Wednesday, April 29, 2009

The most distant object ever seen

On April 23, NASA's Swift satellite detected a gamma-ray burst. It detects one every few days or so, so a new gamma-ray burst is not exciting in and of itself. But when astronomers looked at the burst with optical telescopes on Earth, they learned that it was the most distant object ever seen. Not just that, but it utterly smashed the previous record. Nancy Atkinson of Universe Today wrote a great article explaining this exciting discovery, so I'll be lazy and refer you to her post for more information.

One thing I appreciate about the story, and about this announcement in general, is that the story quoted the redshift of the object. So many astronomy press releases give distances in billions of light years. That practice makes sense for public news stories, because light-years have been the distance unit of choice for public news.

But as an observational astronomer, I rarely think in light-years for distant galaxies, I think in terms of redshift. Redshift is the amount that light has been stretched by the expansion of the Universe; redshift of zero means that the light is unaffected, redshift of one means that its wavelength has been doubled, redshift of two means it's been tripled, and so on. The redshift is the true observable, meaning that when we look at the light from distant objects, we measure this stretching. Astronomers then use models of the Universe to convert the redshift to light years.

Until recent times, astronomers were unsure what the proper model of the Universe was, and so the conversion from redshift to distance was uncertain. Due to the success of missions like WMAP, we now think we understand the fundamental structure of the Universe. But, by then, we'd gotten used to speaking in terms of redshift. So, if you talk about an object that is 12.6 billion light years away or 12.9 billion light years away, that doesn't mean much to my brain. If, instead, you talk about objects that are at redshifts of 6.5 and 8.2, I get it.

There's also always the possibility that we've made some fatal flaw in our understanding of the Universe, which would render our conversions from redshift to light years incorrect. I seriously doubt this is the case, and I'm willing to bet my friends' careers that we've nailed the conversion. But until we know for sure what dark matter and dark energy are, I'm not willing to bet my own career. :)

Congratulations to the astronomers who made this exciting discovery, and thanks a million for the redshifts!

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