1,721,078 research outputs found
Dark matter in modified cosmologies
No full textThe IceCube experiment discovered a high-energy astrophysical neutrino flux with energies of the order of PeV. To explain this phenomenon one may consider the minimal 4-dimensional operator. An interesting possibility is to also explain the relic abundance of DM in the Universe. Assuming that the cosmological background evolves according to the standard cosmological model, this model fails since the rate of DM decay needed to get the correct DM relic abundance differs by many orders of magnitude with respect that one needed to explain the IceCube data. We show that such a discrepancy can be solved if the Universe evolution is governed by a modified cosmology
Hydrogen spin oscillations in a background of axions and the 21-cm brightness temperature
No full textThe 21-cm line signal arising from the hyperfine interaction in hydrogen has an important role in cosmology and provides a unique method for probing of the universe prior to the star formation era. We propose that the spin flip of Hydrogen by the coherent emission/absorption of axions causes a lowering of their spin temperature and can explain the stronger than expected absorption of 21-cm light reported by the EDGES collaboration. We find the analogy of axion interaction with the two-level HI with the Jaynes-Cummings model of a two-level atom in a cavity and we derive the spin-flip frequency in this formalism and show that the coherent oscillations frequency Omega proportional to 1/f(a) in contrast with the incoherent transitions between the HI hyperfine levels where the transition rates proportional to 1/f(a)(2). The axion emission and absorption rates are equal but the spin temperature is still lowered due to different selection rules for the spin-flip transitions compared to the photon process. We show that the axion process goes in the right direction for explaining the EDGES observation. For this mechanism to work, we require a coherent field of relativistic axions with energy E-v peaked at the 21-cm spin-flip energy. Such a coherent background of relativistic axions can arise from the decay of cosmic strings if the decay takes place in the electroweak era
Generalized uncertainty principle and asymptotically safe gravity
No full textWe present a procedure to link the deformation parameter \beta of the generalized uncertainty principle (GUP) to the two free parameters \omega and \gamma of the running Newtonian coupling constant of the asymptotically safe gravity program. To this aim, we compute the Hawking temperature of a black hole in two different ways. The first way involves the use of the GUP in place of the Heisenberg uncertainty relations, and therefore we get a deformed Hawking temperature containing the parameter \beta
. The second way involves the deformation of the Schwarzschild metric due to the Newtonian coupling constant running according to the asymptotically safe gravity prescription. The comparison of the two techniques yields a relation between
\beta and \omega, \gamma. As a particular case, we discuss also the so-called \zeta -model. The relations between \beta and ̃ omega, \zeta allow us to transfer upper bounds from one parameter to the others
Probing Lorentz symmetry violation using the first image of Sagittarius A∗: Constraints on standard-model extension coefficients
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Cosmological curvature acceleration
Full text linkObserved accelerated behavior of the Hubble flow, usually related to the existence of a mysterious unknown fluid dubbed dark energy, can be framed in natural extensions of General Relativity that work at infrared scales. This approach gives rise to a geometric view of the phenomenon that does not require any new particle to be addressed. In this review paper, without claiming for completeness, we will give the main ingredients of such an approach considering models like f(R) gravity, with R the Ricci curvature scalar, and, in general, higher-order theories of gravity including other curvature invariants. In this context, we will review also Teleparallel Equivalent General Relativity and its extensions like f(T) gravity, where T is the torsion scalar. We will show that the curvature picture and the torsion picture can be related each other giving an equivalent paradigm where the observed cosmological acceleration can be addressed by extending the gravitational sector
Baryon asymmetry from the generalized uncertainty principle
No full textThe unexplained observed baryon asymmetry in the Universe is a long-standing problem in physics, with no satisfactory resolution so far. To explain this asymmetry, three Sakharov conditions must be met. An interaction term which couples space-time and the baryon current is considered, which satisfies the first two Sakharov conditions. Furthermore, it is shown that the Generalized Uncertainty Principle (GUP) from quantum gravity induces corrections to the Friedmann equations in cosmology, via the holographic principle. GUP also induces variations of energy and pressure density in the radiation dominated era, which satisfies the third Sakharov condition. Therefore, this construction provides a viable explanation for the observed baryon asymmetry. This also fixes the GUP parameters to alpha(0) approximate to 10(4) and beta(0) approximate to -10(8). (C) 2021 The Author(s). Published by Elsevier B.V
Matter-antimatter asymmetry induced by Barbero-Immirzi parameter
No full textWe review the baryon asymmetry generated in the early Universe in the framework of the metric-spinor gravity with the Holst term, which accounts for the Barbero-Immirzi parameter. For the generation of the matter-antimatter asymmetry, we have considered the model in which the time derivative of the Ricci scalar couples with matter currents (this model is known in the literature as gravitational baryogenesis). By using the current bounds on the parameter of the asymmetry, the ratio of baryon number to the entropy density, we infer a bound on the parameters characterizing the model under consideration (that is, the cosmological model in the presence of the torsion and Barbero-Immirzi parameter).(c) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons .org /licenses /by /4 .0/). Funded by SCOAP3
Ghost-free infinite derivative quantum field theory
No full textIn this paper we will study Lorentz-invariant, infinite derivative quantum field theories, where infinite derivatives give rise to non-local interactions at the energy scale M s , beyond the Standard Model. We will study a specific class, where there are no new dynamical degrees of freedom other than the original ones of the corresponding local theory. We will show that the Green functions are modified by a non-local extra term that is responsible for acausal effects, which are confined in the region of non-locality, i.e. M s−1 . The standard time-ordered structure of the causal Feynman propagator is not preserved and the non-local analog of the retarded Green function turns out to be non-vanishing for space-like separations. As a consequence the local commutativity is violated. Formulating such theories in the non-local region with Minkowski signature is not sensible, but they have Euclidean interpretation. We will show how such non-local construction ameliorates ultraviolet/short-distance singularities suffered typically in the local quantum field theory. We will show that non-locality and acausality are inherently off-shell in nature, and only quantum amplitudes are physically meaningful, so that all the perturbative quantum corrections have to be consistently taken into account
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