Accelerating Expanding Universe

[Old_Geezer]

I just know this will have an obvious explanation and I'll be here with egg all over my face, but there's something I've been pondering for a while, and not thought of the explanation yet, so maybe someone here knows.

We accept that the universe seems to be expanding at an ever increasing rate, and explanations such as dark power (whatever that is) have been suggested to explain it. But surely we should expect this accelerating expansion anyway.

As I understand it, the principle cause (or at least one cause) of expansion is that between any two distant points new space is being created, so the measured separation distance between those points increases regardless of any "real" movement. Have a long enough reference distance and the ends can rush from each other faster than light speed.

So consider, for a distance D, over a corresponding time period P, D is found to have doubled. The new situation is that there is now twice the distance/length creating new space along it at correspondingly twice the rate it did at the start. Monitor for a second period P and the distance has doubled again. And so on.

That isn't linear, that's rushing faster and faster apart. So why doesn't this account for the accelerating expansion phenomenon without resorting to further explanation ? (Bet it's got something to do with time frames and some reason why the situation isn't valid.)

Just asking.

[jim360]

Probably the simplest reason this doesn't quite work is because gravity, which tends to pull space back in on itself. If there's enough matter in the Universe then the effect of gravity could be so strong as to overcome the expansion altogether, and bring things back in on themselves in a "Big Crunch" (this is the actual term if you want to read up on it further). Alternatively there could be not enough matter, or just enough to freeze the size of the Universe in place (these three opens are the "closed", "open" and "flat" universes).

The closest to your picture is the "open" universe, but in this case I think a second reason goes against you, as the way the expansion rate is defined is such that what you are describing would be a "constant" rate in time. Sure, space gets bigger and bigger, but the rate at which it's getting bigger is fixed. What's observed instead is that space is getting bigger and bigger but at an ever increasing rate (so that, in your version, if in time period P the distance D doubled, then in the second time period P the distance might triple or some such).

I'm not sure this is a perfect explanation but I hope it covers the gist of it correctly.

[Old_Geezer]

Thanks. I shall have to give it some thought as I was thinking that there was a set amount of matter, and presently it isn't enough to stop expansion, it's clearly still occurring. Also creation of space increasing distances, as opposed to "real" movement, would continue anyway, and as the expanding universe reaches a particular size, due to the increased distance between matter, gravity is then too weak to reverse the expansion.

Acceleration due to the process I was describing would be constant, but it would still be acceleration. So space would be getting bigger at an increasing rate, by definition. If you are saying that the rate of acceleration is increasing, then fair enough, there's something else going on also.

[jim360]

One problem I have is that it's easily possible to tie yourself up in knots about what you are even measuring when you talk about expansion of the universe. The way Relativity works is that you have to ensure that you are measuring something in the right frame of reference in order for it to make sense, and it's not always clear, to me at least, what that frame is. For the purposes of my day-to-day work gravity isn't even a thing, so I can easily get lost and make mistakes in explaining it when I'm going off half-remembered snippets from lectures years ago. In fact this is the second version of a post, the first version of which I checked and it turned out to be (completely) wrong.

Anyway, I think the key point is that the measurement you describe is an exponential growth rate, and it would not be observed (as far as I know) without something special driving it. If something gave the universe a kick at the start and then gave up (as in, say, a Big Bang + inflation) then the growth rate over a time period P would be, say, a distance D increasing by the same distance D, but then over the second time period the distance would increase by D again, ie a linear growth rate rather than constant doubling.

I don't think you can get exponential growth, as I say, without something special "added on" to otherwise normal physics, such as Inflation (in the early Universe) or Dark Energy or something similar (today).

[Old_Geezer]

That tends to be why I asked. I made an assumption or two; that space is being created (however that occurs) fairly evenly along any long straight line, and at a constant rate. But that new space adds to the length that line (a bit like compound interest adds to a capital balance) so it's creation drives the increased rate of creation in the future. Faster & faster, more & more. (A longer line, so more new space per unit of time. (Sorry about mixing 2D and 3D terms but you know what I mean.))

