It is easy to imagine other universes, governed by slightly different laws of physics, where no intelligent life, or indeed any kind of organized complex systems, could arise. Should we therefore wonder that there exists a universe in which we were able to arise?
It’s a question that physicists, including me, have been trying to answer for decades. But it turns out to be difficult. Although we can confidently trace cosmic history back to one second after the Big Bang, it is more difficult to measure what happened before. Our accelerators simply cannot produce enough energy to replicate the extreme conditions that prevailed in the first nanosecond.
But we expect that it is in that first tiny fraction of a second that the key features of our universe were imprinted.
The conditions of the universe can be described through its “fundamental constants” – fixed quantities in nature, such as the gravitational constant (called G) or the speed of light (called C). There are about 30 of these, representing the size and strength of parameters such as particle masses, forces, or the expansion of the universe. But our theories do not explain what values these constants should have. Instead, we must measure them and plug their values into our equations to accurately describe nature.
The values of the constants are in the range that allows complex systems such as stars, planets, carbon and ultimately humans to evolve. Physicists have discovered that if we adjusted some of these parameters by just a few percent, it would render our universe lifeless. That life exists therefore requires an explanation.
Some claim it’s just a lucky coincidence. However, an alternative explanation is that we live in a multiverse that contains domains with different physical laws and values of fundamental constants. Most may be completely unfit for life. But a few should statistically be life-friendly.
What is the extent of physical reality? We are convinced that it is more extensive than the domain that astronomers can ever observe, even in principle. That domain is definitely limited. This is basically because, like the sea, there is a horizon beyond which we cannot see. And just as we don’t think the ocean stops just beyond our horizon, we expect galaxies beyond the limit of our observable universe. In our accelerating universe, our distant descendants will never be able to observe them either.
Most physicists agree that there are galaxies that we can never see, and that these outnumber the ones we can observe. If they stretched far enough, then anything we could ever imagine happening could be repeated over and over again. Far beyond the horizon, we could all have avatars.
This vast (and largely unobservable) domain would be the aftermath of “our” Big Bang—and would likely be governed by the same physical laws that govern the parts of the universe we can observe. But was our Big Bang the only one?
The theory of inflation, which suggests that the early universe went through a period in which it doubled in size every trillionth of a trillionth of a trillionth of a second, has real observational support. It explains why the universe is so large and smooth, except for fluctuations and ripples that are the “seeds” of galaxy formation.
But physicists including Andrei Linde have shown that under some specific but plausible assumptions about the uncertain physics of this ancient era, there would be an “eternal” production of Big Bangs – each of which would give rise to a new universe.
String theory, which is an attempt to reconcile gravity with the laws of microphysics, assumes that everything in the universe consists of tiny, vibrating strings. But it makes the assumption that there are more dimensions than the ones we experience. These extra dimensions, it suggests, are compressed so closely together that we don’t notice them all. And each type of compression could create a universe with different microphysics—so that other Big Bangs, as they cooled, could be governed by different laws.
The “laws of nature” may therefore, in this even larger perspective, be local statutes governing our own cosmic domain.
If physical reality is like that, then there is a real motivation to explore “counterfactual” universes—places with different gravity, different physics, and so on—to explore what area or parameters would allow complexity to arise, and which would lead to sterile or “stillborn” cosmos. Excitingly, this is ongoing, and recent studies suggest that you could imagine universes that are even friendlier to life than our own. However, most “adjustments” to the physical constants would render a universe stillborn.
That said, some people don’t like the concept of the multiverse. They worry that it would make the hope of a fundamental theory to explain the constants as futile as Kepler’s numerological quest to relate planetary orbits to embedded Platonic solids.
But our preferences are irrelevant to the way physical reality actually is—so we should certainly be open to the possibility of an impending grand cosmological revolution. First we got the Copernican realization that the Earth was not the center of the solar system – it revolved around the Sun. Then we realized that there are zillions of planetary systems in our galaxy and that there are zillions of galaxies in our observable universe.
So could it be that our observable domain – indeed our Big Bang – is a tiny part of a much larger and possibly diverse ensemble?
Physics or metaphysics?
How do we know how atypical our universe is? To answer that, we need to calculate the probability of each combination of constants. And it is a can of worms that we cannot yet open – it must await major theoretical advances.
We ultimately don’t know if there are other Big Bangs. But they are not just metaphysics. We may one day have reason to believe they exist.
Specifically, if we had a theory that described physics under the extreme conditions of the ultra-early Big Bang – and if this theory had been confirmed in other ways, for example by inferring some unexplained parameters in the Standard Model of particle physics – then if it predicted more Big Bangs, we should take it seriously.
Critics sometimes argue that the multiverse is unscientific because we can never observe other universes. But I disagree. We can’t observe the interior of black holes, but we believe what physicist Roger Penrose says about what happens there – his theory has gained credibility by agreeing with many things we can observe.
About 15 years ago, I was on a panel at Stanford where we were asked how seriously we took the multiverse concept—on the “would you bet your goldfish, your dog, or your life” scale on it. I said I was almost at dog level. Linde said he would almost bet his life. Later, after learning this, physicist Steven Weinberg said that he “would happily bet Martin Rees’ dog and Andrei Linde’s life.”
Unfortunately I suspect Linde, my dog and I will all be dead before we have an answer.
In fact, we can’t even be sure we would understand the answer – just as quantum theory is too difficult for monkeys. It is conceivable that machine intelligence could explore the geometric intricacies of some string theories and, for example, spit out some generic features of the Standard Model. Then we would have confidence in the theory and take its other predictions seriously.
But we would never have that “aha” moment of insight that is the greatest satisfaction for a theorist. Physical reality at its deepest level could be so profound that its illumination would have to await posthuman species – depressing or exciting as that may be, according to taste. But that is no reason to dismiss the multiverse as unscientific.