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Hubble constant measurement

  1. Jul 7, 2003 #1

    From what I red (understand??), the Hubble constant is calculated from galaxy's distance (from us and each other).

    1) Is the information speed limit (c) considered constant althrough (through out??) the path?
    2) Is the distance from us mesured using redshift only?
    3) When mesuring galaxy's separation speed as 70 Km/sec for each megaparsec of distance from us, (a) may I interpret that separation speed being also in the direction from us and also (b) considered 6.8E-10 meter/sec2 being the galaxy's acceleration versus the time frame.

    P.S.: h0 = ( H0 * 1000 * c )/ 3.086E+22[meter/Mparsec]
    Last edited by a moderator: Jul 10, 2003
  2. jcsd
  3. Jul 7, 2003 #2
    Bonjour to you too... however else the greeting goes

    The Hubble constant is used to give a measure of how fast space is expanding. Galaxies closer to us, will travel away from us slower
    than the galaxies distant to us.

    So basically we can use it to measure how far galaxies are from us, by just knowing how fast theyre travelling away from us. Make sense?

    1) c - is constant everywhere in a vacuum, always has been, always will be.

    2) There are a number of ways of measuring distances of galaxies from us. eg. Parallax.
    Red Shift only tells us the speed the galaxy travels away from us (sort of like the doppler effect)

    3) a) Yes b) The acceleration of a galaxy away from us is very small!! Otherwise, I'm not sure what youre asking.

    Hope it helped :wink:

    ps. The best known Ho = 65 ms-1/Mpc
  4. Jul 7, 2003 #3


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    Galaxies are too far away to have parallax as a means of measuring distance. Close by galaxies have distance measured by Cepheid variables. Other means are used for those farther out. The most distant can be measured using type Ia supernovae. This last was the means whereby the acceleration of the expansion was observed.
  5. Jul 7, 2003 #4
    Yeah you're right, parallax is only used
    with nearby stars. (I didn't mean galaxies)

    Cepheids are used to predict distances to galaxies,
    and there is one other method (forgot what its called)
    where the speed of a galaxies rotation is measured
    using a shift in the 21cm line, and that is somehow
    related to something else which will ultimately tell
    us how far it is from us.
  6. Jul 9, 2003 #5


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    (1) the speed of light is constant in the vacuum of space

    (2) There are different methods for measuring distance. Check out this link...

    (3a) 70 kps/Mpc on average throughout the visible universe. Works in any direction you look. So something 1 megaparsec away appears to be receding from us at 70 kps. Something 2 megaparecs away appears to be receding from us at 140 kps. (I say "appears to be" because from their viewpoint, we are the ones receding.)

    (3b) I'm not sure what you're asking either. The Hubble constant is not an acceleration. It's a constant rate of expansion (although things appear to be moving faster away from us when they are farther away from us because there is more intervening expanding space). The acceleration of the expansion of space is something else...lambda...the cosmological constant...dark energy...quintessance...whatever.
  7. Jul 9, 2003 #6
    Re: Re: Hubble constant measurement


    "The most distant (galaxies) can be measured using type Ia supernovae": Is this measurement based on chroma? Which way? Links?

    Therefore, "c" is considered constant everywhere in vacuum and is used to "convert" meter & second metrics?

    Ok! 65, 70, 71, doesn't matter to the essence of my question. The point is: Hubble's constant indicates the speed (ms-1) of separation between 2 galaxies upon the distance (Mpc).

