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Thread: Expansion rate: Is this right??

  1. #1
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    Expansion rate: Is this right??

    I've been trying to get my head around the rate at which the universe is currently expanding and have been trying to put a number on it in order to try and visualise how fast this is happening.

    OK so the rate that is now quoted is around 70megaparsecs/sec/sec.

    If a parsec is (roughly) 31million,million kms then a megaparsec is 31million.million.million kms or 31000000000000000000kms
    So x 70 = 21700000000000000000000kms
    How far is this in light years???

    So if I were able to conduct an experiment in the complete abscence of any gravitational forces and hold two tennis balls, one in each hand, and let one go, then in 1second they the space between them will have expanded 21700000000000000000000kms. Thus placing the free tennis ball roughly..where??
    In 2 seconds, double that and in 3 seconds, quadruple it etc.

    Can somebody please check me on this..it sounds way off the mark but I can't think of another way to try and visualise it.
    Thanks in advance.

  2. #2
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    I think it's roughly 70 (kilometers per second ) per Megaparsec.

  3. 2009-Oct-02, 07:54 AM
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    Author requested the post withdrawn from mod queue due to progress of discussion.

  4. #3
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    Quote Originally Posted by agingjb
    I think it's roughly 70 (kilometers per second ) per Megaparsec.
    Yup, that's true. The current best value is 74.2 3.6 (km/s)/Mpc.

  5. #4
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    Lets take a nice round number like 70 km/sec per Megaparsec.

    One parsec (pc) is 3.086 x 10^16 meters.

    So our expansion rate is:

    70,000 m/s per 3.086 x 10^22 meters.

    If the tennis balls start off 1 metre apart then their rate of separation would be:

    (70,000)/(3.086 x 10^22) m/s

    = 2.268 x 10^-18 m/s
    = 7.158 x 10^-11 m/yr
    = 7.159 cm per Gy

    (Would anyone like to check that calc?)

  6. #5
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    Mark = Not a Mathematician.

    Quote Originally Posted by DrWho View Post
    Yup, that's true. The current best value is 74.2 3.6 (km/s)/Mpc.
    I think the way this number is expressed is of some concern to me. I am not correcting the numbers. I except them as true.
    Is it a accelerating rate and, if so should it not be 74.2 + or -3.6 km / sec / sec ... or Mpc / Mps.
    Could this please be clarified ?

    Regardless of this trivial point above, No your tennis balls are not getting further apart.
    Only those objects that are NOT gravitationally bound are being seen as accelerating away. Your tennis balls as part of a gravity field we call planet Earth. The Solar system and this whole Galaxy. Are not expanding at this rate as defined. Go to that image etched forever in our minds of the Hubble Ultra Deep Field... They are leaving us behind so quickly the light from some will no longer reach us.
    .
    Thank you TonyE... thats what I wanted to see.
    Last edited by astromark; 2009-Oct-02 at 08:51 AM. Reason: edit last line in...

  7. #6
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    Doesn't the expansion imply that gravity has some boundary?

    I've heard that if you were to take two tennis balls on either side of the universe and let them go, providing nothing gets in their way, they would eventually collide with each other.

    Since the expansion happens and there is matter all over, doesn't this imply that there is ultimate boundary to any gravity?

    Also doesn't the expansion imply some sort of drag factor on space? If space was just expanding and there was no drag and there was no ultimate boundary to gravity wouldn't we see all galaxies rushing in on themselves and not rushing out?

  8. #7
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    There is no 'boundary' to gravity, it extends to infinity. However, its strength does fall off rapidly with distance, so its effect between distant objects is negligible and expansion can dominate.

  9. #8
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    Quote Originally Posted by astromark View Post
    I think the way this number is expressed is of some concern to me. I am not correcting the numbers. I except them as true.
    Is it a accelerating rate and, if so should it not be 74.2 + or -3.6 km / sec / sec ... or Mpc / Mps.
    Could this please be clarified ?
    While evidence does suggest that the universe is expanding at an accelerating rate, the Hubble constant still represents the current rate of expansion. (Km/s/Mpc)

    I don't know at what rate the expansion is accelerating but I would guess it's much smaller than 74 km/s^2/Mpc.
    Last edited by JimmyRoberts; 2009-Oct-02 at 10:49 AM. Reason: Changed 74 m/s^2 > 74 km/s^2

  10. #9
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    Quote Originally Posted by astromark View Post
    I think the way this number is expressed is of some concern to me. I am not correcting the numbers. I except them as true.
    Is it a accelerating rate and, if so should it not be 74.2 + or -3.6 km / sec / sec ... or Mpc / Mps.
    Could this please be clarified ?
    In SI units H is 2.2x10^-18 s^-1
    Last edited by Noble Ox; 2009-Oct-02 at 11:04 AM. Reason: put end quote in

  11. #10
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    Quote Originally Posted by TonyE View Post
    If the tennis balls start off 1 metre apart then their rate of separation would be:

    (70,000)/(3.086 x 10^22) m/s

    = 2.268 x 10^-18 m/s
    = 7.158 x 10^-11 m/yr
    = 7.159 cm per Gy

    (Would anyone like to check that calc?)
    That looks right:
    70 km/s/Mpc * 1 m = 2.268e-18 m/s

  12. #11
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    Quote Originally Posted by DrWho View Post
    Yup, that's true. The current best value is 74.2 3.6 (km/s)/Mpc.
    I think there is some confusion in this thread. Currently, for two things that are one megaparsec away they are expanding away at a rate of about 74 km/s. Two things that are two megaparsecs away are expanding away at a rate of about 148 km/s. Two things that are three megaparsecs away are expanding away at a rate of about 222 km/s. The expansion is faster the further away one is.

