91 research outputs found
The phenomenology of squeezing and its status in non-inflationary theories
In this paper we skim the true phenomenological requirements behind the
concept of inflationary squeezing. We argue that all that is required is that
at horizon re-entry the fluctuations form standing waves with the correct
temporal phase (specifically, sine waves). We quantify this requirement and
relate it to the initial conditions fed into the radiation dominated epoch by
whatever phase of the Universe produced the fluctuations. The only relevant
quantity turns out to be the degree of suppression of the momentum, , of the
fluctuations, , which we measure by . Even
though equals the squeezing parameter, , in the case of inflation
and bimetric varying speed of light scenarios, this is not true in general,
specifically in some bouncing Universe models. It is also not necessary to
produce a large at the end of the primordial phase: it is enough that
be not too small. This is the case with scenarios based on modified
dispersion relations (MDR) emulating the dispersion relations of
Horava-Lifshitz theory, which produce , enough to comply with the
observational requirements. Scenarios based on MDR leading to a slightly red
spectrum are also examined, and shown to satisfy the observational constraints.Comment: v2: Typos corrected. Matches the accepted version on JCA
Constraints on cosmological birefringence energy dependence from CMB polarization data
We study the possibility of constraining the energy dependence of
cosmological birefringence by using CMB polarization data. We consider four
possible behaviors, characteristic of different theoretical scenarios:
energy-independent birefringence motivated by Chern-Simons interactions of the
electromagnetic field, linear energy dependence motivated by a 'Weyl'
interaction of the electromagnetic field, quadratic energy dependence,
motivated by quantum gravity modifications of low-energy electrodynamics, and
inverse quadratic dependence, motivated by Faraday rotation generated by
primordial magnetic fields. We constrain the parameters associated to each kind
of dependence and use our results to give constraints on the models mentioned.
We forecast the sensitivity that Planck data will be able to achieve in this
respect.Comment: 15 pages, 5 figures. v2 matches JCAP published versio
Reappraisal of a model for deformed special relativity
We revisit one of the earliest proposals for deformed dispersion relations in
the light of recent results on dynamical dimensional reduction and production
of cosmological fluctuations. Depending on the specification of the measure of
integration and addition rule in momentum space the model may be completed so
as to merely deform Lorentz invariance, or so as to introduce a preferred
frame. Models which violate Lorentz invariance have a negative UV asymptotic
dimension and a very red spectrum of quantum vacuum fluctuations. Instead,
models which preserve frame independence can exhibit running to a UV dimension
of 2, and a scale-invariant spectrum of fluctuations. The bispectrum of the
fluctuations is another point of divergence between the two casings proposed
here for the original model
Life of cosmological perturbations in MDR models, and the prospect of travelling primordial gravitational waves
We follow the life of a generic primordial perturbation mode (scalar or
tensor) subject to modified dispersion relations (MDR), as its proper
wavelength is stretched by expansion. A necessary condition ensuring that
travelling waves can be converted into standing waves is that the mode starts
its life deep inside the horizon and in the trans-Planckian regime, then leaves
the horizon as the speed of light corresponding to its growing wavelength
drops, to eventually become cis-Planckian whilst still outside the horizon, and
finally re-enter the horizon at late times. We find that scalar modes in the
observable range satisfy this condition, thus ensuring the viability of MDR
models in this respect. For tensor modes we find a regime in which this does
not occur, but in practice it can only be realised for wavelengths in the range
probed by future gravity wave experiments if the quantum gravity scale
experienced by gravity waves goes down to the PeV range. In this case
travelling---rather than standing---primordial gravity waves could be the
tell-tale signature of MDR scenarios.Comment: 9 pages, 1 figure. v2 matches version accepted for publicatio
Quantum Gravity phenomenology and metric formalism
In this proceedings for the MG14 conference, we discuss the construction of a
phenomenology of Planck-scale effects in curved spacetimes, underline a few
open issues and describe some perspectives for the future of this research
line
Parity at the Planck scale
We explore the possibility that well known properties of the parity operator,
such as its idempotency and unitarity, might break down at the Planck scale.
