Solitons are important nonperturbative excitations in superfluids. For holographic superfluids, we numerically construct “dark” solitons that have the symmetry-restored phase at their core. A central point is that we include the gravitational backreaction of the matter fields, which becomes important at low temperatures. We study in detail the properties of these solitons under variation of the backreaction strength via tuning the gravitational constant. In particular, the depletion fraction of the particle number density at the core of the solitons is carefully investigated. In agreement with the probe-limit analysis, the depletion fraction shows the same qualitative behavior as in Bogoliubov–de Gennes theory, even if the backreaction is included. We find that the depletion decreases with increasing backreaction strength. Moreover, the inclusion of backreaction enables us to obtain the effective energy density of solitons within holography, which together with an evaluation of the surface tension leads to a simple physical explanation for the snake instability of dark solitons.

We explore the suitability of deep learning to capture the physics of subgrid-scale ideal magnetohydrodynamics turbulence of 2D simulations of the magnetized Kelvin-Helmholtz instability. We produce simulations at different resolutions to systematically quantify the performance of neural network models to reproduce the physics of these complex simulations. We compare the performance of our neural networks with gradient models, which are extensively used in the magnetohydrodynamic literature. Our findings indicate that neural networks significantly outperform gradient models in accurately computing the subgrid-scale tensors that encode the effects of magnetohydrodynamics turbulence. To the best of our knowledge, this is the first exploratory study on the use of deep learning to learn and reproduce the physics of magnetohydrodynamics turbulence.

A general Hamiltonian approach to black hole thermodynamics is used to study entropy and conserved charges for Kerr-AdS solutions in general relativity. These thermodynamic variables are first consistently defined by choosing suitable boundary conditions, and then, they are shown to satisfy the first law of black hole dynamics

We propose a matter effect for the gravitational ultraviolet/infrared ($\mathrm{UV}/\mathrm{IR}$) mixing solution to the cosmological constant problem. Previously, the gravitational $\mathrm{UV}/\mathrm{IR}$ mixing model implied a nonstandard equation of state for dark energy, contradicting observation. In contrast, matter effect gravitational $\mathrm{UV}/\mathrm{IR}$ mixing accommodates a standard $\mathrm{\Lambda }\mathrm{CDM}$ cosmology with constant dark energy. Notably, there are new density-dependent predictions for futuristically precise measurements of fundamental parameters, like the magnetic moments of the muon and electron.

We consider the implications of the swampland conjectures on scalar-tensor theories defined in the Einstein frame in which the scalar interaction is screened. We show that chameleon models are not in the swampland provided the coupling to matter is larger than unity and the mass of the scalar field is much larger than the Hubble rate. We apply these conditions to the inverse power law chameleon and the symmetron. We then focus on the dilaton of string theory in the strong coupling limit, as defined in the string frame. We show that solar system tests of gravity imply that viable dilaton models are not in the swampland. In the future of the Universe, if the low-energy description with a single scalar is still valid and the coupling to matter remains finite, we find that the scalar field energy density must vanish for models with the chameleon and symmetron mechanisms. Hence in these models dark energy is only a transient phenomenon. This is not the case for the strongly coupled dilaton, which keeps evolving slowly, leading to a quasi–de Sitter spacetime.

The self-consistent mean field approximation of the two-flavor Nambu–Jona-Lasinio (NJL) model, which introduces a free parameter $\alpha$ ($\alpha$ reflects the weight of different interaction channels), is employed to investigate the contributions of the vector channel at a finite isospin chemical potential ${\mu }_I$, zero baryon chemical potential ${\mu }_B$, and zero temperature $T$. The calculations show that the consideration of the vector-channel contributions leads to a lower value of pion condensate in a superfluid phase, compared with the standard Lagrangian of the NJL model ($\alpha =0$). In a superfluid phase, we also obtain a lower isospin number density, and the discrepancy is getting larger with the increase of the isospin potential. Compared with the recent results from lattice QCD, the isospin density and energy density we obtained with $\alpha =0.5$ agree with the data of the lattice well. In the phase diagram in the $T\hspace{0.17em}–\hspace{0.17em}{\mu }_I$ plane for ${\mu }_B=0$, we can see that the difference of the critical temperatures of a phase transition between the results with $\alpha =0$ and $\alpha =0.5$ is up to 3%–5% for a fixed isospin potential. All of these indicate that the vector channels play an important role in the isospin medium.