    3a...) Therefore, 2 galaxies, at N Mpc from us (suppose H0 constant in space-time) but distant each other by M Mpc, shall separate each other at H*M (ms-1). Is this separation speed exists in all direction, relative to one of the galaxy. (If I live on one of this N Mpc galaxies, shall I see the other galaxy, distant by M Mpc, going faraway at H*M ms-1 ? )

    3b...) If "3a" answer is yes and following unit conversion as per h0[m/s2] = ( H0 * 1000 * c )/ 3.086E+22[m/Mpc], may I consider h0 at 6.8E-10 [m/s2] being some repulsive acceleration?
    Last edited by a moderator: Jul 10, 2003
  8. Jul 10, 2003 #7


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    I believe in your "P.S." you have misunderstood the unit conversion. This has unfortunate consequences later.
    One parsec is 3.086E+16 meters
    One megaparsec is 3.086E+22 meters
    H0 = 70 km/s per Mpc = 7E+4 m/s per 3.086E+22 meters

    This is the reciprocal of a time, one over a certain number of seconds. (this number of seconds is called the "Hubble time")

    One finds the number of seconds by dividing 3.086E+22 meters by 7E+4 meters.

    It is 4.4E+17 seconds

    But often one says 14 billion years.

    This quantity of time is often written 1/ H0
    or H0 -1

    In your calculation, if you do it correctly, you will not
    get an acceleration term---no "repulsive acceleration" as you say.

    The Hubble parameter with the subscript zero H0 refers to the value of H at the present moment. It is instantaneous, so there is no acceleration in the picture.

    Acceleration comes into another calculation, where one examines the change in H over time.

    If one is only looking at H0 then it only describes the intantaneous rate of expansion of space at this present moment, and there is no acceleration to be concerned with.

    Where is Trois-Rivieres and what are the 3 rivers?
    Oh, I see-----PQ is Quebec Province----probably one of the
    rivers is the St Lawrence.
    Last edited: Jul 10, 2003
  9. Jul 10, 2003 #8
    Re: Re: Hubble constant measurement

    3a and 3b are corrected to reflect what I have in mind, not what I had in mind. In fact, I don't know how I made such much wrong statement
    Last edited by a moderator: Jul 10, 2003
  10. Jul 10, 2003 #9


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    the three rivers are the delta branches of the Vermillion River?

    Acceleration comes into another calculation, where one examines the change in H over time.

    Derivative with respect to time ... (Is H0 a function of time...?)

    not exactly. H(t) is a function of time. The present value H(present) is written with a subscript zero H0.

    In cosmology there are two popular meanings of the "time-zero".
    Often t = 0 means the present moment. But also t = 0 can sometimes be the big bang time.


    If one is only looking at H0 then it only describes the intantaneous rate of expansion of space at this present moment, and there is no acceleration to be concerned with.

    ... but parsec IS time. When I draw Hubble's graph, replacing parsec by time, I got a slope corresponding to deceleration! No? Recessing speed decreases as time go by, at the rate of 6.8E-10 m/s2

    Alas, unfortunately parsec IS NOT time! It is an antiquated measure of distance that derives from the miniscule angle
    the "second of arc" that is 1/3600 of a degree.

    "Parsec" is an abbreviation of "parallax second" and it refers to
    the distance 3.086E+16 meters, at which a star will appear to
    shift in the sky by 1/3600 of a degree of angle if one changes one's viewpoint by one Astronomical Unit.

    In my modest opinion the "parsec" unit is an atrocity and its usage should be punishable by a short term in prison. But the conservatism of astronomers, which they learn from the eternal heaven itself, is almost infinite and they are never able to give up such old-fashion terminology. It is the same with their words for the "magnitude" of stars----the scale goes back to a greek named Eudoxus and it is backwards: dim stars have large magnitude.

    As you have said concerning the three rivers, things are not always as they appear at first sight.
  11. Jul 10, 2003 #10
    Interesting topic ..

    I think the Hubble Relation is kind of a dubious figure .. as Marcus pointed out, you cancel out all the like measurement terms, and you come up with "per second". What?? per second!

