For a given amount of mass, there is a radius, called the Schwarzschild Radius, that defines the point at which the mass will collapse into a singularity. After collapse, the Schwarzschild Radius then becomes the new singularity's event horizon. So if you have a lump of clay in your hand, and you compress that lump to a size smaller than its Schwarzschild Radius, then that mass would collapse into a black hole. Of course the Schwarzschild Radius for a lump of clay the size of your fist would be far, far smaller than the radius of even a single proton, but you get the picture. That black holes tend to be big and massive has everything to do with the forces required to produce them, and nothing to do with the inherent requirements for black holes to exist. What works for supermassive stars works in principle for lumps of clay. So if you could generate the necessary force, you really could turn a lump of clay into a tiny black hole.
But will what works for supermassive stars and lumps of clay also work for entire universes? Well, sure, why not? Mass is mass, right? And the mass of an entire universe would make one hellaciously scary black hole.
Or would it? I guess that depends on your point of view. From outside the event horizon, a black hole with the mass of an entire universe would be an unimaginably huge and frightening thing. To even detect it would be to fall prey to its gaping maw. But from inside the event horizon it might be, well, rather pleasant at times. What would it be like inside the event horizon of a black hole with the mass of an entire universe?
To answer that question, just take a look around. Amazingly enough, it turns out that all the mass we've been able to observe and measure in the visible Universe is already within its Schwarzschild Radius. That means that everything we see, as far as our telescopes can reach, should be collapsing into a SuperHumongoMegaGiant black hole. But the Universe is expanding, and at an accelerating rate. If all its mass has been squeezed to fit within its Schwarzschild Radius, then why isn't the Universe collapsing?
The answer may surprise you: it is.
That's right, despite what we observe, the Universe is indeed collapsing. In fact, since all the visible matter in the Universe is already within its Schwarzschild Radius, the SuperHumongoMegaGiant black hole has already been formed. This means that the Schwarzschild Radius is now the event horizon, and we are on the inside!
But don't panic. We're okay for now, and probably will be for a long, long time to come.
So what's the deal? If we're inside a black hole, shouldn't we be crushed into oblivion? Yes, we should, but not just yet. We don't quite know what happens within the event horizon of a black hole, but I think it stands to reason that there is a finite time required for matter to collapse into a central singularity once it has been squeezed within its Schwarzschild Radius. In our case, the radius is so huge that it's taking an unknown billions of years for the Universe's mass to collapse into a singularity. That's plenty of time to form galaxies, give rise to life, fight wars, and watch Doctor Who.
But if the Universe is collapsing, why does it appear to be expanding at an accelerating rate? To answer that question, it will help to have a little background information. When the acceleration of the expansion of the Universe was first observed, we didn't know how uniform it was. Theories involving a “Great Attractor” were concocted to explain the phenomenon of acceleration. It was thought that an unseen, great mass was pulling a significant portion of the Universe's matter in one particular direction. These theories were dismissed mostly because the acceleration of expansion was subsequently observed to be more or less uniform in all directions, but also because the theories were unpalatable. Nothing could be imagined to exist that could have the properties required to tip the entire Universe in such a fashion. Nobody could actually come up with a way for a Great Attractor to exist, much less function. So the Great Attractor theories were replaced by the slightly more palatable Dark Energy theories, though we seem to have done little more than to give the phenomenon a more mysterious name.
But what if the Universe is simply falling? In a waterfall, vertical distance between drops increases at an accelerating rate due to gravitational acceleration. But within localized areas, water droplets can merge and divide under the influence of wind and surface tension. Likewise, if our Universe were falling toward a singularity within the event horizon of a black hole, galaxies and clusters of galaxies would form under the local influence of gravity, while very distant objects would become more distant at an accelerating rate.
Geometrically, of course, the waterfall analogy breaks down. The water drops are pulled apart only vertically, not horizontally. An astronomer on any particular droplet would see droplets below and above accelerating away from him, but droplets to the left and right would be roughly maintaining their distance. Perhaps a better analogy would be a mountainous volcano, with lava flowing from the top, flowing down the sides. As the lava flows, gravity causes it to thin vertically, and the widening girth of the mountain causes it to thin horizontally, so an observer at any one location in the flow would see lava accelerating away from him in all directions. Even this analogy isn't sufficient, but the picture it brings to mind is instructive. We don't really know what geometry applies within a black hole's event horizon, but we do know that the space within – if space within exists at all – is completely wrapped in on itself so that even photons are constrained to suborbital paths around the singularity. So, effectively, the singularity is where every line of sight ends. Therefore, for a sufficiently great distance along any line of sight from any particular position, there is a gravitational gradient that will, no matter which direction we look, result in the observation of distant objects being pulled away by gradient-induced tidal forces. So the reason the Universe is observed to be expanding at an accelerating rate is that tidal forces are actually pulling the Universe asunder in every direction we can possibly look. Rest assured, distant galaxies that appear to be accelerating away from us are already moving in the same direction we are, and will catch up a few billion or trillion years after we have been swallowed by the singularity.
