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Thread: dark energy, obviously

  1. #1 dark energy, obviously 
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    Could anyone please explain to me how can an accelerated expansion take place in a homogenous & isotropic universe?I really can't figure that out - at all. Not only cannot happen radial, but not even linear. Unless I'm terribly wrong - and there's where you'll probably come in.If you even take the time for an (simple !) illustration, I would thank you even more.By the way, not only I cannot figure out dark energy, but I cannot imagine expansion even before DE - how could even a constant expansion, or a deccelerating one, could take place in a homogenous & isotropic three-dimensional universe? As soon as I think of several points in space and try to imagine what would happen (the observations should be the same, but they can't be!), well... I'm really waiting for your input.Thank you so much for your kindness.


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    Quote Originally Posted by alex_ro View Post
    As soon as I think of several points in space and try to imagine what would happen (the observations should be the same, but they can't be!), well... I'm really waiting for your input.
    Those points in space would get further apart from one another. What is expanding is the space between things so any two points will progressively get further apart.

    Lets start with a series of points:

    A . B . C . D . E . . Z .

    After certain amount of time, the space between every point will have increased by 1:

    A . . B . . C . . D . . E . . . . Z . .

    This means that the space between A and B has increased by 1. The space between A and C has increased by 2. And so on.

    But note that the space between B and C has increased by 1 while the space between B and D has increased by 2.

    So everyone sees the same increase (expansion) around them. This maintains the homogeneity. Now expand that to 2 or three dimensions and you should be able to see that it preserve isotropy as well.

    Note that if space between points increases by 1 at a steady rate, then the apparent speed of recession will be 1 between adjacent points, 2 between points separated by 1 and so on. The speed is proportional to distance (which is what we observe).

    Another commonly used example is the surface of a balloon covered in regular spots.

    NOTE: in this analogy we are only concerned with the surface of the balloon, not the size of the balloon itself.

    You can think of the spots representing galaxies (or, more accurately, galaxy clusters). As the balloon inflates, the surface are of the balloon increases and all the spots get further apart. But they stay evenly distributed (homogeneous).

    Now all you have to do is think of a hyperballoon whose surface is 3-dimensional ... easy!

    Does that help at all?

    The other way of thinking about is, is just that the universe becomes less dense over time.

    Dark energy (whatever it is) just accelerates the process.


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    Quote Originally Posted by Strange View Post
    Does that help at all?
    Thank you, but Iím sorry to say that it doesnít. You only gave a linear example, and only with a constant expansion. But letís stick to linear for the moment. In the case of not a constant expansion, an observer in point M (or Z, wherever) will obviously NOT SEE the same as the observer in point A. Because the farther away a cosmic object from point A, the faster it moves away from that observer. So they simply cannot see the same thing. Please correct me if Iím wrong.Now, if we introduce three-dimensions, itís again obvious that no other observer will see what the observer in point A sees. The observer in point A will see, in any direction (radially), what he already saw previously (linearly), but any other observer will see not only something very different compared to the observer in point A, but also something different depending which direction he looks (from his point of view, the universe will not appear isotropic, and thus can neither be homogeneous).Thatís why Iím puzzled. I really canít figure it out.You then bring as example the (surface of a) balloon Ė well, that puzzles me even more. Are we in a three-dimensional space, or not? How does going back to bi-dimensional help?Then, you say:
    Quote Originally Posted by Strange View Post
    Now all you have to do is think of a hyperballoon whose surface is 3-dimensional ... easy!
    Well, it may be for you, but not for me. Some links with illustrations might help.
    Quote Originally Posted by Strange View Post
    The other way of thinking about is, is just that the universe becomes less dense over time.
    How exactly does this affect the evolution the stars etc.?Also, WHEN exactly did dark energy start to get a hold on things? If itís the cosmological CONSTANT, it should run back in time quite a while, shouldnít it?Once again, thank you.P.S. If someone can help with formatting, I don't know what happens: not only all the spaces after "periods" disappear , but also spaces between paragraphs (thus sentences stick together almost gravitationally :-) ! But I do write initial text in MS Word (for spelling, since English is not my native language) - could that be the cause? But then, even if I add spaces in reply window, they will again disappear once I hit "preview post" button!
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    Quote Originally Posted by alex_ro View Post
    But let’s stick to linear for the moment. In the case of not a constant expansion, an observer in point M (or Z, wherever) will obviously NOT SEE the same as the observer in point A. Because the farther away a cosmic object from point A, the faster it moves away from that observer. So they simply cannot see the same thing.
    What I was attempting to show is that whichever point you pick, they will see the same thing.

    Take point A, for example. They will see B move away 1 step per unit time, C move move away 2 steps per unit time, D move move away 3 steps per unit time, E move move away 4 steps per unit time, and so on.

    Now take point M. They will see N move away 1 step per unit time, O move move away 2 steps per unit time, P move move away 3 steps per unit time, Q move move away 4 steps per unit time, and so on.

