A team of scientists are challenging one of the most significant cosmological claims of recent decades: the pace at which the expansion of the universe is accelerating. Measurement of this acceleration led to the discovery of dark energy, and the 2011 Nobel Prize for physics, but a new paper says there is less acceleration, and therefore less dark energy, than we thought.
To measure how fast the universe is expanding, we need to know how far away distant galaxies are. This is done by measuring Type Ia supernovae. While other forms of supernova vary substantially in brightness depending on the size of the exploding star, Ia supernovae are thought to have an intrinsic luminosity related to the width of their light curve. Consequently, by comparing the brightness we see with the expected light emitted, we can estimate their distance.
The notion that all Type 1a supernovae are the same has come under question before. It is thought that “non-standard” Type Ias are caused by two white dwarfs colliding, rather than material from a companion star drawn into a white dwarf until its mass passes the Chandrasekhar Limit, as occurs in “normal” cases.
However, in The Astrophysical Journal, Dr. Peter Milne of the University of Arizona argues that even standard Type Ia supernovae come in two types, which he classifies as NUV-red and NUV-blue. Red events spit out material 12% faster, on average, than blue versions, which in turn affects their light curve. The difference is so small in the visible spectrum that it was missed until Milne used the Swift Ultraviolet space telescope, which revealed the comparison more clearly.
The category difference would be interesting, but wouldn't matter much if each type were evenly distributed. However, Milne says this is not the case.
"We found that the differences are not random, but lead to separating Ia supernovae into two groups, where the group that is in the minority near us are in the majority at large distances—and thus when the universe was younger," Milne said. Two-thirds of nearby Type Ia supernovae are NUV-red, but in the early universe around 90% were NUV-blue, Milne found.
Easily observable supernovae of this type are frustratingly rare, leaving Milne with a sample for “nearby” supernovae (those less than 2.7 billion light-years away) of only 23. After allowing for evidence that earlier supernovae were bluer, and therefore brighter, than we realized, Milne calculates that the universe is accelerating less than previously estimated.
Milne's work does not remove the need for dark energy entirely, since it still suggests an acceleration is occurring, albeit at a slower rate. However, if the work is accepted, it will force a recalculation of the relative amounts of different kinds of matter and energy in the universe.
Type Ias' assumed consistency has been used for other work as well, such as research showing that gravity has not changed with time. If Milne is right, some of these projects may also need reworking.
The reasons for the two types are not known, but may relate to the concentration of heavy elements.