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

Galaxy redshift survey

  1. Feb 3, 2009 #1


    User Avatar
    Gold Member

    arXiv:0902.0229 [ps, pdf, other]
    Title: Large-scale fluctuations in the distribution of galaxies from the Two Degree Field Galaxy Redshift Survey
    Authors: Francesco Sylos Labini, Nikolay L. Vasilyev, Yurij V. Baryshev
    Comments: 20 pages, 31 figures, to be published on Astronomy and Astrophysics. A higher resolution version is available at this link this http URL
    Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
    We study statistical properties of galaxy structures in several samples extracted from the 2dF Galaxy Redshift Survey. In particular, we measured conditional fluctuations by means of the scale-length method and determined their probability distribution. In this way we find that galaxy distribution in these samples is characterized by large amplitude fluctuations with a large spatial extension, whose size is only limited by the sample's boundaries. These fluctuations are quite typical and persistent in the sample's volumes, and they are detected in two independent regions in the northern and southern galactic caps. We discuss the relation of the scale-length method to several statistical quantities, such as counts of galaxies as a function of redshift and apparent magnitude. We confirm previous results, which have determined by magnitude and redshift counts that there are fluctuations of about 30% between the southern and the northern galactic caps and we relate explicitly these counts to structures in redshift space. We show that the estimation of fluctuation amplitude normalized to the sample density is biased by systematic effects, which we discuss in detail. We consider the type of fluctuations predicted by standard cosmological models of structure formation in the linear regime and, to study nonlinear clustering, we analyze several samples of mock-galaxy catalogs generated from the distribution of dark matter in cosmological N-body simulations. In this way we conclude that the galaxy fluctuations present in these samples are too large in amplitude and too extended in space to be compatible with the predictions of the standard models of structure formation.

    If they do not fit the standard model what is the alternative?
  2. jcsd
  3. Feb 4, 2009 #2


    User Avatar
    Science Advisor

    That is the question. It's really difficult to say exactly. It might say something about the temperature of dark matter, for instance (the standard model has dark matter at zero temperature, hence the term "cold dark matter"...obviously this isn't right, but we don't know what its actual temperature is yet). Or it may be saying something about the time-evolution of dark energy. It may also just be telling us that the approximations used to calculate the expected fluctuations are corrupting the result.

    I'd have to look at the paper in more detail to say more, but it's a little bit outside of my field.
  4. Feb 14, 2009 #3
    It is a mathematical question, but I like to know the exact formula to calculate the red shift of remote galaxies. In remote galaxy, the space expansion should be the main factor of red shift and can someone like to explain the formula ? For example what is the red shift for a galaxy moving away with a half speed of light ?
  5. Feb 14, 2009 #4


    User Avatar
    Science Advisor

    Well, you can't really interpret the redshift as a result of expansion directly as a velocity, as the amount of expansion of space between the time the photon was released and the time it is observed sets the redshift, not the velocity.

    However, if we take an object that is moving at high velocity with respect to ourselves in the nearby universe, where the expansion is negligible, one can use the relativistic doppler effect to compute the velocity along the line of sight from the redshift:
  6. Feb 14, 2009 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I take that question to be asking what is the red shift for a galaxy whose recession speed is half the speed of light?

    Recession speed is the rate the distance to the galaxy is increasing. It is better not to think of recession speed as motion. Because for many galaxies that we see, the recession speed is considerably greater than the speed of light. It could not be a motion in the usual sense.

    But I like the question (as long as we realize it is a recession speed, not real motion.)
    I will find out for you.

    Yes. The answer is redshift z = 0.58.
    There is no closed-form formula---you must do a messy numerical calculation. But there is an automatic online calculator that takes care of this for you.
    Put in the standard matter-fraction 0.27 and darkenergy-fraction 0.73 and Hubble rate 71, and then put in z=0.58 and you will see that if you see a galaxy today with redshift 0.58 that means the recession speed NOW is 0.5c. That is what it is today as we are receiving its light. And the recession speed THEN back when the light was emitted is 0.42c.

    You must distinguish between recession speed then and now.

    For example if you put z=0.7 into the calculator it will say that the recession speed then (when the light was emitted) was 0.5c.
    And the recession speed today, as we are receiving the light, is 0.59c.

