In interstellar media characterized by a nonrelativistic plasma of electrons and heavy ions, we study the effect of axion dark matter coupled to photons on the dynamics of an electric field. In particular, we assume the presence of a background magnetic field aligned in a specific direction. We show that there is an energy transfer from the oscillating axion field to photons and then to the plasma induced by forced resonance. This resonance is most prominent for the axion mass <math altimg='si1.svg'><msub><mrow><mi>m</mi></mrow><mrow><mi>&#981;</mi></mrow></msub></math> equivalent to the plasma frequency <math altimg='si2.svg'><msub><mrow><mi>&#969;</mi></mrow><mrow><mi>p</mi></mrow></msub></math>. Requiring that the heating rate of the interstellar medium caused by the energy transfer does not exceed the observed astrophysical cooling rate, we place forecast constraints on the axion-photon coupling <italic>g</italic> for several different amplitudes of the background magnetic field <math altimg='si3.svg'><msub><mrow><mi>B</mi></mrow><mrow><mn>0</mn></mrow></msub></math>. By choosing a typical value <math altimg='si4.svg'><msub><mrow><mi>B</mi></mrow><mrow><mn>0</mn></mrow></msub><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><msup><mrow><mn>10</mn></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>&#8722;</mo><mn>6</mn></mrow></msup></math> G, we find that, for the resonance mass <math altimg='si41.svg'><msub><mrow><mi>m</mi></mrow><mrow><mi>&#981;</mi></mrow></msub><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><msub><mrow><mi>&#969;</mi></mrow><mrow><mi>p</mi></mrow></msub></math>, the upper limit of <italic>g</italic> can be stronger than those derived from other measurements in the literature. With increased values of <math altimg='si3.svg'><msub><mrow><mi>B</mi></mrow><mrow><mn>0</mn></mrow></msub></math>, it is possible to put more stringent constraints on <italic>g</italic> for a wider range of the axion mass away from the resonance point.

Japan

Makoto Amakawa

Tomohiro Fujita

Shinji Tsujikawa

Forecast constraints on the axion-photon coupling from interstellar medium heating

We revisit one-loop corrections to the power spectrum of curvature perturbations ζ in an inflationary scenario containing a transient ultra-slow-roll (USR) period. In ref. [1], it was argued that one-loop corrections to the power spectrum of ζ can be larger than the tree-level one within the parameter region generating the seeds of primordial black holes during the USR epoch, which implies the breakdown of perturbation theory. We prove that this is not the case by using a master formula for one-loop corrections to the power spectrum obtained in ref. [2]. We derive the same formula within the path-integral formalism, which is simpler than the original derivation in [2]. To show the smallness of one-loop corrections, the consistency relations and the effective constancy of tree-level mode functions of ζ for super-Hubble modes play essential roles, with which the master formula gives a simple expression for one-loop corrections. For concreteness, we provide a reduced set of interactions including the leading-order one, while establishing the consistency relations in a self-consistent manner. We also show how the consistency relations of various operators hold explicitly, which plays a key role in proving the absence of large one-loop corrections.

Ryodai Kawaguchi

Yusuke Yamada

Proving the absence of large one-loop corrections to the power spectrum of curvature perturbations in transient ultra-slow-roll inflation within the path-integral approach

We revisit the properties of total time-derivative terms as well as terms proportional to the free equation of motion (EOM) in a Schwinger-Keldysh formalism. They are relevant to the correct calculation of correlation functions of curvature perturbations in the context of inflationary Universe. We show that these two contributions to the action play different roles in the operator or the path-integral formalism, but they give the same correlation functions as each other. As a concrete example, we confirm that the Maldacena's consistency relations for the three-point correlation function in the slow-roll inflationary scenario driven by a minimally coupled canonical scalar field hold in both the operator and path-integral formalisms. We also give some comments on loop calculations.

Roles of boundary and equation-of-motion terms in cosmological correlation functions

The axion-photon coupling allows the existence of a magnetically and electrically charged black hole (BH) solution endowed with a pseudo-scalar hair. For the Reissner-Nordström BH with a given total charge and mass, it is known that the quasinormal modes (QNMs) are independent of the mixture between the magnetic and electric charges due to the presence of electric-magnetic duality. We show that the BH with an axion hair breaks this degeneracy by realizing nontrivial QNMs that depend on the ratio between the magnetic and total charges. Thus, the upcoming observations of BH QNMs through gravitational waves offer an exciting possibility for probing the existence of both magnetic monopoles and the axion coupled to photons.

