We understand our beginnings with broad brush strokes from the Big Bang. Astronomy and physics show there was a beginning. Prior to that there was nothing, during and after that there was something. Theory suggests our universe began around 15 billion years ago as an infinitesimally small, hot dense point of existence.
A favored analogy offers, like the space between raisins expands when baking a loaf of raisin baking bread, through time, space expanded carrying galaxies with it. As our universe inflated and expanded, it cooled, going from unimaginably small and hot, to the size and temperature of things present (image upper right).
What’s the evidence to support a big bang? First, galaxies appear to be moving away from the Milky Way galaxy (shown below) at speeds proportional to their distance. This, the Hubble Law, suggests that the universe was once compacted in that infinitesimally dense point.
Second, if the universe was initially unimaginably hot, people reason there should be some remnant of this heat. In 1965, Nobel Prize winning Radioastronomers Penzias and Wilson discovered a 2.7 Kelvin temperature associated with a ubiquitous universe radiation. This radiation, termed the cosmic microwave background (shown as color enhanced image of detectable universe below) is thought to be the remnant radiation signature of big bang. Finally, the abundance of the light elements hydrogen and helium found in the observable universe are supportive of the big bang model of our origins.
There are a couple notable misconceptions surrounding the big bang. First, we tend to imagine a giant explosion. Forget explosion, think expansion. By analogy, instead of a balloon popping and releasing it's contents, imagine a balloon expanding. Think an infinitesimally small balloon expanding to the size of our current universe.
Another misconception is that we tend to imagine the first instant of the big bang as a little fireball somewhere in space. However, space didn't exist prior to the big bang. In the 1960's Hawking, Ellis and Penrose together published work extending Einstein's Theory of General Relativity to include measurements of time and space. From their calculation conclusions are drawn that spacetime had a finite beginning that corresponds to the origin of matter and energy. The first instant of the big bang didn't appear in space, space began inside it within the Big Bang.
Expansion of our universe continued, when at a point in time around 4.6 billion years ago, Earth and our solar system formed by accretion from the solar nebula.
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