    Maybe you can make a table, and fit an approximate curve to it. That would be interesting. But as far as I know there is no simple perfect formula. One must try out various z and see which is the right one.
    Last edited: Feb 14, 2009
  7. Feb 15, 2009 #6
    Mr Marcus, all.

    The op question is why this study doe's not fit the standard model, do you have any thoughts on that.

    I must say i have a hard time with this expanding or distance increasing space idea, can it even be measured.

    Respect old ned.
  8. Feb 15, 2009 #7


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Hello old ned,

    If you have some difficulty with concepts in cosmology you could start your own thread and ask questions, either here or in the General Relativity forum.

    This thread is about something else. I dropped in just to respond to a side question by V2kkim.

    This discussion thread is about the paper by Labini et al.

    The Labini paper is probably not as important as some remarks made on this thread might lead you to think :biggrin:
    There is indeed one standard cosmological model---the socalled LCDM that everybody uses, though they may adjust parameters up or down some.
    There is AFAIK no one standard model of structure formation. Structure formation is the condensation of clumps out of originally almost uniform cloud. That is something people are currently working on.

    They make computer models to try to imitate how clusters of galaxies curdled and collected out of the initial fog. They are trying several different ways of modeling this.
    Trying for something that gives realistic results.

    Structure formation models are details that take place within the broad context of the LCDM. The LCDM is fairly settled and the Labini paper does not challenge it. It finds nothing that contradicts the main outlines of LCDM.

    Details of structure formation is still work in progress. It is par for the course when Labini finds some statistics that don't match various structure formation schemes that people are studying. Just a tempest in the technical teapot. But potentially helpful to the people who are trying to figure out the process of structure formation (curdling :biggrin:)

    If you are having trouble with the basic concepts, I'd say ask questions about the basic concepts in some more appropriate thread.
  9. Feb 17, 2009 #8
    Regarding recession speed, what is the largest one observed so far? And definitely the weak light signal should be the dominant factor of farther galaxy measurement, but are there other factors making measurement difficult ? If we make a very good telescope say 10 times larger than Hubble scope then will the measurement reach much farther ?
  10. Feb 17, 2009 #9


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Keep in mind that we do not observe the recession speed of very distant objects.

    We observe their redshift. The redshift is not a simpler doppler effect, and there is no simple relation between the redshift and the object's current rate of recession.

    There is also no simple relation between the observed redshift and the earlier rate of recession that the object had back when it emitted the light.

    What you should be be asking is what is the largest observed redshift.

    To answer your question, we observe the CMB which has a redshift of about z=1090.

    You can use Morgan's online calculator to get the "now" recession speed and the "then" recession speed of any object or material observed with redshift 1090.
    The link is in my signature, in small print at the end of the post. If you don't see how to use the calculator, ask. You have to type in 4 numbers: .27, .73, 71, and 1090.

    But you may be wanting to know the highest observed redshift of a galaxy or of a proto-galaxy seen as it is just beginning to form stars.
    In that case, the highest uncontested redshifts measured so far are between 7 and 8.
    Ned Wright usually keeps us up to date on that by reporting in his "News of the Universe" section of his website. They keep finding higher and higher redshifts.

    Again, if you see somebody found a galaxy with redshift 7.6, and you want to derive the speed then, and the speed now, just go to Morgan. It does it automatically:

    If you ever can't remember how to find Morgan calculator it is the top hit when you google "cosmos calculator".
    Remember to type in the first three numbers: .27, .73, 71, and then put in whatever redshift, like 7.6, and press calculate.
  11. Feb 17, 2009 #10


    User Avatar
    Science Advisor

    Well, the largest redshift observed so far is from the cosmic microwave background, at around a redshift of 1089. The largest redshift of a single object so far is from, I believe, a high-redshift quasar at somewhere around z=7 or so.
  12. Feb 22, 2009 #11
    Thanks for info of the red shift. For cosmic micro.. background, what was the original frequency of the emitted light ? gammy ray or x ray ?
  13. Feb 22, 2009 #12


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Mostly visible and infrared. Thermal glow of a black body at 3000 kelvin.
  14. Feb 22, 2009 #13
    Um, is 2dF, also in dS?
    I think it's cool that galaxy redshift is GR in ER, don't u? For the GR survey in 2d dS?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook