Search Results (19 total)

We consider gravitational waves (GWs) generated by primordial inverse-cascade helical magneto-hydrodynamical (MHD) turbulence produced by bubble collisions at the electroweak phase transitions (EWPT). Compared to the unmagnetized EWPT case, the spectrum of MHD-turbulence-generated GWs peaks at lower frequency with larger amplitude and can be detected by the proposed Laser Interferometer Space Antenna.

We study scalar mode perturbations (magnetosonic waves) induced by a helical stochastic cosmological magnetic field and derive analytically the corresponding cosmic microwave background (CMB) temperature and polarization anisotropy angular power spectra. We show that the presence of a stochastic magnetic field, or an homogeneous magnetic field, influences the acoustic oscillation pattern of the CMB anisotropy power spectrum, effectively acting as a reduction of the baryon fraction. We find that the scalar magnetic energy density perturbation contribution to the CMB temperature anisotropy is small compared to the contribution to the CMB E-polarization anisotropy.

We find exact exponentially expanding and contracting de Sitter solutions of the spatially homogeneous TeVeS cosmological equations of motion in the vacuum TeVeS model and a power law accelerated expanding solution in the presence of an additional ideal fluid with equation of state parameter -5/3<ω<-1. A preliminary stability analysis shows that the expanding vacuum solution is stable, while in the ideal fluid case stability depends on model parameter values. These solutions might provide a basis for incorporating early-time inflation or late-time accelerated expansion in TeVeS cosmology.

We show that helical turbulence produced during a first-order phase transition generates circularly polarized cosmological gravitational waves (GWs). The characteristic frequency of these GWs for an extreme case of the phase transition model is around 10^-3–10^-2  Hz with an energy density parameter as high as 10^-12–10^-11. The possibility of detection is briefly discussed.

Measurements of the evolution with redshift of the number density of massive galaxy clusters are used to constrain the energy density of massive neutrinos and so the sum of neutrino masses ∑m_ν. We consider a spatially flat cosmological model with cosmological constant, cold dark matter, baryonic matter, and massive neutrinos. Accounting for the uncertainties in the measurements of the relevant cosmological parameters we obtain a limit of ∑m_ν<2.4  eV (95% C.L.).

Cosmological magnetic fields induce temperature and polarization fluctuations in the cosmic microwave background (CMB) radiation. A cosmological magnetic field with current amplitude of order 10^-9  G is detectable via observations of CMB anisotropies. This magnetic field (with or without helicity) generates vector perturbations through vortical motions of the primordial plasma. This paper shows that magnetic field helicity induces parity-odd cross correlations between CMB temperature and B-polarization fluctuations and between E- and B-polarization fluctuations, correlations which are zero for fields with no helicity (or for any parity-invariant source). Helical fields also contribute to parity-even temperature and polarization anisotropies, canceling part of the contribution from the symmetric component of the magnetic field. We give analytic approximations for all CMB temperature and polarization anisotropy vector power spectra due to helical magnetic fields. These power spectra offer a method for detecting cosmological helical magnetic fields, particularly when combined with Faraday rotation measurements which are insensitive to helicity.

A primordial cosmological magnetic field induces Faraday rotation of the cosmic microwave background polarization. This rotation produces a curl-type polarization component even when the unrotated polarization possesses only gradient-type polarization, as expected from scalar density perturbations. We compute the angular power spectrum of curl-type polarization arising from small Faraday rotation due to a weak stochastic primordial magnetic field with a power-law power spectrum. The induced polarization power spectrum peaks at arc minute angular scales. Faraday rotation is one of the few cosmological sources of curl-type polarization, along with primordial tensor perturbations, gravitational lensing, and the vector and tensor perturbations induced by magnetic fields; the Faraday rotation signal peaks on significantly smaller angular scales than any of these, with a power spectrum amplitude which can be comparable to that from gravitational lensing. Prospects for detection are briefly discussed.

