Tag: Type 1a supernovae

A galaxy surrounded by galaxies

11:12 am

A galaxy surrounded by galaxies
Click for original image.

Cool image time! The picture to the right, cropped, reduced, and sharpened to post here, was taken by the Hubble Space Telescope as part of a survey of galaxies where Type 1a supernovae have occurred, in order to better refine the precise brightness of these explosions.

What makes this galaxy most interesting are the hundreds of other galaxies that appear to surround it. And that ain’t an illusion.

NGC 3285B is a member of the Hydra I cluster, one of the largest galaxy clusters in the nearby Universe. Galaxy clusters are collections of hundreds to thousands of galaxies that are bound to one another by gravity. The Hydra I cluster is anchored by two giant elliptical galaxies at its centre. Each of these galaxies is about 150,000 light-years across, making them about 50% larger than our home galaxy, the Milky Way. NGC 3285B sits on the outskirts of its home cluster, far from the massive galaxies at the centre.

As for the survey program, Type 1a supernovae are the measure cosmologists have used to discover the unexpected acceleration of the universe’s expansion rate at the largest scales, something they dub “dark energy” because they really don’t understand what they have discovered. That discovery however hinges entirely on the assumed intrinsic brightness of Type 1a supernovae. Astronomers have assumed these supernovae all have the same approximate brightness, and extrapolate their distance by that brightness.

The problem are the assumptions. We really don’t know if all Type 1a supernovae are approximately the same brightness. And even if they are equally bright, we also do not have a firm grasp of what that brightness should be.

This survey is an attempt to narrow or eliminate these uncertainties.

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Dark energy evidence found to be uncertain

10:10 am

The uncertainty of science: Astronomers have discovered that the type of supernovae they have used as a standard to measure the accelerating expansion of the universe, which also is evidence for the existence of dark energy, are actually made up of two different types.

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. “We’re proposing that our data suggest there might be less dark energy than textbook knowledge, but we can’t put a number on it,” Milne said. “Until our paper, the two populations of supernovae were treated as the same population. To get that final answer, you need to do all that work again, separately for the red and for the blue population.”

The authors pointed out that more data have to be collected before scientists can understand the impact on current measures of dark energy.

It has always bothered me that the evidence for dark energy was based entirely on measurements of type 1a supernovae from extremely far away and billions of years ago. Not only was that a different time in the universe’s history when conditions could be different, our actual understanding of those supernovae themselves is very tenuous. We really do not have a full understanding of what causes them, or how they even happen. To then assume that these distant explosions are all so similar that their brightness can be used as a “standard” seems untrustworthy. From my perspective, the conclusions, though interesting, are being pushed based on extremely weak data.

The research at the link illustrates just how weak that data was.

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In a paper published today in Science, astronomers show that Type 1a supernovae, the kind used to measure the expansion rate of the universe, can be caused in more than one way, something not previously expected.

11:44 am

The uncertainty of science: In a paper published today in Science, astronomers show that Type 1a supernovae, the kind used to measure the expansion rate of the universe, can be caused in more than one way, something not previously expected.

Andy Howell, second author on the study, said: “It is a total surprise to find that thermonuclear supernovae, which all seem so similar, come from different kinds of stars. It is like discovering that some humans evolved from ape-like ancestors, and others came from giraffes. How could they look so similar if they had such different origins?” Howell is the leader of the supernova group at LCOGT, and is an adjunct faculty member in physics at UCSB.

Recently, some studies have found that Type Ia supernovae are not perfect standard candles –– their brightness depends on the type of galaxy in which they were discovered. The reason is a mystery, but the finding that some Type Ia supernovae come from different progenitors would seem to suggest that the supernova’s ultimate brightness may be affected by whether or not it comes from a nova or a white dwarf merger.

“We don’t think this calls the presence of dark energy into question,” said Dilday. “But it does show that if we want to make progress understanding it, we need to understand supernovae better.”

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Astronomers now believe that Type 1a supernovae — used to discover dark energy — can be produced in two different ways.

9:52 am

The uncertainty of science: Astronomers now believe that Type 1a supernovae — used to discover dark energy — can be produced in two different ways.

Type Ia supernovae are known to originate from white dwarfs – the dense cores of dead stars. White dwarfs are also called degenerate stars because they’re supported by quantum degeneracy pressure. In the single-degenerate model for a supernova, a white dwarf gathers material from a companion star until it reaches a tipping point where a runaway nuclear reaction begins and the star explodes. In the double-degenerate model, two white dwarfs merge and explode. Single-degenerate systems should have gas from the companion star around the supernova, while the double-degenerate systems will lack that gas.

For astronomers, this possibility raises several conflicting questions. If two different causes produce Type 1a supernovae, could their measurement of dark energy be suspect? And if not, why is it that these two different causes produce supernovae explosions that look so much alike?

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