International Journal of Modern Physics D

We provide an update to S. di Serego Alighieri, Int. J. Mod. Phys. D 24 (2015) 1530016, arXiv:1501.06460 [astro-ph.CO], focusing on recent developments regarding constraints on Cosmic Polarization Rotation (CPR), also known as Cosmic Birefringence (CB), derived from Cosmic Microwave Background (CMB) polarization data.

Optical lattice clocks (OLCs) enable us to measure time and frequency with a fractional uncertainty at $10^{-18}$ level, which is 2 orders of magnitude better than Cs clocks. In this paper, after briefly reviewing OLCs and the history of testing the fundamental principles of general relativity, we report our experiments of measuring the gravitational redshift between RIKEN and The University of Tokyo, and at Tokyo Skytree using transportable OLCs. We also discuss a couple of future applications of OLCs such as detecting gravitational waves in space and relativistic geodesy. The possibility of testing second-order parametrized post-Newtonian potential around the Earth is also mentioned.

Within the paradigm of nonperturbative Einstein gravity we study continuous curvature manifolds which possess de Sitter interiors and Kerr exteriors. These manifolds could represent the spacetime of rotating gravastars or other similar black hole mimickers. The scheme presented here allows for a $C^n$ metric transition from the exactly de Sitter interior to the exactly Kerr exterior, with n arbitrarily large. Generic properties that such models must possess are discussed, such as the changing of the topology of the ergosphere from $S^2$ to $S^1\times S^1$. It is shown how in the outer layers of the transition region (the “atmosphere” as it is often called in astrophysics) the dominant/weak and strong energy conditions can be respected. However, much like in the case of its static spherically symmetric gravastar counterpart, there must be some assumptions imposed in the atmosphere for the energy conditions to hold. These assumptions turn out to not be severe. The class of manifolds presented here is expected to possess all the salient features of the fully generic case. Strictly speaking, a number of the results are also applicable to the locally anti-de Sitter core scenario, although we focus on the case of a positive cosmological constant.

In this paper, we investigate the quintessential inflation in the logarithmic Cartan $F(R)$ gravity associated with local Lorenz symmetry. A small logarithmic modification of general relativity has the potential to introduce both inflation and dark energy. We evaluate the time evolution of the Universe such as inflation, reheating, and dark energy. The parameters in the model are fixed to introduce the inflation and the dark energy scales. We show that the CMB fluctuations induced by the inflation are consistent with the current observations. In the reheating process, it is possible to achieve the reheating temperature required for nucleosynthesis in the Big Bang scenario. It can be seen that by choosing an appropriate value for the scalaron field after reheating, the scalaron field again dominates the energy of the Universe and causes the current accelerating expansion as dark energy.

This paper explores a cosmological reconstruction scheme in the background of $f(Q)$ gravity theory from a Holographic perspective. The basic motivation for this work is that the reconstruction is performed from a holographic origin, which has its roots in the black hole thermodynamics and quantum gravity. Dark energy models inspired by holographic prescription are used to reconstruct the $f(Q)$ gravity models. Two such models, namely, the Granda–Oliveros holographic dark energy model and its generalization, the Chen-Jing model, are considered for the study. Different scale factors are used and a thorough reconstruction scheme is set up using the dark energy models. The observationally constrained values of the free model parameters have been used to form the reconstructed models. Finally, a thorough investigation of the energy conditions has been performed to check the cosmological viability of the reconstructed $f(Q)$ models. As an outcome, we get some very promising and cosmologically viable $f(Q)$ models that present some interesting properties and demand further investigation. Finally, a method is discussed how the constructed $f(Q)$ models can be reconciled with a generalized holographic dark energy.

The Second Flavor of Hydrogen Atoms (SFHAs) are hydrogen atoms that have only the S-states, these states being described by the second solution of the standard Dirac equation outside the proton. Their existence is evidenced by four different types of atomic/molecular experiments. The SFHA-based explanations of two perplexing astrophysical puzzles, i.e. of the abnormal absorption in the 21 cm spectral line from the early universe and of the less nubby dark matter distribution in the universe than predicted by the standard cosmology, made the SFHA a compelling candidate for the baryonic dark matter. In distinction to the overwhelming majority of dark matter hypotheses resorting to never discovered subatomic particles or to modifications of the existing physical laws, the SFHA, being based on the standard Dirac equation of quantum mechanics, does not go beyond the Standard Model of particle physics and does not require modifying the existing physical laws, which is why it is favored by the Occam razor principle. In the present paper, first, we show that the SFHA, as the candidate for the baryonic dark matter, does not contradict the known cosmological constraints. Second, based on the cosmological data we demonstrate that the SFHA seems to constitute most of the baryonic dark matter. We also refer to the latest results showing that neutron stars can slowly but continuously generate the baryonic dark matter in the form of the SFHA. Finally, we discuss/motivate some relevant future laboratory experiments and astrophysical observations.

The concept of exotic charged dust is introduced here to represent dark matter. The term “exotic” means that the dust is not composed of normal matter, and the charge — for lack of a better term — is not an electric charge. It is also shown that the often-used approximate expression for dark matter density corresponds to an exact solution of the coupled Einstein–Maxwell equations for charged dust. The Einstein field equations tell us that for a flat universe, the cosmological constant acts as a gravitating negative energy distribution that is gravitationally repulsive; following Schwinger, it is shown that the vacuum must have a negative energy spectrum if QFT is to be gauge invariant.

A comprehensive description is given of a spacetime model of an accelerating massive particle. The particle radiates gravitational waves with optical shear. The wave fronts are smoothly deformed spheres and the particle experiences radiation reaction, similar to an accelerating charged particle, and a loss of mass described by a Bondi mass-loss formula. The spacetime is one of the Bondi–Sachs forms but presented in a form here which is particularly suited to the construction of the model particle. All details of the calculations are given. A detailed examination of the gravitational field of the particle is provided which illustrates the presence of gravitational radiation and also exhibits, in the form of a type of singularity found in some Robinson–Trautman spacetimes, the absence of an external field to supply energy to the particle.