arXiv subjects

Sonia Paban

Publications and source records attributed to Sonia Paban.

At least 19 recordsLinked to original sources

arXiv2026

Stress-Energy Tensor of a Scalar Field on a Product Spacetime with a Time-Dependent Compact Dimension

We compute the vacuum expectation value of the stress-energy tensor of a scalar field on a product spacetime composed of an FLRW background times a compact dimension ($\mathcal{M}^{1, \,d-1} \times \mathcal{S}^1$), where the size of the latter is allowed to vary with time. We modify the standard adiabatic regularization prescription to obtain analytic expressions for both $d=3$ and $d=4$. In the massless and conformally coupled limit, the leading order time-dependent results are consistent with known time-independent Casimir contributions. Furthermore, in this limit the higher-order time-dependent corrections, when the FLRW and compact-dimension scale factors coincide, match known results for ($1+d$)-dimensional FLRW spacetime.

hep-th↗

arXiv2024

Investigating the Origin of CMB Large-Scale Features Using LiteBIRD and CMB-S4

Several missions following Planck are currently under development, which will provide high-precision measurements of the Cosmic Microwave Background (CMB) anisotropies. Specifically, measurements of the E modes will become nearly limited by cosmic variance, which, especially when considering the sharpness of the E-mode transfer functions, may allow for the ability to detect deviations from the concordance model in the CMB data. We investigate the capability of upcoming missions to scrutinize models that have been proposed to address large-scale anomalies observed in the temperature spectra from WMAP and Planck. To this purpose, we consider four benchmarks that modify the CMB angular power spectra at large scales: models producing suppression, a dip, and amplification in the primordial scalar power spectrum, as well as a beyond-Lambda CDM prescription of dark energy. Our analysis shows that large-scale measurements from LiteBIRD will be able to distinguish between various types of primordial and late-time models that predict modifications to the angular spectra at these scales. Moreover, if these deviations from the standard cosmological model are determined to be systematic and do not reflect the true universe model, future experiments could potentially dismiss these features as statistical fluctuations. We also show that additional measurements from CMB-S4 can impose more stringent constraints by probing correlated signals that these models predict at smaller scales (l>100). A byproduct of our analysis is that a recently proposed "Dark Dimension" scenario, featuring power amplification at large scales, is strongly bound by current data, pushing the deviation from the standard model to unobservable scales. Overall, our results demonstrate that future CMB measurements can provide valuable insights into large-scale anomalies that are present in the current CMB data.

astro-ph.CO↗

arXiv2024

Primordial Stochastic Gravitational Wave Backgrounds from a Sharp Feature in Three-field Inflation II: The Inflationary Era

We study the contribution of large scalar perturbations sourced by a sharp feature during cosmic inflation to the stochastic gravitational wave background (SGWB), extending our previous work to include the SGWB sourced during the inflationary era. We focus in particular on three-field inflation, since the third dynamical field is the first not privileged by the perturbations' equations of motion and allows a more direct generalization to $N$-field inflation. For the first time, we study the three-field isocurvature perturbations sourced during the feature and include the effects of isocurvature masses. In addition to a two-field limit, we find that the third field's dynamics during the feature can source large isocurvature transients which then later decay, leaving an inflationary-era-sourced SGWB as their only observable signature. We find that the inflationary-era signal shape near the peak is largely independent of the number of dynamical fields and has a greatly enhanced amplitude sourced by the large isocurvature transient, suppressing the radiation-era contribution and opening a new window of detectable parameter space with small adiabatic enhancement. The largest enhancements we study could easily violate backreaction constraints, but much of parameter space remains under perturbative control. These SGWBs could be visible in LISA and other gravitational wave experiments, leaving an almost universal signature of sharp features during multi-field inflation, even when the sourcing isocurvature decays to unobservability shortly afterwards.

astro-ph.CO↗

arXiv2024

Primordial Stochastic Gravitational Wave Backgrounds from a Sharp Feature in Three-field Inflation I: The Radiation Era

