Dark Energy Is Inconstant: The Universe May Breathe ✛ ✎

Our universe was born in the Big Bang and has been expanding ever since. For a long time, it was thought that the expansion was being slowed by the gravitational pull of all matter, but in the late 20th century, astronomers observing distant supernovae were surprised to discover the opposite: the universe is flying apart faster and faster. This acceleration is driven by mysterious dark energy. Along with dark matter — invisible mass that we learned about thanks to Vera Rubin and her studies of galaxy rotation — it makes up 95% of the cosmos's contents. The simplest guess, going back to Georges Lemaître, says that dark energy is like an inexhaustible property of emptiness and never changes.

However, recent observations, including detailed sky maps from Planck and DESI telescopes, as well as spectroscopic surveys of millions of galaxies (where light is split into colors to measure distances) and gravitational lensing effects (the bending of light rays by massive objects), paint a different picture. When scientists allowed dark energy to evolve, it explained the data better than the constant version. This preference holds at a confidence level of 2–3σ — which is quite compelling for such tasks. At the same time, it turned out that constraints on neutrino mass and on the presence of primordial gravitational waves (ripples in spacetime that could have arisen at the universe's birth) change noticeably depending on whether dark energy is dynamic. For now, these waves haven't been detected, meaning inflation was relatively calm.

There's no practical use here, but the meaning is enormous. If dark energy really does change, predicting the distant future of the universe becomes more difficult: eternal acceleration could give way to deceleration and even contraction. Moreover, this forces us to reconsider the boundaries of the Standard Model of physics and search for deeper theories. The mystery of cosmic acceleration remains open, but now we better understand just how flexible our picture of the world must be.