I'm now wondering what drives the creation of new space. Maybe that's key.

[Zacs-Master]

Dark radiation is thought to invisibly carry energy around the cosmos, altering the expansion. Trouble is cosmology is mind-bogglingly complex and not a suitable subject to get ones head around, unless you're deep, deep into the field as a scientist.

[Peter Pedant]

let me stick my two penny worth in

you arent gonna get an exponential from a power law
( so for exponential expansion you are gonna have to re write a few laws of physics - well why not it has been done before )

BUT - I thought the universe was too large for the time allowed for its existence and expansion at the speed of light or near it
so for a few billionths of a second after the big bang there was a supra normal expansion period

[ could be crap - this is not my day job ]

[Peter Pedant]

jesus Jim the punters on AB do know you are theoretical physicist 9-5 ( and some week ends ) dont they ?

[

[Zacs-Master]

cosmologist Sean Carroll postulates that being human means we always look for explanations from an anthropic perspective, which hinders us and makes our observations too bias. Interesting point.
http://io9.gizmodo.com/why-is-the-scale-of-the-universe-so-freakishly-large-1719048488

[Zacs-Master]

PP, Jim has already qualified his input as limited:
'For the purposes of my day-to-day work gravity isn't even a thing, so I can easily get lost and make mistakes in explaining it when I'm going off half-remembered snippets from lectures years ago'

[Peter Pedant]

if Jim says 1+1 = 2
then I will take it on trust from him

[Zacs-Master]

Jim would only say 1+1=2 if he were sure. In this instance he's saying he doesn't have enough expertise to come to a conclusion. It would seem that the field of science generally is in a similar position, despite all the expertise, demonstrating that this is complex on an almost unimaginable level. One could almost start believing someone / something was controlling it all ;-)

[jim360]

Well on this one I'd like to think that the limits of my knowledge are rather a lot less than the limits of science as a whole, at least!

[Zacs-Master]

I'm certain they are Jim. But, as I said, science as a whole ain't exactly getting A* in the subject at the moment and perhaps the mere fact of being human won't ever let us.

[Khandro]

But as we gaze and ponder the universe, what makes you think it is understandable to the 3 pounds of grey matter developed on the African Savannah to understand all this? There are 'quantum physicists, marvelling at the strange fact that quantum systems don’t seem to be definite objects localized in space until we come along to observe them — whether we are conscious humans or inanimate measuring devices. Experiment after experiment has shown — defying common sense — that if we assume that the particles that make up ordinary objects have an objective, observer-independent existence, we get the wrong answers. The central lesson of quantum physics is clear: There are no public objects sitting out there in some pre-existing space. As the physicist John Wheeler put it, “Useful as it is under ordinary circumstances to say that the world exists ‘out there’ independent of us, that view can no longer be upheld.”

Which is what Buddhist philosophy has been saying for over 2,000 years!

[jim360]

I'm sure that's supposed to be meaningful but it really isn't.

[Zacs-Master]

It is to Buddhists ;-)

[Old_Geezer]

When you get near my size then the objects emerge.
If I don't try to get my 3 pounds of grey matter to understand it I can't expect others to try, and then everything is confusion, forever.

[Old_Geezer]

As the new space is created, so does new moments of time. It's all spacetime isn't it ?. I can't envisage how, but that might throw a spanner into the works.

[Khandro]

//I'm sure that's supposed to be meaningful but it really isn't.//

Wise man say.

[jim360]

Wisdom doesn't come from quotes. You can throw all the quotes from famous physicists you want at me, but unless you actually understand the science -- and, so far, evidence suggests that you don't, really, eg your empty accusations and misplaced cynicism about CERN -- then such quotes carry no weight. You're just taking them out of context, or misunderstanding them, or misusing then, or some combination of all these three and a few other errors I can't think of right now.