The continuum of the Universe Our universe is just a finite number of galaxies rushing away from each other inside this empty ...
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| The continuum of the Universe |
“If the doors of perception were cleansed every thing would appear to man as it is, Infinite. For man has closed himself up, till he sees all things thro’ narrow chinks of his cavern.” -William Blake
13.8 billion years ago, what we know as our Universe began with the hot Big Bang. It’s been expanding and cooling ever since, up through and including the present day. From our point-of-view, we can look back some 46 billion light years in all directions, thanks to the speed of light and the expansion of space. Although that’s a huge distance, it’s not infinitely large. But that’s merely what we can see. What lies beyond that, and could that be infinite? That’s what Buck wants to know, as he asks:
What I’d like to see discussed whether the universe is finite or infinite, and why it might be either. I’ve seen some limited discussion by [Sean Carroll] and [Lisa] Randall to the effect it could be either. We just don’t know.
It’s true that we don’t know whether it’s finite or infinite, but we know a lot more than what we see within the part that’s observable to us.
As we look to greater distances, we also wind up looking back in time. The nearest galaxy, some 2.5 million light years away, appears to us as it was 2.5 million years ago, because the light requires that much time to journey to our eyes from when it was emitted. More distant galaxies appear as they were tens of millions, hundreds of millions or even billions of years ago. As we look ever farther away in space, the light we see from the Universe comes from its progressively younger days. So why not go all the way back to the beginning: to the light that was emitted 13.8 billion years ago? We’ve not only looked, but we’ve found it: the cosmic microwave background, which is the leftover glow from the Big Bang.
What we find is that the Universe was almost perfectly uniform back then, but some regions were more or less dense than average, by only 1-part-in-30,000. That’s enough to grow into the stars, galaxies, galaxy clusters, and cosmic voids we see today. But these early imperfections that we see from this cosmic snapshot encodes an incredible amount of information about the Universe. One such piece of info is a startling fact: the curvature of space, as best as we can tell, is completely flat. If space were positively curved, like we lived on the surface of a 4D sphere, distant light rays would converge. If space were negatively curved, like the surface of a 4D saddle, distant light rays would diverge. Instead, distant light rays move in their original direction, with the fluctuations we have indicating perfect flatness.
From constraints arising from both the cosmic microwave background and the large-scale structure of the Universe combined, we can conclude that if the Universe is finite and loops back in on itself, it needs to be at least 250 times the extent of the part we observe. Because we live in three dimensions, 250 times the radius means (250)3 times the volume, or more than 15 million times as much space. But, big as that is, it still isn’t infinite. A lower bound of the Universe being at least 11 trillion light years in all directions is tremendous, but it’s still finite.
There’s reason to believe our Universe is even bigger than that, though. The hot Big Bang might mark the beginning of the observable Universe as we know it, but it doesn’t mark the birth of space and time itself. Before the Big Bang, the Universe underwent a period of cosmic inflation. Instead of being filled with matter and radiation, and instead of being hot, the Universe was:
- filled with energy inherent to space itself,
- expanding at a constant, exponential rate,
- and creating new space so quickly that the smallest physical length scale, the Planck length, would be stretched to the size of the presently observable Universe every 10–32 seconds.
It’s true that in our region of the Universe, inflation came to an end. But there are three questions we don’t know the answer to that have a tremendous influence on how big the Universe truly is, and whether it’s infinite or not.
1.) How big was the region of the Universe, post-inflation, that created our hot Big Bang? Looking at our Universe today, at how uniform the Big Bang’s leftover glow is, at how flat the Universe is, at the fluctuations stretched across the Universe on all scales, etc., there’s quite a bit we can learn. We can learn the upper limit to the energy scale at which inflation occurred; we can learn how much the Universe must have inflated; we can learn a lower limit how long inflation must have gone on for.
But the pocket of the inflating Universe that gave rise to us could be much, much bigger than that lower limit! It could be hundreds, or millions, or googols of times larger than what we can observe… or even truly infinite. But without being able to observe more of the Universe than we can presently access, we don’t have enough information to decide.
2.) Is the idea of “eternal inflation” correct? If you consider that inflation must be a quantum field, then at any given point during that phase of exponential expansion, there’s a probability that inflation will end, resulting in a Big Bang, and a probability that inflation will continue, creating more and more space. These are calculations we know how to do (given certain assumptions), and they lead to an inevitable conclusion: if you want enough inflation to occur to produce the Universe we see, then inflation will always create more space that continues to inflate compared to the regions that end and produce Big Bangs.
While our observable Universe may have come about from inflation ending in our region of space some 13.8 billion years ago, there are regions where inflation continues — creating more and more space and giving rise to more Big Bangs — continuing to the present day. This idea is known as eternal inflation, and is generally accepted by the theoretical physics community. How big, then, is the entire unobservable Universe by now?
3.) And, finally, how long did inflation go on prior to its end and the resultant hot Big Bang?We can only see the observable Universe created by inflation’s end and our hot Big Bang. We know that inflation must have occurred for at least some ~10–32seconds or so, but it likely went on for longer. But how much longer? For seconds? Years? Billions of years? Or even an arbitrary, infinite amount of time? Has the Universe always been inflating? Did inflation have a beginning? Did it arise from a previous state that was around eternally? Or, perhaps, did all of space and time emerge from nothingness a finite amount of time ago? These are all possibilities, and yet the answer is untestable and elusive at present.
From our best observations, we know that the Universe is an awful lot bigger than the part we can observe. Beyond what we can see, we strongly suspect that there’s plenty more Universe out there just like ours, with the same laws of physics, the same types of physical, cosmic structures, and the same chances at complex life. There should also be a finite size and scale to the “bubble” in which inflation ended, and an exponentially huge number of such bubbles contained within the larger, inflating spacetime. But as inconceivably large as that entire Universe — or Multiverse, if you prefer — may be, it might not be infinite. In fact, unless inflation went on for a truly infinite amount of time, or the Universe was born infinitely large, the Universe ought to be finite in extent.
The biggest problem of all, though? It’s that we don’t have enough information to definitively answer the question. We only know how to access the information available inside our observable Universe: those 46 billion light years in all directions. The answer to the biggest of all questions, of whether the Universe is finite or infinite, might be encoded in the Universe itself, but we can’t access enough of it to know. Until we either figure it out, or come up with a clever scheme to expand what we know physics is capable of, all we’ll have are the possibilities.