    The acceleration of this expansion is not included in the above. I actually haven't sat down to figure it out, but it's not dependent on distance and it takes a fair bit of time to see significant acceleration, on the order of a million years, I think.

  13. #12
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    Quote Originally Posted by TonyE View Post
    Lets take a nice round number like 70 km/sec per Megaparsec.

    One parsec (pc) is 3.086 x 10^16 meters.

    So our expansion rate is:

    70,000 m/s per 3.086 x 10^22 meters.

    If the tennis balls start off 1 metre apart then their rate of separation would be:

    (70,000)/(3.086 x 10^22) m/s

    = 2.268 x 10^-18 m/s
    = 7.158 x 10^-11 m/yr
    = 7.159 cm per Gy

    (Would anyone like to check that calc?)
    This is true for the separation speed at 1 meter but after 1Gy, the separation of the balls would have increase a little bit more than 7.159 cm (neglecting gravitation). This is because the speed of separation increases with distance.

    Neglecting the effects of gravity and of the dark energy and taking the rate of expansion (H) as a constant, we have: v = H*s where v is the velocity at which 2 objects separated by a distance (s) are taking away from each other. Therefore, v = ds/dt = Hs. After integration this yields to s = s0*exp(H*t) where s0 is the original distance separating the 2 objects and t is the time.

    So if you take H = (70,000 m/s per 3.086 x 10^22 meters), s0 = 1 meter and t = 1Gy, you find that the increase in separation is (s s0) = 7.421 cm.

  14. #13
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    Quote Originally Posted by mungoid View Post
    How far is this in light years???

    A megaparsec is 3,262,000 light years. So, if you dropped your tennis balls 3.2 million light years apart, they would recede from each other at around 70 kilometres a second.

  15. #14
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    Quote Originally Posted by astromark View Post

    Only those objects that are NOT gravitationally bound are being seen as accelerating away. Your tennis balls as part of a gravity field we call planet Earth. The Solar system and this whole Galaxy. Are not expanding at this rate as defined.
    Ah yes. that's why I specified
    So if I were able to conduct an experiment in the complete abscence of any gravitational forces
    Thank you all!!
    This now makes a lot more sense to me.
    So to clarify...
    If I start with the tennis balls placed together (no gravity involved) and let one go then I wouldn't see them instanly fly apart or indeed see any appreciable dislocation in my lifetime. Correct?
    However, if they were separated by 3.2million light years when released, and I was standing next to one of them (and somehow I was able to remain stationary relative to the system) then, in 1 second, my tennis ball would be 70km away.
    Sound about right now??

  16. #15
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    Quote Originally Posted by mungoid View Post
    However, if they were separated by 3.2million light years when released, and I was standing next to one of them (and somehow I was able to remain stationary relative to the system) then, in 1 second, my tennis ball would be 70km away.
    Sound about right now??
    When you say you are somehow able to remain "stationary relative to the system" in the scenario you describe, you are really just remaining stationary relative to the distant ball, so that the ball next to you moves away from you.

    In order to remain stationary relative to the distant ball you would have to move towards it at 70 km/s, so the ball next to you would move away from you at 70km/s!

    Of course, it is a lot easier to just remain stationary relative to the ball next to you and let the distant ball recede at 70 km/s due to the expansion of the universe.

  17. #16
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    If you place the two balls 3.2 million light-years apart, stationary relative
    to each other, they will probably stay stationary relative to each other.
    It would be very interesting though, if they start moving apart.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  18. #17
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    Not all things or parts of this universe are receding away from us. This getting away from local effects of gravity is not possible. Whole clusters of galaxies are inexplicably bound in the dance of the spiraling cosmos... As Jeff has eluded to... your balls are not drifting away and might never do so. That 70 km/s /sec / is a fiction locally. Yes I saw that you asked if we could ignore that detail. No, we can not. Your tennis balls would need to be positioned at a far greater distance from us in order for any expansion rate movement to be detectable. That expansion rate you seek is anywhere between 0 and light speed. The 70 is the constant of averages that might never be found.

  19. 2009-Oct-04, 10:56 AM

  20. #18
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    What is 70 km/s /sec exactly?

    ETA: There is nothing wrong with using simplified cases in order to gain a better understanding of the principles in question. For example, ignoring air resistance when learning about free-falling objects. When one learns the basic principles, then the more complex details can be addressed.

    I'm pretty sure most in this thread understand things wouldn't work like this in practice.
    Last edited by JimmyRoberts; 2009-Oct-04 at 05:02 PM.

  21. #19
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    Orphu,

    The figure "70 km/s /sec" is a mistake. Mark should have said "70 km/s/Mpc".
    Two things a megaparsec apart are moving apart at about 70 km/s. For each
    additional megaparsec separation, there is an additional 70 km/s speed.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  22. #20
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    My point in post #16 was that two objects which are initially motionless
    relative to each other, may or may not ever start moving away from each
    other, nomatter how far apart they are. Before the acceleration of the
    expansion was discovered in 1998, I would have said that the objects
    definitely would not start moving apart. With the acceleration, though, it
    is much more likely that they will start moving apart. But since the cause
    of the acceleration isn't known, it isn't yet certain what would happen.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  23. #21
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    Thanks, Jeff. I understand your point.
    With the acceleration, though, it
    is much more likely that they will start moving apart. But since the cause
    of the acceleration isn't known, it isn't yet certain what would happen.
    If they did start moving apart, my guess would be that it would take quite a bit of time before there was any appreciable distance between them.

  24. #22
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    Yes, yes... I did mean to say 70 km/sec/Mps..., and no I have no idea what or if these tennis balls would move at all., and as to why or what is driving this movment...I have no idea. Lets call it 'Dark Energy' ?
    Last edited by astromark; 2009-Oct-04 at 07:48 PM. Reason: edit:)

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