# RELIKE: Reionization Effective Likelihood from Planck 2018 Data

**Authors:** Chen Heinrich, Wayne Hu  
**arXiv:** [2104.13998](https://arxiv.org/abs/2104.13998)  
**Date:** 2021-04-28  
**Summary page:** https://www.summarizepaper.com/en/paper/2104.13998v1/

## Key points

- Authors Chen Heinrich and Wayne Hu introduce RELIKE, an effective likelihood code leveraging Planck 2018 data for reionization models in the redshift range of $6 < z < 30$.
- RELIKE utilizes five constraints from CMB reionization principal components to analyze reionization scenarios rapidly and precisely.
- The code demonstrates strong agreement with exact Planck likelihoods, offering a valuable method for integrating additional reionization datasets.
- Model-independent constraints derived using the PC technique yield results for total Thomson optical depth $\tau_{\rm PC} = 0.0619^{+0.0056}_{-0.0068}$ and place an upper limit on high-redshift component $\tau_{\rm PC}(15, 30) < 0.020$ at a 95% confidence level.
- Upper limits on high-redshift optical depth provided by RELIKE are approximately three times larger than those reported in the Planck 2018 cosmological parameter paper using the FlexKnot method.
- The study validates its findings by analyzing a two-step model capable of accommodating small high-$z$ component, representing a significant advancement in constraining reionization models and offering insights into universe's ionization history across different redshift ranges.

## Summary

In their paper titled "RELIKE: Reionization Effective Likelihood from Planck 2018 Data," authors Chen Heinrich and Wayne Hu introduce RELIKE, a novel effective likelihood code that leverages the latest Planck 2018 data to constrain reionization models within the redshift range of $6 < z < 30$. By utilizing five constraints derived from the CMB reionization principal components (PC), RELIKE offers a rapid and precise method for analyzing various reionization scenarios. The authors validate the efficacy of RELIKE by applying it to two example models, demonstrating strong agreement with exact Planck likelihoods obtained through either a simple Gaussian PC likelihood or a full kernel density estimate. One key advantage of RELIKE is its ability to seamlessly integrate Planck constraints with additional reionization datasets, such as kinetic Sunyaev-Zeldovich effects, line-intensity mapping, luminosity function, star formation history, and quasar spectra. This flexibility is particularly valuable in studies where understanding the redshift dependence of ionization history is crucial. Moreover, the authors derive model-independent constraints using the PC technique, yielding results for the total Thomson optical depth $\tau_{\rm PC} = 0.0619^{+0.0056}_{-0.0068}$ and placing an upper limit on the high-redshift component $\tau_{\rm PC}(15, 30) < 0.020$ at a 95% confidence level. Notably, these upper limits on high-redshift optical depth are approximately three times larger than those reported in the Planck 2018 cosmological parameter paper employing the FlexKnot method. The study further validates its findings by conducting a direct analysis of a two-step model capable of accommodating this small high-$z$ component. Overall, RELIKE represents a significant advancement in constraining reionization models and offers valuable insights into the ionization history of the universe across different redshift ranges.