Physics welcomes the idea that space contains energy whose gravitational effect approximates that of Einstein’s cosmological constant, Λ; today the concept is termed dark energy or quintessence. Physics also suggests that dark energy could be dynamical, allowing for the arguably appealing picture of an evolving dark-energy density approaching its natural value, zero, and small now because the expanding universe is old. This would alleviate the classical problem of the curious energy scale of a millielectron volt associated with a constant Λ. Dark energy may have been detected by recent cosmological tests. These tests make a good scientific case for the context, in the relativistic Friedmann-Lemaître model, in which the gravitational inverse-square law is applied to the scales of cosmology. We have well-checked evidence that the mean mass density is not much more than one-quarter of the critical Einstein–de Sitter value. The case for detection of dark energy is not yet as convincing but still serious; we await more data, which may be derived from work in progress. Planned observations may detect the evolution of the dark-energy density; a positive result would be a considerable stimulus for attempts at understanding the microphysics of dark energy. This review presents the basic physics and astronomy of the subject, reviews the history of ideas, assesses the state of the observational evidence, and comments on recent developments in the search for a fundamental theory.

Motivated by observational indications of low cosmological mass density, we study a spatially open inflation modified hot big bang model whose evolutionary history is divided into three epochs: an early scalar field inflation epoch and the usual radiation and baryon (nonrelativistic matter) epochs. Generalizing techniques previously developed, we derive general solutions of the relativistic linear peturbation equations in each epoch. The constants of integration in the inflation epoch solutions are determined from quantum-mechanical initial conditions under the assumption that the perturbations are in the ground state at early times. The constants of integration in the radiation and baryon epoch solutions are determined from joining conditions derived by requiring that the linear perturbation equations remain nonsingular at the transitions between epochs. Expressions are derived for a number of baryon-epoch statistics which characterize large-scale structure, including the fractional mass, peculiar velocity, and gravitational potential perturbation two-point correlation functions, and the mean square value of the fractional mass and peculiar velocity perturbations. The Sachs-Wolfe relation is generalized to the open model and an expression for the angular fluctuation spectrum of the cosmic microwave background radiation temperature anisotropy is derived; we also determine the dipole velocity perturbation two-point correlation function and mean square value.

The fractional energy density perturbation power spectrum is not a power law; on small scales we find the usual n=+1 scale-invariant flat model law, while on large scales we discover a n=-1 spectrum. The small-scale part of the fractional mass perturbation two-point correlation function agrees with what one finds in the n=+1 scale-invariant flat case, but it has a second zero and at large scales the fractional mass perturbations are weakly positively correlated. Once again, given the form of the fractional mass perturbations are weakly positively correlated. Once again, given the form of the fractional energy density perturbation power spectrum in this model, we find that the slope of the inflation epoch scalar field potential cannot be unduly constrained by observational data.

The functional Schrödinger approach is used to study scalar field theory in hyperbolic (or open) de Sitter spacetime. While on intermediate length scales (small compared to the spatial curvature length scale) the massless minimally coupled scalar field two-point correlation function does have a term that varies logarithmically with scale, as in flat and closed de Sitter spacetime, the spatial curvature tames the infrared behavior of this correlation function at larger scales in the open model. As a result, and contrary to what happens in flat and closed de Sitter spacetime, spontaneously broken continuous symmetries are not restored in open de Sitter spacetime (with more than one spatial dimension).

