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Old 8th October 2017, 09:28 AM   #1
HansMustermann
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'elp a poor bloke understand universe expansion, guv'nor

Well, before I start, I understand Hubble's law and the data it's based on. You look at a type Ia supernova, the farther away it is, the more red-shifted it is. So the universe is not only expanding, it's accelerating.

I get that, no need to link me to Hubble's Law on wikipedia.

But here's what I don't get:

Actually the farther away something is, the older the image we're getting. So if you're looking at a galaxy 1 billion light years away, you're looking at it 1 billion years ago.

So far that should not be controversial. In fact, all astronomers know it. They even say it every time they discover some galaxy from billions of years ago.

Here's where I go off my meds though: doesn't it mean that you could look at Hubble's law as a function of TIME and red-shift instead of SPACE and red-shift? So basically, the farther back in time we observe something, the faster it was going, and the closer to the present we look, the slower things move.

That looks to me more like the image of a universe that exploded at near the speed of light, way back in ye olde Big Bang, and it's been slowing (e.g., due to gravity) ever since.

Which is where I stop and think I might have gotten something about it wrong or I'm missing something. Any smarter folks can explain to me how the conclusion that it's accelerating takes it all into account?
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Old 8th October 2017, 10:04 AM   #2
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Sounds right to me - and logical. On the other hand, I am not an astrophysicist!!!
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Old 8th October 2017, 10:25 AM   #3
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Well, neither am I, so here's to hope that one will show up and tell us what's what.
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Old 8th October 2017, 10:39 AM   #4
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Originally Posted by HansMustermann View Post
Well, before I start, I understand Hubble's law and the data it's based on. You look at a type Ia supernova, the farther away it is, the more red-shifted it is. So the universe is not only expanding, it's accelerating.

I get that, no need to link me to Hubble's Law on wikipedia.

But here's what I don't get:

Actually the farther away something is, the older the image we're getting. So if you're looking at a galaxy 1 billion light years away, you're looking at it 1 billion years ago.

So far that should not be controversial. In fact, all astronomers know it. They even say it every time they discover some galaxy from billions of years ago.

Here's where I go off my meds though: doesn't it mean that you could look at Hubble's law as a function of TIME and red-shift instead of SPACE and red-shift? So basically, the farther back in time we observe something, the faster it was going, and the closer to the present we look, the slower things move.

That looks to me more like the image of a universe that exploded at near the speed of light, way back in ye olde Big Bang, and it's been slowing (e.g., due to gravity) ever since.

Which is where I stop and think I might have gotten something about it wrong or I'm missing something. Any smarter folks can explain to me how the conclusion that it's accelerating takes it all into account?
Vaguely related: does the arrow of time have anything to do with the preponderance of matter over antimatter? Isn't anti-matter sometimes referred to as time-reversed matter?
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Old 8th October 2017, 04:14 PM   #5
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Originally Posted by HansMustermann View Post
I understand Hubble's law and the data it's based on. You look at a type Ia supernova, the farther away it is, the more red-shifted it is. So the universe is not only expanding, it's accelerating.
That's the first misstep in your post, in the order in which you wrote it. But I think it's caused by another misstep which you brought up later in the post even though it seems to be earlier in the chain of logic.

Originally Posted by HansMustermann View Post
That looks to me more like the image of a universe that exploded at near the speed of light, way back in ye olde Big Bang, and it's been slowing (e.g., due to gravity) ever since.
Objects move at speeds of the kind you're thinking of: distance per time. The expansion of space is a different kind of phenomenon, with a different kind of unit. It needs to be described either with two distance units, as in "millimeters added per kilometer per year", or with essentially no simple distances but a percentage or proportionality of distance replacing both, as in "percent/ratio of increase (of distance) per year". (The Hubble Constant is in kilometers per megaparsec per second; in other words, it's how many kilometers longer each megaparsec becomes per second.)

