universe, it should have cooled ever since. Like the cooling cinders of a fire that once blazed red hot, space itself should radiate with some leftover warmth from the Big Bang. Calculations predict that this glow should permeate the universe at just a few degrees above absolute zero. In 1965 the physicists Arno Penzias and Robert Wilson found this residual heat serendipitously, the cosmic microwave background radiation.

A satellite called COBE--the Cosmic Background Explorer--confirmed this finding in 1992. The satellite measured microwaves in nearly all parts of the sky and found a background temperature of 4.9 degrees Fahrenheit above absolute zero. This was firm evidence that the universe has cooled uniformly since the Big Bang. The pattern of radiation was strikingly smooth; it varied by less than 0.0002 degree from one part of the sky to the next. These subtle variations in temperature are like tiny bumps, no more prominent than large grains of sand under a sheet the size of a football field. We interpret these ripples as the signatures of quantum-scale fluctuations during the Big Bang. As the universe expanded and cooled, the patterns served as seeds for galaxies and clusters of galaxies to form.

Finally, we can turn to something as simple as a temperature gauge to provide one more piece of evidence supporting the Big Bang. Distant galaxies--which we see not as they are, but as they were billions of years ago--should be bathed in a hotter cosmic microwave background than today's galaxies. To test this notion, we can use large telescopes to analyze the light from those faraway galaxies. Spectra of these galaxies are crisscrossed by many sets of lines, which reveal the composition of the gases and stars in the galaxies. The lines of certain hydrogen molecules are special: They reveal how quickly the molecules vibrate. The rates of vibration pinpoint the temperature of the environment there, just as reliably as the thermometer in your window. Sure enough, spectra of distant galaxies show that the microwave background in that long-ago era was several degrees warmer. The measurements match the gradual rate of universal cooling predicted by the Big Bang theory.

This array of evidence is impressive. The acceptance of the Big Bang represents an unprecedented unification of astrophysics and particle physics. A coherent cosmic picture, our modern creation story, has emerged from a minimum of assumptions and measurements. Still, most people react to the Big Bang by objecting that it doesn't make sense. How could the entire universe start as an unimaginably energetic speck? It seems a powerful objection, but we must acknowledge that our ability to measure nature long ago outstripped our senses. No longer can we invoke common sense to evaluate whether something is true. For this reason, the fact that the Big Bang is so bizarre should not affect our willingness to embrace it. Quantum mechanics, another highly successful theory, was similarly received when it was first advanced. Quantum mechanics describes a world in which particles act like waves, obeying an odd set of rules that prevents certain motions or states of energy from existing. On these tiniest scales of all, our universe is nonsensical. It should be no surprise that the Big Bang, an explanation for all that exists on the largest scales, is just as strange.

Even so, many questions remain about the Big Bang. For example, how did the energy of the universe expand so smoothly? The tiny lumps seen in the cosmic microwave background are smaller than one would expect from an explosion as fierce as the Big Bang. The American astrophysicist Alan Guth proposed a solution in the early 1980s: "inflation." This scheme has no everyday analog, so don't confuse yourself by thinking of the rising costs of food or fuel. Cosmological inflation refers to an interval when the baby universe expanded at a wild rate. This period of time was incredibly brief--less than 1 billion trillion trillionth of a second. During that cosmic eyeblink, the size of the universe increased by an extraordinary factor of 10⁵⁰. When this furious growth stopped, the cosmos was about the size of a beach ball. Its expansion then "slowed" to its normal explosive pace. The inflationary process would have smoothed out the universe to the extent we see today. The

The acceptance of the Big Bang represents an unprecedented unification of astrophysics and particle physics.

hypothesis also makes specific predictions about the details of the cosmic microwave background and the patterns of gravitational collapse that caused the first groups of galaxies to form. Modern satellites, telescopes aboard high-altitude balloons, and other instruments will map variations in the microwaves with exquisite accuracy to provide a stern test of the inflation hypothesis.

Beyond such scientific details, several basic questions about the Big Bang may seem reasonable to ask. When we ponder the questions carefully, however, we realize that some of them don't make physical sense. For example, what lies beyond the universe? Or, put another way, what is it expanding into? We cannot answer that question because the universe contains all space. Space expands everywhere at once, and there is no "outside." Light beams are forever limited to the confines of our universe. Because nothing can travel faster than light, we cannot probe "beyond." In this way the entire cosmos is like the largest black hole of all, trapping all that exists within its (continued)