Why is the universe being ripped apart? It’s a question that has plagued astronomers since the discovery in the 1990s that the expansion of the universe is accelerating. The story is only further complicated by new observations of distant exploding stars that cast doubt on the leading explanation, called the cosmological constant.
Whatever is causing the universe’s acceleration has been named dark energy, but its origins remain mysterious. Back when Albert Einstein was formulating his general theory of relativity he added a repulsive force to his equations called the cosmological constant, which was meant, at the time, to cause the theory to predict a static universe. Without it, his calculations showed gravity would not result in a steady-state universe, but rather would have caused it to collapse upon itself. When it was later discovered that the universe isn’t static, but expanding, Einstein dropped the constant, reportedly calling it his “biggest blunder.” Decades later, however, when it was revealed that the universe was not merely expanding, but that its dilation was accelerating, scientists retrieved the discarded constant and added it back to the general relativity equations to predict a universe that’s flying apart at increasing speed. The cosmological constant is now the leading idea to account for dark energy, but it only works if what is known as the dark energy equation of state parameter (relating pressure and density), called w, equals –1.
But that is not what the latest experiment, Pan-STARRS (for Panoramic Survey Telescope and Rapid Response System), found. Based on cosmological measurements from other projects combined with Pan-STARRS observations of a special type of stellar explosion called a type Ia supernova, which can be used as a cosmic ruler for measuring astronomical distances, researchers calculated w’s value at −1.186. “If w has this value, it means that the simplest model to explain dark energy is not true,” says Armin Rest of the Space Telescope Science Institute (STScI) in Baltimore, lead author of a paper reporting the results posted October 22 to the astrophysics preprint Web site arXiv. Rest cautioned, however, that the results are too preliminary to seriously doubt the cosmological constant at this point. “I don’t think we can say now that we’ve really found a discrepancy. We still have to look if this is due to some issues with any of these projects.”
The calculation is based on observations of about 150 type Ia supernovae made between 2009 and 2011 by the Pan-STARRS telescope PS1 in Hawaii. This class of supernova occurs when a particular type of star called a white dwarf reaches its maximum mass limit, which is the same for all white dwarfs, and explodes with a standard brightness. By comparing a supernova’s apparent brightness with its known intrinsic brightness, astronomers can deduce how far away it is. Follow-up spectroscopic observations of the supernova, which break light down to its constituent colors, reveal how much the light’s wavelength has been stretched by the expansion of the universe. With these parameters in hand, the Pan-STARRS researchers combined their data with the findings from other probes of dark energy, such as the observations of the cosmic microwave background light from the European Planck satellite, to calculate the dark energy equation of state parameter.
How much to read into the calculation depends on how uncertain it is, and whether systematic errors associated with the telescope and the analysis skewed the result. “It’s generally accepted that telescope calibration, supernova physics and galaxy properties are big sources of uncertainties, so everyone’s trying to figure these out in different ways,” says Daniel Scolnic of Johns Hopkins University, who led an accompanying paper estimating the data’s uncertainties. “I think that Dan did an excellent job characterizing their systematics,” says Alexander Conley of the University of Colorado at Boulder who worked on a different supernova study called the Supernova Legacy Survey that found similar results. “They still have a lot of work to do to improve the characterization for future papers, but they know that and are working on it.” However, another survey researcher, Julien Guy of University Pierre and Marie Curie in Paris, says the team may have underestimated their systematic error by ignoring an extra source of uncertainty from supernova light-curve models. He’s been in touch with the Pan-STARRS researchers, who are looking into that factor. Ultimately, most experts say the new results are tantalizing, but don’t prove the existence of new physics. “The Pan-STARRS paper presents a very thorough, careful analysis and a solid result, but it doesn't qualitatively change our view of the cosmological parameters,” says Joshua Frieman, an astrophysicist at Fermilab in Batavia, Ill., who was not involved in the research.
The fact that multiple cosmology experiments are producing values of w that diverge from –1, however, is causing many to take notice. “This paper is now the third survey of distant supernovae that’s coming to this conclusion,” says STScI astronomer Adam Riess, a member of the Pan-STARRS team who won the 2011 Nobel Prize in Physics for the discovery of dark energy. “We can’t just say this survey or that survey screwed up. It could be something fundamental to one of these measurements. Or it could be that dark energy is more interesting in a way that actually we hope.” Whereas the cosmological constant explains dark energy mathematically, it does not elucidate why such a force exists. An alternative value of w might indicate that dark energy hasn’t been constant over time, but varies—an idea called quintessence. Either way, more data from Pan-STARRS and other surveys are expected soon to either support or refute the latest value of w. “I expect in the next year or two this will probably either become definitive, or go away,” Riess says.