Thus, an expansion rate can't be generally faster or slower than any particular distance-per-time speed. Apparent DPT speed with expansion (the DPT speed at which an object would appear to be moving away from us due to coordinate expansion alone) increases proportionally with distance for a constant expansion rate. In fact, for any given expansion rate and any given DPT speed (including c), there is some distance at which the two will appear to be equal; beyond that distance, the expansion rate appears greater, and within that distance DPT speed appears greater.

To use a fictional example with simple numbers, a 1% expansion rate per whatever time unit means something 10 miles away becomes 10.1 miles away, and something 500 miles away becomes 505 miles away. If you think only in miles, you'd think the farther one is "moving faster" because it went 5 miles in the time that the closer object only went 0.1 mile(s). But the expansion didn't work in miles, and if you think in percentages, you see that the two objects' apparent speeds are exactly proportional with their distances, giving us not two separate "speeds" but one rate of coordinate growth, which can be expressed with no distance units.

Thus, faster recession velocities at longer distances are what would be expected of a constant expansion of the coordinate system, not just an accelerating (or decelerating) one: not the behavior of separate bits & pieces flying apart from a conventional "explosion", but the result of the empty space between objects growing, at a constant rate.

So now, back to the future...
Originally Posted by HansMustermann View Post
I understand Hubble's law and the data it's based on. You look at a type Ia supernova, the farther away it is, the more red-shifted it is. So the universe is not only expanding, it's accelerating.
The acceleration is not what Hubble discovered and is not what Hubble's Law describes. Looking for expansion or contraction anywhere in this is getting a bit ahead of ourselves at this point (and ahead of Hubble).

What Hubble discovered and his law describes is that the recession velocities appeared at least roughly proportional with distance, consistent with a roughly uniform growth rate of the space between here and those other galaxies. The increase in apparent velocity with distance did not, by itself, indicate acceleration.

The acceleration is hiding in that word "roughly" in the previous paragraph. More precise measurements later yielded small deviations from the redshift-to-distance proportionality that would be expected from a simple constant expansion rate.
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Old 8th October 2017, 04:59 PM   #6
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Well, I think I can follow, but that's probably just Dunning-Kruger speaking. But first let's clear up some of those assumptions about what I understand and what I don't about velocities and so on, so we don't end up going in circles around those.

The thing is, nobody ever directly observed space growing. What was observed is individual objects going each at a different speed. Nobody ever observed space growing at a percent per km. Any single object you observe has a SPEED in your frame of reference, and that's what that red shift measures. Hence, my talking about observed speeds.

Sure, after you measure a few of them, you can plot the speed vs distance on a graph and there's a proportionality, so you can draw a line through it. And you can express that PATTERN, the slope of that line that you fit to the points on the graph, in terms of percent per second. In fact, in more mathematical terms, it's just simply the derivative of the speed in respect to distance, for that line plotted through the actual observed data points.

BUT there's a difference between the observed data and that derivative. The data points are directly observed, but the dv/dx is not directly observed. Each individual object you observe, whether it's right nao, or way back when the universe first became transparent to light, will still plain old have a SPEED in your frame of reference.

So, yeah, I don't think I did any major mistake in talking about the speed of actually observed type Ia supernovae. I just didn't take that derivative at that point, is all. Because what I'm asking in the first place is if I'm justified to take it in the first place.


But, anyway, my point is that the way I look at it (which, granted, is probably the wrong way in some way) % per space is kinda based on talking about it as if all of it is happening NOW. Let me illustrate:

- If I launch some firework rockets on Xmas eve, take my binoculars and look at a one at 1 km away and a one at 2km away and so on, and apparent speed RIGHT NOW is proportional to distance, sure, hmm, it looks like some weird pattern of inflation going on. It sure doesn't look like my rockets should do that at a constant acceleration, so some deeper explanation may be in order.

- But if I look at it in terms of TIME, and I see that a rocket 3 seconds ago was going 10m/s faster than the one I observed 2 seconds ago, which in turn was going 10m/s faster than my observation from 1 second ago... actually the most parsimonious conclusion is that the rockets have spent their fuel, and are decelerating at 1g. Exactly what you'd expect if there's nothing accelerating them and they're being pulled back by gravity.

