arXiv subjects

Zhiqi Huang

Publications and source records attributed to Zhiqi Huang.

106 records · Page 6Linked to original sources

arXiv2016

Observational effects of a running Planck mass

We consider observational effects of a running effective Planck mass in the scalar-tensor gravity theory. At the background level, an increasing effective Planck mass allows a larger Hubble constant $H_0$, which is more compatible with the local direct measurements. At the perturbative level, for cosmic microwave background (CMB) anisotropies, an increasing effective Planck mass ($i$) suppresses the unlensed CMB power at $\ell \lesssim 30$ via the integrated Sachs-Wolfe effect and ($ii$) enhances CMB lensing power. Both effects slightly relax the tension between the current CMB data from the Planck satellite and the standard $Λ$CDM model predictions. However, those impacts on the CMB secondary anisotropies are subdominant, and the overall constraints are driven by the background measurements. Combining CMB data from the Planck satellite and an $H_0$ prior from Riess $et$ $al$, we find a $\sim 2σ$ hint of a positive running of effective Planck mass. However, the hint goes away when we add other low-redshift observational data including type Ia supernovae, baryon acoustic oscillations and an Universe age estimation using the oldest stars.

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arXiv2015

Revisiting the cosmological bias due to local gravitational redshifts

A recent article by Wojtak {\it et al} (arXiv:1504.00718) pointed out that the local gravitational redshift, despite its smallness ($\sim 10^{-5}$), can have a noticeable ($\sim 1\%$) systematic effect on our cosmological parameter measurements. The authors studied a few extended cosmological models (nonflat $Λ$CDM, $w$CDM, and $w_0$-$w_a$CDM) with a mock supernova data set. We repeat this calculation and find that the $\sim 1\%$ biases are due to strong degeneracy between cosmological parameters. When cosmic microwave background (CMB) data are added to break the degeneracy, the biases due to local gravitational redshift are negligible ($\lesssim 0.1 σ$).

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arXiv2014

Testing Inflation with Large Scale Structure: Connecting Hopes with Reality

The statistics of primordial curvature fluctuations are our window into the period of inflation, where these fluctuations were generated. To date, the cosmic microwave background has been the dominant source of information about these perturbations. Large scale structure is however from where drastic improvements should originate. In this paper, we explain the theoretical motivations for pursuing such measurements and the challenges that lie ahead. In particular, we discuss and identify theoretical targets regarding the measurement of primordial non-Gaussianity. We argue that when quantified in terms of the local (equilateral) template amplitude $f_{\rm NL}^{\rm loc}$ ($f_{\rm NL}^{\rm eq}$), natural target levels of sensitivity are $Δf_{\rm NL}^{\rm loc, eq.} \simeq 1$. We highlight that such levels are within reach of future surveys by measuring 2-, 3- and 4-point statistics of the galaxy spatial distribution. This paper summarizes a workshop held at CITA (University of Toronto) on October 23-24, 2014.

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arXiv2014

Chameleons in the Early Universe: Kicks, Rebounds, and Particle Production

Chameleon gravity is a scalar-tensor theory that includes a non-minimal coupling between the scalar field and the matter fields and yet mimics general relativity in the Solar System. The scalar degree of freedom is hidden in high-density environments because the effective mass of the chameleon scalar depends on the trace of the stress-energy tensor. In the early Universe, when the trace of the matter stress-energy tensor is nearly zero, the chameleon is very light, and Hubble friction prevents it from reaching the minimum of its effective potential. Whenever a particle species becomes non-relativistic, however, the trace of the stress-energy tensor is temporarily nonzero, and the chameleon begins to roll. We show that these "kicks" to the chameleon field have catastrophic consequences for chameleon gravity. The velocity imparted to the chameleon by the kick is sufficiently large that the chameleon's mass changes rapidly as it slides past its potential minimum. This nonadiabatic evolution shatters the chameleon field by generating extremely high-energy perturbations through quantum particle production. If the chameleon's coupling to matter is slightly stronger than gravitational, the excited modes have trans-Planckian momenta. The production of modes with momenta exceeding 1e7 GeV can only be avoided for small couplings and finely tuned initial conditions. These quantum effects also significantly alter the background evolution of the chameleon field, and we develop new analytic and numerical techniques to treat quantum particle production in the regime of strong dissipation. This analysis demonstrates that chameleon gravity cannot be treated as a classical field theory at the time of Big Bang Nucleosynthesis and casts doubt on chameleon gravity's viability as an alternative to general relativity.

