The success of our cosmological standard model has been threatened by differing measurements of the universe’s expansion rate. This so-called Hubble tension has recently been claimed to have a five sigma significance, excluding the possibility of a statistical fluke. Another discrepancy, albeit less severe, arises from observations of our universe’s large-scale structure.

We investigate the possibility that the cosmic tensions are the signature of a new eV-scale phase transition that occurs in the dark sector. It is triggered shortly before recombination and leads to the decay of an early dark energy fluid. The corresponding energy injection into the cosmic fluid resolves the Hubble tension. This new microscopic scenario opens the door to a more fundamental description of the dark sector where dark matter and early and late dark energy are related to the same dark sector symmetry group with applications to the large-scale structure anomaly.

To establish this idea as a new cosmological paradigm, (i) microscopic and (ii) cosmological model building have to complement and inform each other. (i) is guided by longstanding particle physics challenges such as the origin of neutrino masses, the anomalous magnetic momentum of the muon, and the cosmological constant problem. (ii), on the other hand, is constrained by cosmological datasets with unique signatures arising in the cosmic microwave background, matter power spectrum, gravitational wave observations, and neutrino data.

This project is supported by VR Starting Grant 2022-03160 of the Swedish Research Council.