And that's my comprehension problem in a nutsa... err... nutshell: how do you get to simply leave time out of it?

I assume there's some good reason there, since everyone IS aware of the time component. It's not like everyone just overlooked that there's a time difference. But I don't know that reason, so I'm asking if anyone can tell me what it is.
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Old 9th October 2017, 01:20 AM   #7
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I haven't read you latest post, but I just based on your OP I think you missed something:

Think about the objects not just as their relative velocity relates to us, but also to each other.

So, there are 2 objects 2 billion light years away from us, but say 500,000 light years from each other. They are moving away from us faster than another object (galaxy) 1 billion light years away from us, but they are moving quite slowly relative to each other. Similarly assume that the galaxy 1 billion light years from us is on an approximate straight line path to the others 2 billion light years away. We can see that relative to it those more distant galaxies are moving away less slowly than they are from us (because from our perspective it's moving toward them).

So it can't just be that things were moving apart faster in the past, because we can see how fast they were moving apart from each other, and it's slower than how fast they are moving away from us.

ETA On the other hand, the relationship with respect to distance still holds if we are only comparing distant galaxies with each other. Those close to each other will be moving apart slowly while those separated by larger distances move apart at higher velocities, matching the values predicted by hubble's law.
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Old 9th October 2017, 01:25 AM   #8
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Hans, if all of the universe's observable mass was decelerated by gravity, there'd be a center of gravity somewhere, and we would be affected by it, too. What you say makes sense if we are looking in the direction of that gravitational force vector, but not if looking perpendicular to it. But we see the same phenomenon, at same rates, in all directions. This could only happen in your universe if we were extremely near the gravitational center of the universe, which has a disappearingly small a priori likelihood.
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Old 9th October 2017, 01:31 AM   #9
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I see it in aan analogy of a rubber band.

Say we have a rubber band of 1 meter long. Each second each of the two edges get pulled 1 cm further from the center (let's call that the speed of rubber).
The speed of rubber is the fastest speed possible, no exceptions.

On the rubber band there are a lot of dots.

Dots next to the center move apart, but only slowly.
Relative to a dot in the center, one of the two end dots is moving away from it at the speed of rubber.
But... according to that end dot. The center dot is moving away from it, at te speed of rubber and the other end dot, way on the other end of the rubber band, is moving away at a speed of two times the speed of rubber, even though the rubber band itslef is only stretching at the speed of rubber.

The relative speed is thus only a function of distance. Not of time.

That's as far as I can understand it. (and I may be completely wrong at that)

Now in three dimensions and with the knowledge each and every dot can be thought of as being in the middle.
That's where my analogy breaks down and it gets murky in my head.
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Old 9th October 2017, 02:06 AM   #10
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Originally Posted by Oystein View Post
Hans, if all of the universe's observable mass was decelerated by gravity, there'd be a center of gravity somewhere, and we would be affected by it, too. What you say makes sense if we are looking in the direction of that gravitational force vector, but not if looking perpendicular to it. But we see the same phenomenon, at same rates, in all directions. This could only happen in your universe if we were extremely near the gravitational center of the universe, which has a disappearingly small a priori likelihood.
The thought did occur, and I'll even offer a bigger problem if we assume only gravity, namely that the acceleration ain't quite what you'd expect if MG/r2 and Newton/Birkhoff hold true about spherical distributions. The maths is a bit off for it to be ONLY gravity in an otherwise flat and infinite space. (Though you can make it work for a 4 dimensional space, just for reference.)

That's why the OT merely offers gravity as one example of things that might pull it all back together, rather than claiming it was THE thing.

Still, the basic idea is the same. Even taking the usual, mainstream idea of the universe as a bubble that grows or shrinks together, I can still look at the data as dv/dt (a.k.a., acceleration) instead of dv/dr. And what I actually see there isn't a bubble that's accelerating its expansion, I see a DEcellerating bubble.