    It is entirely symmetrical: every point sees everything else move away (at a rate proportional to distance). Maybe you need to draw out a while series of steps on graph paper to convince yourself.

    Now, if we introduce three-dimensions, it’s again obvious that no other observer will see what the observer in point A sees. The observer in point A will see, in any direction (radially), what he already saw previously (linearly), but any other observer will see not only something very different compared to the observer in point A, but also something different depending which direction he looks (from his point of view, the universe will not appear isotropic, and thus can neither be homogeneous).That’s why I’m puzzled. I really can’t figure it out.
    I am not going to try and draw a three-D version (it can be done). But the same thing applies. Whichever point of view you pick, they will see the same thing. Maybe you need to convince yourself in 1 or 2 dimensions first.

    Or can you explain why the person at point Z will not see the same as the person at point A. Note that it isn't that everything is moving away from A, but everything is moving away from everything else. The space between things is increasing.

    You then bring as example the (surface of a) balloon – well, that puzzles me even more.
    Again, just an analogy. It shows that what works in 1 dimension also works in 2. And is, hopefully, easy to visualise. Or even do yourself with a balloon and a pen.

    Well, it may be for you, but not for me.
    No, not for me either.

    How exactly does this affect the evolution the stars etc.?
    I'm not sure it does. Well, I gather the very earliest stars were bigger and shorter lived than later stars for this reason (but that is not something I know much about).

    Also, WHEN exactly did dark energy start to get a hold on things?
    I think it started to dominate over gravity about 6 billion years ago (but again, not my area).

    P.S. If someone can help with formatting, I don't know what happens: not only all the spaces after "periods" disappear , but also spaces between paragraphs (thus sentences stick together almost gravitationally :-) ! But I do write initial text in MS Word (for spelling, since English is not my native language) - could that be the cause? But then, even if I add spaces in reply window, they will again disappear once I hit "preview post" button!
    You could try going to Settings and changing the Editor option to a more basic one. Or paste into a plain text editor (Notepad?) and then copy and paste into the browser... (not ideal, I know).
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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    Strange, thank you for your time, but I don't think we understand each other.You say:
    Quote Originally Posted by Strange View Post
    Now take point M. They will see N move away 1 step per unit time, O move move away 2 steps per unit time, P move move away 3 steps per unit time, Q move move away 4 steps per unit time, and so on.
    but conveniently or unknowingly you forgot what I said. What you say above as example is the ONLY line of sight for M observer where he will see what A observer sees on the line of sight towards M (and in case of A observer, not exclusively that direction).The M observer, wherever he looks, WILL NOT SEE the same thing again at all. Moreover, looking back at A (the direct line of sight), he will actually see the reversed situation (deceleration?).Perhaps now you'll understand what I meant.So it's not at all how you claim: (remember the acceleration)
    Quote Originally Posted by Strange View Post
    It is entirely symmetrical
    But even without acceleration (constant expansion), the M observer will only see what A observer sees ONLY along the line of sight uniting them (but this time both ways: towards the A observer, and directly away from A observer).I'm not sure how you can even claim that symetry will remain in 3d, since for me it's obvious it won't (again, regardless of acceleration, but even worse with acceleration).
    Quote Originally Posted by Strange View Post
    Note that it isn't that everything is moving away from A, but everything is moving away from everything else. The space between things is increasing.
    "everything is moving away from A" - that's indeed what A observer sees, but, again, another observer simply cannot see the same thing.When you take a multitude of points in a three-dimensional space, points simply can't show the SAME relative motion regardless of point of observation.I suppose we can't understand each other without an illustration, and as I'm rather poor with such things, I'm letting you to do it if you please. :-)Now, about the balloon, I think this is the only thing that, first time I thought about it, SEEMED to work (even with acceleration). Anyway, the universe is not a balloon, is it?But I'm not at all sure the balloon actually works. Because if all observers are on the balloon (IN the bi-dimensional space of the surface of the balloon), their entire observations would take place looking towards inside the balloon (since there's nothing outside the balloon, and also due to curvature). So, plainly, if we are on a balloon, there will be many observations of NOTHING (the empty nothing outside the balloon), contradicted by star observations wherever we look.So while the balloon may indeed show the same acceleration for ALL observers on the balloon, and even throughout different moments in time, I find (excuse me) the balloon thing rather absurd, since in that case the universe won't be isotropic (and homogeneous) NOT EVEN for a single observer.When I asked how does dark energy affect the evolution of the stars, you replied:
    Quote Originally Posted by Strange View Post
    I'm not sure it does. Well, I gather the very earliest stars were bigger and shorter lived than later stars for this reason (but that is not something I know much about).
    It certainly does. What can convince matter to come together and form a star, while working all the time against such a strong "force" pulling everything apart?
    Quote Originally Posted by Strange View Post
    I think it started to dominate over gravity about 6 billion years ago (but again, not my area).
    Do you have a source for that figure?Thank you so much.
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    My entire reply above was written inside this forum page, and those formatting problems, still appear... I don't know what to make of it.
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    It is time, once again, for me to make a toy model to illustrate the expansion of the universe! (Some of you will have seen me post this before)

    Now to model an expanding space we need to assign coordinates within that space. For the moment, forget about any edges to that space, we don't need edges, we just need coordinates in order to measure the expansion of space. Galaxies come later, so for now just imagine a 3 dimensional grid. At each grid intersection we will assign a coordinate, a point, a dot. Let's say each intersection point is 1 meter apart.

