Dark Energy: Not So Constant?

We live in an exciting time for cosmology.

In 1998, a landmark discovery revealed that the universe is not just expanding, but that the expansion rate is accelerating—driven by a mysterious component called dark energy, which makes up most of the cosmos. Yet its nature remains one of the biggest open questions in physics.
 
For a long time, the simplest way to account for this acceleration was Einstein’s cosmological constant, a fixed property of spacetime. Elegant—but perhaps too simple. Now, new results from large-scale surveys are starting to unsettle this picture, hinting that the cosmological ‘constant’ may not be constant after all. What if the force shaping our universe is evolving—and the story of the cosmos turns out to be very different from what we thought?
 
On Thursday May 21, in a special Science Café edition at De Achtertuin, we explore these developments from two complementary perspectives. Cosmologist Guadalupe Cañas Herrera will guide us through the evolution of the universe—from the standard ΛCDM model to the emergence of current observational tensions—and show how cutting-edge surveys such as Euclid are designed to test competing ideas about dark energy. Theoretical physicist Alessandra Silvestri will take us into the landscape of possible explanations, from the cosmological constant to dynamical dark energy and modified gravity, and what these models might reveal about new physics.