Antonio De Felice

Probing the signature of axions through the quasinormal modes of black holes

In a new gravitational theory with the trace anomaly recently proposed by Gabadadze, we study the existence of hairy black hole solutions on a static and spherically symmetric background. In this theory, the effective 4-dimensional action contains a kinetic term of the conformal scalar field related to a new scale <math><mover><mrow><mi>M</mi></mrow><mrow><mo>¯</mo></mrow></mover></math> much below the Planck mass. This property can overcome a strong coupling problem known to be present in general relativity supplemented by the trace anomaly as well as in 4-dimensional Einstein-Gauss-Bonnet gravity. We find a new hairy black hole solution arising from the Gauss-Bonnet trace anomaly, which satisfies regular boundary conditions of the conformal scalar and metric on the horizon. Unlike unstable exact black hole solutions with a divergent derivative of the scalar on the horizon derived for some related theories in the literature, we show that our hairy black hole solution can be consistent with all the linear stability conditions of odd- and even-parity perturbations.

Black holes in a new gravitational theory with trace anomalies

Einsteinian cubic gravity is a higher-order gravitational theory in which the linearized field equations of motion match Einstein's equations on a maximally symmetric background. This theory allows the existence of a static and spherically symmetric black hole solution where the temporal and radial metric components are equivalent to each other (<math><mi>f</mi><mo>=</mo><mi>h</mi></math>), with a modified Schwarzschild geometry induced by cubic curvature terms. We study the linear stability of the static and spherically symmetric vacuum solutions against odd-parity perturbations without dealing with Einsteinian cubic gravity as an effective field theory where the cubic curvature terms are always suppressed relative to the Ricci scalar. Unlike General Relativity containing one dynamical perturbation, Einsteinian cubic gravity has three propagating degrees of freedom in the odd-parity sector. We show that at least one of those dynamical perturbations always behaves as a ghost mode. We also find that one dynamical degree of freedom has a negative sound speed squared <math><mo>−</mo><mn>1</mn><mo>/</mo><mn>2</mn></math> for the propagation of high angular momentum modes. Thus, the static and spherically symmetric hairy black hole solutions realized by unsuppressed cubic curvature terms relative to the Ricci scalar are excluded by ghost and Laplacian instabilities.

Excluding static and spherically symmetric black holes in Einsteinian cubic gravity with unsuppressed higher-order curvature terms

We study vector-tensor theories in which a 4-dimensional vector field <math><msub><mrow><mi>A</mi></mrow><mrow><mi>μ</mi></mrow></msub></math> is coupled to a vector quantity <math><msup><mrow><mi>J</mi></mrow><mrow><mi>μ</mi></mrow></msup></math>, which is expressed in terms of <math><msub><mrow><mi>A</mi></mrow><mrow><mi>μ</mi></mrow></msub></math> and a metric tensor <math><msub><mrow><mi>g</mi></mrow><mrow><mi>μ</mi><mi>ν</mi></mrow></msub></math>. The divergence of <math><msup><mrow><mi>J</mi></mrow><mrow><mi>μ</mi></mrow></msup></math> is equivalent to a Gauss-Bonnet (GB) term. We show that an interacting Lagrangian of the form <math><mi>f</mi><mo>(</mo><mi>X</mi><mo>)</mo><msub><mrow><mi>A</mi></mrow><mrow><mi>μ</mi></mrow></msub><msup><mrow><mi>J</mi></mrow><mrow><mi>μ</mi></mrow></msup></math>, where f is an arbitrary function of <math><mi>X</mi><mo>=</mo><mo>−</mo><mo>(</mo><mn>1</mn><mo>/</mo><mn>2</mn><mo>)</mo><msub><mrow><mi>A</mi></mrow><mrow><mi>μ</mi></mrow></msub><msup><mrow><mi>A</mi></mrow><mrow><mi>μ</mi></mrow></msup></math>, belongs to a scheme of beyond generalized Proca theories. For <math><mi>f</mi><mo>(</mo><mi>X</mi><mo>)</mo><mo>=</mo><mi>α</mi><mo>=</mo><mrow><mi>constant</mi></mrow></math>, this interacting Lagrangian reduces to a particular class of generalized Proca theories. We apply the latter coupling to a static and spherically symmetric vacuum configuration by incorporating the Einstein-Hilbert term, Maxwell scalar, and vector mass term ηX (η is a constant). Under an expansion of the small coupling constant α with <math><mi>η</mi><mo>≠</mo><mn>0</mn></math>, we derive hairy black hole solutions endowed with nonvanishing temporal and radial vector field profiles. The asymptotic properties of solutions around the horizon and at spatial infinity are different from those of hairy black holes present in scalar-GB theories. We also show that black hole solutions without the vector mass term, i.e., <math><mi>η</mi><mo>=</mo><mn>0</mn></math>, are prone to ghost instability of odd-parity perturbations.

