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Chen, Pisin

Publications and source records attributed to Chen, Pisin.

26 records · Page 2
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DOE OSTI2019

Suppression of the long-wavelength CMB spectrum from the Hartle–Hawking wave function in the Starobinsky-type inflation model

The lack of correlations on the large scale cosmic microwave background (CMB) anisotropy provides a potential window to probe beyond the standard inflationary scenario. Herein, we investigate the primordial power spectrum based on the Hartle–Hawking (HH) no-boundary proposal for a homogeneous, isotropic, and spatially-closed universe that leads to Starobinsky-type inflation after the classicalization. While we found that there is no suppression at large scales in the standard $R+R^2$ theory, we also found that it is possible to sufficiently suppress the large-scale power spectrum if a pre-inflationary stage is introduced to the Starobinsky-type model. In addition, such a pre-inflationary stage will be helpful to explain the preference of large e-foldings of the $HH$ proposal. We calculate the $C^{TT}_{ℓ}$ correlation function and show that our proposal gives a better fit to the Planck CMB data. This suggests that our universe might have begun with a compact $HH$ state with a small positive curvature.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Separability of the Klein-Gordon equation for rotating spacetimes obtained from Newman-Janis algorithm

In the literature, the Newman-Janis algorithm (NJA) has been widely used to construct stationary and axisymmetric spacetimes to describe rotating black holes. In addition, it has been recently shown that the general stationary and axisymmetric spacetime generated through NJA allows the complete separability of the null geodesic equations. In fact, the Hamilton-Jacobi equation in this spacetime is also separable if one of the metric functions is additively separable. In this work, we further study the conditions for a separable Klein-Gordon equation in such a general spacetime. Here, the relations between the NJA spacetime and other parametrized axially symmetric spacetimes in the literature are also discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Probing homogeneity with standard candles

We show that standard candles can provide some valuable information about the density contrast, which could be particularly important at redshifts where other observations are not available. We use an inversion method to reconstruct the local radial density profile from luminosity distance observations assuming background cosmological parameters obtained from large scale observations. Using type Ia Supernovae, Cepheids and the cosmological parameters from the Planck mission we reconstruct the radial density profiles along two different directions of the sky. We compare these profiles to other density maps obtained from luminosity density, in particular Keenan et al. 2013 and the 2M++ galaxy catalogue. The method independently confirms the existence of inhomogeneities, could be particularly useful to correctly normalize density maps from galaxy surveys with respect to the average density of the Universe, and could clarify the apparent discrepancy between local and large scale estimations of the Hubble constant. When better observational supernovae data will be available, the accuracy of the reconstructed density profiles will improve and will allow to further investigate the existence of structures whose size is beyond the reach of galaxy surveys.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Dark energy induced anisotropy in cosmic expansion

In order to understand the nature of the accelerating expansion of the late-time universe, it is important to experimentally determine whether dark energy is a cosmological constant or dynamical in nature. If dark energy already exists prior to inflation, which is a reasonable assumption, then one expects that a dynamical dark energy would leave some footprint in the anisotropy of the late-time accelerated expansion. To demonstrate the viability of this notion, we invoke the quintessence field with the exponential potential as one of the simplest dynamical dark energy models allowed by observations. We investigate the effects of its quantum fluctuations (the physical origin of the perturbation being isocurvature) generated during inflation and having fully positive correlation with the primordial curvature perturbations, and estimate the anisotropy of the cosmic expansion so induced. We show that the primordial amplitude of quantum fluctuations of quintessence field δΦ P can be related to the tensor-to-scalar ratio r, and we calculate the perturbed luminosity distance to first order and the associated luminosity distance power spectrum, which is an estimator of anisotropy of late-time accelerated expansion. We find that the gravitational potential at large scales and late times is less decayed in QCDM compared to that in ΛCDM so that the smaller the redshift and multipole, the more relative deficit of power in QCDM. In conclusion, our results of luminosity distance power spectrum also show the similar conclusions of suppression as that of the previous investigation regarding the effect of quantum fluctuations of quintessence field on the CMB temperature anisotropies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Hawking radiation as instantons

There have been various interpretations of Hawking radiation proposed based on the perturbative approach, and all have confirmed Hawking’s original finding. One major conceptual challenge of Hawking evaporation is the associated black hole information loss paradox, which remains unresolved. A key factor to the issue is the end-stage of the black hole evaporation. Unfortunately by then the evaporation process becomes non-perturbative. Aspired to provide a tool for the eventual solution to this problem, here we introduce a new interpretation of Hawking radiation as the tunneling of instantons. We study instantons of a massless scalar field in Einstein gravity. We consider a complex-valued instanton that connects an initial pure black hole state to a black hole with a scalar field that represents the Hawking radiation at future null infinity, where its action depends only on the areal entropy difference. By comparing it with several independent approaches to Hawking radiation in the perturbative limit, we conclude that Hawking radiation may indeed be described by a family of instantons. Since the instanton approach can describe non-perturbative processes, we hope that our new interpretation and holistic method may shed lights on the information loss problem.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Inflationary spectral tilts as a result of the dilatation symmetry breaking

We derive the spectral indices and their runnings of single inflation models by a new approach. We perform a dilatation transformation to the linear cosmological perturbations and derive a current (non)conservation law. Using it, we construct a dilatation charge and a Ward-Takahashi identity for the two-point correlators, and derive two exact expressions for the tree-level spectral indices. First, we apply the slow roll expansion to one of the exact expressions. We calculate the spectral indices and their runnings up to the second and the third order of slow roll parameters, respectively, with use of the “horizon crossing formalism.” By construction, our results are more rigorous and generic than the previous works. Then, we analyze another exact expression to understand how the perturbations and the slow roll parameters contribute to the spectral indices. By a numerical calculation, we confirm that only the behaviors of the slow roll parameters during a few e-folds around the horizon crossing affect significantly to the values of spectral indices. The analysis in this article indicates that if one cannot use the slow roll parameters, regardless of their values, as the expansion parameters around the horizon crossing, then one can no longer apply the slow roll expansion to the spectral indices, and it is thus necessary to apply the more generic method introduced here.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Gravitational perturbations of nonsingular black holes in conformal gravity

It is believed that in the near future, gravitational wave detections will become a promising tool not only to test gravity theories, but also to probe extremely curved spacetime regions in our universe, such as the surroundings of black holes. In this paper, we investigate the quasinormal modes (QNMs) of the axial gravitational perturbations of a class of nonsingular black holes conformally related to the Schwarzschild black hole. Here, these nonsingular black holes can be regarded as the vacuum solution of a family of conformal gravity theories which are invariant under conformal transformations. After conformal symmetry is broken, these black holes produce observational signatures different from those of the Schwarzschild black hole, such as their QNM frequencies. We assume that the spacetime is described by the Einstein equation with the effective energy momentum tensor of an anisotropic fluid. The master equation describing the QNMs is derived, and the QNM frequencies are evaluated with the Wentzel-Kramers-Brillouin (WKB) method up to the 6th order. As expected, the QNM spectra of these nonsingular black holes deviate from those of the Schwarzschild black hole, indicating the possibility of testing these black hole solutions with the help of future gravitational wave detections.

79 ASTRONOMY AND ASTROPHYSICS↗