5,417 research outputs found
Stable Isotropic Cosmological Singularities in Quadratic Gravity
We show that, in quadratic lagrangian theories of gravity, isotropic
cosmological singularities are stable to the presence of small scalar, vector
and tensor inhomogeneities. Unlike in general relativity, a particular exact
isotropic solution is shown to be the stable attractor on approach to the
initial cosmological singularity. This solution is also known to act as an
attractor in Bianchi universes of types I, II and IX, and the results of this
paper reinforce the hypothesis that small inhomogeneous and anisotropic
perturbations of this attractor form part of the general cosmological solution
to the field equations of quadratic gravity. Implications for the existence of
a 'gravitational entropy' are also discussed.Comment: 18 pages, no figure
Association of corotating magnetic sector structure with Jupiters decameter-wave radio emissions
Chree (superposed epoch) analyses of Jupiter's decameter-wave radio emission taken from the new Thieman (1979) catalog show highly significant correlation with solar activity indicated by the geomagnetic Ap index. The correlation effects can be explained in terms of corotating interplanetary magnetic sector features. At times when the solar wind velocity is relatively low, about 300 to 350 km/s, a sector boundary can encounter the Earth and Jupiter almost simultaneously during the period immediately before opposition. After opposition this will not normally occur as the solar wind velocities necessary are too low. The correlation effects are much enhanced for the three apparitions of 1962-1964 during which a relatively stable and long-lived sector pattern was present. Chree analyses for this period indicate periodicities, approximately equal to half the solar rotation period, in the Jupiter data
A comparative study of how political journalists in four European countries reported on the coronavirus pandemic
International comparisons of the culture of political journalism are made more difficult and complex by differences of salience in the chosen countries during the period of measurement. The COVID-19 pandemic offers a rare opportunity to compare national patterns of media coverage and the associated culture of political journalism while controlling for salience, as all countries were experiencing the pandemic in the same way at the same time.
This study uses multiple methods. It begins with qualitative content analysis, measuring and analysing a limited sample of the television and press coverage in the UK, France, Germany and Switzerland in November 2020. The study then contextualises the data within scholars’ familiar classifications of the political, economic and media environment, and finally, applying the methods of thematic research, seeks explanations from 22 semi-structured interviews with senior practitioners in political communication and journalism in the respective countries, and within the EU itself.
France and Germany are members of the EU, the UK and Switzerland had strong associations with the Union. Germany and Switzerland are federations, with traditions of consensual, multi-party government, which in the Swiss case is reinforced by a layer of direct democracy though regular referenda. The political systems in the UK and France while differing, share a majoritarian and indeed adversarial political culture. All have dominant, regulated public broadcasters.
Despite the many differences the evidence of this study suggests that there is a strong correlation between the consensual style of government, civic trust in all its forms and the political culture of the country and that this is expressed through similarities in the media output during this unique period. The UK’s majoritarian parliamentary system and highly competitive media are associated with a significantly different style of political journalism
Cosmologies with Energy Exchange
We provide a simple mathematical description of the exchange of energy
between two fluids in an expanding Friedmann universe with zero spatial
curvature. The evolution can be reduced to a single non-linear differential
equation which we solve in physically relevant cases and provide an analysis of
all the possible evolutions. Particular power-law solutions exist for the
expansion scale factor and are attractors at late times under particular
conditions. We show how a number of problems studied in the literature, such as
cosmological vacuum energy decay, particle annihilation, and the evolution of a
population of evaporating black holes, correspond to simple particular cases of
our model. In all cases we can determine the effects of the energy transfer on
the expansion scale factor. We also consider the situation in the presence of
anti-decaying fluids and so called phantom fluids which violate the dominant
energy conditions.Comment: 12 pages, 1 figur
Spatial Variations of Fundamental Constants
We show that observational limits on the possible time variation of constants
of Nature are significantly affected by allowing for both space and time
variation. Bekenstein's generalisation of Maxwell's equations to allow for
cosmological variation of is investigated in a universe containing
spherically symmetric inhomogeneities. The time variation of is
determined by the local matter density and hence limits obtained in
high-density geophysical enviroments are far more constraining than those
obtained at high redshift. This new feature is expected to be a property of a
wide class of theories for the variation of constants.Comment: 4 page
Bouncing Universes with Varying Constants
We investigate the behaviour of exact closed bouncing Friedmann universes in
theories with varying constants. We show that the simplest BSBM varying-alpha
theory leads to a bouncing universe. The value of alpha increases
monotonically, remaining approximately constant during most of each cycle, but
increasing significantly around each bounce. When dissipation is introduced we
show that in each new cycle the universe expands for longer and to a larger
size. We find a similar effect for closed bouncing universes in Brans-Dicke
theory, where also varies monotonically in time from cycle to cycle.
