Published April 6, 2022
In the beginning, the world was empty and cold.
Prior to the Big Bang — yes, before the Big Bang — the universe underwent a breathtaking cosmic expansion, doubling in size at least 80 times in a fraction of a second. This rapid inflation, fueled by a mysterious form of energy that permeated empty space itself, left the universe desolate and cold.
Only after that did the hot, dense conditions of the Big Bang emerge: As the doubling of the universe ceased, the energy of the vacuum underwent a metamorphosis, transforming into particles of matter and radiation. That metamorphosis flooded space with the superhot plasma of the Big Bang, which forged the primordial elements that went on to make the stars and galaxies we see today.
This is the epic story that UB physicist Will Kinney explores in his new book, “An Infinity of Worlds: Cosmic Inflation and the Beginning of the Universe,” which was published April 5 by MIT Press.
Cosmic inflation — a theory first developed around 1980 by physicists including Alan Guth, Alexei Starobinsky, Andrei Linde and Katsuhiko Sato — is the concept of exponential expansion of the very early universe. Today, it’s one of the leading ideas for what may have occurred in the moments preceding the deeply ancient fires of the Big Bang.
Sprinkled with hand-drawn diagrams by Kinney, “An Infinity of Worlds” introduces readers to the science of cosmic inflation and the evidence for it, as well as the theory’s shortcomings. The book also delves into weird but wondrous consequences of cosmic inflation — like the idea, in a model of eternal inflation, that our universe is one of a possibly infinite number of universes that we’ll never be able to see.
“Human beings have been creating origin stories for as long as human beings have been telling stories, and this is another iteration of that,” says Kinney, professor of physics, College of Arts and Sciences.
But this particular story has a twist: If cosmic inflation correctly describes what happened before the Big Bang, it may push the ultimate answer to the question of where we came from beyond the reach of science.
“We don’t know anything about what came before inflation. It’s doubtful that we’ll ever know,” Kinney says. “One reason is because cosmic inflation is a big eraser. Any trace of the initial conditions of how it got started get diluted because of this exponentially large expansion. Any trace of the circumstances that led to inflation are erased by inflation itself: No matter where it starts, it ends up in the same places.”
In a Q&A with UBNow, Kinney discusses “An Infinity of Worlds” and cosmic inflation.
Standard Big Bang theory doesn’t have any explanation for why the universe is so smooth and so uniform, for all these basic properties of the cosmos that we see. Cosmic inflation sets up this initial state. It’s a theory that explains the initial conditions for the Big Bang.
Inflation tells us that the period of time before the Big Bang was extremely cold, almost at absolute zero, and it was empty of everything but empty space, and that empty space carried energy that stretched the universe out to this enormous size and into the initial state before the Big Bang.
In order to explain the properties of the universe we see today, the universe had to double in size at least 80 times. That’s a lower bound, so you had to have at least that much doubling. To visualize this doubling, imagine a chess board. Place one penny on the first square, two on the next, and four on the next. If you continue doubling the number of pennies on each square, you will be a millionaire at the 28th square, and a billionaire by the 38th square. By the time you fill all 64 squares, you will be a trillionaire thousands of times over. Inflation involves at least 80 doublings, stretching a patch of space about the size of a grapefruit to the size of our entire observable universe in less than a trillionth of a second.
I have been interested in cosmology in general since I was a kid. One of the real formative things for me was when I was in high school, reading Steven Weinberg in “The First Three Minutes,” which was a book about everything that was known about the first moments of the universe at the time that the book was published in 1977.
I started working on inflation as a graduate student. I did my PhD thesis on a certain set of models called “natural inflation.”
The book is for people who aren’t physicists. It is perhaps a little bit different than a lot of lay-oriented science books in the sense that I’m presenting some very complicated ideas, and I don’t shy away from the complexity. I want to give the lay reader a glimpse of current controversies that are very much at the forefront of our understanding.
