Back in the day, the universe could really belt out the supernovae — but no more. A new survey of monster stellar detonations shows that the more distant and ancient the explosions are, the greater their inherent brightness. This discovery has important implications for measuring the rate at which the universe is expanding, and therefore that weird space-making cosmological mystery called dark energy.
The ongoing Supernovae Legacy Survey
, which led to the discovery, involves comparing the supernovae of nearby galaxies to those which can be spotted igniting up to nine billion light-years away.
That distance means the light from these galaxies is nine billion years old, so they are images of the past. The researchers found that the more distant supernovae were on average 12 percent brighter than their modern counterparts.
"It is a little bit of a mystery as to why," said astronomer Andrew Howell, a post-doctoral researcher at the University of Toronto. Howell is the lead author of a paper on the matter published in the September 20 issue of Astrophysical Journal Letters.
The types of supernovae involved in this case are not those gigantic eruptions
which signal one-time, implosion-triggered suicides of super-massive stars. Rather, they are repeating type Ia supernovae
. These are believed to be created by a shrunken stellar corpse called a white dwarf as it sucks material off a companion star — probably a bloated, dying red giant star.
The explosions come about when the material piles up on the white dwarf’s surface until the bottom of the pile reaches critical density and it sets off a gigantic thermonuclear blast.
Theoretically all type Ia supernovae should look pretty similar throughout the history of the universe, making them great bright mile-markers with which to measure how quickly the universe is expanding.
It’s as if every supernova were a person with an identical flashlight on a football field on a very dark night. The closer a flashlight is to an observer, the brighter it appears, while the ever more distant flashlights would look proportionately dimmer. But what if all the nearby flashlights had weaker batteries? That’s pretty much what the astronomers have found.
"It’s a very fundamental issue," Nugent told Discovery News. In order for astronomers to measure the speed at which the universe’s expansion is accelerating — which is attributed to dark energy making more space and driving things apart — they need very precise measurements of these supernovae. But this newfound, unexplained supernovae misbehavior makes the quest to measure dark energy all the more challenging.
"To date it hasn’t affected the cosmology measurements," said Nugent. But as more data is collected on more of the now thousands of known supernovae, that could change.