A model for an expanding noncommutative acoustic fluid analogous to a Friedmann-Robertson-Walker geometry is derived. For this purpose, a noncommutative Abelian Higgs model is considered in a ($3+1$)-dimensional spacetime. In this scenario, we analyze the motion of test particles in this fluid. The study considers a scalar test particle coupled to a quantized fluctuating massless scalar field. For all cases studied, we find corrections due to the noncommutativity in the mean squared velocity of the particles. The nonzero velocity dispersion for particles that are free to move on geodesics disagrees with the null result found previously in the literature for expanding commutative fluid.

We utilize a recently proposed cubic nilpotent superfield to realize inflation in supergravity with the minimal degrees of freedom: the inflaton, graviton, and massive gravitino. As an advantage, the resultant model is free from the catastrophic production of gravitinos due to its vanishing propagation speed. However, the model suffers from the standard gravitino problem, and its viability depends on the mass spectrum and the thermal history of the Universe.

We highlight fundamental differences in the models of light-matter interaction between the behavior of Fock state detection in free space versus optical cavities. To do so, we study the phenomenon of the resonance of detectors with Fock wave packets as a function of their degree of monochromaticity, the number of spatial dimensions, the linear or quadratic nature of the light-matter coupling, and the presence (or absence) of cavity walls in space. In doing so we show that intuition coming from quantum optics in cavities does not straightforwardly carry to the free-space case. For example, in ($3+1$) dimensions the detector response to a Fock wave packet will go to zero as the wave packet is made more and more monochromatic and in coincidence with the detector’s resonant frequency. This is so even though the energy of the free-space wave packet goes to the expected finite value of $\hslash \mathrm{\Omega }$ in the monochromatic limit. This is in contrast to the behavior of the light-matter interaction in a cavity (even a large one) where the probability of absorbing a Fock quantum is maximized when the quantum is more monochromatic at the detector’s resonance frequency. We trace this crucial difference to the fact that monochromatic Fock states are not normalizable in the continuum; thus physical Fock states need to be constructed out of normalizable wave packets whose energy density goes to zero in the monochromatic limit as they get spatially delocalized.

We apply positivity bounds directly to a $U(1)$ gauge theory with charged scalars and charged fermions, i.e., QED, minimally coupled to gravity. Assuming that the massless $t$-channel pole may be discarded, we show that the improved positivity bounds are violated unless new physics is introduced at the parametrically low scale ${\mathrm{\Lambda }}_{\mathrm{new}}\sim (em{M}_{\mathrm{Pl}}{)}^{1/2}$, consistent with similar results for scalar field theories, far lower than the scale implied by the weak gravity conjecture. This is sharply contrasted with previous treatments which focus on the application of positivity bounds to the low energy gravitational Euler-Heisenberg effective theory only. We emphasize that the low cutoff is a consequence of applying the positivity bounds under the assumption that the pole may be discarded. We conjecture an alternative resolution that a small amount of negativity, consistent with decoupling limits, is allowed and is not in conflict with standard UV completions, including weakly coupled ones.

This is an extended version of the previous paper [S. Iso et al., Phys. Rev. D 103, 105010 (2021)] to study entanglement entropy (EE) of a half space in interacting field theories. In the previous paper, we have proposed a novel method to calculate EE based on the notion of ${\mathbb{Z}}_M$ gauge theory on Feynman diagrams, and shown that EE consists of two particular contributions, one from a renormalized two-point correlation function in the two-particle irreducible (2PI) formalism and another from interaction vertices. In this paper, we further investigate them in more general field theories and show that the non-Gaussian contributions from vertices can be interpreted as renormalized correlation functions of composite operators.

