventually converges to either a ground state or an infinitely long-lived metastable stripe state. By applying results from percolation theory, we analytically determine the probability to reach the stripe state as a function of the aspect ratio and the form of the boundary conditions. These predictions agree with simulation results. Our approach generally applies to coarsening dynamics of nonconserved scalar fields in two dimensions.

Simulations of structure formation in interacting dark energy cosmologies

The evidence in favor of a dark energy component dominating the Universe, and driving its presently accelerated expansion, has progressively grown during the last decade of cosmological observations. If this dark energy is given by a dynamic scalar field, it may also have a direct interaction with other matter fields in the Universe, in particular with cold dark matter. Such interaction would imprint new features on the cosmological background evolution as well as on the growth of cosmic structure, like an additional long-range fifth-force between massive particles, or a variation in time of the dark matter particle mass. We review here the implementation of these new physical effects in the N-body code GADGET-2, and we discuss the outcomes of a series of high-resolution N-body simulations for a selected family of interacting dark energy models, as already presented in Baldi et al. [20]. We interestingly find, in contrast with previous claims, that the inner overdensity of dark matter halos decreases in these models with respect to LCDM, and consistently halo concentrations show a progressive reduction for increasing couplings. Furthermore, the coupling induces a bias in the overdensities of cold dark matter and baryons that determines a decrease of the halo baryon fraction below its cosmological value. These results go in the direction of alleviating tensions between astrophysical observations and the predictions of the LCDM model on small scales, thereby opening new room for coupled dark energy models as an alternative to the cosmological constant.

Perturbations of Schwarzschild black holes in dynamical Chern-Simons modified gravity

Dynamical Chern-Simons modified gravity is an attractive, yet relatively unexplored, candidate to an alternative theory of gravity. The dynamical Chern-Simons correction couples a dynamical scalar field to the gravitational field. In this framework, we analyze the perturbation formalism and stability properties of spherically symmetric black holes. Assuming that no background scalar field is present, gravitational perturbations with polar and axial parities decouple. We find no effect of the Chern-Simons coupling on the polar sector, while axial perturbations couple to the Chern-Simons scalar field. The axial sector can develop strong instabilities if the coupling parameter $\ensuremath{\beta}$, associated to the dynamical coupling of the scalar field, is small enough; this yields a constraint on $\ensuremath{\beta}$ which is much stronger than the constraints previously known in the literature.

Dynamics of a self-interacting scalar field trapped in the braneworld for a wide variety of self-interaction potentials

We apply the dynamical systems tools to study the linear dynamics of a self-interacting scalar field trapped in the braneworld, for a wide variety of self-interaction potentials. We focus on Randall-Sundrum and on Dvali-Gabadadze-Porrati braneworld models exclusively. These models are complementary to each other: while the Randall-Sundrum brane produces UV) corrections to general relativity, the Dvali-Gabadadze-Porrati braneworld modifies Einstein's theory at large scales, i e., produces IR modifications of general relativity. This study of the asymptotic properties of both braneworld models, shows---in the phase space---the way the dynamics of a scalar field trapped in the brane departs from standard general relativity behavior.

Inflation with a negative cosmological constant

We find a unique way of realizing inflation through cyclic phases in an universe with negative vacuum energy. According to the second law of thermodynamics entropy monotonically increases from cycle to cycle, typically by a constant factor. This means that the scale factor at the same energy density in consecutive cycles also increases by a constant factor. If the time period of the oscillations remain approximately constant then this leads to an ``over all'' exponential growth of the scale factor, mimicking inflation. A graceful exit from this inflationary phase is possible as a dynamical scalar field can take us from the negative to a positive energy vacuum during the last contracting phase.

Axion isocurvature fluctuations with extremely blue spectrum

We construct an axion model for generating isocurvature fluctuations with blue spectrum, ${n}_{\mathrm{iso}}=2--4$, which is suggested by recent analyses of admixture of adiabatic and isocurvature perturbations with independent spectral indices, ${n}_{\mathrm{ad}}\ensuremath{\ne}{n}_{\mathrm{iso}}$. The distinctive feature of the model is that the spectrum is blue at large scales while scale invariant at small scales. This is naturally realized by the dynamics of the Peccei-Quinn scalar field.

Cosmic age, statefinder, andOmdiagnostics in the decaying vacuum cosmology

As an extension of the cosmological constant model ($\ensuremath{\Lambda}\mathrm{CDM}$), the decaying vacuum (DV) model describes dark energy as a varying vacuum whose energy density decays linearly with the Hubble parameter at late times, ${\ensuremath{\rho}}_{\ensuremath{\Lambda}}(t)\ensuremath{\propto}H(t)$, and produces the matter component. We examine the high-$z$ cosmic age problem in the DV model, and compare it with $\ensuremath{\Lambda}\mathrm{CDM}$ and the Yang-Mills condensate (YMC) dark energy model. Without employing a dynamical scalar field for dark energy, these three models share a similar behavior of late-time evolution. It is found that the DV model, like the YMC, can accommodate the high-$z$ quasar APM $08279+5255$, thus greatly alleviating the high-$z$ cosmic age problem. We also calculate the statefinder $(r,s)$ and the $Om$ diagnostics in the model. It is found that the evolutionary trajectories of $r(z)$ and $s(z)$ in the DV model are similar to those in the kinessence model, but are distinguished from those in $\ensuremath{\Lambda}\mathrm{CDM}$ and the YMC. The $Om(z)$ in the DV model has a negative slope and its height depends on the matter fraction, while the YMC has a rather flat $Om(z)$, whose magnitude depends sensitively on the coupling.

