Universe, Volume 4, Issue 1 (January 2018) – 19 articles

In this article, we start by presenting state-of-the-art methods allowing us to compute moments related to the globally conserved baryon number, by means of first principle resummed perturbative frameworks. We focus on such quantities for they convey important properties of the finite temperature and density equation of state, being particularly sensitive to changes in the degrees of freedom across the quark-hadron phase transition. We thus present various number susceptibilities along with the corresponding results as obtained by lattice quantum chromodynamics collaborations, and comment on their comparison. Next, omitting the importance of coupling corrections and considering a zero-density toy model for the sake of argument, we focus on corrections due to the small size of heavy-ion collision systems, by means of spatial compactifications. Briefly motivating the relevance of finite size effects in heavy-ion physics, in opposition to the compact star physics, we present a few preliminary thermodynamic results together with the speed of sound for certain finite size relativistic quantum systems at very high temperature. Full article

The charged-particle pseudorapidity density ( $\mathrm{d}{N}_{\mathrm{ch}}/\mathrm{d}\eta$ ) for p( $\overline{\mathrm{p}}$ )+p, p+A and A+A(B) collisions and the mean multiplicity $\langle N_{\mathrm{ch}}\rangle$ for e ${}^{-}$ +e ${}^{+}$ , ${\mathrm{e}}^{\pm }+\mathrm{p}$ , and p( $\overline{\mathrm{p}}$ )+p collisions are studied for a wide range of beam energies ( $\sqrt{s}$ ). Characteristic scaling patterns are observed for both $\mathrm{d}{N}_{\mathrm{ch}}/\mathrm{d}\eta$ and $\langle N_{\mathrm{ch}}\rangle$ , consistent with a thermal particle production mechanism for the bulk of the soft particles created in all of these systems. The scaling patterns found also validate an essential role for quark participants in these collisions. The measured values for $\mathrm{d}{N}_{\mathrm{ch}}/\mathrm{d}\eta$ and $\langle N_{\mathrm{ch}}\rangle$ are observed to factorize into contributions that depend on $log\left(\sqrt{s}\right)$ and the number of nucleon or quark participant pairs $N_{\mathrm{pp}}$ . The quantification of these contributions gives expressions that serve to systematize $\mathrm{d}{N}_{\mathrm{ch}}/\mathrm{d}\eta$ and $\langle N_{\mathrm{ch}}\rangle$ measurements spanning nearly 4 orders of magnitude in $\sqrt{s}$ and to predict their values as a function of $\sqrt{s}$ and $N_{\mathrm{pp}}$ . Full article

The Sunyaev–Zel’dovich (SZ) effect is a global distortion of the Cosmic Microwave Background (CMB) spectrum as a result of its interaction with a hot electron plasma in the intracluster medium of large structures gravitationally viralized such as galaxy clusters (GC). Furthermore, this hot gas of electrons emits X-rays due to its fall in the gravitational potential well of the GC. The analysis of SZ and X-ray data provides a method for calculating distances to GC at high redshifts. On the other hand, many galaxies and GC produce a Strong Gravitational Lens (SGL) effect, which has become a useful astrophysical tool for cosmology. We use these cosmological tests in addition to more traditional ones to constrain some alternative dark energy (DE) models, including the study of the history of cosmological expansion through the cosmographic parameters. Using Akaike and Bayesian Information Criterion, we find that the $wCDM$ and $\mathsf{\Lambda }CDM$ models are the most favoured by the observational data. In addition, we found at low redshift a peculiar behavior of slowdown of the universe, which occurs in dynamical DE models when we use data from GC. Full article

Can the holographic principle be extended beyond the well-known AdS/CFT correspondence? During the last couple of years, there has been a substantial amount of research trying to find answers for this question. In this work, we provide a review of recent developments of three-dimensional theories of gravity with higher spin symmetries. We focus in particular on a proposed holographic duality involving asymptotically flat spacetimes and higher spin extended ${\mathfrak{bms}}_3$ symmetries. In addition, we also discuss developments concerning relativistic and nonrelativistic higher spin algebras. As a special case, Carroll gravity will be discussed in detail. Full article