    Put yourself on a point somewhere in this space. Whatever axis you look along you see neighbouring points 1, 2, 3, 4, 5 etc meters away, receding off into the distance. Then we introduce some expansion. Let's say the space grows to 10 times its original size in 1 second! That seems fast perhaps, but this is just a model with easy numbers. The key thing to remember is that the grid expands with the space.

    So, here we are, still sitting on our point (but it could have been any point!) 1 second later. Now lets look along an axis. We see those neighbouring points are now 10, 20, 30, 40, 50 etc meters away. The space increased to 10 times its original size, and so did the distance between each intersection point on that grid.

    Our nearest neighbouring point has receded from 1 to 10 meters in 1 second, so it has receded at 9 meters per second. The next point away has receded from 2 to 20 meters in 1 second, so that point receded at 18 meters per second. The fifth point has moved from 5 to 50 meters away in 1 second, so that one has receded at 45 meters per second. The further away you look, the faster a point will seem to have receded!

    And the view would be the same, whatever viewpoint you choose in the grid! There is no "centre" of expansion, no origin point within that grid - the whole thing, the whole space has expanded from something where the spaces between things were really small to something where the spaces between things are much larger. The expansion of that space has carried matter and energy along for the ride.

    Remember I said the grid of points receded off into the distance.. well a point that was initially 33,000,000 meters away will have moved away to 330,000,000 meters in 1 one second, meaning that it has receded at 300,000,000 meters per second - the speed of light! Any point initially more distant than 33,000,000 meters away from another point will have receded from that point faster than the speed of light. That is the distance were an object recedes at light speed in this "little" model of expansion. If you look at a point that has receded at the speed of light, then from that point, the point you are on has receded at the speed of light. But no object would be moving through space faster than light, no photon would ever overtake another photon, it all just gets carried along by the cosmic flow.

    Now I know this is a very simple model, dealing with a simple 10 times expansion in 1 second. This might seem very different from a universe where the rate of expansion was slowing from immense speed and then starting to accelerate, but if you start your grid very small and apply different rates of expansion to that grid, incrementally, over different lengths of time, to simulate slowing it down and then speeding it up, when you look at the end result it is essentially the same. (Whenever there is a change in the rate of expansion, it is the rate of expansion for the whole grid that changes).

    You might be asking how useful this model actually is. Well you can substitute different distance measures and time-scales if you like but the principle remains. If you sprinkle galaxies throughout the grid and then expand that grid such that the galaxies move with the expansion, you would find that galaxies interact gravitationally with their near neighbours. The further apart galaxies are when they form, the less the gravitational attraction between them. If they are less than a certain distance apart, the galaxies will move towards each other and cluster together, but if there is enough distance they will be moved apart by the expansion of the universe.

    We end up with clusters of gravitationally-bound galaxies and increasing distance between the centres of those clusters, in a universe where there is no "origin point" or centre of expansion.
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    Decelerating expansion.
    If you look at a certain galaxy and calculate how fast it is receding, and then look again at the same galaxy a million years later, it will be more distant but will be receding at a slower rate. The galaxy is moving away at a decelerating rate. The Hubble constant is decreasing - the distance where a galaxy recedes at the speed of light gets larger over time. The "Hubble distance", where objects recede at c, is itself accelerating away from you.


    Constant expansion.
    Whenever you look at a certain galaxy, however distant it becomes, it always recedes at the same speed. The galaxy is moving away at a constant rate. The Hubble constant is still decreasing - the distance where a galaxy recedes at the speed of light still gets larger over time. The Hubble distance recedes at a constant speed. (If the universe was always expanding constantly, the Hubble distance would always equal the age of the universe)


    Accelerating expansion.
    If you look at a certain galaxy and calculate how fast it is receding, and then look again at the same galaxy a million years later, it will be more distant but will be receding at a faster rate. The galaxy is moving away at an accelerating rate. The Hubble constant is still decreasing - the distance where a galaxy recedes at the speed of light still gets larger over time! The Hubble distance recedes at a decelerating speed.


    Exponential expansion.
    Galaxies are accelerating away such that they always reach the speed of light, in relation to you, at the same distance. The Hubble constant remains constant - the distance where a galaxy recedes at the speed light of remains constant. The Hubble distance remains at a constant distance.


