Alan Guth describes Inflation as a theory of the Bang

Alan Guth, Credit: Jacinta Gonzalez

"Our whole universe was in a hot, dense state, then nearly 14 billion years ago expansion started. Wait..." (from The Big Bang Theory TV show intro song).

This intro song of the sitcom The Big Bang Theory has done a fine job synthesizing the main ideas of the Big Bang for the public. The Universe is not static; it is expanding from a hot and dense point at the beginning of everything. However, the Big Bang theory in cosmology is just a theory about the origin of the universe, not its later evolution. What happened right after the Bang? In the words of Alan Guth (born in 1947), a cosmologist at MIT who is one of the pioneers that theorized about an inflationary universe, "Inflation explains the Bang" (from Alan Guth Explains Inflation Theory,

First the Big Bang

The term "Big Bang" was coined by the English cosmologist Fred Hoyle (1915-2001) as a way to describe it as an "irrational process" and pseudoscience, however, the Big Bang is the most favorable theory of the Cosmos ever since the discovery of the cosmic background radiation in the 1960s, the leftover radiation of the Big Bang (Fred Hoyle. In Wikipedia).

Thanks to the work of American astronomer Edwin Hubble (1889-1953), we know that the universe is expanding due to a relationship between the distance to a galaxy and its recessional speed. In general, the farther galaxies are from us, the faster they move away (Edwin Hubble. In Wikipedia). Before Hubble, the less known Catholic Belgian priest and cosmologist Georges Lemaitre (1894-1966) proposed that the universe began as what he called a primeval atom, where "the expansion of the observable universe began with the explosion of a single particle at a definite point in time" (from Cosmic Horizons: Astronomy at the Cutting Edge. Edited by Neil deGrasse Tyson and Steven Soter).

The Universe is "boringly flat"

The Big Bang theory has been around for about 100 years since its formulation, yet the theory went further ahead with the proposal of inflationary cosmology by American cosmologist Alan Guth, among others.

In simple words, the universe increased in size from its subatomic size in the quantum realm to the size of a marble faster than an eye blink or a sneeze (WSU Master Class: Inflationary Cosmology with Alan Guth. That inflation might not seem impressive, but if you inflate a football at the same rate, it will grow to a size larger than the Milky Way in that tiny amount of time (for more insight on the speed of cosmic inflation read the Quora answer posted by Ben McAdam in What was the speed of the cosmic inflation according to inflation theory?.

Afterward, inflation stopped and the expansion rate of the universe slowed down (scientists in the1990s discovered the expansion rate is accelerating, but that is another story).

The "exponential growth" of the universe was introduced in 1979 by Guth because of his interest in solving the "flatness" problem of the universe (flat in the sense of a universe with Euclidean geometry where the sum of the angles in a triangle equals 180 degrees). The universe's measurements to determine if it will be closed or open are incredibly tuned perfectly to keep a balance that is almost a miracle or an absurdity, hence the "problem".

Inflation theory explains why the universe is flat. The extremely rapid expansion in all directions smoothed the universe out as it "doubled in size several times in less than a second" (Quote from How the Big Bang Theory Works by Jonathan Strickland. HowStuffWorks.

A closed universe will eventually collapse on itself due to gravity (the Big Crunch) or an open universe expands forever (the Big Rip). However, our universe has the right numbers to exist the way it does, as a flat universe, and any small change to these measurements would have altered its fate. In other words, cosmologists wondered how the current density of energy in the universe was "so closely fine-tuned" to the density in its early stages; a universe that is balanced perfectly between open and closed making it have a zero-curvature Euclidean geometry if we consider its infinite large scale (Flatness problem. In Wikipedia).

