| A few months ago, I wrote about one of the wildest and most controversial theories of reality, the multiverse. Today I'll describe the less wild but still somewhat controversial theory that birthed it: cosmic inflation. When astronomers observe the sky, they see a few features that cry out for explanation. One is that the universe appears to be as evenly mixed as the milk in a well-stirred cup of coffee. Similar numbers of galaxies lie in every direction, even though the universe doesn't seem to have been around long enough for one side to mix with the other. This is the "horizon problem." Another is that based on observations of ancient light, space-time appears to have no measurable curvature. This "flatness" requires the universe to maintain a very special density of matter and energy for billions and billions of years. This is the "flatness problem." The standard Big Bang theory — the notion that the universe started out as a small, hot, dense patch of space that has been steadily expanding ever since — doesn't address either issue. This gentle expansion doesn't bring new regions of the cosmos into contact in a way that could facilitate mixing. It would also tend to push the density of matter and energy away from the "critical" density that keeps space-time flat. In the 1980s, Alan Guth, Andrei Linde, Paul Steinhardt and others addressed these and other problems by adding a prologue to the Big Bang story. All we see, they posited, sprang from a subatomic speck of space where everything inside was thoroughly mixed up, a condition that solved the horizon problem. Then, for a tiny fraction of a second, this speck swelled exponentially, doubling in size many dozens of times. In this moment, the universe expanded at a rate far faster than the speed of light (a feat possible only for the fabric of space-time itself). This "inflation" stretched out any initial curvature, solving the flatness problem. Decades of astronomical observations have aligned with inflation's predictions, firmly establishing it as the leading theory of how the cosmos came to be. "I don't think that one should seriously question inflation," Zohar Komargodski, a theoretical physicist at Stony Brook University, told me on a recent call. "There is so much evidence." Yet theorists eventually realized that inflation came with an awkward side effect: While the exponential growth may have stopped in our region, it likely would have continued in other regions, leading to the eternal production of bubble universes. This "multiverse" has turned some cosmologists against inflation. More on that in a future newsletter.
What's New and Noteworthy One apparent advance occurred in 2014. Inflation should have stretched tiny ripples in space-time in a way that affects light traveling across the cosmos. And after about a decade of searching for the subtle signal using telescopes located at the South Pole, an international collaboration of scientists announced they had found it. Pioneers of inflation popped champagne in celebration of the theory's most direct proof yet. But within the year, the claim fell apart. Dust in our galaxy had twisted light much as inflation would have. Researchers continue to search for the ripples of inflation but haven't found them yet. The theory doesn't predict how strong these ripples should be. So while detecting them would strongly support the theory, not detecting them does little to rule it out. A selling point of inflation is that it explains why the sky looks the way it does. As the universe swelled in size, tiny quantum fluctuations deposited dollops of energy here and there, which became today's galaxies. So in principle, physicists can learn about the rules governing the quantum realm through careful cosmological measurements. Some of those rules are unknown, because they operate at distances too small for colliders to probe. Theoretical physicists are trying to figure out how to read those unknown rules from the smattering of galaxies in the sky. It's a big challenge, but Quanta has covered developments in this research program in 2019 and 2021. In 2022, two analyses of astronomical data found just such a hint of new quantum laws. By studying huge numbers of tetrads of galaxies, researchers counted more groups of one orientation than another — suggesting that quantum fluctuations during inflation were similarly lopsided in a way that tried-and-true quantum rules can't explain. The discovery may not hold up to scrutiny, but if it's confirmed it could be Nobel-worthy, Quanta reported at the time. | 
