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Galaxy separation faster than the speed of light

  1. Feb 5, 2014 #1
    If two galaxies are separated by a large distance such that radiation transmitted from the mid-point between the two galaxies never reaches either galaxy due to the expansion of space, how can the expansion of space at the mid-point ever affect the distance between the two galaxies. i.e. how does the information that space has expanded at the mid-point reach the two galaxies?

    I have a hard time picturing what the expansion of space means. I'm trying to picture little boxes of space pushing on each other but it doesn't seem to work because of having a finite propagation speed.

    I'll try asking my question another way. Suppose there's two galaxies 100 million light years apart. I don't know what the expansion rate is but suppose that in one second, the distance between the two galaxies increases by one kilometer. Is there any delay between the expansion of space and the reduction in the gravitational force from one galaxy to the other? Is there any delay between the expansion of space and the increase in distance between the two galaxies?
  2. jcsd
  3. Feb 5, 2014 #2
    You have some good questions, unfortunately you also have a few misconceptions about expansion.

    the first one I will clear up is that when scientists refer to expansion increasing faster than the speed of light, they are being inaccurate. In actuality they are referring to the recessive velocity of an extremely far away stellar object as being faster. Nearer to the edges of the observable universe its slightly higher than 3c. The key misunderstanding is not knowing that they are referring to a distance dependent value. Its rate of expansion is the same as it is everywhere else not gravitationally bound. Per a given volume it is extremely small. The volume is simply greater as time goes on.

    Hubble’s Law: The greater the distance of measurement the greater the recessive velocity

    Velocity = H0 × distance.

    the second is the nature of term space. Most everyone who first looks at expansion try to apply a tangible nature to space. Something that grows or stretches, in reality its simply put, the amount of volume available. Other things such as interstellar medium simply fill up that volume. Thinking of it as a void is also incorrect as a void is often used to descriptive the lowest energy state per a given space.(never applied to describe outside of the universe) The amount of volume that is available in the universe can be influenced by several factors. the cosmological constant, inflation, gravity and universe geometry.

    gravity decreases the volume available the cosmological constant and inflation causes an increase in volume. Universe geometry is more complicated as it can do either.

    these points are better explained in these two articles

    Expansion and redshift

    Universe geometry
  4. Feb 5, 2014 #3


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    photonkid, in addition to what Mordred said, I'd recommend that you google "metric expansion"
  5. Feb 5, 2014 #4


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    Changes in gravitational force are represented by changes in geometry. What is called the gravitational field is a function defining the shape, curvature, geometry of 4d world.
    That field has a kind of momentum. Once expansion gets started it will tend to continue although other stuff can gradually slow or speed up.

    Disturbances of the field can only travel at the speed c.

    A little " box" or region of space does not know what is happening this moment to another patch of space a million lightyears away. you are right about that.
    But it does know what has happened to IT in its past, and to the geometry of its immediate surroundings as the news continues to arrive. So large-scale patterns of wholesale distance expansion can get started and persist.

    If two galaxies A and B are a Mly (million ly) apart and A does something unexpected like burp in such a way that should affect the geometry around B i.e. the "gravitational force" on B, then the effect of that will not be felt at B until a million years have passed. The disturbance of the curvature of the geometry has to gradually spread out like ripples on a pond.

    Keep in mind that all I can be telling you about is the model of changing geometry that is summarized in the GR equation. We don't know the nuts and bolts underlying that equation.
    We don't know what geometry is "made of" or WHY it can expand contract ripple. We don't know the mechanism of its interaction with matter. We only know that this interaction is well described by the GR equation (whose LHS is geometry and the change in it and whose RHS is matter and the flow of matter). It relates matter with geometry but it does not tell us the mechanism that hooks them together.

    So I am just telling you what the equation model says, not how the world "is". As far as I know you are absolutely right that info does not travel faster than c. And that goes for the gravitational field.

    BTW the current expansion rate for distances is about 1/144 of one percent per million years.
    So just a check, if the distance between A and B is now 144 Mly then by how much will it increase over the next million years? :biggrin: No need to answer, this is not school, but it helps to be a little quantitative now and then.
    Last edited: Feb 5, 2014
  6. Feb 8, 2014 #5
    Hubble constant is 73.8 km/sec/Mpc (give or take 2.4 km/sec/Mpc including, both random and systematic errors)
  7. Feb 8, 2014 #6


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    Latest I've seen was Planck mission's March 2013 data release
    http://en.wikipedia.org/wiki/Planck_(spacecraft [Broken])
    68% limits 67.80±0.77

    If you just take Planck mission alone the best fit is more like 67.1 but if you combine with other recent studies it comes up slightly to around 67.8 km/s per Mpc

    If you convert this to a percentage growth rate it amounts to around 1/144 of a percent distance growth per million years.

    That is the figure that the Lightcone calculator uses unless you input some other Hubble radius by hand
    Last edited by a moderator: May 6, 2017
  8. Feb 8, 2014 #7
    i just googled it and used what it said don't remember exactly where i got that from. Leonard Susskind, in a lecture said it was 42 km / second / 3 million light years i'm not sure what that works out to be in Mpc
  9. Feb 8, 2014 #8


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    Conceivably, if it was 2012 or earlier, he could have said 72 km/s per Mpc. That was the OLD figure sometimes quoted before the Planck mission results were announced. Planck results supersede the old WMAP numbers.

    One Mpc is about 3.26 million lightyears. So if he was talking informally or to beginning students/layfolks he could even have EQUATED 3 million ly with a megaparsec. It sounds less technical and off-putting. Just a more listener-friendly name for roughly the same distance.

    It is not very likely that he said "42 km / second / 3 million light years."

    Anyway the new standard figure we are trying to get in line with is based on the Planck mission (combined with various other very recent studies that I mentioned) and that's more like 67.8. The new Hubble parameter is somewhat lower than the old WMAP value which like you say was around 72 plus or minus.
  10. Feb 8, 2014 #9


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    Thing one about the Hubble parameter, a basic thing to grasp about it, is how to calculate the Hubble time

    that is 1/H in other words ONE OVER THE HUBBLE PARAMETER.

    And the Hubble radius is just the HUBBLE TIME MULTIPLIED BY c. in other words c/H.

    So if you know the Hubble time in years, you just change years to lightyears, and that tells you the Hubble radius (which is an important distance scale, it is the distance which is currently expanding at the speed of light.)

    So if someone says that 1/H, the Hubble time, is 14.4 billion years
    then you know immediately that c/H, the Hubble distance, is 14.4 billion light years.

    Try pasting this into the Google search window, without the quote marks:

    "1/(67.9 km/s per Mpc)" The search window works as an intelligent calculator. Or if you like an old value like 72, put that in. You will get an older value for the Hubble time. It calculates what you tell it to calculate, if you say it in terms the Google calculator understands.
  11. Feb 8, 2014 #10
    I do believe it was a freshman classical mechanics lecture from a few years ago. Thanks
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