A waveform model for the eccentric binary black holes named SEOBNRE has been used to analyze the LIGO-Virgo’s gravitational wave data by several groups. The accuracy of this model has been validated by comparing it with numerical relativity. However, SEOBNRE is a time-domain model, and the efficiency for generating waveforms is a bottleneck in data analysis. To overcome this disadvantage, we offer a reduced-order surrogate model for eccentric binary black holes based on the SEOBNRE waveforms. This surrogate model (SEOBNRE_S) can simulate the complete inspiral-merger-ringdown waves with enough accuracy, covering eccentricities from 0 to 0.25 (0.1), and mass ratio from $1:1$ to $5:1$ ($2:1$) for nonspinning (spinning) binaries. The speed of waveform generation is accelerated about ${10}^2–{10}^3$ times more than the original SEOBNRE model. Therefore SEOBNRE_S could be helpful in the analysis of LIGO data to find potential eccentricities.

Recent times witnessed a surge of interest in strong gravitational lensing by black holes due to the Event Horizon Telescope (EHT) results, which suggest comparing the black hole lensing in general relativity and modified gravity theories. This may help us to assess the phenomenological differences between these models. A Kerr black hole is also a solution to some alternative theories of gravity, while recently obtained modified Kerr black holes (hairy Kerr black holes), which evade the no-hair theorem, are due to additional sources from surrounding fluid, like dark matter, having conserved energy-momentum tensor. These hairy Kerr black holes may also be solutions to an alternative theory of gravity. We generalize previous work on gravitational lensing by a Kerr black hole in the strong deflection limits to the hairy Kerr black holes, with a deviation parameter $\alpha$ and a primary hair ${\ell }_0$. Interestingly, the deflection coefficient $\overline{a}$ increases and decreases with increasing ${\ell }_0$ and $\alpha$, respectively. $\overline{b}$ shows opposite behavior with ${\ell }_0$ and $\alpha$. We also find that the deflection angle ${\alpha }_D$, angular position ${\theta }_{\infty }$, and $u_m$ decrease, but angular separation $s$ increases with $\alpha$. We compare our results with those for Kerr black holes, and also apply the formalism to discuss the astrophysical consequences in the context of the supermassive black holes Sgr A* and M87*. We observe that the deviations of the angular positions from that of the Kerr black hole are not more than $2.6\mu \mathrm{as}$ for Sgr A* and $1.96\mu \mathrm{as}$ for M87*, which are unlikely to be resolved by the current EHT observations.

We construct few deep generative models of gravitational waveforms based on the semisupervising scheme of conditional autoencoders and its variational extensions. Once the training is done, we find that our best waveform model can generate the inspiral-merger waveforms of binary black hole coalescence with more than 97% average overlap matched filtering accuracy for the mass ratio between 1 and 10. Besides, the generation time of a single waveform takes about one millisecond, which is about 10 to 100 times faster than the effective-one-body-numerical-relativity algorithm running on the same computing facility. Moreover, these models can also help to explore the space of waveforms. That is, with mainly the low-mass-ratio training set, the resultant trained model is capable of generating large amount of accurate high-mass-ratio waveforms. This result implies that our generative model can speed up the waveform generation for the low latency search of gravitational wave events. With improvement of the accuracy in the future work, the generative waveform model may also help to speed up the parameter estimation and can assist the numerical relativity in generating the waveforms of higher mass ratio by progressively self-training.

We obtain a type of Ayón-Beato-García (ABG) related black hole solutions with five parameters: the mass $m$, the charge $q$, and three dimensionless parameters $\alpha$, $\beta$ and $\gamma$ associated with nonlinear electrodynamics. We find that this type of black hole is regular under the conditions: $\alpha \gamma \ge 6$, $\beta \gamma \ge 8$, and $\gamma >0$. Here we focus on the saturated case: $\alpha =6/\gamma$ and $\beta =8/\gamma$, such that only three parameters $m$, $q$ and $\gamma$ remain, which leads to a new family of ABG black holes. For such a family of black holes, we investigate the influence of the charge $q$ and the parameter $\gamma$ on the horizon radius and the Hawking temperature. In addition, we calculate the quasinormal mode frequencies of massless scalar field perturbations by using the sixth-order WKB approximation method and the unstable circular null geodesic method in the eikonal limit. On the one hand, our results show that the increase of the charge $q$ makes the scalar waves decay faster at first and then slowly except for the case of $\gamma =2$. On the other hand, they show that the increase of the parameter $\gamma$ makes the scalar waves decay at first sharply and then slowly. In particular, $\gamma =1$ can be regarded as the critical value for the transition from an unstable configuration to a stable one. Finally, we compute the shadow radius for the new family of ABG black holes and use the shadow data of the $M87^{*}$ black hole detected by the Event Horizon Telescope to provide an upper limit on the charge $q$ of the new black holes. We find that the increase of the charge $q$ makes the shadow radius decrease monotonically, while the increase of the parameter $\gamma$ makes the shadow radius increase at first rapidly and then almost remain unchanged, especially the parameter $\gamma$ has a significant impact on the shadow radius when it is less than one. Moreover, using the shadow data of the $M87^{*}$ black hole, we find that the upper limit of the charge $q$ increases rapidly at first and then slowly but does not exceed the mass of the $M87^{*}$ black hole at last when the parameter $\gamma$ is increasing and going to infinity, and that the data restrict the frequency range of the fundamental mode with $l=1$ to $1.4\times {10}^{-6}\mathrm{Hz}\sim 1.9\times {10}^{-6}\mathrm{Hz}$.