    More than exponential expansion.
    Galaxies are accelerating in the way they accelerate away, such that they reach the speed of light at a distance that decreases over time. The Hubble constant increases. The Hubble distance comes back towards us. Eventually we reach a Big-Rip scenario.
    Last edited by SpeedFreek; February 21st, 2013 at 02:31 PM. Reason: typo!
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    Quote Originally Posted by alex_ro View Post
    Strange, thank you for your time, but I don't think we understand each other.
    Apparently not.

    What you say above as example is the ONLY line of sight for M observer where he will see what A observer sees on the line of sight towards M (and in case of A observer, not exclusively that direction).The M observer, wherever he looks, WILL NOT SEE the same thing again at all. Moreover, looking back at A (the direct line of sight), he will actually see the reversed situation (deceleration?).Perhaps now you'll understand what I meant.So it's not at all how you claim: (remember the acceleration)
    I was ignoring acceleration as an unnecessary complication for the time being.

    I don't really understand your point. If we go back the diagram I drew:
    t = 0: A . B . C . D . E . . Z .

    t = 1: A . . B . . C . . D . . E . . . . Z . .

    Lets consider what A sees at time t=1:
    B has moved 1 unit (kilometre, light-year, megaparsec, whatever) from A.
    C has moved 2 units from A.
    D has moved 3 units from A.
    And so on.

    (Note that this ignores all the points to the left of A. Just because I don't have an infinitely wide piece of paper)

    So the distance each point has moved away from A is proportional to the distance from A. Therefore the speed of recession from A is proportional to distance.

    But now lets look at another point. Lets consider what C sees at time t=1:
    B has moved 1 unit (kilometre, light-year, megaparsec, whatever) from C.
    D has also moved 1 unit from C.
    A has moved 2 units from C.
    E has also moved 2 unit from C.
    F has moved 3 units from C.
    And so on.

    So the distance each point has moved away from C increases the further away they are. Therefore the speed of recession from C is proportional to distance.

    Pick any point and work out how far away all the other points move after 1 time step, 2 steps, etc. Every point will see every other point moving away from them. This is an inevitable consequence of increasing the gap between the points.

    If you can't visualise that then I suggest you get some sheets of graph paper and a pen and see for yourself. You can then confirm it works for 2 dimensions. If you are not convinced that the same thing extends to three dimensions then you are going to have to do some fancy construction with lego or something.

    But it is just kind of obvious.

    "everything is moving away from A" - that's indeed what A observer sees, but, again, another observer simply cannot see the same thing.When you take a multitude of points in a three-dimensional space, points simply can't show the SAME relative motion regardless of point of observation.
    I don't see why not. If the distance between all them is increasing by the same amount, they will see the same thing.

    I suppose we can't understand each other without an illustration, and as I'm rather poor with such things, I'm letting you to do it if you please. :-)
    You will learn much better if you do it yourself. Get a sheet of graph paper, draw a dot on every intersection. Get another sheet draw a dot on every other intersection. Get another sheet draw a dot on every third intersection. (Do you see that pattern). Repeat as many times as it takes you to see that all the dots are moving apart from one another. And the rate at which they separate depends on how far apart they are.

    It certainly does. What can convince matter to come together and form a star, while working all the time against such a strong "force" pulling everything apart?
    Gravity. It dominates in some areas of slightly higher density and causes clouds of gas to collapse. It was once thought that gravity would slow and stop the expansion and cause the entire universe to collapse again. It now seems there is not enough matter to do that.

