Why are all quasars so far away?
If the universe is homogeneous, we should expect to have a homogeneous distribution of quasars, but all of then seem to be far away from Earth. Why is that?
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5 Answers
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There are essentially two reasons.
First, quasars are rare objects, so even though they are homogenously distributed on large scales, the average distance is large. Moreover, the brightest quasars are even rarer, but visible to large distances, so their average distance is even larger.
Second, most quasars were most active at redshift $z\sim2$. This is thought to be because quasar activity is triggered by galaxy mergers, which bring gas into galactic centres so that the supermassive black holes can be fed. Galaxy mergers and star formation peaked at $z\sim2$.
Finally, there is some bias in the sense that astronomers are looking for the most distant (and hence oldest) of objects, because they are more interesting than nearby objects when it comes to learn about the formation history of the universe.
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$\begingroup$ I'm skeptical that there's a distance bias. Nearby quasars would be very interesting. As for quasar activity being triggered by galactic mergers, is it likely that the future collision of our galaxy with Andromeda could create a quasar? $\endgroup$ Commented May 29, 2015 at 21:43
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1$\begingroup$ Nearby quasars would be even more interesting (since they allow detailed observations), but all of those are long known. Intermediate-redshift quasars (at $z\sim1-4$) are plenty and too distant for detailed investigations. $\endgroup$– WalterCommented May 30, 2015 at 12:09
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$\begingroup$ Indeed, the collision of M31 with the Milky Way will most likely trigger some quasar activity. The black hole at the centre of M31 is $~100$ times as massive as that of our galaxy and can make for a pretty bright quasar. $\endgroup$– WalterCommented May 30, 2015 at 12:15
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1$\begingroup$ The mass of the black hole is only one important parameter. More important is how much and how fast you can feed it with gas. Both Andromeda and the Milky Way will have less gas in 4 billion years than they do now. $\endgroup$– ProfRobCommented May 30, 2015 at 14:12
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$\begingroup$ There is still more than enough gas to make for a good quasar. The critical question is how much of that is funneled close enough to the black-hole binary (which will form) and then how much will actually accrete onto one of the holes (rather than being ejected via gravitational slingshot). $\endgroup$– WalterCommented May 31, 2015 at 9:16
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You have stumbled across a profound observation and almost grasped one of its most important consequences.
There are two forms of the so-called cosmological principle. There is the more limited cosmological principle, which to paraphrase, says that the universe will look the same in all directions to any observer anywhere in the universe at the same time (i.e. at the same cosmological epoch). There is also a Perfect Cosmological Principle, that says the universe is homogeneous and isotropic in both space and time.
The Perfect Cosmological Principle was the underpinning for the steady state theory of the universe. However, one of the most obvious objections to this was that we can see that the universe has evolved in time. One of the first realisations of this was indeed the observation that quasars were more common at large distances and hence more common in the past.
Thus this observation tells us that the characteristics of the universe are changing with time and therefore that the Perfect cosmological principle is incorrect.
The more limited Cosmological Principle meanwhile remains. It ony asserts that everything should look the same to all observers at a given cosmological epoch; it does not require the universe to look the same at all times and therefore does not require the density of particular types of astronomical objects to be constant with distance.
Quasar activity peaked at moderate redshifts due to the required feeding processes of active galactic nuclei and the competition between the merger activity of gas-rich galaxies and the quenching caused by massive star formation and negative feedback from the AGN themselves. It appears that the "sweet spot" for comparatively short-lived phases of "quasar activity" is at redshifts of 2-3 where there was significant merger activity and the transport of gas into the central regions of galaxies, but that there had been insufficient time to fully exhaust the gas in galaxies with central black holes.
