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Accounting for time passed in our observation of the universe

  1. Dec 9, 2015 #1
    Greetings - We speak of astronomical objects as being x number of light years away, which is also a statement about the age of the data. So our knowledge of M83, for example, is about 15 million years old, etc, etc. I'm just wondering why there never seems to be any attempt to move this information forward to the current state of the universe. Presumably we could? Since the universe is expanding, what does it really mean to say that M83 is 15 million ly away? At what point in time was this true? Is it true now? Or is this quandary already built in to the calculation? Stated another way: just because the light left M83 15 million years ago, it does not mean that M83 is still in that same location. Of course, it is isn't. So... where is it, and how distant? Or maybe I've got this all mixed up...

  2. jcsd
  3. Dec 9, 2015 #2


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    M83 has a redshift of ~513 km/sec and the 15,210,000 ly distance was its distance when the light left it, so in the 15,210,000 years since, it has moved an additional ~26009 ly further away.
  4. Dec 9, 2015 #3
    In addition to expansion though, the galaxy would also have some amount of proper motion relative to other nearby galaxies.
    That motion can't be simply extrapolated from observations because it's probably gravitationaly bound to several other galaxies and it's future trajectory (from where now see it) would be uncertain because of that.
    Last edited: Dec 9, 2015
  5. Dec 9, 2015 #4
    Red shift would not account for lateral speed, right? I mean, relative to us
  6. Dec 9, 2015 #5
    That was my point, it almost certainly will have a component of motion relative to nearby galaxies, as well as being redshifted due to expansion.
    It would not be possible in practice to predict that 'proper' motion starting from it's presently observed position.
    It's trajectory in relation to nearby galaxies would not be a straight line, gravitational binding to other galaxies could significantly alter it's future path
  7. Dec 9, 2015 #6

    Ken G

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    There are other issues as well. There is actually no single meaning to "distance" in cosmology, by which I mean whenever the distance gets large enough that we would worry about the expansion of the universe. Ned Wright has this list of different empirical meanings of "distance" in cosmology, most of which give different answers: http://www.astro.ucla.edu/~wright/cosmo_02.htm. Matching those empirical distances to the distance when the light was emitted, or the distance now, both require a dynamical model of the universe to carry out, so cannot be considered an observed distance.

    Also, what seems to be the general convention is that when some large distance is quoted in the media, like a billion LY or some such thing, it really doesn't mean a distance at all, it means a lookback time-- they are talking about light that was emitted when the universe was a billion years younger. That also requires a dynamical calculation, all we really observe is a redshift, and that only tells you the total amount of expansion that has occurred during the lookback time, not the lookback time itself. Or other times, they just plug the redshift into the local Hubble law, and get a distance, even though that's a meaningless thing to do for large distances. So you can decide for yourself what distance a 1 billion light year quoted distance or lookback time refers to, but one thing's for sure, it's generally not 1 billion light years away, because the object was closer than that when the light was emitted, and farther than that "now."

    Notice the irony here-- for nearby objects, like stars, we always stress that a light year is a distance, not a time-- yet the convention in cosmology reporting reverses this important distinction!
    Last edited: Dec 9, 2015
  8. Dec 9, 2015 #7


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    With so much uncertainty or hypothesis on the Hubble constant during time how is 13.7 Gy as age computed, the size of the observable part, resp.? I've even read (on PF) about an estimated total size. I forgot the numbers, something between 50 and 80 Gy I think.
  9. Dec 9, 2015 #8

    Ken G

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    The total size could mean how far away right now are just the things we can see, or it could mean how much farther the cosmological principle must apply or we would get a hint of some kind of boundary. So the total distance we can see or infer can vary a lot, but it is indeed at least 50 GLY in all directions. Big!
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