If the falling universe is sufficiently large, our inability to see great distances down-gradient may well make it look flat and virtually endless. In other words, a very large universe falling toward its doom within the formidable confines of a black hole's event horizon may look exactly like what we see when we look at the cosmos. In this environment, normal gravity, instead of slowing the expansion, serves to accelerate the expansion due to the Universe-spanning tidal gradient. And it is this tidal gradient spanning the Universe that explains the uniform acceleration of matter that a Great Attractor in a non-gradient universe cannot.
Incidentally, just because we haven't found the singularity doesn't mean it hasn't begun to form and accumulate mass. Remember, all lines of sight end at the singularity. If the singularity has begun to form, we won't know about it until we smash into it. However, the closer we get to the singularity, the less flat our Universe will appear. If the Milky Way hasn't become a cauldron of boiling neutronium by then, we should be able to measure the apparent flatness of the Universe to roughly predict the moment of impact with the singularity within a hundred billion years or so.
So what happens when all the matter and energy of the Universe does collapse into the singularity? Your guess is as good as mine. It could be that all is annihilated, and everything the Universe once was will be gone forever.
Personally, I rather favor the “steady-state” notion that the black hole containing our Universe is endlessly feeding, swallowing up copious volumes of matter – enough to fill an entire universe every few billion years. The farther we look in any direction, the more “up-gradient” we see, and the closer we see to the event horizon at the edge of the Universe. What would it look like to see matter entering our Universe at the event horizon? That's difficult to say. Outside our event horizon, all bets are off. It may be the case that the fundamental particles outside our Universe are clown shoes, bus tickets, and salad forks. But once those particles fall in, they coalesce into protons, neutrons, electrons (among others) and become subject to the laws of nature as they apply on our side of the event horizon. Of course, we can never see matter entering our Universe in this fashion. Once matter tumbles to our side of the event horizon, it must clump together and form stars before emitting light that we can detect. And then we have to wait for that light to reach us. By the time it does, it's already fallen far from the event horizon, so that is, unfortunately, something we can never see. But the farther we look into the cosmos, the younger the stars and galaxies we see.
If the black hole that contains our Universe is feeding, then that would mean that at any given moment, the Universe consists only of whatever matter is currently between the event horizon and the singularity. As matter falls and is eventually swallowed by the singularity, it is replaced by an endless stream of matter falling through the event horizon from outside the Universe. As new matter falls in through the event horizon, it becomes subject to local gravity and forms filaments of clusters of galaxies, millions or even billions of light years across. These filaments of galaxies are then stretched apart as they continue their descent toward the singularity. This black hole structure that contains our Universe could be beyond ancient, trillions upon trillions of years old, forever flowing, forever renewing, forever growing. The reason the Universe looks like it's fourteen billion years old to us is simply because that's how long we (meaning the matter making up our Milky Way and neighboring galaxies) have been falling. Down-gradient, there may be civilizations who live in a much older, much larger universe than the one we perceive. From their cold, dark, ancient galaxy, they may look up-gradient to our Milky Way and envy our much younger, star-producing spiral wonder. We, due to the geometry of the space within our black hole Universe, have no hope of ever detecting them or knowing of their existence. No matter where you are in the river, you can look only upstream across vast distances and see objects only younger than yourself.
Now we're not supposed to get emotional about scientific theories, but let's face it: we do science because it absolutely thrills us. So, if I may, there is an emotionally compelling aspect to this Falling Universe Theory that I'd like to touch upon. It seems that the more we learn about the fate of the Universe, the more depressing things become. Though the fate of the Universe is something that won't affect life for untold eons, knowing that the end of the Universe will bring an end to life is, for some reason, slightly unsettling to us. Back when we thought the Universe might have enough mass to reverse the Big Bang, collapse, and then perhaps reignite in a new universe, there was at least a hope of renewal, even though life in this Universe would be extinguished and have no hope of contributing to the legacy of life in subsequent Big-Bang cycles. But now that we are quite certain that the Universe will succumb to entropic heat death, we have resigned ourselves to the reality that all life will eventually end forever.
And with this new Falling Universe Theory, it doesn't seem to get any better. Instead of entropic heat death, life ends in a fatal crunch when we slam into the singularity.
But does it have to be that way? If the Universe is truly falling in steady-state fashion, constantly being renewed with a river of matter entering through the event horizon, perhaps life could find a way to leapfrog from galaxy to galaxy, establishing and extending itself to ever younger star systems, thereby avoiding the cataclysmic fate at the singularity. It would be like a tree squirrel hopping from log to log in a sawmill in order to avoid the deadly saw blades at the end of the conveyor. This way, life in this falling, steady-state universe could last, quite literally, forever. (Or at least as “forever” as forever gets.) Of course, this would almost certainly require the development of superluminal-speed ships that can swim upstream faster than we are falling, but I'm not one to say that it's not possible. Indeed, this could be natural selection at play on an intergalactic scale. Though many galaxies may give rise to life, only those galaxies that produce life clever and adaptable enough to find a way to swim upstream will see their progeny outlive them. Life forms that leapfrog the best survive the longest.
So in a very emotionally gratifying turn that seldom accompanies theories regarding the fate of the Universe, it is very interesting to note that the reward in this theoretical universe for such ingenuity is, quite literally, eternal life.
How cool is that?