Biggest Questions in Science: Alan Guth

Portrait of German born US physicist and mathematician Albert Einstein

The big bang theory

Scientists have long pondered the birth of the universe, but for centuries their theories were mostly speculative. Albert Einstein changed this with his General Theory of Relativity. The theory provided a framework for describing the expansion of the universe, and in 1929 Edwin Hubble reported observations showing that the universe is expanding. Hubble’s results led scientists to develop the big bang theory. It explained a lot, like how the universe expanded and cooled, how light chemical elements formed, and how matter coalesced into stars, galaxies and galaxy clusters. But it didn’t explain everything.

Credit: Popperfoto/Getty Images

A COBE morph to WMAP sky map with the galaxy signal removed

Uncomfortable truths

Two important observations seemed incompatible with the big bang theory. First, the universe appeared “smooth,” meaning that the cosmic microwave background—the afterglow of the big bang—seemed to be exactly the same temperature everywhere, even from locations too far apart to have ever interacted. Just as unlikely, the universe seemed “flat,” according to measurements of its total amount of matter. That meant it was poised precisely between a “closed” curvature that would eventually cause it to collapse and an “open” curvature that would drive it to expand forever.

Credit: NASA/WMAP Science Team

Professor Alan Guth, PhD of the MIT Physics Department with a radio telescope on the roof at MIT

Alan Guth proposes cosmic inflation

To improve on the big bang theory, in 1980 astrophysicist Alan Guth of MIT, Andrei Linde of Stanford University and Alexei Starobinsky of the Landau Institute for Theoretical Physics in Moscow developed the theory of cosmic inflation, for which they received the Kavli Prize in Astrophysics in 2014. The theory provided a description of events when the universe was just a fraction of a second old. Cosmic inflation begins with the seeds of all the matter and energy in today’s universe packed into an absurdly hot, dense speck less than a billionth the size of a proton.

Watch Alan Guth talk about the birth of the universe. Credit: Rick Friedman/Corbis via Getty Images

Conceptual illustration representing the origin of the universe

Spacetime breaks out

Such extraordinary conditions, Einstein predicted, would cause gravity to repel objects rather than attract them. This caused the hot, dense speck to expand at a trillion trillion times the speed of light— so forcefully and so fast that it ballooned spacetime itself. Once it was the size of a marble, the universe expanded according to the big bang theory. This theory of early cosmic inflation predicts the universe’s smoothness, its flatness, and a precise pattern of ripples in spacetime that match the observations of today’s telescopes.

Credit: Victor de Schwanberg/Science Photo Library

The Hubble image of the Bubble Nebula, or NGC 7635

“It’s kind of a prequel to the big bang theory. Cosmic inflation sets up the conditions for the big bang. In particular, it provides a possible answer to the question of what propelled the big bang: What caused the explosion? What was it that banged?”

00:15

Listen to the full podcast

Illustration of CR7 (Cosmos Redshift 7), a very distant galaxy discovered using the European Southern Observatory's Very Large Telescope (VLT)

The inflationary universe

Press play to immerse yourself in an interactive 3-D model of the history of our universe. Click on annotations to travel through time, from the birth of the universe 13.8 billion years ago to the present day.

Credit: Wojtek Urbanek

Chalkboard image showing complex equations

Mind-bending implications

While cosmic inflation theory provides answers to a number of persistent questions, it generates many more. As a theorist, Guth spends his days puzzling through some of the theory’s more bizarre implications. What emerges is a view of the universe nearly alien to conventional understanding.

Credit: National Geographic Image Collection/Alamy

Conceptual illustration depicting a multiverse

Do we live in a multiverse?

Although the universe we live in is all we can see, it may not be all that exists. That’s because many versions of inflation theory predict that once a speck of repulsive-gravity material starts inflating, it never stops. Instead, it ends in places, creating a big bang, and a universe like ours take shape. Each of these universes would have the same physics as ours. Outside of them, repulsive gravity material inflates forever, producing an infinite procession of pocket universes that are born, expand, then die.

Credit: Wojtek Urbanek

Can time travel backwards? Time as we know it travels only from past to future, a concept known as the arrow of time. That’s because as the universe evolves, its entropy, or randomness, grows. But cosmic inflation theory suggests that time could travel in two directions. If you travel back past the low-entropy birth of our universe and go back further, entropy will also start to grow again, Guth and Caltech astrophysicist Sean Carroll argue. What’s past to us would be future to someone in that universe. Credit: Wojtek Urbanek

First image of a black hole, using Event Horizon Telescope

Could primordial black holes hold the universe together?

Hybrid inflation, a variant of cosmic inflation theory developed by 2014 Kavli Prize co-winner Andrei Linde, predicts that myriad black holes as small as an asteroid formed in the first second after the big bang. These primordial black holes may make up dark matter—the elusive, invisible matter whose gravity keeps galaxies from flying apart. They may have seeded the supermassive black holes at the center of galaxies, such as this one in galaxy M87.

Credit: Event Horizon Telescope Collaboration

Searching for inflation’s fingerprint

While Guth and other theorists tease out the implications of inflation, other scientists are working to bolster cosmic inflation theory with observations. The theory predicts that inflation produced gravitational waves, which in turn would have left swirly fingerprints called B-mode polarizations in the cosmic microwave background. A large, prominent group of astronomers with the Simons Observatory, which includes the Atacama Cosmology Telescope and Simon’s Array (shown) in Chile’s high desert, will begin observations this year to uncover those fingerprints.

Credit: Giulio Ercolani/Alamy

Dr. Alan Guth, a theoretical physicist and professor at Massachusetts Institute of Technology

“Science has, for me, the very attractive feature of asking the question: What is the world around us actually about? How did it begin? Where is it going? What causes it to exist in the first place? Those, to me, are the big questions that are really fascinating. And even though we don't necessarily expect to answer those questions next year, anything that makes small steps towards understanding the answer to those questions, to me, is very thrilling.”

00:34