In recent literature, one-loop tests of the higher-spin AdS ${}_{d+1}$ /CFT ${}_d$ correspondences were carried out. Here, we extend these results to a more general set of theories in $d>2$ . First, we consider the Type B higher spin theories, which have been conjectured to be dual to CFTs consisting of the singlet sector of N free fermion fields. In addition to the case of N Dirac fermions, we carefully study the projections to Weyl, Majorana, symplectic and Majorana–Weyl fermions in the dimensions where they exist. Second, we explore theories involving elements of both Type A and Type B theories, which we call Type AB. Their spectrum includes fields of every half-integer spin, and they are expected to be related to the $U\left(N\right)/O\left(N\right)$ singlet sector of the CFT of N free complex/real scalar and fermionic fields. Finally, we explore the Type C theories, which have been conjectured to be dual to the CFTs of p-form gauge fields, where $p=\frac{d}{2}-1$ . In most cases, we find that the free energies at $O\left(N^0\right)$ either vanish or give contributions proportional to the free-energy of a single free field in the conjectured dual CFT. Interpreting these non-vanishing values as shifts of the bulk coupling constant $G_N\sim 1/(N-k)$ , we find the values $k=-1,-1/2,0,1/2,1,2$ . Exceptions to this rule are the Type B and AB theories in odd d; for them, we find a mismatch between the bulk and boundary free energies that has a simple structure, but does not follow from a simple shift of the bulk coupling constant. Full article

We investigate cubic interactions between a chiral superfield and higher spin superfields corresponding to irreducible representations of the $4D,\mathcal{N}=1$ super-Poincaré algebra. We do this by demanding an invariance under the most general transformation, linear in the chiral superfield. Following Noether’s method we construct an infinite tower of higher spin supercurrent multiplets which are quadratic in the chiral superfield and include higher derivatives. The results are that a single, massless, chiral superfield can couple only to the half-integer spin supermultiplets $(s+1,s+1/2)$ and for every value of spin there is an appropriate improvement term that reduces the supercurrent multiplet to a minimal multiplet which matches that of superconformal higher spins. On the other hand a single, massive, chiral superfield can couple only to higher spin supermultiplets of type $(2l+2,2l+3/2)$ (only odd values of s, $s=2l+1$ ) and there is no minimal multiplet. Furthermore, for the massless case we discuss the component level higher spin currents and provide explicit expressions for the integer and half-integer spin conserved currents together with a R-symmetry current. Full article

We consider a renormalizable extension of the minimal supersymmetric standard model endowed by an R and a gauged $B-L$ symmetry. The model incorporates chaotic inflation driven by a quartic potential, associated with the Higgs field which leads to a spontaneous breaking of U(1) ${}_{B-L}$ , and yields possibly detectable gravitational waves. We employ quadratic Kähler potential with a prominent shift-symmetric part proportional to $c_{-}$ and a tiny violation, proportional to $c_{+}$ , included in a logarithm with prefactor $-N<0$ . An explanation of the $\mu$ term of the MSSM is also provided, consistently with the low energy phenomenology, under the condition that one related parameter in the superpotential is somewhat small. Baryogenesis occurs via non-thermal leptogenesis which is realized by the inflaton’s decay to the lightest or next-to-lightest right-handed neutrino with masses lower than $1.8\times {10}^{13}$ GeV. Our scenario can be confronted with the current data on the inflationary observables, the baryon asymmetry of the universe, the gravitino limit on the reheating temperature and the data on the neutrino oscillation parameters, for 0.012 ≲ $c_{+}$ / $c_{-}$ ≲ 1/N and gravitino as light as 1 TeV. Full article

In four space-time dimensions, there exists a special infinite-parameter family of chiral modified gravity theories. They are most properly described by a connection field, with space-time metric being a secondary and derived concept. All these theories have the same number of degrees of freedom as general relativity, which is the only parity-invariant member of this family. Modifications of general relativity can be arranged so as to become important in regions with large curvature. In this paper, we review how a certain simple modification of this sort can resolve the Schwarzschild black-hole and Kasner anisotropic singularities of general relativity. In the corresponding solutions, the fundamental connection field is regular in space-time. Full article

The phase diagram of strongly interacting matter can be explored by analyzing data of heavy-ion collisions at different center of mass collision energies. For investigating the space-time structure of the hadron emission source, Bose-Einstein or HBT correlation measurements are among the best tools. In this paper we present the latest results from the PHENIX experiment of the Relativistic Heavy Ion Collider (RHIC) on such measurements in $\sqrt{s_{NN}}=$ 39 GeV and 62 GeV Au + Au collisions. Full article

More and more works deal with statistical systems far from equilibrium, dominated by unidirectional stochastic processes, augmented by rare resets. We analyze the construction of the entropic distance measure appropriate for such dynamics. We demonstrate that a power-like nonlinearity in the state probability in the master equation naturally leads to the Tsallis (Havrda–Charvát, Aczél–Daróczy) q-entropy formula in the context of seeking for the maximal entropy state at stationarity. A few possible applications of a certain simple and linear master equation to phenomena studied in statistical physics are listed at the end. Full article

