Expert: James Gort - 4/2/2010

Question
Dear James Gort,

I am a 38-year-old student of popular science. I don't understand much math, but I am very enthusiastic and curious about popular science. I have a question about entropy in light of the accelerating expansion of the universe. Entropy says that the entropy of any system (incl. the universe) is always increasing, i.e. there is an overall cooling process going on. I am wondering if the expansion of the universe doesn't affect the rate of entropy on a universal scale. If the universe becomes bigger, its energy is getting more dispersed within it, and the overall universe would seem to be getting cooler and cooler, wouldn't it? Doesn't this make general entropy increase more and more??

I hope the question makes sense.

Thanks,
- Tue

Answer
Hi Tue,

Very interesting question.  And one that's the subject of some recent research.  According to some, gravity is a manifestation of the second law of thermodynamics (entropy).  See http://www.technologyreview.com/blog/arxiv/24975/

But to answer your question, you are right that the expansion of the universe results in a cooler universe, and therefore one with increased entropy (as required by the second law).  It's almost as if an expanding universe is required, if the second law is to be obeyed.

The exception would be a contracting universe.  What would happen if the universe contracted to a large black hole, or even if multiple small black holes were formed?  Well, the entropy would also increase!  When matter falls into a black hole, its ordered information is lost to the universe, so the universe becomes more disordered (increases entropy).  This was proven by Kip Thorne and by Hawking.  So it doesn't matter if the universe is expanding (cooling) or collapsing into black holes, it's entropy is always increasing!

Since you're a student of popular science, the best book I know of that explains this (and much more) is "Black Holes and Time Warps" by Kip Thorne, a leading researching in the field.  It doesn't contain much math, but everything in there is the best information we currently have.

Hope that helps.

Prof. James Gort