Do Black Holes Create New Universes? (Script) (Patreon)

What if every single black hole that formed in our universe sparked the big bang of a new universe? Cosmological natural selection proposes exactly this - but even better, it claims to be able to test the hypothesis.

Physicists have been struggling for some time to figure out why our universe is so comfy. Why, for example, are the fundamental constants - like the mass of the electron or the strength of the forces - just right for the emergence of life? Tweak them too much and life, stars, galaxies, the universe as we know it wouldn’t exist. In recent episodes we explored one possible explanation for this - the anthropic principle and the idea of the multiverse. If there are countless universes with different fundamental constants, then it’s not surprising that a few exist with the right numbers for life - and certainly not surprising that we find ourselves in one of those good ones. But if you don’t like the anthropic principle - and many scientists don’t - then rest assured, there’s an alternative. You only need to accept two things: that our universe formed inside a black hole, and that universes can evolve.

Our universe appears, in some sense, designed. It has finely tuned parameters that seem deliberately set for a particular outcome - life. There’s another example in nature where the illusion of design has a perfectly natural explanation - and that’s life itself. We now know that the fantastic complexity of living organisms is an inevitable consequence of evolution by natural selection. 

Inspired by biological evolution, theoretical physicist Lee Smolin came up with Cosmological Natural Selection. It goes like this: the formation of a black hole triggers the formation of a new universe “on the other side” in a new big bang. Those daughter universes go on to expand and make their own black holes, hence their own daughter universes. But in their formation the fundamental constants of the daughter universes are shifted slightly and randomly from their parent - mutations are introduced. Some of those shifts improve the daughter universe’s ability to form new black holes. Those universes have an advantage in propagating their cosmic genetics, and so gradually the ensemble of all universes get better and better at making black holes, just as biological organisms with helpful mutations can get better at surviving and reproducing. Now by happy chance there’s a correlation between making lots of black holes and making life - both require stars. The universe that is better at making stars is better at making planetary systems is better at making us.

Seems fair enough. But is it any more than a cool story? Bro? Let’s take this apart to ask two questions: is it plausible, and is it testable?

First up, for any of this to make sense black holes need to create universes. This is by far the most speculative part. In fact we have no idea, and only very tentative reasons to think so. The idea originated with one of Lee Smolin’s mentors, Bryce deWitt, who postulated that when a black hole collapses, its mass doesn’t all end up stuck in the central, infinitely dense singularity. Rather it sort of bounces - but unable to exit the event horizon of the black hole, it forms a new region of spacetime, effectively creating a new universe. The details of how this happens is presumably buried in the as-yet-unknown theory of quantum gravity. There are various proposals for how such a bounce can happen - all of which are massively speculative, and perhaps we’ll cover another time.

John Archibald Wheeler expanded on the procreating black hole idea by suggesting that the fundamental constants of this new universe could be different to its parent universe. This seems plausible - if fundamental constants can change at all then surely it’s in the highest possible energy environment, which is exactly the end of a black hole collapse. Perhaps the configuration of the geometry string theory’s extra dimensions gets shifted - that would do the job. Inspired by this idea, Smolin added one thing: what if, when universes reproduce, the constants aren’t randomly reconfigured but rather change only slightly - analogous to a small number of genetic mutations. If that were the case, a sort of evolution by natural selection should become as inevitable as biological evolution.

We have no good reason to believe any of this procreating universe stuff - and Lee Smolin has readily admitted that. The point is to instead ask: what if it’s true? What are the consequences? And can we test them?

The exponential nature of the proposed process means that the ensemble of all universes should very quickly be dominated by ones that are extremely good and making black holes. Any given universe may not be totally optimal because its constants varied randomly from its parent - in the same way that any given living organism isn’t the paragon of its kind. So there’s a prediction: the fundamental constants that define black hole production should be close to optimal in a typical universe, at least for a given mechanism for making black holes.

In OUR modern universe, black holes are made when the most massive stars explode as supernovae. There are other ways to make black holes, and we’ll come back to those. So we should expect our universe to be optimized for producing as many of the most massive stars as possible. Is it? It’s actually very hard to say, but it does seem like there’s some fine-tuning there. Stars are formed when giant clouds of gas collapse under their own gravity. But in order for that to happen the gas needs to cool to just a few degrees above absolute zero, rather than the typical 200-kelvin temperature of the typical interstellar nebula. That cooling is extremely slow if the gas only contains the hydrogen and helium produced in the big bang. Heavier elements and molecules allow clouds to cool and stars to form much more quickly, and of these, carbon monoxide is by far the most important. In addition, gas needs to be shielded from the heating effect of other stars - and that seems to require the presence of tiny particles of ice and hydrocarbon dust. So without carbon, oxygen, water, and chemistry in general, far fewer stars and so far fewer black holes would form - and of course these factors also seem to be essential for life.