    I also think it can only mean anything if the two different terms in it have totally independent measurement. Now, that's the idea, you measure the distance by one method, and then assume the redshift is correctly defined by what we think is cosmological expansion, and use that for recession speed. And that's what they try to do, but the standard candles they use for distance only go out so far. Then, they try to scale that back to redshift, to calibrate that and use that for a distance measurement too. So, at the far reaches of the Universe, where the Hubble Relation really shows itself, and kicks in without regard to gravitational flows, they wind up using:

    HR = redshift/redshift

    And that really makes it dubious, IMO.

    And that brings up an idea I've been batting around, that has some implications here. This might get banished to the "Theory Development" forum, but this is really a question, for anybody, to see if I got my ideas right. Here's the question:

    The galaxies in the Universe are said to be filaments on the edges of vast voids, kind of a frothy or bubble like structure. How that is ascertained I don't know, but probably by the HR (redshift/redshift), which might blow up any usage of my idea. But anyway, say this structure of population of galaxies in the Universe is true. Cosmologist now don't know what put that imprint on the structure, but it smacks to me that this structure is the imprint of quantum fluctuations that Inflation took hold of and expanded to cosmic proportions. That is, the seeds of this filamentary structure and galaxy formation is actually quantum in nature.

    So, Inflation just took uneven distribution of matter/energy in the early Universe, and expanded that to huge proportions, and that's the seeds/filamentary structure we see today. If that is true, there are two things I'm trying to get straight in my mind:

    1) Redshift Quantization -- It appears to me this would be the norm, if you are looking at a slice of the Universe in 1 direction. You would be looking at galaxies/objects that are on the surface of these voids, and (almost) nothing in between these voids. The redshifts, if they are indicative of distance, would be quanticized. You're not going to see a redshift pop in there indicating something at a distance in one of these voids! But, if you would take these same measurements in all directions, the voids of each direction should be at random places, and the combined measurements wouldn't show quantization. I don't know why some cosmologists are fighting Halton Arp, and others, over this redshift quantization .. just say "Yes it's true, but because of this filamentary structure of the Universe on the slice you are looking at. If you look at the Universe as a whole, it is not true." I mean, they ought to embrace this, as it seems to me it would help in their proof that the filamentary structure does exist. Maybe what is holding them back, is knowing that the other proof of filamentary structure is based on a redshift/redshift value, and that is something they don't want to get into?

    2) If this filamentary structure is true, and the theory of Inflation yielded a good enough value for how much it expanded the Universe, we ought to be able (maybe) to use that as an independent measurement of how far across these voids are. We know the upper limit of how big a quantum fluxuation is - it is the Planck length. Then we would know how much Inflation expanded the Universe, so we know the size after Inflation. The difference between that figure and the distance of these voids (statistically) now is due to expansion since inflation, and local gravitational influences since Inflation. On the small scale, local galaxy groups, you can ignore expansion and just concentrate on gravitation. On the intermediate groups, you could use the effects of expansion and gravitation. On the fartherest reaches of the Universe, you could ignore gravitation and concentrate on expansion (since Inflation). Each of these numbers, added to that same value after Inflation, should give the same statistical distance of a void -- the distance would be cross checked 2 ways. And that, with quantization of redshift, should give us distance measurements to the fartherest reaches of the Universe.

    Where are the holes?!?!
    Last edited: Jul 10, 2003
  12. Jul 10, 2003 #11

    Nacho, you are too knowledgeable for mine.

    Concerning the delta:St-Maurice river from the north-west arrive at Trois-Rivières into the St-Laurence river

    Concerning "parsec IS time": IMO, actually (now at my time), a N-parsec-far galaxy recess proportionaly to H. Tomorrow, the same galaxy will be farther and will recess more. This constant increases of recession speed relatively to distance (lightyear, parsec, second) is what I call acceleration. No?
  13. Jul 11, 2003 #12


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    Bravo! this is good thinking!

    I also have noticed this sort of "acceleration" in the model
    which the astronomers use.

