In other words, the microwave background is a photon screen that stops us from looking back in time all the way to the Big Bang itself. Clever scientists of the future may devise ways to penetrate that veil. They may learn to detect neutrinos that flashed into space when the first elements fused or gravitational ripples in space-time that were created by the Big Bang. Fortunately, we don't need to wait for new technological advances to study gigantic explosions. Other blasts, quite visible to us today, are the biggest bursts of energy in the cosmos since the primeval Big Bang. We call them gamma-ray bursts.

How bright are gamma-ray bursts? If we floated above Earth's atmosphere and had gamma-ray eyes, we would see flares of light as bright as Venus blaze in the heavens once a day. After mere seconds they would vanish forever. They are so far away that to appear as bright as they do their energy releases must be enormous. The outputs of the most powerful bursts are equivalent to converting the entire mass of the Sun into pure energy in scarcely 10 seconds. For scientists who love explosions, gamma-ray bursts are as exciting as it gets.

Understanding what triggers gamma-ray bursts is one of the foremost challenges in high-energy astrophysics. Theorists have some intriguing ideas. But first some background. We can thank the Cold War for providing us with the satellites that first spotted gamma-ray bursts. The Vela satellites, launched by the United States in the 1960s, were designed to spy gamma rays from clandestine tests of thermonuclear weapons in the atmosphere. But instead of seeing Soviet bombs, the satellites saw one flash after another from space. It took years for military officials to declassify the data. When they did, in 1973, astronomers didn't know how to classify what they saw.

For two decades a debate divided the astrophysics community. Most researchers felt that gamma-ray bursts came from relatively nearby--perhaps from unusual stars in and around our Milky Way. In that case the releases of energy would be impressive but not overly so. A vocal minority maintained that the bursts were fantastically powerful beacons from the depths of the observable universe. Neither camp reached a consensus on how the bursts arose. The confusion was evident in research papers. More than 2,000 were published during those two decades, advancing many notions about how gamma-ray bursts worked.

The logjam broke in the early 1990s with data from a new suite of satellites, including NASA's Compton Gamma Ray Observatory. Measurements of the positions of thousands of gamma-ray bursts showed that they created a random pattern. Their distribution did not follow the hazy band of the Milky Way on the sky, so it seemed unlikely that stars in our galaxy were the culprits. As the evidence mounted, astrophysicists abandoned their biases in favor of the minority view: Gamma-ray bursts indeed were remote in the universe and unimaginably powerful. Near the end of the decade an Italian satellite called BeppoSAX helped solidify that conclusion. BeppoSAX pinpointed the positions of bursts quickly and accurately enough to beam their locations to observatories on the ground. Astronomers who received the messages swung their telescopes to those spots on the sky. For the first time, they saw visible glows from many bursts--occasionally within seconds, with the help of robotic telecopes. The glows, presumably from fireballs accompanying the bursts, often lingered for weeks. The Hubble Space Telescope and large telescopes on the ground followed up by taking images of the glows. Sure enough, they were embedded within faint fuzzy patches: distant galaxies. Spectra of the galaxies revealed that they were hundreds of millions or even billions of light-years away.

Satellite observations also revealed that gamma-ray bursts are as unique as fingerprints. They last anywhere from a few hundredths of a second to a few thousand seconds. Some bursts have multiple peaks of gamma rays, while others have only one. (continued)