So... what am I missing? I know that every single astrophysicist and astronomer is accutely aware of the time axis in those measurements, so they can't all have just missed it.
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Old 9th October 2017, 02:20 AM   #11
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Originally Posted by HansMustermann View Post
The thought did occur, and I'll even offer a bigger problem if we assume only gravity, namely that the acceleration ain't quite what you'd expect if MG/r2 and Newton/Birkhoff hold true about spherical distributions. The maths is a bit off for it to be ONLY gravity in an otherwise flat and infinite space. (Though you can make it work for a 4 dimensional space, just for reference.)

That's why the OT merely offers gravity as one example of things that might pull it all back together, rather than claiming it was THE thing.

Still, the basic idea is the same. Even taking the usual, mainstream idea of the universe as a bubble that grows or shrinks together, I can still look at the data as dv/dt (a.k.a., acceleration) instead of dv/dr. And what I actually see there isn't a bubble that's accelerating its expansion, I see a DEcellerating bubble.

So... what am I missing? I know that every single astrophysicist and astronomer is accutely aware of the time axis in those measurements, so they can't all have just missed it.
Erwin, substitute the rubber band with a rubber balloon being inflated. All of the dots on its surface move away from one another, relative speed a function of distance, but this time, no point has a legitimate claim to be the center, or an end.
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Old 9th October 2017, 02:26 AM   #12
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Hans, it doesn't matter what we call the force that decelerates the universe's masses. What matters is that when you picture this as happening in a static space, there is a center towards which all mass is pulled, and that doesn't conflict with observation only if we are in that center or really really close. Expanding space explains observation without the need to put is in the center.
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Old 9th October 2017, 02:27 AM   #13
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Oops sorry for quoting the wrong post above... Tapatalk!
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Old 9th October 2017, 03:02 AM   #14
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Originally Posted by HansMustermann View Post
So... what am I missing? I know that every single astrophysicist and astronomer is accutely aware of the time axis in those measurements, so they can't all have just missed it.
I'm not sure if I completely understand what you are saying. Are you trying to say that the observations are inconsistent with accelerating expansion, or only that they are consistent with deceleration?

It's worth noting that deceleration was actually what everyone thought was happening until very recently (1998).

I think my post upthread makes it very clear that it can't be as simple as everything was moving apart faster in the past, as we can see objects distant from us in time, but close to each other in space that are moving apart slowly.

Maybe you could be more clear about what sort of model you are thinking about because I think I showed pretty clearly that at least what I thought you were imagining is inconsistent with known observations.
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Old 9th October 2017, 04:50 AM   #15
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Originally Posted by Oystein View Post
Erwin, substitute the rubber band with a rubber balloon being inflated. All of the dots on its surface move away from one another, relative speed a function of distance, but this time, no point has a legitimate claim to be the center, or an end.
I know about the balloon analogy. That does describe the no center point better.
The rubber band helps me better with visualizing the speed of expansion though.

In combination? My brains shut down.
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Old 9th October 2017, 02:03 PM   #16
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The accelerating universe is a recent observation. Supernovae of a certain type have a fairly narrow range of intensities (= number of photons). Intensity, like frequency, is a function of distance, but they work differently when space accelerates its expansion. The actual calculation is a bit of a headache, but acceleration has different effects on frequency and intensity respectively, and the relationship grows nonlinear over time (IIRC distant stars are dimmer than you would expect) because an accelerating universe was slower in the past, hence the light has travelled further and the denominator of the inverse square factor on the intensity is thus larger, roughly speaking.
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Old 9th October 2017, 03:29 PM   #17
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Let's see if I can put this in a way that makes more sense.

You have a supernova at a known distance d, which shines with intensity I.

You observe another supernova, let's call it A, shining with intensity I/4. Assuming a static universe, you'd assume the supernova was at distance 2d so that the light has travelled twice as far. But you know that the universe is expanding, so it was actually closer than 2d. You know the total change in size over time by looking at the redshift.