    Do you have a source for that figure?
    No, I just Googled it when you asked. As I say, not my subject.
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    @SpeedFreekThank you so much for taking the time to talk here. Dark energy really puzzles me, that's why any input is appreciated.Having said that, it honestly didn't help me visualise how observers throughout the universe will observe (WHEREVER they look) what we observe: an accelerating expansion. Sorry.Also, excuse me, but I think you made some errors. You say:
    Quote Originally Posted by SpeedFreek View Post
    The key thing to remember is that the grid expands with the space.
    If that's the case (and the proportion is kept), then we can't detect expansion. At all.Then, in case of decelerating expansion, you say:
    Quote Originally Posted by SpeedFreek View Post
    The Hubble constant is decreasing
    Which means it's not a constant. By the way, have you thought about the impact of accelerating or decelerating expansion on the age of the universe?If what your links say is true, then they should change the age of the universe accordingly.In one exclusive case would be smaller, in the other exclusive case would be larger. If 9 billions (the appearance of dark energy) is correct, then DE would have the upper hand (much more than half the current stated age of the universe) and there should be a (small) decrease of age figure. But I suspect this will be circular, because how will the 9 billions compare to the new age of the universe?(I suspect that's why they left the age unspoken of... :-)Now, about your links:They are interesting, however not quite convincing. For example:"So after the big bang, matter was still relatively dense in the Universe and therefore gravity braked expansion."Well I can't understand this one either. I just thought I understood it, but accelerated expansion has puzzled me.So how can even during a decelerating expansion (post big bang) things stick together gravitationally to form stars or whatever? Hasn't the space expanded (decelerating or accelerating) throughout the history of the universe?They even say: "But as galaxies moved farther apart, dark energy began to exert a more significant influence. "So an accelerated expansion was strong enough to pull galaxies apart from one another (!), but a decelerating expansion wasn't strong enough to work against anything trying to form (specks of dust trying to hold together)?I simply don't get it.The second link is even more puzzling. This is what they say:"Astronomers have long known that galaxies are receding from each other."Note that: "long known".More, the article contradicts itself: once says "9 billion years ago", then says "the expansion rate of the cosmos began speeding up about five to six billion years ago".Since the ONLY evidence for dark energy is accelerated expansion, then obviously the 2 figures should agree with each other.Anyway, why would both articles call dark energy "cosmic jerk"? I mean, they call the dominant force in the universe "cosmic jerk"?! If that's 'jerk', what exactly can the rest of the universe be? Including you and me, by the way...
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    @StrangeWell, again, you ignore ACCELERATED expansion.
    Quote Originally Posted by Strange View Post
    If the distance between all them is increasing by the same amount, they will see the same thing.
    That's the problem, right there. While the distance between the 2 observers will obviously be the same for the 2 observers (whether the first looks at the second one, or vice versa), points along the route between the 2 observers will not show the same thing. If one sees acceleration, the other must see, in my view, deceleration.A farther point from first observer will move away faster from that observer, and thus the second observer cannot see the same thing:this is what A sees:A . B .. C ... D .... E ..... FFor F to see the same thing would mean:F . E .. D ... C .... B ..... Awhich is a clear contradiction.F will only see what A sees in continuation of the line uniting them:F . G .. H ... Now we come to the second thing I'm puzzled about, since DE started puzzling me. Even in the conditions of a decelerated expansion, how can ANYTHING form? You say:
    Quote Originally Posted by Strange View Post
    It was once thought that gravity would slow and stop the expansion and cause the entire universe to collapse again. It now seems there is not enough matter to do that.
    Which means it wasn't enough to form the stars (prior to DE) either. Unless there's yet another mysterious force out there (besides dark energy), changing freely energy into matter (prior to DE) and matter into energy (currently), and moreover "storing" them: matter to form stars, energy for DE to become prominent.Thanks.
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    Quote Originally Posted by alex_ro View Post
    @SpeedFreekThank you so much for taking the time to talk here. Dark energy really puzzles me, that's why any input is appreciated.Having said that, it honestly didn't help me visualise how observers throughout the universe will observe (WHEREVER they look) what we observe: an accelerating expansion. Sorry.Also, excuse me, but I think you made some errors.
    My only error was trying to explain it to you in easy terms. You are the first person I have come across who hasn't understood my toy model explanation. Well done!

    Quote Originally Posted by alex_ro View Post
    You say:
    Quote Originally Posted by SpeedFreek View Post
    The key thing to remember is that the grid expands with the space.
    If that's the case (and the proportion is kept), then we can't detect expansion. At all.
    We can measure the separation between the intersection points. That separation is what is increasing, and that is how we measure the expansion. Our rulers DO NOT expand with the universe.

    Quote Originally Posted by alex_ro View Post
    Then, in case of decelerating expansion, you say:
    Quote Originally Posted by SpeedFreek View Post
    The Hubble constant is decreasing
    Which means it's not a constant.
    Correct. The Hubble constant has NEVER been constant.

    I understand this stuff pretty much completely (it is my area). Sorry I couldn't help you understand it too. But just because you don't understand it, that doesn't mean it is wrong. It isn't wrong.

    Bye.
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    A . B .. C ... D .... E ..... FFor F to see the same thing would mean:F . E .. D ... C .... B ..... Awhich is a clear contradiction.
    There is no contradiction. It's a scaling factor, which means it works both ways. When you see '. . . . . F' that is the distance from A to F, not the the distance between E and F.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by alex_ro View Post
    @StrangeWell, again, you ignore ACCELERATED expansion.
    Yes. That seems an unnecessary complication if you can't even get your head around the idea of uniform expansion. All acceleration means is that the rate of expansion is getting faster. (You have to flip increasingly quickly though the pages of graph paper you have drawn dots on - you have done that, haven't you?)


    That's the problem, right there. While the distance between the 2 observers will obviously be the same for the 2 observers (whether the first looks at the second one, or vice versa), points along the route between the 2 observers will not show the same thing. If one sees acceleration, the other must see, in my view, deceleration.A farther point from first observer will move away faster from that observer, and thus the second observer cannot see the same thing:this is what A sees:A . B .. C ... D .... E ..... FFor F to see the same thing would mean:F . E .. D ... C .... B ..... Awhich is a clear contradiction.F will only see what A sees in continuation of the line uniting them:F . G .. H ...
    The problem is simply your visualisation of this. If you can't imagine it then the only thing to do is draw several stages of expansion (in 1D or 2D) and measure (with a ruler) how the distance between pairs of points differs over time. Whichever two points you choose you will find that their separation will increase at each time step. The rate at which their separation increases will be proportional to their distance.

