I am reading about the accelerating expansion of the universe and something struck me. The observation that things further away were accelerating away faster and that early observations made it look as though we were situated at the center of the universe.

Specifically, it seems as though the universe itself is expanding and not that the things within the universe were moving away from each other. Instead it seems as though the expansion itself is creating more space between the contents.

Recently I also saw an amazing video on the eventual death of everything in the universe. This seems weird to me because as I understand it energy and matter don't go away they just change.

And it got me thinking, what if this eventual heat death of the universe is simply this expansion eventually ripping everything apart until every particle simply is to far apart to interact.

Basically, as things progress the expansion will affect the contents of the universe even down to the particle level.

Is this the case? Or am I misunderstanding this subject?


3 Answers 3


As far as I understand it, the heat death of the universe is a consequence of entropy, not expansion. All processes result in the shifting of some energy to higher entropy. Though the observable universe is an open system, the entire universe is an isolated system, so as more and more energy gets shifted to higher entropy over time the universe will eventually reach a state where effectively no work can be done.


Edit: I believe at least one of cited the authors, Dr. Carrol, no longer holds the position that this answer is based upon.

Is the eventual heat death of the universe due to the expansion of the universe?

Firstly, I'm not a physicist or astronomer, but just enjoy reading popular science books, like those by Prof. Sean Carroll.

There's one idea that says yes, possibly, and vice-versa.

If the vacuum energy exist homogeneously throughout all of space and has a negative pressure, it would help explain why the universe's expansion is accelerating. In empty space where there is nothing but the vacuum energy, it would have a repulsive effect, and push the other empty space away from it, creating more empty space.

In the new empty space, there will still be the vacuum energy, accelerating the process.

In the system of the universe, the smaller systems like galaxies with matter and dark matter overpowering the vacuum energy, they'll remain intact, but the empty space between them and other galaxies will continue to expand.

In one way, I think this makes heat death happen faster, since eventually every galaxy will be so isolated by the expansion, that it effectively will no longer be able to reach any other galaxy once the expansion has grown to the point beyond which light coming from or going to it could travel, given its speed has limit.

Effectively this creates a new system with an observable horizon the size of a universe in its own right. But with much less energy in it. Instead of having all of the energy in the universe, it only has the energy of its galaxy and the empty space up until its observable horizon. It's radiation energy would also more easily dilute away.

As called2voyage pointed out, entropy will always increase, and one way to do this is by finding thermal equilibrium. So when the small amount of energy within the galaxy has its volume replaced with empty space, it would be much easier for it to find thermal equilibrium than if it were in a volume that had other galaxies and patches of dark matter to interact with.

In regards to why this heat death might lead to the acceleration of the universe, it turns out that even when the universe is in thermal equilibrium, it still has a temperature greater than zero due to quantum fluctuations of particles and anti-particles popping in and out of existence.

Given an unimaginable amount of time, instead of a simple particle and anti-particle fluctuating into existence, something more complex might emerge, like a pencil, or a Boltzmann Brain, or another universe.

However, these seem to be going towards a lower state of entropy, going against the second law of thermodynamics.

Instead, it would be much easier1 for an inflaton to fluctuate into existence and cause this sudden low-entropy event, to actually be a way of increasing entropy.

The inflaton has an enormously high energy density, and is rapidly expanding. As it inflates, its energy density gradually decreases. As I understand it, this decrease is not a direct relation to the increasing volume of space, but just the way the inflaton behaves.

At some point, the energy density lowers to a point where energy turns into the types of energy fields we're familiar with, and eventually particles. This is also when inflation stops and reheating, baryogengesis and recombination happen, leaving behind a cosmic microwave background; the moment when the universe cooled enough to become transparent (where as before the CMB's imprint the universe was too hot and dense for light to not bump into something).

What's interesting about this idea is that it allows entropy to always increase by taking the maximum entropy state of the universe (heat death), and further increase it with a quantum fluctuation of an inflaton particle–a much more likely fluctuation than a Boltzmann Brain or another universe–and create another big bang.

Because the entropy density of the background is so low, it is easier to fluctuate into a small proto-inflationary patch than into a universe that looks like ours today.1

This new universe too, will eventually meet its heat death, and find thermal equilibrium; creating the environment for another inflaton to fluctuate into another big bang, and continue increasing entropy. Given enough time it seems imply there will be an infinite number of universes as a result of an ever increasing entropy.

Which one we are in would be mystery, but what matters about this point is that it would explain why the universe started in a low entropy state; it actually started from a maximum entropy state, but fluctuated into a low entropy with a relatively simple quantum fluctuation.

We therefore believe that inflation does provide natural initial conditions for the universe we see, once we place it in the proper context of a larger spacetime that is stubbornly trying to increase its entropy.1

Due to the random nature of quantum fluctuations, this new universe will be different from ours. It will still be largely homogenous, and have a CMB that looks as smooth as ours, but it won't be duplicate of ours.


Carroll, S.M., Che, J. (2005) Does Inflation Provide Natural Initial Conditions for the Universe?

  • $\begingroup$ Thank you for the insightful answer. $\endgroup$
    – Patrick
    Commented Jan 4 at 15:28

Take a balloon and inflate it until it is firm. Then take a soft pen which won't burst the balloon and mark lots of dots all over it, fairly evenly spaced. The balloon is the universe and the dots are the galaxies. Now put an ant on one of the galaxies and begin to inflate the balloon further. From the point of view of the ant, his galaxy is stationary and at the centre, with all the other galaxies moving away and the more distant galaxies moving away fastest of all.

Then put an ant on one of the other galaxies and try to visualise the situation as he sees it. This second ant will also see himself as at the centre of the universe with all the other galaxies rushing away, with the furthest moving away fastest of all. It doesn't matter which dot or galaxy you put the ant on, the picture will always look the same. There is an elastic force in the balloon which is always trying to collapse it, but the expansion resists this force. Its counterpart on a galactic scale is gravity, which is always trying to resist the expansion and gradually slowing it down. Until recently it was thought that eventually gravity would win, and the universe would be brought to a halt and collapse on itself to become a contracting, blue shifted universe, gradually speeding up and hurtling toward destruction in what was called the Big Crunch. But then dark energy threw a spanner in the works.

Dark energy is a mysterious force which astronomers think is speed ing up the expansion too much for gravity to slow it down and halt it. Observations and measurements of the redshift of distant type 1a supernovae are what brought them to this conclusion. However, the measurements are difficult to make and difficult to interpret, so there is always the possibility of error. It also introduces new difficulties, such as a one-off universe with no cause and which will go on expanding forever into the black nothingness of space, making the Big Crunch and the Big Bounce which follows it impossible if current interpretations of the data turn out to be correct. The Big Crunch model has not been entirely abandoned, though dark energy is the favoured theory at present. Dark energy has the disadvantage that no one can explain where it comes from or where it has been hiding, and it also contravenes the 1st Law of Thermodynamics. Some people say that the 1st Law doesn't apply on a cosmic scale, but that sounds like special pleading.

The heat death of the universe, as called2voyage says, is an entropy effect due to the 2nd Law of Thermodynamics, which for some reason can't be abandoned on a cosmic scale, though the 1st Law can. Unlike the 1st Law, the 2nd Law has over a dozen different formulations. From a dark energy, accelerated expansion perspective, it is the cold death of the universe we are talking about, for the black nothingness into which it is reportedly heading is also bitterly cold.

  • 2
    $\begingroup$ This answer would be clearer without the big rant about dark energy which is not directly asked about in the question. $\endgroup$
    – called2voyage
    Commented Aug 26, 2019 at 20:57

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