17 research outputs found
Disc sources for Conformastationary metrics
Conformastationary metrics have been derived by Perjes and by Israel and
Wilson as source-free solutions of the Einstein-Maxwell equations. By analogy
with the conformastatic metrics which have charged dust sources it was assumed
that conformastationary metrics would be the external metrics of charged dust
in steady motion. However for axially symmetric conformastationary metrics we
show that, as well as moving dust, hoop tensions are always necessary to
balance the centrifugal forces induced by the motion. Exact examples of
conformastationary metrics with disk sources are worked out in full.
Generalisations to non-axially symmetric conformastationary metrics are
indicated.Comment: 18 pages, Latex, one postscript figure, accepted for publication by
Class. Quantum Gra
Classical Effective Field Theory for Weak Ultra Relativistic Scattering
Inspired by the problem of Planckian scattering we describe a classical
effective field theory for weak ultra relativistic scattering in which field
propagation is instantaneous and transverse and the particles' equations of
motion localize to the instant of passing. An analogy with the non-relativistic
(post-Newtonian) approximation is stressed. The small parameter is identified
and power counting rules are established. The theory is applied to reproduce
the leading scattering angle for either a scalar interaction field or
electro-magnetic or gravitational; to compute some subleading corrections,
including the interaction duration; and to allow for non-zero masses. For the
gravitational case we present an appropriate decomposition of the gravitational
field onto the transverse plane together with its whole non-linear action. On
the way we touch upon the relation with the eikonal approximation, some
evidence for censorship of quantum gravity, and an algebraic ring structure on
2d Minkowski spacetime.Comment: 29 pages, 2 figures. v4: Duration of interaction is determined in Sec
4 and detailed in App C. Version accepted for publication in JHE
Gravitating discs around black holes
Fluid discs and tori around black holes are discussed within different
approaches and with the emphasis on the role of disc gravity. First reviewed
are the prospects of investigating the gravitational field of a black
hole--disc system by analytical solutions of stationary, axially symmetric
Einstein's equations. Then, more detailed considerations are focused to middle
and outer parts of extended disc-like configurations where relativistic effects
are small and the Newtonian description is adequate.
Within general relativity, only a static case has been analysed in detail.
Results are often very inspiring, however, simplifying assumptions must be
imposed: ad hoc profiles of the disc density are commonly assumed and the
effects of frame-dragging and completely lacking. Astrophysical discs (e.g.
accretion discs in active galactic nuclei) typically extend far beyond the
relativistic domain and are fairly diluted. However, self-gravity is still
essential for their structure and evolution, as well as for their radiation
emission and the impact on the environment around. For example, a nuclear star
cluster in a galactic centre may bear various imprints of mutual star--disc
interactions, which can be recognised in observational properties, such as the
relation between the central mass and stellar velocity dispersion.Comment: Accepted for publication in CQG; high-resolution figures will be
available from http://www.iop.org/EJ/journal/CQ
Counterrotating perfect fluid discs as sources of electrovacuum static spacetimes
The interpretation of some electrovacuum spacetimes in terms of
counterrotating perfect fluid discs is presented. The interpretation is mades
by means of an "inverse problem" approach used to obtain disc sources of known
static solutions of the Einstein-Maxwell equations. In order to do such
interpretation, a detailed study is presented of the counterrotating model
(CRM) for generic electrovacuum static axially symmetric relativistic thin
discs with nonzero radial pressure. Four simple families of models of
counterrotating charged discs based on Chazy-Curzon-type, Zipoy-Voorhees-type,
Bonnor-Sackfield-type, and charged and magnetized Darmois electrovacuum metrics
are considered where we obtain some discs with a CRM well behaved.Comment: 22 pages, 4 figures, LaTeX2
Changing climate both increases and decreases European river floods
Climate change has led to concerns about increasing river floods resulting from the greater water-holding capacity of a warmer atmosphere. These concerns are reinforced by evidence of increasing economic losses associated with flooding in many parts of the world, including Europe. Any changes in river floods would have lasting implications for the design of flood protection measures and flood risk zoning. However, existing studies have been unable to identify a consistent continental-scale climatic-change signal in flood discharge observations in Europe, because of the limited spatial coverage and number of hydrometric stations. Here we demonstrate clear regional patterns of both increases and decreases in observed river flood discharges in the past five decades in Europe, which are manifestations of a changing climate. Our results—arising from the most complete database of European flooding so far—suggest that: increasing autumn and winter rainfall has resulted in increasing floods in northwestern Europe; decreasing precipitation and increasing evaporation have led to decreasing floods in medium and large catchments in southern Europe; and decreasing snow cover and snowmelt, resulting from warmer temperatures, have led to decreasing floods in eastern Europe. Regional flood discharge trends in Europe range from an increase of about 11 per cent per decade to a decrease of 23 per cent. Notwithstanding the spatial and temporal heterogeneity of the observational record, the flood changes identified here are broadly consistent with climate model projections for the next century, suggesting that climate-driven changes are already happening and supporting calls for the consideration of climate change in flood risk management