Early dark energy reconciles the different ‘thermometers’ of the Universe express
For decades, two ways of measuring the speed at which galaxies fly apart gave conflicting numbers, like two thermometers in the same room with different readings. One relied on nearby stars, the other on the ancient light of the Big Bang. Adam Riess used supernovae to get a value of about 73, while the Planck satellite got 67 from the cosmic microwave background (in arbitrary units).
Astronomers added observations of early galaxies from the James Webb Space Telescope and the DESI sound wave map, along with the hypothesis of a brief burst of dark energy at the dawn of time. It’s as if one of the thermometers got caught in a warm draft for a moment: its reading rose slightly, and the discrepancy almost disappeared.
Early dark energy is not the same force that accelerates the Universe today. It was a fleeting push of ‘antigravity’ that vanished before the first stars appeared. Its trace: the patterns of ancient light became slightly blurred (by a few percent), which smoothed out the dispute. Thus, the standard model of the cosmos got a fine-tuning, and the Expansion of the Universe is described without conflict.
Original abstract
as written by the authors arXiv:2606.19090
In the JWST and DESI era, the JWST high-redshift galaxy observations and DESI baryon acoustic oscillation (BAO) measurements severely challenge the standard $$\Lambda$$CDM model, while the $$H_0$$ tension becomes increasingly prominent. In this work, we investigate the capability of the early dark energy (EDE) model to alleviate the $$H_0$$ tension utilizing cosmic microwave background data from Planck, ACT, and SPT, BAO data from DESI, and ultraviolet luminosity function observations from the JWST. Within the canonical axion EDE framework, the CMB+DESI+JWST data significantly increase the $$H_0$$ value to $$71.58\pm1.05\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$$, alleviating the $$H_0$$ tension to the $$1.0\sigma$$ level. Simultaneously, this model improves the fit to the JWST data and exhibits statistical performance significantly better than the $$\Lambda$$CDM model, with $$\Delta\chi^2_{\mathrm{tot}} = -18.26$$ and $$\Delta\mathrm{DIC} = -11.89$$. Our results highlight the complementary advantages of JWST high-redshift galaxy data alongside early- and late-time observations in testing EDE and alleviating the $$H_0$$ tension.
In the brief moment when early dark energy was active, the Universe may have expanded more than it did in the billions of years that followed.