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

Is the H.U.D.F. view in excess of 13 bly's. or not.

  1. Feb 14, 2009 #1
    Regardless of whether the universe expanded by swelling, ballistic motion, expanding space, vacuum energy or reversed gravity, is it the case that the most distant objects seen in the Hubble Ultra Deep field view, (hubblesite.org), are at a distance in excess of 13 billion light years from us, as stated by N.A.S.A.? If so, are we looking at this region as it was something over 13 billion years ago?
     
    Last edited: Feb 14, 2009
  2. jcsd
  3. Feb 14, 2009 #2
    You cannot disregard all of these things to measure the distance to, or the age of the light emitted by, the galaxies in the HUDF using redshift alone which, since you haven't given a direct link to any article, I can only assume is what is used. Therefore, you cannot answer these questions

    with your constraints.
     
  4. Feb 14, 2009 #3

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    First off, I think Matt's replies are right on target.

    Just to mention one point, we would need a link to exactly what you are quoting simply to tell what kind of distance they mean. There's light travel time. That often gets used as a measure of distance in popular science journalism and outreach writing for wide audience. Treating light travel time as a measure of distance is somewhat of a concession to the audience.
    It is not the distance measure you'd normally find used in a journal article.

    Maybe you would like to try an online calculator that converts travel time to redshift (readily convertible to comoving distance)
    http://www.astro.ucla.edu/~wright/DlttCalc.html
    It's easy, just put in 13 billion years and you get that z=7.87 and that the actual present-day distance is 29.7 billion lightyears.
    (press the "general" button, he already has standard model parameters put in as the default.)

    ===============================
    I've been assuming that when you mentioned 13 billion, that was light travel time. If the article actually said that some object in the picture was currently at a distance of 13 billion lightyears (that would be the today distance, the comoving distance) then that means the light travel time would be considerably less. this calculator can help us find out:

    Put in a light travel time of 8.8----8.8 billion years---and again press "general". It will tell you that the today distance is 13.1 billion lightyears.
    And the redshift is z = 1.27. So if we see a galaxy with redshift 1.27 and the light has taken 8.8 billion years to get here, that means that the present distance to the galaxy is about 13 billion lightyears.

    Ask more questions if any of this is puzzling.
     
    Last edited: Feb 14, 2009
  5. Feb 15, 2009 #4
    I'm sorry, my brain is going. What I meant to ask is whether those objects were at a distance of some 13 bly's. at the time that the light from them that is reaching us now, left there? I realise that expansion, or collapse, has continued during the intervening years. N.A.S.A. states that the H.U.D.F. takes us to within 800million years of the big bang, with a possibility, (at that time), of seeing objects that are within 400million years, hence the 13bly question.
    You imply that light years as a distance measure is a sop to the masses. We've always been told that we see the object as it was at the time that the light left. Which is logical. If the light travelled for ten years, then surely we see it as it was ten years earlier, where it was ten years earlier, and at a distance of 10 light years.
     
  6. Feb 15, 2009 #5

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    :biggrin: I hope not, Peter! I use lightyears as a preferred distance unit myself, and it's not a concession to anything or anybody!

    In cosmology, expansion effects are important, so light travel time is not convenient as a measure of distance. It doesn't work in the Hubble law, for example (a proportion relating distance and the rate distance is increasing.)

    The distance measure that works in the Hubble law is called the comoving distance. It is the actual distance measured today as it would be determined by a network of observers at rest with respect to the expansion process (for practical purposes "comoving" observers are those at rest wrt the CMB). You can think of it as the instantaneous today distance to the object.

    It is not proportional to the light travel time. There is no simple correspondence. Light travel time is a very clumsy measure of distance, because distances have expanded at widely differing rates in the past. The light travel time is influenced by the whole history of expansion that went on while the light was in transit. So it isn't a good intuitive geometric ruler. Instead people commonly use the today distance----or the corresponding proper distance at some specified moment back then.
     
  7. Feb 15, 2009 #6

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Matt O. and I both suggested you to give the link. It's easy to misinterpret what one reads or to overlook some key detail. For that reason it's safer and more efficient to give links so we can all be looking at the same page. We are all less likely to make mistakes then.

