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Where did all this come from? In every direction we care to observe, we find stars, galaxies, clouds of gas and dust, tenuous plasmas, and radiation spanning the gamut of wavelengths: from radio to infrared to visible light to gamma rays. No matter where or how we look at the universe, it’s full of matter and energy absolutely everywhere and at all times. And yet, it’s only natural to assume that it all came from somewhere. If you want to know the answer to the biggest question of all — the question of our cosmic origins — you have to pose the question to the universe itself, and listen to what it tells you.
Today, the universe as we see it is expanding, rarifying (getting less dense), and cooling. Although it’s tempting to simply extrapolate forward in time, when things will be even larger, less dense, and cooler, the laws of physics allow us to extrapolate backward just as easily. Long ago, the universe was smaller, denser, and hotter. How far back can we take this extrapolation? Mathematically, it’s tempting to go as far as possible: all the way back to infinitesimal sizes and infinite densities and temperatures, or what we know as a singularity. This idea, of a singular beginning to space, time, and the universe, was long known as the Big Bang.
But physically, when we looked closely enough, we found that the universe told a different story. Here’s how we know the Big Bang isn’t the beginning of the universe anymore.
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The modern cosmic picture of our universe’s history begins not with a singularity that we identify with the Big Bang, but rather with a period of cosmic inflation that stretches the universe to enormous scales, with uniform properties and spatial flatness. The end of inflation signifies the onset of the hot Big Bang. (Credit: Nicole Rager Fuller/National Science Foundation)
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James' World 2 
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