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arXiv2013

The full CMB temperature bispectrum from single-field inflation

We compute the full cosmic microwave background temperature bispectrum generated by nonlinearities after single-field inflation. By integrating the photon temperature at second order along a perturbed geodesic in Newtonian gauge, we derive an expression for the observed temperature fluctuations that, for the first time, clarifies the separation of the gravitational lensing and time-delay effects from the purely second-order contributions. We then use the second-order Boltzmann code CosmoLib$2^{\rm nd}$ to calculate these contributions and their bispectrum. Including the perturbations in the photon path, the numerically computed bispectrum exactly matches the expected squeezed limit. Moreover, the analytic squeezed-limit formula reproduces well the signal-to-noise and shape of the full bispectrum, potentially facilitating the subtraction of the bias induced by second-order effects. For a cosmic-variance limited experiment with $l_{\rm max} = 2000$, the bias on a local signal is $f_{\rm NL}^{\rm loc} =0.73$ negligible for equilateral and orthogonal signals. The signal-to-noise ratio is unity at $l_{\rm max} \sim 3000$, suggesting that second-order effects may hopefully be measured in the future.

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arXiv2013

Catastrophic Consequences of Kicking the Chameleon

The physics of the "dark energy" that drives the current cosmological acceleration remains mysterious, and the dark sector may involve new light dynamical fields. If these light scalars couple to matter, a screening mechanism must prevent them from mediating an unacceptably strong fifth force locally. Here we consider a concrete example: the chameleon mechanism. We show that the same coupling between the chameleon field and matter employed by the screening mechanism also has catastrophic consequences for the chameleon during the Universe's first minutes. The chameleon couples to the trace of the stress-energy tensor, which is temporarily non-zero in a radiation-dominated universe whenever a particle species becomes non-relativistic. These "kicks" impart a significant velocity to the chameleon field, causing its effective mass to vary non-adiabatically and resulting in the copious production of quantum fluctuations. Dissipative effects strongly modify the background evolution of the chameleon field, invalidating all previous classical treatments of chameleon cosmology. Moreover, the resulting fluctuations have extremely high characteristic energies, which casts serious doubt on the validity of the effective theory. Our results demonstrate that quantum particle production can profoundly affect scalar-tensor gravity, a possibility not previously considered. Working in this new context, we also develop the theory and numerics of particle production in the regime of strong dissipation.

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arXiv2012

The CMB bispectrum from recombination

We compute the cosmic microwave background temperature bispectrum generated by nonlinearities at recombination on all scales. We use CosmoLib$2^{\rm nd}$, a numerical Boltzmann code at second-order to compute CMB bispectra on the full sky. We consistently include all effects except gravitational lensing, which can be added to our result using standard methods. The bispectrum is peaked on squeezed triangles and agrees with the analytic approximation in the squeezed limit at the few per cent level for all the scales where this is applicable. On smaller scales, we recover previous results on perturbed recombination. For cosmic-variance limited data to $l_{\rm max} =2000$, its signal-to-noise is $S/N=0.47$ and will bias a local signal by $f_{\rm NL}^{\rm loc}\simeq 0.82$.

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arXiv2012

A Cosmology Forecast Toolkit -- CosmoLib

The package CosmoLib is a combination of a cosmological Boltzmann code and a simulation toolkit to forecast the constraints on cosmological parameters from future observations. In this paper we describe the released linear-order part of the package. We discuss the stability and performance of the Boltzmann code. This is written in Newtonian gauge and including dark energy perturbations. In CosmoLib the integrator that computes the CMB angular power spectrum is optimized for a $\ell$-by-$\ell$ brute-force integration, which is useful for studying inflationary models predicting sharp features in the primordial power spectrum of metric fluctuations. The numerical code and its documentation are available at http://www.cita.utoronto.ca/~zqhuang/CosmoLib.

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arXiv2012

Constraining inflation with future galaxy redshift surveys

With future galaxy surveys, a huge number of Fourier modes of the distribution of the large scale structures in the Universe will become available. These modes are complementary to those of the CMB and can be used to set constraints on models of the early universe, such as inflation. Using a MCMC analysis, we compare the power of the CMB with that of the combination of CMB and galaxy survey data, to constrain the power spectrum of primordial fluctuations generated during inflation. We base our analysis on the Planck satellite and a spectroscopic redshift survey with configuration parameters close to those of the Euclid mission as examples. We first consider models of slow-roll inflation, and show that the inclusion of large scale structure data improves the constraints by nearly halving the error bars on the scalar spectral index and its running. If we attempt to reconstruct the inflationary single-field potential, a similar conclusion can be reached on the parameters characterizing the potential. We then study models with features in the power spectrum. In particular, we consider ringing features produced by a break in the potential and oscillations such as in axion monodromy. Adding large scale structures improves the constraints on features by more than a factor of two. In axion monodromy we show that there are oscillations with small amplitude and frequency in momentum space that are undetected by CMB alone but can be measured by including galaxy surveys in the analysis.

