1,721,089 research outputs found
Cosmological implications of Horava-Lifshitz gravity
No full textMukohyama, Shinji. (2011). Cosmological implications of Horava-Lifshitz gravity. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/118944
Multi-scalar theories of gravity with direct matter couplings and their parametrized post-Newtonian parameters
Full text linkWe study theories of gravity including, in addition to the metric, several
scalar fields in the gravitational sector. The particularity of this work is
that we allow for direct couplings between these gravitating scalars and the
matter sector, which can generally be different for the source and the probe of
gravity, in addition to the universal interactions generated by the Jordan
frame metric. The weak gravity regime of this theory, which would describe
solar-system experiments, is studied using the parametrized post-Newtonian
(PPN) formalism. We derive the expression of the ten parameters of this
formalism. Among them, and are modified with respect to
their values in the theories without direct couplings. This fact holds even
after eliminating the direct couplings between the gravitating scalars and the
energy density of the source, by redefinition of the Jordan frame. All other
PPN parameters are insensitive to the direct couplings once in the correctly
identified Jordan frame. When direct couplings are different for the source and
the probe of gravity, they make non-relativistic probes deviate from the
geodesics of the PPN metric in this frame, already at Newtonian order. Such
couplings would thus be directly detectable and would have excluded by
experiments. This shows that, contrary to the claims in the recent literature,
it is impossible to screen the presence of gravitating scalars relying only on
a curved target space and direct couplings to matter.Comment: 28 pages v2: published version, small modification of the abstract,
subsection 3.4 adde
Are theories really relevant to cosmology?
Full text linkWe examine theories that directly couple the curvature
or with the matter sector in the action, in addition to the
universal coupling. We argue that if the matter sector includes the Standard
Model (SM), such theories are either inconsistent or already excluded by
experiments unless they are a rewriting of gravity or general
relativity. If these theories genuinely couple the SM to curvature, they suffer
from the presence of ghost states at energies within their domain of
application for cosmological purposes. Therefore, we raise questions about
their relevance to cosmology. Moreover, if such theories do not include the SM,
they should just be seen as scalar-tensor, vector-tensor, ..., theories,
depending on the additional degrees of freedom. They should thus be studied
accordingly.}Comment: 24 pages, published version, small corrections and precisions.
Slightly longer abstrac
Singularity at the demise of a black hole
Full text linkWe consider a class of quasiregular singularities characterized by points
possessing two future-directed light cones and two past-directed light cones.
Such singularities appear in the trousers spacetime and the
Deutsch-Politzer spacetime. We argue that these singularities are relevant for
describing the end point of an evaporating black hole, and show that a class of
emergent Lorentz signature theories can provide a microscopic description for
these singularities.Comment: 12 pages, 7 figures. Added and corrected a reference, minor
formatting changes. Matches published versio
Modifikovaná gravitace a urychlené rozpínání kosmu: Nyní a v raném vesmíru
No full textTato práce je shnrutím našich predchozích publikací, v nichž jsem zkoumali problém temné energie a kosmologické konstanty v kontextu modifikované gravitace. V první části představujeme rozšíření mimetické temné hmoty. Mimetická temná hmota je skalární-tensorová teorie invariantní vůči Weylovým tranformacím, která je schopná popisu temné hmoty na kosmologických škálách. V našich publikacích jsme rozšířili mimetickou konstrukci na vektorová pole a Yang-Millsova kalibrační pole. Výsled- né teorie obsahují derivace vyššího řádu a jsou invariantní vůči Weylovým transformacím a představují nové formulace unimodulární gravitace. Dále jsme uvažovali kombinaci této mimetické temné energie s mimetickou temnou hmotou a ukázali jsme, že výsledkem je teorie podobná k-essenci. V druhé části jsme prezentovali minimální modifikaci Einsteinových rovnic, ve kterých jsme trivializovali jejich stopu. Důsledkem tohoto kroku je proměna Newtonovy kon- stanty na globální stupeň volnosti. Následně Newtonova konstanta podléhá kvantovým fluktuacím a relacím neurčitosti. Zjistili jsme, že to samé platí pro efektivní Planckovu konstantu v jedné klasicky ekvivalentní formulaci této modifikace gravitace. V poslední části jsme představili analýzu tensorových perturbací v nedávno objevených modelech minimálně proměnlivé kosmologické...We review our previous works which explored the problems of dark energy and cos- mological constant problem in the context of modified gravity. In the first part, we present extensions of mimetic dark matter. The latter is a Weyl-invariant scalar-tensor theory able to describe dark matter on cosmological scales. In our works we have extended the mimetic construction to vector fields and Yang-Mills gauge fields. The resulting theories provided a novel Weyl-invariant and higher derivative formulations of unimodular gravity. We also introduced a mixture of this mimetic dark energy with mimetic dark matter and showed that it results in k-essence like scalar theory. In the second part we reviewed a minimal modification of Einstein equations, in which their trace part is made trivial. This results in the Newton constant becoming a global degree of freedom. Consequently, the Newton constant is subjected to quantum fluctuations and uncertainty relations. We find that the same applies to the effective Planck constant in a certain classically equivalent formulation of this gravity modification. Finally, we present an analysis of tensor perturbations in the recently proposed models of minimally varying cosmological constant. In these theories, extending Einstein-Cartan gravity, the cosmological constant is allowed to...Matematicko-fyzikální fakultaFaculty of Mathematics and Physic
Modified Gravity and Cosmic Acceleration: Now and in the Early Universe
No full textWe review our previous works which explored the problems of dark energy and cos- mological constant problem in the context of modified gravity. In the first part, we present extensions of mimetic dark matter. The latter is a Weyl-invariant scalar-tensor theory able to describe dark matter on cosmological scales. In our works we have extended the mimetic construction to vector fields and Yang-Mills gauge fields. The resulting theories provided a novel Weyl-invariant and higher derivative formulations of unimodular gravity. We also introduced a mixture of this mimetic dark energy with mimetic dark matter and showed that it results in k-essence like scalar theory. In the second part we reviewed a minimal modification of Einstein equations, in which their trace part is made trivial. This results in the Newton constant becoming a global degree of freedom. Consequently, the Newton constant is subjected to quantum fluctuations and uncertainty relations. We find that the same applies to the effective Planck constant in a certain classically equivalent formulation of this gravity modification. Finally, we present an analysis of tensor perturbations in the recently proposed models of minimally varying cosmological constant. In these theories, extending Einstein-Cartan gravity, the cosmological constant is allowed to..
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