    I can't help you understand this. Only you can. Do the experiment (this is a science forum, after all).

    Which means it wasn't enough to form the stars (prior to DE) either. Unless there's yet another mysterious force out there (besides dark energy), changing freely energy into matter (prior to DE) and matter into energy (currently), and moreover "storing" them: matter to form stars, energy for DE to become prominent.
    Dark energy isn't relevant to that. The early universe was not perfectly homogeneous. There were small variations in density. Gravity made the denser bits get denser.
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    Quote Originally Posted by alex_ro View Post
    More, the article contradicts itself: once says "9 billion years ago", then says "the expansion rate of the cosmos began speeding up about five to six billion years ago".Since the ONLY evidence for dark energy is accelerated expansion, then obviously the 2 figures should agree with each other.Anyway, why would both articles call dark energy "cosmic jerk"? I mean, they call the dominant force in the universe "cosmic jerk"?! If that's 'jerk', what exactly can the rest of the universe be? Including you and me, by the way...
    The rate of expansion is proportional to distance, so locally, there is always negligible expansion, which means that locally, gravity can hold things together. Whilst the rate of expansion was decelerating at first (due to the gravity between distant things gradually slowing the expansion), the effects of gravity fall off with distance. Whilst dark energy started to have an affect on the deceleration 9 billion years ago, the dark energy only managed to completely overcome the gravity between highly separated galaxies and turn the deceleration into an acceleration around 5 or 6 billion years ago, once the expansion had put enough distance between those galaxies. The "jerk" is that change from deceleration into acceleration.

    Simple.
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    but so far, that' only counts for the observable universe.
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    We have no reason to assume the observable universe is any different from any other part of the universe. So it counts for the whole universe, assuming the cosmological principle.
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    Quote Originally Posted by SpeedFreek View Post
    My only error was trying to explain it to you in easy terms. You are the first person I have come across who hasn't understood my toy model explanation. Well done!
    I don't think you explained anything. Certainly not what I was asking. You only claimed things.
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    Quote Originally Posted by SpeedFreek View Post
    My only error was trying to explain it to you in easy terms. You are the first person I have come across who hasn't understood my toy model explanation. Well done!
    Well, it wasn't me saying that, it was you. Read what you wrote previously.
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    Quote Originally Posted by SpeedFreek View Post
    Our rulers DO NOT expand with the universe.
    Of course they do. Aren't any candles you use for distant stars/galaxies, already farther away from Earth?
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    Quote Originally Posted by SpeedFreek View Post
    The Hubble constant has NEVER been constant.
    I am aware of the constant adjustment (last time in sept. last year, if I remember correctly). But anyway, why do they call it "constant" if it's not constant? Just a thought. Even more puzzling when you remember the direct connection to the age of the universe...
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    Quote Originally Posted by SpeedFreek View Post
    I understand this stuff pretty much completely (it is my area). Sorry I couldn't help you understand it too. But just because you don't understand it, that doesn't mean it is wrong. It isn't wrong.
    And what exactly ISN'T wrong? By the way, if it is your area of expertise, then you should have no problems demonstrating preservation of isotropy rergardless of how many observers and where they are placed. I haven't seen that so far.
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    Quote Originally Posted by AlexG View Post
    There is no contradiction. It's a scaling factor, which means it works both ways. When you see '. . . . . F' that is the distance from A to F, not the the distance between E and F.
    No offense, but I don't think you're making any sense.
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    @Strange: Well, before the grid (any grid), we should settle on the linear example, shouldn't we? So, how is my example (A...F) wrong?
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    Quote Originally Posted by SpeedFreek View Post
    The rate of expansion is proportional to distance
    Precissely my point. So my example A...F is valid, and therefore what observer F sees (along the way to A) is very different from what A sees.
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    Quote Originally Posted by SpeedFreek View Post
    The "jerk" is that change from deceleration into acceleration.
    You missed my point. I didn't ask what "jerk" refers to, I asked WHY did they call it "jerk".
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    Quote Originally Posted by SpeedFreek View Post
    Simple.
    Actually, it's not simple at all. Your entire paragraph it's only a series of claims postulated to keep formal cosmology intact in your mind. For none of those things you actually presented any evidence. While mainstreamers are desperately trying to figure in dark energy to avoid contradictions, you simply make some claims and hope that it will go away. They are so desperate in fact, that even best cosmologists in the world, for example George Ellis, lately are focusing more on the micro (quantum) and less on the macro.So, sorry, things are not at all simple. Far from it...
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    Your usage of the term "mainstreamers" says it all really. I am wasting my time here. You have made up your mind already, based on your own incomplete understanding. Hence my tone.
    Last edited by SpeedFreek; February 22nd, 2013 at 03:45 PM.
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    Quote Originally Posted by alex_ro View Post
    Quote Originally Posted by SpeedFreek View Post
    The Hubble constant has NEVER been constant.
    I am aware of the constant adjustment (last time in sept. last year, if I remember correctly). But anyway, why do they call it "constant" if it's not constant? Just a thought. Even more puzzling when you remember the direct connection to the age of the universe...
    You obviously do not understand that the Hubble constant is an average of the whole history of the expansion, which is why it relates to the age of the universe, rather than it being the expansion rate "now".

