“Entropy shakes its angry fist at you for being clever enough to organize the world.” -Brandon Sanderson

The universe was born hot, dense, expanding, full of matter, antimatter and radiation… and in a low-entropy state. If entropy is a measure of disorder, though, that sure does sound like an awfully high-entropy state, not a low-entropy one. So why, when we talk about the Universe, do we say that the early Universe had such low entropy?

Our Universe, from the hot Big Bang until the present day, underwent a huge amount of growth and evolution, and continues to do so. Image credit: NASA / CXC / M. Weiss.

Our Universe, from the hot Big Bang until the present day, underwent a huge amount of growth and evolution, and continues to do so. Image credit: NASA / CXC / M. Weiss.

The answer has much more to do with a comparison to the entropy of the Universe today, as well as in the far future. Today, the entropy is some quadrillion (or 10^15) times larger than it was around 13.8 billion years ago. And if we extrapolate into the very far future, it will be another 100 quintillion (10^20) times bigger than it is today. But what’s responsible for the difference?

A system set up in the initial conditions on the left and let to evolve will become the system on the right spontaneously, gaining entropy in the process. Image credit: Wikimedia Commons users Htkym and Dhollm.

A system set up in the initial conditions on the left and let to evolve will become the system on the right spontaneously, gaining entropy in the process. Image credit: Wikimedia Commons users Htkym and Dhollm.

It isn’t stars, galaxies, gravitational clustering or any of the other major changes you probably imagine. Find out the answer on this week’s Ask Ethan!

(And sorry, no Comments of the Week tomorrow; come back next week for a double dose, and I’ll see you for more science here on Monday!)

Comments

  1. #1 John
    Baltimore
    April 15, 2017

    How about that! I never before thought of Black Holes as being a measure of entropy.

  2. #2 CFT
    April 15, 2017

    Once again, let’s speculate using made up numbers on measureless calculations about how much entropy the universe had almost 14 billion years ago…with our IMAGINARY entropy detector. Never mind that such calculations of entropy can not be performed en toto on the universe we live in today, (you really don’t know how much entropy you have in even a small closed system unless you have an exactly measured volume and compare it to a previous state with which you can also measure with some accuracy…which in the case of our universe – you simply don’t have the ability to measure, just guesses).
    Ethan (or anyone else) really doesn’t know exactly how large the universe is right now outside of a WAG, or was 14 billion years ago outside of an even more uninformed WAG he is trying to fit with still other speculations. He forgot to mention the pesky detail that with each more powerful telescope that sees further out, the estimates of size of the universe shift upwards to keep up with observation. Where the size of the universe is concerned, estimates have been consistently wrong. In the 1950’s the size of the universe was calculated to be far smaller than it is now based on the limits of what they could observe (I’m ignoring inflation nonsense, but you can include it if it makes you feel better). He also really doesn’t know how much entropy (outside of a super-ultra-mega-massive-data-hole WAG) would have been present at his idea of a big bang, as that would depend upon whether he is speculating about the BB being a repeating process, or not. Details, details, details…they really are the devils that do make all the difference even in wild ass guesses and data-free calculations based on next to useless estimates.
    .
    There can be no detected or measured change in size or volume except in relation to something else which is measureable, a pesky little detail which Ethan ignores as he swears up and down there is no ‘outside’ of the universe for it to change/expand in relation to (as he has claimed numerous times), which makes the assumption of a change in size of the universe utterly meaningless. It is just as logically likely in such a ridiculous speculation (sans any actual relative measurement) to assume that everything is actually getting smaller within the same volume (compressed by DE or DM no doubt!!!;o)~) giving the relative appearance that things are growing further apart in an increasing volume. This is why size-changing taffy space is scientifically such a bad idea, you can do ANYTHING with it, and it can’t be falsified, as you can always tack another lame excuse on the end to bring it back into agreement with whatever you like.

  3. #3 Johnny
    April 16, 2017

    (1) why do black holes contain so much entropy? Once a black hole forms we lose all the quintillion particles that could have different arrangements and have a single entity with no structure.
    (2) why didn’t we have any black holes at or soon after the Big Bang? I know the standard answer is “inflation” – but this suggests (if we don’t want to give us on the 2nd Law) the entropy was even LOWER then. And it’s turtles all the way down

  4. #4 eric
    April 17, 2017

    So why was the early Universe so low-entropy? Because it didn’t have any black holes.

