International Journal of Modern Physics A

We revisit the decoupling effects associated with heavy particles in the renormalization group running of the vacuum energy in a mass-dependent renormalization scheme. We find the running of the vacuum energy stemming from the Higgs condensate in the entire energy range and show that it behaves as expected from the simple dimensional arguments, meaning that it exhibits the quadratic sensitivity to the mass of the heavy particles in the infrared regime. The consequence of such a running to the fine-tuning problem with the measured value of the Cosmological Constant is analyzed and the constraint on the mass spectrum of a given model is derived. We show that in the Standard Model (SM) this fine-tuning constraint is not satisfied while in the massless theories this constraint formally coincides with the well-known Veltman condition. We also provide a remarkably simple extension of the SM where saturation of this constraint enables us to predict the radiative Higgs mass correctly. Generalization to constant curvature spaces is also given.

I discuss a general class of models where the inflation is driven by supersymmetry breaking with the superpartner of the goldstino (sgoldstino) playing the role of the inflaton. Imposing an R-symmetry allows to satisfy easily the slow-roll conditions, avoiding the so-called $\eta$-problem, and leads to two different classes of small field inflation models; they are characterized by an inflationary plateau around the maximum of the scalar potential, where R-symmetry is either restored or spontaneously broken, with the inflaton rolling down to a minimum describing the present phase of our universe. Inflation can be driven by either an F- or a D-term, while the minimum has a positive tuneable vacuum energy. The models agree with cosmological observations and in the simplest case predict a tensor-to-scalar ratio of primordial perturbations $10^{-9}\lesssim r\lesssim 10^{-4}$ and an inflation scale $10^{10}\mathrm{\text{GeV}}\lesssim H_{\ast }\lesssim 10^{12}\mathrm{\text{GeV}}$.

After a brief introduction to the AdS/CFT holography, a tachyon inflation will be discussed in the framework of holographic cosmology. The model is based on a holographic braneworld scenario with an effective tachyon field on a D3-brane located at the holographic bound of an asymptotic ADS₅ bulk.

I will discuss how to use precision data from orbital motion of binary pulsars to constrain models of ultra-light dark matter (masses in the range $10^{-21}$ eV to $10^{-18}$ eV).

Inspired by recent studies of high-scale decay constant or flavorful QCD axions, we review and clarify their existence in effective string models with anomalous U(1) gauge groups. We find that such models, when coupled to charged scalars getting vacuum expectation values, always have one light axion, whose mass can only come from nonperturbative effects. If the main nonperturbative effect is from QCD, then it becomes a Peccei–Quinn axion candidate for solving the strong CP problem. We then study simple models with universal Green–Schwarz mechanism and only one charged scalar field: in the minimal gaugino condensation case the axion mass is tied to the supersymmetry breaking scale and cannot be light enough, but slightly refined models maintain a massless axion all the way down to the QCD scale. Both kinds of models can be extended to yield intermediate scale axion decay constants. Finally, we gauge flavorful axion models under an anomalous U(1) and discuss the axion couplings which arise.

It is shown that, contrary to a widespread prejudice, massless higher spin (HS) field theories can be defined in flat space–time. Examples of Yang–Mills-like theories with infinite many local fields of any spin are constructed explicitly in any dimension, along with Chern–Simons-like models in any odd dimension. These theories are defined via actions invariant under HS gauge transformations and their equations of motion are derived. It is also briefly explained why these theories circumvent well-known no-go theorems.

We consider the consequences of the leading quartic corrections to the Einstein–Hilbert action of gravity at low energy. Using the equivalence between the scalar $R^2$ contribution and a scalar-tensor field theory, we analyze the possible ways of detecting the associated scalaron and suggest that short distance tests of gravity, and in particular future tests of Newton’s law aboard satellites, would provide the best environment to detect such a modification of gravity. We also analyze the regimes for which the $R^2$ theory would result as a low energy manifestation of putative high energy UV completions involving extra dimensions. In the four-dimensional $N=1$ supergravity limit of such extra-dimensional models, the $R^2$ models would emerge from the stabilization of a nearly no-scale superfield such as the ones associated to the Kähler modulus corresponding to the breathing mode of a six-dimensional compactification.

We aim to explain some astrophysical phenomena in low density astronomical regions without invoking dark matter. We propose that the effective theory of gravity could have individual terms which become dominant in respective regions, and effectively describe the gravitational phenomena there. In the outer region of galaxies, the effective theory of gravity could be the Weyl gravity, which is the unique local scale symmetric metric theory. Inside the bulge, however, general relativity is the effective description of gravity. Hence, we propose the Einstein–Weyl gravity as a scale-dependent effective description of the gravitational phenomena. At high densities or high curvature regions, the effective description for gravity is provided by Einstein’s general relativity, whereas in the low density regions it is provided by the Weyl gravity. We then solve the field equations of Weyl gravity in the outer region of galaxies and determine the geometry in which stars move with the same rotational velocity independent of their distance from the center of the galaxy. We then utilize this metric solution to calculate the deflection angle of light from a distant source by a galaxy cluster. The deflection angle calculated in Schwarzschild–de Sitter-like space includes contributions from both the cosmological constant $\mathrm{\Lambda }$ and the Mannheim–Kazanas parameter $\gamma$. There are conflicting results on the deflection angle for light in Weyl gravity in the literature. Our result agrees with some and disagrees with many. We point out possible reasons for this discrepancy.

According to the standard cosmological model, thermodynamic conditions of the early Universe were such that nuclear matter existed in the state of quark–gluon plasma, rather than hadrons. On the other hand, it is generally believed that quantum gravity effects become ever more stronger as we approach the Big Bang, in particular, we expect that the phenomenon of space–time noncommutativity will be significant. Thus we are led to consider the properties of quarks and gluons in noncommutative space–time. For this, we employ the $SO{(2,3)}_{\star }$ model of noncommutative gravity. As a first step towards the full theoretical treatment of the effects of noncommutativity on quark–gluon plasma, our main goal in this paper is to consistently incorporate Yang–Mills gauge fields in the $SO{(2,3)}_{\star }$ framework and investigate their coupling to gravity that arises due to space–time noncommutativity. We construct an action that is invariant under deformed $SO{(2,3)}_{\star }\times SU{(N)}_{\star }$ gauge transformations and expand it perturbatively in orders of the canonical deformation parameter ${𝜃}^{\alpha \beta }$ via Seiberg–Witten map. In particular, we analyze the flat-space–time limit and demonstrate that residual noncommutativity induces various new couplings of quarks and gluons.

We analyze a tachyon cosmological model based on the dynamics of a 3-brane in the bulk of the second Randall–Sundrum model. This model contains extended general warp functions, i.e. generalized bulk geometry. We study a power law warp factor in cosmological context. This type of warp factor generates an inverse power law tachyonic potential.

First, I summarize the current status of dark energy, including methods to use data to separate between general-relativity and modified-gravity scenarios for the accelerating universe. Then, I discuss recent results from the Dark Energy Survey, currently the world’s leading experiment mapping large-scale structure in the universe. Year-1 DES analysis performed in 2017 included the combination of galaxy clustering, cosmic shear, and their cross-correlation to impose constraints on key cosmological parameters, while upcoming Year-3 and -5 analyses will dramatically improve those constraints. I discuss some of the challenges in this complex analysis, its results, and the more general path forward toward better understanding of dark matter and dark energy in the universe. I also comment on the foremost tension in the field of cosmology today: between local measurements of the Hubble constant from type Ia supernovae, and global measurements from the cosmic microwave background anisotropies.

The relativistically broadened Fe K$\alpha$ line, originating from the accretion disc in a vicinity of a super massive black hole, is observed in only less than 50% of type 1 Active Galactic Nuclei (AGN). In this study, we investigate could this lack of detections be explained by the effects of certain parameters of the accretion disc and black hole, such as the inclination, the inner and outer radius of disc and emissivity index. In order to determine how these parameters affect the Fe K$\alpha$ line shape, we simulated about 60,000 Fe K$\alpha$ line profiles emitted from the relativistic disc.

Based on simulated line profiles, we conclude that the lack of the Fe K$\alpha$ line detection in type 1 AGN could be caused by the specific emitting disc parameters, but also by the limits in the spectral resolution and sensitivity of the X-ray detectors.

Motivated by the study of soluble models of quantum field theory, we illustrate a new type of topological effect by comparing the constructions of canonical Klein–Gordon quantum fields on the two-dimensional de Sitter spacetime as opposed to its double covering. We show that while the commutators of the two fields coincide locally, the global topological differences make the theories drastically different. Many of the well-known features of de Sitter quantum field theory disappear. In particular, there is nothing like a Bunch–Davies vacuum. Correspondingly, even though the local horizon structure is the same for the two universes, there is no Hawking–Gibbons thermal state. Finally, there is no complementary series of fields.

We focus on the integrable properties in low-dimensional holography. The motivation is that most of the integrable structures underlying holographic duality survive weak-strong coupling transition. We found relation between certain integrable structures in low-dimensional holography and key characteristics of the theories. We propose generalizations to higher spin (HS) theories including Sachdev–Ye–Kitaev (SYK) model. We comment on some of the intriguing relations found in this study.