    Assuming your paraphrase is right, though, the light travel time from the most distant object in the picture should be roughly 13.0 billion years.
    (Because the age of expansion is about 13.8, the deepfield view goes back to 0.8, so the difference is 13.0.)

    Look back at post #3 which says how far away those objects are. Their distance today is 29.7 billion lightyears. And their redshift is z = 7.87.

    The answer to your question is NO. You are talking about the most distant objects in the field, which emitted the light when expansion was only 0.8 billion years old, and whose light has taken 13.0 billion years in transit.
    You want to know how far those objects were back then.

    For that, go to Wright's usual cosmo calculator
    http://www.astro.ucla.edu/~wright/CosmoCalc.html
    and put in the redshift z = 7.87
    You will get that the angular size distance (essentially the distance back then when the light was emitted) is 3.35 billion lightyears. We can round it off to 3.3 so as not to seem too precise.

    ========================
    Actually a quicker way to get the same answer---3.35 billion lightyears---is just a one-stop trip to this calculator:
    http://www.astro.ucla.edu/~wright/DlttCalc.html
    where you put in 13 for the travel time and press general and it tells you that the angularsize distance is 3.35 billion lightyears.
    Sorry, I took the long way round because I didn't register at first that what you wanted was the distance from us back then when the light was emitted---or rather the distance of that object from the matter that eventually became the solar system and us.
    ========================

    Another calculator you might like to try
    http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html
    On the left there are boxes for matter density, cosmological constant, Hubble rate, and redshift.
    Type in .27, .73, 71, and 7.87, and press calculate.

    It will tell you light travel time 13 billion years, distance then 3.3 billion lightyears, distance now 29.7 billion lightyears. It will also tell you recession speeds then and now.
     
    Last edited: Feb 15, 2009
  8. Feb 17, 2009 #7
    Thank you for your replies. I have no particular interest in where these galaxies are today, it is where they were then that is of interest. In reply #6 Marcus concedes that that the distance from where we are now to the position that the light emanated from, is indeed some 13 bly's. Which takes me to a separate question posted earlier regarding the size of the early universe. If the light that can be seen to have come from two galaxies, each were in the opposite direction from the other, and each were at a distance of 12bly's from where we now are at the time that the light departed, then that can only mean that 12by's ago, the, (visible to us), universe had a spread of 24bly's. How is this possible in a universe that is "only" 13.7 by's old?
    Also, in your email reply you state that light travel time is influenced by a whole history of varying rates of expansion expansion whilst the light was in transit. Elsewhere I also read of "tired" light or light that is losing it's energy. Does this mean that light speed is inconstant and that Einstein was wrong regarding the constancy of light speed in a vacuum?
     
    Last edited: Feb 18, 2009
  9. Feb 23, 2009 #8
    Marcus, any response to the above reply?
     
    Last edited: Feb 23, 2009
  10. Feb 23, 2009 #9

    Jorrie

    User Avatar
    Science Advisor
    Gold Member

    Your reply to Marcus said:
    If Marcus will pardon the interference, he did not say what you stated, but rather that the light travel time was some 13 by. Light travel time has little to do with the distance that those objects were from us when the light left them. The cosmo calculators give their "then distance" from us as 3.34 bly. The cosmos has expanded in the meantime, hence light took quite a bit longer...
     
    Last edited: Feb 23, 2009
  11. Feb 24, 2009 #10

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Hi Jorrie! Glad to see you. Don't you have your own cosmo calculator at your website, or am I mistaking?
    And/or some cosmo tutorial pages?
    If you do, how about posting a link (or PM if you prefer)?

    I was awfully busy yesterday and didn't see PeterW's post. Glad you jumped in. Don't ever stand on ceremony in a case like that:cool:
     
  12. Feb 24, 2009 #11

    Jorrie

    User Avatar
    Science Advisor
    Gold Member

    Hi Marcus. No, I have only spreadsheet versions, but in a minor way I "collaborated" with http://www.geocities.com/alschairn/cc_e.htm". It does particularly well up to (hypothetically) high redshifts of about one million or so. IMO, The Wright and Morgan calculators are equally good (and simpler to understand) for real measured redshifts up to ~1100, where radiation density plays a minor role.

    As for a tutorial - well, no, not quite. Methinks your "https://www.physicsforums.com/showthread.php?t=261161"" thread does an excellent job! It has become a bit long and unwieldy, so perhaps a separate thread to summarize the salient points may be a good idea.

