Quantum simulation of Hawking radiation and curved spacetime with a superconducting on-chip black hole.

Hawking radiation is one of the quantum features of a black hole that can be understood as a quantum tunneling across the event horizon of the black hole, but it is quite difficult to directly observe the Hawking radiation of an astrophysical black hole. Here, we report a fermionic lattice-model-type realization of an analogue black hole by using a chain of 10 superconducting transmon qubits with interactions mediated by 9 transmon-type tunable couplers. The quantum walks of quasi-particle in the curved spacetime reflect the gravitational effect near the black hole, resulting in the behaviour of stimulated Hawking radiation, which is verified by the state tomography measurement of all 7 qubits outside the horizon. In addition, the dynamics of entanglement in the curved spacetime is directly measured. Our results would stimulate more interests to explore the related features of black holes using the programmable superconducting processor with tunable couplers.

Constraining First-Order Phase Transitions with Curvature Perturbations.

The randomness of the quantum tunneling process induces superhorizon curvature perturbations during cosmological first-order phase transitions. We for the first time utilize curvature perturbations to constrain the phase transition parameters, and find that the observations of the cosmic microwave background spectrum distortion and the ultracompact minihalo abundance can give strict constraints on the phase transitions below 100 GeV, especially for the low-scale phase transitions and some electroweak phase transitions. The current constraints on the phase transition parameters are largely extended by the results of this work, therefore provide an novel approach to probe related new physics.

Eccentricity of Long Inspiraling Compact Binaries Sheds Light on Dark Sirens.

The localization and distance inference of gravitational waves are two crucial factors for dark sirens as precise probes of cosmology, astrophysics, and fundamental physics. In this Letter, for the first time we investigate the parameter estimation of gravitational waves emitted by the eccentric compact binaries in the midfrequency (0.1-10 Hz) band. Based on the configuration of one cluster of DECIGO (B-DECIGO), we simulate five types of typical compact binaries in GWTC-3 with component mass ranging from O(1∼100) M_{⊙}. For each type of binaries, we assign discrete eccentricities from 0 to 0.4 at 0.1 Hz in 10^{3} random orientations. The multiple harmonics induced by eccentricity can break the degeneracy between parameters. We find that with eccentricity e_{0}=0.4, these typical binaries can achieve O(10^{2}-10^{4}) improvement for the distance inference in the near face-on orientations, compared to the circular case. More importantly, a nonvanishing eccentricity (0.01-0.4) can significantly improve the source localization of the typical binary black holes, most by 1.5-3.5 orders of magnitude. Our result shows the remarkable significance of eccentricity for dark sirens in the midband as precise probes of the Universe.

Hubble parameter estimation via dark sirens with the LISA-Taiji network.

The Hubble parameter is one of the central parameters in modern cosmology, and describes the present expansion rate of the universe. The values of the parameter inferred from late-time observations are systematically higher than those inferred from early-time measurements by about [Formula: see text]. To reach a robust conclusion, independent probes with accuracy at percent levels are crucial. Gravitational waves from compact binary coalescence events can be formulated into the standard siren approach to provide an independent Hubble parameter measurement. The future space-borne gravitational wave observatory network, such as the LISA-Taiji network, will be able to measure the gravitational wave signals in the millihertz bands with unprecedented accuracy. By including several statistical and instrumental noises, we show that, within a five-year operation time, the LISA-Taiji network is able to constrain the Hubble parameter within [Formula: see text] accuracy, and possibly beats the scatters down to [Formula: see text] or even better.

Magnetic Field and Gravitational Waves from the First-Order Phase Transition.

We perform the three-dimensional lattice simulation of the magnetic field and gravitational wave productions from bubble collisions during the first-order electroweak phase transition. Except for the gravitational wave, the power-law spectrum of the magnetic field strength is numerically calculated for the first time, which is of a broken power-law spectrum: B_{ξ}∝f^{0.91} for the low-frequency region of ff_{⋆} in the thin-wall limit, with the peak frequency being f_{⋆}∼5 Hz at the phase transition temperature 100 GeV. When the hydrodynamics is taken into account, the generated magnetic field strength can reach B_{ξ}∼10^{-7} G at a correlation length ξ∼10^{-7} pc, which may seed the large scale magnetic fields. Our study shows that the measurements of cosmic magnetic field strength and gravitational waves are complementary to probe new physics admitting electroweak phase transition.

Large Anisotropies of the Stochastic Gravitational Wave Background from Cosmic Domain Walls.

