Quote:
Originally Posted by clarknova
7 billion years is half the (believed) lifetime of the universe. I'd say that's pretty long. Who knows how many of those galaxies have drifted away, or collided, &c.?
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I should perhaps say that cosmology was my field as a professional astronomer, before I discovered that IT was a lot better paid
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Galaxies are among the oldest things in the universe - pretty much all the galaxies we see today had formed within a billion years of the Big Bang; our own Milky Way galaxy appears to have formed within 100
million years of the BB (you can estimate the age of a galaxy by looking at the amount of beryllium in its globular clusters - it's an element whose abundance has increased pretty much linearly throughout the lifetime of the universe).
Galaxies don't really "die". They are gravitationally bound structures which "stick together" pretty much forever. They may cease star formation eventually (although none has yet done so), but the galaxy carries on, and its mass really doesn't change.
Hence my view that if this galaxy cluster contained 800 trillion solar mass' worth of material 7 billion years ago, it's still going to do so today.