AliCPT Sensitivity to Cosmic Reheating

TL;DR

This work forecasts AliCPT-1's sensitivity to cosmic reheating by analyzing three inflation scenarios: α-attractor T-model, radion gauge inflation (RGI), and a minimal QCD‑driven warm inflation model. Using a Fisher matrix framework with AliCPT-1 combined with Planck and a fiducial tensor-to-scalar ratio $r=0.01$, it shows that $T_{ m re}$ can be constrained to about 10% precision in favorable cold-inflation cases and the inflation-scale $M$ to ~1% when $r$ is measured; the results also map $T_{ m re}$ to the inflaton–SM couplings, notably $g$ in cold reheating. In warm inflation, AliCPT-1 can constrain the inflaton's gluon-coupling–driven parameters $(f, ext{λ})$ and $T_{ m re}$, though theoretical uncertainties lead to method-dependent precision. Overall, the paper demonstrates AliCPT's potential to illuminate the initial hot Universe and the microphysics bridging inflation and particle physics, with complementary implications for axion searches and laboratory tests.

Abstract

We present the first assessment of the Ali Cosmic Microwave Background Polarization Telescope's (AliCPT) sensitivity to the reheating epoch after cosmic inflation, based on its ability to detect primordial gravitational waves. We consider three models of inflation, an $α$-attractor T-model, RGI inflation and QCD-driven warm inflation. Assuming a fiducial value of $r=0.01$, we find that AliCPT-1, in its fully loaded focal plane detector configuration and combined with Planck, can provide measurements of the order of magnitude of the reheating temperature with an accuracy around $10\%$. For QCD-driven warm inflation this can be translated into a constraint on the inflaton coupling to gluons, which can be probed independently in axion search experiments. Our results constitute the first demonstration of AliCPT's ability to probe the initial temperature of the hot big bang and the microphysical parameter connecting cosmic inflation and particle physics.

Figures (9)

• Figure 1: The red ellipses indicate the 68% and 95% CL regions corresponding to the principal observational benchmark \ref{['it:A']} used throughout this work, cf. Table \ref{['PrincipalBenchmark']}. Details are given in Sec. \ref{['sec:forecasts']}. For comparison, we plot the corresponding regions from the Planck's $MCMC$ chains Planck:2018vyg.
• Figure 2: The predictions of the $\alpha$-T model \ref{['alpha V']} for $n_s$ and $r$ lie on the lines defined by \ref{['AlphaInAlphaT']}. The position along this line is determined by $N_{\rm re}$, hence each point corresponds to a value of $T_{\rm re}$ obtained from \ref{['Tre']}. The gray region on the right corresponds to $N_{\rm re} < 0$. The dotted lines correspond to $N_k=50$ and $N_k=60$$e$-folds between the horizon-crossing of the pivot scale $k=0.05 \ {\rm Mpc}^{-1}$ and the end of inflation. The blue areas display marginalized joint confidence regions for $(n_s, r)$ at the $1\sigma$$(68\%)$ CL (dark) and $2\sigma$$(95\%)$ CL (light) from Planck+BICEP/Keck 2018 data (Fig. 5 in BICEP:2021xfz). The red lines indicate the $1\sigma$ and $2\sigma$ contours of the likelihood function \ref{['Eq:Likelihood']} in benchmarks \ref{['it:A']} (upper panel) and \ref{['it:B']} (lower panel) in table \ref{['PrincipalBenchmark']}.
• Figure 3: Predictions from the RGI model \ref{['RGI V']}, with gray lines defined by \ref{['Nepenthe']} and all other conventions analogue to Fig. \ref{['fig:ellipsesalphaT']}.
• Figure 4: After imposing the current measurement of $A_s$ the warm inflation model \ref{['eq:lag']} contains only one free combination of parameters, which defines a line in the $n_s$-$r$ plane. However, the precise shape of this line and the relation to the model parameters still suffer from theoretical uncertainties that are comparable in to the current observational uncertainty in $n_s$. To illustrate this, we plot the results of three different computational methods labeled as WI1, WI2 and WI3, cf. Table \ref{['WIcomputations']}. We compare this to current Planck+BK constraints BICEP:2021xfz (in blue) and the expected sensitivity from AliCPT-1 in Table \ref{['PrincipalBenchmark']} (red ellipses corresponding to benchmark \ref{['it:A']} in the upper panel and \ref{['it:C']} in the lower panel).
• Figure 5: Posterior distributions for the reheating temperature $T_{\rm re}$ and Yukawa coupling $y$ in the $\alpha$-attractor model \ref{['alpha V']} with fixed $\alpha=4$ (upper panel) and $\alpha=2.7$ (lower panel) for the benchmarks \ref{['it:A']} and \ref{['it:B']}. The dark gray area on the left is excluded by BBN, that on the right by the requirement that energy density during reheating cannot exceed the scale of inflation. The standard deviations for the posteriors are summarized in Table \ref{['ErrorBars']}.