Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Time-dependent mass of cosmological perturbations in the hybrid and
  dressed metric approaches to loop quantum cosmology   Loop quantum cosmology has recently been applied in order to extend the
analysis of primordial perturbations to the Planck era and discuss the possible
effects of quantum geometry on the cosmic microwave background. Two approaches
to loop quantum cosmology with admissible ultraviolet behavior leading to
predictions that are compatible with observations are the so-called hybrid and
dressed metric approaches. In spite of their similarities and relations, we
show in this work that the effective equations that they provide for the
evolution of the tensor and scalar perturbations are somewhat different. When
backreaction is neglected, the discrepancy appears only in the time- dependent
mass term of the corresponding field equations. We explain the origin of this
difference, arising from the distinct quantization procedures. Besides, given
the privileged role that the big bounce plays in loop quantum cosmology, e.g.
as a natural instant of time to set initial conditions for the perturbations,
we also analyze the positivity of the time-dependent mass when this bounce
occurs. We prove that the mass of the tensor perturbations is positive in the
hybrid approach when the kinetic contribution to the energy density of the
inflaton dominates over its potential, as well as for a considerably large
sector of backgrounds around that situation, while this mass is always
nonpositive in the dressed metric approach. Similar results are demonstrated
for the scalar perturbations in a sector of background solutions that includes
the kinetically dominated ones; namely, the mass then is positive for the
hybrid approach, whereas it typically becomes negative in the dressed metric
case. More precisely, this last statement is strictly valid when the potential
is quadratic for values of the inflaton mass that are phenomenologically
favored.

14
The categories of this paper are: math-ph
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The categories of this paper are: stat
18
The categories of this paper are: econ
26
The categories of this paper are: bayes-an
13
The categories of this paper are: quant-ph
35
The categories of this paper are: mtrl-th
37
The categories of this paper are: supr-con
36
The categories of this paper are: cmp-lg
15
The categories of this paper are: nucl-ex
7
The categories of this paper are: hep-th
33
The categories of this paper are: plasm-ph
28
The categories of this paper are: alg-geom
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The categories of this paper are: cond-mat
32
The categories of this paper are: atom-ph
8
The categories of this paper are: hep-ex
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The categories of this paper are: eess