688 research outputs found
Generation of helical magnetic fields from inflation
The generation of helical magnetic fields during single field inflation due
to an axial coupling of the electromagnetic field to the inflaton is discussed.
We find that such a coupling always leads to a blue spectrum of magnetic fields
during slow roll inflation. Though the helical magnetic fields further evolve
during the inverse cascade in the radiation era after inflation, we conclude
that the magnetic fields generated by such an axial coupling can not lead to
observed field strength on cosmologically relevant scales.Comment: 4 pages, 1 figure; Contribution to the proceedings of the
International Conference on Gravitation and Cosmology (ICGC), Goa, India,
December, 201
Can slow roll inflation induce relevant helical magnetic fields?
We study the generation of helical magnetic fields during single field
inflation induced by an axial coupling of the electromagnetic field to the
inflaton. During slow roll inflation, we find that such a coupling always leads
to a blue spectrum with , as long as the theory is treated
perturbatively. The magnetic energy density at the end of inflation is found to
be typically too small to backreact on the background dynamics of the inflaton.
We also show that a short deviation from slow roll does not result in strong
modifications to the shape of the spectrum. We calculate the evolution of the
correlation length and the field amplitude during the inverse cascade and
viscous damping of the helical magnetic field in the radiation era after
inflation. We conclude that except for low scale inflation with very strong
coupling, the magnetic fields generated by such an axial coupling in single
field slow roll inflation with perturbative coupling to the inflaton are too
weak to provide the seeds for the observed fields in galaxies and clusters.Comment: 33 pages 6 figures; v4 to match the accepted version to appear in
JCA
Imperfect Dark Energy from Kinetic Gravity Braiding
We introduce a large class of scalar-tensor models with interactions
containing the second derivatives of the scalar field but not leading to
additional degrees of freedom. These models exhibit peculiar features, such as
an essential mixing of scalar and tensor kinetic terms, which we have named
kinetic braiding. This braiding causes the scalar stress tensor to deviate from
the perfect-fluid form. Cosmology in these models possesses a rich
phenomenology, even in the limit where the scalar is an exact Goldstone boson.
Generically, there are attractor solutions where the scalar monitors the
behaviour of external matter. Because of the kinetic braiding, the position of
the attractor depends both on the form of the Lagrangian and on the external
energy density. The late-time asymptotic of these cosmologies is a de Sitter
state. The scalar can exhibit phantom behaviour and is able to cross the
phantom divide with neither ghosts nor gradient instabilities. These features
provide a new class of models for Dark Energy. As an example, we study in
detail a simple one-parameter model. The possible observational signatures of
this model include a sizeable Early Dark Energy and a specific equation of
state evolving into the final de-Sitter state from a healthy phantom regime.Comment: 41 pages, 7 figures. References and some clarifying language added.
This version was accepted for publication in JCA
Zero-point quantum fluctuations in cosmology
We re-examine the classic problem of the renormalization of zero-point
quantum fluctuations in a Friedmann-Robertson-Walker background. We discuss a
number of issues that arise when regularizing the theory with a momentum-space
cutoff, and show explicitly how introducing non-covariant counter-terms allows
to obtain covariant results for the renormalized vacuum energy-momentum tensor.
We clarify some confusion in the literature concerning the equation of state of
vacuum fluctuations. Further, we point out that the general structure of the
effective action becomes richer if the theory contains a scalar field phi with
mass m smaller than the Hubble parameter H(t). Such an ultra-light particle
cannot be integrated out completely to get the effective action. Apart from the
volume term and the Einstein-Hilbert term, that are reabsorbed into
renormalizations of the cosmological constant and Newton's constant, the
effective action in general also has a term proportional to F(phi)R, for some
function F(phi). As a result, vacuum fluctuations of ultra-light scalar fields
naturally lead to models where the dark energy density has the form
rho_{DE}(t)=rho_X(t)+rho_Z(t), where rho_X is the component that accelerates
the Hubble expansion at late times and rho_Z(t) is an extra contribution
proportional to H^2(t). We perform a detailed comparison of such models with
CMB, SNIa and BAO data.Comment: 23 pages, 9 figures. v3: refs added. To appear in Phys. Rev.
Mechanical evaluation of a tissue-engineered zone of calcification in a bone-hydrogel osteochondral construct
The objective of this study was to test the hypothesis that mechanical properties of artificial osteochondral constructs can be improved by a tissue-engineered zone of calcification at the bone-hydrogel interface. Experimental push-off tests were performed on osteochondral constructs with or without a tissue-engineered zone of calcification. In parallel, a numerical model of the osteochondral defect treatment was developed and validated against experimental results. .Experimental results showed that the shear strength at the bone-hydrogel interface increased by 100% with the tissue-engineered zone of calcification. Numerical predictions of the osteochondral defect treatment showed that the shear stress at the bone-hydrogel interface was reduced with the tissue-engineered zone of calcification. We conclude that a tissue-engineered zone of calcification in osteochondral constructs can provide two improvements. First, it increases the strength of the bone-hydrogel interface, and second, it reduces the stress at this interface
Future of the universe in modified gravitational theories: Approaching to the finite-time future singularity
We investigate the future evolution of the dark energy universe in modified
gravities including gravity, string-inspired scalar-Gauss-Bonnet and
modified Gauss-Bonnet ones, and ideal fluid with the inhomogeneous equation of
state (EoS). Modified Friedmann-Robertson-Walker (FRW) dynamics for all these
theories may be presented in universal form by using the effective ideal fluid
with an inhomogeneous EoS without specifying its explicit form. We construct
several examples of the modified gravity which produces accelerating
cosmologies ending at the finite-time future singularity of all four known
types by applying the reconstruction program. Some scenarios to resolve the
finite-time future singularity are presented. Among these scenarios, the most
natural one is related with additional modification of the gravitational action
in the early universe. In addition, late-time cosmology in the non-minimal
Maxwell-Einstein theory is considered. We investigate the forms of the
non-minimal gravitational coupling which generates the finite-time future
singularities and the general conditions for this coupling in order that the
finite-time future singularities cannot emerge. Furthermore, it is shown that
the non-minimal gravitational coupling can remove the finite-time future
singularities or make the singularity stronger (or weaker) in modified gravity.Comment: 25 pages, no figure, title changed, accepted in JCA
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