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What is the current radius of the observable universe?

  1. The radius of the observable universe is 14 billion LY

    7 vote(s)
    63.6%
  2. No, it is more like 42 billion LY

    2 vote(s)
    18.2%
  3. It sounds better to say 12 or 13 Gigaparsecs

    0 vote(s)
    0.0%
  4. [8)] none of the above

    2 vote(s)
    18.2%
  1. May 15, 2003 #1

    marcus

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    How far away are the most distance objects we can see?

    What is currently the volume of space that we can study with the light that is reaching us?

    You might want to look at this brief Cosmology FAQ paragraph:

    http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN

    *******footnote stuff, background on distance scales used****

    Cosmologists use several different indicators of distance and
    for a discussion of the various sorts of distance, as well as
    a Javascript calculator that converts between them (if you specify
    what assumptions you are making about the model), see:

    http://www.astro.ucla.edu/~wright/cosmo_02.htm

    http://www.astro.ucla.edu/~wright/CosmoCalc.html

    Among the most commonly used indices are redshift (z)
    and "comoving distance".

    The latter is measured at the present from rest with respect to the CMB and it is the type of distance that works in the
    Hubble law relating distance to velocity

    v = H0 D

    the present comoving distance to an object multiplied by the
    current value of the Hubble parameter is equal to the present radial comoving velocity

    Other types of distance are "angular distance" (angular smallness of an object of known size) and "luminosity distance"
    (dimness of a source of known brightness) and "light travel time".
    The last does not work in the Hubble law and may be difficult to determine because different parts of the light's path have undergone different amounts of stretching---the travel time is hard to relate consistently to other measures of distance. But it is one of the indices that is calcuated by the "Cosmo Calculator" at Ned Wright's site.
     
  2. jcsd
  3. May 16, 2003 #2
    The edge of the universe expanding at the speed of light should be a distance r = c/H if you just analyze H=dr/rdt for r with dr/dt=c.
     
  4. May 16, 2003 #3

    marcus

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    Why should the edge of the obs. universe be receding from us at only the speed of light?

    In GR lots of things go faster than light (the limit only belongs
    to SR and very local contexts)

    A quasar has been observed with z=6.4

    According to the most standard picture of things (Astro 101)
    this quasar is currently receding at twice the speed of light.
    And it is certainly part of the observable universe. Indeed we can see well past redshift 6.4!

    Please explain why you say these things!

    god save the empire and Albert Einstein too
     
  5. May 16, 2003 #4

    chroot

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    Because, for a location to be observable, information must have travelled to us from that location. Information cannot propagate faster than light.
    I'm not quite sure you understand general relativity...
    Perhaps you should double-check this conclusion.

    - Warren
     
  6. May 16, 2003 #5

    marcus

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    Chroot has done it again!

    You are off-base Chroot, the simplest way to get up to speed would probably be to look at John Baez gen rel tutorial and
    Ned Wright's cosmology tutorial. I will try to get some links for you.

    Just for refs here is what you said:

    ******************

    quote:
    --------------------------------------------------------------------------------
    Originally posted by marcus
    Why should the edge of the obs. universe be receding from us at only the speed of light?
    --------------------------------------------------------------------------------


    Because, for a location to be observable, information must have travelled to us from that location. Information cannot propagate faster than light.

    quote:
    --------------------------------------------------------------------------------
    In GR lots of things go faster than light (the limit only belongs
    to SR and very local contexts)
    --------------------------------------------------------------------------------


    I'm not quite sure you understand general relativity...

    quote:
    --------------------------------------------------------------------------------
    According to the most standard picture of things (Astro 101)
    this quasar is currently receding at twice the speed of light.
    --------------------------------------------------------------------------------


    Perhaps you should double-check this conclusion.

    - Warren

    ********************

    It is not my conclusion. It is the standard expert view
    that I am just relaying. I have double-checked it
    because when first encountered it is mindboggling
    that parts of the U are currently receding faster than light.
    I have repeatedly checked it and (counterintuitively enough)
    it is the case----i.e. a characteristic built into the prevailing models.