Now you can make massive stars without this chemistry. In fact we think in the very early universe MOST stars turned into black holes because pure hydrogen and helium allows collapse into gigantic stars. Those gigantic stars produced gigantic black holes.  Without chemistry, more of the gas ends up in black holes, but because those black holes are more massive you get fewer of them over all, and so, perhaps fewer universes. It’s not clear whether our universe comes close to optimizing the number of black holes formed from supernovae, but it’s at least remotely plausible.

But what about other sources of black holes? Theoretical physicist and cosmologist Alexander Vilenkin proposed that if a universe lasts forever then in the distant future, quantum fluctuations of that near vacuum will cause black holes to spontaneously appear - and given infinite time these will eventually outnumber those produced by stars - stellar black holes. If all this is true then the most black holes would be produced by the biggest universes - more space means more chances for these quantum fluctuations. That favours lots of dark energy generating rapid expansion. And that’s definitely not our universe.

Lee Smolin has various arguments against this: for example, we don’t know that our physics can really be extrapolated to the insanely long timescales required for these quantum fluctuations. I would also add that even if Vilenkin’s argument holds, there are no doubt different regions in the landscape of possible fundamental constants where different types of black hole are optimized. This would lead to multiple branches of the cosmic genetic tree - some of which correspond to producing lots of stellar black holes. And naturally we’d find ourselves on one of those branches because those also happen to be the ones that favour life. But, whoopsie, I just invoked the anthropic principle, which is exactly what were trying to avoid with this whole idea.

As speculative as all of this is, Smolin claims there’s a concrete test of the idea. If cosmological natural selection is true, then the fundamental parameters favouring black hole production should be optimized completely independently to those that also favour the appearance of life. And he suggests there is one. But first some background. When massive stars die, they actually mostly produce neutron stars - a planet sized ball of neutrons so dense that teeters on the edge of collapsing into a black hole. Black holes only form when the neutron stars is above a certain mass limit.

Now it may be that in the cores of the most massive neutron stars, some particles can convert into strange quarks. The resulting material is even denser than the original neutron star, and so brings the star closer to collapse. And the lower the mass of the strange quark, the easier it is to convert lighter particles into strange quarks. That in turn means less massive neutron stars are able to collapse into black holes. Surely, then, if universes evolve to maximize the number of black holes, the strange quark mass should be optimized to make the cutoff between neutron stars and black holes as low as possible.

Lee Smolin calculates that optimized cutoff at around 2 times the mass of the Sun. So, if this universe is optimized for black hole production then there should be no neutron stars more massive than 2 solar masses. And? The most massive known neutron star is 2.17 solar masses, discovered just this year. Perhaps the extra .17 can be factored into the uncertainties of the theory... Or perhaps this is the falsification we were looking for. We await Smolin’s comments on this.

I’d like to add my own objection: cosmological natural selection is meant to explain the fine tuning in the fundamental constants, which appear either set by design or by extreme luck. It tries to avoid the anthropic principle by proposing a natural selection that favours black hole production, and it’s just a happy coincidence that the same factors favour life. But then do we really gain anything? It just so happens that carbon and oxygen are good for both black hole production and organic molecules ... but what if it was, I dunno, beryllium and boron that helped stars form - or other elements useless to life. If we causally disconnect the selection process for cosmic reproduction from the emergence of life then it seems we still have to invoke a lot of good luck?

Overall, cosmological natural selection is an appealing idea because it seeks a natural explanation for fine tuning, and one that parallels a known process in nature - biological evolution by natural selection. It also seems to give us predictions that we can try to test and falsify. And even though this idea probably isn’t true, it’s really important to remember that speculative ideas like this are exactly how we probe the edges of science. No one of them is likely to be true, but they help us explore the vast space of all possible realities - where somewhere is hidden the true nature of OUR reality. Or, you know, our momma might be a black hole, and we live in an endlessly evolving, proliferating space time.