Now, let's say you have a supernova B with intensity I/16. In a static universe, you'd assume the supernova was at a distance 4d, so that the light has travelled four times as far. But... you know the universe is expanding at a particular rate. Here's where the calculations get too messy for me to attempt blindly. But, you knew the other star was closer by a certain fraction by looking at the redshift. Without looking at the redshift of THIS star, you can calculate the fraction of the distance it was at, assuming the rate of expansion is constant. Then, since you know how much smaller the universe has was at emission, you can calculate how much the light should have been been redshifted.

What you find is that it is less redshifted than you had expected. How do you explain this?

Well, the redshift is very firmly grounded in theory - the universe was smaller than you thought at the time of emission. This is fairly unambiguous.

So, what has happened, according to the acceleration theory? You have used a recent average of expansion. Let's consider a photon from star A emitted just as one from star B flies by. In a contrived case, let's say there was no expansion before this point. Then the photons would have the exact same redshift. If there was only a little expansion, then photon B would have a little more redshift, etc. But in general, you'd need to use a different, smaller average to get the correct redshift for B.

So, what happens when we look at the redshift first instead? The stars look dimmer than we'd expect.


This, by the way, is a good example of a problem where a solid grasp of linear algebra makes everything so much easier to figure out and intuitive.
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Old 9th October 2017, 06:03 PM   #18
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Well, my point was that more than one thing causes red shift. Velocity also does. In fact, I'm pretty sure we as a species knew about Doppler effect more than half a century before we knew that space stretches. And not just about sound. The book where Doppler proposes the effect is actually about the colour of stars. That it applies to sound too was more like a side-effect of thinking too hard about red-shift.

The OTHER thing that causes red-shift, well, it still boils down to stretched space, and it's gravity. It's what's 100% responsible for the temperature of Hawking radiation, for example, no dark energy needed there. But let's leave that one be for now.

The only real objection to it just being velocity and an imploding universe is really the one mentioned by Oystein. We either have to be relatively close to the centre of it all, or the universe must have a stranger geometry that, well, I'm not qualified enough to support.

But then SOMEONE would have to be close to the centre. It WOULD be a big fluke if, say, the centre happened to be as close as, say, the Great Attractor, and we just pulled the lottery ticket to be close to it. But again, SOMEONE has to.

I mean, it's even more vanishingly improbable that one given person would win over 750 million in the lottery, than that we'd happen to be about 250 million lightyears from the centre of a normal universe, but SOMEONE did.


Well, that was just to clarify what I'm talking about. In the meantime I'm actually starting to figure it out. I think.

Thanks to everyone who took the time to answer.
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Old 9th October 2017, 06:45 PM   #19
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OK, I re-read your question. You're thinking "what if space was static and things in the past moved quickly", basically? There's an elementary basis-independent vector algebra exercise about this. Generally, everyone thinks they're at the centre of an explosion, unless they can observe an inhomogenity of mass distribution. It's geometry - things close to you move away slowly, faraway things quickly (think of a gas cloud expanding into a void).

But by all accounts space is homogeneous, not an expanding shell of matter in a void. Second, I believe we can observe redshifts that would require speeds that would be significant fractions of the speed of light to achieve velocity redshift.