The 2 + 1 + 1 decomposition of space-time is useful in monitoring the temporal evolution of gravitational perturbations/waves in space-times with a spatial direction singled-out by symmetries. Such an approach based on a perpendicular double foliation has been employed in the framework of dark matter and dark energy-motivated scalar-tensor gravitational theories for the discussion of the odd sector perturbations of spherically-symmetric gravity. For the even sector, however, the perpendicularity has to be suppressed in order to allow for suitable gauge freedom, recovering the 10th metric variable. The 2 + 1 + 1 decomposition of the Einstein–Hilbert action leads to the identification of the canonical pairs, the Hamiltonian and momentum constraints. Hamiltonian dynamics is then derived via Poisson brackets. Full article

There was an obvious parallelism between the lives of the two Hungarian men of science: János Bolyai a genial geometer and Frigyes Károlyházy a missionary of education in the field of modern physics. Both of them selected and solved a complex problem that was too difficult for the majority of scientists of their times. Their work was criticized authoritatively by high rank scientists of their discipline, which led to their withdrawal from science. The belated recognition of the Non-Euclidean geometry and the gravity inspired quantum-mechanical state reduction put these excellent achievements to their deserved right place. Full article

We review various methods for finding exact solutions of higher spin theory in four dimensions, and survey the known exact solutions of (non)minimal Vasiliev’s equations. These include instanton-like and black hole-like solutions in (A)dS and Kleinian spacetimes. A perturbative construction of solutions with the symmetries of a domain wall is also described. Furthermore, we review two proposed perturbative schemes: one based on perturbative treatment of the twistor space field equations followed by inverting Fronsdal kinetic terms using standard Green’s functions; and an alternative scheme based on solving the twistor space field equations exactly followed by introducing the spacetime dependence using perturbatively defined gauge functions. Motivated by the need to provide a higher spin invariant characterization of the exact solutions, aspects of a proposal for a geometric description of Vasiliev’s equation involving an infinite dimensional generalization of anti de Sitter space are revisited and improved. Full article

We study a class of non-unitary $\mathfrak{so}(2,d)$ representations (for even values of d), describing mixed-symmetry partially massless fields which constitute natural candidates for defining higher-spin singletons of higher order. It is shown that this class of $\mathfrak{so}(2,d)$ modules obeys of natural generalisation of a couple of defining properties of unitary higher-spin singletons. In particular, we find out that upon restriction to the subalgebra $\mathfrak{so}(2,d-1)$ , these representations branch onto a sum of modules describing partially massless fields of various depths. Finally, their tensor product is worked out in the particular case of $d=4$ , where the appearance of a variety of mixed-symmetry partially massless fields in this decomposition is observed. Full article

Universal hyperbolic geometry gives a purely algebraic approach to the subject that connects naturally with Einstein’s special theory of relativity. In this paper, we give an overview of some aspects of this theory relating to triangle geometry and in particular the remarkable new analogues of midpoints called sydpoints. We also discuss how the generality allows us to consider hyperbolic geometry over general fields, in particular over finite fields. Full article

It was argued recently that the holographic higher spin theory features non-local interactions. We further elaborate on these results using the Mellin representation. The main difficulty previously encountered in this method is that the Mellin amplitude for the free theory correlator is ill-defined. To resolve this problem, instead of literally applying the standard definition, we propose to define this amplitude by linearity using decompositions, where each term has the associated Mellin amplitude well-defined. Up to a sign, the resulting amplitude is equal to the Mellin amplitude for the singular part of the quartic vertex in the bulk theory and, hence, can be used to analyze bulk locality. From this analysis we find that the scalar quartic self-interaction vertex in the holographic higher spin theory has a singularity of a special form, which can be distinguished from generic bulk exchanges. We briefly discuss the physical interpretation of such singularities and their relation to the Noether procedure. Full article

Knowledge of the equation of state (EoS) of cold and dense baryonic matter is essential for the description of properties of neutron stars (NSs). With an increase of the density, new baryon species can appear in NS matter, as well as various meson condensates. In previous works, we developed relativistic mean-field (RMF) models with hyperons and $\Delta$ -isobars, which passed the majority of known experimental constraints, including the existence of a $2M_{\odot }$ neutron star. In this contribution, we present results of the inclusion of ${\rho }^{-}$ -meson condensation into these models. We have shown that, in one class of the models (so-called KVOR-based models, in which the additional stiffening procedure is introduced in the isoscalar sector), the condensation gives only a small contribution to the EoS. In another class of the models (MKVOR-based models with additional stiffening in isovector sector), the condensation can lead to a first-order phase transition and a substantial decrease of the NS mass. Nevertheless, in all resulting models, the condensation does not spoil the description of the experimental constraints. Full article