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  1. Mar 18, 2009 #1
    Hello everyone!! I'm new to the board, so thought I'd take a moment and introduce myself. I have a degree in Liberal Arts, but like so many of us, the first time I gazed upward on a dark night and wondered in amazement, I was hooked. I have enjoyed perusing so many of your contributions, and hope in some small way I will be able to add a bit of knowledge to the forum.
    Now I pose a question. I understand that due to the accelerated expansion of space, that the edge of the observable universe is about 46 bly. Now, a galaxy omits a photon aproximately 13.7 billion years ago. Space is expanding, so the space traversed by the photon expands behind it during the trip. Consequently, by the time the photon reaches us, the galaxy is about three times further. My question is this. Even though the photon has been stretched, and the galaxy is at a greater distance than 13.7 bly, won't the galaxy we observe when the photon reaches us be the galaxy as it appeared when it first emitted the photon? At 46 bly, and space still expanding, we may never see the photons emitted at that distance. Correct? In fact, our galaxy at 46 bly may have long merged with other galaxies.
  2. jcsd
  3. Mar 19, 2009 #2


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    Actually, the edge of the visible universe is set by the cosmic microwave background, which was emitted at the time that the universe became transparent. The universe was just over 1000 times smaller then, so that radiation was emitted a mere 46 million light years away. It's just that due to the expansion it has taken 13.7 billion years to get here (for every light year the photon moved, the space between us and the photon expanded significantly, so that it took so long).

    Yes, basically. But bear in mind that there were no galaxies when the CMB was emitted. The first galaxies don't appear until the universe was somewhere around the time that the universe had expanded by a factor of 50 from the emission of the CMB (I'd have to look up where this was in time and distance).

    If the accelerated expansion continues, we can never see anything that is currently beyond about 16 billion light years, because beyond that there will always be more space left to travel than the photon will pass.
  4. Mar 19, 2009 #3


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    Correct. According to standard cosmo, light emitted today, at that distance will never reach us. Or indeed light emitted today from any distance beyond the 16 billion lightyears that Chalnoth mentioned.

    I concur with each point in Chalnoth's response.
    Good questions, iflexit. Welcome. I hope to see more of your posts.

    (Most of the time on this forum we are talking about details features consequences of the standard cosmological model, in its various versions. It gets tedious always repeating this and you probably realized it already :biggrin:. So I guess we should just assume it unless otherwise specified. The standard model is called LambdaCDM, and the cosmo constant Lambda is the thing that causes the very gradual acceleration resulting in the presence of that 16 billion lightyear horizon.)
    Last edited: Mar 19, 2009
  5. Mar 19, 2009 #4
    Kudos to you, gentlemen. You have certainly provided me with a better understanding.
    Chalnoth, I believe the distance for galaxy formation is about 13.2 bly, and therefore created when the universe was only about 500 million years old.
    I just have to throw one more inane one out. My high school physics teacher (1968) asked this question of the class, and I've never forgotten it. If we get into a car and accelerate to the speed of light, then turn the headlights on, will they work?
  6. Mar 19, 2009 #5
    You can't quite accelerate all the way to the speed of light (basically you would need an infinite fuel supply to offset your car's dilating mass), but the lights would work as normal (the rules of physics are relative), although they mightn't be terribly helpful (for navigating obstacles that have almost reached you by the time you see the light that they reflected).

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