Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why does the furthest stars seem to be moving away faster than light?

  1. May 18, 2014 #1
    I heard somewhere that the amount of redshift in the most distant stars show that they are moving away faster than light and its due to the shape of the universe. I was wondering how do we even know what she shape of the universe is, I thought we couldn't see any boundaries.
  2. jcsd
  3. May 18, 2014 #2
    your right we don't see any boundaries, the shape is a energy-density relation between matter and the cosmological constant. http://cosmology101.wikidot.com/universe-geometry this article covers geometry.
    now onto superluminal velocity. this is a consequence of separation distance or Hubble's law

    Hubble's law the greater the separation, the greater the recessive velocity.

    Now here is Phind's balloon analogy it will explain there is no boundaries, no inside or outside the universe
    http://www.phinds.com/balloonanalogy/ : A thorough write up on the balloon analogy used to describe expansion
    and here is an article on superluminal velocity and how it relates to the Hubble sphere
    http://tangentspace.info/docs/horizon.pdf :Inflation and the Cosmological Horizon by Brian Powell
    more related articles can be found on my signature, check the misconceptions section
    the questions are best answered with those article than try to explain completely in a single post

    by the way welcome to PF
    Last edited: May 18, 2014
  4. May 18, 2014 #3


    User Avatar
    Science Advisor
    Gold Member

    This has nothing to do with the "shape of the universe". Would you mind giving us the source where you heard this? The notion "moving away from us faster than the speed of light" is a slightly subtle notion due to the problem of measuring speed of distant objects in General Relativity. But, given this notion, it is the same no matter the "shape" of the universe. It depends on only the observed Hubble's law.
  5. May 19, 2014 #4
    Quite simply, from our frame of reference, the stars appear to moving away faster than light because spacetime expands at such a rate. Since we appear to be at rest, and spacetime expands in such a way, the objects appear to move faster than light, although they are not actually moving that fast (from their own reference frame, they would actually appear to be stationary, and would appear significantly slower the closer we got to them)
  6. May 20, 2014 #5
    ?. They don't "seem" to be. You seem to be confusing what you heard, that or what you heard is wrong. We can't see (normal) stars at cosmological distances, they're too small/dim. We do see galaxies and supernovae. We are able to measure the redshift of the light we detect from them. IF you were to (incorrectly) plug these redshifts into the equation for Doppler shift, we would NOT get a velocity greater than the speed of light.
    For instance given a redshift, z, of 8.6, if γ=v/c the doppler equation 1+z = √((1+γ)/(1-γ)) gives 97.85%. A redshift of 100 would give 99.98%. It IS reasonable to ASSUME that if we measure an object with a cosmological redshift of 8.6, which would mean that the light we are NOW measuring was emitted 13+ billion years ago, it IS reasonable to assume that that object is NOW beyond our ability to ever see again, and its distance is increasing at faster than the speed of light, c. In fact, I don't know of any theories of cosmology which would not accept that as being 'correct' ("true"). This is a difficult subject, especially since the words we use are often used in different ways. Distance, speed, velocity, even universe all have various DIFFERENT meanings depending on context. Most of the confusion arises in thinking that there is a single (intuitive) meaning of these concepts. For instance, most cosmologists accept the fact that the Universe is much larger than our Observable Universe, and many accept the assumption that it is infinite. The Observable Universe is what we can (or could or will ever) see. Speaking about what goes on outside of that region is more philosophy/religion than science (at least the way I define science).
    I should note that a cosmological redshift of the 'most distant' object ever measured is 8.6, although I am not sure if that has been accepted by the consensus yet. We'll NEVER see something with a cosmological z of 100!
  7. May 20, 2014 #6

    I meant to say galaxies, not stars. I couldn't find the direct link to the information. I feel silly now.
  8. May 20, 2014 #7
    did you read this article,? it does a good job clarifying what is meant by superluminal velocities with regard to expansion

    http://tangentspace.info/docs/horizon.pdf :Inflation and the Cosmological Horizon by Brian Powell

    another good article is this one

    http://arxiv.org/abs/astro-ph/0310808 :"Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe" Lineweaver and Davies
  9. May 20, 2014 #8


    User Avatar
    Science Advisor

    This quantity is called β. We have β = v/c, and γ = 1/√(1 - β2).

    There is no meaningful way to define the word NOW in relativity, because which distant event happens at the same time as your present time is frame dependent. The oft-quoted example is that if you start walking toward the Andromeda Nebula, the definition of NOW for events there jumps forward by 4 days.

    Wikipedia lists four objects with greater redshift than this, up to z = 11.9. The redshift of the CMB is estimated to be about 1000.
  10. May 23, 2014 #9
    Well, yes and no. That happens is because the reference we are chosen. Take a a balloon for example, first you should draw some dots on the surface of it. Then blowing it up, once you have blown it up, you will find the further distance between two dots, the faster speed of two dots. Because they are both moving away from each other at a certain speed, for instance, a litter more bit greater than the half of speed of light, thus the speed you measure at one point is greater than the speed of light. So that is the reason why we observe that stars are moving away from us than the speed of light.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook