And if there are other universes, what would they be like? Can they be habitable?
It may feel like speculation that’s deep on speculation, but it’s not as crazy as you might think.
My colleagues and I have been exploring what other parts of the multiverse might be like—and what these hypothetical neighboring universes can tell us about the conditions that make life possible and how they arise.
What if universes
Some physicists argue that a burst of rapid expansion at the cosmic dawn known as inflation makes some form of multiverse inevitable. Our universe would really just be one of many.
In this theory, each new universe crystallizes out of the seething background of inflation, imprinted with its own unique blend of physical laws.
If physical laws similar to ours govern these other universes, then we can grasp 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 constants of nature, such as the speed of light, that dictate the strengths of those interactions.
So we can imagine hypothetical “what if” universes where we change these properties and explore the consequences within mathematical equations.
It may sound simple, but the rules we mess with are the basic makeup of the universe. If we imagine a universe where e.g. the electron is a hundred times heavier than in our universe, what consequences would this have for stars, planets and even life?
What does life need?
We recently addressed this question in a series of articles considering habitability across the multiverse. Naturally, livability is a complex concept, but we believe that life requires a few select ingredients to get started.
Complexity is one of those ingredients. For life on Earth, that complexity comes from the elements of the periodic table, which can be mixed and arranged into a myriad of different molecules. We are living molecular machines.
But a stable environment and a constant flow of energy are also essential. It is no surprise that terrestrial life began on the surface of a rocky planet with an abundance of chemical elements, bathed in the light of a long-lived stable star.
Adjustment of the basic forces
Are similar environments found across the extent of the multiverse? We started our theoretical exploration by considering the abundance of chemical elements.
In our universe, apart from primordial hydrogen and helium, which were formed in the Big Bang, all elements are created through the lives of stars. They are either generated through nuclear reactions in stellar cores or in the ultimate 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 of the universe. Gravity squeezes the star’s core, driving it to enormous temperatures and densities. Electromagnetism tries to force atomic nuclei apart, but if they can get close enough, the strong nuclear force can bind them into a new element. Even the weak nuclear force, which can turn a proton into a neutron, plays an important role in the ignition of the stellar furnace.
The masses of the fundamental particles, such as electrons and quarks, may also play a central role.
So to explore these hypothetical universes we have many dials we can adjust. The changes in the fundamental universe flow through to the rest of physics.
The carbon-oxygen balance
To tackle the enormous complexity of this problem, we chopped 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 pieces together to tell an overarching story of habitability across the multiverse.
A complex picture emerges. Some factors can greatly affect the habitability of a universe.
For example, the ratio of carbon to oxygen, something set by a particular chain of nuclear reactions in the heart of a star, seems to be particularly important.
Going too far from the value in our universe, where there are roughly equal amounts of the two elements, results in environments where it would be extremely difficult for life to emerge and thrive.
But the abundance of other elements seems to be less important. As long as they are stable, which depends on the balance of the fundamental forces, they can play a central role in the building blocks of life.
More complexity to explore
We’ve only been able to take a broad brush approach to unraveling habitability across the multiverse and sample the space of possibilities in very discrete increments.
In order to make the problem manageable, we also had to take several theoretical shortcuts and approximations. So we are only at the first stage of understanding the conditions for life across the multiverse.
In the next steps, the full complexity of alternative physics in other universes must be considered. We will need to understand the influence of the fundamental forces on the small scale and extrapolate it to the large scale, to the formation of stars and eventually planets.
The notion of a multiverse is still only a hypothesis, an idea that has yet to be tested. In truth, we don’t yet know if this is an idea that can be tested.
And we don’t know if the physical laws might be different across the multiverse, and if they are, how different they might be.
We may be at the start of a journey that will reveal our ultimate place in infinity—or we may be headed for a scientific dead end.
Geraint Lewis, Professor of Astrophysics, University of Sydney
This article is republished from The Conversation under a Creative Commons license. Read original article.