Again, not a closed universe, not an open one either, but a universe where "the curvature created in space by the countless stars, black holes, dust clouds, galaxies, and so on constitutes just a bunch of little bumps on a space that is, overall, boringly flat" (Davide Castelvecchi. What Do You Mean, The Universe Is Flat? Part 1. Scientific American,

I see a problem on the horizon

When you text or call somebody in another state or country, think about the opportunity of being able to communicate between such large distances. Very small distances if we consider the distance to the nearest star (about 4 light-years) or the distance across the Milky Way (about 100,000 light-years) or even the distance to the nearest largest galaxy, the Andromeda nebula (over 2,000,000 light-years). The light we see tonight when we look at the Andromeda nebula took over 2 million years to reach us here on Earth. Humans were emerging out of Africa and they belonged to a different species, Homo erectus, when the light from Andromeda started its journey (How Far Back Are We Looking in Time? Universe Today,

These vast distances make us question, why is the universe homogeneous (the same in all locations)? One side of the cosmos isn't supposed to be in communication with the other side, so far away, in terms of billions of light-years. A light-year is the distance light travels in one year (approximately 6,000,000,000,000 miles), so how can the universe remain so uniform when its parts are so far apart? This is called the Horizon Problem.

The universe is about 13.8 billion years old, so if you observe two galaxies that are 20 billion light-years apart (that is, the Earth in the middle and each galaxy 10 billion light-years in opposite directions), then the total distance between the galaxies is 20 billion light-years (Horizon problem. In Wikipedia). This brings a problem because the time of communication between those two galaxies is longer than the age of the universe itself, yet they inhabit an overall smooth universe. The universe is not supposed to be homogeneous, that is unless there was a time when all of the parts of the universe were in touch.

Enter the homogeneous universe

Guth explains that if the universe was much smaller, right after the Big Bang, then all of its parts were in touch with each other, so there was a time when it was homogeneous (the same). This uniformity was preserved when the singularity was inflated in a sudden expansion, hence places in the cosmos that haven't had the time to interact will share an approximate temperature because they were in close contact when the universe was young and, loosely speaking, the expansion to a much larger size was faster than the speed of light.
The Inflationary Universe, Credit: Ida Lee

Going back to the galaxies that are 20 billion light-years apart, the space they inhabit had time to interact and reach equilibrium before inflation. The incredible fast inflation of the universe is a reasonable explanation of why the large universe still shows almost uniform temperatures. Alan Guth explains: "In inflationary models, the universe begins so small that uniformity is easily established...Then inflation stretches the region to be large enough to include the visible universe" (from WSU Master Class: Inflationary Cosmology with Alan Guth, This leads us to the question: What did cause the sudden inflation?


We may know about thesis and antithesis in literature and philosophy, a hero and an antihero in a story, or even about matter and antimatter in physics. In cosmology, there is also something called antigravity, thought of by many as sci-fi. We are mostly accustomed to ordinary gravity, an attractive force responsible for what makes objects fall and keeps the planets in orbit. However, in Albert Einstein's General Relativity, gravity can also be repulsive (Cosmological constant,

It turns out that the main force driving Inflation is antigravity, more specifically, "a repulsive-gravity material", which existed in a very tiny "patch" or region of the early universe, and due to its repulsiveness, this patch grew rapidly to a larger size in extremely small fractions of a second. Because this "repulsive-gravity material" is unstable, it decays as radioactive substances decay, causing inflation to stop and the released energy produces ordinary matter and radiation. Throughout the entire process, the law of conservation of energy is conserved because the positive matter cancels the negative gravity. "The total energy of the universe is zero", as Alan Guth puts it, "I often say that the universe is the ultimate free lunch, since it actually requires no energy to produce a universe. At some point the inflation ends because the repulsive-gravity material becomes metastable...At this point the repulsive gravity turns off, but the region continues to expand in a coasting pattern for billions of years to come. Thus, inflation is a prequel to the era that cosmologists call the Big Bang" (from 3 Questions: Alan Guth on new insights into the ‘Big Bang’ (

Inflation has inspired many scientists and there are alternate inflationary models of the universe. It has even led to the possibility of a multiverse, where different regions of the universe have continued inflating and creating new universes as bubbles in a foam. Others think Inflation is too speculative, perhaps a reason why Alan Guth has not been awarded a Nobel prize for his work (he has won many other prestigious awards). However, the consequences of Inflation will continue to amaze me because they reverberate for all eternity.

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