The universe we can see and measure is about 13.8 billion years old. However, the universe is larger than 13.8 billion light years in diameter due to the expansion and subsequent inflation of space, in accordance with the Big Bang theory. In fact, our best current estimate, taking expansion and inflation of space into account, puts the edge of the observable universe at about 46–47 billion light-years away from Earth. This “edge” would represent our current cosmological horizon.

If you assume that the universe is infinite, then logically it would extend beyond the current cosmological horizon. Scientists have termed this infinite universe a “super-universe.” If the infinite universe theory is correct, our universe may be one universe out of uncountable billions in the super-universe. We cannot see the other universes because our current observation technology is unable to look through the cosmic microwave-background radiation, which originated when the matter in the universe was plasma (hot, ionized gas), and thus opaque. In theory, if we develop more advanced observation technology, such as a neutrino telescope (one capable of detecting neutrinos) or even a gravitational telescope (one capable of detecting the yet-undiscovered gravitation particle called a “graviton”), we would be able to look beyond the cosmic microwave-background radiation and see older events. We would have a new cosmological horizon, but we would never be able to examine the “edge” of an infinite universe. Why? It has no edge—and advances in cosmic observation technology will not matter. Even the hypothetical graviton (the theoretical particle of gravity), traveling at the speed of light, would never reach us from an infinitely distant universe.

Why is an infinite universe even plausible? We know from actual observations that the universe’s expansion is accelerating. The farther out our instruments allow us to observe, we can measure that the expansion is accelerating, and even exceeding the speed of light. The accelerating expansion is termed “inflation,” and was confirmed in the late 1990s. Until inflation’s confirmation, scientists believed that gravity would eventually slow the universe’s expansion, and even eventually cause the universe to contract in a “Big Crunch,” since gravity causes everything to pull on everything.

Long before we had any observable proof of the universe’s inflationary expansion, two scientists independently postulated its existence in 1979. Unfortunately, one scientist, Alexei Starobinsky of the L.D. Landau Institute of Theoretical Physics in Moscow, was unable to communicate his work to the worldwide scientific community due to the political policies of the former Soviet Union. Fortunately, the other scientist, Alan Guth, Professor of Physics at the Massachusetts Institute of Technology, developed an inflationary model independently, and communicated it worldwide. Guth’s model, however, was not able to reconcile itself with the isotropic, homogeneous universe we observe today. In other words, to the best of our current ability to measure it, the universe essentially looks the same in every direction. Andrei Linde, Russian-American theoretical physicist and Professor of Physics at Stanford University, solved Guth’s theoretical dilemma in 1986. Linde published an alternative model entitled “Eternally Existing Self-Reproducing Chaotic Inflationary Universe” (known as “Chaotic Inflation theory”). In Linde’s model, our universe is one of countless others. A prediction of the chaotic inflation theory is an infinite universe with bubble universes within it. Would they be the same as our universe? No one knows. Perhaps one or more universes would be different from ours. However, being infinite, an infinite number of universes would be identical to ours, even down to the last atom, obeying the same physical laws.

The concept of an infinite universe would also imply an infinite number of us (you, me, and everyone else) are out there somewhere beyond the cosmic horizon. Given an infinite number of us, we are living out every possible scenario. This is difficult to comprehend because infinite numbers cannot be comprehended. Here is a simple way to think about this. If you play poker, what are the odds that you will be dealt a royal flush (Ace, King, Queen, Jack, Ten, all in the same suit) in the first five cards? They are 2,598,960 to 1. That means you will get a royal flush about once every 2,598,960 hands of five-card poker (known as five-card stud poker). Even if you play every day, and for numerous hours a day, you may never get one. However, if you have forever, and continue playing, eventually you will get one, then another, and with infinite time, an uncountable number (an infinite number). Using this example, if there are an infinite number of us in the universe, then each of us in some part of the universe will experience a possible scenario. Since there are an infinite number of us, as a group we will experience every conceivable scenario. For example, in one of these possible scenarios, you would be the President of the United States.

I recognize the implications of an infinite universe are difficult to comprehend. A natural question to ask is, is it possible? The fact is, it’s theoretically possible, but there is no conclusive physical evidence. Recently, it’s been suggested that irregularities observed in the cosmic microwave background may be evidence of another universe bumping into ours. However, there is no scientific consensus regarding that hypothesis, so I am going to leave that discussion for a future post. Currently, it is scientifically valid to assert we do not know if the universe is finite or infinite.