28 research outputs found
Neutron Stars in Palatini and Theories
We study solutions of the stellar structure equations for spherically
symmetric objects in Palatini and in the mass-radius region associated to neutron stars. We
illustrate the potential impact of the and terms by studying a range
of viable values of and . Similarly, we use different equations
of state (SLy, FPS, HS(DD2) and HS(TMA)) as a simple way to account for the
equation of state uncertainty. Our results show that for certain combinations
of the and parameters and equation of state, the effect of
modifications of general relativity on the properties of stars is sizeable.
Therefore, with increasing accuracy in the determination of the equation of
state for neutron stars, astrophysical observations may serve as discriminators
of modifications of General Relativity.Comment: 13 pages, 18 figures. References added. Discussion around Fig. 18
extended. Agrees with published versio
Beyond Rainbow-Ladder in a covariant three-body Bethe-Salpeter approach: Baryons
We report on recent results of a calculation of the nucleon and delta masses
in a covariant bound-state approach, where to the simple rainbow-ladder
gluon-exchange interaction kernel we add a pion-exchange contribution to
account for pion cloud effects. We observe good agreement with lattice data at
large pion masses. At the physical point our masses are too large by about five
percent, signaling the need for more structure in the gluon part of the
interaction.Comment: 4 pages, 3 figures, Proceedings of The 13th International Conference
on Meson-Nucleon Physics and the Structure of the Nucleon (MENU 2013), Rom
Spectrum of scalar and pseudoscalar glueballs from functional methods
We provide results for the spectrum of scalar and pseudoscalar glueballs in
pure Yang-Mills theory using a parameter-free fully self-contained truncation
of Dyson-Schwinger and Bethe-Salpeter equations. The only input, the scale, is
fixed by comparison with lattice calculations. We obtain ground state masses of
and for the scalar and pseudoscalar
glueballs, respectively, and and for the
corresponding first excited states. This is in very good quantitative agreement
with available lattice results. Furthermore, we predict masses for the second
excited states at and . The quality of the
results hinges crucially on the self-consistency of the employed input. The
masses are independent of a specific choice for the infrared behavior of the
ghost propagator providing further evidence that this only reflects a
nonperturbative gauge completion.Comment: 13 pages, 10 figs.; v2: extended version with a meson calculation to
illustrate the extrapolation, unified scale setting in comparison, agrees
with published versio
Dynamical Aspects of Generalized Palatini Theories of Gravity
We study the field equations of modified theories of gravity in which the
lagrangian is a general function of the Ricci scalar and Ricci-squared terms in
Palatini formalism. We show that the independent connection can be expressed as
the Levi-Civita connection of an auxiliary metric which, in particular cases of
interest, is related with the physical metric by means of a disformal
transformation. This relation between physical and auxiliary metric boils down
to a conformal transformation in the case of f(R) theories. We also show with
explicit models that the inclusion of Ricci squared terms in the action can
impose upper bounds on the accessible values of pressure and density, which
might have important consequences for the early time cosmology and black hole
formation scenarios. Our results indicate that the phenomenology of
f(R_{ab}R^{ab}) theories is much richer than that of f(R) and f(R_{ab}R^{ab})
theories and that they also share some similarities with Bekenstein's
relativistic theory of MOND.Comment: 8 pages, no figure
Glueballs from bound state equations
Glueballs are bound states in the spectrum of quantum chromodynamics which consist only of gluons. They belong to the group of exotic hadrons which are widely studied experimentally and theoretically. We summarize how to calculate glueballs in a functional framework and discuss results for pure Yang-Mills theory. Our setup is totally self-contained with the scale being the only external input. We enumerate a range of tests that provide evidence of the stability of the results. This illustrates the potential of functional equations as a continuum first-principles method complementary to lattice calculations
Bouncing Cosmologies in Palatini Gravity
We consider the early time cosmology of f(R) theories in Palatini formalism
and study the conditions that guarantee the existence of homogeneous and
isotropic models that avoid the Big Bang singularity. We show that for such
models the Big Bang singularity can be replaced by a cosmic bounce without
violating any energy condition. In fact, the bounce is possible even for
pressureless dust. We give a characterization of such models and discuss their
dynamics in the region near the bounce. We also find that power-law lagrangians
with a finite number of terms may lead to non-singular universes, which
contrasts with the infinite-series Palatini f(R) lagrangian that one needs to
fully capture the effective dynamics of Loop Quantum Cosmology. We argue that
these models could also avoid the formation of singularities during stellar
gravitational collapse.Comment: 8 pages, 4 figures; added references and a short comment in sec.I
Approximating the Steady-State Temperature of 3D Electronic Systems with Convolutional Neural Networks
Thermal simulations are an important part of the design process in many engineering disciplines. In simulation-based design approaches, a considerable amount of time is spent by repeated simulations. An alternative, fast simulation tool would be a welcome addition to any automatized and simulation-based optimisation workflow. In this work, we present a proof-of-concept study of the application of convolutional neural networks to accelerate thermal simulations. We focus on the thermal aspect of electronic systems. The goal of such a tool is to provide accurate approximations of a full solution, in order to quickly select promising designs for more detailed investigations. Based on a training set of randomly generated circuits with corresponding finite element solutions, the full 3D steady-state temperature field is estimated using a fully convolutional neural network. A custom network architecture is proposed which captures the long-range correlations present in heat conduction problems. We test the network on a separate dataset and find that the mean relative error is around 2% and the typical evaluation time is 35 ms per sample (2 ms for evaluation, 33 ms for data transfer). The benefit of this neural-network-based approach is that, once training is completed, the network can be applied to any system within the design space spanned by the randomized training dataset (which includes different components, material properties, different positioning of components on a PCB, etc.)
Approximating the Steady-State Temperature of 3D Electronic Systems with Convolutional Neural Networks
Thermal simulations are an important part of the design process in many engineering disciplines. In simulation-based design approaches, a considerable amount of time is spent by repeated simulations. An alternative, fast simulation tool would be a welcome addition to any automatized and simulation-based optimisation workflow. In this work, we present a proof-of-concept study of the application of convolutional neural networks to accelerate thermal simulations. We focus on the thermal aspect of electronic systems. The goal of such a tool is to provide accurate approximations of a full solution, in order to quickly select promising designs for more detailed investigations. Based on a training set of randomly generated circuits with corresponding finite element solutions, the full 3D steady-state temperature field is estimated using a fully convolutional neural network. A custom network architecture is proposed which captures the long-range correlations present in heat conduction problems. We test the network on a separate dataset and find that the mean relative error is around 2% and the typical evaluation time is 35 ms per sample (2 ms for evaluation, 33 ms for data transfer). The benefit of this neural-network-based approach is that, once training is completed, the network can be applied to any system within the design space spanned by the randomized training dataset (which includes different components, material properties, different positioning of components on a PCB, etc.)