43 research outputs found
Cosmological perturbations in a family of deformations of general relativity
We study linear cosmological perturbations in a previously introduced family
of deformations of general relativity characterized by the absence of new
degrees of freedom. The homogeneous and isotropic background in this class of
theories is unmodified and is described by the usual Friedmann equations. The
theory of cosmological perturbations is modified and the relevant deformation
parameter has the dimension of length. Gravitational perturbations of the
scalar type can be described by a certain relativistic potential related to the
matter perturbations just as in general relativity. A system of differential
equations describing the evolution of this potential and of the stress-energy
density perturbations is obtained. We find that the evolution of scalar
perturbations proceeds with a modified effective time-dependent speed of sound,
which, contrary to the case of general relativity, does not vanish even at the
matter-dominated stage. In a broad range of values of the length parameter
controlling the deformation, a specific transition from the regime of modified
gravity to the regime of general relativity in the evolution of scalar
perturbations takes place during the radiation domination. In this case, the
resulting power spectrum of perturbations in radiation and dark matter is
suppressed on the comoving spatial scales that enter the Hubble radius before
this transition. We estimate the bounds on the deformation parameter for which
this suppression does not lead to observable consequences. Evolution of scalar
perturbations at the inflationary stage is modified but very slightly and the
primordial spectrum generated during inflation is not noticeably different from
the one obtained in general relativity.Comment: 45 pages, version published in JCAP; minor changes, one section moved
to the appendi
A 4D gravity theory and G2-holonomy manifolds
Bryant and Salamon gave a construction of metrics of G2 holonomy on the total
space of the bundle of anti-self-dual (ASD) 2-forms over a 4-dimensional
self-dual Einstein manifold. We generalise it by considering the total space of
an SO(3) bundle (with fibers R^3) over a 4-dimensional base, with a connection
on this bundle. We make essentially the same ansatz for the calibrating 3-form,
but use the curvature 2-forms instead of the ASD ones. We show that the
resulting 3-form defines a metric of G2 holonomy if the connection satisfies a
certain second-order PDE. This is exactly the same PDE that arises as the field
equation of a certain 4-dimensional gravity theory formulated as a
diffeomorphism-invariant theory of SO(3) connections. Thus, every solution of
this 4-dimensional gravity theory can be lifted to a G2-holonomy metric. Unlike
all previously known constructions, the theory that we lift to 7 dimensions is
not topological. Thus, our construction should give rise to many new metrics of
G2 holonomy. We describe several examples that are of cohomogeneity one on the
base.Comment: 25 page
Pure-connection gravity and anisotropic singularities
In four space-time dimensions, there exists a special infinite-parameter family of chiral modified gravity theories. They are most properly described by a connection field, with space-time metric being a secondary and derived concept. All these theories have the same number of degrees of freedom as general relativity, which is the only parity-invariant member of this family. Modifications of general relativity can be arranged so as to become important in regions with large curvature. In this paper, we review how a certain simple modification of this sort can resolve the Schwarzschild black-hole and Kasner anisotropic singularities of general relativity. In the corresponding solutions, the fundamental connection field is regular in space-time
Chiral perturbation theory for GR
We describe a new perturbation theory for General Relativity, with the chiral
first-order Einstein-Cartan action as the starting point. Our main result is a
new gauge-fixing procedure that eliminates the connection-to-connection
propagator. All other known first-order formalisms have this propagator
non-zero, which significantly increases the combinatorial complexity of any
perturbative calculation. In contrast, in the absence of the
connection-to-connection propagator, our formalism leads to an effective
description in which only the metric (or tetrad) propagates, there are only
cubic and quartic vertices, but some vertex legs are special in that they
cannot be connected by the propagator. The new formalism is the gravity analog
of the well-known and powerful chiral description of Yang-Mills theory.Comment: 28 pages, multiple feynmp diagram
A 4D gravity theory and G2-holonomy manifolds
© 2019 International Press of Boston, Inc. Bryant and Salamon gave a construction of metrics of G2 holonomy on the total space of the bundle of anti-self-dual (ASD) 2-forms over a 4-dimensional self-dual Einstein manifold. We generalise it by considering the total space of an SO(3) bundle (with fibers 3) over a 4-dimensional base, with a connection on this bundle. We make essentially the same ansatz for the calibrating 3-form, but use the curvature 2-forms instead of the ASD ones. We show that the resulting 3-form defines a metric of G2 holonomy if the connection satisfies a certain second-order PDE. This is exactly the same PDE that arises as the field equation of a certain 4-dimensional gravity theory formulated as a di_eomorphism-invariant theory of SO(3) connections. Thus, every solution of this 4-dimensional gravity theory can be lifted to a G2-holonomy metric. Unlike all previously known constructions, the theory that we lift to 7 dimensions is not topological. Thus, our construction should give rise to many new metrics of G2 holonomy. We describe several examples that are of cohomogeneity one on the base
Anisotropic singularities in chiral modified gravity
In four space-time dimensions, there exists a special infinite-parameter
family of chiral modified gravity theories. All these theories describe just
two propagating polarizations of the graviton. General Relativity with an
arbitrary cosmological constant is the only parity-invariant member of this
family. We review how these modified gravity theories arise within the
framework of pure-connection formulation. We introduce a new convenient
parametrisation of this family of theories by using certain set of auxiliary
fields. Modifications of General Relativity can be arranged so as to become
important in regions with large Weyl curvature, while the behaviour is
indistinguishable from GR where Weyl curvature is small. We show how the Kasner
singularity of General Relativity is resolved in a particular class of modified
gravity theories of this type, leading to solutions in which the fundamental
connection field is regular all through the space-time. There arises a new
asymptotically De Sitter region `behind' the would-be singularity, the complete
solution thus being of a bounce type.Comment: v2: published version, 42 pages, 4 figure
Neuronal nitric oxide synthase contributes to the regulation of hematopoiesis
Nitric oxide (NO) signaling is important for the regulation of hematopoiesis. However, the role of individual NO synthase (NOS) isoforms is unclear. Our results indicate that the neuronal NOS isoform (nNOS) regulates hematopolesis in vitro and in vivo. nNOS is expressed in adult bone marrow and fetal liver and is enriched in stromal cells. There is a strong correlation between expression of nNOS in a panel of stromal cell lines established from bone marrow and fetal liver and the ability of these cell lines to support hematopoietic stem cells; furthermore, NO donor can further increase this ability. The number of colonies generated in vitro from the bone marrow and spleen of nNOS-null mutants is increased relative to wild-type or inducible- or endothelial NOS knockout mice. These results describe a new role for nNOS beyond its action in the brain and muscle and suggest a model where nNOS, expressed in stromal cells, produces NO which acts as a paracrine regulator of hematopoietic stem cells
Seabirds reveal mercury distribution across the North Atlantic
Author contributionsC.A. and J.F. designed research; C.A., B. Moe, A.T., S.D., V.S.B., B. Merkel, J.Å., and J.F. performed research; C.A., B. Moe, M.B.-F., A.T., S.D., V.S.B., B. Merkel, J.Å., J.L., C.P.-P., and J.F. analyzed data; C.A., B.M., V.S.B., and J.F. sample and data collection, data coordination and management, statistical methodology; H.S. sample and data contribution and Data coordination and management; D.G., M.B.-F., F. Amélineau, F. Angelier, T.A.-N., O.C., S.C.-D., J.D., K.E., K.E.E., A.E., G.W.G., M.G., S.A.H., H.H.H., M.K.J., Y. Kolbeinsson, Y. Krasnov, M.L., J.L., S.-H.L., B.O., A.P., C.P.-P., T.K.R., G.H.S., P.M.T., T.L.T., and P.B. sample and data contribution; A.T., P.F. and S.D. sample and data contribution and statistical methodology; J.Å. statistical methodology; J.F. supervision; and C.A., B. Moe, H.S., D.G., A.T., S.D., V.S.B., B. Merkel, J.Å., F. Amélineau, F. Angelier, T.A.-N., O.C., S.C.-D., J.D., K.E., K.E.E., A.E., P.F., G.W.G., M.G., S.A.H., H.H.H., Y. Kolbeinsson, Y. Krasnov, S.-H.L., B.O., A.P., T.K.R., G.H.S., P.M.T., T.L.L., P.B., and J.F. wrote the paper.Peer reviewe