Parity might then do more than just swap right and left polarized states and
reverse the sign of spatial momentum : it might generate
superpositions of right and left handed states, as well as mix momenta of
different magnitudes. We lay down the general formalism, but also consider the
concrete case of the Planck scale kinematics governed by -Poincar\'e
symmetries, where some of the general features highlighted appear explicitly.
We explore some of the observational implications for cosmological
fluctuations. Different power spectra for right handed and left handed tensor
modes might actually be a manifestation of deformed parity symmetry at the
Planck scale. Moreover, scale-invariance and parity symmetry appear deeply
interconnected
Quantization of fluctuations in DSR: the two-point function and beyond
We show that the two-point function of a quantum field theory with de Sitter
momentum space (herein called DSR) can be expressed as the product of a
standard delta function and an energy-dependent factor. This is a highly
non-trivial technical result in any theory without a preferred frame. Applied
to models exhibiting running of the dimensionality of space, this result is
essential in proving that vacuum fluctuations are generally scale-invariant at
high energies whenever there is running to two dimensions. This is equally true
for theories with and without a preferred frame, with differences arising only
as we consider higher order correlators. Specifically, the three-point function
of DSR has a unique structure of "open triangles", as shown here.Comment: 5 pages, 1 figur
Including birefringence into time evolution of CMB: current and future constraints
We introduce birefringence effects within the propagation history of CMB,
considering the two cases of a constant effect and of an effect that increases
linearly in time, as the rotation of polarization induced by birefringence
accumulates during photon propagation. Both cases result into a mixing of E and
B modes before lensing effects take place, thus leading to the fact that
lensing is acting on spectra that are already mixed because of birefringence.
Moreover, if the polarization rotation angle increases during propagation,
birefringence affects more the large scales that the small scales. We put
constraints on the two cases using data from WMAP 9yr and BICEP 2013 and
compare these results with the constraints obtained when the usual procedure of
rotating the final power spectra is adopted, finding that this dataset
combination is unable to distinguish between effects, but it nevertheless hints
for a non vanishing value of the polarization rotation angle. We also forecast
the sensitivity that will be obtained using data from Planck and PolarBear,
highlighting how this combination is capable to rule out a vanishing
birefringence angle, but still unable to distinguish the different scenarios.
Nevertheless, we find that the combination of Planck and PolarBear is sensitive
enough to highlight the existence of degeneracies between birefringence
rotation and gravitational lensing of CMB photons, possibly leading to false
detection of non standard lensing effects if birefringence is neglected.Comment: 20 pages, 10 figures. New version matching the one accepted by JCAP.
Corrected typos in equations 2.17-2.1
On the initial singularity problem in rainbow cosmology
It has been recently claimed that the initial singularity might be avoided in
the context of rainbow cosmology, where one attempts to account for
quantum-gravitational corrections through an effective-theory description based
on an energy-dependent ("rainbow") space-time metric. We here scrutinize this
exciting hypothesis much more in depth than previous analyses. In particular,
we take into account all requirements for singularity avoidance, while
previously only a subset of these requirements had been considered. Moreover,
we show that the implications of a rainbow metric for thermodynamics are more
significant than previously appreciated. Through the analysis of two
particularly meaningful examples of rainbow metrics we find that our concerns
are not merely important conceptually, but actually change in quantitatively
significant manner the outcome of the analysis. Notably we only find examples
where the singularity is not avoided, though one can have that in the regime
where our semi-classical picture is still reliable the approach to the
singularity is slowed down when compared to the standard classical scenario. We
conclude that the study of rainbow metrics provides tantalizing hints of
singularity avoidance but is inconclusive, since some key questions remain to
be addressed just when the scale factor is very small, a regime which, as here
argued, cannot be reliably described by an effective rainbow-metric picture.Comment: v3: typo in Eq. (9) corrected, results unchanged. 10 pages, 5
figures, v2 matches published versio
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