New Phenomena and New States of Matter in the Universe

The following sections are included:

• Dedication
• Foreword
• Preface
• Contents
• List of Figures
• List of Tables

Gravitational waves are the only messenger able to probe physical processes that have occurred in the early universe, since the cosmic plasma is opaque to electromagnetic radiation at high redshift (z > 1100). Quantum fluctuations of a putative scalar field that drives inflation are able to produce a stochastic gravitational wave background. Additional mechanisms contributing to such a background are bubble collisions, sound waves and turbulence generated during the electroweak or the QCD phase transitions. Gravitational waves produced by these mechanisms could be detected by the planned Big Bang Observer space-based interferometer (phase 2) but unfortunately their signals are immersed in the astrophysical background generated by binary systems constituted by compact objects, despite different techniques proposed for separation of the signals. If the QCD transition is crossover and if the turbulence level is high enough, the expected signal is above the astrophysical background and detection would be possible. Modifications of the standard model leading to a strong electroweak transitions would also render possible the detection of the signal. A cosmic string network is able to generate a detectable gravitational wave background if the string tension is larger than Gµ/c² ≃ 1.4 × 10⁻¹⁷.

In the binary-driven hypernova model of long gamma-ray bursts, a carbon–oxygen star explodes as a supernova in presence of a neutron star binary companion in close orbit. Hypercritical (i.e. highly super-Eddington) accretion of the ejecta matter onto the neutron star sets in, making it reach the critical mass with consequent formation of a Kerr black hole. We have recently shown that, during the accretion process onto the neutron star, fast neutrino flavor oscillations occur. Numerical simulations of the above system show that a part of the ejecta keeps bound to the newborn Kerr black hole, leading to a new process of hypercritical accretion. We address here the occurrence of neutrino flavor oscillations given the extreme conditions of high density (up to 10¹² g cm⁻³) and temperatures (up to tens of MeV) inside this disk. We estimate the evolution of the electronic and non-electronic neutrino content within the two-flavor formalism (ν_eν_x) under the action of neutrino collective effects by neutrino self-interactions. We find that neutrino oscillations inside the disk have frequencies between ∼ (10⁵ − 10⁹) s⁻¹, leading the disk to achieve flavor equipartition. This implies that the energy deposition rate by neutrino annihilation $(\nu +\overline{\nu }\to e^{-}+e^{+})$ in the vicinity of the Kerr black hole, is smaller than previous estimates in the literature not accounting for flavor oscillations inside the disk. The exact value of the reduction factor depends on the ν_e and ν_x optical depths but it can be as high as ∼5.

Thirty years since the detection of the first TeV source, thanks to the successful observations by the Whipple Observatory in 1989, very-high-energy (VHE) gamma-ray astronomy has finally reached full maturity. The recent observational breakthroughs in the detection of sub-Teraelectronvolt emission from gamma-ray bursts, the searches for counterparts of the VHE neutrinos from IceCube, as well as the fast expansion of the sensitivity of observations at the highest energies, in its pursuit of Galactic PeVatrons and the origin of high-energy cosmic rays — all of these accomplishments reveal a picture of the high-energy cosmos that goes much beyond previous expectations, and demonstrate how far the field has evolved in terms of its technological capabilities. Ground-based gamma-ray astronomy is in fact a pivotal player in the nascent multi-messenger astronomy, bridging the non-electromagnetic events with the electromagnetic signals and consequently their associated astrophysical sources. In this contribution I will review the main recent observational results of the field, which are closely connected to the search for the origin of cosmic-rays, concentrating on detailing the efforts towards establishing the counterparts to multi-messenger events. I will also sketch the landscape of future experiments and observatories that will enter operation in the next decade, and the perspective they lay out for the field of Astroparticle Physics.

The current view of the universe is the result of two major theories elaborated in the 20th century: General Relativity and Quantum Field theory. According to such theories the basic stuff of the universe is formed by space–time and matter fields existing on space–time. Such a view, articulated in the standard cosmological model of the Hot Big Bang, is under question because of the tension created by the discrepancies between the observed dynamics of visible matter and the cinematic expectations from General Relativity…

In this chapter, we investigate the structure and composition of hot neutron star matter and proto-neutron stars. Such objects are made of baryonic matter that is several times denser than atomic nuclei and tens of thousands of times hotter than the matter in the core of our Sun. The relativistic finite-temperature Green function formalism is used to formulate the expressions that determine the properties of such matter in the framework of the density-dependent mean-field approach. Three different sets of nuclear parameterizations are used to solve the many-body equations and to determine the models for the equation of state of ultra-hot and dense stellar matter. The meson–baryon coupling schemes and the role of the Δ(1232) baryon in proto-neutron star matter are discussed in great detail. The use of the non-local three-flavor Nambu–Jona-Lasinio model to describe quark matter, the hadron-quark composition of dense baryonic matter at zero temperature is discussed. General relativistic models of non-rotating as well as rotating proto-neutron stars are presented in part two of our study.

A review will be presented on the algebraic extension of the standard Theory of General Relativity (GR) to the pseudo-complex formulation (pc-GR). The pc-GR predicts the existence of a dark energy outside and inside the mass distribution, corresponding to a modification of the GR-metric. The structure of the emission profile of an accretion disk changes also inside a star. Discussed are the consequences of the dark energy for cosmological models, permitting different outcomes on the evolution of the universe.

According to the Standard Model of Cosmology, ∼70% of the universe is composed by dark energy, ∼25% of dark matter and only the remaining ∼5% of known baryonic matter. Although we have some clues about the main properties of dark energy and dark matter, we still do not know exactly what they are, neither have we detected them in laboratory. The effects of dark energy and dark matter can be explained in an alternative form, by modifying the Einstein–Hilbert gravitational action, such that the resulting field equations contain new terms which, in principle, are capable of describing such dark sector effects. A well-behaved alternative gravity theory must also work in the stellar scales. As long as we still do not know the equation of state of super dense matter within neutron stars, a possibility to explain some recently detected massive pulsars comes exactly from those new terms in the field equations of Alternative Gravity Theories. Moreover, the recently emerged gravitational wave astrophysics may also be a field of applications to Alternative Gravity. In this chapter we review the importance of Alternative Gravity and its applications in the equilibrium configurations of neutron stars. We show how the recent gravitational wave astrophysics can be used to constraint among different Alternative Gravity Theories.

In this contribution, we present for the first time a scenario according to which early quark deconfinement in compact stars is triggered by the Bose-Einstein condensation (BEC) of a light sexaquark (S) with a mass m_S < 2054 MeV, that has been suggested as a candidate particle to explain the baryonic dark matter in the universe. The onset of S BEC marks the maximum mass of hadronic neutron stars and it occurs when the condition for the baryon chemical potential μ_b = m_S/2 is fulfilled in the center of the star, corresponding to M_onset < 0.7M_⊙. In the gravitational field of the star the density of the BEC of S increases until a new state of the matter is attained, where each of the S-states got dissociated into a triplet of color-flavor-locked (CFL) diquark state. These diquarks are the Cooper pairs in the color superconducting CFL phase of quark matter, so that the developed scenario corresponds to a BEC–BCS transition in strongly interacting matter. For the description of the CFL phase, we develop here for the first time the three-flavor extension of the density-functional formulation of a chirally symmetric Lagrangian model of quark matter where confining properties are encoded in a divergence of the scalar self-energy at low densities and temperatures.