A spatially flat cosmological scalar field Φ model with the scalar field potential ∝exp(-Φ / sqrt[p]), p>1, provides a simple class of inflationary cosmologies (which includes the usual exponential expansion inflation) that may be used as an analytical testing ground to help understand the predictions of the inflation model of the very early Universe. We divide the evolution of this model into three distinct epochs: scalar-field dominance and conventional radiation and baryon dominance; in each epoch we only account for irregularities in the dominant form of matter. We present closed-form solutions of the (synchronous gauge) relativistic linear perturbation equations that govern the evolution of inhomogeneities. These classical solutions, augmented with quantum-mechanically motivated initial conditions and joining conditions to match the expressions for the irregularities at the scalar-field-radiation and radiation-baryon transitions, are used to estimate the large-time form of the spectrum of energy-density irregularities, of the local departure velocity from homogeneous expansion, of large-scale fluctuations in the microwave background temperature, and of the gravitational-wave energy density. The inflation epoch results agree with those found from a purely quantum-mechanical analysis. Depending on the value of p this model can have more large-scale power than the usual scale-invariant spectrum (at the expense of less small-scale power) and would seem to be marginally better at forming large-scale structure than the canonical model; however, the decrease in small-scale power serves to exacerbate the problem of late galaxy formation. As the model approaches the exponential expansion inflation limit, the power spectrum tends towards the scale-invariant form, although, in this limit the numerical prefactor diverges. We find that transverse peculiar velocity perturbations are not generated. Normalizing by fitting to the observed large-scale departure velocity, we find that models which stop inflating around 10⁷-10¹⁶ GeV are not obviously observationally inconsistent.

Estimates for the baryon-dominated epoch form of the large-scale adiabatic energy-density irregularities generated during an early scalar-field-dominated inflation epoch, in simple inflation-modified hot-big-bang models, are compared to the widely used approximate general expression, which is proportional to the large-scale, gauge-invariant part of H²〈φφ^*〉 / (Φ̇_b)² evaluated at the first Hubble radius crossing (here Φ_b and φ are the spatially homogeneous and inhomogeneous parts of the scalar field, H is the Hubble parameter, and an overdot represents a derivative with respect to time). In the de Sitter inflation limit, if the inflation-epoch background scalar-field solution is an "attractor," or if there is sufficient inflation before the scale of interest leaves the Hubble radius, the approximate general expression identically reproduces what we have found. It is also less than an order of magnitude away from our expression in a large fraction of the parameter space of the inflation model we study and is within 2 orders of magnitude of our result in almost all of parameter space. We also show that the more accurate general expression (which the above formula is an approximation of) identically reproduces our results in the simple models studied, provided the inflation-epoch background scalar-field solution is an "attractor" or if there is sufficient inflation. The approximate general formula is used to restudy energy-density inhomogeneities in the quartic-potential scalar-field de Sitter inflation model; the difference between the standard result in this model and our result in related models is traced to a difference in the form of the part of the potential used to model "reheating" and the end of inflation. Very roughly, if (as we have assumed) the inflation-"reheating" transition occurs rapidly on the relevant Hubble time scale, observational data do not unduly constrain the parameters of the inflation model.

A spatially flat cosmological model dominated by a massive scalar (or pseudoscalar) field (Φ) with the scalar field potential V(Φ)=m²(Φ-Φ⁽²⁾)² (where Φ⁽²⁾ is the value of the scalar field at which the potential vanishes), in the limit in which the length scale corresponding to the scalar field mass m is very much smaller than length scales of cosmological interest (and, in particular, the length scale set by the Hubble parameter), is a tractable approximation of the macrophysics of both the late-time axion-dominated scenario as well as the early-time "reheating" subscenario of the inflation picture. Extending techniques we have previously developed, we derive solutions of the synchronous-gauge relativistic linear perturbation equations which govern the evolution of spatial irregularities, about a spatially homogeneous and isotropic background, in this model.

Joining conditions for cosmological spatial irregularities at an equation-of-state transition spatial hypersurface are derived by requiring that the relativistic linear perturbation theory equations of motion remain singularity-free at the transition spatial hypersurface. The transition spatial hypersurface is taken to be a spatially homogeneous local energy-density spatial hypersurface, between two spacetime-dependent "speed-of-sound" fluid-dominated epochs of a spatially flat Friedmann-Lemaître-Robertson-Walker cosmological model. Since the spacetime-dependent "speed-of-sound" fluid model includes, as special cases, both the scalar field model and the ideal fluid model, these joining conditions may be used at the scalar-field-radiation and radiation-baryon transitions in simple inflation-modified hot big-bang models. These joining conditions differ from two earlier sets of joining conditions (which differ from each other).