The detection of a primordial stochastic gravitational wave background has the potential to reveal unprecedented insights into the early universe, and possibly into the dynamics of inflation. Generically, UV-complete inflationary models predict an abundance of light scalars, so any inflationary stochastic background may well be formed in a model with several interacting degrees of freedom. The stochastic backgrounds possible from two-field inflation have been well-studied in the literature, but it is unclear how similar they are to the possibilities from many-field inflation. In this work we study stochastic backgrounds from more-than-two field inflation for the first time, focusing on the scalar-induced background produced during the radiation era by a brief turn in three-field space. We find an analytic expression for the enhancement in the power spectrum as a function of the turn rate and the torsion, and show that unique signatures of three-field dynamics are possible in the primordial power spectrum and gravitational wave spectrum. We confirm our analytic results with a suite of numerical simulations and find good agreement in the shape and amplitude of the power spectra. We also comment on the detection prospects in LISA and other future detectors. We do not expect the moderately large growth of the inflationary perturbations necessary for detection to cause a breakdown of perturbation theory, but this must be verified on a case-by-case basis for specific microphysical models to make a definitive claim.

astro-ph.CO↗

arXiv2021

Rapid-turn inflation in supergravity is rare and tachyonic

Strongly non-geodesic, or rapidly turning trajectories in multifield inflation have attracted much interest recently from both theoretical and phenomenological perspectives. Most models with large turning rates in the literature are formulated as effective field theories. In this paper we investigate rapid-turn inflation in supergravity as a first step towards understanding them in string theory. We find that large turning rates can be generated in a wide class of models, at the cost of high field space curvature. In these models, while the inflationary trajectories are stable, one Hessian eigenvalue is always tachyonic and large, in Hubble units. Thus, these models satisfy the de Sitter swampland conjecture along the inflationary trajectory. However, the high curvatures underscore the difficulty of obtaining rapid-turn inflation in realistic string-theoretical models. In passing, we revisit the $η$-problem in multifield slow-roll inflation and show that it does not arise, inasmuch as the inflatons, $ϕ^i$, can all be heavier (in absolute value) that the Hubble scale: $|m_i|/H>1$, $\forall\, i$.

hep-th↗

arXiv2020

The Multi-Field, Rapid-Turn Inflationary Solution

There are well-known criteria on the potential and field-space geometry for determining if slow-roll, slow-turn, multi-field inflation is possible. However, even though it has been a topic of much recent interest, slow-roll, rapid-turn inflation only has such criteria in the restriction to two fields. In this work, we generalize the two-field, rapid-turn inflationary attractor to an arbitrary number of fields. We quantify a limit, which we dub extreme turning, in which rapid-turn solutions may be found efficiently and develop methods to do so. In particular, simple results arise when the covariant Hessian of the potential has an eigenvector in close alignment with the gradient -- a situation we find to be common and we prove generic in two-field hyperbolic geometries. We verify our methods on several known rapid-turn models and search two type-IIA constructions for rapid-turn trajectories. For the first time, we are able to efficiently search for these solutions and even exclude slow-roll, rapid-turn inflation from one potential.

hep-th↗

arXiv2020

Multi-field Inflation in High-Slope Potentials

We present two families of multi-field potentials that support inflation while satisfying the refined de Sitter and the distance swampland conjectures. Both families feature Planck-compatible phenomenology. The first is a helix-type potential, in a flat field-space metric, that satisfies the conjectures via a high turning rate. This model has a tensor-toscalar ratio close to, but below, the current experimental limits and small non-gaussianities. The second family, an example of orbital inflation, utilizes a negatively curved field metric to achieve prolonged inflation with nontrivial turning in the presence of a tachyonic direction. Although perturbations in this model undergo an exponential growth before horizon exit, it is always possible to match the measured amplitude of the power spectrum by lowering the scale of inflation if the turning rate is low enough. We identify a Planck-compatible region of parameter space in which the scale of inflation is above that of nucleosynthesis. Due to the rapid growth, this model predicts an exponentially suppressed value for the tensor-to-scalar ratio.

