Initial Field Value Research Articles

Dynamical realization of the small field inflation of Coleman-Weinberg type in the post supercooled universe

The small field inflation (SFI) of Coleman-Weinberg (CW) type suffers from precise tuning of the initial inflaton field value to be away from the true vacuum one. We propose a dynamical trapping mechanism to solve this problem: an ultra-supercooling caused by an almost scale-invariant CW potential traps the inflaton at the false vacuum, far away from the true vacuum dominantly created by the quantum scale anomaly, and allows the inflaton to dynamically start the slow-roll down due to a classical explicit-scale breaking effect. To be concrete, we employ a successful CW-SFI model and show that the proposed mechanism works consistently with the observed bounds on the inflation parameters. The proposed new mechanism thus provides new insights for developing small field inflation models.

Axion free-kick misalignment mechanism

We propose an alternative scenario for the axion misalignment mechanism based on the nontrivial interplay between the axion and a light dilaton in the early universe. Dark matter abundance is still sourced by the initial misalignment of the axion field, whose motion along the potential kicks the dilaton field away from its minimum, and dilaton starts to oscillate later with a delayed onset time for oscillation and a relatively large misalignment value due to the kick; eventually the dilaton dominates over the axion in their energy densities, and the dilaton is identified as dark matter. The kick effect due to axion motion is the most significant if the initial field value of dilaton is near its minimum; therefore, we call this scenario axion free-kick misalignment mechanism, where axion plays the role similar to a football player. Dark matter abundance can be obtained with a lower axion decay constant compared to the conventional misalignment mechanism.

On the Trans-Planckian Censorship Conjecture and the generalized non-minimal coupling

We investigate the Trans-Planckian Censorship Conjecture (TCC) and the arising bounds on the inflationary cosmology caused by that conjecture. In that investigation, we analyze TCC bounds for both Jordan and Einstein frames in the presence of a generic non-minimal coupling (to gravity) term. That term allows us to use the functional freedom it brings to the inflationary Lagrangian as an effective Planck mass. In this sense, we argue one should consider the initial field value of the effective Planck mass for the TCC. We show that as a result, one can remove the TCC upper bounds without the need to produce a new process or go beyond the standard inflation mechanism, with the generalized non-minimal coupling, and for Higgs-like symmetry-breaking potentials.

Primordial black holes from QCD axion bubbles

We propose a scenario in which a strong Peccei-Quinn (PQ) symmetry breaking in the early universe results in large inhomogeneities of the initial QCD axion field value, leading to the formation of very dense axion bubbles. Some of the axion bubbles subsequently collapse into primordial black holes (PBHs). The spatially homogeneous part of the QCD axion explains dark matter of the universe, while the PBHs arising from the axion bubbles can explain the LIGO events or the seed of supermassive black holes. Interestingly, the mass of PBH is determined by the axion decay constant; for fa = 1017 (1016) GeV, the PBH mass is heavier than about 10 (104) M⊙. In addition, axion miniclusters are also formed from the axion bubbles more abundantly than PBHs, and their masses are expected to be heavier than in the usual scenario based on the spontaneous breaking of the PQ symmetry after inflation.

Axion kinetic misalignment and parametric resonance from inflation

Axion cold dark matter from standard misalignment typically requires a decay constant fa ≳ 1011 GeV . Kinetic misalignment and parametric resonance easily allow lower values of fa when the radial Peccei-Quinn (PQ) symmetry breaking field takes large initial values. Here, we consider the effects of inflation on kinetic misalignment and parametric resonance. We assume that the initial PQ field value is determined by quantum fluctuations, and is set by the Hubble parameter during inflation, HI, and the PQ field mass. PQ field oscillations begin before or after the completion of reheating after inflation at a temperature TR. We determine the range of fa and the inflationary parameters (HI, TR) consistent with axion dark matter for a quartic potential for the PQ field. We find that 108 GeV < fa < 1011 GeV can consistently produce axion dark matter. A significant portion of the allowed parameter space predicts rare kaon decays, KL → (π0 + missing energy), and/or suppression of structure formation on small scales.

