By Dr David Whitehouse
BBC News Online science editor
Observations of distant supernovae - exploding stars - made by the Hubble telescope give new information on the so-called dark energy of the cosmos.
Robert Knop and a "supernova"
This mysterious force explains the 1998 discovery that the expansion of the Universe is actually accelerating.
The amazing finding was produced by examining the acceleration's effect on the light coming from supernovae.
Now, Hubble has provided the best data to confirm the phenomenon and limit the nature of the dark energy itself.
Supernovae (SN) are among the most cataclysmic events in the Universe.
For a few days or weeks, these exploding stars can outshine an entire galaxy of their companions, and be visible from the other side of the cosmos.
They come in two types. The Type 1a supernovae are used by astronomers as "standard candles"; their similarity means their peak brightness can provide a dependable measure of their distance.
In a galaxy far, far away...
When using them to measure the large-scale structure of the Universe, it is vital to have observations that are as detailed as possible.
This means using the Hubble Space Telescope (HST) which has a view that is unhindered by the Earth's distorting atmosphere.
"It is particularly important to separate the SN from their host galaxy. You have to have as sharp an image as is possible, and this is best done from space," Professor Robert Knop of Vanderbilt University told BBC News Online.
"From the HST you have much lower sky background and much higher resolution."
The new Hubble study involves 11 SN observed in unprecedented detail.
Hubble at the ready
It reinforces the remarkable discovery announced by the Supernova Cosmology Project (SCP) early in 1998 that the expansion of the Universe is accelerating due to a mysterious energy that pervades all space.
That finding was based on data from over three dozen Type 1a supernovae, all but one of them observed from the ground.
Now, the search has gone into space as the SCP researchers have devised a method to identify supernovae using ground-based telescopes, and then move to the HST for more detailed follow-up observations.
While the data on the new supernovae are the best yet, the core finding remains the same.
"The SN appear fainter than they should be. This means they are further away than we expected because the expansion of the Universe has been accelerating," says Professor Knop.
The only way to explain this, according to most researchers, is to invoke so-called dark energy - a repulsive form of gravity which has its origin in the fabric of space itself.
Among the numerous attempts to explain the nature of dark energy, some are allowed by these new Hubble measurements - including an idea originally proposed by Albert Einstein.
The next phase could be a snap
However, others have now been ruled out, including some of the simplest models of the theories known as quintessence.
Knop and colleagues now hope the SN sample will grow to the point where it can help them determine some of the properties of the dark energy and even tell them exactly what it is.
As they use the HST, the astronomers are looking forward to the next phase of supernova research. This is the proposed SuperNova/Acceleration Probe, or Snap satellite, which should discover thousands of Type 1a supernovae and measure their properties until their light has died away.