The Universe May Have Switched to a New Energy State express
A compressed spring can sit for years without moving. It seems frozen forever, but inside the metal, atoms slowly rearrange, and one day the spring clicks, settling into a new, more relaxed position. Physicists think of the vacuum in a similar way: it might not be truly empty and unchanging, but rather a temporarily frozen state that could suddenly switch to another — with a different energy density.
The authors of the new study built several models of such a late transition. In some scenarios, only the energy of the vacuum changes: that mysterious dark energy that pushes galaxies apart. In others, part of the dark matter turns into radiation, and domain walls (thin layers where vacuum properties change abruptly) appear in space. They then compared these scenarios with three sets of real data: distances to distant galaxies measured via baryon acoustic oscillations (sound waves from the early Universe frozen in the distribution of matter), supernova brightness, and the cosmic microwave background map — ancient light that preserves the imprint of the first moments.
The most interesting result: a model where about 10% of dark matter participates in the transition, and domain walls are born in space, agrees remarkably well with observations. It favors a transition time around redshift 7—that is, roughly 800 million years after the big bang. A pure drop in vacuum energy without additional effects is more tightly constrained: if the transition happened later than when the Universe was half its current age, the vacuum energy could have dropped by at most half. Otherwise, the cosmic microwave background map would look different — bubbles of new vacuum would have left noticeable distortions.
Thus, the hypothesis of vacuum instability transforms from a philosophical speculation into a testable part of cosmology. Just as the click of a spring reveals hidden tension, future cosmic microwave background maps and new distance measurements will either capture the traces of such a transition or rule it out definitively.
Original abstract
as written by the authors arXiv:2605.30259
The existence of a landscape of metastable vacua raises the possibility that our Universe may have undergone quantum vacuum decay at late times. This work explores how such a transition can be tested with cosmological observables, focusing on precision distance measurements and cosmic microwave background (CMB) anisotropies. A set of phenomenological models is constructed in which late-time quantum tunneling changes the vacuum energy and may convert a subcomponent of dark matter into dark radiation, possibly accompanied by domain-wall production. The resulting expansion histories are compared with DESI DR2 baryon acoustic oscillation data; supernova distance measurements from DES-Dovekie, Pantheon+, and Union3; and a compressed CMB likelihood. For quantum-tunneling models, current cosmological distance measurements still allow a 50% decrease in the total vacuum energy for a transition redshift $$z_t<1$$. The model with dark-matter conversion and domain-wall production provides a good fit to resolve the tension between cosmological observables and the $$\Lambda$$CDM model, with a preferred transition around $$z_t \sim 7$$ and about 10% of dark matter participating in the transition. Additionally, CMB anisotropy constraints from bubble nucleation and the associated domain-wall network are derived and shown to strongly restrict slow or sparse late transitions. Applied to the minimal quantum-tunneling model, these constraints allow an $$\mathcal{O}(10\%)$$ decrease in the total vacuum energy for a transition redshift $$z_t$$ of order unity. For nonminimal models, dark-matter-density-dependent tunneling can proceed rapidly enough to evade such bounds. These results demonstrate that late-time quantum vacuum decay is a testable cosmological phenomenon and provide a concrete observational handle on metastable-vacuum physics motivated by landscape scenarios.
Vacuum energy is not an empty abstraction: if you gathered it from a cubic centimeter of empty space, it would be enough to boil a kettle, but by current estimates its density is $10^{120}$ times smaller than theory predicts.
In Greg Egan's novel Schild's Ladder, the vacuum suddenly transitions to a lower energy state, and the boundary of the new vacuum expands at the speed of light, destroying everything in its path.