Katsuki Aoki

Coupled vector Gauss-Bonnet theories and hairy black holes

We propose a new model of scalarized neutron stars (NSs) realized by a self-interacting scalar field ϕ nonminimally coupled to the Ricci scalar R of the form <math><mi>F</mi><mo>(</mo><mi>ϕ</mi><mo>)</mo><mi>R</mi></math>. The scalar field has a self-interacting potential and sits at its vacuum expectation value <math><msub><mrow><mi>ϕ</mi></mrow><mrow><mi>v</mi></mrow></msub></math> far away from the source. Inside the NS, the dominance of a positive nonminimal coupling over a negative mass squared of the potential leads to a symmetry restoration with the central field value <math><msub><mrow><mi>ϕ</mi></mrow><mrow><mi>c</mi></mrow></msub></math> close to 0. This allows the existence of scalarized NS solutions connecting <math><msub><mrow><mi>ϕ</mi></mrow><mrow><mi>v</mi></mrow></msub></math> with <math><msub><mrow><mi>ϕ</mi></mrow><mrow><mi>c</mi></mrow></msub></math> whose difference is significant, whereas the field is located in the vicinity of <math><mi>ϕ</mi><mo>=</mo><msub><mrow><mi>ϕ</mi></mrow><mrow><mi>v</mi></mrow></msub></math> for weak gravitational stars. The Arnowitt-Deser-Misner mass and radius of NSs as well as the gravitational force around the NS surface can receive sizable corrections from the scalar hair, while satisfying local gravity constraints in the Solar system. Unlike the original scenario of spontaneous scalarization induced by a negative nonminimal coupling, the catastrophic instability of cosmological solutions can be avoided. We also study the cosmological dynamics from the inflationary epoch to today and show that the scalar field ϕ finally approaches the asymptotic value <math><msub><mrow><mi>ϕ</mi></mrow><mrow><mi>v</mi></mrow></msub></math> without spoiling a successful cosmological evolution. After ϕ starts to oscillate about the potential minimum, the same field can also be the source for cold dark matter.

Portugal

Masato Minamitsuji

Symmetry restoration in the vicinity of neutron stars with a nonminimal coupling

In gravitational theories where a canonical scalar field ϕ with a potential <math><mi>V</mi><mo>(</mo><mi>ϕ</mi><mo>)</mo></math> is coupled to a Gauss-Bonnet (GB) term <math><mi>G</mi></math> with the Lagrangian <math><mi>f</mi><mo>(</mo><mi>ϕ</mi><mo>,</mo><mi>G</mi><mo>)</mo></math>, we study the cosmological stability of tensor and scalar perturbations in the presence of a perfect fluid. We show that, in decelerating cosmological epochs with a positive tensor propagation speed squared, the existence of nonlinear functions of <math><mi>G</mi></math> in f always induces Laplacian instability of a dynamical scalar perturbation associated with the GB term. This is also the case for <math><mi>f</mi><mo>(</mo><mi>G</mi><mo>)</mo></math> gravity, where the presence of nonlinear GB functions <math><mi>f</mi><mo>(</mo><mi>G</mi><mo>)</mo></math> is not allowed during the radiation- and matter-dominated epochs. A linearly coupled GB term with ϕ of the form <math><mi>ξ</mi><mo>(</mo><mi>ϕ</mi><mo>)</mo><mi>G</mi></math> can be consistent with all the stability conditions, provided that the scalar-GB coupling is subdominant to the background cosmological dynamics.

Cosmological stability in Gravity

In regularized 4-dimensional Einstein-Gauss-Bonnet (EGB) gravity derived from a Kaluza-Klein reduction of higher-dimensional EGB theory, we study the existence and stability of black hole (BH) solutions on a static and spherically symmetric background. We show that asymptotically-flat hairy BH solutions realized for a spatially-flat maximally symmetric internal space are unstable against linear perturbations for any rescaled GB coupling constant. This instability is present for the angular propagation of even-parity perturbations both in the vicinity of an event horizon and at spatial infinity. There is also a strong coupling problem associated with the kinetic term of even-parity perturbations vanishing everywhere.