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Register a new userNearly scale-invariant curvature modes from entropy perturbations during graceful exit
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In this Letter, we describe how a spectrum of entropic perturbations generated during a period of slow contraction can source a nearly scale-invariant spectrum of curvature perturbations on length scales larger than the Hubble radius during the transition from slow contraction to a classical non-singular bounce (the `graceful exit phase). The sourcing occurs naturally through higher-order scalar field kinetic terms common to classical (non-singular) bounce mechanisms. We present a concrete example in which, by the end of the graceful exit phase, the initial entropic fluctuations have become negligible and the curvature fluctuations have a nearly scale-invariant spectrum with an amplitude consistent with observations.
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The observed temperature fluctuations in the cosmic microwave background can be traced back to primordial curvature modes that are sourced by adiabatic and/or entropic matter perturbations. In this paper, we explore the entropic mechanism in the context of non-singular bouncing cosmologies. We show that curvature modes are naturally generated during `graceful exit, i.e., when the smoothing slow contraction phase ends and the universe enters the bounce stage. Here, the key role is played by the kinetic energy components that come to dominate the energy density and drive the evolution towards the cosmological bounce.
A seemingly simple question, how does warm inflation exit gracefully?, has a more complex answer than in a cold paradigm. It has been highlighted here that whether warm inflation exits gracefully depends on three independent choices: The form of the potential, the choice of the warm inflation model (i.e., on the form of its dissipative coefficient) and the regime, of weak or strong dissipation, characterizing the warm inflation dynamics. Generic conditions on slow-roll parameters and several constraints on the different model parameters required for warm inflation to exit gracefully are derived.
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It is known that some cosmological perturbations are conformal invariant. This facilitates the studies of perturbations within some gravitational theories alternative to general relativity, for example the scalar-tensor theory, because it is possible to do equivalent analysis in a certain frame in which the perturbation equations are simpler. In this paper we revisit the problem of conformal invariances of cosmological perturbations in terms of the covariant approach in which the perturbation variables have clear geometric and physical meanings. We show that with this approach the conformal invariant perturbations are easily identified.
We discuss the difference between various gauge-invariant quantities typically used in single-field inflation, namely synchronous $zeta_s$, comoving $zeta_c$, and unitary $zeta_u$ curvatures. We show that conservation of $zeta_c$ outside the horizon is quite restrictive on models as it leads to conservation of $zeta_s$ and $zeta_u$, whereas the reverse does not hold. We illustrate the consequence of these differences with two inflationary models: ultra-slow-roll (USR) and braiding-ultra-slow-roll (BUSR). In USR, we show that out of the three curvatures, only $zeta_s$ is conserved outside the horizon, and we connect this result to the concepts of separate universe and the usage of the $delta N$ formalism. We find that even though $zeta_s$ is conserved, there is still a mild violation of the separate universe approximation in the continuity equation. Nevertheless, the $delta N$ formalism can still be applied to calculate the primordial power spectrum of some gauge-invariant quantities such as $zeta_u$, although it breaks down for others such as the uniform-density curvature. In BUSR, we show that both $zeta_u$ and $zeta_s$ are conserved outside the horizon, but take different values. Additionally, since $zeta_u ot=zeta_c$ we find that the prediction for observable curvature fluctuations after inflation does not reflect $zeta_c$ at horizon crossing during inflation and moreover involves not just $zeta_u$ at that epoch but also the manner in which the braiding phase ends.
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