100 research outputs found
The Possibility of a Non-Lagrangian Theory of Gravity
General Relativity resembles a very elegant crystal glass: If we touch its
principles, that is, its Lagrangian, there is a risk of breaking everything.
Or, if we will, it is like a short blanket: Curing some problems creates new
problems. This paper is devoted to bring to light the reasons why we pursue the
possibility of a non-Lagrangian theory of gravity under the hypothesis of an
extension of the original general relativity with an ansatz inspired in the
fundamental principles of classical and quantum physics.Comment: 6 pages, 1 figure. Version accepted in Universe MDP
Some Remarks on Alternative (or Modified) Theories of Gravity
A seminar given about 30 years ago by Ruben Aldrovandi motivates this text
where some reflexions about constructing theories that modify General
Relativity are made. Two particular cases, the Brans-Dicke and Unimodular
Gravity ones, are discussed, in a quite qualitative way, showing on how they
can address some of the most outstanding problems of General Relativity,
specially the transplanckian physics and the cosmological constant problem.Comment: Latex file, 14 pages. To appear in the volume "Tribute to Ruben
Aldrovandi" (Editora Livraria da F\'isica, S\~ao Paulo, 2024
Modified gravity models and the central cusp of dark matter haloes in galaxies
The N-body dark matter (DM) simulations point that DM density profiles, e.g. the Navarro Frenk White (NFW) halo, should be cuspy in its centre, but observations disfavour this kind of DM profile. Here we consider whether the observed rotation curves close to the galactic centre can favour modified gravity models in comparison to the NFW halo, and how to quantify such difference. Two explicit modified gravity models are considered, Modified Newtonian Dynamics (MOND) and a more recent approach renormalization group effects in general relativity (RGGR). It is also the purpose of this work to significantly extend the sample on which RGGR has been tested in comparison to other approaches. By analysing 62 galaxies from five samples, we find that (i) there is a radius, given by half the disc scale length, below which RGGR and MOND can match the data about as well or better than NFW, albeit the formers have fewer free parameters; (ii) considering the complete rotation curve data, RGGR could achieve fits with better agreement than MOND, and almost as good as a NFW halo with two free parameters (NFW and RGGR have, respectively, two and one more free parameters than MOND)
Renormalization Group approach to Gravity: the running of G and L inside galaxies and additional details on the elliptical NGC 4494
We explore the phenomenology of nontrivial quantum effects on low-energy
gravity. These effects come from the running of the gravitational coupling
parameter G and the cosmological constant L in the Einstein-Hilbert action, as
induced by the Renormalization Group (RG). The Renormalization Group corrected
General Relativity (RGGR model) is used to parametrize these quantum effects,
and it is assumed that the dominant dark matter-like effects inside galaxies is
due to these nontrivial RG effects. Here we present additional details on the
RGGR model application, in particular on the Poisson equation extension that
defines the effective potential, also we re-analyse the ordinary elliptical
galaxy NGC 4494 using a slightly different model for its baryonic contribution,
and explicit solutions are presented for the running of G and L. The values of
the NGC 4494 parameters as shown here have a better agreement with the general
RGGR picture for galaxies, and suggest a larger radial anisotropy than the
previously published result.Comment: 9 pages, 2 figs. Based on a talk presented at the VIII International
Workshop on the Dark Side of the Universe, June 10-15, 2012, Buzios, RJ,
Brazil. v2: typos removed, matches published versio
Evolution of the phase-space density and the Jeans scale for dark matter derived from the Vlasov-Einstein equation
We discuss solutions of Vlasov-Einstein equation for collisionless dark
matter particles in the context of a flat Friedmann universe. We show that,
after decoupling from the primordial plasma, the dark matter phase-space
density indicator Q remains constant during the expansion of the universe,
prior to structure formation. This well known result is valid for
non-relativistic particles and is not "observer dependent" as in solutions
derived from the Vlasov-Poisson system. In the linear regime, the inclusion of
velocity dispersion effects permits to define a physical Jeans length for
collisionless matter as function of the primordial phase-space density
indicator: \lambda_J = (5\pi/G)^(1/2)Q^(-1/3)\rho_dm^(-1/6). The comoving Jeans
wavenumber at matter-radiation equality is smaller by a factor of 2-3 than the
comoving wavenumber due to free-streaming, contributing to the cut-off of the
density fluctuation power spectrum at the lowest scales. We discuss the
physical differences between these two scales. For dark matter particles of
mass equal to 200 GeV, the derived Jeans mass is 4.3 x 10^(-6) solar masses.Comment: 18 pages, 2 figures. Accepted for publication in JCA
The growth factor parametrization versus numerical solutions in flat and non-flat dark energy models
In the present investigation we use observational data of
to determine observational constraints in the plane
using two different methods: the growth factor parametrization and the
numerical solutions method for density contrast, . We verified the
correspondence between both methods for three models of accelerated expansion:
the model, the model and the running
cosmological constant model. In all case we consider also curvature as
free parameter. The study of this correspondence is important because the
growth factor parametrization method is frequently used to discriminate between
competitive models. Our results we allow us to determine that there is a good
correspondence between the observational constrains using both methods. We also
test the power of the data to constraints the curvature
parameter within the model. For this we use a non-parametric
reconstruction using Gaussian processes. Our results show that the data with the current precision level does not allow to distinguish
between a flat and non-flat universe
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