hep-th↗

arXiv2018

Inflation in Multi-field Modified DBM Potentials

We study multi-field inflation in random potentials generated via a non-equilibrium random matrix theory process. We make a novel modification of the process to include correlations between the elements of the Hessian and the height of the potential, similar to a Random Gaussian Field (RGF). We present the results of over 50,000 inflationary simulations involving 5-100 fields. For the first time, we present results of $\mathcal{O}(100)$ fields using the full `transport method', without slow-roll approximation. We conclude that Planck compatibility is a common prediction of such models, however significant isocurvature power at the end of inflation is possible.

astro-ph.CO↗

arXiv2017

Robustness of Inflation to Inhomogeneous Initial Conditions

We consider the effects of inhomogeneous initial conditions in both the scalar field profile and the extrinsic curvature on different inflationary models. In particular, we compare the robustness of small field inflation to that of large field inflation, using numerical simulations with Einstein gravity in 3+1 dimensions. We find that small field inflation can fail in the presence of subdominant gradient energies, suggesting that it is much less robust to inhomogeneities than large field inflation, which withstands dominant gradient energies. However, we also show that small field inflation can be successful even if some regions of spacetime start out in the region of the potential that does not support inflation. In the large field case, we confirm previous results that inflation is robust if the inflaton occupies the inflationary part of the potential. Furthermore, we show that increasing initial scalar gradients will not form sufficiently massive inflation-ending black holes if the initial hypersurface is approximately flat. Finally, we consider the large field case with a varying extrinsic curvature K, such that some regions are initially collapsing. We find that this may again lead to local black holes, but overall the spacetime remains inflationary if the spacetime is open, which confirms previous theoretical studies.

hep-th↗

arXiv2017

Generalized surface tension bounds in vacuum decay

Coleman and De Luccia (CDL) showed that gravitational effects can prevent the decay by bubble nucleation of a Minkowski or AdS false vacuum. In their thin-wall approximation this happens whenever the surface tension in the bubble wall exceeds an upper bound proportional to the difference of the square roots of the true and false vacuum energy densities. Recently it was shown that there is another type of thin-wall regime that differs from that of CDL in that the radius of curvature grows substantially as one moves through the wall. Not only does the CDL derivation of the bound fail in this case, but also its very formulation becomes ambiguous because the surface tension is not well-defined. We propose a definition of the surface tension and show that it obeys a bound similar in form to that of the CDL case. We then show that both thin-wall bounds are special cases of a more general bound that is satisfied for all bounce solutions with Minkowski or AdS false vacua. We discuss the limit where the parameters of the theory attain critical values and the bound is saturated. The bounce solution then disappears and a static planar domain wall solution appears in its stead. The scalar field potential then is of the form expected in supergravity, but this is only guaranteed along the trajectory in field space traced out by the bounce.

hep-th↗

arXiv2017

von Neumann's Formula, Measurements and the Lindblad Equation

We previously remarked that when an observable A has a continuous spectrum, then von Neumann's formula for the post-measurement state needs to be extended and the correct formula ineluctably involves the resolution of the detector used in the measurement. We generalize previous results to compute the uncertainties in successive measurements of more general pairs of observables. We also show that this extended von Neumann's formula for the post-measurement state is a completely positive map and, moreover, that there is a completely-positive interpolation between the pre- and post-measurement states. Weinberg has advocated that the time-evolution during the measurement process should be modeled as an open quantum system and governed by a Lindblad equation. We verify that this is indeed the case for an arbitrary observable, A, and a fairly general class of interpolations.

quant-ph↗

arXiv2016

Tunneling from a Minkowski vacuum to an AdS vacuum: A new thin-wall regime

Using numerical and analytic methods, we study quantum tunneling from a Minkowski false vacuum to an anti-de Sitter true vacuum. Scanning the parameter space of theories with quartic and non-polynomial potentials, we find that for any given potential tunneling is completely quenched if gravitational effects are made sufficiently strong. For potentials where $ε$, the energy density difference between the vacua, is small compared to the barrier height, this occurs in the thin-wall regime studied by Coleman and De Luccia. However, we find that other potentials, possibly with $ε$ much greater than the barrier height, produce a new type of thin-wall bounce when gravitational effects become strong. We show that the critical curve that bounds the region in parameter space where the false vacuum is stable can be found by a computationally simple overshoot/undershoot argument. We discuss the treatment of boundary terms in the bounce calculation and show that, with proper regularization, one obtains an identical finite result for the tunneling exponent regardless of whether or not these are included. Finally, we briefly discuss the extension of our results to transitions between anti-de Sitter vacua.

hep-th↗

arXiv2016

On primordial equation of state transitions

We revisit the physics of transitions from a general equation of state parameter to the final stage of slow-roll inflation. We show that it is unlikely for the modes comprising the cosmic microwave background to contain imprints from a pre-inflationary equation of state transition and still be consistent with observations. We accomplish this by considering observational consistency bounds on the amplitude of excitations resulting from such a transition. As a result, the physics which initially led to inflation likely cannot be probed with observations of the cosmic microwave background. Furthermore, we show that it is unlikely that equation of state transitions may explain the observed low multipole power suppression anomaly.

astro-ph.CO↗

arXiv2015

Tunneling in Theories with Many Fields

The possibility of a landscape of metastable vacua raises the question of what fraction of vacua are truly long lived. Naively any would-be vacuum state has many nearby decay paths, and all possible decays must be suppressed. An interesting model of this phenomena consists of $N$ scalars with a random potential of fourth order. Here we show that the scaling of the typical minimal bounce action with $N$ is readily understood, and differs from statements in the literature. We discuss the extension to more realistic landscape models.

hep-th↗

arXiv2014

Analyzing Multi-Field Tunneling With Exact Bounce Solutions

We study multi-field tunneling using exact solutions for additive potentials. We introduce a binomial potential with non-integer powers that could be considered a generalization of the $4D$ Fubini instanton potential. Using scaling arguments, we show that for multi-field potentials taller and wider barriers may still lead to a smaller bounce action.

hep-th↗

arXiv2014

Bogoliubov Excited States and the Lyth Bound

We show that Bogoliubov excited scalar and tensor modes do not alleviate Planckian evolution during inflation if one assumes that $r$ and the Bogoliubov coefficients are approximately scale invariant. We constrain the excitation parameter for the scalar fluctuations, $β$, and tensor perturbations, $\tildeβ$, by requiring that there be at least three decades of scale invariance in the scalar and tensor power spectrum. For the scalar fluctuations this is motivated by the observed nearly scale invariant scalar power spectrum. For the tensor fluctuations this assumption may be shown to be valid or invalid by future experiments.

astro-ph.CO↗

arXiv2014

Power Spectrum and Non-Gaussianities in Anisotropic Inflation

We study the planar regime of curvature perturbations for single field inflationary models in an axially symmetric Bianchi I background. In a theory with standard scalar field action, the power spectrum for such modes has a pole as the planarity parameter goes to zero. We show that constraints from back reaction lead to a strong lower bound on the planarity parameter for high-momentum planar modes and use this bound to calculate the signal-to-noise ratio of the anisotropic power spectrum in the CMB, which in turn places an upper bound on the Hubble scale during inflation allowed in our model. We find that non-Gaussianities for these planar modes are enhanced for the flattened triangle and the squeezed triangle configurations, but show that the estimated values of the f_NL parameters remain well below the experimental bounds from the CMB for generic planar modes (other, more promising signatures are also discussed). For a standard action, f_NL from the squeezed configuration turns out to be larger compared to that from the flattened triangle configuration in the planar regime. However, in a theory with higher derivative operators, non-Gaussianities from the flattened triangle can become larger than the squeezed configuration in a certain limit of the planarity parameter.

hep-th↗

arXiv2014

Lower Bound for the Multi-Field Bounce Action

We present a lower bound for the multi-field bounce action with a quartic potential in the absence of gravity. We find that for a large number of fields the lower bound decreases with the number of fields as N^{-3}. This work clarifies previous statements made in numerical studies which found that the bounce action scales as N^{-2.66} and discusses some subtleties of studying field space trajectories.

hep-th↗