Slow Rollover Research Articles

Inflation with F(T) teleparallel gravity

We study early universe with a particular form of F(T) Telleparallel gravity theory, in which inflation is driven by a scalar field. To ensure slow rollover, two different potentials are chosen in a manner, such that they remain almost flat for large initial value of the scalar field. Inflationary parameters show wonderful fit with the presently available Planck's data set. The energy scale of inflation is sub-Planckian and graceful exit from inflation is also administered. The chosen form of F(T) administers late-time cosmic acceleration too. In the process, unification of the early inflation with late-time acceleration is ensured. Unfortunately, a decelerated radiation dominated era is only possible with a different form of (quartic) potential, which being devoid of a flat section does not admit slow rollover.

Magnification Bias of Distant Galaxies in the Hubble Frontier Fields: Testing Wave Versus Particle Dark Matter Predictions

Abstract Acting as powerful gravitational lenses, the strong lensing galaxy clusters of the deep Hubble Frontier Fields (HFF) program permit access to lower-luminosity galaxies lying at higher redshifts than hitherto possible. We analyzed the HFF to measure the volume density of Lyman-break galaxies at z > 4.75 by identifying a complete and reliable sample up to z ≃ 10. A marked deficit of such galaxies was uncovered in the highly magnified regions of the clusters relative to their outskirts, implying that the magnification of the sky area dominates over additional faint galaxies magnified above the flux limit. This negative magnification bias is consistent with a slow rollover at the faint end of the UV luminosity function and it indicates a preference for Bose–Einstein condensate dark matter with a light boson mass of m B ≃ 10 − 22 eV over standard cold dark matter. We emphasize that measuring the magnification bias requires no correction for multiply-lensed images (with typically three or more images per source), whereas directly reconstructing the luminosity function will lead to an overestimate unless such images can be exhaustively matched up, especially at the faint end that is only accessible in the strongly lensed regions. In addition, we detected a distinctive downward transition in galaxy number density at z ≳ 8, which may be linked to the relatively late reionization reported by Planck. Our results suggests that JWST will likely peer into an “abyss” with essentially no galaxies detected in deep NIR imaging at z > 10.

Nanostructured ultrafast silicon-tip optical field-emitter arrays.

Femtosecond ultrabright electron sources with spatially structured emission are an enabling technology for free-electron lasers, compact coherent X-ray sources, electron diffractive imaging, and attosecond science. In this work, we report the design, modeling, fabrication, and experimental characterization of a novel ultrafast optical field emission cathode comprised of a large (>100,000 tips), dense (4.6 million tips·cm(-2)), and highly uniform (<1 nm tip radius deviation) array of nanosharp high-aspect-ratio silicon columns. Such field emitters offer an attractive alternative to UV photocathodes while providing a direct means of structuring the emitted electron beam. Detailed measurements and simulations show pC electron bunches can be generated in the multiphoton and tunneling regime within a single optical cycle, enabling significant advances in electron diffractive imaging and coherent X-ray sources on a subfemtosecond time scale, not possible before. At high charge emission yields, a slow rollover in charge is explained as a combination of the onset of tunneling emission and the formation of a virtual cathode.

Energy-momentum tensor of cosmological fluctuations during inflation

We study the renormalized energy-momentum tensor (EMT) of cosmological scalar fluctuations during the slow-rollover regime for chaotic inflation with a quadratic potential and find that it is characterized by a negative energy density which grows during slow rollover. We also approach the back-reaction problem as a second-order calculation in perturbation theory, finding no evidence that the back reaction of cosmological fluctuations is a gauge artifact. In agreement with the results for the EMT, the average expansion rate is decreased by the back reaction of cosmological fluctuations.

