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Do Quasars exist right now?

  1. Jan 3, 2004 #1
    I currently understand that quasars are billions of light years away from us. So the light from those distant quasars that reaches us now, started its journey a billion years ago.

    When we examine quasars today we are really looking at something that existed a very long time ago and very far away (billions of units for both of those scales).

    So how do we know that the quasars are still around after these billions of years? It seems that we really can't, unless I am missing something.

    My other question. When Cosmologists calculate stuff, like the amount of matter in the universe, or the rate of expansion, how exactly are they accounting for the fact that we can't observe the universe as it is right now? Because the farther we look, the more in the past it really is.

    Maybe I need a thought experiment. Say that a metallurgist is examining an straight iron rod that is 15 billion light years long by standing at one end. This rod has had a tough life and is continuously rusting from neglect. (uniformly across the entire rod). The end she is close appears rusty, but if she remains still and looks out toward the other end, the rod would get progressively less rusty. That is because the light that has traveled from that distant part of the rod is was transmitted at a time when the rod was less rusty. How does the cosmologist account for that?

    This is keeping me up at night, and I would appreciate some websites or books to clear this up.

  2. jcsd
  3. Jan 3, 2004 #2
    I think we can only assume that the processes in the Universe are more or less the same - if those quasars are not around anymore, perhaps some other are.

    More important, though, is another question: why does this "time" matter? What is "now"? Is that what you experience? Every particle of your body experiences time a bit differently. There is no past or future. Or there are both at the same time, all the time, if you prefer.
  4. Jan 3, 2004 #3


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    strange as it may seem cosmologists do have an idea of the universe "at the present moment" and the metric, or distance function, that they use gives the distance at the present moment (not some easier to understand thing like the "time the light took to get here from it"

    so a cosmologist might say that the distance to (the remains of?) a quasar that made the light we see 10 billion years ago is (at the present moment) 30 billion years.

    You can see diagrams explaining this and read about it at Ned Wright's UCLA Cosmology Tutorial website or in Chuck Lineweaver's survey article---HTML at CalTech or in PDF for download at
    I think Googling with the name Ned Wright, or Ned Wright cosmology, would get the UCLA tutorial.

    The distance function they use, called FRW or "Robertson Walker", that gives distance at the present moment, or at any moment in the past SOUNDS like an extra complication but actually makes things a lot simpler and easier in the end, once you get used to it.

    The reason it can be defined is that there is the CMB, the Background, that makes it possible to define when a galaxy is sitting still with respect to Background, or with respect to the expansion of the universe. Or if a galaxy IS moving wrt Background then you can say what direction how fast and compensate for it. And any two galaxies that are sitting still, or two that are "co-moving", share approximately the same idea of time. It is not perfect but a cosmologist will talk about this with you for a while and then say there is a workable idea of simultaneity and the state of things "at the present moment" in cosmology.

    Quasars are qualitatively the same as "active galactic nuclei", just bigger, and there are some nearer ones of those. They arent all 10 billion years old! they are supermassive black holes, probably, with stuff spiraling into them and the stuff gets hot and radiates. The gravitational energy given up by the stuff as it spirals into the hole is converted at pretty good efficiency into Xrays and other radiation and sometimes jets coming out the north and south poles of the spiral. Pretty dramatic. Lots of power. Maybe in the old days when galaxies were just forming there were more. The universe was denser then. More matter per cubic mile. And those very old ones are maybe all burnt out. But I believe there are recent quasars too, such as they call "active galactic nuclei". Even our own Milky Way has a black hole in it that is a couple of million solar masses. I guess it could someday grow to be a billion solar masses as might be given quasar rating, as long as there was plenty of stuff around for it to gobble up. Black holes must be fed if they are going to radiate quasar-style.

    Hope somebody shows up who has the facts down pat. I am just remembering and not too sure how big a BH has to be to become a quasar. Our Milky Way one of a couple of million solar is piddling.
    It makes burps of Xrays from time to time but is not really in the active galactic nucleus league. Anybody got some sure facts?
    Last edited: Jan 3, 2004
  5. Jan 4, 2004 #4
    Anybody got some sure facts?

    no just a guess

    the quasars are a subform of a super massive blackhole in the process of formation
    ie several smaller BH's oribiting each other
    as they spirial inward to form the final super BH
    hence moving very fast and contacting alot of mass in the dust disk, much more then a large static hole would or could incounter do to both the speed and the movement
    therefore creating the quasar
    once the smaller BHs drop inside the event horizen of the forming super BH the show [the quasar] is over
    or maybe the actual impact of the BH's with each other as they combine

    anyway the quasar is the product of the formation of the super BH and not a random event so is limited to the time frame of the super black holes birth
    and is not repeated later in it's life
    Last edited: Jan 4, 2004
  6. Jan 6, 2004 #5


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    Welcome to PF!

    One of the early results from the landmark (Australian!) 2dF project was the discovery (?) of the quasar luminosity function. In a word, quasars seem to have been born with a range of (optical) brightnesses, and after that each quasar's brightness declined gracefully as it aged, more or less in step with every other quasar. Scroll down to "Luminosity Function" on this page:

    As marcus and ray b said, the current thinking is that quasars, AGNs (active galaxy nuclei), blazars, Seyferts, and the supermassive black holes at the centre of 'classical' galaxies are various manifestations of the same thing.

    There are probably a dozen papers which draw all this together, but I don't have any to hand

    So, another way of looking at this is that the nearest quasar isn't 3C273, it's M87 (the giant elliptical at the heart of the Virgo cluster), or perhaps Sag A* (the supermassive BH at the heart of dear old Milky Way).

    BTW, love your name; are you from the land of Oz by any chance?
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