Pure natural quintessential inflation and dark energy

We propose the pure natural quintessential inflation model which is motivated by Witten’s conjecture, where the axion couples to pure Yang–Mills [Formula: see text] gauge field at large [Formula: see text] limit. This modifies the standard cosine potential which is presented in the natural inflation, making it compatible with current CMB data. Our model gives a successful inflation as well as acceleration at the late times by quintessence inflaton field ([Formula: see text]). Here the inflaton field is responsible for inflation, after that the field enters into peculiar type of reheating and then they act as dynamical dark energy field which follows the same inflation potential and same model parameters. The dynamical field slowly rolls until the Hubble drops to mass of the quintessence field and it reaches the current dark energy field value. Here the dark energy scale is field dependent. Our quintessence model follows thawing frozen approach, therefore the frozen quintessence field evolves with respect to cosmic time from initial field value [Formula: see text] to present nonzero minimum field value [Formula: see text]. The obtained field value turned into ultralight and it satisfies the present dark energy density which is [Formula: see text].

Axion Kinetic Misalignment Mechanism.

In the conventional misalignment mechanism, the axion field has a constant initial field value in the early Universe and later begins to oscillate. We present an alternative scenario where the axion field has a nonzero initial velocity, allowing an axion decay constant much below the conventional prediction from axion dark matter. This axion velocity can be generated from explicit breaking of the axion shift symmetry in the early Universe, which may occur as this symmetry is approximate.

Initial conditions of inflation in a Bianchi I Universe

We investigate the initial conditions of inflation in a Bianchi~I universe that is homogeneous but not isotropic. We use the Eisenhart lift to describe such a theory geometrically as geodesics on a field space manifold. We construct the phase-space manifold of the theory by considering the tangent bundle of the field space and equipping it with a natural metric. We find that the total volume of this manifold is finite for a wide class of inflationary models. We therefore take the initial conditions to be uniformly distributed over it in accordance with Laplace's principle of indifference. This results in a normalisable, reparametrisation invariant measure on the set of initial conditions of inflation in a Bianchi~I universe. We find that this measure favours an initial state in which the inflaton field is at or near its minimum, with a mild preference for some initial anisotropy. Since inflation requires an initial field value with a large displacement from its minimum, we therefore conclude that the theory of inflation requires finely tuned initial conditions.

Vacuum decay in real time and imaginary time formalisms

We analyze vacuum tunneling in quantum field theory in a general formalism by using the Wigner representation. In the standard instanton formalism, one usually approximates the initial false vacuum state by an eigenstate of the field operator, imposes Dirichlet boundary conditions on the initial field value, and evolves in imaginary time. This approach does not have an obvious physical interpretation. However, an alternative approach does have a physical interpretation: in quantum field theory, tunneling can happen via classical dynamics, seeded by initial quantum fluctuations in both the field and its momentum conjugate, which was recently implemented in Ref. [1]. We show that the Wigner representation is a useful framework to calculate and understand the relationship between these two approaches. We find there are two, related, saddle point approximations for the path integral of the tunneling process: one corresponds to the instanton solution in imaginary time and the other one corresponds to classical dynamics from initial quantum fluctuations in real time. The classical approximation for the dynamics of the latter process is justified only in a system with many degrees of freedom, as can appear in field theory due to high occupancy of nucleated bubbles, while it is not justified in single particle quantum mechanics, as we explain. We mention possible applications of the real time formalism, including tunneling when the instanton vanishes, or when the imaginary time contour deformation is not possible, which may occur in cosmological settings.