A spatially flat cosmological scalar field φ model with an exponential scalar field potential V(φ)∝exp(-φ / sqrt[p]), with p>1, has a time-dependent fixed-point solution which describes an inflating universe with the scale factor evolving as a(t)∝t^p. In the limit p→∞ this model reduces to the usual exponential expansion inflation model. In this paper we derive the inflationary epoch spectra of energy density and gravitational-wave irregularities by perturbatively solving the quantum-mechanical Dirac-Wheeler-DeWitt equations for this model. Our results agree with those found using an essentially classical analysis. We also show, by using two different orderings of factors in the quantum-mechanical Hamiltonian, that operator-ordering ambiguities do not affect the spectrum of irregularities to the order to which we work; they do, however, affect the normalization of the wave function.

We present closed-form solutions of the relativistic linear perturbation equations (in synchronous gauge) that govern the evolution of inhomogeneities in homogeneous, spatially flat, ideal-fluid, cosmological models. These expressions, which are valid for irregularities on any scale, allow one to analytically interpolate between the known approximate solutions which are valid at early times and at late times.

The cosmological consequences of a pervasive, rolling, self-interacting, homogeneous scalar field are investigated. A number of models in which the energy density of the scalar field red-shifts in a specific manner are studied. In these models the current epoch is chosen to be scalar-field dominated to agree with dynamical estimates of the density parameter, Ω_dyn∼0.2, and zero spatial curvature. The required scalar-field potential is ‘‘nonlinear’’ and decreases in magnitude as the value of the scalar field increases. A special solution of the field equations which is an attractive, time-dependent, fixed point is presented. These models are consistent with the classical tests of gravitation theory. The Eötvös-Dicke measurements strongly constrain the coupling of the scalar field to light (nongravitational) fields. Nucleosynthesis proceeds as in the standard hot big-bang model. In linear perturbation theory the behavior of baryonic perturbations, in the baryon-dominated epoch, do not differ significantly from the canonical scenario, while the presence of a substantial amount of homogeneous scalar-field energy density at low red-shifts inhibits the growth of perturbations in the baryonic fluid. The energy density in the scalar field is not appreciably perturbed by nonrelativistic gravitational fields, either in the radiation-dominated, matter-dominated, or scalar-field-dominated epochs. On the basis of this effect, we argue that these models could reconcile the low dynamical estimates of the mean mass density with the negligibly small spatial curvature preferred by inflation.

We present an on-shell superspace formulation of ten-dimensional N=1 supergravity coupled to N=1 super Yang-Mills theory. The coupling is completely specified in superspace by the Bianchi identity dH=c₁trF², where H is the gauge-invariant three-form field strength of supergravity and F is the two-form super Yang-Mills field strength. We also briefly discuss the theory that results from modifying this Bianchi identity by the addition of a piece proportional to the square of the super curvature two-form.

We formulate a functional approach to scalar quantum field theory in (n+1)-dimensional de Sitter spacetime and solve the functional Schrödinger equation for the conformally and minimally coupled scalar fields in both the k=0 and k=1 gauges. We show that there is a natural initial condition, the requirement that the field energy remain finite as the scale factor a becomes small, which specifies a unique, time-dependent, de Sitter vacuum state. This initial condition is closely related to Hawking’s prescription of including in the functional integral only those field configurations which are regular on the Euclidean section. The Green’s functions constructed using this initial condition are explicitly shown to be the analytic continuation of those derived using the Euclidean path-integral formalism and the regularity (boundary) condition. These Green’s functions are used to study the Hawking effect and the restoration of continuous symmetries. In particular we study the restoration of a broken O(2) symmetry of a Φ⁴ theory. We argue that spontaneously broken continuous symmetries are always dynamically restored in de Sitter spacetime.