4 ms·

Actually, the parts of the universe receding from us faster than the speed of light can still be causally connected to us. It’s a known “paradox” that has the f

Actually, the parts of the universe receding from us faster than the speed of light can still be causally connected to us. It’s a known “paradox” that has the following analogy: an ant walks on an elastic band toward us at speed c, and we stretch the band away from us by pulling on the far end at a speed s > c. Initially the ant despite walking in our direction gets farther, but eventually it does reach us (in exponential time). The same is true for light coming from objects that were receding from us at a speed greater than c when they emitted it. See https://en.m.wikipedia.org/wiki/Ant_on_a_rubber_rope https://en.m.wikipedia.org/wiki/Ant_on_a_rubber_rope

adastra22 2y ago
They will never reach us because the rate of expansion is accelerating.
- ziofill 2y ago
  https://arxiv.org/abs/astro-ph/0310808 https://arxiv.org/abs/astro-ph/0310808
  - adastra22 2y ago
    That article doesn't back up your claim.
    - ziofill 2y ago
      Yes it does, look at the caption of Fig. 1: "Photons we receive that were emitted by objects beyond the Hubble sphere were initially receding from us (outward sloping lightcone at t <∼ 5 Gyr). Only when they passed from the region of superluminal recession vrec > c (gray crosshatching) to the region of subluminal recession (no shading) can the photons approach us". I can’t reply to your last reply. I agree, in fact I said those regions can be still causally connected to us, not that they are.
      - adastra22 2y ago
        It shows that SOME “superluminal” photons can reach us, not that ALL can. With accelerating expansion, eventually all galaxies fall out of that interval and become unreachable.
      - nyrikki 2y ago
        Those photons aren't superluminal, the are in our past light cone, they were headed out way before the emitter was beyond the horizon. It gets complicated because the concept of 'now' is a local property and because those objects aren't moving away ftl, space is expanding.

adastra22 2y ago

They will never reach us because the rate of expansion is accelerating.

ziofill 2y ago
https://arxiv.org/abs/astro-ph/0310808 https://arxiv.org/abs/astro-ph/0310808
- adastra22 2y ago
  That article doesn't back up your claim.
  - ziofill 2y ago
    Yes it does, look at the caption of Fig. 1: "Photons we receive that were emitted by objects beyond the Hubble sphere were initially receding from us (outward sloping lightcone at t <∼ 5 Gyr). Only when they passed from the region of superluminal recession vrec > c (gray crosshatching) to the region of subluminal recession (no shading) can the photons approach us". I can’t reply to your last reply. I agree, in fact I said those regions can be still causally connected to us, not that they are.
    - adastra22 2y ago
      It shows that SOME “superluminal” photons can reach us, not that ALL can. With accelerating expansion, eventually all galaxies fall out of that interval and become unreachable.
    - nyrikki 2y ago
      Those photons aren't superluminal, the are in our past light cone, they were headed out way before the emitter was beyond the horizon. It gets complicated because the concept of 'now' is a local property and because those objects aren't moving away ftl, space is expanding.