Re: will the universe stop expanding and begin to contract, and expand again?

Hello Coleman,

Scientists are really hesitant to say what happened "before the big bang" or "after the big crunch". On one hand it's because, experimentally, we will never test this sort of thing. More importantly, our understanding of General Relativity tells us that the end of the universe, if it ever happens, would be a singularity: everything coming together and collapsing to a point with infinitely strong gravity. And, under infinitely strong gravity, the passage of time must stop. Let me try to explain.

The laws of General Relativity (our modern theory of gravity) work in interesting ways. Under the old theory (Newton's), gravity was a force that caused attractions between objects. Under Einstein, we say that gravity distorts space and time. To describe the motion of objects under gravity, you use ordinary mechanics (like Newton's laws) but replacing all of the straight lines by curved lines in this new distorted space. Imagine playing pool on a curved or wavy table and you'll get intuition for part of this ...

Gravity also distorts time. A person in an intense gravitational field experiences the passage of time at a different rate than someone far away from gravity. For Earthlings this is a tiny, tiny effect - for example, the clocks on satellites should run a barely-measureable bit faster since they are farther away from the Earth's gravity. (The clocks run faster, radioactive particles decay faster, people think faster ... everything) That's not particularly exciting in the Earth's weak gravity, but it does interesting things near Black Holes, or Big Bangs. For example, if you were to take a telescope and watch someone falling into a black hole, you'd see that person's watch slow down and actually stop as they entered stronger and stronger gravitational fields. In fact, you'd never see them actually enter the black hole ... you'd see their experience of time glide to a halt, and leave them hovering on the verge of falling in.

Weird.

Anyway, inside the black hole things are as bad or worse. No matter where you are and where you are looking, the immensely strong gravity has slowed everything down. Ultimately, in the instant before you enter the black hole itself, you are moving at essentially the speed of light, you are in an infinitely strong gravitational field, and your watch is exactly frozen. The "Big Crunch" at the end of the universe would be just like this - every particle, photon, clock, and tiddlywink falling into a singularity with the mass of the Universe. None of this "falling" material has any experience of time, and anybody far away is outside of the black hole and cannot see what is going on.

So that's the source of the trouble. Your idea about everything continuing to move past the singularity is fine in classical physics, but how do you describe this "motion" if you cannot describe the passage of time? I've heard people describe this as "The laws of physics break down". This is really true - we do not know how to describe anything for which time does not pass.

Anyway. While we're sitting on this Singularity with time all stopped and everything, it's also hard to see what form "density fluctuations" and "groups of particles" and even "individual particles" might take. It's hard, even, to see whether quantum mechanics can function (at first glance: no). Right now, all speculation about what came before the Big Bang (and what might come after the Big Crunch) is on hold until we have a quantum theory of gravity. Since we also still don't understand the source of density fluctuations, your idea may fit in somewhere ... take a look at Paul Davies' "The Last Three Minutes", Hawkings' "Brief History of Time", or (if you're ready for the real math of GR) Misner, Thorne and Wheeler's "Gravitation"

Thanks for the question! And happy new year!

-Ben Monreal