    It is what the rate of expansion would have been all along if the expansion were constant. But the expansion is not constant. The end result, of course, is the same.
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    Quote Originally Posted by alex_ro View Post
    Quote Originally Posted by SpeedFreek View Post
    The "jerk" is that change from deceleration into acceleration.
    You missed my point. I didn't ask what "jerk" refers to, I asked WHY did they call it "jerk".
    That is the technical term for a change in the nature of an acceleration.

    http://en.wikipedia.org/wiki/Jerk_(physics)
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    Quote Originally Posted by alex_ro View Post
    Quote Originally Posted by SpeedFreek View Post
    Our rulers DO NOT expand with the universe.
    Of course they do. Aren't any candles you use for distant stars/galaxies, already farther away from Earth?
    Yes, they are farther away, as measured by a ruler that does not expand with the universe.

    You should do your homework before you argue against something you obviously don't understand properly.

    (These subsequent replies are for other people who are interested in how we came to these conclusions. You obviously aren't.)
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    Quote Originally Posted by alex_ro View Post
    Quote Originally Posted by SpeedFreek View Post
    The rate of expansion is proportional to distance
    Precissely my point. So my example A...F is valid, and therefore what observer F sees (along the way to A) is very different from what A sees.
    This is patent nonsense. Observer F sees the same relationship, but in reverse.

    A..B...C....D.....E........F

    F..E...D....C.....B........A

    This is cosmology 101 folks.
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  35. #34  
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    Quote Originally Posted by alex_ro View Post
    @Strange: Well, before the grid (any grid), we should settle on the linear example, shouldn't we? So, how is my example (A...F) wrong?
    What this example:
    A . B .. C ... D .... E ..... F

    You points are not equally space. Is this supposed to show that more space has been added between some points than others? If so, that is what is wrong.

    Let me try one more time

    T=0: ABCDEFGHIJKLMNOPQRSTUVWXYZ
    T=1: A.B.C.D.E.F.G.H.I.J.K.L.M.N.O.P.Q.R.S.T.U.V.W.X.Y. Z
    T=2: A..B..C..D..E..F..G..H..I..J..K..L..M..N..O..P..Q. .R..S..T..U..V..W..X..Y..Z
    T=3: A...B...C...D...E...F...G...H...I...J...K...L...M. ..N...O...P...Q...R...S...T...U...V...W...X...Y... Z

    The space between adjacent points increases by 1 at each step.

    Look at the distance A to B at each time step.
    Now look at the distance M to N, and M to L at each time step.

    See some similarities?

    Compare the distance from A to M at each time step with distance from M to A and M to Z at each time step.

    See some similarities?

    But it is already obvious you are not "just asking questions" and I don't really care if you are more interested in remaining ignorant than actually learning. That is your choice.
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    Yes, it should be pointed out that the fact that the further away something is the faster it recedes is NOT an indicator of accelerating expansion. I should have emphasised that earlier. With all forms of expansion, be it decelerating, constant, accelerating, or exponential (and exponential expansion is the only case where the Hubble constant actually remains constant, as I described in post #8), the further away something is, the faster it recedes.

    Strange's illustration of expansion is more useful here, for sure.

    If galaxies were equally spaced, they would remain equally spaced, but the spaces between them get larger. That is the overall picture of expansion.

    However, our view of the universe cannot see things as they are at any one time - our "past light cone" cuts through different epochs with differing expansion rates.

    So whilst we might see a view like I showed in post #33 in a universe that has decelerated more than it accelerated, when we calculate the overall picture we find the expansion to act as Strange illustrated.
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    Quote Originally Posted by alex_ro View Post
    Actually, it's not simple at all. Your entire paragraph it's only a series of claims postulated to keep formal cosmology intact in your mind.
    It is not a series of claims, it is an attempt to explain something quite simple to some who is (or is pretending to be) too stupid to understand. Or maybe it is some weird religious thing. I don't know and I don't care. You are obviously not interested in science. Just pretending it doesn't make sense.

    For none of those things you actually presented any evidence.
    You haven't shown any ability understand a simple explanation so why should you care about evidence. There is, of course, loads of eidence. But I assume you will just dismiss it as not making sense because it doesn't fit your prejudices.
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    Quote Originally Posted by Strange View Post

    What this example:
    A . B .. C ... D .... E ..... F

    You points are not equally space. Is this supposed to show that more space has been added between some points than others? If so, that is what is wrong.
    What might be confusing things here is the difference between considering comoving points, equally spaced, as in your above illustration (which is an overview of the situation, calculated from what we see, and what I was describing in my toy model) and considering what we actually see, which is different galaxies at different times and scale factors, from which we have to make that calculation.

