Dark Energy: The Shadowy Reflection of Dark Matter?

In this article, we review a series of recent theoretical results regarding a conventional approach to the dark energy (DE) concept. This approach is distinguished among others for its simplicity and its physical relevance. By compromising General Relativity (GR) and Thermodynamics at cosmological scale, we end up with a model without DE. Instead, the Universe we are proposing is filled with a perfect fluid of self-interacting dark matter (DM), the volume elements of which perform hydrodynamic flows. To the best of our knowledge, it is the first time in a cosmological framework that the energy of the cosmic fluid internal motions is also taken into account as a source of the universal gravitational field. As we demonstrate, this form of energy may compensate for the DE needed to compromise spatial flatness, while, depending on the particular type of thermodynamic processes occurring in the interior of the DM fluid (isothermal or polytropic), the Universe depicts itself as either decelerating or accelerating (respectively). In both cases, there is no disagreement between observations and the theoretical prediction of the distant supernovae (SNe) Type Ia distribution. In fact, the cosmological model with matter content in the form of a thermodynamically-involved DM fluid not only interprets the observational data associated with the recent history of Universe expansion, but also confronts successfully with every major cosmological issue (such as the age and the coincidence problems). In this way, depending on the type of thermodynamic processes in it, such a model may serve either for a conventional DE cosmology or for a viable alternative one.Comment: Review article, 38 pages, 5 figures, accepted for publication in Entrop

Geodesic motions versus hydrodynamic flows in a gravitating perfect fluid: Dynamical equivalence and consequences

Stimulated by the methods applied for the observational determination of masses in the central regions of the AGNs, we examine the conditions under which, in the interior of a gravitating perfect fluid source, the geodesic motions and the general relativistic hydrodynamic flows are dynamically equivalent to each other. Dynamical equivalence rests on the functional similarity between the corresponding (covariantly expressed) differential equations of motion and is obtained by conformal transformations. In this case, the spaces of the solutions of these two kinds of motion are isomorphic. In other words, given a solution to the problem "hydrodynamic flow in a perfect fluid", one can always construct a solution formally equivalent to the problem "geodesic motion of a fluid element" and vice versa. Accordingly, we show that, the observationally determined nuclear mass of the AGNs is being overestimated with respect to the real, physical one. We evaluate the corresponding mass-excess and show that it is not always negligible with respect to the mass ofthe central dark object, while, under circumstances, can be even larger than the rest-mass of the circumnuclear gas involved.Comment: LaTeX file, 22 page

Newtonian nonlinear hydrodynamics and magnetohydrodynamics

We use covariant methods to analyse the nonlinear evolution of self-gravitating, non-relativistic media. The formalism is first applied to imperfect fluids, aiming at the kinematic effects of viscosity, before extended to inhomogeneous magnetised environments. The nonlinear electrodynamic formulae are derived and successively applied to electrically resistive and to highly conductive fluids. By nature, the covariant equations isolate the magnetic effects on the kinematics and the dynamics of the medium, combining mathematical transparency and physical clarity. Employing the Newtonian analogue of the relativistic 1+3 covariant treatment, also facilitates the direct comparison with the earlier relativistic studies and helps to identify the differences in an unambiguous way. The purpose of this work is to set the framework and take a first step towards the detailed analytical study of complex nonlinear systems, like non-relativistic astrophysical plasmas and collapsing protogalactic clouds.Comment: Typos corrected, references added and updated (MNRAS in press

Exploiting neutron-rich radioactive ion beams to constrain the symmetry energy

The Modular Neutron Array (MoNA) and 4 Tm Sweeper magnet were used to measure the free neutrons and heavy charged particles from the radioactive ion beam induced 32Mg + 9Be reaction. The fragmentation reaction was simulated with the Constrained Molecular Dynamics model(CoMD), which demonstrated that the of the heavy fragments and free neutron multiplicities were observables sensitive to the density dependence of the symmetry energy at sub-saturation densities. Through comparison of these simulations with the experimental data constraints on the density dependence of the symmetry energy were extracted. The advantage of radioactive ion beams as a probe of the symmetry energy is demonstrated through examination of CoMD calculations for stable and radioactive beam induced reactions