We investigate the stochastic gravitational wave background (SGWB) from cosmic domain walls (DWs) caused by quantum fluctuations of a light scalar field ϕ during inflation. Large-scale perturbations of ϕ lead to large-scale perturbations of DW energy density and anisotropies in the SGWB. We find that the angular power spectrum of this SGWB is scale invariant and at least of the order of 10^{-2}, which is a distinctive feature of observational interest. Since we have not detected primordial gravitational waves yet, anisotropies of the SGWB provide a nontrivial opportunity to verify the rationality of inflation and detect the energy scale of inflation, especially for low-scale inflationary models. Square kilometer array has the opportunity to detect the anisotropies of such SGWBs. The common-spectrum process observed recently by NANOGrav could also be interpreted by the SGWB from cosmic DWs.

The LISA-Taiji Network: Precision Localization of Coalescing Massive Black Hole Binaries.

We explore a potential LISA-Taiji network to fast and accurately localize the coalescing massive black hole binaries. For an equal-mass binary located at redshift of 1 with a total intrinsic mass of 105 M ⊙, the LISA-Taiji network may achieve almost four orders of magnitude improvement on the source localization region compared to an individual detector. The precision measurement of sky location from the gravitational-wave signal may completely identify the host galaxy with low redshifts prior to the final black hole merger. Such identification of the host galaxy is vital for the follow-up variability in electromagnetic emissions of the circumbinary disc when the binary merges to a new black hole and enables the coalescing massive black hole binaries to be used as a standard siren to probe the expansion history of the Universe.

Gravitational Waves Induced by Non-Gaussian Scalar Perturbations.

We study gravitational waves (GWs) induced by non-Gaussian curvature perturbations. We calculate the density parameter per logarithmic frequency interval, Ω_{GW}(k), given that the power spectrum of the curvature perturbation P_{R}(k) has a narrow peak at some small scale k_{*}, with a local-type non-Gaussianity, and constrain the nonlinear parameter f_{NL} with the future LISA sensitivity curve as well as with constraints from the abundance of the primordial black holes (PBHs). We find that the non-Gaussian contribution to Ω_{GW} increases as k^{3}, peaks at k/k_{*}=4/sqrt[3], and has a sharp cutoff at k=4k_{*}. The non-Gaussian part can exceed the Gaussian part if P_{R}(k)f_{NL}^{2}≳1. If both a slope Ω_{GW}(k)∝k^{ß} with ß∼3 and the multiple-peak structure around a cutoff are observed, it can be recognized as a smoking gun of the primordial non-Gaussianity. We also find that if PBHs with masses of 10^{20} to 10^{22} g are identified as cold dark matter of the Universe, the corresponding GWs must be detectable by LISA-like detectors, irrespective of the value of P_{R} or f_{NL}.

Gravitational Waves from Oscillons with Cuspy Potentials.

We study the production of gravitational waves during oscillations of the inflaton around the minimum of a cuspy potential after inflation. We find that a cusp in the potential can trigger copious oscillon formation, which sources a characteristic energy spectrum of gravitational waves with double peaks. The discovery of such a double-peak spectrum could test the underlying inflationary physics.

Intertwined Order and Holography: The Case of Parity Breaking Pair Density Waves.

We present a minimal bottom-up extension of the Chern-Simons bulk action for holographic translational symmetry breaking that naturally gives rise to pair density waves. We construct stationary inhomogeneous black hole solutions in which both the U(1) symmetry and spatially translational symmetry are spontaneously broken at a finite temperature and charge density. This novel solution provides a dual description of a superconducting phase intertwined with charge, current, and parity orders.

Phase transitions in a holographic s [Formula: see text] p model with back-reaction.

In a previous paper (Nie et al. in JHEP 1311:087, arXiv:1309.2204 [hep-th], 2013), we presented a holographic s [Formula: see text] p superconductor model with a scalar triplet charged under an SU(2) gauge field in the bulk. We also study the competition and coexistence of the s-wave and p-wave orders in the probe limit. In this work we continue to study the model by considering the full back-reaction. The model shows a rich phase structure and various condensate behaviors such as the "n-type" and "u-type" ones, which are also known as reentrant phase transitions in condensed matter physics. The phase transitions to the p-wave phase or s [Formula: see text] p coexisting phase become first order in strong back-reaction cases. In these first order phase transitions, the free energy curve always forms a swallow tail shape, in which the unstable s [Formula: see text] p solution can also play an important role. The phase diagrams of this model are given in terms of the dimension of the scalar order and the temperature in the cases of eight different values of the back-reaction parameter, which show that the region for the s [Formula: see text] p coexisting phase is enlarged with a small or medium back-reaction parameter but is reduced in the strong back-reaction cases.