**The rate of expansion is accelerating…[b/]
erm, why? does anyone know?
I read somewhere that we know it’s accelerating because when we look at distant objects, they are moving faster than close objects…*
Surely this would mean that it’s slowing wouldn’t it? - for distant objects, we’d be seeing the light from an earlier time than close ones, so earlier in time it was moving fast, now it’s slower. Eh?
*(Or did I read an article that had this back to front?)
The last I hear the Rate of Change of the universe was slowing … .
The Big bang propelled the universe outward, but the acceleration was only for a very short time, But powerful so the universe still continues to move outwards.
Gravity is currently the only active force, towards the centre of the universe (as the explosion has finished) so the rate of expansion of the Universe is slowing.
The are three possible conclusions to this.
If the gravitational force is large enough the universe will eventually slow, stop and then begin to contract.
If the Gravitational force is just right the universe will slow and then all the forces will balance and the universe will become static.
Or the force is not large enough and the Universe will continue to expand.
Or the force is not large enough and the Universe will continue to expand.
hmmm. somthing doesn’t seem right here but after a few pints at lunch time I can put my finger on it
I’m sure someone (or many people) will correct me If I’m wrong.
The ‘accelerating’ phase of the universe is called Inflationary Theory. The idea is if, when the universe formed at the Big Bang, we should have had a smooth, homogenous universe and not the lumpy one we see with galaxies here and there (they would never have formed). Inflationary Theory, where the Universe went through a rapid expansion before starting to slow down, accounts for the lumpiness we see. Why this is so I don’t know…I’m just taking the cosmologists word for it but they seem (mostly) satisfied with this explanation.
Frankly, to me it sounds like cosmologists fudging to make the observations fit the theory since it is counterintuitive. However, the last I heard (just recently) is that there is ever more evidence to support this idea. I’m not qualified to second guess these people so I’ll go with it too (that and the fact that quantum theory can be seriously counterintuitive yet seems to work so I’m getting used to it).
All of that said the last I checked cosmologists believed our universe was open. That is to say, there won’t be a Big Crunch when the universe reverses its expansion and shrinks back to zero size. Instead, the universes expansion will forever slow down but never quite reach zero expansion.
I don’t know why but I’m sad about there not being a Big Crunch. It just seems like a nice closure to the universe with the possibility of another Big Bang following the Big Crunch. Instead we are left with the heat death of the universe. We get a whimper instead of a bang. The universe will ever so slowly cool off till there is nothing but an occasional particle aimlessly flitting about all alone.
What’d I’d like to know (since this is in the same vein as the OP) is that if the Big Bang was this huge explosion that sent the entire universe outward at a certain acceleration for an extremely short amount of time (and is continuing to move outward because of the massive power of that initial explosion), and if we on Earth look toward the center of the universe we can see the universe as it was in the past, then ostensibly on the other side of this center, the universe is also expanding outward, right? So if we somehow developed the means to look all the way past the center of the universe, we’d eventually see younger and younger galaxies, right?
First things first: There is no one center to the expansion. Every point is expanding away from every other point. When you look at something far away, in any direction, you’re seeing it as it was long ago, when it was younger.
Second things second: In the past few years, it has come to light that the Universe is, in fact, speeding up in its expansion, not slowing down like everyone thought. This means that, contrary to the belief a few years ago, gravity is not the dominant force at work in the Universe on a large scale. The evidence for this is currently very strong, and getting stronger every day. Why is this happening? Well, we’ve got a few names for the phenomenon (dark energy, cosmological constant, quintessence), but that’s not really an explanation. There’s a lot of research being done, though, and a lot of very smart people trying their durndest. On the bright side, this is a very good time for a budding cosmologist to be in grad school and thinking about a doctoral thesis…
Regarding the expansion of the universe: Is it thought now that the universe’s expansion has been speeding up for a long time now (say, millions of years), or do scientists believe it’s a more-recent occurrence? If so, what do scientists speculate could be the cause of the change?
And finally, if the expansion is accelerating, do scientists now believe it will someday stop expanding all together? Is this what the so-called “Big Crunch” is? (I know that’s supposed to be a long, long ways off, though.)
And if the universe will experience a Big Crunch someday, will it happen after our sun dies out? (I’m guessing way, waaaaaaay after…)
We can’t look toward ‘the centre of the universe’. The structure of the universe is similar to the structure of a balloon, only with one more dimension. Imagine you’re a two-dimensional being, and your universe is an inflating balloon. You live inside the skin of the balloon, and the only directions you can look are inside the skin because you’re two-dimensional. You can’t look toward the (three-dimensional) ‘centre of the balloon’ because, even if you can conceive of a third spatial dimension, you can’t build a device that lets you observe into it.
