A crucial question along the way is, why did the matter in the universe evolve over billions of years in such a way as to create us?
Our modern picture of our cosmos was painstakingly pieced together through data collected by astronomers, who frequently brought back results that defied conventional theoretical wisdom of the time. A century ago, in 1915, Albert Einstein put the finishing touches on his general theory of relativity, which conceives of spacetime itself as a dynamic object whose curvature gives rise to the force we know as gravity. Before that point, it’s safe to say that we knew next to nothing about what the universe was really like on large scales. Spacetime was thought to be absolute and eternal, in accordance with Newtonian mechanics, and astronomers were divided on whether the Milky Way was the only galaxy in the universe, or merely one of countless many.
Now the basics have been well established. The Milky Way we see stretching across the dark night sky is a galaxy—a collection of stars orbiting under their mutual gravitational attraction. It’s hard to count precisely how many, but there are over 100 billion stars in the Milky Way. It’s not alone; scattered throughout observable space we find at least 100 billion galaxies, typically with sizes roughly comparable to that of our own. (By coincidence, the number 100 billion is also a very rough count of the number of neurons in a human brain.) Recent studies of relatively nearby stars suggest that most of them have planets of some sort, and perhaps one in six stars has an “Earth‐like” planet orbiting around it.
One of the most striking features of the universe is the contrast between its uniformity in space and its dramatic evolution over time. We seem to live in a universe with a pronounced temporal imbalance: about 14 billion years between the Big Bang and now, and perhaps an infinite number of years between now and the eventual future. To the best of our knowledge, there’s a legitimate sense in which we find ourselves living in a young and vibrant period in the universe’s history—a history that will mostly be cold, dark, and empty.
Why is that? Maybe there’s a deeper explanation, or maybe that’s just how it is. The best a modern cosmologist can do is to take these observed features of the universe as clues to its ultimate nature, and keep trying to put it all into a more comprehensive picture. A crucial question along the way is, why did the matter in the universe evolve over billions of years in such a way as to create us?
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