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arXiv2011

The Art of Lattice and Gravity Waves from Preheating

The nonlinear dynamics of preheating after early-Universe inflation is often studied with lattice simulations. In this work I present a new lattice code HLATTICE. It differs from previous public available codes in the following three aspects: (i) A much higher accuracy is achieved with a modified sixth-order symplectic integrator; (ii) scalar, vector, and tensor metric perturbations in synchronous gauge and their feedback to the dynamics of scalar fields are all included; (iii) the code uses a projector that completely removes the scalar and vector components defined by the discrete spatial derivatives. Such a generic code can have wide range of applications. As an example, gravity waves from preheating after inflation are calculated with a better accuracy.

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arXiv2010

Parameterizing and Measuring Dark Energy Trajectories from Late-Inflatons

Bulk dark energy properties are determined by the redshift evolution of its pressure-to-density ratio, $w_{de}(z)$. An experimental goal is to decide if the dark energy is dynamical, as in the quintessence (and phantom) models treated here. We show that a three-parameter approximation $w_{de}(z; ε_s, ε_{ϕ\infty}, ζ_s)$ fits well the ensemble of trajectories for a wide class of late-inflaton potentials $V(ϕ)$. Markov Chain Monte Carlo probability calculations are used to confront our $w_{de}(z)$ trajectories with current observational information on Type Ia supernova, Cosmic Microwave Background, galaxy power spectra, weak lensing and the Lyman-$α$ forest. We find the best constrained parameter is a low redshift slope parameter, $ε_s \propto (\partial \ln V / \partial ϕ)^2$ when the dark energy and matter have equal energy densities. A tracking parameter $ε_{ϕ\infty}$ defining the high-redshift attractor of $1+w_{de}$ is marginally constrained. Poorly determined is $ζ_s$, characterizing the evolution of $ε_s$, and a measure of $\partial^2 \ln V / \partial ϕ^2$ . The constraints we find already rule out some popular quintessence and phantom models, or restrict their potential parameters. We also forecast how the next generation of cosmological observations improve the constraints: by a factor of about five on $ε_s$ and $ε_{ϕ\infty}$, but with $ζ_s$ remaining unconstrained (unless the true model significantly deviates from $Λ$CDM). Thus potential reconstruction beyond an overall height and a gradient is not feasible for the large space of late-inflaton models considered here.

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arXiv2009

Preheating After Modular Inflation

We study (p)reheating in modular (closed string) inflationary scenarios, with a special emphasis on Kahler moduli/Roulette models. It is usually assumed that reheating in such models occurs through perturbative decays. However, we find that there are very strong non-perturbative preheating decay channels related to the particular shape of the inflaton potential (which is highly nonlinear and has a very steep minimum). Preheating after modular inflation, proceeding through a combination of tachyonic instability and broad-band parametric resonance, is perhaps the most violent example of preheating after inflation known in the literature. Further, we consider the subsequent transfer of energy to the standard model sector in scenarios where the standard model particles are confined to a D7-brane wrapping the inflationary blow-up cycle of the compactification manifold or, more interestingly, a non-inflationary blow up cycle. We explicitly identify the decay channels of the inflaton in these two scenarios. We also consider the case where the inflationary cycle shrinks to the string scale at the end of inflation; here a field theoretical treatment of reheating is insufficient and one must turn instead to a stringy description. We estimate the decay rate of the inflaton and the reheat temperature for various scenarios.