Three-body correlations in the ground-state decay of 26O

Background: Theoretical calculations have shown that the energy and angular correlations in the three-body decay of the two-neutron unbound O26 can provide information on the ground-state wave function, which has been predicted to have a dineutron configuration and 2n halo structure. Purpose: To use the experimentally measured three-body correlations to gain insight into the properties of O26, including the decay mechanism and ground-state resonance energy. Method: O26 was produced in a one-proton knockout reaction from F27 and the O24+n+n decay products were measured using the MoNA-Sweeper setup. The three-body correlations from the O26 ground-state resonance decay were extracted. The experimental results were compared to Monte Carlo simulations in which the resonance energy and decay mechanism were varied. Results: The measured three-body correlations were well reproduced by the Monte Carlo simulations but were not sensitive to the decay mechanism due to the experimental resolutions. However, the three-body correlations were found to be sensitive to the resonance energy of O26. A 1{\sigma} upper limit of 53 keV was extracted for the ground-state resonance energy of O26. Conclusions: Future attempts to measure the three-body correlations from the ground-state decay of O26 will be very challenging due to the need for a precise measurement of the O24 momentum at the reaction point in the target

The relationship between climate risk, climate policy uncertainty, and CO2 emissions: empirical evidence from the US

This paper examines the relationship between climate risk and climate policy uncertainty, and CO2 emissions in the US over the 2000–2022 period using a structural Factor- Augmented Vector AutoRegression (FAVAR) model with a two-step principal component analysis based on monthly observations. We employ a very recent measure to proxy for uncertainty regarding climate policy based on the Climate Policy Uncertainty Index (CPU) of Gavriilidis (2021), while Climate Risk is proxied by financial cost of natural disasters and number of deaths. We use different variables for CO2 emissions, based on total and sectoral emission (commercial, electric power, residential sector, transportation, and industrial sector). The results indicate that a significant percentage of the variance of CO2 emissions in the US, is explained by Natural Disasters Cost, which also seem to account for a significant percentage of the US Total Renewable Energy Consumption variance. Shocks to disaster costs seem to decrease all type of emissions significantly and also increase renewable energy use significantly. Natural disasters increase political disagreement among U.S. politicians, as well as, the climate policy uncertainty, highlighting the need for efficient policymaking and regulations. In further results, we find that an increase in Partisan Conflict decreases emissions and explains a significant amount of renewable energy variance

Screening enhancement factors for laboratory CNO and rp astrophysical reactions

Cross sections of laboratory CNO and rp astrophysical reactions are enhanced due to the presence of the multi-electron cloud that surrounds the target nuclei. As a result the relevant astrophysical factors are overestimated unless corrected appropriately. This study gives both an estimate of the error committed if screening effects are not taken into account and a rough profile of the laboratory energy thresholds at which the screening effect appears. The results indicate that, for most practical purposes, screening corrections to past relevant experiments can be disregarded. Regarding future experiments, however, screening corrections to the CNO reactions will certainly be of importance as they are closely related to the solar neutrino fluxes and the rp process. Moreover, according to the present results, screening effects will have to be taken into account particularly by the current and future LUNA experiments, where screened astrophysical factors will be enhanced to a significant degree.Comment: 6 RevTex pages + 2 ps figures. (Revised version). Accepted for publication in Journal of Physics

Search for unbound 15Be states in the 3n+12Be channel

15Be is expected to have low-lying 3/2+ and 5/2+ states. A first search did not observe the 3/2+ [A. Spyrou et al., Phys. Rev. C 84, 044309 (2011)], however, a resonance in 15Be was populated in a second attempt and determined to be unbound with respect to 14Be by 1.8(1) MeV with a tentative spin-parity assignment of 5/2+ [J. Snyder et al., Phys. Rev. C 88, 031303(R) (2013)]. Search for the predicted 15Be 3/2+ state in the three-neutron decay channel. A two-proton removal reaction from a 55 MeV/u 17C beam was used to populate neutron-unbound states in 15Be. The two-, three-, and four-body decay energies of the 12Be + neutron(s) detected in coincidence were reconstructed using invariant mass spectroscopy. Monte Carlo simulations were performed to extract the resonance and decay properties from the observed spectra. The low-energy regions of the decay energy spectra can be described with the first excited unbound state of 14Be (E_x=1.54 MeV, E_r=0.28 MeV). Including a state in 15Be that decays through the first excited 14Be state slightly improves the fit at higher energies though the cross section is small. A 15Be component is not needed to describe the data. If the 3/2+ state in 15Be is populated, the decay by three-neutron emission through 14Be is weak, less than or equal to 11% up to 4 MeV. In the best fit, 15Be is unbound with respect to 12Be by 1.4 MeV (unbound with respect to $14Be by 2.66 MeV) with a strength of 7%.Comment: 6 pages, 5 figures, accepted in Physical Review