Is our universe all there is, or could there be more? Is our universe just one of countless many, all together in an all-encompassing multiverse?
And if there are other universes, what would they look like? Could they be habitable?
This may feel like speculation upon speculation, but it’s not as crazy as you might think.
My colleagues and I have been studying what other parts of the multiverse might look like – and what these hypothetical neighboring universes can tell us about the conditions that support life and how it arises.
Some physicists claim that a burst of rapid expansion at cosmic dawn, known as inflation, makes some form of multiverse inevitable. Our universe would really be just one of many.
In this theory, each new universe crystallizes out of the seething backdrop of inflation, shaped by its own unique blend of physical laws.
If these other universes are governed by physical laws similar to ours, then we can control them. Well, at least in theory.
Within our universe, physics is governed by rules that tell us how things should interact with each other and by natural constants like the speed of light that determine the strength of those interactions. So we can imagine hypothetical “what-if” universes in which we change these properties and study the consequences within mathematical equations.
This may sound simple, but the rules we tinker with are the basic makeup of the universe. If we imagine a universe in which the electron is, for example, 100 times heavier than in our universe, what would be the consequences for stars, planets and even life?
What does life need?
We recently addressed this question in a series of articles looking at habitability across the multiverse. Of course, habitability is a complex concept, but we think life needs a select few ingredients to get going.
Complexity is one of those ingredients. For life on Earth, this complexity comes from the elements of the periodic table, which can be mixed and arranged into myriad different molecules. We are living molecular machines.
But a stable environment and a constant flow of energy are also essential. It’s no surprise that terrestrial life began on the surface of a rocky planet, teeming with chemical elements, bathed in the light of a long-lived stable star.
Optimization of the fundamental forces
Do similar environments exist throughout the multiverse? We began our theoretical exploration by considering the abundance of chemical elements.
In our universe, all elements except the original hydrogen and helium, which were formed in the Big Bang, are created by stellar life. They are formed either by nuclear reactions in stellar cores or by the violent violence of supernovae when a massive star tears itself apart at the end of its life.
All these processes are governed by the four fundamental forces in the universe. Gravity compresses the stellar core, pushing it to immense temperatures and densities. Electromagnetism tries to force atomic nuclei apart, but if they get close enough, the strong nuclear force can bind them into a new element. The weak nuclear force, which can convert a proton into a neutron, also plays an important role in the ignition of the star furnace.
The masses of elementary particles such as electrons and quarks can also play a crucial role.
So, to explore these hypothetical universes, we have many watch faces that we can customize. The changes in the fundamental universe feed into the rest of physics.
The carbon-oxygen balance
To address the immense complexity of this problem, we’ve chopped up the various pieces of physics into manageable chunks: stars and atmospheres, planets and plate tectonics, the origins of life, and more. And then we put the chunks together to tell an overall story about habitability across the multiverse.
A complex picture emerges. A few factors can greatly affect the habitability of a universe.
For example, the ratio of carbon to oxygen, which is determined by a specific chain of nuclear reactions in the heart of a star, seems to be particularly important.
Too large a deviation from the value in our universe, where roughly equal amounts of the two elements exist, results in environments in which life would be extremely difficult to arise and thrive.
But the abundance of other elements seems less important. As long as they are stable, which depends on the balance of fundamental forces, they can play a central role in the building blocks of life.
More complexity to explore
We could only take a rough approach to unraveling habitability across the multiverse by exploring the space of possibility in very discrete steps.
In addition, to make the problem manageable, we had to take several theoretical shortcuts and approximations. So we are only in the first phase of understanding the living conditions throughout the multiverse.
The next steps must take into account the full complexity of the alternative physics of other universes. We need to understand the influence of the fundamental forces on the small scale and extrapolate it on the large scale, on the formation of stars and eventually planets.
A word of caution
The notion of a multiverse is still just a hypothesis, an idea yet to be tested. In truth, we don’t yet know if it’s an idea that may be tested.
And we don’t know if the laws of physics could be different in the multiverse, and if so, how different they could be.
We may be at the beginning of a journey that will reveal our ultimate place in infinity—or we may be heading for a scientific dead end.
Geraint Lewis, Professor of Astrophysics, University of Sydney
This article was republished by The Conversation under a Creative Commons license. Read the original article.