• Question: I think this is a curious question!! Is it a common misconception that recession velocities cannot exceed the speed of light? Here is my view on the topic. A common misconception is that the expansion of the universe cannot be faster than the speed of light. Since Hubble\\\'s law predicts superlumnial recession at large distances it is sometimes stated that Hubble\\\'s law needs special relativistic corrections when the recession velocity approaches the speed of light. However it is well accepted that general relativity, not special relativity, is necessary to describe cosmological observations. Supernovae surveys calculating cosmological parameters, galaxy-redshift surveys the cosmic microwave background anisotropy tests, all use general relativity to explain their observations. When observables are calculated using special relativity contradictions with other observations quickly arise. Moreover, we know that there is no contradiction in special relativity when faster than light motion occurs outside the observers inertial frame. General relativity was specifically derived to be able to predict motion when global inertial frames were not available. Galaxies that are receding from us superluminally are at rest locally (their peculiar velocity, Vpec=0) and motion in their local inertial frame remains well described by special relativity. There are in no sense catching up with photons (Vpec=c). Rather, the galaxies and the photons are both receding from us at recession velocities greater than the speed of light.

    Asked by jarsden to Meeks, Pete, Stephen, Steve, Tom on 22 Jun 2010 in Categories: .
    • Photo: Steve Roser

      Steve Roser answered on 18 Jun 2010:

      Its all relative, as my auntie says. And your question is?

    • Photo: Tom Hartley

      Tom Hartley answered on 18 Jun 2010:

      OK jarsden – you’ve beaten me with that. I seem to remember answering an earlier question with the idea that recession velocities were less than the speed of light. If I was wrong – I stand to be corrected. I am afraid I didn’t really follow the argument above, but I guess a physicist would.

      I still don’t think they are because i) I would expect no light to reach us from a star or galaxy exceeding the speed of light, and ii) I learned (but do not fully understand) that nothing can travel faster than c. However relativity deals with this kind of issue in a clever way that probably requires a physics degree to understand, but I am quite willing to listen to a dumbed down explanation of why I am wrong.

    • Photo: Stephen Curry

      Stephen Curry answered on 18 Jun 2010:

      Um – could you repeat the question?

      I don’t think there are any serious reasons to doubt special relativity – which is the theory that places a limit on the speed that matter can attain.

      Does this help in any way?

    • Photo: Pete Edwards

      Pete Edwards answered on 21 Jun 2010:

      I refer you to my earlier answer on this. As you know distant galaxies, like our own Milky Way are carried along by the expansion of the Universe, and will move apart from every other galaxy. As you look at galaxies further and further away, they appear to be moving faster and faster away from us. So is it possible that they could eventually appear to be moving away from us faster than the speed of light? Don’t panic Einstein’s theory is not broken! The galaxies themselves aren’t actually moving very quickly through space, it’s the space itself which is expanding away, and the galaxy is being carried along with it. As long as the galaxy doesn’t try to move quickly through space, no physical laws are broken.

    • Photo: Marieke Navin

      Marieke Navin answered on 22 Jun 2010:

      ???

Comments

  • Photo: yourmum

    yourmum commented on :

    I think Steve, that the question Jarsden is posing towards you, is whether or not the universe can expand at the speed of light. Something I too am very curious to find out, by also having expressed my interest in submitting my own question toward you scientists, regarding the fact that red shifting galaxies cant possibly be moving away at the speed of light if physics states this as impossible.

  • Photo: jarsden

    jarsden commented on :

    I was wondering what your viwes on the topic are