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Shi Pi

Publications and source records attributed to Shi Pi.

52 records · Page 3Linked to original sources
arXiv2018

Quintessence Saves Higgs Instability

We study a model where quintessence potential $e^{-ξϕ}$ coupled to Higgs potential. We calculate the evolution of the quintessence, and track the running of the effective Higgs self-coupling. We find it slightly larger than that of the standard model in the past. Requiring the electroweak vacuum to be absolutely stable in inflationary era, we find a lower bound $ξ> 0.35\pm 0.05$, where the uncertainty is mainly from the measurement of the top quark mass. This lower bound, together with the upper bound from the observation for dark energy $ξ\lesssim0.6$, narrows down the parameter space and makes it possible to test this model in the near future. Interestingly, the bound on $ξ$, if actually shown to be the case by observation, supports the recently proposed Swampland Conjecture.

hep-ph↗
arXiv2018

Scalaron from $R^2$-gravity as a Heavy Field

We study a model of inflation in which a scalar field $χ$ is non-minimally coupled to Starobinsky's $R^2$ gravity. After transforming it to the Einstein frame, a new scalar field, the scalaron $ϕ$, will appear and couple to $χ$ with a nontrivial field metric, while $χ$ acquires a positive mass via the non-minimal coupling. Initially inflation occurs along the $ϕ$ direction with $χ$ trapped near its origin by this induced mass. After $ϕ$ crosses a critical value, it starts rolling down rapidly and proceeds todamped oscillations around an effective local minimum determined by the value of $χ$, while inflation still continues, driven by the $χ$ field at this second stage where the effect of the non-minimal coupling becomes negligible. The presence of the damped oscillations during the transition from the first to second stage of inflation causes enhancement and oscillation features in the power spectrum of the curvature perturbation. Assuming that the oscillations may be treated perturbatively, we calculate these features by using the $δN$ formalism, and discuss its observational implications to large scale CMB anomalies or primordial black hole formation, depending on the scale of the features.

astro-ph.CO↗
arXiv2017

Strongly Coupled Quasi-Single Field Inflation

We study the power spectrum of quasi-single field inflation where strong coupling is considered. The contribution from the massive propagator can be divided into local and non-local contributions. The local one is the leading contribution and is power-law suppressed as a function of mass, while the non-local contribution is exponentially suppressed in the large mass limit. For the local contribution, it is possible to use the effective field theory approach to study the power spectrum in the strongly coupled region of the parameter space. For the non-local contribution, we developed a partial effective field theory method to simplify the calculation: When there are multiple massive propagators, one can fully compute it after integrating out all but one massive propagator by effective field theory. The result retains the "standard clock" signal, which is interesting for probing the expansion history of the primordial universe and the physics of a "cosmological collider". The error involved compared to the full calculation is power law suppressed by the effective mass of the heavy field.

hep-th↗
arXiv2017

Oscillating scalar fields in extended quintessence

We study a rapidly-oscillating scalar field with potential $V(ϕ) = k|ϕ|^n$ nonminally coupled to the Ricci scalar $R$ via a term of the form $(1- 8 πG_0 ξϕ^2) R$ in the action. In the weak coupling limit, we calculate the effect of the nonminimal coupling on the time-averaged equation of state parameter $γ= (p + ρ)/ρ$. The change in $\langle γ\rangle$ is always negative for $n \ge 2$ and always positive for $n < 0.71$ (which includes the case where the oscillating scalar field could serve as dark energy), while it can be either positive or negative for intermediate values of $n$. Constraints on the time-variation of $G$ force this change to be infinitesimally small at the present time whenever the scalar field dominates the expansion, but constraints in the early universe are not as stringent. The rapid oscillation induced in $G$ also produces an additional contribution to the Friedman equation that behaves like an effective energy density with a stiff equation of state, but we show that, under reasonable assumptions, this effective energy density is always smaller than the density of the scalar field itself.

gr-qc↗
arXiv2015

Trail of the Higgs in the primordial spectrum

We study the effects of the Higgs directly coupled to the inflaton on the primordial power spectrum. The quadratic coupling between the Higgs and the inflaton stabilizes the Higgs in the electroweak vacuum during inflation by inducing a large effective mass for the Higgs, which also leads to oscillatory features in the primordial power spectrum due to the oscillating classical background. Meanwhile, the features from quantum fluctuations exhibit simple monotonic k-dependence and are subleading compared to the classical contributions. We also comment on the collider searches.

hep-ph↗
arXiv2015

On the possibility of blue tensor spectrum within single field inflation

We present a series of theoretical constraints on the potentially viable inflation models that might yield a blue spectrum for primordial tensor perturbations. By performing a detailed dynamical analysis we show that, while there exists such possibility, the corresponding phase space is strongly bounded. Our result implies that, in order to achieve a blue tilt for inflationary tensor perturbations, one may either construct a non-canonical inflation model delicately, or study the generation of primordial tensor modes beyond the standard scenario of single slow-roll field.

