Cosmic Ruler Flawed
Type 1a supernovae, vital to estimates of the size and expansion of the universe, are not uniform. This has cosmic implications.
A team from the University of Arizona has news of cosmic proportions. For many years, Type 1a supernovae have been considered “standard candles” at all distances. This allows astronomers to calculate cosmic distances, and in fact was used to deduce the accelerated expansion of the universe in the late 1990s that won three astronomers a Nobel Prize. Without a known cause for cosmic acceleration, astronomers have proposed some unknown kind of “dark energy” as the cause. All of this has relied on the assumption of uniformity of Type 1a supernovae (see 9/30/12, 3/15/08, and 11/01/06).
A UA press release now calls that assumption into question. Using data from the Swift satellite, which measures stars in ultraviolet light, the UA astronomers found that Type 1a’s fall into two classes. The nearest ones are redder than the more distant ones. Reporter Daniel Stolte titled his article, “Accelerating universe? Not so fast.”
“Since nobody realized that before, all these supernovae were thrown in the same barrel. But if you were to look at 10 of them nearby, those 10 are going to be redder on average than a sample of 10 faraway supernovae.”
The authors conclude that some of the reported acceleration of the universe can be explained by color differences between the two groups of supernovae, leaving less acceleration than initially reported. This would, in turn, require less dark energy than currently assumed.
The team is unable to put a number on how much the figures will need to be adjusted, saying further work is needed. Meanwhile, though, another mission is seeking to measure dark energy if it exists. The DESI spectroscope (Dark Energy Spectroscopic Instrument) will be fit at the prime focus of the Mayall 4-meter telescope at Kitt Peak (see photo) to measure 30 million galaxies’ worth of the universe in 3-D.
PhysOrg reports on the instrument built at University of Michigan:
Cosmologists suspect a mysterious property called dark energy. Although it is thought to comprise 75 percent of the universe, its nature and the physics behind it are still mysteries.
DESI will create a high-definition, 3-D map of a swath of the universe going back 10 billion light-years. By exploring how structure in the universe has evolved through time, scientists hope to uncover the tug-of-war between the forces of gravity and dark energy.
The UM astronomers need to talk to the UA astronomers.
If you win a Nobel Prize for a false conclusion, do you have to give the money back?
Note once again how assumptions play crucial roles in models that try to understand observations. Perhaps dark energy is real, though less than expected. There’s an outside chance though, if supernova populations are not as uniform as even the UA astronomers believe, that there is no dark energy at all. This should be a lesson in assumptions and unknowns in science. Conclusions in theory are only tentative. They can always be overthrown by later findings.