    -J
     
    Last edited by a moderator: Apr 24, 2017
  13. Feb 25, 2009 #12
    Thank you gentlemen. As it is unlikely that any PF contributor would raise his head above the parapet, tread on the hallowed ground that is Einstein, and state that the speed of light in a vacuum is not constant, Can I safely assume that the light from the galaxies, as configured in box #7 above, each had a travel time of 12 by's in order to reach here, now, at this place that we currently occupy, in 2009, and that 12 by's of light travel time translates into 12 bly's of distance? And that this in turn means that that particular piece of the universe had a spread of 24 bly's when the universe was approx 1.7 by's old?
    Just checking out your ballon analogy, (Marcus), and I do realise that the space between ourselves the aforementiond galaxies has increased, or expanded, since the light left them.
     
    Last edited: Feb 25, 2009
  14. Feb 25, 2009 #13

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Nope. Sorry Peter you are still confused. Because light has a constant instantaneous local speed, there is no simple way to translate a large scale travel time into a large scale distance between sender and receiver.

    Obviously, because after the light has traveled between two points along its way, the distance between those two can increase. So the light will be further from its source than it could have traveled on its own (without the help of distance expansion.)

    So you cannot simply translate 12 billionyears of travel time into 12 billionyears of distance from source.

    You should watch the balloon model animated film
    You will see photons of light (pictured as little wigglers) traveling across the face of the balloon always at the same speed of about 1 millimeter per second. And if you pay close attention you will see that due to expansion each little wiggler manages to get much farther away from its source than it would be able to on its own, unaided by the changing geometry.

    Google "wright balloon model" and see if it helps you think along slightly different lines.

    Here
    http://www.astro.ucla.edu/~wright/Balloon2.html

    Try not to think only with words, because our ordinary language has the assumption of static geometry built into the words themselves. In nature, geometry is not static, so the words themselves can obstruct assimilation.

    The white spiral whirlygigs are galaxies and the wrigglers that gradually lengthen wavelength and change color are photons.
     
    Last edited: Feb 25, 2009
  15. Feb 26, 2009 #14
    Yes, this is a great source of confusion. Well put, Marcus.

    In my opinion the frequent use of the words "is" and "are" by all and sundry also confuses, because cosmologists use them having, in the back of their minds, (and with a great deal of observational justification!) an Einsteinian-based, General Relativistic MODEL of the universe, that the calculators Marcus has referred express.

    Wheras you, Peter, probably have your own rather different ideas about "distance" in the back of your mind, based on everyday experiences with rulers, radar and perspective.

    But, on the scale of the universe, "distance" is a fanciful concept impossible for us to actually measure. For example even the "network of users" referred to by Marcus earlier is a purely imaginary idea, impossible to actually implement, but useful in that it allows us to project our local ideas of "distance" (rather dangerously, I think) onto the cosmic scene.

    Astronomers have a better practice. They don't talk much about distances, light years etc, but just classify far-away objects with the measurable quantity z, which tells how big their redshift is. This can be related by well-established theory and modelling to make quantitative the the concept of remoteness.

    Another confusing feature of the universe is that it is in practice impossible to be sure when far-apart events occur "now", or are even separated by a defined time. Cosmologists define a model-dependent "universal time" to do this, which as far as I can figure out could in practice best be measured locally (to modest accuracy relative to cosmic time-spans) only from the temperature of the locally observed cosmic background radiation, making (quite reasonable) assumptions about the origin of this radiation.
     
  16. Feb 26, 2009 #15

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed


    Yes, that is an excellent way to observe the universal time! Each person in the cosmos is given a free standard time-piece, all the clocks having been started at the same moment (about 380,000 years into the expansion) when the medium filling space cooled enough to become transparent.
    It's very democratic----one standard issue clock, pre-set and free of charge. :biggrin:
    Just provide your own thermometer.


    ===============
    notice however that the deeper you are in a gravity well, the hotter the CMB becomes as it falls in. So your clock runs slower. People outside the well see a cooler therefore older universe. You see a hotter therefore younger. It's just a small correction. Something a time specialist could probably explain how to adjust for :biggrin:
    ===============

    Remoteness, that is the distance I was talking about. In other words distance can be calculated from redshift z by well-established theory and modeling. Making a definite idea of distance (the distance basic to stating and applying Hubble law) quantitative, as you say.

