2,543 research outputs found
Cosmic magnetic fields and dark energy in extended electromagnetism
We discuss an extended version of electromagnetism in which the usual gauge
fixing term is promoted into a physical contribution that introduces a new
scalar state in the theory. This new state can be generated from vacuum quantum
fluctuations during an inflationary era and, on super-Hubble scales, gives rise
to an effective cosmological constant. The value of such a cosmological
constant coincides with the one inferred from observations as long as inflation
took place at the electroweak scale. On the other hand, the new state also
generates an effective electric charge density on sub-Hubble scales that
produces both vorticity and magnetic fields with coherent lengths as large as
the present Hubble horizon.Comment: 4 pages, 2 figures. Contribution to the proceedings of Spanish
Relativity Meeting 2010, Granada, Spain, 6-10 September 201
Spacetimes with vector distortion: Inflation from generalised Weyl geometry
Spacetime with general linear vector distortion is introduced. Thus, the
torsion and the nonmetricity of the affine connection are assumed to be
proportional to a vector field (and not its derivatives). The resulting
two-parameter family of non-Riemannian geometries generalises the conformal
Weyl geometry and some other interesting special cases. Taking into account the
leading order quadratic curvature correction to the Einstein-Hilbert action
results uniquely in the one-parameter extension of the Starobinsky inflation
known as the alpha-attractor. The most general quadratic curvature action
introduces, in addition to the canonical vector kinetic term, novel ghost-free
vector-tensor interactions.Comment: 5 pages, 1 table, 0 figures. V2: Added the general quadratic theor
The canonical frame of purified gravity
In the recently introduced gauge theory of translations, dubbed Coincident
General Relativity, gravity is described with neither torsion nor curvature in
the spacetime affine geometry. The action of the theory enjoys an enhanced
symmetry and avoids the second derivatives that appear in the conventional
Einstein-Hilbert action. While it implies the equivalent classical dynamics,
the improved action principle can make a difference in considerations of
energetics, thermodynamics, and quantum theory. This essay reports on possible
progress in those three aspects of gravity theory. In the so-called purified
gravity, 1) energy-momentum is described locally by a conserved, symmetric
tensor, 2) the Euclidean path integral is convergent without the addition of
boundary or regulating terms and 3) it is possible to identify a canonical
frame for quantisation.Comment: 7 pages, no figure
The bimetric variational principle for General Relativity
The bimetric variational principle is a subtle reinterpretation of general
relativity that assumes the spacetime connection to be generated by an
independent metric. Unlike the so called Palatini formalism that promotes the
connection into a fundamental field, the new variational principle results in a
physically distinct theory since the potential for the connection carries new
degrees of freedom. The connection-generating metric naturally allows also an
antisymmetric component. This sets torsion propagating! It is also shown here
that while in the most straightforward generalization of the Einstein-Hilbert
action the nonmetric degrees of freedom become ghosts, there exist very simple
actions which give rise to viable theories at the linearised level when
subjected to the bimetric variational principle. However, the non linear
interactions might bring unpleasant features like the Boulware-Deser ghost.
