Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Seeing red, again.

  1. Jun 25, 2004 #1
    In my other thread I learned that gravitation red-shift does exists. Now I'm thinking of an explaination of why most of the universe seems "redder." Maybe its not because most of the stars and galaxies are moving away from us, but maybe its because some of the stars are massive enough to red-shift the light they emit to make it look like their moving away from us at great speeds.

    I'm probably wrong but its just a thought.
  2. jcsd
  3. Jun 25, 2004 #2


    User Avatar
    Science Advisor

    Not a completely ridiculous conjecture, although it does pose some problems. One of these would be the necessity of stars further away from us being more massive than nearby stars, since they are red shifted more. Since more distant stars appear quite similar in all other ways, it would be difficult to explain why they would be so much more massive than those in our immediate neighborhood.
  4. Jun 26, 2004 #3
    i think the whole source of confusion and controversy in the red-shift discussion is from the fact that current theory pretty much assumes that the degree of the red-shift we see is due entirely to the Doppler effect

    while some or most of the redshift from some or all objects should most definitely be attributed to that, why should we assume that a very massive object that has a very high redshift should be accelerating away from us faster than a less massive object with a lower redshift?

    i think the reason so many people bash the big bang and inflation is because it seems to them that the theory is based an too many simplistic assumptions like redshift=distance=velocity and then because of this, exotic crap like dark energy/matter are apparently dreamed up to explain why observations are deviating from theory

    maybe someone would like to educate us about this?
  5. Jun 26, 2004 #4
    Well put shrumeo, you took the words right out of my mouth.
  6. Jun 26, 2004 #5
    Not true. The redshift due to the Doppler effect, that due to the peculiar velocity of an object, it's not the principal component of the total redshift observed. The principal component comes from the stretching of the wavelentgh by expansion of spacetime

    Gravitational redshift indeed exist, as proved in the Pound-Rebka-Snider experiment, but it only accounts for a very small fraction of the total redshift. And not only the source can cause it, also other stars and bodies in the path from the source to us
  7. Jun 27, 2004 #6
    One other aspect is the location of where one is viewing from, the 'line of sight'. Take our Sun, we see the light emited at a near constant frequency, we are always on the recieving end of its emissions, but when we gaze away from the Sun we do not see the light propergating away with a high Redshift signal.

    If we look at a local nearby Star where our Suns light falls upon it, then it has to be Redshifted form 'Our' pespective viewpoint as it's light is moving away from us, so how do we distinguish light at far away locations as being 'totally' emited from the object in view?..when light from our Sun falls onto another celestial body, that light has to be Redshifted at 'the' location of the object being investigated, we do not observe light in flight through space, only at source or end-source.

    So can we ask how much of our Suns light is falling upon any celestial object we view, and how much if any of this light adds to the Red-Shift signal of other celestial bodies?..how do we distinguish our Stars light from 'other' Stars light signals?
  8. Jun 27, 2004 #7


    User Avatar
    Gold Member

    How do we know that gravitational redshift accounts for a very small fraction of total redshift? We know that clocks run slower the more deeply they are imbedded in a gravitational field - even atomic clocks. This was predicted by Einstein, and is confirmed by the offset needed to keep GPS satellites synched with ground-based clocks. How do we know that quasars are not near-singularities and that the light emitted from them is not redshifted by being emitted within a very strong gravitational field? This mechanism would at least explain how frequencies are redshifted without resorting to tired-light band-aids. There is a lot about redshifts that we do not know.
    Last edited: Jun 27, 2004
  9. Jun 27, 2004 #8
    turbo-1, I also believed that gravitaional redshifts could be important, but then i asked a question in this forum (i'm iron4 there), and JS Princeton, that is a knowledgeable person, answered this
    http://www.badastronomy.com/phpBB/viewtopic.php?t=3636&highlight=gravitational+redshift+iron4 [Broken]

    Note: Although JS Princeton seems to have been banned, and that surprises me very much, he used to be one of the leaders of the forum. Russ Watters also posts there, he can confirm that
    Last edited by a moderator: May 1, 2017
  10. Jun 28, 2004 #9
    Ah, yes, the new one is that it's the expanding space elongating the light waves, i forgot

    so, when did IBBT start? did it start with Hubble's observations or did it start with Einstein and his "cosmological constant"? or was one feeding into the other and we can't get an expanding universe out of our heads (where everything we see "points" to an expanding universe?)

    So now, we have apparently said that the gravitational redshift isn't enough, and the Doppler redshift isn't enough to produce the redshifts we see, and it must be expanding space.

    Was there ever a point when we thought that the Doppler shift was all we needed?
    When was the transition made from all Doppler to some Doppler, some Compton, and mostly space expansion?

