The history of the universe is depicted in this diagram, which begins with the Big Bang at the far left and ends with the present-day universe at right. The imprint of the Big Bang was left at 380,000 years, when the universe became thin and cool enough for particles to stick together to form atoms. The earliest stars yet seen formed about 180 million years after the Big Bang. Since then, galaxies have continued to form and spread out as the univere expands as the result of the Big Bang and dark energy, which increases the rate of acceleration. [N.R. Fuller/NSF]
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The Big Bang
The universe contains hundreds of billions of galaxies. All but a few of them are moving away from us, with galaxies that are farther moving faster than those that are closer. That’s because the universe is expanding as a result of the Big Bang.
In fact, the discovery that the universe is expanding is what caused astronomers to think of the Big Bang in the first place. If all the galaxies are moving away from each other today, then they must have been closer together in the past. And that suggests that in the far-distant past, everything was jammed together into a single point.
There’s no agreement about what that point was. But scientists do agree that 13.8 billion years ago, it began to expand. It wasn’t a giant explosion in space, though. Instead, scientists like to say that it was an explosion of space — space didn’t exist until the Big Bang. Neither did time, matter, or energy.
Time and energy sprang into existence first. Some of the energy was converted to the most basic particles of matter. And as the universe expanded and cooled, some of those particles combined to form atoms — mainly hydrogen and helium, the simplest elements. That period left an “afterglow” in the sky — a background of radiation that’s seen across the entire universe. Both the radiation and the chemistry match what Big Bang theory predicts — supporting this model of how the universe was born.
We’ll talk about what came before the Big Bang tomorrow.