How did the universe begin? Most astronomers now believe it began with the so-called Big Bang, more than 13.8 billion years ago:
The universe went from a small and dense ball of nothing to a rapidly expanding chaotic mess of a lot of things.
By the way, I just found out that the term "Big Bang" was coined by the astrophysicist Fred Hoyle --- coined derisively because he thought this theory was wrong.
He himself believed in the Steady State theory, the idea that, as Wikipedia puts it, "... new matter is continuously created as the Universe expands, thus adhering to the perfect cosmological principle (the principle that the observable universe is basically the same in any time as well as any place)." The idea is that even though the universe is expanding, it doesn't change its appearance over time.
How can one believe in a theory that was concocted by three guys who had just watched the movie "Dead of Night" and were inspired by the movie's circular plot*?
Really? I mean, really? So, what inspired the Big Bang theory? Did some scientists watch "Fantasia" or "Flash Gordon Conquers the Universe"? (Both of these movies came out in 1940.)
Just kidding. Actually, the Big Bang theory has become accepted by scientists because it is borne out by observation.
Here's a basic (by which I mean that I can understand it) explanation of the Big Bang theory.
Wikipedia also has a very complete (at least I think it's complete---it's certainly more than I've ever known before about this subject, and more than I thought I would ever want to know) summary of the theory and its development and expansion.
From the Wikipedia article, I learned that the idea was first noted by George Lemaitre in 1927 that an expanding universe "might be traced back in time to an originating single point." Since then, observations have led scientists to accept this theory, rather than the Steady State theory. As early as 1929, Edwin Hubble discovered evidence that galaxies are drifting apart at high speeds, evidence of the Big Bang theory. Then, in 1964, cosmic microwave background radiation was discovered, as predicted by the Big Bang model.
And, as noted in Wikipedia, "The known physical laws of nature can be used to calculate the characteristics of the Universe in detail back in time to an initial state of extreme density and temperature."
One more thought about Hoyle and his Steady State theory, again, from Wikipedia:
"Dr. Virginia Trimble, an astrophysicist at the University of California at Irvine, said that Dr. Hoyle's opposition to the Big Bang, while considered a mistake, 'was significant in that it went a long way toward making cosmology a true science'' in which competing theories were tested by observations."
In the past year, our understanding of the expansion of the universe from the time of the Big Bang has changed again, and then again. I'll be writing about that next.
*Here's a summary of the plot, and, in case you're supremely bored or need some help getting to sleep, you can buy this ancient (1945) movie from Amazon.com, if you still have a VHS player. (Just kidding: It's also available on DVD. But it won't play on most North American DVD systems.) (BTW, the Screenonline people note: "Warning: screenonline full synopses contain 'spoilers' which give away key plot points. Don't read on if you don't want to know the ending!."