Why are all quasars so far away?

  • Why are all quasars so far away?

    If the universe is homogeneous, we could expect to have a homogeneous distribution of quasars, but all of then seem to be far away from Earth. Why is that?

  • adrianmcmenamin

    adrianmcmenamin Correct answer

    7 years ago

    The discussion of the Cosmological Principle above is very relevant, but it is possible that so is a (weak) application of the anthropic principle - in other words if we were in a region of extremely energetic physical phenomena, such as quasars, we would be unlikely to exist - as the evidence suggests that the development of intelligent life takes a considerable time and highly energetic events are likely to disrupt that.

    You suggest that our region of space coincidentally happens to have fewer quasars and so is more hospitable to life. That would imply that other regions of space will have more quasars than our local region does *when observed at the same cosmological epoch*. An interesting idea, but not yet verifiable, since assuming the Big Bang was 13.8 billion years ago, we simply can't see regions of space that are more than 12.8 billion years old unless they're within 1 billion light-years.

    But observations of quasar distributions could be illuminating. If nearby quasars are rare because *old* quasars are rare, the region of rarity should be roughly a sphere centered on us; if it's random, it's more likely to be some other shape, and there could well be other regions with few quasars.

    You are right. I should not have used the word "likely" but rather "possible" and will edit it to reflect that.

    +1 for being interesting. You are effectively saying that the cosmological principle may not apply on intermediate scales. I suspect the effects on us of quasars at $0.1

    The AGN argument is stronger, yes. But if quasars were formed in a region where, say as a result of quantum effects and inflation, the initial density of the universe was slightly greater (and hence galatic collisions were more likely) that could manifest itself, in the current epoch, at a very large scale - couldn't it?

    You certainly don't want to be in the same galaxy as a quasar, but they're not *that* dangerous. If Andromeda went quasar we'd be fine. (The absolute magnitude of a quasar is about -26, which means they'd be as bright as the Sun at 10 pc distance, so at 1,000,000 pc (typical nearby Galaxy) it would be 10**10 dimmer or -1 mag. Just a bright star. No prob. Quasars simply don't sterilize a big enough volume of space for this argument to be valid.

    @RobJeffries I beg to differ. In the past the Milky Way was certainly a quasar from time to time (though a relatively small one owing to the small mass of Sgr A*) and yet here we are. I think if you go through the math, you find that the sterilizing effect of a quasar is limited to a relatively small region, <1kpc for a $10^8M_\odot$ SMBH

    @Walter "In the past" means at a non-negligible redshift right? Has the Milky Way been a quasar/AGN that would be detectable at say z=0.5 sometime in the last 4.5 billion years?

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