The little rip is a cosmological abrupt event predicted by some phantom dark energy models that could describe the future evolution of our Universe. This event can be interpreted as a big rip singularity delayed indefinitely, although in those models bounded structures will be destroyed in a finite cosmic time in the future. In this work, we analyze the little rip cosmology from a classical and quantum point of view within the scheme of alternative metric $f(R)$ theories of gravity. The quantum analysis is performed in the framework of $f(R)$ quantum geometrodynamics by means of the modified Wheeler-DeWitt equation. In this context, we show that the DeWitt criterion can be satisfied. Similar to what happens in general relativity, this result points toward the avoidance of the little rip in $f(R)$ quantum cosmology.

We report on critical phenomena in the gravitational collapse of electromagnetic waves. Generalizing earlier results that focused on dipole electromagnetic waves, we compare here with quadrupole waves in axisymmetry. We perform numerical simulations of dipole and quadrupole wave initial data, fine tuning both sets of data to the onset of black hole formation in order to study the critical solution and related critical phenomena. We observe that different multipole moments have different symmetries, indicating that the critical solution for electromagnetic waves cannot be unique, at least not globally. This is confirmed in our numerical simulations: while dipole data lead to a single center of collapse, at the center of symmetry, quadrupole data feature two separate centers of collapse on the symmetry axis, above and below the center of symmetry—reminiscent of similar findings reported for critical collapse of vacuum gravitational waves. While the critical solution for neither the dipole nor the quadrupole data is exactly self-similar, we find that their approximate echoing periods appear to differ, as do the critical exponents. We discuss whether the centers of collapse found for dipole and quadrupole data might all have the same properties, which would suggest a “local uniqueness” of the critical solution. Instead, we provide some evidence—including the differing echoing periods and critical exponents—suggesting that the critical solutions are distinct even locally. We speculate on the implications of our findings for critical phenomena in the collapse of vacuum gravitational waves, which share with electromagnetic waves the absence of a spherically symmetric critical solution.

Gravitational-wave sources can serve as standard sirens to probe cosmology by measuring their luminosity distance and redshift. Such standard sirens are also useful to probe theories beyond general relativity with a modified gravitational-wave propagation. Many of previous studies on the latter assume multimessenger observations so that the luminosity distance can be measured with gravitational waves while the redshift is obtained by identifying sources’ host galaxies from electromagnetic counterparts. Given that gravitational-wave events of binary neutron star coalescences with associated electromagnetic counterpart detections are expected to be rather rare, it is important to examine the possibility of using standard sirens with gravitational-wave observations alone to probe gravity. In this paper, we achieve this by extracting the redshift from the tidal measurement of binary neutron stars that was originally proposed within the context of gravitational-wave cosmology (another approach is to correlate “dark sirens” with galaxy catalogs that we do not consider here). We consider not only observations with ground-based detectors (e.g., Einstein Telescope) but also multiband observations between ground-based and space-based (e.g., DECIGO) interferometers. We find that such multiband observations with the tidal information can constrain a parametric non-Einsteinian deviation in the luminosity distance (due to the modified friction in the gravitational wave evolution) more stringently than the case with electromagnetic counterparts by a factor of a few. We also map the above-projected constraints on the parametric deviation to those on specific theories and phenomenological models beyond general relativity to put the former into context.

