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I Galaxies moving away at the speed of light?

  1. Aug 1, 2017 #1
    I have as advised done some catching up on basic cosmology, looking at various discussion and lectures on the nature of the universe.
    Something that has always played on my mind, is that i remember reading and watching a documentary about the potential to achieve the velocity of light.
    It was discussed that to accelerate an object to the speed of light would require infinite energy, as the object would exponentially gain mass. it is my understanding that photons are massless so there is no issue with lights ability, but when it comes to galaxies it is more confusing. is the acceleration due to the kinetic energy of the initial bang( if thats the model) or is it due the material and it's density pushing them away at that velocity?
    And if we are not at the centre of the universe are we not moving away from someone else's galaxy at the speed of light?
  2. jcsd
  3. Aug 1, 2017 #2
    Where do you see galaxies accelerating at the speed of light? My understanding is two very distant galaxies move away from each other at greater than the speed of light if they are beyond each other's cosmological horizon, but this is due to the expansion of space and not acceleration of the galaxies themselves.
  4. Aug 1, 2017 #3
    does that mean beyonds lights ability to reach them and is that an issue concerning distance ? and is the expansion of space at the velocity >light ? and does that mean we are static on a balloon surface that is expanding? i am confused !!!!!!
  5. Aug 1, 2017 #4


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    It's best not to think of recession velocities as regular velocities.
    The former have no upper limit. The limit on the latter works only locally, because in the expanding universe - which is a general relativistic effect - special relativity applies only locally where space-time is approximately flat and static.
    This means that while you can have galaxies receding at arbitrarily large velocities, and while you can have light being carried away by expansion faster than the speed of light, in any given spot in the universe light is the fastest thing. Or to put it differently, expansion cannot make anything overtake a beam of light that is just passing by.

    If you were to use the balloon analogy, and imagine two ants ('galaxies') on that balloon - their recession velocity would be the result of the evolution of the space they're embedded in (= the balloon expanding), and have nothing to do with how fast either ant (or even the Fastest Ant Ever = light) can go.

    Particle horizon does not mark the boundary where recession velocity exceeds c - that's the Hubble radius (aka Hubble sphere), which is much closer.
  6. Aug 1, 2017 #5
    Yes, the most distant galaxies are moving away from us so quickly that they are moving away faster than light - but however ...
    they are not moving through space, but with the overall expansion of space.
    Therefore they are not violating the cosmic speed limit.
  7. Aug 1, 2017 #6
    so expansion is not movement? and are the ants accelerating independently of the expansion ?
  8. Aug 1, 2017 #7


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    The ants can locally do whatever they like, providing they don't try to walk faster than the Fastest Ant Ever (well, they can try, but they won't), but that is independent of the expansion carrying them away - and which in turn couldn't care less about ants and their limitations. It's doing its own thing.

    I'm not sure if it's not movement - distances are still increasing. But it's definitely a different kind of movement than what we deal with everyday. For example, relative positions of objects carried away by the expansion don't change - if one galaxy was at your five o'clock, and another at eleven, they'll stay there forever (unless they or you move locally).
  9. Aug 1, 2017 #8
    it's hard to get your head around the physics of it. so only light/Electromagnetic radiation can traverse the expansion, and how localised is the expansion? is the space between every object expanding and if not why not?
  10. Aug 1, 2017 #9


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    This isn't quite accurate. The recession velocity being greater than the speed of light has nothing to do with the cosmological horizon.

    The issue here is that you're thinking of the world in terms of special relativity. Things get a bit more tricky in General Relativity.

    In GR, the curvature of space-time throws a wrinkle into the mix: the velocity of far-away objects becomes ambiguous.

    Think, for example, of a vehicle moving North on the opposite side of the Earth, with both you and this vehicle on the equator. Is it moving towards or away from you? If you measure distance from the south, it's getting moving away. If you measure distance around the north, it's getting closer. If you measure along the equator, it's not moving towards or away.

    The curvature of space-time introduces an analogous ambiguity for far-away objects. Because the velocity of far-away objects is ambiguous, it's just not possible to have a speed of light limitation in the math.

    Instead, the limitation is local: no object can outrun a light ray.

    Recession velocity, then, isn't the velocity of the far-away galaxy. It's just one way of writing down a number that we call the velocity. We could choose other definitions of velocity if we wanted (analogous to measuring the change in distance of the vehicle on the other side of the Earth). And it turns out that by the definition of recession velocity, most of the visible galaxies are far enough away that their recession velocities are now and always have been faster than the speed of light.
  11. Aug 1, 2017 #10
    thanks that makes the picture clearer for me, appreciated
  12. Aug 1, 2017 #11


    Staff: Mentor

    And just to clarify how the local limitation ties into this: light emitted away from us by any of those galaxies has a "recession velocity" from us that is even faster than that of the galaxy itself--so the galaxy is not outrunning light rays in its local vicinity.
  13. Aug 1, 2017 #12
    so escape velocity for light from blackholes in various galaxies differ? and the speed of light is dependant on local conditions of a galaxy and it's curvature of space time ?
  14. Aug 1, 2017 #13


    Staff: Mentor

    No. How did you get that out of what has been said?

    The term "speed of light" is ambiguous.

    The "recession velocity" of light from us is dependent on how far away from us the galaxy is, but that's a coordinate-dependent quantity that doesn't have any physical meaning; it's just a convenient number for cosmologists to quote.

    The speed that anyone would measure a light ray in their local vicinity to have, relative to them, is the same everywhere; it doesn't depend on local conditions or spacetime curvature.
  15. Aug 1, 2017 #14
    understood so c is constant and unchangeable regardless of matter distribution okay i think i have it now.
  16. Aug 2, 2017 #15
    Yes. The two things needed to measure velocity is distance and time. General Relativity links them in such a way that if you change speed through one, the other one changes too, allowing for the proportion to always be the same.
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