# New approach to dark energy might explain our cooling universe

NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI) By Anil Ananthaswamy OUR universe’s relentless march towards cold, empty darkness could be causing its expansion to accelerate, rather than the other way around. The finding could help cosmologists think differently about dark energy, and possibly explain why it has the value it does. In the late 1990s, astronomers observed that the universe’s expansion is accelerating. They attributed this to dark energy – an inherent property of the vacuum of space-time. One idea is that dark energy is really the cosmological constant, a quantity arising in Einstein’s general relativity. But when we calculate its theoretical value, the answer is about 120 orders of magnitude larger than the observed one. The mismatch has vexed cosmologists for decades. “Why does the cosmological constant have the value it does? Why is it so small?” says Sean Carroll at the California Institute of Technology in Pasadena. “Dark energy emerges from quantum space-time, and then drives the accelerated expansion of the universe” Although they can’t solve that yet, Carroll and his student Aidan Chatwin-Davies suggest we could make headway using something quite different: the laws of thermodynamics. “We are contributing to a movement to change that question to something else,” says Carroll. We have had an idea of the universe’s end state since 1983, when Robert Wald of the University of Chicago showed that a universe with a positive cosmological constant will end up as a flat, empty, featureless void, called de Sitter space. Wald did this using general relativity. But some physicists had long suspected that you could reach the same end state using thermodynamics. The link with thermodynamics also dates back to the 1980s. Tom Banks at the University of California, Santa Cruz, suggested then that the value of dark energy could be related to the entropy of space-time. Entropy is a measure of the disorder of a system: it’s low for a solid with rigidly organised atoms, and high for a hot gas with chaotically moving atoms. What if the system is “closed”, in that it can’t exchange energy with its surroundings? According to the second law of thermodynamics, its entropy will keep growing until it reaches an equilibrium. If we regard the entire universe as closed, the law suggests that it too will eventually reach a state of peak entropy and just stay there. That sounded a lot like de Sitter space to Carroll and others. Having a thermodynamic route to the same universal end game could help break the stalemate over the mismatch problem. But they couldn’t prove the route existed. Some clues came from black-hole physics. In 1974, Israeli physicist Jakob Bekenstein showed that the entropy of a system containing a black hole and its immediate environment grows – a result now called the generalised second law of thermodynamics. This hinted that the final state of the universe predicted by general relativity was related to growing entropy. “The missing ingredient was some way of formulating the [generalised] second law in a way that was applicable to the whole universe all at once,” says Carroll. That came last year, when Raphael Bousso at the University of California, Berkeley, and Netta Engelhardt, now at Princeton University, applied Bekenstein’s idea to a patch of space-time in an expanding universe like ours. They conjectured that its entropy increases. Carroll and Chatwin-Davies took Bousso and Engelhardt’s definition of entropy – which uses a quantum mechanical description of space-time – as their starting point. They then calculated what happens to the geometry of space-time as it evolves. Lo and behold, once a universe has reached peak entropy it is effectively one described by de Sitter geometry, they proved (arxiv.org/abs/1703.09241). “The universe will approach de Sitter space and stay there forever,” says Carroll. Bousso is impressed. “This is a beautiful application of our cosmological second law,” he says. This thermodynamic way of thinking turns the standard view of dark energy on its head: dark energy emerges from the quantum structure of space-time and then drives the accelerated expansion. Solving the mystery of dark energy’s value then becomes a case of justifying the choice of a particular quantum mechanical description of space-time. Carroll is careful not to overstate the implications. “There’s what we proved, and there is the coffee-shop chatter about what it might imply, whichis of course much more speculative,” he says. But the work could offer a new way to grapple with the cosmological constant’s tiny value. This article will appear in print under the headline “Dark energy flipped upside down” More on these topics: