GRB 090423 FAQ

Embargoed until 18.00 GMT 28th Oct 2009

1. HOW WAS THE BURST DETECTED?

Gamma-ray bursts are incredibly bright explosions (much brighter than anything else we know of) and emit all forms of light -- not just visible light, but also radio, infrared, ultraviolet, x-rays and in particular gamma-rays. Detecting this burst was a multi-step process: firstly the Swift satellite saw the bright flash of (high energy) gamma-rays. This only gives a rough location for the burst, so the next step was for Swift to slew around, as it is designed to do, and take a photograph in X-ray light of this location. The X-ray image gives a much more precise position. At this point the search moved to large telescopes on the Earth. In particular, the burst could not be seen in visible light, but was detected in infrared light with a number of telescopes (the United Kingdom Infared Telescope, the Gemini-North Telescope, both in Hawaii, and then the European Southern Observatory Very Large Telescope in Chile). The properties of this light, in particular, the "redshift" indicate how far away the burst was, and hence how far back in time we are seeing it.

2. HOW DO WE DETERMINE THE AGE OF THE UNIVERSE WHEN IT OCCURRED?

The redshift of the light really tells us the fraction of its present age, that the universe was when the light set out. This fraction turned out to be about 4.6%, and since we have fairly good measurements of the age of the universe from other observations (namely 13.7 billion years) that means the burst must have occurred when the universe was just about 630 million years old. Hence we arrive at the number of just over 13 billion years for the age of the light from the burst.

3. WHAT DO WE LEARN FROM THIS EVENT?

From this observation the main thing we learn is very simply that stars were being formed at that time. Up till now we have only had circumstantial evidence that that was the case. We also know that some of them are massive stars which do sometimes explode to produce gamma-ray bursts. That is important for the future since it confirms that GRBs can be used to study this era.

4. WHAT WERE THE DARK AGES OF THE UNIVERSE?

After the Big Bang it is thought that the universe was filled with a very smooth gas, which gradually expanded and cooled. Only after some period of time, perhaps a one or two hundred million years (but we don't really know) some stars started to form in this gas, as clumps began to contract under the action of gravity. This era before and during the formation of the first stars is called the "dark ages", because there were no luminous sources and also because radiation from the new stars ionized the gas between the galaxies, eventually making it more transparent to ultra-violet light.

5. CAN WE SEE THE FIRST GALAXIES?

The early stars, and the small galaxies and star clusters, that they presumably formed in, are so faint and distant that they would be incredibly hard to see and study. One of the hopes of NASA's new space telescope (JWST) is that it may pick up some evidence of these very early stars, when it is launched in about 5 years. But even with JWST it will be hard. Till now, all we have to go on are the properties of galaxies we see when the universe was about a billion years old, and then we have to use theoretical ideas to try to figure out how they became to be like that. Some very faint galaxies seen in other surveys may be from similar redshifts to GRB 090423, but since they are too faint to measure redshifts for, we can't be sure. GRBs are so bright that they can be much more easily seen, even at very high redshifts, and subsequently we can search deeply at their locations to attempt to identify their host galaxies.

6. WHAT MIGHT WE LEARN FROM GRBS ABOUT THE EARLY UNIVERSE?

Gamma-ray bursts, because they are so hugely bright, offer a new route to seeing some of these first stars directly (ie. the ones which explode), and from their properties, even to find out what the universe was like then. If we can get very good data on future gamma-ray bursts at the same distance, then we hope to learn things like, how much of the hydrogen in the universe was in a neutral (as opposed to ionised) form, and whether there were any elements around other than hydrogen and helium. Both these two things are important since we believe it was the first generations of stars which both ionised hydrogen and also produced other elements (such as oxygen, carbon etc.) which were not produced in the Big Bang. Hence, as I say, observing GRBs can in principle tell us something about these very first stars.