We consider Galileon models on curved spacetime, as well as the counterterms introduced to maintain the second-order nature of the field equations of these models when both the metric and the scalar are made dynamical. Working in a gauge-invariant framework, we first show how all the third-order time derivatives appearing in the field equations—both metric and scalar—of a Galileon model or one defined by a given counterterm can be eliminated to leave field equations which contain at most second-order time derivatives of the metric and of the scalar. The same is shown to hold for arbitrary linear combinations of such models, as well as their k-essence-like/Horndeski generalizations. This supports the claim that the number of degrees of freedom in these models is only 3, counting 2 for the graviton and 1 for the scalar. We comment on the arguments given previously in support of this claim. We then prove that this number of degrees of freedom is strictly lower than 4 in one particular such model by carrying out a full-fledged Hamiltonian analysis. In contrast to previous results, our analyses do not assume any particular gauge choice of restricted applicability.

The $(1+1)$-dimensional Friedmann-Robertson-Walker universe filled with a perfect fluid with equation of state $p=\omega \rho$ is analyzed through the view of Hořava-Lifshitz (HL) theory of gravity. In this theory, the anisotropic scaling of space and time breaks Lorentz invariance of general relativity in such a way that the gravitational action is no longer a topological invariant and the theory becomes dynamical. With the introduction of a perfect fluid through Schutz formalism, it is shown that the resulting dynamical theory is very similar to the two-dimensional Jackiw-Teitelboim model, where a dilatonic degree of freedom is introduced to force a dynamical theory. However, in HL theory, the introduction of a dilaton field is not necessary.

We investigate scalar field theories in de Sitter space by means of nonperturbative renormalization group techniques. We compute the functional flow equation for the effective potential of $\mathrm{O}(N)$ theories in the local potential approximation and we study the onset of curvature-induced effects as quantum fluctuations are progressively integrated out from subhorizon to superhorizon scales. This results in a dimensional reduction of the original action to an effective zero-dimensional Euclidean theory. We show that the latter is equivalent both to the late-time equilibrium state of the stochastic approach of Starobinsky and Yokoyama and to the effective theory for the zero mode on Euclidean de Sitter space. We investigate the immediate consequences of this dimensional reduction: symmetry restoration and dynamical mass generation.

The tensor-vector-scalar (TeVeS) model is considered a viable theory of gravity. It produces the Milgrom’s modified Newtonian dynamics in the nonrelativistic weak field limit and is free from ghosts. This model has been tested against various cosmological observations. Here, we investigate whether new observations such as the galaxy velocity power spectrum measured by 6dF and the kinetic Sunyaev Zel’dovich effect power spectrum measured by ACT/SPT can put further constraints on the TeVeS model. Furthermore, we perform the test of TeVeS cosmology with a sterile neutrino by confronting to Planck data, and find that it is ruled out by cosmic microwave background measurements from the Planck mission.

We investigate simplified models of dark matter with scalar mediators at hadron colliders using the final state topology with two jets and missing energy. These models can arise in a wide variety of BSM scenarios including the possibility of the mediator mixing with the Higgs. Our aim is first to determine the projected reach of the LHC and the future circular hadron collider for excluding such models, and we also compare these to the relic density and direct detection constraints. We use the kinematic distributions to extract information on mediator masses at colliders. At the 13 TeV LHC we can probe mediator masses up to 750 GeV, and at a 100 TeV collider the reach is increased to 2.5 TeV mediators. We also explain how individual models with different values of mediator masses can be differentiated from each other.

We attempt to simultaneously explain the recently observed 3.55 keV x-ray line in the analysis of XMM-Newton telescope data and the Galactic Center gamma ray excess observed by the Fermi Gamma Ray Space Telescope within an Abelian gauge extension of the standard model. We consider a two component dark matter scenario with tree level mass difference 3.55 keV such that the heavier one can decay into the lighter one and a photon with energy 3.55 keV. The lighter dark matter candidate is protected from decaying into the standard model particles by a remnant $Z_2$ symmetry into which the Abelian gauge symmetry gets spontaneously broken. If the mass of the dark matter particle is chosen to be within 31–40 GeV, then this model can also explain the Galactic Center gamma ray excess if the dark matter annihilation into $b\overline{b}$ pairs has a cross section of $⟨\sigma v⟩\simeq (1.4-2.0)\times 10^{-26}{\mathrm{cm}}^3/\mathrm{s}$. We constrain the model from the requirement of producing correct dark matter relic density, 3.55 keV x-ray line flux, and Galactic Center gamma ray excess. We also impose the bounds coming from dark matter direct detection experiments as well as collider limits on additional gauge boson mass and gauge coupling. We also briefly discuss how this model can give rise to subelectron volt neutrino masses at tree level as well as the one-loop level while keeping the dark matter mass at a few tens of giga-electron volts. We also constrain the model parameters from the requirement of keeping the one-loop mass difference between two dark matter particles below a kilo-electron volt. We find that the constraints from light neutrino mass and kilo-electron volt mass splitting between two dark matter components show more preference for opposite $CP$ eigenvalues of the two fermion singlet dark matter candidates in the model.

