arXiv subjects

Yi-Fu Cai

Publications and source records attributed to Yi-Fu Cai.

At least 91 records · Page 5Linked to original sources

arXiv2020

Dirac-Born-Infeld realization of sound speed resonance mechanism for primordial black holes

We present a concrete realization of the sound speed resonance (SSR) mechanism for primordial black hole (PBH) formation within a specific model of Dirac-Born-Infeld (DBI) inflation. We perform a perturbative approach to phenomenologically construct such a viable DBI inflation model that involves the nonoscillating stage and the oscillating stage, with a type of specific forms of the warp factor and the potential. We show that the continuous but nonsmooth conjunction of sound speed between two stages does not yield manifest effects on the phenomenology of SSR, and thus, our model gives rise to the same PBH mass spectrum as the original predictions of SSR. Additionally, we also demonstrate that the violation of adiabaticity of the Mukhanov-Sasaki equation does not affect the comoving curvature perturbation after Hubble crossing in the nonresonant region. Making use of observational data, we derive various cosmological constraints on the parameter space. Our analyses show that the predicted tensor-to-scalar ratio is typically small, while the amplitude of primordial non-Gaussianity can meet with cosmic microwave background bounds, and additionally, the consistency relation for single-field slow-roll inflation is softly violated in our case due to the small sound speed variations.

astro-ph.CO↗

arXiv2020

NANOGrav results and Dark First Order Phase Transitions

The recent NANOGrav evidence of a common-source stochastic background provides a hint to Gravitational Waves (GW) radiation from the Early Universe. We show that this result can be interpreted as a GW spectrum produced from First Order Phase Transitions (FOPTs) around a temperature in the KeV-MeV window. Such a class of FOPTs at temperatures much below the electroweak scale can be naturally envisaged in several Warm Dark Matter models such as Majoron Dark Matter.

hep-ph↗

arXiv2020

Emergent Universe and Genesis from the DHOST Cosmology

In this article, we present an emergent universe scenario that can be derived from DHOST cosmology. The universe starts asymptotically Minkowski in the far past just like the regular Galileon Genesis, but evolves to a radiation dominated period at the late stage, and therefore, the universe has a graceful exit which is absent in the regular Galileon Genesis. We analyze the behavior of cosmological perturbations and show that both the scalar and tensor modes are free from the gradient instability problem. We further analyze the primordial scalar spectrum generated in various situations and discuss whether a scale invariance can be achieved.

gr-qc↗

arXiv2020

Experimental Constraint on an Exotic Parity-Odd Spin- and Velocity-Dependent Interaction with a Single Electron Spin Quantum Sensor

n improved laboratory bound on the exotic spin- and velocity-dependent interaction at micrometer scale is established with a single electron spin quantum sensor. The single electron spin of a near-surface nitrogen-vacancy center in diamond is utilized as the quantum sensor and a vibrating half-sphere lens is taken as the source of the moving nucleons. The exotic interaction between the polarized electron and the moving nucleon source is explored by measuring the possible magnetic field felt by the electron spin quantum sensor. Our experiment set improved constraints on the exotic spin- and velocity-dependent interaction within the force range from 1 to 330 $μ$m. The upper limit of the coupling $g_A^eg_V^N $ at $200 ~μm$ is $| g_A^e g_V^N| \leq 8.0\times10^{-19}$, significantly improving the current laboratory limit by more than four orders of magnitude.

quant-ph↗

arXiv2020

DHOST Bounce

We present a new class of nonsingular bounce cosmology free from instabilities, using a single scalar field coupled to gravity within the framework of the Degenerate Higher-Order Scalar-Tensor (DHOST) theories. In this type of scenarios, the gradient instability that widely exists in nonsingular bounce cosmologies in the framework of scalar-tensor and Horndeski/Galileon theories is removed by the effects of new operators introduced by the DHOST, due to the modification that they later bring about to the dispersion relation of perturbations. Hence, our results demonstrate that there is indeed a loophole for this type of bounce scenarios to be free from pathologies when primordial perturbations evolve through the bounce phase, and thus the theoretical {\it no-go} theorem for nonsingular bounce cosmology of Horndeski/Galileon theories can be delicately evaded in DHOST extensions.

gr-qc↗

arXiv2020

Interpreting cosmological tensions from the effective field theory of torsional gravity