    I dont want to argue because it is not my baby-- it's just the ordinary educated view---but I will try to find you some URLs.
     
  7. May 16, 2003 #6

    marcus

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    A URL for Chroot

    Go to Wright's javascript cosmology calculator

    http://www.astro.ucla.edu/~wright/CosmoCalc.html

    put in z = 6.4 and press "flat"

    (a quasar with redshift 6.4 was observed last year by
    Bob Becker at UC Davis et al. so you are finding the
    comoving distance to that particular object, with the
    assumption that the universe is flat)

    The calculator will go:

    "The comoving radial distance, which goes into Hubble's law, is 8589.1 Mpc or 28.014 Gly."

    Now hubbles law is just v = H0 D
    where D is the comoving distance and v is the speed of recession.

    So just plug in 28 billion LY and you will get that the speed
    is 600 thousand kilometers of second-----twice the speed of light.

    that is the current speed of recession of that part of the observable universe where that particular quasar lives

    explanations can be found aplenty in the Usenet Physics FAQ
    and various tutorials: Wright's and others.
     
  8. May 16, 2003 #7
    Marcus I have to call you on this

    A Z=6.4 doesn't mean the Quasar is moving at twice the speed of light, it means it has two units of Velocity Parameter/Rapidity. There is a hyperbolic relationship between Rapidity and Velocity. Rapidity is expressed in units of C but the true velocity is the hyperbolic assymtote of the Rapidity.
     
    Last edited: May 16, 2003
  9. May 16, 2003 #8

    chroot

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  10. May 16, 2003 #9

    chroot

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    Re: Marcus I have to call you on this

    Well said. I didn't actually feel like trying to explain it.. marcus tends to be rather full of himself.

    - Warren
     
  11. May 16, 2003 #10
    Simply because space that is outside the radius at which the expansion = c has "velocity" > c assuming homogeneous space expanding everywhere and thus is unobservable, like the inside of a black hole.

    *edit
    tyler is right if v=c then z=infinity
     
    Last edited: May 16, 2003
  12. May 16, 2003 #11

    Actually, in the course of time, do we see more or less of the observable universe? Or is our horizon of observation always limited to a fixed part of the material world (even though the size of it increases in time)

    In other words can material objects which previously were not within our horizon, become visible one day? Can material objects which now are visible to us, become invisble one day?

    And another thing, I would like to propose a new outlook on the universe in total, founded on the assumption that space itself does not in anyway expand (just by defining the unit of measurement to be exactly proportional to the expansion of space, provided that the expansion rate is a global feauture of the universe).

    See my post Expansion? Contraction?.

    Some may say, it is a weird idea or even disallowed by Nature.
    However a well established principle of physics and physics laws is that physics does not depend on our choice of measuring units.
    This is a valid proposition, and could only be rejected if one assumed that nature comes up with a preferred set of measuring units, which are hold to be constants of Nature.

    The simple thought of using a measuring unit for length that is proportional to the overall expansion of space, and taken the speed of light to be constant, means also that the time unit must be proportional to that.

    In fact that means that the age of the universe is infinite, and there was no Big Bang singularity at all.

    However a measuring unit for length that is proportional to the space expansion, does mean - while on one side it excludes the phenomena of expansion of space - on the other side it introduces a new, yet unexplained phenomena, that of material contraction, within all material objects (from galaxies to atoms and below).

    This new outlook presents us with another contradiction.
    While on one side removing the "beginning of time", making time in fact of infinite extend in both directions, it introduces the fact that space is to be thougt of finite extend (always the same size).

    No matter how we look at it, and independend of our units of measurement, the universe as a whole will always present us a contradiction.

    However, the fact that within all forms of material existence, contradictions are inherently there, is an unavoidable property of material existence, because it's the only way in which matter can exist. Without contradiction matter would not exist, and that would cause an even more profound contradiction, namely that nothing would exist.
     
