Blue Marble Space Institute cosmologist examines multiverse hypothesis

Science fiction writes about and postulates alternative universes to ours, but these are fictional scenarios. 

What is the scientific thinking about the actual existence of other universes, perhaps with different physical laws than ours, that could sustain life, the multiverse? And how can we evaluate scientific hypotheses about multiverse habitability?

The multiverse is not a new concept. It's been discussed and debated from ancient times. Today the concept is still alive, and controversial, among scientists, philosophers, and theologians.

A contemporary group of cosmologists, scientists who study how the universe began and developed, is examining aspects of the multiverse hypothesis and how to make predictions about it that are testable. 

Their sixth in a series of articles exploring the multiverse appears in the Dec. 21, 2022, peer-reviewed online journal Universe, titled "Multiverse Predictions for Habitability: Planetary Characteristics."

Testable predictions

The international group of authors notes at the outset that recent astronomical discoveries of possibly habitable planets that orbit sun-like stars outside the solar system demand a deeper exploration of the physical conditions necessary for life. Their article looks in detail at assumptions about habitability and considers how each of these might be compatible with the multiverse hypothesis.

In the article researchers focus on two areas, whether a large moon is necessary to stabilize the Earth's tilt and keep a planet habitable and the role of water. In particular, they look at how water might get to a planet. Was it from an asteroid impact, a comet, or "the oxidation of a primordial hydrogen atmosphere"?

In a coming paper, they consider the multiverse framework in terms of the laws of physics on Earth, and the mathematical constants that describe and measure physical relationships. These include the fine structure constant that governs the strength of electromagnetic interactions, and the mass ratios of atomic and subatomic particles.

Some conclusions

The present paper concludes that a large moon and stable obliquity are not necessary for habitability. As for water availability and its relationship to sustaining life, the conclusion is more complex.

In summing up their discussion of a moon and obliquity and water on habitability, the researchers write:

"Planetary habitability is rife with subtleties, as even confined to our own universe, selection effects play a large role in explaining why we are in this particular location," they said. "Disentangling the selection effects inherent in our universe from the variation in planetary characteristics throughout the multiverse is challenging, and requires a thorough investigation of many subtle effects for each habitability condition. However, once this is done, we are capable of shedding extra light on which habitability criteria are reasonable, and which are not.

"As we account for possible habitability factors more thoroughly, we are beginning to uncover a clearer picture of which aspects of our universe are unique, which perceived oddities are meaningful, and which purported prerequisites for life are sensible."

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McCullen Sandora, et al. "Multiverse Predictions for Habitability: Planetary Characteristics," Universe, Dec. 21, 2022.

https://doi.org/10.3390/ universe9010002

An interview with McCullen Sandora:

Is there a multiverse, and could it support life?

McCullen Sandora received his Ph.D in theoretical cosmology from California Davis. He's worked dark matter, dark energy, the early universe, the nature of gravity, and the multiverse. Currently he is a researcher at the Blue Marble Space Institute of Science, where he works on the intersection of astrobiology and cosmology, models of habitability and strategies for biosignature searches.

This is a complex theoretical topic for non-specialists. Can you explain your work in simple terms?

My work tries to tackle the question of whether a multiverse exists or not, that is, whether there are other universes aside from our own, with different laws of physics.

The multiverse has been a topic of debate for decades. Some of the fundamental physical constants, such as the electron mass or strength of the gravitational force, appear to be somewhat arbitrary, in that it seems possible that they could have been different. People really started taking the idea of a multiverse seriously when physicists noticed that if you change the values of some of these constants even by a few percent, then some of the large scale properties of our universe change.

A classic example is that if you change the strength of the electric force by a few percent, there would be way less carbon. This is quite an odd feature, because many biologists think that carbon is necessary for life, but it can be explained if there are lots of different possible universes, and we find ourselves in the one suitable for life.

Since we can't observe another universe directly, how is it possible to validate your hypothesis or to disprove it?

There are several examples like this that make the multiverse a compelling hypothesis to take seriously scientifically. But this type of reasoning gives people angst because, number one, it presupposes what life needs (in this case, carbon), and number two, this theory is impossible to test directly, because in all likelihood we are confined to our own universe, and we will never get direct confirmation that other universes exist.

Nevertheless, this hypothesis may be true, and my aim is to outline a framework for determining whether the multiverse exists or doesn't.

Referring back to the example above can shed light on the proposed method. While biologists are still debating whether carbon is necessary for life or not, the multiverse theory actually makes a prediction that carbon is necessary. This is because, if carbon were not needed, there would be no reason to find ourselves in a universe that happens to produce it in such abundance. Any of the thousands more carbon-free universes could equally well host life, and indeed most life would be present in those universes.

So, the strategy for testing this prediction is to go out and check. If we scour our universe for signs of life on other planets, and find that life does indeed need carbon, this prediction of the multiverse will be validated. Conversely, if we find life that is not carbon-based, the prediction will be falsified.

However, even if the carbon prediction does turn out to be true, it won't amount to very convincing evidence that the multiverse exists. 

What about other predictions for habitability?

There are currently dozens of habitability hypotheses. Does life need to be on an Earth-like planet? Around a sunlike star? Can it be in a binary star system? Can the planet be tidally locked? Does it need liquid water? For each of these, we can check whether or not they are compatible with the multiverse, and that's exactly what I've been doing.

I've come up with over a dozen predictions the multiverse makes about what life needs. Now, when we eventually find other instances of life out there, we'll be able to test all of them. If the multiverse is true, we expect all of these predictions to be true, and if just one prediction is false, it would rule out the multiverse. 

Can you explain how one false prediction rules out the multiverse?

I just meant the standard asymmetry, that it's much easier to disprove a theory than prove it.

You can never really prove a theory, because even if you verify 100 of its predictions, there's always the possibility that its 101st prediction will be wrong. 

Do you have more predictions?

The current paper considers several ideas about whether the architecture of planetary systems is important for life in the multiverse context. Many people have proposed that a large moon is necessary for life, as it stabilizes Earth's tilt. This is intriguing, since only a few percent of Earth-like planets are expected to have large moons like ours.

Without our moon, the Earth's tilt would swing wildly, making the poles and the equator swap places every few thousand years. It's conceivable that this would make it much harder for complex life to take hold to the point where it develops intelligence.

On the other hand, optimists say it may not make much of a difference. From the multiverse perspective, though, this idea doesn't pass muster, because it turns out that our universe is one of the only ones where planets' tilts are unstable in the first place. If a stable tilt is necessary, then there are other universes out there where every planet is automatically stable, and so these universes would be able to host much more life than ours.

What about the amount of water and habitability?

Earth, again, seems special, since it has just enough water to almost cover the surface. However, in other universes, much more or much less water could typically be delivered during planet formation.

The interesting thing about this is we still don't know for certain how Earth originally got its water; it could have either been via asteroids, comets, or chemical reactions from an initial magma ocean phase. Even if we know the source, it remains an open question whether the majority of water was delivered as a result of the precise arrangement of the outer gas giant planets, or not.

And lastly, there are differing views as to how water affects habitability: whether partially covered planets like Earth are optimal, or whether ocean worlds or desert worlds may be better. When we take all these considerations into account, we find that some are incompatible with the multiverse theory. 

What are you working on next?

All these predictions will not be easy to check. I don't expect them to be fulfilled in my lifetime, and for all I know it may take thousands or even millions of years. The aim is to accumulate as many arguments like this as possible so that we can get as much evidence either for or against the multiverse as possible.