Now at some time in the past, someone started inflating the balloon. I’m a mathematician, so I’m going to assume the balloon started out as a single point, exactly six thousand years ago. Say you look out along the skin of the balloon. It’s a very big balloon, and the guy inflating it makes Pavarotti look like he has asthma. If you look out for ten light-years, you will be seeing the balloon as it was ten years ago. Right? And the farther out you look, the smaller the balloon was when whatever-it-was-that-you’re-seeing emitted the light that’s just hit your eyes. Because of when the balloon started, there is nothing that has ever existed, that could have emitted light from six-thousand-one light years away so that its light could be hitting your eyes right now. It doesn’t matter how big across your universe is—from your vantage point, and everyone else’s for that matter, it looks like the universe is a disc six thousand light years across. And moreover, once your two-dimensional theoretical physicists factor in all the relativistic effects, it looks like the skin gets ‘thicker’ the farther away you go, because the stuff you’re seeing now is in fact from when the balloon was less inflated, and therefore less stretched.
Well, except for that very last bit, it’s a pretty good model for our universe. [sub]I don’t know if the universe looks exactly like a four-dimensional balloon, all I’m saying is that it could.[/sub] We can’t look ‘toward the centre of the universe’, because we can only look along the ‘skin’, and the centre is outside our field of vision. But we can see parts of the universe the way they looked in the past, simply because light takes time to travel from there to here. For that matter, we can’t see anything in ‘the universe as it is this instant’, for the same reason. So ironically, to see the universe as it looked in the past, we don’t look toward its centre, or even directly away from its centre; we look across that axis, because it’s the only direction we can look, and the farther away we look, the younger the things we see.
Whew! A lot to handle. I think I’m grasping some of it, though. Thanks, MrDeath (boy, how often do you get to say that?).
So we cannot look at the center of the universe because such a place no longer exists (essentially every single point in the universe is the center).
If we can look at younger and younger parts of the universe according to the light that reaches us from those parts of the universe, is it conceivable that we would be able to examine the universe as it was when it was created? That’s really a rhetorical question, I guess, since in theory we can do this - but not yet in practice, right?
Your balloon analogy is most apt, because it keeps me from thinking thusly: that if we look from point A toward the center C, we see younger galaxies; if we look from point A away from center C, we should then see older galaxies, or the future of our own. But your analogy neatly defeats this, and I’m glad you mentioned it.
Whew! A lot to handle. I think I’m grasping some of it, though. Thanks, MrDeath (boy, how often do you get to say that?).
So we cannot look at the center of the universe because such a place no longer exists (essentially every single point in the universe is the center).
If we can look at younger and younger parts of the universe according to the light that reaches us from those parts of the universe, is it conceivable that we would be able to examine the universe as it was when it was created? That’s really a rhetorical question, I guess, since in theory we can do this - but not yet in practice, right?
Your balloon analogy is most apt, because it keeps me from thinking thusly: that if we look from point A toward the center C, we see younger galaxies; if we look from point A away from center C, we should then see older galaxies, or the future of our own. But your analogy neatly defeats this, and I’m glad you mentioned it.
I think I can answer a couple questions here, while doing more good than harm.
Quoth dantheman: “And if the universe will experience a Big Crunch someday, will it happen after our sun dies out?”
Yes, I think. Though I’ve never heard it stated explicitly, I think the Universe would collapse back at no more than the speed at which it expanded. So, even if it turned around now, it would take more than 12 Billion years to Crunch, and that’s an absolute minimum; it could easily be more like 1,000,000 Billion years. The Sun only has about 5 Billion years to go. But, as has been mentioned, a Big Crunch does not appear likely.
“If we can look at younger and younger parts of the universe according to the light that reaches us from those parts of the universe, is it conceivable that we would be able to examine the universe as it was when it was created?”
Again, I believe that the farthest back we can hope to see is 300,000 years after the Big Bang. At that point, all the light in the Universe got scattered, so you can’t directly observe anything before it. But up to that point, yeah, I think we can see as far back as we want, in principle.
It seems to be very recent indeed, only in the past billion years or two. We can’t pin it down very precisely yet, though. Millions? Ha! The Universe laughs at millions of years.
We have no idea what caused the current inflation to “turn on”, or if it’ll ever “turn off”. If it never turns off, then the Universe will certainly never re-contract and go through a Big Crunch. If it does turn off, it still looks like the density of the Universe is just barely exactly low enough that it’ll never quite stop expanding, but there’s room for some error here. If, on the other hand, the mysterious force causing the acceleration reverses (no reason to suppose that it couldn’t), then the Universe might be able to collapse anyway, regardless of density.
I agree, by the way, that “This is the way the world ends: not in a bang, but a whimper” is rather a depressing thought, and I would consider a bang-crunch universe to be much more aesthetic, but unfortunately, the Universe doesn’t care what you or I may think.
