# Can the universe build itself back together after the Heat Death? [closed]

We all know that due to the 2nd Law of Thermodynamics the universe after a certain long period of time will eventually have all its energy spread out homogeneously and reach a Heat Death where nothing interesting ever happens anymore.

However, given that the universe's lifespan is infinite, and the 2nd Law of Thermodynamics is a statistical law (meaning that decreases in entropy are low in probability but still possess nonzero chance), isn't it reasonable to assume that at some point, very far down the line in the Heat Death, the universe puts itself back together and forms stars, galaxies and everything interesting again?

Similar to how a monkey on a typewriter, given an infinite amount of time, will type out the complete works of Shakespeare, isn't the large- scale reduction in entropy just an extremely, extremely rare, but inevitable event which will eventually occur given an infinite timespan?

If so, the universe can never really die, can it?

## closed as primarily opinion-based by StephenG, user191954, Jon Custer, John Rennie, Kyle KanosOct 17 '18 at 10:10

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

• There is no such thing as infinity. It does not exist. It simply is a term for something very large. Using it literally creates silly arguments like monkeys typing something meaningful. It is not a serious argument that can be scientifically tested. – safesphere Oct 16 '18 at 9:31
• Your argument ignores completely the fact that the universe is expanding. If the typewriter's keys slowly start breaking, the monkeys are never going to type out even Hamlet. – Peter Shor Oct 16 '18 at 13:01

This was originally proposed by Boltzmann, who suggested that the entire universe could be a low-probability low-entropy fluctuation in a high-entropy world:

We assume that the whole universe is, and rests for ever, in thermal equilibrium. The probability that one (only one) part of the universe is in a certain state, is the smaller the further this state is from thermal equilibrium; but this probability is greater, the greater is the universe itself. If we assume the universe great enough, we can make the probability of one relatively small part being in any given state (however far from the state of thermal equilibrium), as great as we please. We can also make the probability great that, though the whole universe is in thermal equilibrium, our world is in its present state. It may be said that the worlds is so far from thermal equilibrium that we cannot imagine the improbability of such a state. But can we imagine, on the other side, how small a part of the whole universe this world is? Assuming the universe great enough, the probability that such a small part of it as our world should be in its present state, is no longer small.

If this assumption were correct, our world would return more and more to thermal equilibrium; but because the whole universe is so great, it might be probable that at some future time some other world might deviate as far from thermal equilibrium as our world does at present.

Are Boltzmann worlds plausible? Current cosmological models have unbounded futures and are at finite temperature due to horizon radiation, so it looks like they are a prediction. This doesn't sit well with some physicists, of course. As Susskind points ut:

This would mean not that the future is totally empty space but that the world will have all the features of an isolated finite thermal cavity with finite temperature and entropy. Thermal equilibrium for such a system is not completely featureless. On short time scales not much can be expected to happen but on very long time scales everything happens.

Smaller fluctuations are exponentially more likely than larger fluctuations. That means that occasionally field fluctuations will not just randomly produce particle-antiparticle pairs, but atoms, molecules, and entire organisms that briefly persist. This includes randomly generated observers that can have brains in arbitrary states ("Boltzmann brains", a headache to some philosophers and physicists) and domains that have low entropy (the Boltzmann worlds). But note the probability distribution: finding ourselves in a big universe with lots of remote galaxies is far, far less likely than being in a single solar system floating in nothingness. So our observations seem to rule out that we live in a Boltzmann world.

So if our observations rule out living in a Boltzmann world but theory predicts that most observers live in them, there is something awry. But there is no general agreement on whether anthropic arguments, the unboundedness of the future, the way we measure probability across "big worlds" or something else is the source of the contradiction.

A somewhat related, but distinct, phenomenon is Poincare recurrence. In systems that has a dynamic that maps their bounded phase spaces to themselves and conserve phase space volume the system state will after a sufficiently long but finite time return to a state very close to the initial state. It may hence seem plausible that the universe will eventually return to a state close to the initial state and history will repeat.

Contrarily, the expansion of the universe means the phase space is expanding and the theorem does not apply. In a very real sense the expansion makes the evolution of the universe irreversible even if local regions may experience Boltzmann recurrences due to random chance and relative isolation.

But see also Lubos Motl's answer to this question and Susskind's paper on de Sitter space. Motl claims the recurrence time is $$\exp(10^{120})$$ because by the holographic principle we have a dynamics on the horizon - which is unchanging in size.

Whether Poincare recurrence will happen hence appears to hinge on somewhat unsettled theoretical issues.

• My question addresses more of if perhaps one considers an extremely low entropy fluctuation over the entire universe which kind of "resets" the universe to how it was before the heat death - yes the probability would be extremely low - but over the "unbounded futures" which you stated, surely it will occur eventually? – Hexiang Chang Oct 16 '18 at 12:44
• Ah, you mean Poincare recurrence. I was considering discussing it in my answer since it is somewhat related, but thought you mostly cared about random fluctuations. Will update. – Anders Sandberg Oct 16 '18 at 13:39