    Consider the particle horizon as point F. Between A and F, the universe has scaled up by a factor of ~1100 since the light from F was emitted. If we consider point E as the most distant galaxy we have seen, then the universe has only scaled up by a factor of ~8.

    If we consider ourselves as A, then from the point of view of an observer at F, they only see the CMB coming from A, emitted 13.7 Gyr ago, and to them the universe has scaled up by a factor of ~1100 since, whereas to F the universe has scaled up only 8 times since the light left B.

    So, A sees F as F sees A.
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    Quote Originally Posted by SpeedFreek View Post
    What might be confusing things here is the difference between considering comoving points, equally spaced, as in your above illustration (which is an overview of the situation, calculated from what we see, and what I was describing in my toy model) and considering what we actually see, which is different galaxies at different times and scale factors, from which we have to make that calculation.
    Indeed. And what I was trying to show (initially) was, basically, the reverse. IF all adjacent points were moving apart at the same rate then the results would match what we see.

    But, ultimately, all I wanted to get across was the more abstract point that if space between points increases steadily, it will maintain the original homogeneity. Even if you forget all about the universe.

    For whatever reason, alex_ro has decided that modern cosmology is wrong. And he is so wedded to this idea that even simple geometry on paper, which has nothing to do with cosmology, has to go out the window. How daft is that!
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    Quote Originally Posted by Strange View Post
    How daft is that!
    Yup.

    Don't you just love how these people come along, arguing that our cosmology makes no sense to them and ascribing a collective name to people who do understand it ("mainstreamers" or another one is "Big-Bangers") as if there is a choice of sides, like a battle to be fought.

    What they do not understand is that they do not understand. The overwhelming observational evidence, accumulated over the past 80 years or so, is what has led science to these conclusions. What they don't get is that calling us "mainstreamers" in a obviously pejorative sense is like using "people who adhere to the scientific method" as a form of insult! How daft is that!

    I am willing to predict that any follow up on behalf of the OP will now include terms like "closed-minded", "sheeple", "brainwashed" or "self appointed defenders of the orthodoxy".
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    And now, in the interests of fairness and balance:

    Quote Originally Posted by alex_ro View Post
    Could anyone please explain to me how can an accelerated expansion take place in a homogenous & isotropic universe?I really can't figure that out - at all.
    For a pretty thorough explanation, I might recommend the following book:

    Relativistic Cosmology - Academic and Professional Books - Cambridge University Press

    Relativistic Cosmology - George Ellis, Roy Maartens and Malcolm MacAllum.

    Cosmology has been transformed by dramatic progress in high-precision observations and theoretical modelling. This book surveys key developments and open issues for graduate students and researchers. Using a relativistic geometric approach, it focuses on the general concepts and relations that underpin the standard model of the Universe. Part I covers foundations of relativistic cosmology whilst Part II develops the dynamical and observational relations for all models of the Universe based on general relativity. Part III focuses on the standard model of cosmology, including inflation, dark matter, dark energy, perturbation theory, the cosmic microwave background, structure formation and gravitational lensing. It also examines modified gravity and inhomogeneity as possible alternatives to dark energy. Anisotropic and inhomogeneous models are described in Part IV, and Part V reviews deeper issues, such as quantum cosmology, the start of the universe and the multiverse proposal.
    Here is an excerpt - http://www.cambridge.org/servlet/fil...54_excerpt.pdf

    Quote Originally Posted by alex_ro View Post
    While mainstreamers are desperately trying to figure in dark energy to avoid contradictions, you simply make some claims and hope that it will go away. They are so desperate in fact, that even best cosmologists in the world, for example George Ellis, lately are focusing more on the micro (quantum) and less on the macro. So, sorry, things are not at all simple. Far from it...
    You are completely misrepresenting the situation. There is no "desperation" at all. The acceleration of the expansion is in fact necessary in order to explain our observations, if the universe is homogeneous and isotropic - a fact you obviously do not understand as stated in your OP and a fact that is explained in great detail in the above volume. Acceleration is not introduced to "avoid contradictions" (here you show your bias - dark energy is just the placeholder for that which causes an acceleration), it is the predicted result of our observations, based on the cosmological principle. Now George Ellis is a specialist in modelling inhomogeneous and anisotropic universes and discusses the possibility that what appears to be cosmic acceleration may in fact be a detection of spatial inhomogeneity, after explaining how we came to the conclusion that the expansion is now accelerating if our universe is homogeneous and isotropic. There is no desperation or "lately focussing" on the micro. Good science involves considering all the possibilities. It is like you only want to focus on half the story.
    Last edited by SpeedFreek; February 22nd, 2013 at 06:48 PM.
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    Quote Originally Posted by alex_ro View Post
    Quote Originally Posted by AlexG View Post
    There is no contradiction. It's a scaling factor, which means it works both ways. When you see '. . . . . F' that is the distance from A to F, not the the distance between E and F.
    No offense, but I don't think you're making any sense.
    You're a trolling crank, as well demonstrated over on scienceforums.net. No offense of course.
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