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THROUGHOUT all of human history, people have looked up at the night sky and wondered about the universe and how it came to be. But in one respect, we’re very different from our ancestors: we more or less understand what we’re looking at.
Take an image from the Hubble Ultra Deep Field, for example. We know the blotches of light on it are not stars, but entire galaxies similar to own Milky Way. And because it takes time for light to travel through space, we’re not seeing what these galaxies look like today, but rather what they were like over 13 billion years ago, a few hundred million years after the big bang.
A little over a century ago, scientists didn’t have the faintest understanding of our universe’s distant past, and they certainly knew nothing about its origin. We didn’t have the tools even to conceptualise questions about how the universe might change or evolve. All of that changed with Albert Einstein. With his general theory of relativity, he how showed how space isn’t static and unchanging. It can be curved: it can warp and deform: it can expand and contract.
In 1929, Edwin Hubble observed that the universe is in fact changing. Every galaxy is receding from us: every two points in space are getting farther apart from each other as time advances. The universe is expanding.
If the universe were smaller in the past, and we know how much matter and energy it contains, we can deduce that its matter and energy density must once have been higher. Billions of years ago, it must have been in…
