###### By Emily Levesque, University of Washington

## Studying the cosmic microwave background gives us a glimpse of how the universe looked about 300,000 years after the Big Bang. But what happened in the first fractions of a second after the Big Bang, and how did that shape the universe? This question ultimately led to an innovative new theory called cosmic inflation that could explain the earliest moments of the universe, as proposed by physicist Alan Guth.

### Alan Guth

Alan Guth was born in New Jersey in 1947. A first-generation college student, he left high school a year early to attend MIT and stayed there to earn his bachelor’s, master’s, and doctoral degrees. Six years later, he was working as a researcher at Cornell University, studying particle physics, when he attended a presentation by Robert Dicke.

By 1978, the cosmic microwave background had been a topic of study for more than a decade, and physicists like Dicke were beginning to explore inconsistencies between the theory of the Big Bang and the properties they were observing in this light from the early universe.

### The Big Bang

According to the Big Bang theory, the density of the universe—how much matter and energy appeared to be packed into a given space—seemed absolutely perfect. The universe was exactly as dense as it needed to be to give us what physicists call a flat universe, one that doesn’t crunch back on itself or rip itself apart but instead smoothly expands. Such a perfect density seemed too good to be true, and it also struck Dicke as odd.

Studying the cosmic microwave background had shown him that even the smallest fluctuations in the properties of the early universe would be amplified and show up as big deviations today. This meant that if the density of the universe was perfect now, it must have also been perfect in the universe’s earliest moments; if it had deviated even a little bit at those early moments the universe would look dramatically different today.

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### Why Match So Perfectly?

Another detail of the cosmic microwave background was also curious. Astronomers in 1978 believed that the cosmic microwave background was homogenous; it looked pretty much the same no matter where we observed it.

However, that fact was strange when they thought about it: this light had been emitted in the early universe, and from all over the early universe. Yet, different expanses of the universe were completely disconnected from one another. So, why did they match so perfectly? This would be like two people shopping for blue paint in different hardware stores in different cities and picking out the exact same color.

Setting coincidence aside, it’s impossible to explain such perfect agreement unless the two had been together previously, exchanging information to reach a shared result. For our two paint shoppers, we can bring them together with a big book of color samples to help them coordinate a plan. But how can we bring different parts of the universe together?

By 1980, Alan Guth thought he had a solution to both of these problems: cosmic inflation. The idea is deceptively simple and focuses on the universe in the first fleeting moments after the Big Bang itself. This period of inflation is crucial, but extremely brief.

### Cosmic Inflation

You might be familiar with the mathematical concept of exponents: a number raised to the power of two means we multiply the number by itself twice, turning 10 squared into 100. Raising a number to the power of 10 means we multiply it by itself 10 times, turning 10 into 10 billion, or a 1 followed by 10 zeroes. You can see that positive exponents can increase a number’s value dramatically.

Negative exponential growth does the opposite, decreasing a number’s value and letting us express very tiny numbers. Ten to the power of negative 2 gives us a fraction of 100th, or a one sitting in the second position after a decimal point. Ten to the power of negative 10 gives us a fraction of one 10 billionth, or a one sitting in the 10th position after a decimal point. Exponents are crucial for understanding both how brief and how dramatic inflation was in the early universe.

We think that inflation began at a time of 10 to the minus 36 seconds, a minuscule fraction of a second after the Big Bang, and it only lasted about 10 to the minus 32 seconds. In that brief flash of time, the universe expanded by a factor of around 100 trillion trillion times its previous size—that’s a one followed by 26 zeroes.

This dramatic expansion explains how the universe could have gotten so enormous so quickly, and in doing so it explained why vastly disparate corners of the universe could look so incredibly similar. This rapid expansion would also help suppress the effects of any small perturbations in density at these early times, allowing the universe to look perfect and flat today without implying that it had to start and stay perfect in those earlier moments of the universe.

### Common Questions about Alan Guth and the Cosmic Inflation Theory

**Q: Who came up with the idea of cosmic inflation?**Alan Guth got acquainted with the inconsistencies of Big Bang Theory and observations of the universe. This ultimately led to him coming up with his theory of cosmic inflation.

**Q: Why were physicists such as Robert Dicke suspicious of the Big Bang theory’s model of a flat universe?**The model suggested that the universe was expanding smoothly and was exactly as dense as it needed to be, which seemed too good to be true. Robert Dicke and others knew that even the smallest fluctuations at the beginning of the universe would have ripple effects as the universe expanded, making the model of a flat universe all the more puzzling.

**Q: When do we think the universe’s inflation began?**We think that cosmic inflation began 10 to the minus 36 seconds after the big bang and only lasted 10 to the minus 32 seconds. But in that brief period after the big bang, the universe expanded 100 trillion trillion times.