If the universe is alive, how would we tell?
The question of whether the universe is alive comes down to what we think life is. One of the primary features of nature is that living things are ultimately composed of non-living things and so it is the organization of those things, the process by which they persist, and the information that they contain that determines whether they are alive.
The simplest forms of life on Earth are viruses and bacteria and these contain basic means of self-replication and consumption of nourishment. Prions, on the other hand, fragments of proteins, thus containing the same material as living things, also replicate, but they are judged not to be alive.
The Encyclopedia Britannica provides definitions of life based on several considerations (growth, change, reproduction, active resistance to external perturbation, and evolution) but states that a conclusive definition is impossible.
The universe certainly grows and changes, not only expanding but forming complex structures like galaxies, galaxy clusters, and webs of interlinked clusters. It may also evolve from other universes and produce “spores” for new ones via black holes.
My own definition of life, explained here, is anything that can travel from one ordered state to another with a higher probability than a non-living thing. In other words, the primary definition of life is that which can keep itself alive. While this definition seems circular because it defines life based on non-life, it might be better to say that life and non-life cluster in distinct groupings depending on their tendency to maintain an ordered state.
A rock can stay in an ordered, intact state for billions of years if nothing external interferes with it. Therefore, another requirement for something to be alive is that it continues in an ordered state despite external inteference.
These definitions make it diffcult to say whether the universe is alive, even if it is something akin to a fungus, because we cannot observe other universes nor are we aware of any attempts to interfere with ours. In addition, we have no evidence that our universe reproduces which would be an important characteristic for it to be alive.
The universe does have an internal order that causes it to persist. Measurements of the expansion of the universe show that it is flat, meaning that it will not collapse back in on itself, nor will it rip apart. This means that it will persist indefinitely. Yet, this could be no different than the rock. If nothing external interferes with the universe, there is no reason to think it wouldn’t persist.
Mathematically, however, it is possible to imagine what would happen if another universe were to interfere with ours. In a multiverse with many universes popping into existence such as the theory of Eternal Inflation proposes, universes may collide with one another.
One paper explores the possibility that our universe might be a “bubble” and what would we see if another such bubble collided with ours. For example, the patch of sky where the collision occurred would show discontinuities with the rest of the sky. The appearance of such observations is discussed in this 2008 paper where it is proposed that these collisions produce cosmic wakes which look like symmetric discs in the Cosmic Microwave Background, the earliest light we can see.
What this means is that, although there is no conclusive evidence that other universes are interfering with ours, we do have theories about what those might look like.
The next question then is, if our universe resists such collisions in some way, as a living thing might, how would it do that?
If the universe does this very effectively, unfortunately, we might see nothing at all because no collisions are observable. For example, if the universe is like a plant cell with the particle horizon (the furthest we can observe) being a cell wall, then we will observe nothing of these other universes.
If, however, our universe has ways of repairing its walls after a collision, this might be present in the universe’s DNA. That DNA would be roughly what we call the laws of physics. These are rules that the universe follows, which may not be the same from universe to universe.
As mentioned in the 2008 paper, inflation, the rapid expansion of the early universe, limits the effects of bubble collisions. It also erases influence from the parent universe and creates most of the matter and energy in the universe at the same time. This doesn’t necessarily mean that inflation evolved to produce those effects, but it is at least one mechanism by which interference from other universes is limited within our universe while allowing our universe to form.
As Lee Smolin pointed out in a 2006 paper, Eternal Inflation is not mutually exlusive with his Cosmic Natural Selection hypothesis for evolutionary universes from black holes. Therefore, there is no reason to believe that we could not have an entire ecosystem of universes that reproduce, evolve, and inflate from false vacua.
Unfortunately, it is impossible to say conclusively whether the universe is alive without observing other universes. Whether our universe has simply been very effective at shielding us from those universes or whether they exist at all is unknown. We can, however, say that at one time human beings were in the same situation relative to planets outside our solar system (exoplanets). Indeed, I’m old enough to remember when we didn’t know whether other stars had planets. Now we can directly image them and in some cases determine some of their chemical composition. With new instruments such as the JWST and gravitational wave detectors, there is no telling what the future will bring.