Little red dots as a cosmological probe: constraining $H_0$ with quasi-periodic pulsations

Zijian Zhang, Kohei Inayoshi, Masamune Oguri, Linhua Jiang, Fengwu Sun, Mingyu Li, Xiaojing Lin

First listed 2026-06-05 | Last updated 2026-06-03

Abstract

The James Webb Space Telescope (JWST) has uncovered a population of ``little red dots'' (LRDs) at $z \gtrsim 4$, potentially representing early supermassive black holes embedded in dense gaseous envelopes. The recent discovery of the lensed LRD RXJ2211-RX1 reveals significant variability on rest-frame timescales of decades, which may be interpreted as quasi-periodic variation that has a potential physical parallel to stellar pulsations. In this work, we derive an idealized, self-consistent period-luminosity-temperature ($P$-$L$-$T_{\rm eff}$) relation based on the hydrostatic envelope model. If this theoretical relation holds and can be empirically validated/calibrated, it would offer a novel framework for constraining the Hubble constant ($H_0$). The current sparse sampling of \tgta\ yields a preliminary $H_0 = 120.7_{-46.5}^{+47.0} \text{ km s}^{-1}\text{ Mpc}^{-1}$ as a proof-of-concept, with the error budget dominated by the uncertainty of the pulsation period. Our forecasting analysis shows that continuous monitoring over a 10-year baseline can reduce the $H_0$ uncertainty to 3-20\%, depending on the intrinsic pulsation period, while the systematic uncertainty floor remains to be fully characterized. This method offers a potential independent probe to measure luminosity distances in the early universe.

Short digest

This Letter proposes an idealized route to turn pulsating little red dots into high-redshift distance indicators by deriving a self-consistent period-luminosity-temperature relation for black-hole envelopes in hydrostatic equilibrium. Motivated by the lensed LRD R2211-RX1, whose intrinsic variability may be quasi-periodic on rest-frame decade timescales, the authors show how the reconstructed pulsation period and time-averaged photometric observables could be mapped to H0. With the current sparse sampling, the paper presents only a proof-of-concept and finds that the H0 uncertainty is dominated by the period measurement. Forecasts indicate that extending monitoring to a 10-year baseline could improve the precision to about 3-20%, provided the relation can be empirically calibrated and the systematic floor is brought under control.

Key figures to inspect

• Figure 1. This is the paper’s core synthesis figure: the left panel shows how the posterior on the rest-frame pulsation period for R2211-RX1 tightens as the monitoring baseline is extended, while the right panel translates that directly into H0 constraints. It is the clearest single visualization of the proof-of-concept, the dominant role of period uncertainty, and the practical payoff of longer time-domain coverage. The comparison against existing SH0ES, Planck, TRGB, and strong-lensing measurements also makes explicit where this proposed LRD-based method would sit among current H0 probes.
• Figure 2. This figure is the key caveat-and-forecast companion to Figure 1 because it tests how the inferred precision changes for different intrinsic pulsation periods, here 27 and 40 years. It shows that the method’s near-term performance is strongly period-dependent, which is central to the paper’s argument that decade-long monitoring can yield very different returns depending on the true variability timescale. Use this figure to understand the robustness of the forecasting exercise rather than just the headline result for one assumed period.