2,754 research outputs found
Moisture resistance of SU-8 and KMPR as structural material for integrated gaseous detectors
This paper treats the moisture resistance of SU-8 and KMPR, two photoresists considered as structural material in microsystems. Our experiments focus on the moisture resistance of newly developed radiation imaging detectors containing these resists. Since these microsystems will be used unpackaged, they are susceptible to all kinds of environmental conditions. Already after one day of exposure to a humid condition the structural integrity and adhesion of SU-8 structures, measured by a shear test is drastically reduced. KMPR photoresist shows much stronger moisture resistance properties, making it a suitable alternative in our application. © 2008 Elsevier Science. All rights reserved
Some not-so-common ideas about gravity
Most of the approaches to the construction of a theory of quantum gravity
share some principles which do not have specific experimental support up to
date. Two of these principles are relevant for our discussion: (i) the
gravitational field should have a quantum description in certain regime, and
(ii) any theory of gravity containing general relativity should be relational.
We study in general terms the possible implications of assuming deviations from
these principles, their compatibility with current experimental knowledge, and
how can they affect future experiments.Comment: 12 pages (+ references). Invited talk at DICE2014, Castiglioncello,
September 201
Weyl relativity: A novel approach to Weyl's ideas
In this paper we revisit the motivation and construction of a unified theory
of gravity and electromagnetism, following Weyl's insights regarding the
appealing potential connection between the gauge invariance of electromagnetism
and the conformal invariance of the gravitational field. We highlight that
changing the local symmetry group of spacetime permits to construct a theory in
which these two symmetries are combined into a putative gauge symmetry but with
second-order field equations and non-trivial mass scales, unlike the original
higher-order construction by Weyl. We prove that the gravitational field
equations are equivalent to the (trace-free) Einstein field equations, ensuring
their compatibility with known tests of general relativity. As a corollary, the
effective cosmological constant is rendered radiatively stable due to Weyl
invariance. A novel phenomenological consequence characteristic of this
construction, potentially relevant for cosmological observations, is the
existence of an energy scale below which effects associated with the
non-integrability of spacetime distances, and an effective mass for the
electromagnetic field, appear simultaneously (as dual manifestations of the use
of Weyl connections). We explain how former criticisms against Weyl's ideas
lose most of their power in its present reincarnation, which we refer to as
Weyl relativity, as it represents a Weyl-invariant, unified description of both
the Einstein and Maxwell field equations.Comment: 34 pages, no figure
Black holes turn white fast, otherwise stay black: no half measures
Recently, various authors have proposed that the first ultraviolet effect on
the gravitational collapse of massive stars to black holes is the transition
between a black-hole geometry and a white-hole geometry, though their proposals
are radically different in terms of their physical interpretation and
characteristic time scales [1,2]. Several decades ago, it was shown by Eardley
that white holes are highly unstable to the accretion of small amounts of
matter, being rapidly turned into black holes [3]. Studying the crossing of
null shells on geometries describing the black-hole to white-hole transition,
we obtain the conditions for the instability to develop in terms of the
parameters of these geometries. We conclude that transitions with long
characteristic time scales are pathologically unstable: occasional
perturbations away from the perfect vacuum around these compact objects, even
if being imperceptibly small, suffocate the white hole explosion. On the other
hand, geometries with short characteristic time scales are shown to be robust
against perturbations, so that the corresponding processes could take place in
real astrophysical scenarios. This motivates a conjecture about the transition
amplitudes of different decay channels for black holes in a suitable
ultraviolet completion of general relativity.Comment: 24 pages, 3 figures. V2: Minor changes and updated references.
Matches the published versio
Where does the physics of extreme gravitational collapse reside?
The gravitational collapse of massive stars serves to manifest the most
severe deviations of general relativity with respect to Newtonian gravity: the
formation of horizons and spacetime singularities. Both features have proven to
be catalysts of deep physical developments, especially when combined with the
principles of quantum mechanics. Nonetheless, it is seldom remarked that it is
hardly possible to combine all these developments into a unified theoretical
model, while maintaining reasonable prospects for the independent experimental
corroboration of its different parts. In this paper we review the current
theoretical understanding of the physics of gravitational collapse in order to
highlight this tension, stating the position that the standard view on
evaporating black holes stands for. This serves as the motivation for the
discussion of a recent proposal that offers the opposite perspective,
represented by a set of geometries that regularize the classical singular
behavior and present modifications of the near-horizon Schwarzschild geometry
as the result of the propagation of non-perturbative ultraviolet effects
originated in regions of high curvature. We present an extensive exploration of
the necessary steps on the explicit construction of these geometries, and
discuss how this proposal could change our present understanding of
astrophysical black holes and even offer the possibility of detecting genuine
ultraviolet effects on future gravitational wave experiments.Comment: 43 pages, 1 figure. Review article with new results on the black to
white hole transition. Prepared for the special issue "Open Questions in
Black Hole Physics" edited by Gonzalo J. Olm
Tomography of high-redshift clusters with OSIRIS
High-redshift clusters of galaxies are amongst the largest cosmic structures.
Their properties and evolution are key ingredients to our understanding of
cosmology: to study the growth of structure from the inhomogeneities of the
cosmic microwave background; the processes of galaxy formation, evolution, and
differentiation; and to measure the cosmological parameters (through their
interaction with the geometry of the universe, the age estimates of their
component galaxies, or the measurement of the amount of matter locked in their
potential wells). However, not much is yet known about the properties of
clusters at redshifts of cosmological interest. We propose here a radically new
method to study large samples of cluster galaxies using microslits to perform
spectroscopy of huge numbers of objects in single fields in a narrow spectral
range-chosen to fit an emission line at the cluster redshift. Our objective is
to obtain spectroscopy in a very restricted wavelength range (~100 A in width)
of several thousands of objects for each single 8x8 square arcmin field.
Approximately 100 of them will be identified as cluster emission-line objects
and will yield basic measurements of the dynamics and the star formation in the
cluster (that figure applies to a cluster at z~0.50, and becomes ~40 and ~20
for clusters at z~0.75 and z~1.00 respectively). This is a pioneering approach
that, once proven, will be followed in combination with photometric redshift
techniques and applied to other astrophysical problems.Comment: 4 pages, 3 figures. Proceedings of "Science with the GTC", Granada
(Spain), February 2002, RMxAA in pres
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