What are the most distant objects seen in the Universe?

If you had asked this question before September 2005, most astronomers would have said that quasars are, by far, the most distant objects in the Universe. This has changed with the detection of a gamma-ray burst originating at the edge of the visible Universe.

A quasar is a distant galaxy with a supermassive black hole at its core. The black hole is pulling in interstellar gas at a furious rate. This releases energy---that is, light. The quasar, short for quasi-stellar object, is so far that this brilliant light looks like a pinpoint, or a star. There were other objects in the early Universe, to be sure. But the quasars are so bright that these are all we can see from this era... until now.

On September 4, 2005, NASA's Swift satellite detected a very distant gamma-ray burst. These bursts are the most powerful explosions known. They occur from our perspective at a rate of about one per day. They are random and only last for a few seconds, though, so they are hard to detect. Scientists say they arise from the explosion of very massive stars, more massive than a supernova. Gamma-ray bursts signal the birth of a black hole, created as massive stars explode. They temporarily outshine quasars.

Scientists have known that gamma-ray bursts are distant, just like quasars. Quasars, however, have been the record holders. About a dozen quasars have been detected over 12 billion light years away, from when our 13.7-billion-year-old Universe was quite young. The most distant gamma-ray bursts have been about 10 billion light years away.

Then came GRB 050904, the September 4 burst. Scientists measure cosmic distances via redshift, the extent to which light is "shifted" toward the red, or lower energy, part of the electromagnetic spectrum during the light's long journey across the Universe. The greater the distance, the higher the redshift. GRB 050904 had a redshift of 6.29, which translates to a distance of about 13 billion light-years from Earth. Only a handful of quasars are over redshift 5.5.

One quasar is at redshift 6.4, so technically this quasar is still the record holder. But here's the catch: Scientists have mapped out hundreds of thousands of quasars, and only three are beyond redshift 6. In fact, the quasar population seems to peak around redshift 2 (about 10 billion light years away) and teeters out at redshift 5 (about 12 billion light years away). Swift has detected fewer than 100 gamma-ray bursts, and already it has found one at redshift 6.29. Surely there are more distant ones to be found.

Star formation began about 200 million years after the Big Bang, at a redshift between 20 and 10. Swift could detect these very first stars when they explode. Of course, these stars have already exploded billions of years ago. Their light takes 13.5 billion years to reach us. It could arrive tomorrow in the form of a gamma-ray burst!

Swift isn't working alone. Swift detects bursts and provides a location. Powerful ground-based telescopes then view the burst afterglow to determine the distance. All the tools are now in place to find exceedingly distant gamma-ray bursts. Through the study of gamma-ray bursts, scientists hope to understand when and how the first stars formed. Each burst also serves as a probe, because the light contains information of all the material it passes through on the journey towards Earth---like a well-traveled suitcase with stickers from around the world.

This week's question comes from Christopher Wanjek. Mr. Wanjek is a science writer supporting the Beyond Einstein initiative, a roadmap to understand the forces of nature beyond General Relativity and Quantum Mechanics through the study of the Universe from the Big Bang to black holes.