The proximity of the Peccei-Quinn scale to the scale of supersymmetry breaking in models of pure gravity mediation hints at a common dynamical origin of these two scales. To demonstrate how to make such a connection manifest, we embed the Peccei-Quinn mechanism into the vectorlike model of dynamical supersymmetry breaking à la Izawa and Yanagida as well as Intriligator and Thomas (IYIT). Here, we rely on the anomaly-free discrete $Z_4^R$ symmetry required in models of pure gravity mediation to solve the $\mu$ problem to protect the Peccei-Quinn symmetry from the dangerous effect of higher-dimensional operators. This results in a rich phenomenology featuring a QCD axion with a decay constant of $\mathcal{O}({10}^{10})\mathrm{GeV}$ and mixed dark matter in the form of weakly interacting massive particles and axions. In addition, exactly five pairs of extra $\mathbf{5}$ and ${\mathbf{5}}^{*}$ matter multiplets, directly coupled to the supersymmetry breaking sector and with masses close to the gravitino mass, $m_{3/2}\sim 100\mathrm{TeV}$, are needed to cancel the $Z_4^R$ anomalies.

We compare the low-energy partial-wave analyses of $\pi N$ scattering with high-energy data via finite-energy sum rules. We construct a new set of amplitudes by matching the imaginary part from the low-energy analysis with the high-energy, Regge parametrization and reconstruct the real parts using dispersion relations.

Motivated by recent lattice QCD studies, we explore the effects of interactions on strangeness fluctuations in strongly interacting matter at finite temperature. We focus on S-wave $K\pi$ scattering and discuss the role of the $K_0^{*}(800)$ and $K^{*}(1430)$ resonances within the S-matrix formulation of thermodynamics. Using the empirical $K\pi$ phase shifts as input, we find that the $K\pi$ S-wave interactions provide part of the missing contribution to the strangeness susceptibility. Moreover, it is shown that the simplified treatment of the interactions in this channel, employed in the hadron resonance gas approach, leads to a systematic overestimate of the strangeness fluctuations.

The hydrostatic equilibrium and the stability against radial perturbation of charged strange quark stars composed of a charged perfect fluid are studied. For this purpose, it is considered that the perfect fluid follows the MIT bag model equation of state and the radial charge distribution follows a power-law. The hydrostatic equilibrium and the stability of charged strange stars are investigated through the numerical solutions of the Tolman-Oppenheimer-Volkoff equation and the Chandrasekhar’s pulsation equation, being these equations modified from their original form to include the electrical charge. In order to appreciably affect the stellar structure, it is found that the total charge should be of order ${10}^{20}[\mathrm{C}]$, implying an electric field of around ${10}^{22}[\mathrm{V}/\mathrm{m}]$. We found the electric charge that produces considerable effect on the structure and stability of the object is close to the star’s surface. We obtain that for a range of central energy density the stability of the star decreases with the increment of the total charge and for a range of total mass the electric charge helps to grow the stability of the stars under study. We show that the central energy density used to reach the maximum mass value is the same used to determine the zero eigenfrequency of the fundamental mode when the total charge is fixed, thus indicating that the maximum mass point marks the onset of instability. In other words, when fixing the total charge, the conditions $\frac{dM}{d{\rho }_c}>0$ and $\frac{dM}{d{\rho }_c}<0$ are necessary and sufficient to determine the stable and unstable equilibrium configurations regions against radial oscillations. We also consider another charge distribution, charge density proportional to the energy density, and show that our results do not depend on this choice and the conditions used to determine regions made of the stable and unstable charged equilibrium configurations are maintained.

We compute the one-loop photon contribution to the graviton self-energy on a de Sitter background and use it to solve the linearized Einstein equation for a point mass. Our results show that a comoving observer sees a logarithmic spatial running Newton’s constant. Equivalently, a static observer reports a secular suppression of the Newtonian potential.

Quantum graphity (QG) is a model of emergent geometry in which space is represented by a dynamical graph. The graph evolves under the action of a Hamiltonian from a high-energy pregeometric state to a low-energy state in which geometry emerges as a coarse-grained effective property of space. Here we show the results of numerical modeling of the evolution of the QG Hamiltonian, a process we term “ripening” by analogy with crystallographic growth. We find that the model as originally presented favors a graph composed of small disjoint subgraphs. Such a disconnected space is a poor representation of our Universe. A new term is introduced to the original QG Hamiltonian, which we call the hypervalence term. It is shown that the inclusion of a hypervalence term causes a connected latticelike graph to be favored over small isolated subgraphs.

By considering $f(R)$ gravity models, the cosmic evolution is modified with respect to the standard $\mathrm{\Lambda }\mathrm{CDM}$ scenario. In particular, the thermal history of particles results is modified. In this paper, we derive the evolution of relics particles (weakly interacting massive particles) assuming a reliable $f(R)$ cosmological solution and taking into account observational constraints. The connection to the PAMELA experiment is also discussed. Results are consistent with constraints coming from BICEP2 and PLANCK experiments.

