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Clearing up my Cosmological Confusion - what is expanding?

  1. Feb 12, 2016 #1


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    As a layman, I have come to understand basic expansion (meaning without dark energy) as objects which are not locally gravitationally bound are moving away from each other due to some inertia created from an initial period of inflation. This rate of recession is the Hubble constant and it changes over time as the universe has changed from being radiation / matter dominated and the energy density has diluted over time with expansion.

    However, there are no ‘forces’ acting upon these objects causing them to recede and space is not a physical thing that expands causing this recession.

    There is however also Dark energy, which can be modelled as a force which causes this rate of expansion to accelerate. Dark energy being like a scalar field which means it’s energy density doesn’t dilute with expansion. So as these objects recede, there are greater distances between them and hence more dark energy between them, so more ‘force’ and this is what causes the rate of expansion to accelerate at greater distances.

    However, I also read that a lot that ‘space’ itself is expanding and this expanding space has physical consequences. The most frequent references to this are:

    1. cosmological redshift, which explains that the wave length of a photon is ‘stretched’ due to space expanding.

    2. Recession velocities are allowed to be >c because recession is not ‘proper motion’ it is caused by space between objects growing.

    3. Light from distant galaxies which are receding from us at speeds >c will never reach us as the space between them and us is expanding at a rate greater than c.

    As a counter to those there points I have come to understand the following:

    As regards to a), cosmological redshift is not from ‘space expanding’ but more from of the system consisting of the emitter, the photon, and the receiver. These posts here and here by Peter in a different thread were very useful for me.

    As regards b) I started a thread here where I am trying to understand more about how GR allows distant objects to recede >c. Not because of space between them expanding but because of something to do with the curvature of spacetime over large distances allows for recession speeds to be great than c. (Still trying to understand this)

    As regards c) I have read that light from galaxies moving away from us > will still reach us one day. Which makes sense if there is nothing ‘growing’ in between.

    So I’d be very grateful for any help to clarify these points and correct my understanding!
  2. jcsd
  3. Feb 12, 2016 #2


    Staff: Mentor

    It looks to me like there are already threads clarifying a) and b), so I'll focus here on c).

    It depends. The term for the distance at which an object is "moving away from us at c" (I put this in quotes to emphasize all the caveats to that interpretation, which I have talked about in other threads and won't rehash here) is the "Hubble distance". Now consider a galaxy which, right now, is further away from us than the Hubble distance, and emits a light pulse towards us. The "distance" (again, I put this in quotes to emphasize that it's not a distance we directly measure, it's a coordinate distance in a particular chart) of this light pulse from us will actually be increasing with time (but more slowly than the distance of the emitting galaxy from us), so initially it doesn't seem like it will ever reach us.

    However, the Hubble distance itself is also increasing with time, which means that it is possible that at some point, the distance of the light pulse from us, which starts out being greater than the Hubble distance, will become less than the Hubble distance. (This can happen as long as the Hubble distance is increasing faster than the distance of the light pulse from us.) Once the light pulse is closer to us than the Hubble distance, its distance from us will be decreasing with time, so it will eventually reach us, even though the galaxy that emitted it is further from us than the Hubble distance (even at that future time), and always will be.

    This does not mean that light from all galaxies will eventually reach us, no matter how far away they are. According to our best current model, the expansion of the universe is accelerating, and that means the Hubble distance will not increase without bound; rather, it will asymptotically approach a particular finite maximum value, which is determined by the density of dark energy. (If there were no dark energy, the Hubble distance would increase without bound, assuming the universe kept on expanding forever.) This means that there are galaxies that are far enough away from us that the expansion of the universe will always keep the light they emit in our direction beyond the Hubble distance, so it will never be able to reach us. (At a particular instant of time, the distance from us beyond which galaxies are unable to emit light that will ever reach us is called the "cosmological event horizon" distance. It is further away from us than the Hubble distance, but the difference between them gets smaller over time.)

    The following article has a discussion of the above that includes some useful diagrams:

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