We study the response of a static Unruh-DeWitt detector outside an exotic compact object (ECO) in $3+1$ dimensions. The horizonless ECO, whose surface is extremely close to the would-be event horizon, acts as a black hole mimicker. ECOs that have an origin provide a physical means by which all waves, including gravitational waves, are reflected (off the origin). They also provide a source of damping of a wave as it traverses the material in the ECO. We study here the case of a probe scalar field for simplicity. We find that the response rate is notably distinct from the black hole case, even when the wave is lost completely to damping. For partial or no damping we find resonance structures in the response rate that depend on the compactness of the ECO and the detector location. We provide a detailed analysis in connection with the ECO’s vacuum mode structure and the transfer function.

We make forecasts for the impact a future “midband” space-based gravitational wave experiment, most sensitive to ${10}^{-2}-10\mathrm{Hz}$, could have on potential detections of cosmological stochastic gravitational wave backgrounds (SGWBs). Specific proposed midband experiments considered are TianGo, B-DECIGO, and AEDGE. We propose a combined power-law integrated sensitivity (CPLS) curve combining GW experiments over different frequency bands, which shows the midband improves sensitivity to SGWBs by up to two orders of magnitude at ${10}^{-2}-10\mathrm{Hz}$. We consider GW emission from cosmic strings and phase transitions as benchmark examples of cosmological SGWBs. We explicitly model various astrophysical SGWB sources, most importantly from unresolved black hole mergers. Using Markov Chain Monte Carlo, we demonstrated that midband experiments can, when combined with LIGO $\mathrm{A}+$ and LISA, significantly improve sensitivities to cosmological SGWBs and better separate them from astrophysical SGWBs. In particular, we forecast that a midband experiment improves sensitivity to cosmic string tension $G\mu$ by up to a factor of 10, driven by improved component separation from astrophysical sources. For phase transitions, a midband experiment can detect signals peaking at 0.1–1 Hz, which for our fiducial model corresponds to early Universe temperatures of $T_{*}\sim {10}^4–{10}^6\mathrm{GeV}$, generally beyond the reach of LIGO and LISA. The midband closes an energy gap and better captures characteristic spectral shape information. It thus substantially improves measurement of the properties of phase transitions at lower energies of $T_{*}\sim O({10}^3)\mathrm{GeV}$, potentially relevant to new physics at the electroweak scale, whereas in this energy range LISA alone will detect an excess but not effectively measure the phase transition parameters. Our modeling code and chains are publicly available.

https://github.com/sbird/grav_midband

Weak gravitational lensing of the cosmic microwave background (CMB) is an important cosmological tool that allows us to learn about the structure, composition and evolution of the Universe. Upcoming CMB experiments, such as the Simons Observatory (SO), will provide high-resolution and low-noise CMB measurements. We consider the impact of instrumental systematics on the corresponding high-precision lensing reconstruction power spectrum measurements. We simulate CMB temperature and polarization maps for an SO-like instrument and potential scanning strategy, and explore systematics relating to beam asymmetries and offsets, boresight pointing, polarization angle, gain drifts, gain calibration and electric crosstalk. Our analysis shows that the majority of the biases induced by the systematics we modeled are below a detection level of $\sim 0.6\sigma$. We discuss potential mitigation techniques to further reduce the impact of the more significant systematics, and pave the way for future lensing-related systematics analyses.

The increasing tension between the different local direct measurements of the Hubble expansion rate and that inferred from the cosmic microwave background observation by the $\mathrm{\Lambda }$-cold-dark-matter model could be a smoking gun of new physics, if not caused by either observational systematics or local bias. We generalize previous investigation on the local bias from a local void by globally fitting the Pantheon sample over all parameters in the radial profile function of a local void described by an inhomogeneous but isotropic Lemaître-Tolman-Bondi metric with a cosmological constant. Our conclusion strengthens the previous studies that the current tension on Hubble constant cannot be saved by a local void alone.