    I think I want to start a thread about:

    which refers to soon-to-be-published results with implications to the age of the oldest stars (they could be older than 13.7 by)
  11. Jun 28, 2004 #10


    User Avatar
    Science Advisor
    Gold Member

    Last edited by a moderator: May 1, 2017
  12. Jun 29, 2004 #11
    This is interesting. I had never heard of it before. Of course, we all had to learn the Lyman, Paschen, and Balmer Series in school.

    from: http://cfa-www.harvard.edu/~jcohn/lya.html [Broken]
    Distribution of matter: The Lyman alpha clouds are formed by gas falling into gravitational potential wells of all the matter, not just the luminous matter. So they provide another tracer of dark matter.

    They don't provide enough information here. Do they mean non-luminous matter (big clouds of gas, etc.), or the mysterious dark matter that's supposed to be most of everything?

    this from: http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1998A&A...329...30R
    We study the evolution with redshift, from z ~ 5 to z=0, of the Lymanalpha forest in a CDM model using numerical simulations including collisionless particles only. The baryonic component is assumed to follow the dark matter distribution.

    sounds like Aristotle's concentric spheres
    Last edited by a moderator: May 1, 2017
  13. Jun 30, 2004 #12
    Last edited: Jun 30, 2004
  14. Jul 1, 2004 #13
    so a quasar is supposed to have a mass comparable to a galaxy
    i will say a typical galaxy holds 10b stars,
    let's just say our hypothetical galaxy has 10b solar masses
    IIRC, a typical quasar has a radius comparable to the solar system
    I will just say 30 AU

    mass of sun = 2.3 x 10^33 g (from the Wikepedia page)
    10b x this = 2.3 x 10^43 g
    radius of 30 AU ~ 4.5 x 10^14 cm

    plugging these into the equation above, I get a gravitational redshift from a typical quasar to be z=3.78 !

    Am i wrong here?
  15. Jul 1, 2004 #14


    User Avatar
    Science Advisor
    Gold Member

    The emission source of quasars is thought to be infall of matter into a supermassive black hole. This occurs at a respectable distance from the event horizon: probably much greater than 30 AU. 10 billion solar masses is probably too large. There is no large gravitational redshift component in radiation emitted by supermassive black holes of known size, so no reason to think there it would be a major component of quasar emissions.
  16. Jul 1, 2004 #15
    Anyway, Shrumeo, I stated that that is the formula for a star! I don't know the formula for a quasar
    If you scroll down the wikipedia page, there's a general formula for an spherical object, not necessarily a star. But given that quasars are really black holes (in General relativity volumeless pointlike or ringlike objects), the formula can't apply. Perhaps if you believe (like me) that black holes really have volume, you can try to apply this other formula...
    Last edited: Jul 1, 2004
  17. Jul 2, 2004 #16


    User Avatar
    Science Advisor
    Gold Member

    The 'size' of a black hole is defined by the event horizon. All observable consequences [mainly Hawking radiation] end there. It occupies an observable [finite] volume of space in that respect. It is, however, an interesting feature. Due to quantum fluctuations the precise location of the event horizon is always fuzzy [always in a state of expansion or contraction]. If we apply this model to the concept of the universe as a hyper massive black hole [and it does contain similar features], we would be forced, at any given time, to conclude it is either expanding or contracting. Assuming that to be the case, time dilation effects would cause us to conclude that whatever state we happen to currently observe endures for an enormous period of time [by our clocks].

    So this is the same as saying we exist in a universe with an event horizon radius of 13.7 billion light years. Anyone care to calculate the mass required to produce an event horizon of this size.. and see how it compares to the estimated mass of the observable universe? ... I'm feeling lazy at the moment.
  18. Jul 2, 2004 #17
    So what the general equation is saying is that M/R can never be more than
    1.35 x 10^28 g/cm, otherwise you will have a black hole? Since as you approach that ratio z --> infinity.

    Why am I told then, when I read things about quasars, that they have the mass of a galaxy (maybe I'm confusing luminosity) but the size of the solar system?
  19. Jul 2, 2004 #18
    The event horizon incompasses the area the size of the solar system not the singularity where the actual mass is held. And actual black holes don't have densities of 1.35 x 10^28 g/cm, thats just the threshold volume where after you pass it, all the matter is compressed down to a near point. So you can have a supermassive black hole with the mass of a galaxy but all the mass will still be all be in the central point.
  20. Jul 2, 2004 #19
  21. Jul 3, 2004 #20


    User Avatar
    Gold Member

    It is often said that some quasars are as luminous as a thousand galaxies, even though the variability of the light output of such a quasar can limit its physical size to the diameter of our solar system. If quasars are at the cosmological distance suggested by their redshifts, this is the kind of awesome energy output that is calculated by relating their brightness to their percieved remoteness. If the extreme redshift of quasars is NOT due to their distance from us, but is caused by some other mechanism, they don't have to be as luminous (by many orders of magnitude).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook