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Ultra Deep Field raises questions

  1. Apr 22, 2004 #1


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    Hubble's UDF image is visually stunning. It also raises questions for me that I hope can be answered here in terms that I can grasp. I am an amateur astronomer (dabbling in astrophotography) with limited math skills. I marvel at the enormity of our universe every time I step out under dark skies, but the UDF has taken that awe to a whole new level, because it brings up some incongruities (to me anyway) that are impossible for me to resolve without some help.

    Official descriptions of the image state that it contains galaxies at z=6 and perhaps even to z=12, which if true means that Hubble has directly observed galaxies that formed only a couple of hundred million years after the Big Bang. There is also the implication that with sufficient telescope time (millions of years!) Hubble could demonstrate to us that the universe is remarkably homogeneous in all directions. In other words, no matter which direction we look, we would see multitudes of galaxies, including highly redshifted faint ones that by inference are very old (a few hundred million years after the Big Bang).

    Question 1 - If the above is true, how did the irregularities arise that precipitated the formation of all these galaxies, and why is the distribution (in every direction) so uniform? It seems like a couple of hundred million years is a very brief time for so much galactic structure to develop. Also, I'm guessing that the irregularities must have arisen when the universe was VERY young and compact, or the results would not be so uniform in every direction.

    Question 2 - As we survey space with our telescopes, we get the illusion that as we increase the sensitivity of our 'scopes, we are looking farther outward. It appears as if we are at the center of a huge sphere (the observable universe) that appears to grow in volume as our telescopes and sensors improve. This impression is hard to shake, but it must be fundamentally wrong, since if the universe is expanding, the light we see from the faintest, oldest galaxies must have been emitted when the universe was much smaller and more compact. Does anybody have a topological model that can reconcile this problem to a person with limited math skills? Philosophically, I can tell myself that as we look outward far enough in any direction, we are essentially looking back toward a singularity which is the beginning of the universe, but I have trouble visualizing the convergence necessary to work back toward that much younger universe.

    Thanks to anybody who can help, or point me to some good resources.
  2. jcsd
  3. Apr 24, 2004 #2


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    Sensible and good questions that come up often. Instead of typing a long reply, just take a look at: http://www.astro.ucla.edu/~wright/cosmology_faq.html#ct2 and I think most of your questions will be covered there somewhere. It is a good "keeper" site, and I would bet that about everyone who posts here has it bookmarked.

    Come back with more if you don't find what you need; lots of PF posters can answer your questions.
  4. Apr 24, 2004 #3


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    Thank you

    Thank you for the link to Ned Wright's pages. I had bookmarked his home page from links on other posts but had been unable to reach it and assumed that it had been abandoned. Yesterday, I found a link to his cosmology tutorial that bypasses his home page and was able to get through, and indeed found sone nice diagrams that demonstrate how the universe might well be infinite, while our "observable" universe is bounded by the time since the big bang and the distances that the photons from distant galaxies have had to travel to reach us.

    That would moot one of my questions (2), although others are popping up. The first question still looms large, though. If the universe is the same in all directions (Mr. Wright says "homogeneous and isotropic", IIRC) and is infinite in size, when did the initial conditions arise that initiated all this uniform galactic structure? If the universe is infinite, it is hard to think of it as being anything less than infinite at any time other than at "zero time". I've got a lot of digging to do. I wish now I had kept pursuing Engineering instead of switching to Liberal Arts all those many years ago, so I'd have better math skills. Oh, well.
    Last edited: Apr 24, 2004
  5. Apr 24, 2004 #4
    Maths skills help but there are few concepts that scientists can't explain in words.Richard Feynman,the nobel laureate, said if we can't explain it , we don't understand it.And he's right.
  6. Apr 25, 2004 #5


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    Welcome to Physics Forums turbo-1!

    Labguy gave a good answer to your questions, and you've already found the Ned Wright cosmology tutorial useful.

    At the top of this General Astronomy & Cosmology sub-forum there's a sticky thread, started by Phobos (with lots of contributions from marcus), the A&C Reference Library. There are many, many links in the posts in this thread. While not all of them are relevant to your question on how proto-galaxies arose, and how the universe came to be isotropic and homogeneous (at large scales), I'm sure by reading the relevant ones you'll get lots of answers. Any that you still have, please ask away here!

    Some 'jargon' may help you find things a bit faster ...
    - 'inflation' is how the early universe was made 'flat' and homogeneous. Guth's first inflation model is now thought unlikely to be a good model of what happened, and as there's little yet to constrain the details of inflation, a veritable 'inflation cottage industry' has sprung up; theories galore
    - 'large scale structure' is the lumpiness of universe; there's a hierarchy of scales, with cosmology mostly concerned with the galaxy-cluster-supercluster-chains&sheets&voids levels.
    - 'dark matter' (DM) is widely accepted as critical for cosmological models, if consistency and match to observations are required. However, while the bulk characteristics of DM can be described, and its distribution within clusters and superclusters has been worked out to some extent, what DM is comprised of is a mystery (lots of ideas!)

    Very broadly, the idea is that inhomogeneities in matter density (caused by ... ah, that's a good question!) at the time of matter-radiation decoupling (somewhat of a misnomer) gave rise to the observed large-scale structure "today". Although the general outline of how the regions of over-density subsequently resulted in the first generation of stars and proto-galaxies is more or less OK, many details have yet to be worked out (or, if you prefer, constrained by good data). From time to time you will see PRs breathlessly announcing an observation which breaks the consensus model (or some such drama); the context and details of what have been found are usually more prosaic.
  7. Apr 25, 2004 #6
    another cosmological paradox

    hi its a very interesting post

    this is another paradox that was arised to me when thinking about great scale universe
    suppse we observe two galaxies at the edge of the universe but in opposed extremes , said,north and south, and suppose they are very near the big bang, so at this time the universe was thousands times small than it is now,
    ¿ is not possible that this two galaxies have a gravitational effect one over another and that this effect could in fact be observed?

  8. Apr 25, 2004 #7


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    I think you'll find this goes by the name of the 'horizon problem', a term which you may have seen in some of the webpages in the links in Phobos' sticky post. You don't need deep maths to understand this, though there are some subtleties which can easily trip you up.
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