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Abstract

The thermal state of the intergalactic medium (IGM) is an important probe of physical properties for the bulk of gas in the universe. Here, we perform a new measurement of the thermal state for redshift z ≤ 5.4 covering 12 billion years from the endstage of reionization to the present day. For this purpose we measure the Lyman-α forest flux power spectrum based on high resolution quasar spectra from different ground- and space-based spectrographs, combine this analysis with archival measurements of percent level precision, analyze hydrodynamical simulations, use powerful statistical techniques for interpolation, and perform Bayesian inference via Markov chain monte carlo. We observe a rise in the temperature at mean density from 6000 K at z = 5.4 towards 14 000 K at z = 3.4 followed by a cooldown phase reaching 6000 K at z = 0.03. This evolution is provides conclusive evidence for photoionization heating due to reionization of He II, as well as the subsequent cooling of the IGM due to an expanding universe in concordance with model predictions. The agreement with previous measurements is good as well, but our analysis supercedes those by accounting for additional parameters that we marginalize over, and by the vast cosmological timespan our measurement spans. At the highest redshifts z > 5 we infer lower temperatures than expected from the standard picture of IGM heating allowing leaving little room for additional smoothing due to warm dark matter free streaming. Additionally, our measurement for z < 0.5 allows additional constraints on the ultraviolet background in contradiction to previous claims of a UV underproduction crisis.