Bounds on very low reheating scenarios after Planck (1511.00672v1)

Abstract: We consider the case of very low reheating scenarios ($T_{\rm RH}\sim\mathcal{O}({\rm MeV})$) with a better calculation of the production of the relic neutrino background (with three-flavor oscillations). At 95% confidence level, a lower bound on the reheating temperature $T_{\rm RH}>4.1$ MeV is obtained from Big Bang Nucleosynthesis, while $T_{\rm RH}>4.3$ MeV from Planck data for very light ($\sum m_i = 0.06$ eV) neutrinos. If neutrino masses are allowed to vary, Planck data yield $T_{\rm RH}>4.7$ MeV, the most stringent bound on the reheating temperature to date. Neutrino masses as large as 1 eV are possible for very low reheating temperatures.

• The paper presents revised lower bounds on the reheating temperature, establishing T₍RH₎ > 4.1 MeV (BBN) and T₍RH₎ > 4.7 MeV (CMB) under low reheating scenarios.
• The study reveals that low reheating temperatures can lead to insufficient neutrino thermalization, significantly affecting the effective number of neutrinos (Nₑff) through flavor oscillations.
• The paper indicates that while neutrino mass constraints are relaxed in these scenarios, Planck observations continue to provide robust limits on the cumulative neutrino mass.

Bounds on Very Low Reheating Scenarios After Planck

The paper examines cosmological scenarios characterized by very low reheating temperatures, particularly around the order of MeV, and investigates the implications for the production of relic neutrinos and subsequent effects on Big Bang Nucleosynthesis (BBN) and Cosmic Microwave Background (CMB) observations. This analysis is motivated by the possibility that the Universe underwent periods of reheating due to the decay of unstable non-relativistic particles in addition to the traditional reheating following primordial inflation.

Objective and Methodology

The authors aim to explore the boundary conditions on the reheating temperature following inflation, especially at the very low end, taking into account the latest data from the Planck satellite and other cosmological observations. The research reevaluates the production and thermalization of neutrinos during low reheating scenarios, making use of three-flavor neutrino oscillation calculations. Using a modified cosmological model interfaced with Boltzmann and Markov Chain Monte Carlo codes, the paper incorporates new observations and refined theoretical calculations to bracket reheating temperatures more accurately.

Key Findings

1. Lower Bound from Big Bang Nucleosynthesis: The lower bound on the reheating temperature $T_{\rm RH}$ deduced using BBN comes out to be $T_{\rm RH} > 4.1$ MeV with 95% confidence level. This boundary is determined primarily through deuterium abundance measurements, asserted as more precise than previous analyses.
2. Reheating Effects on Neutrino Background: The research identifies that insufficient thermalization at very low reheating temperatures causes the effective number of neutrinos, $N_{\rm eff}$, to fall below the standard value. The comprehensive analysis, leveraging enhanced calculation techniques, indicates that flavor oscillations influence neutrino yields and distributions more profoundly in low-reheating regimes.
3. Constraints from CMB Data: From the CMB data obtained from Planck, the paper provides a more stringent boundary of $T_{\rm RH} > 4.7$ MeV. Simultaneously, constraints on the sum of neutrino masses remain relatively robust, though slightly loosened under scenarios of low reheating temperatures.
4. Neutrino Mass Constraints: Even though cosmological neutrino mass limits are somewhat relaxed in low-reheating scenarios, Planck findings uphold a credible constraint on the sum of neutrino masses, remaining one of the few observational probes sensitive to cumulative neutrino properties amidst altered early Universe conditions.

Implications and Future Directions

The exploration of very low reheating scenarios expands the landscape of early Universe cosmology, potentially accommodating alternative compositions of dark radiation or non-standard particle models such as sterile neutrinos or axions. This work underscores the need for precision in neutrino decoupling calculations and highlights possible extensions of the standard model that may be revealed through detailed studies constrained by next-generation cosmological surveys.

The implications are twofold. Practically, the bounds on reheating temperatures refine the permissible models of post-inflationary dynamics, while theoretically, they provide a framework to assess new physics impacting the neutrino sector within the early Universe. Moving ahead, more detailed analyses of neutrino flavor dynamics and interactions under different reheating paradigms can open pathways for addressing the residual enigmas of dark matter composition and primordial nucleosynthesis within the cosmological framework. Additionally, upcoming improvements in neutrino experimental data might further confine the parameters discussed, fortifying—or necessitating revisions to—the standard cosmological model.