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1$\begingroup$ I think Olber's paradox is the most obvious objection to the perfect cosmological principle. It can only be resolved if the universe is finite, either in space or in time (or both or some strange physics such as tiring light). $\endgroup$– WalterCommented May 30, 2015 at 12:13
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1$\begingroup$ The steady state theory of course came along long after Olber's paradox was identified. The steady state proponents simply argued that the expansion of the universe just redshifted away the light from distant objects. The perfect cosmological principle is not contradicted by a simple expansion. Olber's paradox cannot be explained by a static universe that is infinite in space and time. $\endgroup$– ProfRobCommented May 30, 2015 at 12:49
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3$\begingroup$ Steady state theory is of course contradicted by many things, including the topic of my answer. However, the creation of matter from nothing can hardly be considered a major obstacle if the alternative is the creation of everything in one go in a big-bang! Indeed, the origin of the term "big-bang" is as a term to ridicule the notion that everything can be created in an instant from nothing. The physics required by the big-bang is far more "strange" (or certainly no less strange) than that required by steady state theory. $\endgroup$– ProfRobCommented May 30, 2015 at 13:53
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1$\begingroup$ Note that the term "Big Bang" was invented by Fred Hoyle, the main proponent of the steady-state theory to mock the Big Bang. (It didn't work...) $\endgroup$ Commented Jul 7, 2018 at 11:57
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1$\begingroup$ @kenG Hoyle's use of the term "big bang theory" in radio broadcasts appeared to be derisive. He did claim otherwise in later interviews. This is what your source discusses on pages 15-18. I guess it's all in the ear of the beholder as it were. $\endgroup$– ProfRobCommented Dec 28, 2018 at 16:58
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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.
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$\begingroup$ 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. $\endgroup$ Commented Feb 17, 2015 at 20:55
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3$\begingroup$ 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. $\endgroup$ Commented Feb 17, 2015 at 20:55
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$\begingroup$ You are right. I should not have used the word "likely" but rather "possible" and will edit it to reflect that. $\endgroup$ Commented Feb 18, 2015 at 9:15
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1$\begingroup$ +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 $0.1<z<0.5$ would be negligible though? Your answer I think is more possibly an answer to why isn't our galaxy an AGN? $\endgroup$– ProfRobCommented Feb 18, 2015 at 10:58
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3$\begingroup$ 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. $\endgroup$ Commented Jul 7, 2018 at 11:53
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in the early universe there was more diffuse matter in gaseous form available to the black holes. The galaxies were more foggy with diffuse matter and less void, so a BH travelling through a galaxy could catch a continuous cloud of matter as it went along, resulting in huge capture rates. Back then, most gas had not yet collapsed to form stars, there was more fuel available for both the feeding of black holes and the forming of new stars. Much of that fuel was subsequently consumed in the formation of stars during the first few billion years after the big bang.
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1$\begingroup$ Can you maybe add a source for this graph (to both attribute it, and provide people with a source of more information) if you found it elsewhere - or make a note, if you constructed it on your own? Thanks. $\endgroup$– HDE 226868 ♦Commented Mar 10, 2019 at 4:47
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$\begingroup$ graph from here courses.lumenlearning.com/astronomy/chapter/… found it again by searching "relative number quasars time graph" $\endgroup$ Commented Mar 10, 2019 at 8:56
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"Far away" = "very old" in cosmology.
All quasars are far away because all of them are old. These are objects that occurred when our bubble of Universe was young. So when our telescopes look far into space, they look back in time, and see a lot of quasars then.
They're mostly giant black holes devouring gas and dust and cosmic junk, of which there was lots back then near these black holes. After they vacuum clean their environment, they calm down and the quasars basically turn off.
There are essentially no quasars forming in our time. Therefore there are no quasars visible nearby.
answered Dec 28, 2018 at 18:56
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$\begingroup$ Exactly. What you see is what was before. And "before" means it was in different place. When wave is stuck into wall, it reflects. Same as with Universe as closed pool of water, whenever waves reflect, you see them again and again. But these ones are really "old" waves. Just imagine water pool, but remove walls (boundless, but not indefinite). $\endgroup$– sanarisCommented Jan 2, 2019 at 21:15