hep-th↗
arXiv2015

Probing reheating with primordial spectrum

We study the impacts of reheating temperature on the inflationary predictions of the spectral index and tensor-to-scalar ratio. Assuming sinusoidal oscillations and that reheating process is very fast, the reheating temperature can be constrained for sinusoidal oscillation within a factor of 10 - 100 or even better with the prospect of future observations. Beyond this, we find that the predictions can also be insensitive to the reheating temperature in certain models, including Higgs inflation.

hep-ph↗
arXiv2014

Inflation beyond T-models and primordial B-modes

We describe extended theories which shares the gauge transformation symmetry of the T-models, and takes the T-models as well as Starobinsky model as special cases. We derive a general relation between the two slow-roll parameters, and find that a large class of models can be embedded. Such models include more general Starobinsky-like inflation as well as the chaotic inflation with a large tensor-to-scalar ratio consistent with the BICEP2 result.

hep-th↗
arXiv2013

Equilateral non-Gaussianity from heavy fields

The effect of self-interactions of heavy scalar fields during inflation on the primordial non-Gaussianity is studied. We take a specific constant-turn quasi-single field inflation as an example. We derive an effective theory with emphasis on non-linear self-interactions of heavy fields and calculate the corresponding non-Gaussianity, which is of equilateral type and can be as relevant as those computed previously in the literature. We also derive the non-Gaussianity by directly using the in-in formalism, and verify the equivalence of these two approaches.

hep-th↗
arXiv2012

Curvature Perturbation Spectrum in Two-field Inflation with a Turning Trajectory

We revisit a two-component inflaton model with a turning trajectory in the field space, where the field slowly rolls down along the trajectory. We consider the case when the effective mass in the direction perpendicular to the trajectory, namely the isocurvature direction, is either of the same order as or much larger than the Hubble parameter. Assuming that the turning angular velocity is small, we compute analytically the corrections to the power spectrum of curvature perturbation caused by the mediation of the heavy isocurvature perturbation, and compare our analytic results with the numerical ones. Especially, when M_{eff}^2>>H^2, we find that it is proportional to M_{eff}^{-2}. This result is consistent with the one obtained previously by an effective field theory approach.

hep-th↗
arXiv2011

Dynamics of Cosmological Perturbations in Multi-Speed Inflation

We study a multi-field inflationary theory with separable Lagrangian, which has different speed of sound for each field. We find that the fields always coupled at perturbative level through gravitational interaction. We show that if the coupling terms among the perturbation fields are weak enough, these fields can be treated as a combination of decoupled fields, which are similar to normal modes in coupled oscillation. By virtue of such fields, the curvature perturbation at the horizon-crossing can be calculated up to the leading order of slow variation parameters via $δ\mathcal{N}$ formalism. Explicitly, we consider a model of multi-speed DBI inflation, and calculate the power spectrum in detail. The result depends on the ratio of different speeds of sound, and shows an apparent amplification when the ratio deviates from unity.

hep-th↗
arXiv2009

Power spectra of scalar and tensor modes in modified Horava-Lifshitz gravity

In Horava-Lifshitz gravity, the extra dynamical scalar mode could play significant role in cosmology. However it has been pointed out that such a scalar may suffer from the strong coupling problem in IR. We address this issue in this paper. Our analysis shows that the scalar mode could decouple naturally at λ=1 due to the extra gauge symmetry. On the other hand, the fact that the scalar mode becomes ghost when 1/3<λ<1 is a real challenge to the theory. We try to overcome this problem by modifying the action such that the RG flow lies outside the problematic region. We discuss the cosmological implications of the action and calculate the power spectra of scalar and tensor modes.

hep-th↗
arXiv2009

1/R Correction to Gravity in the Early Universe

To explain the accelerated expansion of late universe, the 1/R correction to Einstein gravity is usually considered, where R is the Ricci scalar. This correction term is generally believed to be negligible in the early universe. However, if the 1/R term is inflaton-dependent, it will dramatically change the story of inflation. The entropy perturbation will naturally appear and drive the evolution of curvature perturbation outside the Hubble horizon. In a large class of models, the entropy perturbation can be made nearly scale-invariant. In Einstein gravity the single-field inflation with a quartic potential has been ruled out by recent observations, but it revives when the 1/R term is turned on. The evolution of non-Gaussianities on large scale are also studied and applied to inflation with 1/R correction. In some specific models, a large non-Gaussianity can be naturally generated outside the horizon.

astro-ph.CO↗
arXiv2009

Scale Invariant Power Spectrum in Hořava-Lifshitz Cosmology without Matter

In this paper we investigate the physical implications of the dynamical scalar mode in pure Horava-Lifshitz gravity on cosmology. We find that it can produce a scale-invariant power spectrum in UV era if the detailed balance condition on the action is relaxed. This indicates that the physical scalar mode may seed the large scale structure and Horava-Lifshitz cosmology could be a qualified alternative to inflationary scenario.

hep-th↗