$\texttt{21cmLSTM}$: A Fast Memory-based Emulator of the Global 21 cm Signal with Unprecedented Accuracy

TL;DR

The paper introduces 21cmLSTM, a recurrent neural network emulator for the global 21 cm signal that achieves unprecedented accuracy and speed, enabling precise Bayesian parameter inference from observational data.

Contribution

It presents 21cmLSTM, a novel LSTM-based neural network emulator that outperforms previous feedforward models in accuracy and captures long-term dependencies in the 21 cm signal.

Findings

Average relative rms error of 0.22% (0.39 mK)

Posterior 1σ rms error about 3× less than observational noise

Effective in Bayesian inference with various noise levels

Abstract

Neural network (NN) emulators of the global 21 cm signal need emulation error much less than the observational noise in order to be used to perform unbiased Bayesian parameter inference. To this end, we introduce $\texttt{21cmLSTM}$ -- a long short-term memory (LSTM) NN emulator of the global 21 cm signal that leverages the intrinsic correlation between frequency channels to achieve exceptional accuracy compared to previous emulators, which are all feedforward, fully connected NNs. LSTM NNs are a type of recurrent NN designed to capture long-term dependencies in sequential data. When trained and tested on the same simulated set of global 21 cm signals as the best previous emulators, $\texttt{21cmLSTM}$ has average relative rms error of 0.22% -- equivalently 0.39 mK -- and comparably fast evaluation time. We perform seven-dimensional Bayesian parameter estimation analyses using $\texttt{21cmLSTM}$ to fit global 21 cm signal mock data with different adopted observational noise levels, $\sigma_{21}$. The posterior $1\sigma$ rms error is $\approx3\times$ less than $\sigma_{21}$ for each fit and consistently decreases for tighter noise levels, showing that $\texttt{21cmLSTM}$ can sufficiently exploit even very optimistic measurements of the global 21 cm signal. We made the emulator, code, and data sets publicly available so that $\texttt{21cmLSTM}$ can be independently tested and used to retrain and constrain other 21 cm models.