202 research outputs found
Stochastic background of gravitational waves from cosmological sources
Gravitational waves (GW) can constitute a unique probe of the primordial
universe. In many cases, the characteristic frequency of the emitted GW is
directly related to the energy scale at which the GW source is operating in the
early universe. Consequently, different GW detectors can probe different energy
scales in the evolution of the universe. After a general introduction on the
properties of a GW stochastic background of primordial origin, some examples of
cosmological sources are presented, which may lead to observable GW signals.Comment: Proceedings of LISA Symposium X, accepted for publication in Journal
of Physics: Conference Series. Typos corrected, two references adde
Adding helicity to inflationary magnetogenesis
The most studied mechanism of inflationary magnetogenesis relies on the
time-dependence of the coefficient of the gauge kinetic term
. Unfortunately, only extremely finely tuned versions
of the model can consistently generate the cosmological magnetic fields
required by observations. We propose a generalization of this model, where also
the pseudoscalar invariant is multiplied by a
time dependent function. The new parity violating term allows more freedom in
tuning the amplitude of the field at the end of inflation. Moreover, it leads
to a helical magnetic field that is amplified at large scales by
magnetohydrodynamical processes during the radiation dominated epoch. As a
consequence, our model can satisfy the observational lower bounds on fields in
the intergalactic medium, while providing a seed for the galactic dynamo, if
inflation occurs at an energy scale ranging from to GeV. Such
energy scale is well below that suggested by the recent BICEP2 result, if the
latter is due to primordial tensor modes. However, the gauge field is a source
of tensors during inflation and generates a spectrum of gravitational waves
that can give a sizable tensor to scalar ratio even if
inflation occurs at low energies. This system therefore evades the Lyth bound.
For smaller values of , lower values of the inflationary energy scale are
required. The model predicts fully helical cosmological magnetic fields and a
chiral spectrum of primordial gravitational waves.Comment: 17 pages, 4 figures. Minor changes to match the version accepted for
publication in JCA
Limits on stochastic magnetic fields: A defense of our paper [1]
In their recent paper ``Faraday rotation of the cosmic microwave background
polarization by a stochastic magnetic field'', Kosowsky et al. Phys.Rev. D71,
043006 (2005) have commented about our paper [C.Caprini and R.Durrer, Phys.
Rev. D65, 023517 (2002)], in which we derived very strong limits on the
amplitude of a primordial magnetic field from gravitational wave production.
They argue that our limits are erroneous. In this short comment we defend our
result.Comment: 2 pages, no figure
No-go theorem for k-essence dark energy
We demonstrate that if k-essence can solve the coincidence problem and play
the role of dark energy in the universe, the fluctuations of the field have to
propagate superluminally at some stage. We argue that this implies that
successful k-essence models violate causality. It is not possible to define a
time ordered succession of events in a Lorentz invariant way. Therefore,
k-essence cannot arise as low energy effective field theory of a causal,
consistent high energy theory.Comment: 4 pages, 2 figures. Replaced with revised version accepted for
publication in Phys. Rev. Let
Can the observed large scale magnetic fields be seeded by helical primordial fields?
Gravitational wave production induces a strong constraint on the amplitude of
a primordial magnetic field. It has been shown that the nucleosynthesis bound
for a stochastic gravitational wave background implies that causally generated
fields cannot have enough power on large scales to provide the seeds necessary
for the observed magnetic fields in galaxies and clusters, even by the most
optimistic dynamo amplification. Magnetic fields generated at inflation can
have high enough amplitude only if their spectrum is very red. Here we show
that helicity, which leads to an inverse cascade, can mitigate these limits. In
particular, we find that helical fields generated at the QCD phase transition
or at inflation with red spectrum are possible seeds for the dynamo. Helical
fields generated at the electroweak phase transition are instead excluded as
seeds at large scales. We also calculate the spectrum of gravitational waves
generated by helical magnetic fields.Comment: 41 pages, 12 figure
On the frequency of gravitational waves
We show that there are physically relevant situations where gravitational
waves do not inherit the frequency spectrum of their source but its wavenumber
spectrum.Comment: 4 pages, 2 figures. In v2 minor corrections and a full sentence
changed in section III in the paragraph about turbulence. Version accepted
for publication by PR
Magnetic fields from inflation: the transition to the radiation era
We compute the contribution to the scalar metric perturbations from
large-scale magnetic fields which are generated during inflation. We show that
apart from the usual passive and compensated modes, the magnetic fields also
contribute to the constant mode from inflation. This is different from the
causal (post inflationary) generation of magnetic fields where such a mode is
absent and it might lead to significant, non-Gaussian CMB anisotropies.Comment: 19 pages, no figures. v2: Substantially revised version with
different conclusions. v3: one reference added, matches version accepted for
publication in PR
CMB temperature anisotropy at large scales induced by a causal primordial magnetic field
We present an analytical derivation of the Sachs Wolfe effect sourced by a
primordial magnetic field. In order to consistently specify the initial
conditions, we assume that the magnetic field is generated by a causal process,
namely a first order phase transition in the early universe. As for the
topological defects case, we apply the general relativistic junction conditions
to match the perturbation variables before and after the phase transition which
generates the magnetic field, in such a way that the total energy momentum
tensor is conserved across the transition and Einstein's equations are
satisfied. We further solve the evolution equations for the metric and fluid
perturbations at large scales analytically including neutrinos, and derive the
magnetic Sachs Wolfe effect. We find that the relevant contribution to the
magnetic Sachs Wolfe effect comes from the metric perturbations at
next-to-leading order in the large scale limit. The leading order term is in
fact strongly suppressed due to the presence of free-streaming neutrinos. We
derive the neutrino compensation effect dynamically and confirm that the
magnetic Sachs Wolfe spectrum from a causal magnetic field behaves as
l(l+1)C_l^B \propto l^2 as found in the latest numerical analyses.Comment: 31 pages, 2 figures, minor changes, matches published versio
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