193 research outputs found
Dynamics of a lattice Universe
We find a solution to Einstein field equations for a regular toroidal lattice
of size L with equal masses M at the centre of each cell; this solution is
exact at order M/L. Such a solution is convenient to study the dynamics of an
assembly of galaxy-like objects. We find that the solution is expanding (or
contracting) in exactly the same way as the solution of a
Friedman-Lema\^itre-Robertson-Walker Universe with dust having the same average
density as our model. This points towards the absence of backreaction in a
Universe filled with an infinite number of objects, and this validates the
fluid approximation, as far as dynamics is concerned, and at the level of
approximation considered in this work.Comment: 14 pages. No figure. Accepted version for Classical and Quantum
Gravit
Observables in a lattice Universe
We explore observables in a lattice Universe described by a recently found
solution to Einstein field equations. This solution models a regular lattice of
evenly distributed objects of equal masses. This inhomogeneous solution is
perturbative, and, up to second order in a small parameter, it expands at a
rate exactly equal to the one expected in a dust dominated
Friedmann-Lema\^itre-Robertson-Walker (FLRW) model with the equivalent,
smoothed, energy density. Therefore, the kinematics of both cosmologies are
identical up to the order of perturbation studied. Looking at the behaviour of
the redshift and angular distance, we find a condition on the compactness of
the objects at the centre of each cell under which corrections to the FLRW
observables remain small, i.e. of order of a few percents at most.
Nevertheless, we show that, if this condition is violated, i.e. if the objects
are too compact, our perturbative scheme breaks down as far as the calculations
of observables are concerned, even though the kinematics of the lattice remains
identical to its FLRW counter-part (at the perturbative order considered). This
may be an indication of an actual fitting problem, i.e. a situation in which
the FLRW model obtained from lightcone observables does not correspond to the
FLRW model obtained by smoothing the spatial distribution of matter. Fully
non-perturbative treatments of the observables will be necessary to answer that
question.Comment: 23 pages, 4 figures. Replaced to matched version accepted in Class.
Quantum Grav. Results unchanged but interpretation clarifie
Gravitational anomalies signaling the breakdown of classical gravity
Recent observations for three types of astrophysical systems severely
challenge the GR plus dark matter scenario, showing a phenomenology which is
what modified gravity theories predict. Stellar kinematics in the outskirts of
globular clusters show the appearance of MOND type dynamics on crossing the
threshold. Analysis shows a ``Tully-Fisher'' relation in these systems,
a scaling of dispersion velocities with the fourth root of their masses.
Secondly, an anomaly has been found at the unexpected scales of wide binaries
in the solar neighbourhood. Binary orbital velocities cease to fall along
Keplerian expectations, and settle at a constant value, exactly on crossing the
threshold. Finally, the inferred infall velocity of the bullet cluster
is inconsistent with the standard cosmological scenario, where much smaller
limit encounter velocities appear. This stems from the escape velocity limit
present in standard gravity; the ``bullet'' should not hit the ``target'' at
more than the escape velocity of the joint system, as it very clearly did.
These results are consistent with extended gravity, but would require rather
contrived explanations under GR, each. Thus, observations now put us in a
situation where modifications to gravity at low acceleration scales cease to be
a matter of choice, to now become inevitable.Comment: 10 pages, 5 figures, Astrophysics and Space Science Proceedings 38,
4
Pure kinetic k-essence as the cosmic speed-up
In this paper, we consider three types of k-essence. These k-essence models
were presented in the parametric forms. The exact analytical solutions of the
corresponding equations of motion are found. It is shown that these k-essence
models for the presented solutions can give rise to cosmic acceleration.Comment: 10 pages, typos corrected, main results remain the same, minor
changes to match IJTP accepted versio
Non-minimally coupled dark matter: effective pressure and structure formation
We propose a phenomenological model in which a non-minimal coupling between
gravity and dark matter is present in order to address some of the apparent
small scales issues of \lcdm model. When described in a frame in which gravity
dynamics is given by the standard Einstein-Hilbert action, the non-minimal
coupling translates into an effective pressure for the dark matter component.
We consider some phenomenological examples and describe both background and
linear perturbations. We show that the presence of an effective pressure may
lead these scenarios to differ from \lcdm at the scales where the non-minimal
coupling (and therefore the pressure) is active. In particular two effects are
present: a pressure term for the dark matter component that is able to reduce
the growth of structures at galactic scales, possibly reconciling simulations
and observations; an effective interaction term between dark matter and baryons
that could explain observed correlations between the two components of the
cosmic fluid within Tully-Fisher analysis.Comment: 18 pages, 6 figures, references added. Published in JCA
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Fake artesunate could compromise the hope that artemisinin-based combination therapy offers for malaria control in Africa and Asia
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