Cosmological tensions can arise within $Λ$CDM scenario amongst different observational windows, which may indicate new physics beyond the standard paradigm if confirmed by measurements. In this article, we report how to alleviate both the $H_0$ and $σ_8$ tensions simultaneously within torsional gravity from the perspective of effective field theory (EFT). Following these observations, we construct concrete models of Lagrangians of torsional gravity. Specifically, we consider the parametrization $f(T)=-T-2Λ/M_P^2+αT^β$, where two out of the three parameters are independent. This model can efficiently fit observations solving the two tensions. To our knowledge, this is the first time where a modified gravity theory can alleviate both $H_0$ and $σ_8$ tensions simultaneously, hence, offering an additional argument in favor of gravitational modification.

astro-ph.CO↗

arXiv2020

Updated constraints on superconducting cosmic strings from the astronomy of fast radio bursts

In this article we update constraints on superconducting cosmic strings (SCSs) in light of the recent observational developments of fast radio bursts (FRBs) astronomy. Assuming strings follow an exponential distribution characterized by current, we show that two parameters in our context, which are the characteristic tension ($Gμ$) and a parameter which describes the aforementioned exponential distribution ($I_c$), can be constrained by FRB experiments. Particularly, we investigate data sets from Parkes and ASKAP. We looked through a parameter space where $Gμ\sim [10^{-17}, 10^{-12}]$ and $I_c\sim [10^{-1}, 10^{2}]$ GeV, and found our results show that Parkes jointly with ASKAP can constrain the parameter space for SCSs.

astro-ph.HE↗

arXiv2020

Testing the equivalence principle via the shadow of black holes

We study the equivalence principle, regarded as the cornerstone of general relativity, by analyzing the deformation observable of black hole shadows. Such deformation can arise from new physics and may be expressed as a phenomenological violation of the equivalence principle. Specifically, we assume that there is an additional background vector field that couples to the photons around the supermassive black hole. This type of coupling yields impact on the way the system depends on initial conditions, and affects the black hole shadow at different wavelengths by a different amount, and therefore observations of the shadow in different wavelengths could constrain such couplings. This can be tested by future multi-band observations. Adopting a specific form of the vector field, we obtain constraints on model parameters from Event Horizon Telescope observations and measurements of gas/stellar orbits.

astro-ph.CO↗

arXiv2020

Model-independent reconstruction of $f(T)$ gravity from Gaussian Processes

We apply Gaussian processes and Hubble function data in $f(T)$ cosmology, to reconstruct for the first time the $f(T)$ form in a model-independent way. In particular, using $H(z)$ datasets coming from cosmic chronometers as well as from the radial BAO method, alongside the latest released local value $H_{0} = 73.52 \pm 1.62$ km/s/Mpc, we reconstruct $H(z)$ and its derivatives, resulting eventually in a reconstructed region for $f(T)$, without any assumption. Although the cosmological constant lies in the central part of the reconstructed region, the obtained mean curve follows a quadratic function. Inspired by this we propose a new $f(T)$ parametrization, i.e. $f(T) = -2Λ+ξT^2$, with $ξ$ the sole free parameter that quantifies the deviation from $Λ$CDM cosmology. Additionally, we confront three viable one-parameter $f(T)$ models of the literature, which respectively are the power-law, the square-root exponential, and the exponential one, with the reconstructed $f(T)$ region, and then we extract significantly improved constraints for their model parameters, comparing to the constraints that arise from usual observational analysis. Finally, we argue that since we are using the direct Hubble measurements and the local value for $H_0$ in our analysis, with the above reconstruction of $f(T)$, the $H_0$ tension can be efficiently alleviated.

astro-ph.CO↗

arXiv2019

Implications of the possible 21-cm line excess at cosmic dawn on dynamics of interacting dark energy

In this paper we study implications of the possible excess of 21-cm line global signal at the epoch of cosmic dawn on the evolutions of a class of dynamically interacting dark energy (IDE) models. We firstly summarize two dynamical mechanisms in which different background evolutions can exert considerable effects on the 21-cm line global signal. One is the decoupling time of Compton scattering heating, the other stems from the direct change of optical depth due to the different expansion rate of the Universe. After that, we investigate the IDE models to illustrate the tension between the results of Experiment to Detect the Global Epoch of reionization Signature (EDGES) and other experiments. To apply the analyses of these two mechanisms to IDE models, we find that only the optical depth can be significantly changed. Accordingly, in order to relieve the tension by including the effects of the decoupling time of Compton scattering heating, we deduce a possible evolution form for the Hubble parameter within IDE that begins at an early stage around $z\sim 100$ and then smoothly evolves to a value at $z\sim 17$ which is smaller than that obtained in the standard paradigm. Eventually, we fulfill this scenario by adding an early dark energy dominated stage to the cosmological paradigm described by IDE models, which can alleviate the tension between EDGES and other cosmological observations but cannot completely solve it.

