2,102 research outputs found
Dark Energy in vector-tensor theories of gravity
We consider a general class of vector-tensor theories of gravity and show
that solutions with accelerated expansion and a future type III singularity are
a common feature in these models. We also show that there are only six
vector-tensor theories with the same small scales behavior as General
Relativity and, in addition, only two of them can be made completely free from
instabilities. Finally, two particular models as candidates for dark energy are
proposed: on one hand, a cosmic vector that allows to alleviate the usual
naturalness and coincidence problems and, on the other hand, the
electromagnetic field is shown to give rise to an effective cosmological
constant on large scales whose value can be explained in terms of inflation at
the electroweak scale.Comment: 4 pages, 1 table. Contribution to the proceedings of Spanish
Relativity Meeting 2009, Bilbao, Spain, 7-11 September 200
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
A cosmic vector for dark energy
In this work we show that the presence of a vector field on cosmological
scales could explain the present phase of accelerated expansion of the
universe. The proposed theory contains no dimensional parameters nor potential
terms and does not require unnatural initial conditions in the early universe,
thus avoiding the so called cosmic coincidence problem. In addition, it fits
the data from high-redshift supernovae with excellent precision, making
definite predictions for cosmological parameters. Upcoming observations will be
able to clearly discriminate this model from standard cosmology with
cosmological constant.Comment: 5 pages, 3 figures, 1 table. New comments and references included.
Final version to appear in Phys. Rev.
El legado crĂtico de Northrop Frye
El legado de Northop Frye se muestra, diecisĂ©is años despuĂ©s de su muerte, en pleno esplendor. De ese legado resulta fundamental su compromiso con el contexto social de la literatura que Frye resumiĂł en la fĂłrmula del equilibrio entre la dimensiĂłn trascendente (el “concern”) y la libertad. Esa fĂłrmula constituye la gran verdad del legado de Frye, pero al lado de esta fĂłrmula el autor de este artĂculo encuentra un vacĂo: el recelo de Frye a construir de forma explĂcita una filosofĂa de la historia literaria, lo que ha dado lugar a una lectura de Frye que ofrece conceptos
mecánicos y abstractos. Sixteen years after Northrop Frye’s death, his legacy is still splendid. The essential of this legacy is the commitment with the social context of literature, condensed in Frye’s formula of equilibrium between concern and freedom. This formula is the great truth in Frye’s legacy. At its side, however, the author of this essay finds a vacuum: Frye’s reluctance to an explicit philosophy of literary history, from which a reading of Frye arises that sets out mechanical and abstract concepts
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
Cosmology with moving dark energy and the CMB quadrupole
We study the consequences of a homogeneous dark energy fluid having a
non-vanishing velocity with respect to the matter and radiation large-scale
rest frames. We consider homogeneous anisotropic cosmological models with four
fluids (baryons, radiation, dark matter and dark energy) whose velocities can
differ from each other. Performing a perturbative calculation up to second
order in the velocities, we obtain the contribution of the anisotropies
generated by the fluids motion to the CMB quadrupole and compare with
observations. We also consider the exact problem for arbitrary velocities and
solve the corresponding equations numerically for different dark energy models.
We find that models whose equation of state is initially stiffer than
radiation, as for instance some tracking models, are unstable against velocity
perturbations, thus spoiling the late-time predictions for the energy
densities. In the case of scaling models, the contributions to the quadrupole
can be non-negligible for a wide range of initial conditions. We also consider
fluids moving at the speed of light (null fluids) with positive energy and show
that, without assuming any particular equation of state, they generically act
as a cosmological constant at late times. We find the parameter region for
which the models considered could be compatible with the measured (low)
quadrupole.Comment: 23 pages, 6 figures. Confidence intervals calculated from WMAP data,
new references and comments included. Final version to appear in PR
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&
The hyperyoung HII region in G24.78+0.08 A1
Context. G24.78+0.08 A1 is a 20 Msun star surrounded by a hypercompact (HC)
HII region, driving a CO bipolar outflow, and located at the center of a
massive rotating toroid undergoing infall towards the HC region. Recent water
maser observations suggest that the HC region is expanding and accretion onto
the star is halted. Aims. This study aims to confirm the expansion scenario
proposed for the HC region on the basis of recent H2O maser observations.
Methods. We carried out continuum VLA observations at 1.3cm and 7mm with the A
array plus Pie Town configuration to map the HC region towards G24 A1. Results.
The emission of the HC region has been resolved and shows a ring shape
structure. The profiles of the emission obtained by taking slices at different
angles passing through the barycenter of the HC region confirm the shell
structure of the emission. The ratio between the inner and the outer radius of
the shell, Ri/Ro, derived fitting the normalized brightness temperature profile
passing through the peak of the 7mm emission, is 0.9, which indicates that the
shell is thin. The deconvolved outer radius estimated from the fit is 590 AU.
These results imply that the HC region in G24 A1 cannot be described in terms
of a classical, homogeneous HII region but is instead an ionized shell. This
gives support to the model of an expanding wind-driven, ionized shell suggested
by the kinematics and distribution of the H2O masers associated with the HC
region. According to this model, the HC region is expanding on very short times
scales, 21-66 yr.Comment: 4 pages, 5 figures. Accepted for publication by A&A Lette
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