New inflation in the landscape and typicality of the observed cosmic perturbation

We investigate if the observed small and nearly scale-invariant primordial cosmic perturbation, i.e. the perturbation amplitude $P_\zeta\sim10^{-9}$ and the spectral index $n_s \simeq 0.965$, is typical in the landscape of vacua after imposing anthropic selections on them. We consider the situation where the universe begins from a metastable vacuum driving a precedent inflation, a curvature-dominated open universe is created by tunneling, and the curvature energy is inflated away by new inflation. We argue that the initial inflaton field value is homogeneous but typically non-zero because of the quantum fluctuation of long wavelength modes created during the precedent inflation, and only the universe which accidentally has a small inflaton field value is anthropically selected. We show that this bias, together with certain distributions of inflation model parameters that are physically well-motivated, makes the observed small and nearly scale-invariant spectrum typical.

Initial conditions for critical Higgs inflation

It has been pointed out that a large non-minimal coupling ξ between the Higgs and the Ricci scalar can source higher derivative operators, which may change the predictions of Higgs inflation. A variant, called critical Higgs inflation, employs the near-criticality of the top mass to introduce an inflection point in the potential and lower drastically the value of ξ. We here study whether critical Higgs inflation can occur even if the pre-inflationary initial conditions do not satisfy the slow-roll behavior (retaining translation and rotation symmetries). A positive answer is found: inflation turns out to be an attractor and therefore no fine-tuning of the initial conditions is necessary. A very large initial Higgs time-derivative (as compared to the potential energy density) is compensated by a moderate increase in the initial field value. These conclusions are reached by solving the exact Higgs equation without using the slow-roll approximation. This also allows us to consistently treat the inflection point, where the standard slow-roll approximation breaks down. Here we make use of an approach that is independent of the UV completion of gravity, by taking initial conditions that always involve sub-planckian energies.

Gravitino production suppressed by dynamics of sgoldstino

In supersymmetric theories, the gravitino is abundantly produced in the early Universe from thermal scattering, resulting in a strong upper bound on the reheat temperature after inflation. We point out that the gravitino problem may be absent or very mild due to the early dynamics of a supersymmetry breaking field, i.e. a sgoldstino. In models of low scale mediation, the field value of the sgoldstino determines the mediation scale and is in general different in the early Universe from the present one. A large initial field value since the era of the inflationary reheating suppresses the gravitino production significantly. We investigate in detail the cosmological evolution of the sgoldstino and show that the reheat temperature may be much higher than the conventional upper bound, restoring the compatibility with thermal leptogenesis.

Axion string dynamics I: 2+1D

If the axion exists and if the initial axion field value is uncorrelated at causally disconnected points, then it should be possible to predict the efficiency of cosmological axion production, relating the axionic dark matter density to the axion mass. The main obstacle to making this prediction is correctly treating the axion string cores. We develop a new algorithm for treating the axionic string cores correctly in 2+1 dimensions. When the axionic string cores are given their full physical string tension, axion production is about twice as efficient as in previous simulations. We argue that the string network in 2+1 dimensions should behave very differently than in 3+1 dimensions, so this result cannot be simply carried over to the physical case. We outline how to extend our method to 3+1D axion string dynamics.

Viable dark energy models using pseudo-Nambu-Goldstone bosons

A canonical pseudo-Nambu-Goldstone boson (pNGB) can play the role of a dark energy field responsible for present cosmic acceleration. Confronted with the recent cosmological data, we find that the pNGB field requires a spontaneous symmetry breaking scale $f$ close to ${M}_{P}$ and that the initial field value be fine-tuned. It is difficult to achieve a large $f$ in a theoretically consistent setup. A possible resolution can be achieved by increasing the Hubble friction in well-motivated particle physics models in the general setup of modified gravity theories. We show two phenomenological examples of this setup where the standard pNGB action has been modified by introducing terms motivated by Galileon cosmology. We confront those examples with the recent supernovae, PLANCK and baryon acoustic oscillation data. We find that moderate values of the dimensionless constants that increase the friction make $f\ensuremath{\ll}{M}_{P}$ and the generic initial conditions also favorable by the data. We also comment on how the fifth-force constraints arising in these modified theories can be evaded.

