Original title: The SRG/eROSITA All-Sky Survey: Early dark energy and Hubble constant from the cluster mass function
J. Strunk, E. Artis, E. Bulbul, S. Grandis +22 · September 2026
Astronomers face a major puzzle: measurements of how fast the universe expands, known as the Hubble constant, disagree depending on whether scientists look at the early or late universe. To help solve this, researchers studied an axion-like Early Dark Energy model. In this setup, a temporary field briefly speeds up cosmic expansion around the epoch of recombination before rapidly fading away, altering how large structures grow over time. The team tested this idea by counting massive galaxy clusters observed by the eROSITA X-ray instrument. By seeing how many clusters formed, they set the first limits on Early Dark Energy using cluster counts alone, finding its maximum fraction is under 0.3. When they combined their cluster counts with other cosmic datasets, the analysis pointed to a nonzero presence of Early Dark Energy, yielding expansion rates that agree with late-Universe measurements.
Imagine you are baking a cake and, without warning, the oven temperature spikes for a few minutes before returning to normal. The cake might rise too fast and collapse, leaving a different structure than expected. In this paper, the early universe acts like that oven—a temporary burst of energy (early dark energy) alters how galaxy clusters formed, leaving a signature in their numbers today.
The Hubble constant tension—a disagreement between early and late universe expansion rates—is one of cosmology's biggest puzzles. This work uses 5259 galaxy clusters to test a proposed solution: early dark energy. If confirmed, it would change our understanding of dark energy and the universe's early history.
The analysis only uses clusters at redshifts 0.1-0.45, not the full sample. Mass calibration depends on external weak lensing data. The most significant detection (3.2σ) is not definitive. The model tested is only one type of early dark energy. Constraints rely on combining several datasets, and systematics are not fully addressed in the abstract.
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The SRG/eROSITA All-Sky Survey: Early dark energy and Hubble constant from the cluster mass function — J. Strunk, E. Artis, E. Bulbul et al.
Abstract licence: CC0
Explanation generated by Synthesis from the abstract — it is not the original paper. This is a preprint, so it hasn't necessarily been peer reviewed.