The Arrow of Time in the Cosmos

Q&A from my TEDx talk

This is a discussion about the arrow of time which is a follow-up on my TEDx talk. You probably will want to be familiar with that material before looking at these notes.

On the YouTube website for this talk “tompwhu” posted the following comment:

I have a slight problem with the smoothness of the Universe statement. As the expansion of the Universe is accelerating, would not there be a time when a certain measure of smoothness a few billion years ago regains that same measure of smoothness again at some point into the future? Can I infer from that also, that ultimately - after many, many aeons into the future - the state of the Universe will be a precursor to another Big Bang? And in a similar vein, was "our" Big Bang caused in like manner? Or am I naively conjuring up a scenario that does not follow from Prof. Albrecht's statement?

My reply:

This is a wonderful question. Roughly speaking, it is the reason I am so interested in the arrow of time to begin with. I decided it was too complicated to try to include in my TEDx talk, but I'm glad you asked. Here is my attempt at an answer.

The destruction I discuss is basically a way to talk about processes that we see going one way, but not in the time reverse direction. It is a stand-in for the more technical concept of "increasing entropy". Which processes have this "irreversible" property can depend on when and where you look.

I like the example of a water wheel. It harnesses the "irreversible" flow of water from high to low elevations, except of course thanks to the Earth's Water Cycle, water can come back up and keep the flow coming. (In fact, the water wheel is really harnessing “solar power” via the water cycle.) Some billion or so years in the future the natural evolution of the Sun will most likely bake off all the water on Earth, making the water wheel entirely useless (and also incinerating it!). So the flow of time harnessed by the water wheel can only be captured in certain locations (by a stream) and at certain times (when the Earth’s water cycle is successfully keeping that steam going).

We simply do not understand the cosmos well enough to bring a similar “big picture” analysis to the arrow of time in the cosmos as a whole. That of course makes it a topic of interest for research, and as it turns out, one that is linked with other important problems in cosmology as well.

You rightly point out that the accelerating universe seems to signal an end to the "destruction of smoothness" trend. The universe is entering a new phase where the trend will be the reverse, namely the destruction of the lumpiness (the creation of which I discuss in my TEDx talk). The period of cosmic inflation that many think created the earlier smoothness operates in a very similar way. The switch between these different trends happens because major changes take place in the universe (such as the onset of "cosmological constant domination" of the current epoch, or the start and end of "inflaton domination" for the inflationary epoch). These major changes are somewhat analogous to way the explosion of the Sun in a nova will radically change the Earth and the possibility of hydro power.

Understanding how all this fits together in a big picture of the universe is a really interesting unsolved problem that occupies a lot of the research time of me and my colleagues. I have attempted to tie things together the way you suggest in your comment, as have others (see this book for example). But it is far from clear at this point which (if any) of these ideas may be correct.

More about my work on these topics (including both accessible and advanced material) can be found here