Quantum field dynamics of the slow rollover in the linear delta expansion

We show how the linear delta expansion, as applied to the slow-roll transition in quantum mechanics, can be recast in the closed time-path formalism. This results in simpler, explicit expressions than were obtained in the Schr\"odinger formulation and allows for a straightforward generalization to higher dimensions. Motivated by the success of the method in the quantum-mechanical problem, where it has been shown to give more accurate results for longer than existing alternatives, we apply the linear delta expansion to four-dimensional field theory. At small times all methods agree. At later times, the first-order linear delta expansion is consistently higher that Hartree-Fock, but does not show any sign of a turnover. A turnover emerges in second-order of the method, but the value of $<\hat{\Phi}^2(t)>$ at the turnover is larger that that given by the Hartree-Fock approximation. Based on this calculation, and our experience in the corresponding quantum-mechanical problem, we believe that the Hartree-Fock approximation does indeed underestimate the value of $<\hat{\Phi}^2(t)>$ at the turnover. In subsequent applications of the method we hope to implement the calculation in the context of an expanding universe, following the line of earlier calculations by Boyanovsky {\sl et al.}, who used the Hartree-Fock and large-N methods. It seems clear, however, that the method will become unreliable as the system enters the reheating stage.

Quantum dynamics of the slow rollover transition in the linear delta expansion

We apply the linear delta expansion to the quantum mechanical version of the slow rollover transition which is an important feature of inflationary models of the early universe. The method, which goes beyond the Gaussian approximation, gives results which stay close to the exact solution for longer than previous methods. It provides a promising basis for extension to a full field theoretic treatment.

Generic inflationary and noninflationary behavior in toy-cosmology

Cosmological solutions of a toy homogeneous isotropic universe filled with a superfluid Bose condensate described by a complex scalar field (with relativistic barotropic fluid interpretation) are studied. The eigenvalues of the tangent map for the resulting Hamiltonian system are used to classify the phase space regions and to understand the typical toy-universe evolution. After a transient, inflation is obtained in the hyperbolic (real eigenvalues) region. This new independent eigenvalue-based inflationary criterion is shown to be compatible and complementary to the standard slow roll-over conditions. For the later evolution of the toy-universe, a family of adiabatic trajectories oscillating about a conventional cosmology filled with a relativistic fluid is obtained once the system falls into the elliptic (imaginary eigenvalue) regions. The corresponding thermodynamic functions are computed.

Exact scalar field cosmologies in a higher derivative theory

We obtain exact cosmological solutions of a higher derivative theory described by the Lagrangian L=R+2αR 2 in the presence of interacting scalar field. The interacting scalar field potential required for a known evolution of the FRW universe in the framework of the theory is obtained using a technique different from the usual approach to solve the Einstein field equations. We follow here a technique to determine potential similar to that used by Ellis and Madsen in Einstein gravity. Some new and interesting potentials are noted in the presence of R 2 term in the Einstein action for the known behaviours of the universe. These potentials in general do not obey the slow rollover approximation.

Inflation and the nature of supersymmetry breaking

The scale at which supersymmetry is broken and the mechanism by which supersymmetry breaking is fed down to the observable sector has rich implications on the way Nature may have chosen to accomplish inflation. We discuss a simple model for slow rollover inflation which is minimal in the sense that the inflation may be identified with the field responsible for the generation of the μ-term. Inflation takes place at very late times and is characterized by a very low reheating temperature. This property is crucial to solve the gravitino problem and may help to ameliorate the cosmological moduli problem. The COBE normalized value of the vacuum energy driving inflation is naturally of the order of 1011 GeV. This favors the N = 1 supergravity scenario where supersymmetry breaking is mediated by gravitational interactions. Nonetheless, smaller values of the vacuum energy are not excluded by present data on the temperature anisotropy and the inflationary scenario may be implemented in the context of new recent ideas about gauge mediation where the standard model gauge interactions can serve as the messengers of supersymmetry breaking. In this class of models supersymmetry breaking masses are usually proportional to the F-term of a gauge singlet superfield. The same F-term may provide the vacuum energy density necessary to drive inflation. The spectrum of density perturbations is characterized by a spectral index which is significantly displaced from one. The measurements of the temperature anisotropies in the cosmic microwave background radiation at the accuracy expected to result from the planned missions will be able to confirm or disprove this prediction and to help in getting some deeper insight into the nature of supersymmetry breaking.