By “speed” you mean “rate”, right? I’m not sure if the terms are interchangeable or not. (I’m not sure of a lot of things.) It seems to me there are two actions at work here: the Big Bang itself, which took a teeny weeny bit of time, and this ongoing expansion, which has gone on for, oh, a long time. Right? So if the universe expanded at Q speed, then it’ll collapse at Q speed. Which means, of course, that we wouldn’t even know it (if we’re still alive and kicking), at least not at the time it changes from expansion to contraction. … Right?
Now, at the point at which the light in the universe was scattered (thereby making it impossible for us to observe it)… Do we need light in order for something to be observed? Then again, I guess there’s the matter of what was there to be observed, for if we’re talking about observing the light itself, I can see how that might well be impossible, what with the scattering and all.
Sure doesn’t. And you know, even with all the physics in the world, the Universe could suddenly say, “Thank you all for your patience through our Massive Expansion. Things have worked out right smartly for us all, but owing to a dip in the Universe’s stock, we shall now go through a Massive Contraction over the next several billion years. If you are an employee of Universe, Inc. (NYSE: UNIVS), please be aware we shall lay off no more than 1 quintillion of you. Thank you, and good day.”
Quoth dantheman: “By ‘speed’ you mean ‘rate’, right?”
Excellent question. If you look at anything far enough away, then it will be receding from us at a certain speed, something which you can tell by its redshift. This speed is proportional to the expansion rate of the Universe, so the two are related like that. If we watched one particular object over time, we’d see that its speed is increasing, so we would know that the Universe is accelerating. (Nota Bene: We’d have to do this for a really long time to notice any change - in practice, this isn’t how it’s done.) Now, if the Universe were to start contracting, we would know it (after a few million years, or however long it takes the light to reach us) because this thing would be moving back toward us - its redshift would turn into a blueshift.
That whole wishing the Universe was a certain way thing always bothers me. Relying on aesthetics was a big part of why Cosmology was held back for centuries.
Quoth dantheman: “By ‘speed’ you mean ‘rate’, right?”
Excellent question. If you look at anything far enough away, then it will be receding from us at a certain speed, something which you can tell by its redshift. This speed is proportional to the expansion rate of the Universe, so the two are related like that. If we watched one particular object over time, we’d see that its speed is increasing, so we would know that the Universe is accelerating. (Nota Bene: We’d have to do this for a really long time to notice any change - in practice, this isn’t how it’s done.) Now, if the Universe were to start contracting, we would know it (after a few million years, or however long it takes the light to reach us) because this thing would be moving back toward us - its redshift would turn into a blueshift.
That whole wishing the Universe was a certain way thing always bothers me. Relying on aesthetics was a big part of why Cosmology was held back for centuries.
Just for reference thinking of the big bang as an explosion is going to cause you headaches. I tend to use a proablistic distribution model, where by the bounds on the position of a particle are expanding. According to this model random variations in the rate of exansion are perfectly acceptable, but the maths gets bad very fast. personnelly I like the attempts looking at tying the rate of increase to the concentration of matter at the edge of the current the system.
I remember attending a lecture on this way back in … 95 I think. I first instinct was to guess that something outside our view was pulling it but they all seemed pretty adament about it being something from within the visible universe.
Every force but gravity has both a positive and negative forces, so a lot of people are theorizing that this is the negative force of gravity, which only becomes apparant when the scales get truely grand. Maybe but I’m skeptical. I still like my something pulling from the outside better. A lot of people will probably jump on me for saying stuff is outside but clearly there is. If we were to have some sort of transport that could take us 10 billion light years in one direction then at that point which we reached we would still see a spherical event horizon and lots of galaxies not visible from where we were because ht elight hasn’t had enough time to reach us yet.
I believe this is possible because if we take that balloon idea and blow it up faster then light travels (those are some lungs huh?) and if you’re standing on a particular point on the ballon you would not be able to see all of the balloon, even though originally it was all in the same point.
Although the problem with my theory is that it contradicts the … that theory about no part of the universe being special, and for something outside to succeed in pulling the middle parts toward it instead of us, it would have to be heavier then us.
So if the Universe is expanding at speed n, then an object fixed in space (having its own orbit) will recede from us at a speed 1/n?
How long would it take for an object to change from redshift to blueshift? Is it a discernable change, or something we’d have to approximate? As in, we’d have to look at an object, see it’s now blueshifting, then calculate when the change took place - or could we predict when it will blueshift? (My guess is the former.)
[Thanks, all you scientific-types… Hope this isn’t too much slumming on your part to dumb this down for us yokels…]
I hope so, because that’s the year we make contact. We need to know our shit before that happens!
BTW, when you say such an object would have to be heavier than us, do you mean it would need to have more mass or that it would need a stronger gravity field? Or do the two notions go together anyway?
