An implicit numerical algorithm general relativistic hydrodynamics

We present an astrophysical science-case study of IndIGO-D, a decihertz gravitational-wave mission concept, focusing on compact binary sources across the 0.1-10 Hz band bridging LISA and next-generation ground-based interferometers. For these science projections, we consider three spacecraft forming an L-shaped interferometer in a heliocentric orbit, with two orthogonal 1000 km arms sharing a common vertex. We calculate the time-dependent antenna response of this geometry, including modulation produced by the orbital motion of the constellation. We treat the strain-sensitivity curve supplied by the IndIGO-D collaboration as an input. For the assumed sensitivity, we estimate a BNS detection rate of $\sim76$-$767\,{\rm yr}^{-1}$. We compute horizon distances for different source classes and find that intermediate-mass black-hole binaries with masses $10^2$-$10^3\,M_\odot$ are detectable throughout their formation history, complementing the reach of LISA and terrestrial detectors. Binary neutron star systems are observable to a horizon redshift of $z\lesssim0.3$, allowing continuous multi-band coverage with Voyager-class interferometers from the decihertz regime to merger. A Bayesian parameter-estimation study of a GW170817-like binary shows that the sky localization area improves from $\sim21\,{\rm deg}^2$ at one month, $\sim1\,{\rm deg}^2$ at one week, down to $0.3\,{\rm deg}^2$ at six hours pre-merger. These sky areas are readily tiled by the Rubin Observatory, whose $9.6\,{\rm deg}^2$ field of view and r-band depth enable high-cadence, repeated coverage of GW170817-like kilonovae at this distance and beyond. IndIGO-D bridges the gap between millihertz and terrestrial observations, enabling early warnings on timescales from months to hours and enhancing the prospects for multi-band and multi-messenger discoveries. (abridged)

gr-qc↗

arXiv2026

Analog regular black holes and black hole mimickers for surface-gravity waves in fluids

Recent advances in the observation of black-hole candidates have renewed interest in probing their near-horizon structure and in searching for departures from the standard singular solutions of general relativity. In this context, significant effort has been devoted to regular black holes and to horizonless black-hole mimickers, motivated primarily by quantum-gravitational effects. Depending on the value of the regularization parameter relative to the object mass, typical spherically symmetric solutions can describe either of these two scenarios. Regular black-hole configurations generically feature an outer and an inner horizon surrounding a maximally symmetric core; the inner horizon in turn triggers mass inflation and semiclassical instabilities. The horizonless branch of the same solutions, by contrast, supports stable inner light rings when sufficiently compact, yet is itself subject to instabilities associated with long-lived quasinormal modes. Here we investigate how to emulate these spacetimes in an analogue-gravity platform based on surface-gravity waves in a shallow-water basin, with the aim of reproducing these instabilities experimentally. We begin by identifying the flow profiles and boundary conditions required to replicate the relevant effective geometries. In particular, we show that the inner-core metrics can be simulated with a non-rotating central-drainage configuration, and we propose a graded-drainage profile to connect them to an asymptotically flat exterior region. We then assess the experimental feasibility of studying the instabilities mentioned above with current technology. Our conclusion is that, while the required setup is realizable in principle, alternative media, such as Bose-Einstein condensates, may offer a more practical route to faithfully capturing the targeted physical features.

gr-qc↗

arXiv2026

Quantum gravitational contrast in creating Schrödinger cat state

In this paper, we illustrate how a Schrödinger cat state created via a matter-wave interferometer can be viewed as the simplest quantum-gravity setup where we can treat both matter and gravity on an equal footing at a perturbative level. Here we treat Einstein's theory of general relativity using an effective field theory approach, quantising the massless spin-2 graviton in the presence of a quantum spatial superposition of matter that creates a matter-wave interferometer in the non-relativistic limit. We show that, due to the matter-graviton coupling, the graviton vacuum is displaced analogous to the coherent state. We study the contrast/overlap between the coherent states of the left and right superpositions in the matter-wave interferometer. We also study the entanglement between matter and the graviton in this setup and relate it to a gravitational contrast, or the overlap of the quantum geometries led by the coherent states. In the appendix, we provide an example of a time-dependent harmonic oscillator and study the contrast/overlap of such coherent states of the graviton.

gr-qc↗