Large-Angle Cosmic Microwave Background Anisotropy in an Open Universe in the One-Bubble Inflationary Scenario

We consider an alternative scenario of inflation which can account for a spatially open universe. It is similar to the old inflation in which the bubble nucleation occurs in the sea of false vacuum, but differs from it in that the second slow rollover inflation occurs inside a nucleated bubble. Hence our observable universe is entirely contained in one nucleated bubble. The significance of the scenario is that apart from a variance due to model parameters it gives us a definite prediction on the spectrum of the primordial density perturbations, hence is observationally testable. Here we investigate the spectrum of CMB anisotropies on large angular scales. We find that the contribution from peculiar modes which never appear in the usual harmonic analysis is significant in the case $\Omega_0\lesssim0.1$.

Quantum rolling down out of equilibrium.

In a scalar field theory, when the tree level potential admits broken symmetry ground states, the quantum corrections to the static effective potential are complex. (The imaginary part is a consequence of an instability towards phase separation and the static effective potential is not a relevant quantity for understanding the dynamics). Instead, we study here the equations of motion obtained from the one loop effective action for slow rollover out of equilibrium. We considering the case in which a scalar field theory undergoes a rapid phase transition from $T_i>T_c$ to $T_f<T_c$. We find that, for slow rollover initial conditions (the field near the maximum of the tree level potential), the process of phase separation controlled by unstable long-wavelength fluctuations introduces dramatic corrections to the dynamical evolution of the field. We find that these effects slow the rollover even further

Limits on reheating in inflationary cosmology models.

We show that the decay of a scalar particle into ordinary matter particles is restricted by the mass and by the vacuum expectation value of the scalar field. If this scalar field is responsible for inflation, difficulties with reheating the Universe may ensue. For the case of induced-gravity models, where the magnitude of the vacuum expectation value is of order the Planck mass, the scalar field decays predominantly into gravitons and adequate reheating does not occur in the usual slow rollover scenario. We also discuss the effect of the energy stored in the scalar-field oscillations.

The condition for classical slow rolling in new inflation

By means of the stochastic description of inflation we investigate the dynamics of a fixed comoving domain in a continuously inflating universe on a global scale, both analytically and numerically. Particular attention is paid to the condition for a domain to enter the classical slow rolling phase. New inflationary universe models with the potential form V( ϕ)≈ V 0− cϕ 2 n at ϕ∼0 are considered. The critical value of the scalar field beyond which the field slowly rolls down the potential hill is estimated. We find that for all models under consideration, the condition for classical slow rolling is a sufficient condition for the expected amplitude of density perturbations to be smaller than unity. In other words, the density perturbation amplitude at the later Friedmann stage is always smaller than unity if the universe experienced the classical slow roll-over phase.

Stochastic stage of an inflationary universe model

Using the stochastic description of a scalar field on its phase space, we analyse the early stage of a new inflationary universe model with a Higgs-type potential. At this stage, quantum fluctuations play an important role for the dynamics of the scalar field as well as the classical potential force. We find that the velocity dispersion of the field 〈 ϕ ̇ 2〉 becomes negligible within the Hubble timescale and the slow roll-over phase is realized, provided that 〈 ϕ ̇ 2〉≲〈V(ϕ)〉 and λ〈ϕ 2〉 2⪡〈V(ϕ)〉 initially, where λ is the quartic coupling constant. This supports the picture that inflation can set in for a wide class of pre-inflationary states, including thermal states, and the behavior of the scalar fields is independent of such states. It also verifies the use of the slow roll-over approximation for most states of inflation.

Models for inflation with a low supersymmetry-breaking scale

We present models where the same scalar field is responsible for inflation and for the breaking of supersymmetry. The scale of supersymmetry breaking is related to the slope of the potential in the plateau region described by the scalar field during the slow rollover, and the gravitino mass can therefore be kept as small as M W, the mass of the weak gauge boson. We show that such a result is stable under radiative corrections. We describe the inflationary scenario corresponding to the simplest of these models and show that no major problem arises, except for a violation of the thermal constraint (stabilization of the field in the plateau region at high temperature). We discuss the possibility of introducing a second scalar field to satisfy this constraint.

Kaluza-Klein cosmological inflation

We discuss cosmological inflation resulting from slow rollover to equilibrium of the radius of the compact manifold in Kaluza-Klein theories. Reheating of the universe after this inflation, and the spectrum of density fluctions are also considered.