astro-ph.CO↗

arXiv2019

Primordial black holes from sound speed resonance in the inflaton-curvaton mixed scenario

We study sound speed resonance (SSR) mechanism for primordial black hole (PBH) formation in an early universe scenario with inflaton and curvaton being mixed. In this scenario, the total primordial density perturbations can be contributed by the fluctuations from both the inflaton and curvaton fields, in which the inflaton fluctuations lead to the standard adiabatic perturbations, while the sound speed of the curvaton fluctuations are assumed to be oscillating during inflation. Due to the narrow resonance effect of SSR mechanism, we acquire the enhanced primordial density perturbations on small scales and it remains nearly scale-invariant on large scales, which is essential for PBH formation. Finally, we find that the PBHs with specific mass spectrum can be produced with a sufficient abundance for dark matter in the mixed scenario.

astro-ph.CO↗

arXiv2019

When Primordial Black Holes from Sound Speed Resonance Meet a Stochastic Background of Gravitational Waves

As potential candidates of dark matter, primordial black holes (PBHs) are within the core scopes of various astronomical observations. In light of the explosive development of gravitational wave (GW) and radio astronomy, we thoroughly analyze a stochastic background of cosmological GWs, induced by over large primordial density perturbations, with several spikes that was inspired by the sound speed resonance effect and can predict a particular pattern on the mass spectrum of PBHs. With a specific mechanicsm for PBHs formation, we for the first time perform the study of such induced GWs that originate from both the inflationary era and the radiation-dominated phase. We report that, besides the traditional process of generating GWs during the radiation-dominated phase, the contribution of the induced GWs in the sub-Hubble regime during inflation can become significant at critical frequency band because of a narrow resonance effect. All contributions sum together to yield a specific profile of the energy spectrum of GWs that can be of observable interest in forthcoming astronomical experiments. Our study shed light on the possible joint probe of PBHs via various observational windows of multi-messenger astronomy, including the search for electromagnetic effects with astronomical telescopes and the stochastic background of relic GWs with GW instruments.

astro-ph.CO↗

arXiv2019

Evading the theoretical no-go theorem for nonsingular bounces in Horndeski/Galileon cosmology

We show that a nonsingular bounce, free of ghosts and gradient instabilities, can be realized in the framework of Horndeski or generalized Galileon cosmology. In particular, we first review that the theoretical no-go theorem, which states that the above is impossible, is based on two very strong assumptions, namely that a particular quantity cannot be discontinuous during the bounce, and that there is only one bounce. However, as we show in the present work, the first assumption not only can be violated in a general Horndeski/Galileon scenario, but also it is necessarily violated at the bounce point within the subclass of Horndeski/Galileon gravity in which $K(ϕ,X)$ becomes zero at $X=0$. Additionally, concerning the second assumption, which is crucial in improved versions of the theorem which claim that even if a nonlinear free of pathologies can be realized it will lead to pathologies in the infinite past or infinite future, we show that if needed it can be evaded by considering cyclic cosmology, with an infinite sequence of nonsingular bounces free of pathologies, which forbids the universe to reach the "problematic" regime at infinite past or infinite future. Finally, in order to make the analysis more transparent we provide explicit examples where nonsingular bounces without theoretical pathologies can be achieved.

gr-qc↗

arXiv2019

Testing the scale-dependent hemispherical asymmetry with the 21-cm power spectrum from the epoch of reionization

Hemispherical power asymmetry has emerged as a new challenge to cosmology in early universe. While the cosmic microwave background (CMB) measurements indicated the asymmetry amplitude $A \simeq 0.07$ at the CMB scale $k_{\rm CMB}\simeq 0.0045\,{\rm Mpc}^{-1}$, the high-redshift quasar observations found no significant deviation from statistical isotropy. This conflict can be reconciled in some scale-dependent asymmetry models. We put forward a new parameterization of scale-dependent asymmetric power spectrum, inspired by a multi-speed inflation model. The 21-cm power spectrum from the epoch of reionization can be used to constrain the scale-dependent hemispherical asymmetry. We demonstrate that an optimum, multi-frequency observation by the Square Kilometre Array (SKA) Phase 2 can impose a constraint on the amplitude of the power asymmetry anomaly at the level of $ΔA \simeq 0.2$ at $0.056 \lesssim k_{\rm 21cm} \lesssim 0.15 \,{\rm Mpc}^{-1}$. This limit may be further improved by an order of magnitude as $ΔA \simeq 0.01$ with a cosmic variance limited experiment such as the Omniscope.