The evidence just doesn't suggest it whereas the GR equations themselves hint at expansion. But there's almost never a single golden piece of proof in theoretical physics. There are always different models, tests, simulations, computations, that suggest one interpretation over the other.
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Old 9th October 2017, 08:01 PM   #20
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Originally Posted by HansMustermann View Post
The only real objection to it just being velocity and an imploding universe is really the one mentioned by Oystein.
A spelling mistake: it is a universe explodding away from us that gives red shift and produces Oystein's "we are not the center of the universe" objection. To which we can add:
  • The fragments (galaxies) from an explosion would be moving at constant velocity once away from the blast zone.
    We observe that the velocity of galaxies varies with distance from us. That needs some kind of continuous, invisible "explosion" like expanding spacetime.
  • An explosion propels different masses at different velocities.
    There is no dependence on galaxy mass in Hubble's law.
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Old 10th October 2017, 12:20 AM   #21
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Originally Posted by Reality Check View Post
A spelling mistake: it is a universe explodding away from us that gives red shift and produces Oystein's "we are not the center of the universe" objection. To which we can add:
  • The fragments (galaxies) from an explosion would be moving at constant velocity once away from the blast zone.
    We observe that the velocity of galaxies varies with distance from us. That needs some kind of continuous, invisible "explosion" like expanding spacetime.
  • An explosion propels different masses at different velocities.
    There is no dependence on galaxy mass in Hubble's law.
Well, I was kinda getting ahead of myself, but that's what you get when writing stuff after midnight and after one beer too many

My point since message one was that my layman (and probably wrong) interpretation would be that the expansion is decelerating, rather than accelerating. Which means that at some point it turns into an implosion, actually. You know, ye olde "big crunch" idea.

And indeed, if I turn distance into time, the closest stuff to us in time (and distance, thanks to c) are actually currently moving TOWARDS each other. E.g., the whole Laniakea Supercluster is moving towards the Great Attractor. So basically I'm suspecting that we're just past the point of stop and start falling back in. Hence implosion.

Basically, EXplosion, hence red-shift, gradually decelerate for a dozen billion years, turn into an IMplosion eventually.

But really it was just thinking all that when writing about the explosion, and, well, I ended up writing implosion. You know how it is. Thoughts are coming and going in my head like trains in a railways station, and some blow the horn; you can imagine what's in my head

EDIT: anyway, the point was that if you turn the "but it depends on distance" into "but it depends on time", because distance in light years is 1 to 1 equivalent to time in years, then you probably understand why I'm not exactly moved by your first point. We're just observing those fragments at different times.
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Old 10th October 2017, 12:39 AM   #22
HansMustermann
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Originally Posted by TubbaBlubba View Post
Second, I believe we can observe redshifts that would require speeds that would be significant fractions of the speed of light to achieve velocity redshift.
1. So does the current interpretation. In fact, in the current interpretation, the parts outside the observable universe are moving faster than light in our frame. In fact, the vast majority of the actual universe, would be moving faster than light from our point of view, and be outside the observable universe.

2. They're 100% equivalent anyway. You'd see the same redshift in both cases.

3. The big bang would in fact start at the speed of light, since it was a bunch of photons. Things being near the speed of light after matter condenses and is pushed away by even more photons, wouldn't really be anything special.

Originally Posted by TubbaBlubba View Post
The evidence just doesn't suggest it whereas the GR equations themselves hint at expansion. But there's almost never a single golden piece of proof in theoretical physics. There are always different models, tests, simulations, computations, that suggest one interpretation over the other.
If just GR was enough, we wouldn't need Dark Energy to explain it all.
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Old 10th October 2017, 04:13 AM   #23
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Originally Posted by HansMustermann View Post
1. So does the current interpretation. In fact, in the current interpretation, the parts outside the observable universe are moving faster than light in our frame. In fact, the vast majority of the actual universe, would be moving faster than light from our point of view, and be outside the observable universe.
It's a different type of moving at the speed of light.

Quote:
2. They're 100% equivalent anyway. You'd see the same redshift in both cases.
No. Cosmic redshift does not translate directly to Doppler shifts. The functions are very different. Notably, Doppler shift approaches infinity as v approaches c. But cosmic redshift is implies superluminal recession already by a factor of 2 or so.