For the first time, we compute three-loop contributions to all triple vertices in QCD at the symmetric point. The analytic results are obtained in massless QCD with an arbitrary color group in the Landau gauge. All new loop integrals are expressed in terms of harmonic polylogarithms at the sixth root of unity. These corrections allow us to derive expressions for the four-loop QCD beta function in a set of momentum-subtraction schemes.

We study the effect of Lorentz symmetry violation (LSV) on the behavior at high energy of $SU(N)$ gauge theory with quarks in the fundamental representation. The approach is similar to that for QED treated in a previous paper. In contrast to QED, standard Lorentz invariant QCD is asymptotically free. Our aim is to explore the structure of the renormalization group at high energy and hence weak coupling without requiring the Lorentz symmetry breaking to be small. The simplest type of LSV leaves the theory invariant under a subgroup of the Lorentz group that preserves a (timelike) 4-vector. We examine this case in detail and find that asymptotic freedom is frustrated. That is, at sufficiently high energy, the running coupling constant attains a minimum value before increasing again, while the LSV parameter increases without bound.

We obtain a newly charged BBMB (Bocharova-Bronnikov-Melnikov-Bekenstein) black hole solution from the Einstein-Weyl-Maxwell conformal (conformally coupled) scalar theory with a positive Weyl coupling parameter $m_2^2$ numerically. This solution has primary scalar hair, compared to secondary scalar hair in the charged BBMB black hole solution. The limiting case of $m_2^2\to \infty$ leads to the series form of the charged BBMB black hole solution. However, for $m_2^2<0$, we could not obtain any asymptotically flat black hole solution with scalar hair.

In this paper, we present an iterative method to generate an infinite class of new nonlocal field theories whose propagators are ghost-free. We first examine the scalar field case and show that the pole structure of such generalized propagators possesses the standard two derivative pole and in addition can contain complex conjugate poles which, however, do not spoil at least tree level unitarity as the optical theorem is still satisfied. Subsequently, we define analogous propagators for the fermionic sector which is also devoid of unhealthy degrees of freedom. As a third case, we apply the same construction to gravity and define a new set of theories whose graviton propagators around the Minkowski background are ghost-free. Such a wider class also includes nonlocal theories previously studied and Einstein’s general relativity as a peculiar limit. Moreover, we compute the linearized gravitational potential generated by a static pointlike source for several gravitational theories belonging to this new class and show that the nonlocal nature of gravity regularizes the singularity at the origin.

The decay timescales of the quasinormal modes of a massive scalar field have an intriguing behavior: they either grow or decay with increasing angular harmonic numbers $\ell$, depending on whether the mass of the scalar field is small or large. We identify the properties of the effective potential of the scalar field that leads to this behavior and characterize it in detail. If the scalar field is nonminimally coupled, considered here, the scalar quasinormal modes will leak into the gravitational wave signal and will have decaying times that are comparable or smaller than those typical in general relativity. Hence, these modes could be detectable in the future. Finally, we find that the anomalous behavior in the decay timescales of quasinormal modes is present in a much larger class of models beyond a simple massive scalar field.

Nonlinear generalizations of teleparallel gravity entail the modification of a Lagrangian that is pseudoinvariant under local Lorentz transformations of the tetrad field. This procedure consequently leads to the loss of the local pseudoinvariance and the appearance of additional degrees of freedom (d.o.f.). The constraint structure of $f(T)$ gravity suggests the existence of one extra d.o.f. when compared with general relativity, which should describe some aspect of the orientation of the tetrad. The purpose of this article is to better understand the nature of this extra d.o.f. by means of a toy model that mimics essential features of $f(T)$ gravity. We find that the nonlinear modification of a Lagrangian $L$ possessing a local rotational pseudoinvariance produces two types of solutions. In one case the original gauge-invariant variables—the analogue of the metric in teleparallelism—evolve like when governed by the (nondeformed) Lagrangian $L$; these solutions are characterized by a (selectable) constant value of its Lagrangian, which is the manifestation of the extra d.o.f. In the other case, the solutions do contain new dynamics for the original gauge-invariant variables, but the extra d.o.f. does not materialize because the Lagrangian remains invariant on-shell. Coming back to $f(T)$ gravity, the first case includes solutions where the torsion scalar $T$ is a constant, to be chosen at the initial conditions (extra d.o.f.), and no new dynamics for the metric is expected. The latter case covers those solutions displaying a genuine modified gravity; $T$ is not a constant, but it is (on-shell) invariant under Lorentz transformations depending only on time. Both kinds of $f(T)$ solutions are exemplified in a flat Friedmann-Lemaître-Robertson-Walker universe. Finally, we present a toy model for a higher-order Lagrangian with rotational invariance [analogous to $f(R)$ gravity] and derive its constraint structure and number of d.o.f.

