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

Particle and event horizons

  1. May 5, 2005 #1
    In a flat universe with dark energy (cosmological constant) and normal matter like ours, the sum of the density parameter for matter (Omega m) with the density parameter for the cosmological constant (Omega lambda) at the present time equals 1.

    If Omega lambda is larger then a certain value the size of the particle horizon is larger then the size of the cosmological event horizon. What does this mean? What happens in a Universe where the particle horizon is bigger then the event horizon?
  2. jcsd
  3. May 19, 2005 #2


    User Avatar
    Science Advisor

    I would say there is no relation between them. The particle horizon is related to the past and the event horizon is related to the future. To define our particle horizon one shall consider which would be the current position of a particle of zero mass sent at the beginning of time from our comoving position. On the other hand, our event horizon is the current position of objects whose light will never reach us (in future). The particle horizon grows always and the event horizon tends to the Hubble sphere (c / H) in case of a cosmological model dominated by the cosmological constant (like the current one). Objets beyond the event horizon but inside the particle horizon may be for example very far quasars; their light, emitted in past, is reaching us now, but the light they are supposedly emitting now, will never reach us in future.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook