Reheating predictions in single field inflation (1502.04673v2)

Abstract: Reheating is a transition era after the end of inflation, during which the inflaton is converted into the particles that populate the Universe at later times. No direct cosmological observables are normally traceable to this period of reheating. Indirect bounds can however be derived. One possibility is to consider cosmological evolution for observable CMB scales from the time of Hubble crossing to the present time. Depending upon the model, the duration and final temperature after reheating, as well as its equation of state, may be directly linked to inflationary observables. For single-field inflationary models, if we approximate reheating by a constant equation of state, one can derive relations between the reheating duration (or final temperature), its equation of state parameter, and the scalar power spectrum amplitude and spectral index. While this is a simple approximation, by restricting the equation of state to lie within a broad physically allowed range, one can in turn bracket an allowed range of $n_s$ and $r$ for these models. The added constraints can help break degeneracies between inflation models that otherwise overlap in their predictions for $n_s$ and $r$.

• The paper demonstrates how reheating dynamics and its constant EOS can constrain inflationary observables in single-field models.
• It employs a simplified approach to derive relations between reheating duration, terminal temperature, and key inflationary parameters like the scalar power spectrum and spectral index.
• Detailed numerical results on models such as polynomial, Starobinsky, and Higgs inflation reveal tighter constraints on nₛ and r, advancing early Universe physics.

Insights into Reheating Predictions in Single Field Inflation

The paper, "Reheating predictions in single field inflation" by Cook et al., addresses the intricacies of the reheating period following the inflationary epoch in the early Universe. The research focuses on relating the dynamics of reheating to inflationary observables, specifically within the framework of single-field inflation models.

Core Concepts and Approach

Reheating is a critical transition phase where the inflaton field's energy is transferred to standard particles, contributing to the Universe's subsequent thermal history. This period is not directly observable through cosmological phenomena; however, indirect constraints can be applied by examining observable Cosmic Microwave Background (CMB) scales from the Hubble crossing time. The paper emphasizes the importance of the reheating period's duration, its terminal temperature, and the equation of state (EOS) of reheating, linking these factors to inflationary observables.

Under the assumption of a constant EOS during reheating, the authors derive relationships between the duration of reheating (or its terminal temperature), its EOS, and inflationary parameters such as the scalar power spectrum amplitude and spectral index. This simplification allows for bracketing potential values for these parameters, adding constraints that help distinguish between overlapping inflation models, which may otherwise result in similar predictions for the scalar spectral index $n_s$ and the tensor-to-scalar ratio $r$.

Numerical and Theoretical Results

The paper provides detailed numerical predictions for several popular single-field inflationary models, such as polynomial, Starobinsky, Higgs, natural, and hilltop inflation models. For each model, key relationships between the spectral index, reheating parameters, and the potential are derived and analyzed, leading to constraints on the EOS parameter of reheating.

Significant emphasis is placed on the implications of these results for testing and differentiating between inflation models. For instance, within polynomial potentials, models with exponents $\alpha = 2/3, 1, 2$, and 4 show compatibility with observational data, but prefer particular EOS values to fit current constraints effectively. The models project tighter constraints on $n_s$ and $r$ than those previously determined, notably making $\phi^2$ models with $r \geq 0.11$ challenging to reconcile with observational data when reheating scenarios with realistic EOS are considered.

Implications and Future Directions

The paper highlights the potential of using reheating constraints as a discriminating tool among different inflationary models. As CMB observations become increasingly precise, particularly those constraining $n_s$ and $r$, this methodology allows for refining models' viability beyond existing Planck mission constraints. Such advancements in our understanding of reheating dynamics promise to enhance our comprehension of early Universe physics and the inflationary epoch's nuances.

The results indicate that focusing on the EOS during the reheating phase offers novel avenues for probing inflationary scenarios and cosmological parameters. The implications for refining the parameter space of models could potentially enable distinguishing factors that lead to observable variations in CMB.

In conclusion, Cook et al. provide a comprehensive framework connecting reheating's complex physical processes to observable inflationary parameters, extending the constraints on early Universe models. This work foregrounds reheating as a vital component in the broader context of cosmological model building and testing, pointing to potential for future theoretical and observational advances in the field.