ALL supernovae are not created equal, and those that exploded roughly 5
billion years ago may have shone less brightly than the supernovae blasting away
today, claims a team of astronomers. The result calls into question the idea
that the Universe is expanding ever more rapidly.
Until two years ago, most astronomers assumed that the speed at which the
Universe is expanding was slowing down because of the gravity of the matter
within it. But then two teams of astronomers studying supernovae made a
mind-bending discovery. They found that the Universe is apparently growing at an
ever-increasing rate. Theorists have sought to explain this by suggesting that
the cosmos must be filled with a strange form of energy that spreads and
stretches space and time.
However, the evidence for an accelerating Universe rests on the assumption
that all so-called type Ia supernovae emit nearly the same amount of light. This
premise allows researchers to calculate the relative distance of two far-off
type Ia supernovae by comparing their apparent brightness. When astronomers used
this technique to measure the distances of ancient supernovae they found them to
be farther away than expected, indicating that expansion of the Universe is
speeding up.
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But is it really safe to assume that type Ia supernovae are the unwavering
beacons of the cosmos? Perhaps not, says a team of British and Australian
astronomers. They say in an upcoming edition of Astrophysical Journal
Letters that one popular model for the workings of type Ia supernovae
suggests that ancient supernovae may be naturally dimmer than younger
ones.
This model says that type Ia supernovae arise when a white
dwarf鈥攖he carbon and oxygen core of a dead star鈥攇obbles up a
companion made mostly of helium, says Christopher Tout of the University of
Cambridge. The helium builds up on the surface of the white dwarf, and when
thick enough this coating of nuclear fuel will begin to undergo nuclear fusion.
The reaction causes an inward-moving wave of heat or pressure that ignites a
runaway fusion reaction at the heart of the white dwarf and blows it to
bits.
The explosion, called an edge-lit detonation, releases an amount of light
that depends on the 鈥渕etallicity鈥� of the star that produced the white
dwarf鈥攖hat is, the percentage of its mass that consists of elements
heavier than helium. Astronomers know from studies of distant galaxies that the
metallicity of ancient stars was lower than the 2 per cent found in today鈥檚
stars, so type Ia supernova may have been dimmer billions of years ago.
But this picture has problems of its own, says Mario Livio of the Space
Telescope Science Institute in Baltimore, Maryland. Edge-lit detonations should
produce bluer light than is seen in type Ia supernovae. They should also eject
large quantities of helium moving at great speed, which type Ia supernovae do
not. But the new finding, and the doubt it casts on the accelerating Universe
scenario, is still worth taking seriously, says Livio. 鈥淚 think it鈥檚 healthy to
be sceptical.鈥