20 research outputs found
Magnetohydrodynamics in the Inflationary Universe
Magnetohydrodynamic (MHD) waves are analysed in the early Universe, in the
inflationary era, assuming the Universe to be filled with a nonviscous fluid of
the Zel'dovich type () in a metric of the de Sitter form. A spatially
uniform, time dependent, magnetic field is assumed to be present.
The Einstein equations are first solved to give the time dependence of the
scale factor, assuming that the matter density, but not the magnetic field,
contribute as source terms. The various modes are thereafter analysed; they
turn out to be essentially of the same kind as those encountered in
conventional nongravitational MHD, although the longitudinal magnetosonic wave
is not interpretable as a physical energy-transporting wave as the group
velocity becomes superluminal. We determine the phase speed of the various
modes; they turn out to be scale factor independent. The Alfv\'{e}n velocity of
the transverse magnetohydrodynamic wave becomes extremely small in the
inflationary era, showing that the wave is in practice 'frozen in'.Comment: 19 pages, LaTeX, no figures. Minor additions to the Summary section
and Acknowledgments section. Two new references. Version to appear in Phys.
Rev.
Magnetic Knots as The origin of Spikes in the Gravitational Waves Backgrounds
The dynamical symmetries of hot and electrically neutral plasmas in a highly
conducting medium suggest that, after the epoch of the electron-positron
annihilation, magnetohydrodynamical configurations carrying a net magnetic
helicity can be present. The simultaneous conservation of the magnetic flux and
helicity implies that the (divergence free) field lines will possess
inhomogeneous knot structures acting as source seeds in the evolution equations
of the scalar, vector and tensor fluctuations of the background geometry. We
give explicit examples of magnetic knot configurations with finite energy and
we compute the induced metric fluctuations. Since magnetic knots are
(conformally) coupled to gravity via the vertex dictated by the equivalence
principle, they can imprint spikes in the gravitational wave spectrum for
frequencies compatible with the typical scale of the knot corresponding, in our
examples, to a present frequency range of -- Hertz. At
lower frequencies the spectrum is power-suppressed and well below the COBE
limit. For smaller length scales (i.e. for larger frequencies) the spectrum is
exponentially suppressed and then irrelevant for the pulsar bounds. Depending
upon the number of knots of the configuration, the typical amplitude of the
gravitational wave logarithmic energy spectrum (in critical units) can be even
four orders of magnitude larger than the usual flat (inflationary) energy
spectrum generated thanks to the parametric amplification of the vacuum
fluctuations.Comment: Accepted for publication in Physical Review D, 20 pages in RevTex
style, 4 Encapsulated figure
Hypermagnetic Knots, Chern-Simons Waves and the Baryon Asymmetry
At finite hyperconductivity and finite fermionic density the flux lines of
long range hypermagnetic fields may not have a topologically trivial structure.
The combined evolution of the chemical potentials and of pseudoscalar fields
(like the axial Higgs), possibly present for temperatures in the TeV range, can
twist the hypercharge flux lines, producing, ultimately, hypermagnetic knots
(HK). The dynamical features of the HK depend upon the various particle physics
parameters of the model (pseudoscalar masses and couplings, strength of the
electroweak phase transition, hyperconductivity of the plasma) and upon the
magnitude of the primordial flux sitting in topologically trivial
configurations of the hypermagnetic field. We study different cosmological
scenarios where HK can be generated. We argue that the fermionic number sitting
in HK can be released producing a seed for the Baryon Asymmetry of the Universe
(BAU) provided the typical scale of the knot is larger than the diffusivity
length scale. We derive constraints on the primordial hypermagnetic flux
required by our mechanism and we provide a measure of the parity breaking by
connecting the degree of knottedness of the flux lines to the BAU. We rule out
the ordinary axion as a possible candidate for production (around temperatures
of the order of the GeV) of {\em magnetic} knots since the produced {\em
electromagnetic} helicity is negligible (for cosmological standard) if the
initial amplitude of the axion oscillations is of the order of the Peccei-Quinn
breaking scale.Comment: 30 pages in Revtex style, 8 figure
Primordial Hypermagnetic Fields and Triangle Anomaly
The high-temperature plasma above the electroweak scale GeV may
have contained a primordial hypercharge magnetic field whose anomalous coupling
to the fermions induces a transformation of the hypermagnetic energy density
into fermionic number. In order to describe this process, we generalize the
ordinary magnetohydrodynamical equations to the anomalous case. We show that a
not completely homogeneous hypermagnetic background induces fermion number
fluctuations, which can be expressed in terms of a generic hypermagnetic field
configuration. We argue that, depending upon the various particle physics
parameters involved in our estimate (electron Yukawa coupling, strength of the
electroweak phase transition) and upon the hypermagnetic energy spectrum,
sizeable matter-antimatter fluctuations can be generated in the plasma. These
fluctuations may modify the predictions of the standard Big Bang
nucleosynthesis (BBN). We derive constraints on the magnetic fields from the
requirement that the homogeneous BBN is not changed. We analyse the influence
of primordial magnetic fields on the electroweak phase transition and show that
some specific configurations of the magnetic field may be converted into net
baryon number at the electroweak scale.Comment: Latex, 53 pages, 8 eps figure
Large-scale magnetic fields from hydromagnetic turbulence in the very early universe
We investigate hydromagnetic turbulence of primordial magnetic fields using
magnetohydrodynamics (MHD) in an expanding universe. We present the basic,
covariant MHD equations, find solutions for MHD waves in the early universe,
and investigate the equations numerically for random magnetic fields in two
spatial dimensions. We find the formation of magnetic structures at larger and
larger scales as time goes on. In three dimensions we use a cascade (shell)
model, that has been rather successful in the study of certain aspects of
hydrodynamic turbulence. Using such a model we find that after
times the initial time the scale of the magnetic field fluctuation (in the
comoving frame) has increased by 4-5 orders of magnitude as a consequence of an
inverse cascade effect (i.e. transfer of energy from smaller to larger scales).
Thus {\it at large scales} primordial magnetic fields are considerably stronger
than expected from considerations which do not take into account the effects of
MHD turbulence.Comment: 10 pages uuencoded LATeX, 4 figures include
The Magnetized Universe
Cosmology, high-energy physics and astrophysics are converging on the study
of large-scale magnetic fields. While the experimental evidence for the
existence of large-scale magnetization in galaxies, clusters and superclusters
is rather compelling, the origin of the phenomenon remains puzzling especially
in light of the most recent observations. The purpose of the present review is
to describe the physical motivations and some of the open theoretical problems
related to the existence of large-scale magnetic fields.Comment: 147 pages, 10 included figures. Few corrected typos and added
reference
Amplification of hypercharge electromagnetic fields by a cosmological pseudoscalar
If, in addition to the standard model fields, a new pseudoscalar field exists
and couples to hypercharge topological number density, it can exponentially
amplify hyperelectric and hypermagnetic fields in the symmetric phase of the
electroweak plasma, while coherently rolling or oscillating. We present the
equations describing the coupled system of a pseudoscalar field and hypercharge
electromagnetic fields in the electroweak plasma at temperatures above the
electroweak phase transition, discuss approximations to the equations, and
their validity. We then solve the approximate equations using assorted
analytical and numerical methods, and determine the parameters for which
hypercharge electromagnetic fields can be exponentially amplified.Comment: 14 pages, 6 figure