How real is the multiverse?

Imagine setting off in a rocket and leaving Earth. Leaving the solar system. Leaving our galaxy. Breaking the edge of the observable universe and leaving our cosmos behind (which would be impossible, since you’d have to go faster than the speed of light, but work with me here).

Now you are navigating the unfathomable void for eons, only to find another universe, another galaxy within it, another solar system, another Earth … and another you, sitting there, reading this article.

This is the multiverse and it could be a natural prediction of the physical theories that define the beginning of the universe. Or maybe not. It’s hard to say, as new research has shown.

Related: What is the multiverse theory?

A great ancient universe

Cosmologists largely believe that when our universe was extremely young, less than a trillionth of a trillionth of a second, it went absolutely insane. In the smallest fraction of a moment of time (again, involving trillionths and trillionths of a second), the universe got really, really big.

How big? It’s hard to say exactly, because this concept is highly hypothetical, but “much bigger than you think” should be enough. Most models of this event, called inflation, require a universe that is at least 10 ^ 52 times larger than the observable volume of the cosmos. Given that that observable patch is already 90 billion light-years across, that means that the true extent of our universe is so great that it is almost incomprehensible.

An illustration of the expansion of the universe after the Big Bang. (Image Credit: MARK GARLICK / SCIENCE PHOTO LIBRARY via Getty Images)

Inflation solves many problems in the standard Big Bang cosmology, a model that describes how the universe began, such as the fact that very distant regions of the universe have roughly the same temperature. According to inflation theory, these regions were once much more welcoming and knew each other quite well, before inflation ripped them apart.

There is another potential consequence of inflation: it may not be done. In fact, it may never be done. This is called “eternal inflation,” and this idea describes how the universe on the grandest scales can always be inflating, with only small pockets pinching each other into normal, quiet patches like ours. Each pinched island universe would be separated by a vast chasm from nothing, with the islands flying away from each other faster than light (because that’s what inflation does).

These island universes, embedded within the larger “multiverse,” would never meet and could never talk to each other. In fact, it would be impossible to find direct evidence of its existence.

To inflate or not to inflate

Without that direct evidence, could we at least make an educated guess as to whether the multiverse is likely or not? If we are just a bubble in a giant tub filled with foam that expands faster than light, how could we solve this?

The first step is to test for inflation. The jury is still out on that, but there is some evidence that something like inflation happened in the early universe. Fluctuations in the cosmic microwave background, or the light released when our universe began to cool down when it was 380,000 years old, have a pattern that matches what you would see if inflation had occurred. No other theory of the early universe matches that pattern of light.

So that’s good. But “inflation” is not a single theory. It is more like a class or category of theories. Different models assume different physics, different drivers, different causes, and different effects of this event. Since all of these theories are based on hypothetical models of the extreme physics of the early universe, it is too early to say which of the theories, if any, is correct.

Physicists suspect that eternal inflation is generic, that is, a consequence of most, if not all, inflation models. So, following this suspicion, if inflation is correct, then eternal inflation is probably correct as well, and the multiverse could be real.

Judging the multiverse

Needless to say, the existence of the multiverse is a pretty big pill to swallow. If eternal inflation is correct, then there is not just one universe, or many universes, but an infinite number of pocket universes. Each would potentially support its own laws of physics and particle arrangements. So if the number of ways to organize matter and energy is finite, there are only so many ways to build a universe, then an infinite multiverse demands repeated copies of the same physical situation, even if any particular combination of physical configurations is incredibly rare. . .

That means there is a copy of you, at a finite (but very far) distance. And another copy beyond that. And other. And other. Countless of you are doing the exact same thing.

But we can only say that the multiverse is likely if eternal inflation is indeed generic (that is, a common feature of most, if not all, inflation models), which is exactly what a team of physicists claim in a recent article, published in preprint from the arXiv database and submitted to the Journal of Cosmology and Astroparticle Physics. They ran a large number of inflation models through a grinder, varying the model types and model parameters, counting which ones were a single affair and which ones led to eternal inflation and a multiverse.

His answer: it is complicated.

First, they discovered that eternal inflation was not as common as originally thought. His explanation of why cosmologists had thought eternal inflation was generic was because early cosmologists had studied only a limited set of models. They found that many viable inflation models (“viable” here means they obviously did not contradict the observations) did not lead to an eternal inflation scenario.

However, the researchers found that it is difficult to even handle measuring the “common” of something like eternal inflation, since we do not have a good understanding of inflation models and how they work. They argued that it is impossible to answer the question of genericness with a single answer, because there is still much to learn about the physics of inflation.

So is there another you out there, reading this exact same article? Science says: hard to say.

Originally posted on Live Science.

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