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Paul M. Sutter is an astrophysicist at The Ohio State University, host of Ask a Spaceman and Space Radio, and author of "Your Place in the Universe." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.
In the 1920s, astronomer Edwin Hubble discovered something absolute groundbreaking and unprecedented in the history of really all of humanity. He found that our universe is alive. It's dynamic, it's changing, it's evolving with time.
Universe - Past - Future - Difference - Expansion
Our universe was different in the past and will be different in the future. And that difference is due to expansion. Over time, galaxies on average get farther away from us and farther away from each other. We live in an expanding universe. But exactly how fast is it expanding today? Astronomers are struggling with this answer because different techniques are giving different results, and the resolution to this problem may come in the form of brand-new physics. Or maybe we're just getting something wrong. Hard to tell.
We understand the expansion of the universe through Einstein's theory of general relativity. This is also how we understand all gravitational interactions in said universe. General relativity connects the amount and kinds of stuff you have in space with distortions of that space, giving rise to bends and folds and features. Then that geometry of space-time tells all the stuff how to move. Voila: gravity.
Universe - Definition - Stuff - Relativity - Amount
The universe is by definition made of stuff, and therefore is subject to general relativity. The amount and kinds of stuff at grand scales tell space-time how to bend at those same grand scales, and that bending of space-time tells all the matter how to move. So the ingredients of the universe affect the expansion rate. And in different points in cosmic history, different components have been in charge; the universe is made of many things...
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