We consider a big class of chiral four-dimensional string models which bear $SO(10)\times {SO(8)}^2\times {U(1)}^2$ gauge symmetry. Supersymmetry (SUSY) is spontaneously broken via the Scherk–Schwarz mechanism associated with two large internal dimensions. We study two central issues in nonsupersymmetric string phenomenology: the suppression of the cosmological constant and the decompactification problem. We identify a subclass of chiral nonsupersymmetric models with fermion–boson degeneracy leading to exponentially suppressed cosmological constant. In addition, we propose a novel solution to the decompactification problem related to the cancellation of the linear growth of the gauge part of the thresholds with a similar growth of the universal part. We show that the proposed mechanism resolves the decompactification problem related to the non-Abelian gauge couplings in approximately 40% of the models under consideration.

Quantum spinor matter in extremal conditions (high densities and temperatures, presence of strong magnetic fields) have drawn the attention of researchers in diverse areas of contemporary physics, ranging from cosmology, high-energy and astroparticle physics to condensed matter physics. We study an impact of the confining boundary conditions on the properties of physical systems with hot dense magnetized ultrarelativistic spinor matter and elucidate a significant role of boundaries for such systems.

Double-beta decay (DBD) is a rare nuclear process of great interest due to its potential to provide information about physics beyond the Standard Model (BSM). For example, the discovery of the neutrinoless double-beta $(0\nu \beta \beta )$ decay mode could give information about important issues such as possible violations of Lorentz symmetry and lepton number, nature of neutrinos (are they Dirac- or Majorana-like particles?), neutrino absolute masses, neutrino mass hierarchy, existence of heavy (sterile) neutrinos, etc. In the theoretical study of DBD, one needs a precise calculation of the nuclear matrix elements (NMEs) and phase space factors (PSFs) entering the half-lives formulas, for different decay modes, transitions and mechanisms of occurrence. Reliable computations of these quantities may result in reliable predictions of DBD half-lives and constrains of the BSM parameters related to the possible mechanisms that can contribute to the $0\nu \beta \beta$ decay.

In this paper, I briefly review the theoretical challenges in the study of $0\nu \beta \beta$ decay. I describe the computation of the NMEs and PSFs and present results for a number of selected nuclei. Then, I show the broader potential of this process to provide information about BSM physics and present new upper limits for parameters associated with light neutrino, heavy neutrino and SUSY exchange mechanisms. Finally, I suggest a more consistent approach to calculate the NMEs and PSFs, namely to compute directly their product and discuss some possibilities to reduce the errors related to the uncertain value of the axial-vector constant.

We know about Dark Matter (DM) only thanks to Gravity. I will review the idea that this must be the case because DM is nothing else than a manifestation of Gravity itself, in the guise of an additional, massive, spin-2 particle.

Informational dependence between statistical or quantum subsystems can be described with Fisher information matrix or Fubini-Study metric obtained from variations/shifts of the sample/configuration space coordinates. Using these (noncovariant) objects as macroscopic constraints, we consider statistical ensembles over the space of classical probability distributions (i.e. in statistical space) or quantum wave functions (i.e. in Hilbert space). The ensembles are covariantized using dual field theories with either complex scalar field (identified with complex wave functions) or real scalar field (identified with square roots of probabilities). We construct space–time ensembles for which an approximate Schrodinger dynamics is satisfied by the dual field (which we call infoton due to its informational origin) and argue that a full space–time covariance on the field theory side is dual to local computations on the information theory side. We define a fully covariant information-computation tensor and show that it must satisfy certain conservation equations.

Then we switch to a thermodynamic description of the quantum/statistical systems and argue that the (inverse of) space–time metric tensor is a conjugate thermodynamic variable to the ensemble-averaged information-computation tensor. In (local) equilibrium, the entropy production vanishes, and the metric is not dynamical, but away from the equilibrium the entropy production gives rise to an emergent dynamics of the metric. This dynamics can be described approximately by expanding the entropy production into products of generalized forces (derivatives of metric) and conjugate fluxes. Near equilibrium, these fluxes are given by an Onsager tensor contracted with generalized forces and on the grounds of time-reversal symmetry, the Onsager tensor is expected to be symmetric. We show that a particularly simple and highly symmetric form of the Onsager tensor gives rise to the Einstein–Hilbert term. This proves that general relativity is equivalent to a theory of nonequilibrium (thermo)dynamics of the metric, but the theory is expected to break down far away from equilibrium where the symmetries of the Onsager tensor are to be broken.

We study the transverse Kähler structure of the Sasaki–Einstein space $T^{1,1}$. A set of local holomorphic coordinates is introduced and a Sasakian analogue of the Kähler potential is given. We investigate deformations of the Sasaki–Einstein structure preserving the Reeb vector field, but modifying the contact form. For this kind of deformations, we consider the Sasaki–Ricci flow which converges in a suitable sense to a Sasaki–Ricci soliton. Finally, it is described the constructions of Hamiltonian holomorphic vector fields and Hamiltonian function on the $T^{1,1}$ manifold.