    But I don't agree with your tone. Cepheid distance to the Virgo Cluster is not "fanciful" even tho not practical to measure by radar. All measures of distance outside the solar system involve some type of inference. They are all steps on the astronomers ladder of distance definitions. It serves little purpose to call them "fanciful."

    I hope you are not intentionally giving the impression that because astronomers make use of redshift they do not use distance. Redshift is directly measured and used in galaxy catalogs---it is a useful auxilliary, but it does not replace distance at the equation level or in any fundamental sense. Astronomers constantly make essential use of several interrelated distance concepts---brightness distance, angular size distance, proper distance and so on.
     
    Last edited: Feb 26, 2009
  17. Feb 26, 2009 #16

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    :surprised

    You shock me, oldman. A central effort throughout the last century was determining how far away things were. Distance has been a major obsession of astronomers. Constructing the successive rungs of the distance ladder. Determining various standard candles.

    Great ingenuity has gone into this. What you say is categorically false. It would be more accurate to say that a good many of them talk almost non-stop about distance.

    Off course the catalogs list redshift---that is directly measured.
    But the differential equations and the formulas do not for the most part work with redshift.
    The physical reasoning does not. Redshift is what you catalog and classify with, and you whip out your calculator and covert it, as needed, to the distance you need to think with, and model physics with.
     
  18. Feb 26, 2009 #17

    Jorrie

    User Avatar
    Science Advisor
    Gold Member

    Marcus, I'm not quite as shocked as you are: we old folks have learned to accept that one of the few things we can trust from the astronomers is the redshift - at least it is very directly measured. The distance conversions are mostly model and parameter (H_0, Omega) dependent and we have seen soooo many model/parameter revisions.:wink: Lambda-CDM is most likely not the final model...

    That said, the large scale distances that we have today cannot be very far out - there are some corroborating evidence from independent measurements, as you hinted.

    -J
     
  19. Feb 26, 2009 #18
    Actually, on re-reading what I said, I shock myself. Especially since I am the son of an astronomer who spent a large proportion of his working life making parallax observations to support his family, including me! But ungrateful child I am not.

    Yes, of course what you say is correct.

    Nevertheless on the scale under discussion here, the cosmological scale, which I had in mind as I wrote and thought about modern astronomers who use satellite observatories, there is always a screen of theory between the observation of redshift and distances in SI units, light years or light-travel times. Sometimes quite sophisticated theory, even if calculators handle it easily.

    As you say, you whip out a calculator or use a web calculator. And also plot redshift against distance measured by a ladder of indirect deductions (which always starts with non-GR parallax measurements) to deduce the Hubble law. But distance, GR teaches us, is a difficult and dynamic concept. Better here, I believe, to stick to an operational approach in matters didactic and talk of redshift plain and simple, and also to stress that cosmological NOW is just as difficult a concept as a bunch of distances that morph. It should be treated carefully .
     
  20. Feb 26, 2009 #19
    I think we need to be careful here to distinguish between theory dependence and mere coordinate dependence.

    1) Theory dependence: It is recognised that the interpretation of a lot of observations depend on the theory they are supporting. This is sometimes used as a criticism of the theory, but it shouldn't be. This is the way science works, you make a theoretical model which predicts what the observations should be and then you compare this with what is actually observed

    2) Coordinate dependence. Anyone who has taken a course in GR should know that it allows you a lot of freedom in choice of coordinates. This isn't a problem if you do calculations using GR, but it tends to be a problem in popular accounts of cosmology, where readers tend to have an intuitive idea of what 'distance' or 'now' means and think that this extends to the farthest reaches of the universe
     
  21. Feb 26, 2009 #20

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Now with the helpful contributions of Jorrie and chronon we have the makings of an implicit balanced consensus and I basically agree with the general drift of Oldman's posts and what I imagine to be the collective sense.

    Oldman it's illuminating to learn your father was an astronomer. You have a sophisticated view of cosmology and it's measurements, and I think we understand you better with that bit of background.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Is the H.U.D.F. view in excess of 13 bly's. or not.
  1. Horizon of view (Replies: 3)

  2. Star Viewing (Replies: 2)

Loading...