# How inevitable is the heat death of the universe?

Original question: **how inevitable is the heat death of the universe?**

What does that actually mean now?Inevitable?

Of course it simply means ‘*not to avoid*‘!But what is this in the context of this question?

What I am trying to work on is simply that it is very important in these kinds of questions to distinguish between what the universe — that really thing beyond what we go into — and our **model of that reality**.

The extent to which our predictions about the future of our reality are accurate depends, of course, on how good our models actually are.One has to beware of the idea that science has some ‘*eternal truth*‘ in their hands.We can *leave these kinds* of claims to the many religions that are already there.

There is no one who thinks or expects that our current insights into the workings of nature are both true and complete!And of course any deviation between our models and the reality they are a model of can lead to radically different answers to questions about the future, especially the long future.

To make a long story short, we have no reason to believe that we already know all the phenomena that exist in the universe and that future discoveries do not simply sweep our current models off the map or at least important Will impose a restriction on our models.

That said, if we assume that we are a little bit close to our models, then it seems very likely that the future of our universe looks a bit sad.

Observations tested against the current models[1 indicate that the universe is out of the thighs, and it is getting faster.The expansion itself is already a consequence of the general theory of relativity which predicts that a static universe is particularly unstable, something like a pencil that balances on its tip, the smallest hit…

This means that every universe must be either expanding or shrinking.

In such a universe, the eventual future depends on how much ‘*stuff*‘ There is in the universe.That is to say all the ‘*stuff*‘ that causes gravity: energy, momentum (which has to do with energy movement) and stress.Technically speaking, the elements of energy momentum[2 tensor.

- If there is enough ‘
*stuff*‘, gravity will eventually reverse the initial expansion and will bring everything together again. - If there is too little ‘
*stuff*‘, the expansion will continue forever (= Never stop), but slower, as gravity tog slightly sputters. - And then we have the magical limit between these two other possibilities. If we have exactly enough stuff then the expansion in the limit to infinity will be exactly zero.

But what we perceive is that this expansion seems to be getting faster!This cannot **be explained** within the model, with the foregoing.At least not without the assumption that a special kind of ‘*stuff*‘ is in the universe that behaves a little strange, namely dark energy[3[4.

The strange thing about this dark energy is how it dilute itself by extending the available space.Think of a gas. If we make volume of the barrel bigger, the density of the gas becomes smaller, the pressure drops. This actually applies to all the ‘*normal stuff*‘ that we know.This also applies to radiation, as the precise formula is something else. But this dark energy **does not dilute**.

This has a peculiar effect.If we view the breakdown of all sorts of stuff in a cubic metre of space in terms of their density, then it appears that upon expansion, eventually dark energy becomes the largest part of all the stuff.

A simple example:

Suppose we have two types of ‘*stuff*‘: [Matha [/Math and [MATHB [/math.Let’s say that stuff [Matha [/math is like just gas thinning, that will say, if the volume becomes twice as large, the density halves.But stuff [MATHB [/math doesn’t dilute.Look at the table below.

[Math\\begin array c \\textbf{Doubling} & \\textbf{A} [m ^ {-3} & \\textbf{B} [m ^ {-3} & \\textbf{Percentage B} [\ \% \ \ \ \ \\hline 0 & 16 & 1 & 1/17 = \\phantom{0}5.88 \ \ \ \ 1 & 8 & 1 & 1/9 = 11.11 \ \ \ 2 & 4 & 1 & 1/5 = 20.00 \ \ \ 3 & 2 & 1 & 1/3 = 33.33 \ \ \ \ 4 & 1 & 1 & 1/2 = 50.00 \ \ \ \ \\vdots & \\vdots & \\vdots & \\vdots \ \ \ \ n & 2 ^ {4-n} & 1 & 100/({1 + 2 ^ {4-n}}) \ \ \ \ \\vdots & \\vdots & \\vdots & \\vdots \ \ \ \ \\li m_ {n \\to\\infty} & 0 & 1 & 100 \ \ \ \ \\hline \\end{array} [/math

So the long-term future of the the universe according to this model is mainly determined by the further properties of dark energy, as it becomes the dominant factor overall in space.

And it is precisely this stuff that takes care of the acceleration of the expansion.

So in the long term, it does not look very cosy.But keep in mind that we are talking about the extremely long term here. Many orders of magnitude larger than the current age of the universe, not billions but triljarden, quadriljarden or even longer.

Is that inevitable?Well, if the models are accurate and complete…

Footnotes