“The closer we get to the real world, the fewer tools we have and the less we understand the rules of the game,” said Daniel Baumann, a cosmologist at the University of Amsterdam.
But some Escher advances may finally be starting to bleed through. The universe’s first moments have always been a mysterious era when the quantum nature of gravity would have been on full display. Now multiple groups are converging on a novel way to indirectly evaluate descriptions of that flash of creation. The key is a new notion of a cherished law of reality known as unitarity, the expectation that all probabilities must add up to 100%. By determining what fingerprints a unitary birth of the universe should have left behind, researchers are developing powerful tools to check which theories clear this lowest of bars in our shifty and expanding space-time.
Unitarity in de Sitter space “was not understood at all,” said Massimo Taronna, a theoretical physicist at the National Institute for Nuclear Physics in Italy. “There is a huge jump that has happened in the last couple of years.”
Spoiler Alert
The unfathomable ocean that theorists aim to plumb is a brief but dramatic stretch of space and time that many cosmologists believe set the stage for all we see today. During this hypothetical era, known as inflation, the infant universe would have ballooned at a truly incomprehensible rate, inflated by an unknown entity akin to dark energy.
Cosmologists are dying to know exactly how inflation might have happened and what exotic fields might have driven it, but this era of cosmic history remains hidden. Astronomers can see only the output of inflation — the arrangement of matter hundreds of thousands of years after the Big Bang Seed, as revealed by the cosmos’s earliest light. Their challenge is that countless inflationary theories match the final observable state.
Patterns in the ever-expanding arrangement of galaxies might reveal secrets of the universe’s first moments.
Yet the task may not be impossible. Just as currents in the Escher-like ocean can be deciphered from their shadows on its boundary, perhaps theorists can read the inflationary story from its final cosmic scene. In recent years, Baumann and other physicists have sought to do just that with a strategy called bootstrapping.
Cosmic bootstrappers strive to winnow the crowded field of inflationary theories with little more than logic. The general idea is to disqualify theories that fly in the face of common sense — as translated into stringent mathematical requirements. In this way, they “hoist themselves up by their bootstraps,” using math to evaluate theories that can’t be distinguished using current astronomical observations.
One such commonsense property is unitarity, an elevated name for the obvious fact that the sum of the odds of all possible events must be 1. Put simply, flipping a coin must produce a heads or a tails. Bootstrappers can tell at a glance whether a theory in the Escher-like “anti-de Sitter” space is unitary by looking at its shadow on the boundary, but inflationary theories have long resisted such simple treatment, because the expanding universe has no obvious edge.
Physicists can check a theory for unitarity by laboriously calculating its predictions from moment to moment and verifying that the odds always add up to 1, the equivalent of watching a whole movie with an eye for plot holes. What they really want is a way to glance at the end of an inflationary theory — the film’s final frame — and instantly know whether unitarity has been violated during any previous scene.
But the concept of unitarity is linked closely to the passage of time, and they’ve struggled to understand what shape the fingerprints of unitarity would take in this final frame, which is a static, timeless snapshot. “For many years the confusion was, ‘How the hell can I get information about time evolution … in an object where time doesn’t exist at all?’” asked Enrico Pajer, a theoretical cosmologist at the University of Cambridge.
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