astro-ph.CO↗

arXiv2019

Searching for an Exotic Spin-Dependent Interaction between Electrons at the Nanometer Scale with Molecular Rulers

We propose that a type of molecular rulers, which contains two electron spins with precisely adjustable distance by varying the length of the shape-persistent polymer chains, can be utilized to constrain the axial-vector mediated interaction between electron spins at the nanometer scale. With measurements of the coupling strengths between two electron spins within different molecular rulers, an improved laboratory bound of exotic dipole-dipole interaction between electrons is established within the force range from 3 to 220 nm. The upper bound of the coupling $g_A^e g_A^e/4π\hbar c $ at 200 nm is $|g_A^e g_A^e/4π\hbar c| \leq 4.9\times10^{-13}$, which is about 20 times more stringent than previous limits.

physics.atom-ph↗

arXiv2019

Two-field cosmological phase transitions and gravitational waves in the singlet Majoron model

In the singlet Majoron model, we study cosmological phase transitions (PTs) and their resulting gravitational waves (GWs), in the two-field phase space, without freezing any of the field directions. We first calculate the effective potential, at one loop and at finite temperature, of the Standard Model Higgs doublet together with one extra Higgs singlet. We make use of the public available Python package CosmoTransitions to simulate the two-dimensional cosmological PTs and evaluate the gravitational waves generated by first-order PTs. With the full 2D simulation, we are able not only to confirm the PTs' properties previously discussed in the literature, but also we find new patterns, such as strong first-order PTs tunneling from a vacuum located on one axis to another vacuum located on the second axis. The two-field phase space analysis presents a richer panel of cosmological PT patterns compared to analysis with a single-field approximation. The PTGW amplitudes turn out to be out of the reach for the space-borne gravitational wave interferometers such as LISA, DECIGO, BBO, TAIJI and TianQin when constraints from colliders physics are taken into account.

hep-ph↗

arXiv2018

Numerical study of inflationary preheating with arbitrary power-law potential and a realization of curvaton mechanism

During inflationary preheating, the energy stored in the inflaton field is rapidly converted into excitations of other entropy fields. This stage is characterized by exponential particle production due to parametric resonance and is notoriously difficult to analyze using analytic methods. We develop a detailed numerical simulation to investigate inflationary preheating when the potential of the inflaton is a power-law function with arbitrary power index. To achieve a successful graceful exit from a primordial inflationary phase to a smooth, oscillatory phase during preheating, we assume the inflaton potential reduces to a quadratic function in the infrared regime, which may be regarded as a mass term at low energy scales. With this simplification, our numerical method may be applied to unconventional chaotic inflation models. To demonstrate its validity, we numerically analyze the preheating stage of axion-monodromy inflation which is inspired by string theory. By performing perturbation analyses, our result shows that the power spectrum of primordial curvature perturbation can be dominated by fluctuations of entropy field rather than those of inflaton, which can be regarded as a particular realization of the curvaton mechanism through a preheating process.

astro-ph.CO↗

arXiv2018

Searching for the Dark Force with 21-cm Spectrum in Light of EDGES

The EDGES Collaboration has recently announced the detection of the 21-cm spectrum with an absorption profile centred at $78$ megahertz, of which the depth is deeper than that expected by the standard cosmological paradigm. To enrich the heating process of baryons due to scattering with dark matter during dark ages, we in this Letter explore the possibility of extra heat transfer between dark sector compositions and their observational signatures on the 21-cm cosmological spectrum. By parameterizing interaction models of the dark Universe, we find that the observational constraint on the parameter space of dark matter can be slightly relaxed but the discrepancy with the commonly predicted parameter space of weakly interacting massive particles remains. Our analyses also reveal that the interaction between dark compositions may leave observational signatures on the 21-cm spectrum during dark ages and thus would become detectable in the forthcoming 21-cm cosmology.

astro-ph.CO↗