    I suspect this is just an important example of a more general statement. The initial universe was too hot for protons and neutrons to be stable, for nuclei to form, for atoms to form, and for molecules to form. All of these things are ‘additional ways’ to distribute energy. Since entropy is a measure of how evenly distributed energy is amongst possible states, a hot universe with fewer stable/accessible states is going to be lower entropy.

    So I’d argue it’s not ‘lack of BH’s’ per se that was the cause of the early universe’s low entropy. It was the temperature, which did not permit macrostates/structures like atoms and molecules and BH’s to form.

  5. #5 CFT
    April 17, 2017

    @Johnny,
    eric knows his physics PR just fine, but he isn’t being very honest by omission.
    Humanity knows next to nothing about actual black holes (whatever they might be), much less how they operate internally, it’s just theoretical speculation based on Hilbert’s flawed math (he had to empty a space time matrix of all matter and energy, then magically re-insert in a volumeless mass of infinite density to get one) about something admittedly beyond the reach of examination. Inferring so much about something never actually confirmed to exist out side of theory is absurdity. I do not question that incredibly dense objects of incredible mass have been detected, I do openly question (until further data is provided) that these objects are actually black holes as theoretically proposed. If planet X is massive, and you find a massive planet, have you in fact found planet X? Not necessarily at all.
    .
    Physicists are presently fond of black holes because they can publish lots of papers about them with little chance of refutation due to experiment or actual data in the near future (No, I didn’t make that up, Dr. Sabine Hossenfelder did). As physicists consider the number of papers published their currency of ‘accomplishment’, their fascination with BHs is understandable sociologically within a peer driven community, but not scientifically. Black holes are in fact actually data holes, as we have next to no information about them internally. Attributing entropy to them is meaningless, un-measureable, and quite untestable at present.
    Wait for more data before you leap.

  6. #6 eric
    April 18, 2017

    CFT:

    [black holes are] something admittedly beyond the reach of examination. Inferring so much about something never actually confirmed to exist out side of theory is absurdity

    LOL, here is an example examination. 18 years of direct observations through a telescope of stellar movement. Add that to our understanding of celestial mechanics and it is trivial to see how we have confirmed the existence of an approximately 4-million-sun-mass object that doesn’t radiate EM the way the stars orbiting it do.

    What exactly is your alternative explanation for what this object is?

  7. #7 rrutis
    syracuse
    April 18, 2017

    Eric:
    Clearly the stars are orbiting some dense object. Anyone know where CFT is actually located?

    I couldn’t resist, sorry.

  8. #8 Johnny
    Milky Way
    April 19, 2017

    @eric
    Playing devil’s advocate: all we can observe is a very dense and massive object. Everything beyond that is based on theory.
    Unfortunately we also know our theory breaks down when a black hole is created (I.e singularity, infinite time etc)

    This does not mean black holes aren’t real – but there’s a reason Hawking and Benkstein have not visited Stockholm.

  9. #9 eric
    April 19, 2017

    Playing devil’s advocate: all we can observe is a very dense and massive object.

    …that doesn’t radiate the way stars do. That’s an important observation, because it rules out alternative ‘normal matter’ hypotheses, since the gravitational compression in a normal matter object with a mass of 4 million Sol’s is certainly going cause fusion. Maybe a neutron star? They also emit detectable EM…but even worse for that hypothesis, neutron stars are not gravitationally stable above about 3 Sol masses.

    A BH fits the data. No other known or (AFAIK) even hypothesized object does. Thus “BH” is the current, provisional, best supported conclusion. Could it be wrong? Sure. But I think at this point it is on the skeptics to come up with a viable alternative explanation, not just throw stones. The just-throwing-stones strategy is typical of creationists and cranks.

  10. #10 Johnny
    April 22, 2017

    We know very dense and massive objects exist. These objects are not powered by fusion and do not emit EM radiation. Fine. But what are those objects?

    Applying GR we get an infinitely dense, infinitely small point – the “singularity’ – which cannot be seen from outside the black hole. Every time a theory predicts infinities it means one thing – the theory is out of its depth and cannot apply to the situation.

  11. #11 Frank
    Omaha,NE
    April 22, 2017

    No. It just means it does apply to the center of the BH.
    I think your logic is called hasty generalization fallacy.

  12. #12 Frank
    April 22, 2017

    It should be “No. It just means it does NOT apply to the center of the BH.”