Scientists Have Hunted Dark Energy for Decades. They May Be Chasing a Cosmic Ghost.

In 1916, Albert Einstein published his foundational paper on the General Theory of Relativity in the German-language science journal Annalen der Physik. Now, 110 years later, scientists are still debating just how right Einstein’s conception of the universe really was, especially as it relates to the accelerated expansion of the universe. Of course, at the time of his revolutionary publication, the world still didn’t know that the universe was expanding, so a year later, Einstein introduced a “lambda” cosmological constant to his equations to form a stable, non-collapsing universe.Thanks to the pioneering work of Edwin Hubble in 1929, the world learned that the universe was actually expanding, and in response Einstein discarded lambda, calling it his “greatest blunder.” What Einstein couldn’t have known was that, decades after his death, lambda would make a comeback. When astronomers discovered in the late 1990s that the universe’s expansion was speeding up, the constant became central to the modern model of the cosmos, known as “Lambda-Cold Dark Matter,” or ΛCDM. Rising along with the renaissance of the cosmological constant was a concept known as dark energy, the force that made this accelerated expansion possible.This ΛCDM is a pretty tidy description of the universe with one glaring plot hole—we’ve never directly seen or measured dark energy. Because of this lack of evidence, some scientists have searched for other possible explanations, even disregarding Einstein entirely, but a new study by researchers at the University of California, Davis takes the opposite approach. According to their research, published earlier this year in the journal Proceedings of the Royal Society A, Einstein was actually more right than he ever knew.To build their case for a universe without dark energy, UC Davis mathematics professor emeritus Blake Temple and his colleagues turned to Friedmann spacetimes. These are a family of solutions to Einstein’s field equations originally developed by Alexander Friedmann, the Soviet mathematician who first proposed that the universe is expanding.“The Friedmann family of spacetimes describe a uniform three-dimensional Universe of galaxies expanding in time from an initial Big Bang singularity at a rate determined by Einstein’s field equations,” the authors write in the study.The researchers rewrote Einstein's equations in a “self-similar” form, meaning the equations keep the same structure at any scale, much like a fractal. This version of the equations let them test a key question: if the smooth, even spread of matter in a Friedmann universe gets slightly disturbed, does it settle back to normal or move further away from it? They found that the small disturbances don’t fade away. Instead, they grow, at both large and small scales. In other words, a perfectly uniform Friedmann universe can’t stay that way.“Unstable solutions in physics and science are considered not physical. You’ll never observe them in nature,” Temple said in a press statement. “The instability of all Friedmann spacetimes to accelerated expansion suggests a simpler, more natural explanation for the acceleration of the universe than dark energy.”That instability is the heart of the argument. Einstein’s own static universe had a similar flaw: it was so delicately balanced that the slightest nudge would tip it over. “We prove that, like Einstein’s static model, the Friedmann spacetimes are all unstable to radial perturbation at large length scales,” Temple added. As the universe drifts away from a perfect Friedmann state, the team argues, its expansion naturally speeds up. That would mean Einstein’s original equations, without lambda, may explain the acceleration we see today, without dark energy.The work is purely mathematical, though, and it comes with a catch. The team’s model has a center, so we’d need to live near a special spot in the universe for it to match what astronomers see. That goes against a basic assumption of modern cosmology. Temple argues that ΛCDM has the same problem, however.For now, scientists around the world continue to probe for the elusive dark matter and dark energy that presumably dominates the universe (with some detectors reporting encouraging results). But the study raises an intriguing possibility: we might be chasing a cosmological ghost.Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.