arXiv subjects

Paolo M Bassani

Publications and source records attributed to Paolo M Bassani.

4 recordsLinked to original sources

arXiv2026

Galactic dark matter halos: From anisotropic fluids in general relativity to Horava-Lifshitz gravity

We deform the GR Hamiltonian by adding an extra weight $+1$ density to the potential. We show that potential deformations of this type leave the (reduced) Dirac algebra unchanged and the modification is naturally reinterpreted as an effective anisotropic stress-energy contribution. While the fluid reproduces an isothermal-like mass scaling, its pressure anisotropy prevents it from giving flat rotation curves in this reduced phenomenological toy model. We then turn to HL gravity, where the absence of a local Hamiltonian constraint leaves a non-vanishing local Hamiltonian density, giving a controlled nonconservation law for the emergent dust component. Generalizing earlier results, we identify a restricted class of LTB backgrounds for which the HL source term yields a positive scaling dark matter density, consistent with ghost freedom and with the continuous $λ\to 1$ limit, in which the effective dust amplitude vanishes together with the deviation from GR. The analysis is conditional on a prescribed background: obtaining a fully backreacted areal-radius solution consistent with the HL field equations is left as a natural direction for future work.

gr-qc↗

arXiv2025

Violations of energy conservation in Horava-Lifshitz gravity: a new ingredient in the dark matter puzzle

We investigate the interplay between Horava-Lifshitz (HL) gravity and more general theories where the local Hamiltonian constraint is lost, for example due to the time variability of the Lagrangian (e.g. via its parameters) where time is defined on a foliation according to a prescription mimicking Lambda and 4-volume time in unimodualr gravity. In one direction we subject the multitude of parameters in HL to this variability game, mimicking RG flow in a cosmological setting. In the opposite direction, we examine the evolution on the left-over Hamiltonian should the HL algebra of constraints be still applicable, rather than the algebra of General Relativity being restored. Within the projectable theory, the non-vanishing Hamiltonian can be reinterpreted as a pressureless fluid, resulting in essentially the same phenomenologies at macroscopic scales as in the standard cold dark matter paradigm. At high energies and short distances, however, unlike in theories with similar variability based on GR, violations of stress-energy tensor conservation persist, and these are computed here for the full class of projectable HL models. The phenomenological implications are examined: remarkably the driven solution resulting from these energy conservation violations is shown to be the attractor of the system during a free-fall collapse as far as the backreaction is negligible. When the backreaction is taken into account, the driven solution is expected to play an important role towards our understanding of microscopic caustic avoidance, which is one of the most significant issues in many alternatives to particle dark matter scenarios.

gr-qc↗

arXiv2025

How to make a Universe

We establish the general conditions under which evolution in the laws of physics and matter creation or destruction are closely intertwined. They make use of global time variables canonically dual to the constants of Nature. Such times flow at a rate determined by what can be interpret as the chemical potential of the fundamental constants (in analogy with phenomenological clocks based on isentropic fluids). The general condition for violations of energy conservation is then that a matter parameter evolves as a function of a gravity clock or vice-versa. This framework can be envisaged as the environment within which a natural selection scenario operates, powered by random mutations in the values of the constants of nature (or indeed any other variability in the laws in terms of the times defined above). The prize function is the creation of matter, followed by its preservation. This can be accomplished in an environment where diffeomorphism invariance is among the possible theories, with mutations modelled, for example, on the absorbing Markov chain. In such a set-up the diffeormorphism invariant state with fixed constants (or any nearby state) should be the absorbing state. John Wheeler's ``higgledy-piggledy'' chaotic cosmic start therefore finds a realization in this model, where its own demise and the establishment of order and seemingly immutable laws is also a predection of the model.

gr-qc↗

arXiv2023

Unimodular-like times, evolution and Brans-Dicke Gravity

In unimodular-like theories, the constants of nature are demoted from pre-given parameters to phase space variables. Their canonical duals provide physical time variables. We investigate how this interacts with an alternative approach to varying constants, where they are replaced by dynamical scalar fields. Specifically we investigate the Brans-Dicke theory of gravity and its interaction with clocks dual to the cosmological constant, the Planck mass, etc. We crucially distinguish between the different role of Newton's G in this process, leading to the possibility of local Lorentz invariance violation. A large number of possible theories emerge, for example where the Brans-Dicke coupling, omega, depends on unimodular-like times (in a generalization of scalar-tensor theories), or even become the dual variable to unimodular-like clocks ticking variations in other demoted constants, such as the cosmological constant. We scan the space of possible theories and select those most interesting regarding the joint variations of the Brans-Dicke omega and other parameters, (such as the cosmological constant); and also regarding their energy conservation violation properties. This ground work is meant to provide the formalism for further developments, namely regarding cosmology, black holes and the cosmological constant problem.

hep-th↗