945 research outputs found
Multiplicity of closed characteristics on symmetric convex hypersurfaces in
Let be a compact hypersurface in bounding a convex
set with non-empty interior. In this paper it is proved that there always exist
at least geometrically distinct closed characteristics on if
is symmetric with respect to the origin.Comment: 16 page
Pulsar timing array observations as possible hints for nonsingular cosmology
Recent pulsar timing array (PTA) experiments have reported strong evidence of
the stochastic gravitational wave background (SGWB). If interpreted as
primordial gravitational waves (GWs), the signal favors a strongly blue-tilted
spectrum. Consequently, the nonsingular cosmology, which is able to predict a
strongly blue-tilted GW spectrum with on certain scales, offers
a potential explanation for the observed SGWB signal. In this paper, we present
a Genesis-inflation model capable of explaining the SGWB signal observed by the
PTA collaborations while also overcoming the initial singularity problem
associated with the inflationary cosmology. Furthermore, our model predicts
distinctive features in the SGWB spectrum, which might be examined by
forthcoming space-based gravitational wave experiments.Comment: 20 pages, 8 figures; references added, published in EPJ
Solving and Completing the Rabi-Stark Model in the Ultrastrong Coupling Regime
In this work,we employ a unitary transformation with a suitable parameter to
convert the quantum Rabi-Stark model into a Jaynes-Cummings-like model.
Subsequently, we derive the analytical energy spectra in the ultrastrong
coupling regime. The energy spectra exhibit a phenomenon known as spectral
collapse, indicating the instability of the model due to the unboundedness of
its energy from below at higher coupling parameters. To stabilize the
Rabi-Stark model, we introduce a nonlinear photon-photon interaction term. We
then compare the modified model with the original model in the classical
oscillator (CO) limit. Interestingly, we observe a regular "staircase" pattern
in the mean photon number of the ground state. This pattern exhibits a fixed
slope and equal step width, which we determine analytically. Moreover, we
analytically determine the phase boundary, which slightly differs from that in
the original Rabi-Stark model. These findings offer insights into the
investigation of those superradiant phase transitions that are unbounded from
below due to the phenomenon of spectral collapse.Comment: 11 pages, 7 figure
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