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.