Oscillons After Inflation (1106.3335v2)

Abstract: Oscillons are massive, long-lived, localized excitations of a scalar field. We show that in a large class of well-motivated single-field models, inflation is followed by self-resonance, leading to copious oscillon generation and a lengthy period of oscillon domination. These models are characterized by an inflaton potential which has a quadratic minimum and is shallower than quadratic away from the minimum. This set includes both string monodromy models and a class of supergravity inspired scenarios, and is in good agreement with the current central values of the concordance cosmology parameters. We assume that the inflaton is weakly coupled to other fields, so as not to quickly drain energy from the oscillons or prevent them from forming. An oscillon-dominated universe has a greatly enhanced primordial power spectrum on very small scales relative to that seen with a quadratic potential, possibly leading to novel gravitational effects in the early universe.

• The paper demonstrates that self-resonance in single-field inflation models leads to prolific oscillon formation dominating the early universe’s energy density.
• It employs Floquet analysis and three-dimensional simulations to reveal the conditions under which oscillons persist for roughly a Hubble time.
• Implications include modifications to the primordial power spectrum and potential avenues for primordial black hole formation.

Oscillons After Inflation: An Overview

The paper "Oscillons After Inflation" by Amin et al. explores a specific class of post-inflationary dynamics involving massive, long-lived, localized structures in a scalar field known as oscillons. The authors propose that, within certain inflationary models characterized by single-field potentials, a self-resonance mechanism can lead to the prolific generation of oscillons, resulting in a period where oscillons dominate the energy density of the universe.

The authors focus on single-field inflationary models where the inflaton potential features a quadratic minimum, but is less steep than quadratic at larger field values. Such models align with several string monodromy and supergravity scenarios and are consistent with current cosmological observations. Specifically, they investigate models where the inflaton potential transitions from being $\phi^2$-like near the minimum to being of the form $\phi^{2\alpha}$ during inflation, with $\alpha$ less than one. These scenarios emerge naturally in certain string theory and supergravity-based models.

Mechanism of Oscillon Formation

Following the end of inflation, the universe transitions into a phase where the inflaton field can exhibit resonances, a process driven by the intrinsic properties of the scalar field potential, known as self-resonance. The authors demonstrate that this can lead to the generation of oscillons, which are spatially localized configurations with non-trivial stability features. The dynamics facilitating oscillon production depend significantly on the inflaton potential's parameters, which influence the resonance's onset and intensity.

By performing a Floquet analysis, the authors ascertain that strong resonance—a requisite for efficient oscillon production—occurs under specific conditions regarding the potential's characteristics, notably the parameters $\alpha$ and the crossover scale $M$. Numerical simulations further reinforce these findings, showing that for particular parameter values, the universe can indeed enter an oscillon-dominated phase.

Numerical Simulations and Results

The paper details three-dimensional simulations that help explore the nonlinear dynamics following inflation in these models. The simulations reveal that with suitable potential parameters, inhomogeneities and localized field configurations (oscillons) emerge rapidly, comprising a non-negligible fraction of the total energy density. Notably, the authors find that oscillons can persist for periods on the order of a Hubble time, effectively leading to a transient matter-dominated phase in the early universe.

Implications and Future Directions

A significant implication of this work is the alteration of the post-inflationary expansion history, which could influence the predicted primordial power spectrum. An oscillon-dominated phase can affect the growth of density perturbations, potentially leading to phenomena like primordial black hole formation due to the enhanced power on small scales.

While the paper focuses on the self-resonance and formation of oscillons in single-field potential scenarios, it opens avenues for further exploration of the interactions and potential stability conditions of oscillons, especially their gravitational interactions and decay channels. Future work could also investigate the impact of coupling these oscillons to other fields and explore scenarios where the inflaton may be part of a more complex field arrangement.

In summary, this paper enriches our understanding of the post-inflationary universe by identifying conditions where oscillons contribute significantly to the universe's energy density, while suggesting profound implications for early universe cosmology and the subsequent evolution of cosmic structures.