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arXiv2009

Particle Production During Inflation: Observational Constraints and Signatures

In a variety of inflation models the motion of the inflaton may trigger the production of some non-inflaton particles during inflation, for example via parametric resonance or a phase transition. Particle production during inflation leads to observables in the cosmological fluctuations, such as features in the primordial power spectrum and also nongaussianities. Here we focus on a prototype scenario with inflaton, ϕ, and iso-inflaton, χ, fields interacting during inflation via the coupling g^2 (ϕ-ϕ_0)^2χ^2. Since several previous investigations have hinted at the presence of localized "glitches" in the observed primordial power spectrum, which are inconsistent with the simplest power-law model, it is interesting to determine the extent to which such anomalies can be explained by this simple and well-motivated model. Our prototype scenario predicts a bump-like feature in the primordial power spectrum, rather than an oscillatory "ringing" pattern as has previously been assumed. We discuss the observational constraints on such features. We find that bumps with amplitude as large as 10% of the usual scale invariant fluctuations from inflation are allowed on scales relevant for Cosmic Microwave Background experiments. Our results imply an upper limit on the coupling g^2 which is crucial for assessing the detectability of the nongaussianity produced by inflationary particle production. We also discuss more complicated features that result from superposing multiple instances of particle production. Finally, we point to a number of microscopic realizations of this scenario in string theory and supersymmetry and discuss the implications of our constraints for the popular brane/axion monodromy inflation models.

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arXiv2009

Cosmological Fluctuations from Infra-Red Cascading During Inflation

We propose a qualitatively new mechanism for generating cosmological fluctuations from inflation. The non-equilibrium excitation of interacting scalar fields often evolves into infra-red (IR) and ultra-violet (UV) cascading, resulting in an intermediate scaling regime. We observe elements of this phenomenon in a simple model with inflaton ϕand iso-inflaton χfields interacting during inflation via the coupling g^2 (ϕ-ϕ_0)^2 χ^2. Iso-inflaton particles are created during inflation when they become instantaneously massless at ϕ=ϕ_0, with occupation numbers not exceeding unity. We point out that very quickly the produced χparticles become heavy and their multiple re-scatterings off the homogeneous condensate ϕ(t) generates bremschtrahlung radiation of light inflaton IR fluctuations with high occupation numbers. The subsequent evolution of these IR fluctuations is qualitatively similar to that of the usual inflationary fluctuations, but their initial amplitude is different. The IR cascading generates a bump-shaped contribution to the cosmological curvature fluctuations, which can even dominate over the usual fluctuations for g^2>0.06. The IR cascading curvature fluctuations are significantly non-gaussian and the strength and location of the bump are model-dependent, through g^2 and ϕ_0. The effect from IR cascading fluctuations is significantly larger than that from the momentary slowing-down of ϕ(t). With a sequence of such bursts of particle production, the superposition of the bumps can lead to a new broad band non-gaussian component of cosmological fluctuations added to the usual fluctuations. Such a sequence of particle creation events can, but need not, lead to trapped inflation.

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arXiv2009

Non-Gaussian Spikes from Chaotic Billiards in Inflation Preheating

A new class of non-Gaussian curvature fluctuations ζ_{pr} (\bx) \equiv δN(χ_i) arises from the post-inflation preheating behaviour of a non-inflaton field χ_i. Its billiard-like chaotic dynamics imprints regular log-spaced narrow spikes in the number of preheating e-folds N(χ_i). We perform highly accurate lattice simulations of SUSY-inspired quartic inflaton and coupling potentials in a separate-universe approximation to compute N(χ_i) as a function of the (nearly homogeneous) initial condition χ_i. The super-horizon modes of χ_i(\bx) result in positive spiky excursions in ζ_{pr} and hence negative gravitational potential fluctuations added to the usual sign-independent inflaton-induced perturbations, observably manifested in large cosmic structures and as (polarized) temperature CMB cold spots.

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arXiv2009

Cosmological Constraints on Decaying Dark Matter

We present a complete analysis of the cosmological constraints on decaying dark matter. Previous analyses have used the cosmic microwave background and Type Ia supernova. We have updated them with the latest data as well as extended the analysis with the inclusion of Lyman-$α$ forest, large scale structure and weak lensing observations. Astrophysical constraints are not considered in the present paper. The bounds on the lifetime of decaying dark matter are dominated by either the late-time integrated Sachs-Wolfe effect for the scenario with weak reionization, or CMB polarization observations when there is significant reionization. For the respective scenarios, the lifetimes for decaying dark matter are $Γ^{-1} \gtrsim 100$ Gyr and $ (f Γ) ^{-1} \gtrsim 5.3 \times 10^8$ Gyr (at 95.4% confidence level), where the phenomenological parameter $f$ is the fraction of the decay energy deposited in baryonic gas. This allows us to constrain particle physics models with dark matter candidates through investigation of dark matter decays into Standard Model particles via effective operators. For decaying dark matter of $\sim 100$ GeV mass, we found that the size of the coupling constant in the effective dimension-4 operators responsible for dark matter decay has to generically be $ \lesssim 10^{-22}$. We have also explored the implications of our analysis for representative models in theories of gauge-mediated supersymmetry breaking, minimal supergravity and little Higgs.

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