Numerical Simulation of Flow around a Cylinder under Different Reynolds Number

Based on fully structured grids parallel numerical simulations of flow around a cylinder under different Reynolds number are carried out. Two-dimensional and three-dimensional models are established at the same time under specific Reynolds number, and further analyze of three-dimensional flow characteristics as well as the generated influence to overall physical quantities are presented. In order to explore efficient high Reynolds number turbulence models, a comparative research of the LES model without wall functions and the Spalart-Allmaras turbulence model is carried out. In order to improve the computational efficiency, a domain decomposition parallel computing strategy is used, and a calculation strategy that results of coarse grid was assigned to fine grid as initial field value by 3D linear interpolation is presented. Simulation results show that: Drag coefficient and Strouhal number have very good consistency with the experimental data, which verifies the correctness of the calculation method; Even if at low Reynolds number (200≤Re≤300), using a three-dimensional model is still necessary; While in the high Reynolds number stage, compared to LES model without wall functions, Spalart-Allmaras model is more applicable and more efficient.

Universality in a class of Q-ball solutions: An analytic approach

The properties of Q-balls in the general case of a sixth order potential have been studied using analytic methods. In particular, for a given potential, the initial field value that leads to the soliton solution has been derived and the corresponding energy and charge have been explicitly evaluated. The proposed scheme is found to work reasonably well for all allowed values of the model parameters.

Can dark energy be decaying?

We explore the fate of the Universe given the possibility that thedensity associated with `dark energy' may decay slowly with time.Decaying dark energy is modelled by a homogeneous scalar field whichcouples minimally to gravity and whose potential has at least one local quadratic maximum. Dark energy decays as the scalar fieldrolls down its potential, consequently the current acceleration epochis a transient. We examine two models of decaying dark energy. Inthe first, the dark energy potential is modelled by an analytical formwhich is generic close to the potential maximum. The second potentialis the cosine, which can become negative as the field evolves,ensuring that a spatially flat Universe collapses in the future. Weexamine the feasibility of both models using observations of highredshift type Ia supernovae. A maximum likelihood analysis is used tofind allowed regions in the {m, ϕ0} plane (m isthe tachyon mass modulus and ϕ0 the initial scalar field value;m ∼ H0 and ϕ0 ∼ MP by order of magnitude). For thefirst model, the time for the potential to drop to half its maximumvalue is larger than ∼8 Gyr. In the case of the cosinepotential, the time left until the Universe collapses is alwaysgreater than ∼18 Gyr (both estimates are presented for Ω0m = 0.3, m/H0 ∼ 1, H0 ≃ 70 km s−1 Mpc−1, and at the 95.4%confidence level).

New results with superconducting 500 MHz cavities at CERN

New experimental results with three 500 MHz single cell Nb cavities of “spherical” shape are described. At 4.2 K, acceleration fields up to 6.5 MV/m and Q o values up to 3 × 10 9 (1.7 × 10 10 at 2.2 K) have been reached. Rf He ion sputtering was used and allowed to decrease the electron loading sufficiently so that the fields were always limited by a thermal instability driven by a small region of increased rf losses. A stepwise removal of different lossy regions by mechanical grinding and milling allowed field levels to be increased. It was found that the cavity performance depends strongly on the arrangement of the cavity and the vacuum system. After an accidental exposure to laboratory air no strong Q 0 degradation was observed and the initial field value could be restored after a short He ion sputtering.

On a Stability of a Q-Switched Laser Pulse Form

The effect of an inhomogeneity of population inversion at an initial Q -switched time on the stability of the giant pulse from of a laser in an axial multimode oscillation was analysed numerically for the laser equation with an unrealistic assumption that the ratio of a cavity relaxation constant to a frequency difference between neighbouring modes is unity, principally Results indicate that for three mode oscillation there may be an initial spatial distribution of population which would uniform the giant pulse from irrespective of initial field value.