Seeing Halfway to the Beginning of Time

Posted by jns on 26 September 2008

Once again Physics News Update delivered what I thought was a really cool story. It concerns an incredibly bright gamma-ray burst from an incredibly distant object–more distant than anything ever seen before: 7,000,000,000 light years away!

The object exploding was an old star that had used up all its fuel, the end of the sequence in which hydrogen is fused into helium, helium is fused into carbon and oxygen, later products fuse into ever heavier elements until the end of the line is reached with lead as a final product in that sequence. When the fusion sequence ends, there is no outward pressure to balance gravity and the star collapses. Its outer shell is blown off, the core can be compressed to a black hole, and the violence can create shock waves of outward expanding gases (that are dispersing those heavy elements around the universe) that interact explosively with each other and give off exceedingly brilliant bursts of electromagnetic radiation — the gamma-ray bursts. For a fantastic description of these last moments in the life of a supernova, you might like to read my posting “A Star Explodes in Slow Motion“.

So brilliant was the end of this start that, as mentioned below, it would have been visible to the naked eye in full sunlight, had you been looking. Fortunately for science, the satellite known as Swift did see it, and had time to let other observers now. And how interesting to remember that this explosion happened 7 billion years ago.

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FURTHEST SEEABLE THING.

For the first time in history you could have looked half way back to the origin of the universe with your naked eye. On the night of March 19, 2008 a telescope mounted in space observed a flash from a gamma ray burst, an extremely explosive celestial object, which set several records.

First, if you’d been looking in that direction you would have been able to see, with your own unaided eyes, something at a distance further–seven billion light years–than anything a human being has ever seen in history. Second, since looking out into space is equivalent to looking back in time (it takes the light from distant objects many years to reach the Earth), you would have been witnessing the earliest thing ever seeable by the naked eye.

A new report describes observations made of the explosion by an orbiting telescope called Swift and by some of ground-based telescopes that got in on the action once they were notified by Swift. Swift has three onboard detectors which look not at ordinary visible light but at much more energetic light in the form of x rays and gamma rays. One feature of Swift’s mission is that as soon as it sees something interesting it alerts controllers on the ground so that other telescopes can be turned in that direction. In this way the explosive outburst, whose official name is GRB 080319B, could be tracked by telescopes sensitive to other kinds of light, such as infrared and even radio waves.

The March 19 event is an example of a gamma ray burst. This comes about when certain heavy old stars have used up all their internal fuel. When a star has no more fuel, the force of gravity causes it to contract. If this process is violent enough, the star can blow apart as a supernova. In some special cases, what is left behind is a black hole, and outward going shock waves which, when they criss-cross, can create a brilliant flash of light. For a short time this light is more powerful than that coming from an entire galaxy of stars. The cone of energy flying away from the explosion can be quite narrow, so to be observed from far away, as this object was, it had to be aligned just right to be seen by Swift.

This gamma burst was not the furthest ever observed with a telescope, but it was the brightest in terms of the energy released. So bright, in fact, that it could have been seen unaided in areas of North and South America the night of March 19, if only for about 40 seconds. The splash of light arriving at Swift’s place in orbit that two of Swift’s three detectors were temporarily blinded. Fortunately several telescopes quickly maneuvered into position and could study the stellar explosion as it unfolded. By then the gamma rays, the most energetic part of the light blast, would have died down.

But other types of light continued to issue from the scene. According to Swift scientist Judith Racusin, an astronomer at Penn State, this has become the best-observed gamma ray burst, and the observations have already changed the way we think about bursts work. When you look out at the night sky about 3000 stars are visible. Everything you can see at night is either a planet in our home solar system or one of those stars, all of which are located in our home galaxy, the Milky Way.

The furthest thing you can normally see with the naked eye, and with some difficulty, is the Andromeda Galaxy, about 2.5 million light years away. Only about once a century is a supernova visible from any further galaxy. And by now it’s been 400 years since we’ve seen one of those. That makes GRB 080319B all the more impressive. It breaks the record of most distant seeable-with-the-naked-eye thing by a factor of a thousand.

Located in the Bootes Constellation, the gamma burst is at a distance of 7 billion light years, which means that it took light seven billion years to come from the blast to Earth. That means that a person seeing the visible portion of the blast would have been looking halfway back toward the time of the big bang, when, according to modern cosmology, the universe began. When the blast occurred the sun hadn’t even appeared yet, much less the Earth, much less the human species. (The results appeared [in] Nature magazine, 11 September 2008.)

[Phillip F. Schewe, James Dawson, and Jason S. Bardi, "Physics News Update -- The American Institute of Physics Bulletin of Research News", American Institute of Physics, issue #873, 25 September 2008.]