Assuming the universe is infinitely large now (big assumption, I realise), it must always have been infinitely large. Otherwise there would have been a moment when it transitioned from being of finite extent to being of infinite extent, which is not possible.
Is it the case that the expansion of the universe since the Big Bang is not a case of the universe getting wider, but getting less dense?
My understanding is it’s possible that there are multiple branes that compose the universe, and that a collision between two of them is what set off the Big Bang, I suppose these branes could be infinite in extent.
Another possibility, which I think move cosmologists think is true is that the universe was infinitely small at the time of the big bang and then inflated to a large size and has continued to expand.
Correct on all counts. Assuming that it’s infinite now (which isn’t actually known, but is at the very least a reasonable assumption), it was always infinite, at any time past t=0. And the main manifestation of the expansion is that it’s getting less dense.
There is (according to my understanding) no such time as “after the big bang”. Any dividing line between “this is the big bang happening, right now” and “okay we are now in the post-big-bang universe” is utterly arbitrary. I suppose you could define one and get some nods — “when the plasma cooled down enough to have starlike objects”, for example — but it isn’t really intrinsic that it’s now a different event or sequence of events, it’s all a continuum.
It’s not analogous to your birth (or mine), where we all agree that when you’re outside of the birth canal you’re no longer being born, you have been born, so a birth has a specified ending, a crossover point. (It helps that birth happens to everyone, human and other-species alike, whereas with the origins of the universe there’s just one occurrence so who is to say that this or that point represents The End of the Big Bang?)
If any of you dissent with that, feel free to enlighten me and explain to me what the celestial crossover point is/was.
Didn’t cosmic inflation take some time? Granted, not much (10^-32 seconds) but still…
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Did cosmic inflation come to a halt? Obviously it was taking place at a jaw-dropping rate in the first few microattowhateverseconds but was it a phenomenon that came to an end or is the ongoing expansion of the universe a continuation of that?
Well yes, me too, but that doesn’t negate the possibility that the Big Bang is still taking place now.
As I understand it, the term “cosmic inflation” as used in astrophysics does not refer to the ongoing expansion of the universe but specifically to the very brief (around 10e-30 seconds) but very rapid (from a diameter the size of subatomic particles to a diameter of a few inches) expansion very early on. So even if the universe continues to expand, cosmic inflation is over.
There is some rearguard renaming suggestion that the big bang was part of inflation, and that there was a pre-inflationary period that now, as a matter of nomenclature, preceded this. Given the big-bang is an awful name (no thanks to Fred Hoyle on this one) I wouldn’t get too hung up on the desire to partition things into such stages. Even inflation is a postulate with little more going for than that it is a reasonably economical way of explaining why the universe is so flat, and nobody has anything better. But it is something that really does need explaining.
Use of infinity is at best a very wobbly idea when it comes to assertions about the universe, or anything physical. It might help some philosophical discourse, at least as a handle to assign to things we know we have no clue about currently, and things we may never know. Size as a meaningful concept may not exist at some stages of the universe, or indeed the concept of time. Or not work the way they do now.
There is no shortage of navel gazing about this, but I very much doubt we are close to an answer. One also doubts that an answer may ever be fully within our grasp.
Whatever drove inflation did come to a halt. This is the usual diagrammatic way of displaying the phases. Note that the time scale is highly non-linear. It isn’t even logarithmic. The entire jump from ab-initio to the end of inflation took an unimaginably tiny amount of time. 10^{-32}s.
It’s a bit more than that though AIUI, as the inflation model has made predictions about the CMB and gravitational waves that were subsequently confirmed. It’s way, way short of the level of proof of something like the big bang, but it’s also not just something accepted only as some kind of placeholder.
As a slight nitpick: inflation has halted for this bubble of spacetime that we inhabit. We cannot yet make more general claims about it.
(further nitpick of my nitpick incoming…
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I am currently reading Thomas Hertog’s On the Origin of Time, Stephen Hawking’s Final Theory, so I think I can clearly state that I don’t understand any of this. I mean I’m halfway through the book and I can barely grasp the concept of there being a beginning of time in the first place.
It seems there are some reasons why it should be impossible, but it is easy to find articles in theoretical physics in which just that happens
so it needs to be considered before being ruled out, not dismissed offhand.
“The Big Bang” can refer to the singular moment t=0 (“Inflation ended 10^-32 seconds after the Big Bang”), or it can refer to a very early period in the Universe’s history with a vaguely-defined ending point (“Hydrogen, deuterium, and helium-3 atoms were formed during the Big Bang”), or it can refer to the entire, ongoing, expansion of the Universe, which is still continuing. In the first two senses, it’s certainly possible to talk about “after the Big Bang”.
Although it’s also worth pointing out that there might not actually have been a t=0: It’s increasingly considered plausible that the inflationary era was infinitely long (a model called “eternal inflation”). The inflationary state of the Universe is unstable, and was bound to eventually collapse into something like what we have now, but meanwhile, it also expands at a phenomenally rapid rate. So it’s possible that there’s always inflation going on, and every so often (for extremely quick values of “every so often”), a bubble of non-inflationary space forms and starts expanding, but meanwhile new inflationary space is being created so rapidly that most of spacetime is still inflationary.
Only one of those three definitions is common (the moment of instability that preceded inflation), and the other very common definition is missing here (i.e., the moment at the end of inflation when the phase transition to the “true vacuum” happens, leading to the dumping of potential energy to form the dense hot plasma with the desired initial conditions for everything that comes after.)
In either case, the broader term “Big Bang cosmology” or similar refers to the whole picture which includes things like the era of nucleosynthesis and the continued expansion. But the Big Bang is almost always one of the above two definitions. Which one you find depends on who is speaking, with inflationary theorists (largely interested in the pre-inflation physics) using the first and observational cosmologists (largely interested in the post-inflation evolution) using the second.
(Terminology is still evolving. Given the inconsistent usage and the fact that one of the definitions isn’t even always applicable in a given inflationary model, some people have started avoiding the term while others have started using the phrase “hot Big Bang” to indicate that they mean the second definition.)
It is a weird concept to wrap your head around but if you really want to bake your noodle consider the alternative…infinite time (no beginning and no end).
In fact, the notion of there being a beginning of time dates all the way back to St. Augustine. One question that philosophers were bandying about was “What did God do in all of the infinite time before creating the world?”, and Augustine’s answer was “There wasn’t any time before creating the world, because time is part of the world and was itself created by God”.
I suppose that does make littler sense, when you think about it.
Actually, the book is pretty absorbing even if you’re not a hardcore physicist and I do grasp some of the higher level theories to a degree. It’s a pretty mindbending book … and concept.
So complete layman here, but would it be wrong to say “Since there was no instant where a particle in the universe could have traversed from one ‘side’ of the universe to the other in anything less than infinite time” that the universe has always been effectively infinite?
Recognizing that size in the above is vague and undefined, but I think you get the gist.
Okay, I could be wrong (see above) but as I understand at the instant the Big Bang happened (the beginning of space-time) there were no particles to travel from one side to the other, so that definition is kind of moot.