We have studied photon motion around axially symmetric rotating Kerr black holes in the presence of a plasma with radial power-law density. It is shown that in the presence of a plasma, the observed shape and size of the shadow changes depending on the (i) plasma parameters, (ii) black hole spin, and (iii) inclination angle between the observer plane and the axis of rotation of the black hole. In order to extract the pure effect of the plasma influence on the black hole image, the particular case of the Schwarzschild black hole has also been investigated and it has been shown that the photon sphere around the spherically symmetric black hole is left unchanged under the plasma influence; however, the Schwarzschild black hole shadow size in the plasma is reduced due to the refraction of the electromagnetic radiation in the plasma environment of the black hole. The study of the energy emission from the black hole in plasma environment shows that in the presence of the plasma the maximal energy emission rate from the black hole decreases.

We investigate spherically symmetric, static spacetimes in Eddington-inspired Born-Infeld gravity coupled to Born-Infeld electrodynamics. The two constants, $b^2$ and $\kappa$ which parametrize the Born-Infeld structures in the electrodynamics (matter) and gravity sectors, characterize the features of our analytical solutions. Black holes or naked singularities are found to arise, depending on the values of $b^2$ and $\kappa$, as well as charge and mass. Several such solutions are classified and understood through the analysis of the associated metric functions for fixed $\kappa$, varying $b^2$ and vice versa. Further, we also compare the new metric functions with those for the known $b^2\to \infty$ (Maxwell) and the $\kappa \to 0$ (geonic black hole) cases. Interestingly, for a particular relation between these two parameters, $b^2=1/4\kappa$, $\kappa >0$, we obtain a solution resembling the well-known Reissner-Nordström line element, albeit some modifications. Using this particular solution as the background spacetime, we study null geodesics for Born-Infeld photons and also gravitational lensing. Among the interesting features we note (i) an increase in the radius of the photon sphere with increasing $\kappa$ and (ii) a net positive contribution in the leading order correction term involving $\kappa$, in the weak lensing formula for the deflection angle. We also investigate the effective potential and light propagation for various other solutions through numerics and plots. In summary, our paper is the first attempt towards figuring out how Born-Infeld structures in both the matter and gravity sectors can influence the nature and character of resulting gravitational fields.

Recently, an excess of high-energy positrons in our Galaxy has been observed by AMS-02. The spectrum obtained can be best fitted with the annihilation of $\sim \mathrm{TeV}$ dark matter particles. However, recent analysis of dwarf galaxies by Fermi/LAT observations highly constrains the TeV dark matter annihilation cross section and rules out the $b\overline{b}$ and all the leptophilic channels except the $4-\mu$ channel. In this article, I show that the remaining possible $4-\mu$ channel is also ruled out by using the observational data from cool-core clusters. Therefore, all the leptophilic channels that can account for the excess positrons seen in AMS-02, HEAT, and PAMELA are ruled out.

We investigate nonthermal gravitino production after tribrid inflation in supergravity, which is a variant of supersymmetric hybrid inflation where three fields are involved in the inflationary model and where the inflaton field resides in the matter sector of the theory. In contrast to conventional supersymmetric hybrid inflation, where nonthermal gravitino production imposes severe constraints on the inflationary model, we find that the “nonthermal gravitino problem” is generically absent in models of tribrid inflation, mainly due to two effects: (i) With the inflaton in tribrid inflation (after inflation) being lighter than the waterfall field, the latter has a second decay channel with a much larger rate than for the decay into gravitinos. This reduces the branching ratio for the decay of the waterfall field into gravitinos. (ii) The inflaton generically decays later than the waterfall field, and it does not produce gravitinos when it decays. This leads to a dilution of the gravitino population from the decays of the waterfall field. The combination of both effects generically leads to a strongly reduced gravitino production in tribrid inflation.

We show that inflationary models with broken rotational invariance generate testable off-diagonal signatures in the correlation between the $\mu$-type distortion and temperature fluctuations of the cosmic microwave background. More precisely, scenarios with a quadrupolar bispectrum asymmetry, usually generated by fluctuations of primordial vector fields, produce a nonvanishing $\mu \text{−}T$ correlation when $|{\ell }_1-{\ell }_2|=2$. Since spectral distortions are sensitive to primordial fluctuations up to very small scales, a cosmic variance limited spectral distortion experiment can detect such effects with a high signal-to-noise ratio.

We consider a broad class of interactions between radiation and a light scalar field, including both conformal and disformal couplings. Such a scalar field potentially acts on cosmological scales as dark energy and could also appear in modified gravity theories. We study the consequences of these couplings on the mixing between the scalar field and photons in galaxy clusters in the presence of a magnetic field. In particular we focus on the resulting turbulence-induced irregularities in the x-ray and UV bands. We find new bounds on the photon-to-scalar couplings, both conformal and disformal, which complement laboratory experiments and other astrophysical constraints.