The Universe Without Its Longest Waves express
The universe is like an ocean, where waves of different lengths roll onto the shore. The longest waves are the giant fluctuations in the distribution of matter that we see in the cosmic microwave background. Standard theory says such waves should exist on all scales. But on the very largest scales, comparable to the size of the visible universe, observations do not give a clear answer: is there even a single wave?
The authors of the paper tested what would happen if there were no waves at all on those scales, as if the ocean suddenly ended at an invisible wall. They considered several versions of such a cutoff and compared them with the latest data of three types: the cosmic microwave background map, distances to distant galaxies from baryon acoustic oscillations, and supernova brightness. It turned out that a cutoff does slightly improve agreement with observations, but the improvement is so small that it does not outweigh the added model parameters.
The scientists also checked whether a cutoff could explain a recent discrepancy in reionization data — the epoch when the first stars ignited and ionized hydrogen. It turns out it does not: suppressing large-scale waves does not provide enough freedom to reconcile all the data. Thus, current observations do not require abandoning the standard spectrum, where waves exist on all scales.
Original abstract
as written by the authors arXiv:2602.16659
The nearly scale-invariant primordial power spectrum provides the standard initial conditions for cosmological perturbations. However, the largest scales remain only weakly constrained by CMB observations, leaving room for deviations such as an infrared (IR) cut-off. This possibility is further motivated by the persistence of large-scale CMB anomalies, most notably the low quadrupole power. In this work, we revisit several broad classes of phenomenologically motivated IR cut-off scenarios using parametrised functional forms of the primordial power spectrum. We confront these models with the latest CMB, BAO, and supernova data and derive updated constraints on the cut-off scale and associated features. Our results remain consistent with earlier studies, showing that although such models suppress power at low multipoles, the improvement in fit is marginal and does not overcome the associated parameter penalties. We therefore find no statistically significant evidence favouring IR cut-off models over the standard power-law spectrum with current data. We further explore the interplay between IR cut-off features and a possible increase in the reionisation optical depth, motivated by the recent CMB-BAO tension highlighted by DESI DR2 within the $$\Lambda$$CDM framework. We show that the additional freedom introduced by large-scale suppression is generally insufficient to support a substantial increase in optical depth, owing to the weak statistical preference for suppressed large-scale temperature power. Finally, we examine the implications of IR cut-off models for large-scale CMB anomalies by analysing the corresponding anomaly statistics within a Bayesian framework.
The lowest mode of the cosmic microwave background, the quadrupole, turns out to be noticeably weaker than the standard model predicts. This is one of the long-standing puzzles of cosmology, but statistically it is not strong enough to demand new physics.
In Carl Sagan's novel Contact, the protagonist hears a signal that repeats itself — a hint that the universe might be finite and closed, like the surface of a sphere. A cutoff in the spectrum on large scales would be similar to such finiteness: beyond the horizon, there are simply no waves.