Dynamics of a scalar field in Robertson-Walker spacetimes

We analyze the dynamics of a single scalar field in Friedmann-Robertson-Walker universes with spatial curvature. We obtain the fixed point solutions which are shown to be late time attractors. In particular, we determine the corresponding scalar field potentials which correspond to these stable solutions. The analysis is quite general and incorporates expanding and contracting universes with both positive and negative scalar potentials. We demonstrate that the known power law, exponential, and de Sitter solutions are certain limits of our general set of solutions.

Self-interacting scalar field trapped in a Randall–Sundrum braneworld: The dynamical systems perspective

We apply the dynamical systems tools to study the linear dynamics of a self-interacting scalar field trapped on a Randall–Sundrum brane. The simplest kinds of self-interaction potentials are investigated: (a) constant potential, and (b) exponential potential. It is shown that the dynamics of the Randall–Sundrum model significantly differs from the standard four-dimensional behavior at early times: in all cases of interest the (singular) empty universe is the past attractor for every trajectory in phase space, meanwhile the kinetic energy-dominated solution is always a saddle critical point. The late-time dynamics is not affected by the brane effects.

Precision calculations of the gravitational wave background spectrum from inflation

The spectrum of the gravitational wave background originating from quantum fluctuations during inflation is calculated numerically for various inflation models over a wide range of frequencies. We take into account four ingredients: the scalar field dynamics during inflation making no use of the slow-roll approximation, the fermionic decay of the scalar field with a small coupling constant during the reheating process, the change of the effective number of degrees of freedom ${g}_{*}$ in the radiation-dominated era, and the anisotropic stress of free-streaming neutrinos. By numerically solving the evolution of gravitational waves during and after inflation up to the present, all of these effects can be examined comprehensively and accurately over a broad spectrum, even at very high frequencies. We find that the spectrum shows (i) a large deviation from the spectrum less accurate obtained by Taylor expanding around the CMB scale using the slow-roll approximation, (ii) a characteristic frequency dependence due to the reheating effect, and (iii) damping due to the ${g}_{*}$ changes and the neutrino anisotropic stress. We suggest that future analysis of the gravitational wave background should take into consideration the fact that analytical estimates using the Taylor expansion overestimate the amplitude of the spectrum.

Competing bounds on the present-day time variation of fundamental constants

We compare the sensitivity of a recent bound on time variation of the fine structure constant from optical clocks with bounds on time varying fundamental constants from atomic clocks sensitive to the electron-to-proton mass ratio, from radioactive decay rates in meteorites, and from the Oklo natural reactor. Tests of the Weak Equivalence Principle also lead to comparable bounds on present variations of constants. The "winner in sensitivity" depends on what relations exist between the variations of different couplings in the standard model of particle physics, which may arise from the unification of gauge interactions. WEP tests are currently the most sensitive within unified scenarios. A detection of time variation in atomic clocks would favour dynamical dark energy and put strong constraints on the dynamics of a cosmological scalar field.

New perspective on the relation between dark energy perturbations and the late-time integrated Sachs-Wolfe effect

The effect of quintessence perturbations on the integrated Sachs-Wolfe (ISW) effect is studied for a mixed dynamical scalar field dark energy (DDE) and pressureless perfect fluid dark matter. A new and general methodology is developed to track the growth of the perturbations, which uses only the equation of state (EoS) parameter ${w}_{\mathrm{DDE}}(z)\ensuremath{\equiv}{p}_{\mathrm{DDE}}/{\ensuremath{\rho}}_{\mathrm{DDE}}$ of the scalar field DDE, and the initial values of the relative entropy perturbation (between the matter and DDE) and the intrinsic entropy perturbation of the scalar field DDE as inputs. We also derive a relation between the rest-frame sound speed ${\stackrel{^}{c}}_{s,\mathrm{DDE}}^{2}$ of an arbitrary scalar field DDE component and its EoS ${w}_{\mathrm{DDE}}(z)$. We show that the ISW signal differs from that expected in a $\ensuremath{\Lambda}\mathrm{CDM}$ cosmology by as much as $+20%$ to $\ensuremath{-}80%$ for parametrizations of ${w}_{\mathrm{DDE}}$ consistent with SNIa data, and about $\ifmmode\pm\else\textpm\fi{}20%$ for parametrizations of ${w}_{\mathrm{DDE}}$ consistent with $\mathrm{SNIa}+\mathrm{CMB}+\mathrm{BAO}$ data, at 95% confidence. Our results indicate that, at least in principle, the ISW effect can be used to phenomenologically distinguish a cosmological constant from DDE.

Time variation of fundamental couplings and dynamical dark energy

Scalar field dynamics may give rise to a nonzero cosmological variation of fundamental constants. Within different scenarios based on the unification of gauge couplings, the various claimed observations and bounds may be combined in order to trace or restrict the time history of the couplings and masses. If the scalar field is responsible for a dynamical dark energy or quintessence, cosmological information becomes available for its time evolution. Combining this information with the time variation of couplings, one can determine the interaction strength between the scalar and atoms, which may be observed by tests of the Weak Equivalence Principle. We compute bounds on the present rate of coupling variation from experiments testing the differential accelerations for bodies with equal mass and different composition and compare the sensitivity of various methods. In particular, we discuss two specific models of scalar evolution: crossover quintessence and growing neutrino models.