Similar behaviour occurs also in varying speed of light theories
Cosmological milestones and energy conditions
Until recently, the physically relevant singularities occurring in FRW
cosmologies had traditionally been thought to be limited to the "big bang", and
possibly a "big crunch". However, over the last few years, the zoo of
cosmological singularities considered in the literature has become considerably
more extensive, with "big rips" and "sudden singularities" added to the mix, as
well as renewed interest in non-singular cosmological events such as "bounces"
and "turnarounds". In this talk, we present an extensive catalogue of such
cosmological milestones, both at the kinematical and dynamical level. First,
using generalized power series, purely kinematical definitions of these
cosmological events are provided in terms of the behaviour of the scale factor
a(t). The notion of a "scale-factor singularity" is defined, and its relation
to curvature singularities (polynomial and differential) is explored. Second,
dynamical information is extracted by using the Friedmann equations (without
assuming even the existence of any equation of state) to place constraints on
whether or not the classical energy conditions are satisfied at the
cosmological milestones. Since the classification is extremely general, and
modulo certain technical assumptions complete, the corresponding results are to
a high degree model-independent.Comment: 8 pages, 1 table, conference proceedings for NEB XII conference in
Nafplio, Greec
Direct evidence for solar wind control of Jupiter's hectometer-wavelength radio emission
Observations of the solar wind close to Jupiter, by the Voyager 1 and Voyager 2 spacecraft in 1978 and 1979, are compared with the hectometer wavelength radio emission from the planet. A significant positive correlation is found between variations in the solar wind plasma density at Jupiter and the level of Jovian radio emission output. During the 173-day interval studied for the Voyager 2 data, the radio emission displayed a long term periodicity of about 13 days, identical to that shown by the solar wind density at Jupiter and consistent with the magnetic sector structure association already proposed for groundbased observations of the decameter wavelength emission
Some Late-time Asymptotics of General Scalar-Tensor Cosmologies
We study the asymptotic behaviour of isotropic and homogeneous universes in
general scalar-tensor gravity theories containing a p=-rho vacuum fluid stress
and other sub-dominant matter stresses. It is shown that in order for there to
be approach to a de Sitter spacetime at large 4-volumes the coupling function,
omega(phi), which defines the scalar-tensor theory, must diverge faster than
|phi_infty-phi|^(-1+epsilon) for all epsilon>0 as phi rightarrow phi_infty 0
for large values of the time. Thus, for a given theory, specified by
omega(phi), there must exist some phi_infty in (0,infty) such that omega ->
infty and omega' / omega^(2+epsilon) -> 0 as phi -> 0 phi_infty in order for
cosmological solutions of the theory to approach de Sitter expansion at late
times. We also classify the possible asymptotic time variations of the
gravitation `constant' G(t) at late times in scalar-tensor theories. We show
that (unlike in general relativity) the problem of a profusion of ``Boltzmann
brains'' at late cosmological times can be avoided in scalar-tensor theories,
including Brans-Dicke theory, in which phi -> infty and omega ~ o(\phi^(1/2))
at asymptotically late times.Comment: 14 page
Cosmological Constraints on a Dynamical Electron Mass
Motivated by recent astrophysical observations of quasar absorption systems,
we formulate a simple theory where the electron to proton mass ratio is allowed to vary in space-time. In such a minimal theory only
the electron mass varies, with and kept constant. We find
that changes in will be driven by the electronic energy density after
the electron mass threshold is crossed. Particle production in this scenario is
negligible. The cosmological constraints imposed by recent astronomical
observations are very weak, due to the low mass density in electrons. Unlike in
similar theories for spacetime variation of the fine structure constant, the
observational constraints on variations in imposed by the weak
equivalence principle are much more stringent constraints than those from
quasar spectra. Any time-variation in the electron-proton mass ratio must be
less than one part in since redshifts This is more than
one thousand times smaller than current spectroscopic sensitivities can
achieve. Astronomically observable variations in the electron-proton must
therefore arise directly from effects induced by varying fine structure
'constant' or by processes associated with internal proton structure. We also
place a new upper bound of on any large-scale spatial
variation of that is compatible with the isotropy of the microwave
background radiation.Comment: New bounds from weak equivalence principle experiments added,
conclusions modifie
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