It is easy to imagine other universes, governed by slightly different laws of physics, in which intelligent life, or any organized complex systems at all, could not arise. So should we be surprised that there is a universe that we could appear in?
That’s a question that physicists, including myself, have been trying to answer for decades. But it’s proving difficult. Although we can certainly trace cosmic history back to a second after the Big Bang, what happened before is harder to assess. Our accelerators simply cannot produce enough energy to reproduce the extreme conditions that existed in the first nanosecond.
But we assume that in that first tiny fraction of a second, the key features of our universe were imprinted.
The conditions of the universe can be described by its “fundamental constants” – fixed quantities in nature, such as the gravitational constant (called G) or the speed of light (called C). About 30 of them represent sizes and strengths 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 for the evolution of complex systems such as stars, planets, carbon, and ultimately humans. Physicists have discovered that changing some of these parameters by just a few percent would render our universe lifeless. The fact that life exists therefore requires some explanation.
Some argue it’s just a happy coincidence. However, an alternative explanation is that we live in a multiverse that contains realms with different physical laws and values of fundamental constants. Most could be totally unfit for life. But a few should, statistically speaking, be livable.
What is the extent of physical reality? We’re confident it’s larger than the range astronomers can ever observe, even in principle. This area is definitely finite. This is essentially because, like the ocean, there is a horizon beyond which we cannot see. And just as we don’t think the ocean ends just beyond our horizon, we expect galaxies beyond the limit of our observable universe. In our accelerating universe, even our distant descendants will never be able to observe them.
Most physicists would agree that there are galaxies we can never see and that there are more galaxies than we can observe. If they stretch far enough, anything we can imagine can repeat itself over and over again. Far beyond the horizon we could all have avatars.
This vast (and largely unobservable) area 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 that 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 of doubling every trillionths of a trillionth of a trillionth of a second, has real observational support. It explains why the universe is so big and smooth, except for fluctuations and waves that are the “seeds” for galaxy formation.
But physicists like Andrei Linde have shown that under certain but plausible assumptions about the uncertain physics in this ancient era, there would be an “eternal” production of big bangs, each of which would give birth to a new universe.
String theory, which is an attempt to unify gravity with the laws of microphysics, posits that everything in the universe is made up of tiny, vibrating threads. But it assumes there are more dimensions than what we experience. These extra dimensions are squeezed together so tightly that we don’t all notice them. And each type of densification could create a universe with different microphysics – so that other big bangs could obey different laws as they cool.
The “laws of nature” can therefore, in this even larger perspective, be local laws governing our own cosmic patch.
If physical reality is like that, then there’s real motivation to explore “counterfactual” universes – places with different gravity, different physics, and so on – to explore which areas or parameters would create complexity and which ones would become sterile or ” “counterfactual” universes. “stillborn” cosmos. Excitingly, this continues, with recent research suggesting that you can imagine universes even more livable than our own. However, most “tweaks” to the physical constants would result in a universe being stillborn.
However, some dislike the concept of the multiverse. They fear that this would render futile any hope for a fundamental theory to explain the constants, as would Kepler’s numerological quest to relate planetary orbits to nested Platonic solids.
But our preferences are irrelevant to the way physical reality actually is – so we should certainly be open to the possibility of a great cosmological revolution to come. First we had the Copernican insight that the earth is not the center of the solar system – it revolves around the sun. Then we realized that there are millions of planetary systems in our galaxy and that there are millions of galaxies in our observable universe.
So could it be that our observable region – indeed, our Big Bang – is a tiny part of a much larger and potentially diverse ensemble?
Physics or Metaphysics?
How do we know how atypical our universe is? To answer that, we need to calculate the probabilities of each combination of constants. And that’s a can of worms that we can’t open yet—it’ll have to wait for major theoretical advances.
Ultimately, we don’t know if there are other big bangs. But they are not just metaphysics. We might someday have reasons to believe they exist.
In particular, if we had a theory describing physics under the extreme conditions of the ultra-early Big Bang – and if that theory had been confirmed in some other way, for example by deriving some unexplained parameters in the Standard Model of particle physics – then if it has multiple big bangs predicted we should take it seriously.
Critics sometimes argue that the multiverse is unscientific because we could never observe other universes. But I disagree. We can’t observe the interiors of black holes, but we believe what physicist Roger Penrose says about what’s happening there—his theory has gained credibility because it agrees with a lot of things we can observe.
About 15 years ago I was on a panel at Stanford where we were asked how seriously we take the concept of the multiverse – on the scale “you would bet your goldfish, your dog, or your life”. I said I was almost at dog level. Linde said he would almost bet his life. Later, when told this, physicist Steven Weinberg said 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 that we understand the answer – just like quantum theory is too difficult for monkeys. It is conceivable that machine intelligence could explore the geometric intricacies of some string theories and spit out some generic features of the Standard Model, for example. We would then have faith in the theory and take its other predictions seriously.
But we would never have the “aha” insight that is a theorist’s greatest satisfaction. Physical reality at its deepest level could be so profound that it would have to await post-human species to unravel it – however depressing or exhilarating that is, depending on one’s taste. But that’s no reason to dismiss the multiverse as unscientific.