We study linear perturbations of the Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological model in the Regge-Wheeler formalism, which is a standard framework to study perturbations of spherically symmetric black holes. In particular, we show that the general solution of linear perturbation equations can be given in terms of two copies of a master scalar satisfying the scalar wave equation on the FLRW background (with a Regge-Wheeler/Zerilli type potential), thus representing two gravitational degrees of freedom and one scalar satisfying a transport type equation representing matter perturbation. We expect the Regge-Wheeler formalism to be easily extended to include nonlinear perturbations, akin to the recent work [A. Rostworowski, Phys. Rev. D 96, 124026 (2017)].

Axions are some of the best motivated particles beyond the Standard Model. We show how the attractive self-interactions of dark matter (DM) axions over a broad range of masses, from ${10}^{-22}\mathrm{eV}$ to ${10}^7\mathrm{GeV}$, can lead to nongravitational growth of density fluctuations and the formation of bound objects. This structure formation enhancement is driven by parametric resonance when the initial field misalignment is large, and it affects axion density perturbations on length scales of order the Hubble horizon when the axion field starts oscillating, deep inside the radiation-dominated era. This effect can turn an otherwise nearly scale-invariant spectrum of adiabatic perturbations into one that has a spike at the aforementioned scales, producing objects ranging from dense DM halos to scalar-field configurations such as solitons and oscillons. We call this class of cosmological scenarios for axion DM production “the large-misalignment mechanism.” We explore observational consequences of this mechanism for axions with masses up to 10 eV. For axions heavier than ${10}^{-5}\mathrm{eV}$, the compact axion halos are numerous enough to significantly impact Earth-bound direct detection experiments, yielding intermittent but coherent signals with repetition rates exceeding one per decade and crossing times less than a day. These episodic increases in the axion density and kinematic coherence suggest new approaches for axion DM searches, including for the QCD axion. Dense structures made up of axions from ${10}^{-22}\mathrm{eV}$ to ${10}^{-5}\mathrm{eV}$ are detectable through gravitational lensing searches, and their gravitational interactions can also perturb baryonic structures and alter star formation. At very high misalignment amplitudes, the axion field can undergo self-interaction-induced implosions long before matter-radiation equality, producing potentially-detectable low-frequency stochastic gravitational waves.

Dark matter may be in the form of nonbaryonic structures such as compact subhalos and boson stars. Structures weighing between asteroid and solar masses may be discovered via gravitational microlensing, an astronomical probe that has in the past helped constrain the population of primordial black holes and baryonic massive astrophysical compact halo objects. We investigate the nontrivial effect of the size of and density distribution within these structures on the microlensing signal and constrain their populations using the eros-2 and ogle-iv surveys. Structures larger than a solar radius are generally constrained more weakly than pointlike lenses, but stronger constraints may be obtained for structures with mass distributions that give rise to caustic crossings or produce larger magnifications.

The large dark cores of common dwarf galaxies are unexplained by the standard heavy particle interpretation of dark matter. This puzzle is exacerbated by the discovery of a very large but barely visible, dark matter dominated galaxy Antlia II orbiting the Milky Way, uncovered by tracking star motions with the Ĝaia satellite. Although Antlia II has a low mass, its visible radius is more than double any known dwarf galaxy, with an unprecedentedly low density core. We show that Antlia II favors dark matter as a Bose-Einstein condensate, for which the ground state is a stable soliton with a core radius given by the de Broglie wavelength. The lower the galaxy mass, the larger the de Broglie wavelength, so the least massive galaxies should have the widest soliton cores of lowest density. An ultralight boson of $m_{\psi }\sim 1.1\times {10}^{-22}\mathrm{eV}$ accounts well for the large size and slowly moving stars within Antlia II and agrees with boson mass estimates derived from the denser cores of more massive dwarf galaxies. For this very light boson, Antlia II is close to the lower limiting Jeans scale for galaxy formation permitted by the uncertainty principle, so other examples are expected but none significantly larger in size. This simple explanation for the puzzling dark cores of dwarf galaxies implies dark matter as an ultralight boson, such as an axion generic in string theory.