In 2011, the Nobel Prize for Physics was shared by astrophysicists Saul Perlmutter, Adam Riess, and Brian Schmidt. Their accomplishment: uncovering evidence for the accelerating expansion of the universe.
This acceleration was what would come to be associated with a mysterious repulsive force called dark energy, which acts as its driver and which should monopolize the vast majority of all energy in existence.
Their discovery was based on observations of type 1a supernovae, the exploding stars considered to be "standard candles" by astronomers—their luminescence is constant enough such that it's possible to safely calculate distances between Earth and the supernovae based on brightness alone. By observing a related property known as redshift, in which the frequency of light arriving at Earth from a distant source is stretched as the source moves away from Earth, it's possible to calculate the speed at which a standard candle is moving away from us.
What Perlmutter, Riess, and Schmidt found is that this redshift doesn't quite match up to what should be expected given supernovae that are moving away from us at a constant speed. Indeed, they seemed to be speeding up—the universe isn't just expanding, it's expanding faster and faster the more it's flung apart.
A paper published Friday in the open-access journal Scientific Reports suggests that this conclusion may not be so assured as had been previously thought. Simply, we now have many more type 1a supernovae to observe than we did when the accelerating cosmos was first posited in the late-1990s. The original batch of standard candles consisted of 51 supernovae, while the new paper examined some 740. Its conclusion: "We find, rather surprisingly, that the data are still quite consistent with a constant rate of expansion." No acceleration, no dark energy. Welp.
OK, well, it's not quite that easy. It's not that astrophysicists have just been ignoring the presence of all these extra supernovae. Generally, the discovery of more standard candles has only helped confirm acceleration. What the new study actually offers is an alternative mode of statistical analysis, one that arguably better takes into account varying light curves (light is bent all around by gravity as it travels through space) and the effects of cosmic dust. We've been treating things too simply, the study argues. A more complex view offers perhaps a simpler, less terminal cosmos.
To be clear, there is other evidence for the accelerating expansion of the universe besides the redshifts of standard candles. For example, observations of the Cosmic Microwave Background indicate that matter can contribute only a small portion of the total energy density of the universe—it seems there is a missing component that dark energy fits very nicely. Moreover, the formation of structure in the early-universe would seem to suggest that cosmological expansion could not have been constant since the beginning of time and at some point must have been much less.
I should admit some bias here. I'm a huge fan of dark energy because I'm a huge fan of the idea that at some finite point in the future the universe will become an elegant inverse of its Big Bang birth: a quiet, extinguished darkness of boundless nothing, flung apart to infinity. The alternative, to me, just seems like sprawl.