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Why such strange units of measure?

  1. Apr 20, 2012 #1
    Why do astronomers and cosmologists prefer to use the Distance Modulus instead of -- rather than in addition to -- Bly (billion light-years) or Mpc (mega parsecs)? Being a LOG10 it distorts relative distances. You can find Bly plotted on a graph (but not specified), and even in studies comparing z with distances according to standard candles, you won't find tables listing z alongside metric distances.

    Why do astronomers and cosmologists prefer to use "temperature" rather than redshift when discussing CMB (cosmic microwave background radiation)? Why not either call it "cosmic temperature background" or specify the length of the microwave? Space being a vacuum, it seems unnecessarily weird to talk of temperature.
  2. jcsd
  3. Apr 20, 2012 #2
    The fact that it's logarithmic is exactly why it's used; in astronomy, we are often talking about things on huge ranges of distance scales, so carrying around (or even saying) big units is ungainly. Also, from a technical standpoint, distance modulus doesn't always correspond to actual distance (in case we are discussing one that is uncorrected for extinction) and redshift doesn't always correspond to actual distance either (because strictly speaking, the distance inferred is based on how accurate our cosmological models are).

    As for the CMB, the shape of the spectrum is that of blackbody radiation, and the spectrum for blackbody radiation is entirely dependent on the temperature of what's radiating.

  4. Apr 21, 2012 #3
    I can understand using both, and that all extreme distant measures are more or less inaccurate. But to go from distance modulus to Mly is only to go from 2 digits to 4 or 5, and though a LOG10 might reduce relative inaccuracies (only because it's a LOG10 reduction), to telescope relative distances is itself to impose an inaccuracy. What's especially frustrating to me is that I've seen Hubble diagrams that plot z against Mpc, I've never found a corresponding table that speicifies the data points.

    Regarding the CMB, isn't what's most significant about it not the temperature of the source but how long it's been traveling? Doesn't it invite confusion to use the term "microwave" but measure it as a temperature? Doesn't it suggest that what's being measured is energetic particles, as-if remnants of the original plasma?
  5. Apr 23, 2012 #4
    You can calculate the distances in Mpc quite straightforwardly if you know the value of Hubble parameter. You need to remember though, that this is the luminosity distance, which is different from angular diameter distance, which is different from comoving distance.

    Well, it's not the absolute temperature which is interesting, but the fluctuations. At least I recall seeing redshift given more often than the absolute temperature.

    The point kind of is that what is being measured is exactly the remnants of the original plasma. As the plasma was tightly coupled with photons, you see exactly (modulo the changes happening while photons travel through the universe) the temperature of the original plasma.
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