Quote:
3. The big bang would in fact start at the speed of light, since it was a bunch of photons. Things being near the speed of light after matter condenses and is pushed away by even more photons, wouldn't really be anything special.
Yes, but in our hypothetical "dust cloud" scenario the relative recession speeds locally would only be a tiny fraction of the initial expansion speed.
Quote:
If just GR was enough, we wouldn't need Dark Energy to explain it all.
Dark Energy is a factor in the Cosmological Constant of GR. We need DE to better understand acceleration, but GR can incorporate it naturally.
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Old 10th October 2017, 04:53 AM   #24
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Well, yes, but the cosmological constant can lead to widely different results, if it has different values. You can get an expanding universe, a brake-and-implode universe, or a static universe just the same. It's not like you can say that GR is a priori telling you that the universe is expanding.
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Old 10th October 2017, 08:30 AM   #25
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Originally Posted by HansMustermann View Post
Well, yes, but the cosmological constant can lead to widely different results, if it has different values. You can get an expanding universe, a brake-and-implode universe, or a static universe just the same. It's not like you can say that GR is a priori telling you that the universe is expanding.
Which is why I said "hints at".
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Old 10th October 2017, 01:18 PM   #26
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Originally Posted by HansMustermann View Post
My point since message one was that my layman (and probably wrong) interpretation would be that the expansion is decelerating, rather than accelerating. ...
We measure an acceleration of the expansion of the universe so the interpretation is somehow wrong.

We measure the distance to a galaxy. That light takes time to get from that galaxy to us does not "turn distance into time". What it means is that we can express that distance in a unit of measurement related to light - a light year.
That may be where the interpretation goes wrong - a light year is a distance.
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Old 10th October 2017, 01:31 PM   #27
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Originally Posted by HansMustermann View Post
It's not like you can say that GR is a priori telling you that the universe is expanding.
That is correct. What GR tells us is that even without a cosmological constant (i.e. up until 1998 ), a homogeneous, isotropic universe will be contracting, static or expanding. It is a big body of empirical evidence that tells us that the universe is expanding.

Then along comes 1998 and we measure from supernovae data that the expansion is accelerating which matches the case of a tiny positive cosmological constant ("10−52 m−2, in metric units"). We call this dark energy. Some years later and we have measurements of the CMB from WMAP and Planck that confirm that dark energy exists and matches that cosmological constant. There are a couple of other lines of evidence for dark energy.
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Old 10th October 2017, 02:30 PM   #28
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Originally Posted by Reality Check View Post
We measure an acceleration of the expansion of the universe so the interpretation is somehow wrong.
Yep. Particularly we see that sorted by redshift (a function of total expansion) we see that distant stars are relatively dimmer than we'd expect them to be if expansion was constant. If it were decelerating, they would be brighter than expected.

There is no other viable explanation, except spacetime expansion, for the redshift observed, so the conclusion follows. Doppler redshift etc works completely differently.
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Old 10th October 2017, 04:18 PM   #29
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Well, see, that's the kind of thing that I'm interested in. Works differently... how? I mean, I understand the difference in how it's produced, but does it mean I can tell which is which?

let's take the basic scenario: I observe a type 1a supernova, it has a certain observed magnitude (hence distance), and a certain amount of red-shift. How do I tell if that's because of space stretching, or if it's plain old Doppler?

EDIT: because that's what I meant when I said they're 100% equivalent. As far as I know (but it might be wrong) the spectrum looks the same in both cases. You may need a different speed for Doppler, but it looks the same in the end.
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Old 10th October 2017, 04:24 PM   #30
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You cannot until you measure the redshift and distance for other type 1a supernovae. It is the variation with distance (Hubble's law) that says it is not plain old Doppler shift.

ETA: The distance to a type 1a supernova is determined by estimating its intrinsic magnitude from how the light varies during the nova. This is compared to its observed magnitude to give a distance from the inverse square law.

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Old 10th October 2017, 09:24 PM   #31
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Well, that still brings me back to my original question: how do you know it's actually varying with distance, as in, at the same time, as opposed to just witnessing the universe brake down over time?

I mean, both seem to me about as supportable, and both need some form of dark energy (or the braking version doesn't respect Birkhoff, and thus GR), so it seems like a toss.

Is it the CMB that needs the space to expand? Or, well, what's the deal? How do you know it's one and not the other
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Old 10th October 2017, 09:47 PM   #32
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Originally Posted by HansMustermann View Post
Well, that still brings me back to my original question: how do you know it's actually varying with distance, as in, at the same time, as opposed to just witnessing the universe brake down over time?

I mean, both seem to me about as supportable, and both need some form of dark energy (or the braking version doesn't respect Birkhoff, and thus GR), so it seems like a toss.

Is it the CMB that needs the space to expand? Or, well, what's the deal? How do you know it's one and not the other
I feel like you should probably get a doctoral degree in theoretical physics, and then come school us on whatever questions you still have.

As it is, it really looks like you're asking for all the benefits of an advanced degree, without having to put in any of the work yourself.
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Old 10th October 2017, 11:04 PM   #33
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What an arrogant (lack of) answer! So now you're no longer supposed to ask complicated questions in fields where you don't have a degree? Well, goodbye to science communication!
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Old 11th October 2017, 12:12 AM   #34
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Originally Posted by theprestige View Post
As it is, it really looks like you're asking for all the benefits of an advanced degree, without having to put in any of the work yourself.
Pretty much. I wish I could take several years off to get first the M.Sc in physics, and then do the post-grad study, but alas I can't really afford to.
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Old 11th October 2017, 12:29 AM   #35
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Originally Posted by theprestige View Post
As it is, it really looks like you're asking for all the benefits of an advanced degree, without having to put in any of the work yourself.
If that's possible, it sounds like a good thing to me. Is there something you find objectionable about him wanting to have a better understanding of cosmology?
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Old 11th October 2017, 12:34 PM   #36
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Originally Posted by HansMustermann View Post
Well, that still brings me back to my original question: how do you know it's actually varying with distance, as in, at the same time, as opposed to just witnessing the universe brake down over time?
Because we measure it as already stated multiple times. We measure the distance to galaxies (not only via type 1a supernovae). We measure the redshift of these galaxies. This gave us Hubble's law (the universe is expanding). We then got data from galaxies further away and these showed that the expansion was accelerating (not braking).

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Old 11th October 2017, 12:45 PM   #37
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Originally Posted by HansMustermann View Post
Is it the CMB that needs the space to expand?
cosmic microwave background
Evidence for dark energy, e.g. CMB, which gives an accelerating expansions of the universe.
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Old 11th October 2017, 12:48 PM   #38
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Originally Posted by Reality Check View Post
Because we measure it as already stated multiple times. We measure the distance to galaxies (not only via type 1a supernovae). We measure the redshift of these galaxies. This gave us Hubble's law (the universe is expanding). We then got data from galaxies further away and these showed that the expansion was accelerating (not braking).
I think you're missing what he's asking. Let me try and clarify the question as I understand it, to see if that helps any:

1. Because light travels at light speed, that means things we see farther away are older. The light from a galaxy 1,000,000 light years away is 1,000,000 years old.
2. If the acceleration was faster 1,000,000 years ago and slower now, then you'd expect 1,000,000 year-old light to be more red-shifted than closer, "newer" light.
3. This would indicate that the factor involved is time, rather than distance: older light is more red-shifted, so was moving faster then "newer" light.

That's what he's asking about, and asking for what the flaw in the reasoning is. I'm pretty sure there's something wrong with that too, but don't know enough to explain it clearly myself.

And Hans, if I've bungled it even worse, feel free to tell me to STFU and but out .
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Old 11th October 2017, 01:52 PM   #39
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Originally Posted by Hellbound View Post
I think you're missing what he's asking...
Then we need to wait for Hans to clearly state what he is asking about.
I think he is talking about the expansion of the universe as measured by several means, e.g. distances from type 1a supernova giving + velocities from redshift. In that case there are multiple, agreeing lines of evidence that the expansion is accelerating not braking as he asserts.
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Old 11th October 2017, 02:56 PM   #40
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Nah, Hellbound got it. Well, and #2 it was speed rather than acceleration, but otherwise it's an exact summary.
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