    It is a little different from an accelerating motion of something
    moving through space

    because here we are talking about the expansion of the space itself (the galaxy or other massive object may perhaps be sitting absolutely still in the space around it----may be at rest, not moving or accelerating at all----but may be carried away from us by the expansion at an ever-increasing rate)

    the expansion of space is called the "Hubble flow" by astronomers and the best diagrams of it that I have seen on the WWW are in a pedagogical article by Charles Lineweaver, an Australian.
    Also Ned Wright has a cosmology tutorial on the WWW with many good diagrams.

    You can see several acceleration effects in these diagrams.
    Both Wright and Lineweaver are recognized authorities in cosmology so you get the mainstream standard picture from them
    of what the Hubble constant represents and how the expansion works.

    what the actual truth is, one does not know!!!

    however what you say agrees with the prevailing model
    the galaxy will be receding more rapidly tomorrow
    because it will be farther away

    before one bakes a loaf of bread one lets it "rise"---the bread dough expands by the action of the yeast

    In an expanding loaf of RAISIN bread the raisins are all receding from each other

    and they are actually accelerating away from each other as well

    because the more distant raisins are moving away more rapidly
    in proportion to their distance
  14. Jul 11, 2003 #13
    True, but wouldn't the combined gravitational effect that all other raisins have on any 1 raisin be the same after the loaf of bread raised as before it raised?

    If so, none of the raisins can consider that acceleration, especially in a "Machian" or Relative Universe. If it is truely acceleration, we ought to be able to feel the effects of that acceleration. I don't think we would be able to feel any effects due to expansion, assuming there aren't any objects receding from view .. and I'm not sure that would even matter.
  15. Jul 12, 2003 #14


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    I think, as you pointed out, I just tripped up on my own shoelaces so to speak. should not have said "accelerating"
    For simplicity I'm thinking of those distant galaxies as not moving--just sitting still in the space around them.

    the space around them is getting more distant from us
    and the rate of recession is proportional to distance
    so it is greater at greater distances
    and any one patch of space will be receding at a greater rate tomorrow (because it will be farther away then)

    but this is not motion THRU space
    so the Einst. Equiv. principle that equates effects of gravity with
    those of acceleration does not apply here
    because there is no motion thru space and (a fortiori) no accelerated motion thru space

    the way this happens is that in GR the metric is a dynamic changing entity and in the simplification of the GR equation called the Friedman equation there is a "scale factor" a(t)
    that governs the change in the spatial part of the metric
    (just scales the dx2 + dy2 + dz2
    in the simplest way imaginable)

    and just to get stability and be able to solve the Friedman eqn. you have to allow some breathing room to a(t). It has to either
    gradually increase or decrease.

    But this is not motion! It is just a gradual change in the metric, thru its scale factor which multiplies the spatial part of it.
    So none of that Equivalence principle stuff applies to it.

    I have no illusion that I have made this clearer! Whole thing's
    a bit enigmatic. But your post was right and had to respond
    as best I could with something! Or does this make it clearer?
    Depends on you and how you picture things, I think.
  16. Jul 12, 2003 #15

    I didn't mean to imply anything like that .. sorry if I did .. because you also said at the top of the post that I replied to:

    I thought with that, and the quotes around acceleration it was clear you weren't talking about "matter things" in the Universe accelerating. It was just that I like a good analogy .. and the "raisin bread" analogy was the perfect one to bring out how matter in the Universe would see/react to that "acceleration".
  17. Jul 14, 2003 #16

    Here in Canada, we use international measurement system but it took about 10 years to transit from US to Metric system. If we hypothetically consider that this was a slow and progressive transition, our vehicule's speedometer shall show a slow and progressive metric expansion as per 100 mph equals 160 km/h.

    Looking at the absolute value, we could presume that everybody increase their speed by 60% over 10 year due to fuel crisis but, in fact, it was due to scale factor.

    IMO, irrelevantly to how we look to the phenomena, something is causing "scale factor" or the "deceleration".
    Last edited by a moderator: Jul 15, 2003
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