We assess comprehensively the potential of galaxy cluster edges as a standard ruler in measuring cosmological distances and probing exotic physics. Confronting five alternative cosmological models with cluster edges from the near-future Dark Energy Spectroscopic Instrument survey, we conclude that cluster edges can serve as a promising probe to constrain models beyond $\mathrm{\Lambda }\mathrm{CDM}$. In particular, the constraining precision of the equation of state of dark energy from cluster edges is just about 2 times as large as that from the Pantheon type Ia supernovae sample. We find that the constraining power of cluster edges can be exhibited better by combining it with other probes. Combining cluster edges with the cosmic microwave background, baryon acoustic oscillations, type Ia supernovae, cosmic chronometers, and simulated gravitational-wave events from the space-based Einstein Telescope, we constrain $\mathrm{\Lambda }\mathrm{CDM}$ and find that the data combination of the cosmic microwave background and cluster edges gives the best constraints on the Hubble constant $H_0$, with 0.5% precision, and the matter density ratio ${\mathrm{\Omega }}_m$, with 1.6% precision, among these five pair datasets, and that the data combination of gravitational waves and cluster edges shares almost the same constraining power as that of type Ia supernovae and cluster edges. We also give the most stringent constraint on $\mathrm{\Lambda }\mathrm{CDM}$ by combining cluster edges with available cosmological data. With the help of other probes, galaxy cluster edges can give new insights on exotic physics better at cosmological scales.

The swampland de Sitter conjecture in combination with upper limits on the tensor-to-scalar ratio $r$ derived from observations of the cosmic microwave background endangers the paradigm of slow-roll single-field inflation. This conjecture constrains the first and the second derivatives of the inflationary potential in terms of two $\mathcal{O}(1)$ constants $c$ and $c^{\prime }$. In view of these restrictions, we reexamine single-field inflationary potentials with $S$-duality symmetry, which ameliorate the unlikeliness problem of the initial condition. We compute $r$ at next-to-leading order in slow-roll parameters for the most general form of $S$-dual potentials and confront model predictions to constraints imposed by the de Sitter conjecture. We find that $c\sim \mathcal{O}({10}^{-1})$ and $c^{\prime }\sim \mathcal{O}({10}^{-2})$ can accommodate the 95% C.L. upper limit on $r$. By imposing at least $50e$-folds of inflation with the effective field theory description valid only over a field displacement $\mathcal{O}(1)$ when measured as a distance in the target space geometry, we further restrict $c\sim \mathcal{O}({10}^{-2})$, while the constraint on $c^{\prime }$ remains unchanged. We comment on how to accommodate the required small values of $c$ and $c^{\prime }$.

The waveform templates of the matched filtering-based gravitational-wave search ought to cover wide range of parameters for the prosperous detection. Numerical relativity (NR) has been widely accepted as the most accurate method for modeling the waveforms. Still, it is well known that NR typically requires a tremendous amount of computational costs. In this paper, we demonstrate a proof-of-concept of a novel deterministic deep learning (DL) architecture that can generate gravitational waveforms from the merger and ringdown phases of the non-spinning binary black hole coalescence. Our model takes $\mathcal{O}(1)$ seconds for generating approximately 1500 waveforms with a 99.9% match on average to one of the state-of-the-art waveform approximants, the effective-one-body. We also perform matched filtering with the DL-waveforms and find that the waveforms can recover the event time of the injected gravitational-wave signals.

A new dynamic is identified between dark matter and nuclei. Nuclei accelerated to MeV energies by the internal potential of composite dark matter can undergo nuclear fusion. This effect arises in simple models of composite dark matter made of heavy fermions bound by a light scalar field. Cosmologies and detection prospects are explored for composites that catalyze nuclear reactions in underground detectors and stars, including bremsstrahlung radiation from nuclei scattering against electrons in hot plasma formed in the composite interior. If discovered and collected, this kind of composite dark matter could in principle serve as a ready-made, compact nuclear fusion generator.