    Last edited: May 16, 2003
  13. May 16, 2003 #12
    In Quantum Mechanics a displacement of our coordinates to the left is equivalent to a displacement of the system to the right. They are physically and mathematically indistinguishable interpretations. Since the represntation of the Universe expanding while matter remains the same size is a mathematical convention I don't see why we couldn't view the space of the Universe as remaining the same size while the matter in it shrinks.

    However I don't think that can be used to infer that there was no "beginning of time".

    In my opinion we just don't know enough experimentally or theoretically to close the books on the shape and future history of the Universe. We don't know where the Matter in it came from, or even whether it might be being produced today. Fresh Hydrogen from deeper space is moving into spiral galaxies along the arms, that is what fuels new star formation. We don't know if various "constants" are changing with time, throwing our calculations of past history off.

    We have to always contend with the fact that Nature has a bigger and more precise imagination than we inside observers of her have.
     
  14. May 16, 2003 #13
    When we change our length unit to that comprising the expansion of space, and when considering the speed of light to be constant, this necessitates us to change the unit of time as well. Seen from the old units of time and length, the new units of time and space were both in a proportinal way smaller in the past. When measured in the new time and length units, it follows that the size of the universe is constant and that the age of the universe is infinite.

    It's a simple conclusion. But not necessrarily true, but just legimated on the basis (assumption) of the validity of the new length and time units. But it is argued that this or that system of measuring units, are in fact "equal", which means we can not make absolute judgement about that, only relative ones. We have to take into account that one truth (based on one system of measuring units) contradicts another (based on another system of measuring units).
    That is just what the world is.

    And here is one more analogy from geometry, to accomodate this idea of a universe being finite in time in one time unit, and infinite in time in another time unit...

    Think of paralle lines. In flat eucledian space, the lines never intersect and extend to the infinite. In non-flat space, the lines wlll intersect within a finite distance however. I hope this can make you bridge these opposing thougths.

    Matter (using the philosophical term matter) can not be created or destroyed, but only be transformed. But notice that I use matter to denote all forms of matter (particles, waves, energy, fields), not just the physical matter.


    We can imagine that Nature can imagine, but I realy doubt if Nature itself (apart from us) can imagine anything at all. Nature is just the reality outside of our imagination and independend of it, and our imaginations are the projections of Nature/reality in our brains.
     
    Last edited: May 16, 2003
  15. May 16, 2003 #14

    marcus

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    Re: Re: A URL for Chroot

    People, check out this page that chroot refers to! It is very instructive. In fact chroot I am rather familiar with this one of
    Wright's FAQ and not long ago was discussing it in another thread at PF with someone else. What you point to begins:

    "If the Universe is only 10 billion years old, how can we see objects that are now 30 billion light years away?"

    "When talking about the distance of a moving object, we mean the spatial separation NOW, with the positions of both objects specified at the current time. In an expanding Universe this distance NOW is larger than the speed of light times the light travel time due to the increase of separations between objects as the Universe expands. This is not due to any change in the units of space and time, but just caused by things being farther apart now than they used to be...."

    Wright's tutorial and FAQ contain plenty of references to the fact that parts of the observable universe are receding from us faster than c----this is routine. Don't see how this FAQ reference supports your claims---tends more to help dismiss them."

    Someone should do a tutorial at PF on the Hubble law and how the Hubble law distance is defined
     
  16. May 16, 2003 #15
    A star that is near the edge of the visible universe will have global velocity wrt us as well as local velocity, so might escape after time if local velocity is < global.
     
