Looking beyond lambda

Abstract

Widening cracks are appearing in the Λ cold dark matter (ΛCDM) model (where Λ is the cosmological constant) and it is becoming increasingly clear that the standard cosmological model struggles to describe the full expansion history of the Universe as revealed by the cosmic microwave background, baryon acoustic oscillation measurements, and locally calibrated type Ia supernovae. Taken at face value, recent results suggest a dark sector that may be more complex than commonly assumed. We must prepare for the possibility of moving beyond the ΛCDM era, where merely testing if the dark energy equation of state w = −1 is no longer sufficient, and embrace the challenge of unravelling the physics of dark matter, dark energy and gravity on cosmic scales. Guided by increasingly robust data—secured through considerable investment—we should pursue deeper understanding while being open to complexity in the dark sector, rather than settling for the simplest phenomenology. Data from new facilities and a new Dark Energy Task Force could help illuminate the path forward while changes to our scientific practices will be essential to navigate the potentially rocky road ahead. The Dark Energy Spectroscopic Survey’s latest results indicate a tension between observations and the cosmological constant model of dark energy. This Perspective puts these findings in context and suggests a way forward for dark energy research.