This remains to be explored since this new type of bimetric theories does not,
in principle, lie in the class of usual bimetric theories where non-linear
interactions inevitably come in with new ghost-like degrees of freedom.Comment: 5 pages, no figures. Version 2: corrected sign error, considerably
extended discussion
Model d'inundacions en ventalls al.luvials: el cas de les planes costaneres valencianes
Els ventalls al.luvials que conformen les planes costaneres valencianes estan sotmesos a inundacions periòdiques, produĂŻdes per les revingudes sobtades dels rius i barrancs que les recorren. Els diferents episodis analitzats en la darrera dècada han servit de base per a dissenyar un model d'inundacions, que intenta caracteritzar tant els processos com les formes inundables. Els ventalls sĂłn formes convexes, on hi ha una divergència dels fluxos desbordats a les corbes i als punts d'intersecciĂł. Els paleocaixers, normalment desconnectats dels rius actuals, drenen la superfĂcie del ventall, mentre que els espais intercons i les marjals sĂłn zones còncaves que arrepleguen les aigĂĽes desbordades dels cons i les dels caixers que hi desemboquen. Aquest funcionament natural es complica enormement degut a l'acciĂł antròpica que modifica les condicions naturals d'aquest espais. AixĂ, el ponts, les carreteres, els desviaments, els cobriments i les ocupacions de caixers sĂłn obres que poden interferir en els fluxos canalizats i en els desbordaments, modificant contĂnuament les zones inundables. Paraules clau : ventall al.luvial, risc d'inundaciĂł, processos d'inundaciĂł, concavitats, convexitats, acciĂł antròpica
Sobre la restauraciĂł fluviali la complexitat dels rius efĂmers: algunes consideracions crĂtiques
En aquest article s"analitzen les actuacions de restauraciĂł fluvial dutes a terme a la ConfederaciĂł HidrogrĂ fica del XĂşquer entre 2008 i 2009, especialment les intervencions de restauraciĂł geomorfològica, la qual cosa ens permet una sèrie de consideracions crĂtiques sobre les obres. Es fan tambĂ© algunes reflexions teòriques sobre els canvis ambientals i antròpics esdevinguts al llarg del segle XX i les implicacions que tenen sobre la restauraciĂł fluvial, en la mesura que aporten complexitat i incertesa a la restauraciĂł. En particular s"analitzen algunes actuacions de restauraciĂł que comprometen la connectivitat hidrològica i sedimentĂ ria, que sĂłn especialment febles en rius efĂmers. This paper presents a critical review of the geomorphological river restoration works carried out in the JĂşcar River Basin between 2008 and 2009, considering the impacts on the hydro-geomorphic systems. The scope of the environmental and anthropogenic changes occurred in the Mediterranean rivers throughout the twentieth century adds considerable complexity and uncertainty to the restoration actions. In particular several considerations are done about some restoration works that compromise the hydrological and sediment connectivity, which are particularly weak in ephemeral rivers
Teaching about Madrid: A Collaborative Agents-Based Distributed Learning Course
Interactive art courses require a huge amount of computational resources to be running on real time. These computational resources are even bigger if the course has been designed as a Virtual Environment with which students can interact. In this paper, we present an initiative that has been develop in a close collaboration between two Spanish Universities: Universidad Politécnica de Madrid and Universidad Rey Juan Carlos with the aim of join two previous research project: a Collaborative Awareness Model for Task-Balancing-Delivery (CAMT) in clusters and the “Teaching about Madrid” course, which provides a cultural interactive background of the capital of Spain
On the kinematics of massive star forming regions: the case of IRAS 17233-3606
Direct observations of accretion disks around high-mass young stellar objects
would help to discriminate between different models of formation of massive
stars. However, given the complexity of massive star forming regions, such
studies are still limited in number. Additionally, there is still no general
consensus on the molecular tracers to be used for such investigations. Because
of its close distance and high luminosity, IRAS 17233-3606 is a potential good
laboratory to search for traces of rotation in the inner gas around the
protostar(s). Therefore, we selected the source for a detailed analysis of its
molecular emission at 230 GHz with the SMA. We systematically investigated the
velocity fields of transitions in the SMA spectra which are not affected by
overlap with other transitions, and searched for coherent velocity gradients to
compare them to the distribution of outflows in the region. Beside CO emission
we also used high-angular H2 images to trace the outflow motions driven by the
IRAS 17233-3606 cluster. We find linear velocity gradients in many transitions
of the same molecular species and in several molecules. We report the first
detection of HNCO in molecular outflows from massive YSOs. We discuss the CH3CN
velocity gradient taking into account various scenarios: rotation, presence of
multiple unresolved sources with different velocities, and outflow(s). Although
other interpretations cannot be ruled out, we propose that the CH3CN emission
might be affected by the outflows of the region. Higher angular observations
are needed to discriminate between the different scenarios. The present
observations, with the possible association of CH3CN with outflows in a few
thousands AU around the YSOs' cluster, (i) question the choice of the tracer to
probe rotating structures, and (ii) show the importance of the use of H2 images
for detailed studies of kinematics.Comment: accepted for publication in A&
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