  17. May 16, 2003 #16

    marcus

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    Re: Marcus I have to call you on this

    Hello Tyger,
    Have a look at FAQ "Can objects move away from us faster than the speed of light?"

    http://www.astro.ucla.edu/~wright/cosmology_faq.html#FTL

    Also you might find part 2 of Wright's cosmology tutorial interesting. The four types of distance are discussed including the "comoving" distance that works in the Hubble law.

    http://www.astro.ucla.edu/~wright/cosmo_02.htm


    Did you actually try out the calculator?
    This is important. Please try it.
    Based on the best model we have at present if you put in z=6.4 it will calculate the "radial comoving distance that goes into the Hubble law" as 28 billion LY.
    THAT is what tells us that the rate that space around the quasar is receding from us is 2c at the present time.
    It is something that must be calculated---when you use the calculator leave the parameters that he put in it alone and just press the "flat" button.

    Your answer suggests you are confusing this with the special relativity doppler formula. In SR there are no velocities greater than c and IIRC you can get beta from w=z+1 by the inverse doppler
    formula
    beta = (w2-1)/ (w2+1)
    If you think THIS is what is involved you are missing the point.
    It sounded like you thought I didnt know this "hyperbolic" formula or whatever and that you were telling me about it.
    Indeed this formula never gives a beta bigger than one!!!:smile:
    But that is beside the point.

    The situation in GR is different and it is routine for parts of the observable universe to be currently receding at speeds greater than c and the tutorials on the web make this point over and over again. It does not violate SR, which is a local theory.

    BTW you are right to say z=6.4 does not MEAN that the quasar is receeding at 2c. What it means is what it says, redshift 6.4. But from that redshift which concerns some light emitted maybe 11 or 12 billion years ago one can, using what is currently known about the universe, CALCULATE where the quasar is now. And how fast is is currently receding, or if you prefer to think of it as sitting still, how fast the space it is sitting in is receding from us.
     
    Last edited: May 16, 2003
  18. May 16, 2003 #17

    chroot

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    marcus,

    You really have no idea what you're talking about.

    - Warren
     
  19. May 16, 2003 #18
    Quote by heusdens

    Nature imagined you!
     
  20. May 16, 2003 #19
    OK Marcus

    I'm beginning to see what you're talking about. Near a gravitating body the space around it is being drawn in, in effect it is shrinking around the body. But on the largest scale space itself is expanding, just the opposite of the shrinking. And that has to be taken into account in our calculations. Whereas I was looking at space as being essentially flat with just the objects in it moving apart. That is certainly a different picture.
     
  21. May 16, 2003 #20

    marcus

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    Tyger thanks for the kind words

    I am glad you are feeling like you understand things better.
    The root cause of the misunderstanding is the existence of two different ideas of distance.

    Cosmologists like to use the "comoving distance" that works with the Hubble law and refers to measurement by observers who are at rest with respect to the CMB. It turns out to be a good clear idea of distance.

    Journalists like to use "light travel time" as an idea of distance but this has many pitfalls and leads to confusion. It doesnt measure distance between things in the present. And different parts of the light's path get stretched out by different amounts (early parts have time to be stretched more). so it is a fickle unreliable disaster as a distance measure.

    So you get FAQ by for instance Ned Wright who teaches cosmology and who explains the good kind of distance---FAQ like
    "if the age of the universe is 13 billion Y then how come we can see things that are 39 billion LY away?" He does it with 10 and 30 but it would be more realistic to say 13 and 39. And he
    explains "comoving distance" in quite a bit of detail. It is
    distance IN THE PRESENT from the standpoint of observers who are at rest with respect to the CMB i.e. to the expansion of space.
    In cosmology there is a preferred frame----different from SR where there is no preferred frame.

    And he explains why it is that the current boundary of the observable universe is receding at 3 times the speed of light and so on. It is pretty simple once you get used to it.

    Like, a quasar that we see with redshift 6.4 was not so far
    away THEN when the light left it and was not receding so fast THEN, but by now, in the present moment, it is 28 billion LY away and is receding at twice the speed of light.

    And that is a fairly routine speed for some portion of space to be receding. The speed of expansion of space is PROPORTIONAL to distance---that is what the Hubble Law v=HD says. So since
    expansion has that linearity or proportionality there MUST be lots of space receding from us at speeds greater than c or 2c, etc.

    I dont know why people sqwawk so much when one says this.
     
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