GoKawiilIn theory, a universe can come in any shape or size, but scientists prefer to think about three basic kinds of universes: one that’s expanding, one that’s collapsing, and one that stays the same. Out of these three simplified models, an expanding universe is the hardest for physicists to understand. Yet it’s exactly the one our real world most resembles.
When physicists calculate what’s going on with particles at the smallest, quantum levels in a universe that is collapsing or static, they can get their results to make sense. Unfortunately for physicists, our real universe is not collapsing or static but expanding — being pushed apart by dark energy.
When scientists try to make sense of quantum theory in an expanding universe, they are met with one confusing paradox after another. In expanding space, physicists cannot square the world we experience with the way things work at the smallest levels.
Now physicists trying to make sense of how the quantum world works within our expanding universe are hoping to learn from an unexpected source: black holes.
The Shapes of Space-Time
In 1915, Albert Einstein’s theory of gravity, called general relativity, introduced the idea that space and time are inextricably linked.
Space and time react to the contents of the universe: If the universe is filled with matter, then over time, the attractive pull of that matter’s gravity should cause space to contract. If the universe is filled with enough dark energy — or what in Einstein’s day was called a cosmological constant — then over time, its push should cause space to expand.
When Einstein first wrote his general theory of relativity, he believed that our universe must be eternal and unchanging. In geometric terms, he believed that the universe should be infinite and flat, and that the forces that push and pull on space-time should exactly cancel out.
But a Dutch physicist named Willem de Sitter was more open-minded. He realized that it was a natural consequence of relativity for the universe to evolve. In 1916-17, de Sitter published three papers exploring relativity’s possibilities. (In the process, he introduced many English speakers to Einstein’s theories, which, originally written in German, had been siloed off because of severed scientific communications during World War I.)
Albert Einstein famously believed in an eternal and unchanging universe. Linda Hall Library of Science, Engineering and Technology
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