When I first read “The First Three Minutes” by Steven Weinberg in high school, I only understood about a third of it, but it fired up my imagination. I liked the fact that he made it accessible and used plain language but took on complex ideas. It was a hard read, in the best sense of the word. I have tried to emulate this, and I expect my book will be something that undergraduate and graduate students in physics can read and get something out of, and certain parts are aimed at my colleagues, so this is part of a larger conversation in the field. But the goal is to make that conversation accessible to a lay audience.
Another thing I try to do in the book is point out the current limitations of cosmic inflation, where we don’t know things, and where we’re speculating. It’s just basic honesty. You’re telling the truth. I think that hyping speculative things ultimately undermines people’s faith in science as a rigorous undertaking.
It’s only recently that there has been real observational evidence that supports the model, so up until the last 10 or 15 years, it’s been a playground for theorists.
People are starting to take it more seriously because there is data that supports the theory now. One example is precision measurements of the cosmic microwave background, which built support for inflation and killed off a lot of competing theories.
When I started writing the book, I didn’t have such a clear conception of the degree to which I would ultimately structure it around a Copernican viewpoint — the idea that we’re not special, that we’re in some sense ordinary.
Copernicus proposed that the Earth is not in any privileged position in the cosmos, that the Earth is one of many planets revolving around the sun. Giordano Bruno built on Copernicus’ ideas and took them even further. Bruno said, “God is infinite, so His universe must be too. … He is glorified not in one, but in countless suns; not in a single earth, a single world, but in a thousand thousand, I say in an infinity of worlds.”
“The title of my book is from Bruno. I think that he was really hundreds of years ahead of his time in grappling with a lot of the problems that we are grappling with today in cosmology.
A cosmic horizon is a boundary beyond which we cannot see. Our observable universe — what’s inside our horizon — is a tiny patch of an infinitely larger space. We can’t see further than the distance light has had time to travel since the Big Bang, but there is actually more of the universe beyond what we can see. The outer boundary of the observable universe is called the cosmic horizon.
Here’s how I put it in the book: “As with the horizon on Earth, the cosmic horizon is an artifact of our viewpoint, not a property of the universe itself. Observers in different locations see different horizons, always with themselves at the center. The observable universe as seen from any point is finite, but the universe as a whole continues outward forever. The universe outside the horizon of an observer at any point is invisible because the light from there has not had time to reach them yet.”
During inflation, space expands so fast that portions of the universe are swept outside the cosmic horizon as objects recede from each other at faster than the speed of light; the horizon contains less and less. Inflation and its relationship to the cosmic horizon help to explain a number of properties of the early universe.
The presence of horizons in the universe presents an interesting problem for science because they represent a limit to our knowledge. We cannot, even in principle, see beyond the edge of the observable universe. This raises a lot of interesting questions about the nature of scientific knowledge itself, which I explore in the book.
“The universe at the first moments of time involved unimaginably high energy, which is the realm of particle physics. This is where the physics of the largest things we can describe, at the scale of the cosmos itself, meets the physics of the smallest things, elementary particles and fields.
Inflation is believed to have occurred at an extreme energy scale, about a hundred billion times larger than the energies probed by particle collisions at the Large Hadron Collider in Geneva, Switzerland, where many of my colleagues work. We can use the universe as a laboratory to probe physics far beyond the reach of particle accelerators on Earth.
Dark energy is causing an accelerating expansion of the universe today. If dark energy is stable, if it’s really a cosmological constant — and we don’t know if that’s true, but that’s the simplest idea — then the future of the universe is that everything that we see that isn’t in our direct vicinity will move away from us. Distant galaxies will move away faster and faster, faster than the speed of light due to the expansion, and they’ll disappear from view.
The Milky Way will run into the other large spiral galaxy in the local group, the Andromeda Galaxy, about 4 billion years from now, right about the time that the sun runs out of fuel and explodes, so we won’t be around for this.
Eventually, the only thing that will be left in the universe that we can see will be the few dozen galaxies that are closest to us, and our observable universe will just be dark and empty. That’s the ultimate fate. Just this little island of stars in a completely empty and black space. After a hundred trillion years or so, the last stars will burn out, and the universe will descend into darkness forever.