Ever wonder if the universe is flat?
No, I haven't taken Wingnut Pills and decided that the Flat Earthers make sense. This is an honest-to-Einstein problem in physics, one that not only raises eyebrows about the supposed "fine-tuning" of the universe but has a huge effect on its ultimate fate.
By this time most people who are reasonably scientifically literate (or at least watch Star Trek) know about curved space -- that the presence of mass warps space-time, a little like the way a heavy weight on a trampoline stretches and deforms the flexible sheet it's sitting on. The trampoline analogy isn't a bad one; if you have a bowling ball in the middle of a trampoline, and you roll a marble on the surface, the marble's path will be deflected in such a way that it appears the bowling ball is attracting the marble. In reality, however, there's no attraction involved; the bowling ball has warped the space around it, and the marble is only following the contours of the space it's traveling through.
Bump up the number of dimensions by one, and you've got an idea of how curved space-time works. The trampoline is a 2-D surface warped into a third dimension; where you're sitting right now is a 3-D space warped into a fourth dimension.
The "flatness problem" asks a seemingly simple question; okay, matter deforms space locally, but what's the shape of space as a whole? In our trampoline analogy, you can visualize that although the bowling ball deflects the surface nearby, as a whole the trampoline is flat. Harder to picture, perhaps, is that the trampoline could be a different shape; the surface of the entire trampoline could be spherical, for example, and still have indentations on the surface corresponding to places where massive objects were located.
That, in a nutshell, is the flatness problem. The key is the matter/energy density of the entire universe. If the universe is flat as a whole, the matter/energy density is exactly right for the outward expansion from the Big Bang to slow down, asymptotically approaching zero, but never quite getting there (and never reversing direction). A universe with a higher matter/energy density than the critical value would eventually halt, then fall inward again, resulting in a "Big Crunch" as all the stuff in the universe collapses back to a singularity. (This is sometimes called a "spherical universe" because space-time would be warped into a four-dimensional hypersphere. If you can't picture this, don't worry, neither can anyone else.) If the matter/energy density is lower than the critical value, the universe would continue to expand forever, getting thinner and more spread out, eventually reaching the point where any particular cubic light year of space would have very little chance of having even a single atom in it somewhere. (This is known as a "hyperbolic universe," for analogous reasons to the "spherical universe" mentioned above, but even harder to visualize.)
So, which is it?
There doesn't seem to be a good reason, argued from first principles, that the universe has to be any particular one of the three. When I first ran into this concept, in high school physics class, I was rooting for the spherical universe solution; ending the universe with an enormous collapse seemed (and still seems) preferable to the gradual attenuation of matter and energy that would occur with the other two. Plus, it also raised the possibility of a rebounding second Big Bang and a new start, which was kind of hopeful-sounding even if nothing much would survive intact through the cusp.
Because there seemed to be no reason to expect the value of the matter-energy density -- known to physicists as Ω -- to be constrained, figuring out what it actually is occupied a great deal of time and effort by the astrophysicists. It was a matter of some shock when by their best measurements, the value of Ω was:
To save you the trouble, that's exactly one, out to the 62nd decimal place.
So in other words, the universe is flat, or so close to it that we can't tell the difference.
This engenders more than a few other problems. For one thing, why is Ω exactly 1? Like I said earlier, nothing from the basic laws of physics seems to require it. This brings up the issue of cosmological fine-tuning, which understandably makes us science-types a little twitchy. Then there's the problem that the outer reaches of the universe that we can see -- so places farther away in space, and further back in time -- are moving away from us a lot faster than they should if the universe was flat. This has given rise to a hypothesized repulsive "dark energy" to account for this, but what exactly dark energy is turns out to be even more problematic than the "dark matter" that appears to comprise over a quarter of the overall mass/energy of the universe even though we haven't been able to detect it other than by its gravitational bending of space-time.
The reason this warped topic comes up is research by the groundbreaking and often controversial Nobel laureate Roger Penrose, who published a paper in Monthly Notices of the Royal Astronomical Society this summer that identified six "warm spots" that had been detected in the background radiation of the universe, and which Penrose believes are "Hawking points" -- places where a black hole evaporated due to its "Hawking radiation" eventually bleeding off mass (a topic that deserves a whole other post). The problem is, the evaporation of a black hole by Hawking radiation generates theoretical lifetimes for your average black hole of many times the current age of the universe, so the presence of six of them indicates something funny must be going on.
What that funny business is, Penrose claims, is that we're seeing the ghosts of black holes that evaporated before the Big Bang that formed our universe.
In other words, in a previous universe."The Big Bang was not the beginning," Penrose said in an interview with Sarah Knapton in The Telegraph. "There was something before the Big Bang and that something is what we will have in our future. We have a universe that expands and expands, and all mass decays away, and in this crazy theory of mine, that remote future becomes the Big Bang of another aeon. So our Big Bang began with something which was the remote future of a previous aeon."