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

Origins of the Universe

  1. Dec 27, 2008 #1
    G'day from the land of ozzzzz

    Is the universe Expanding or accelerating?

    Is the Universe Contracting or deccelerating?

    Simple observation of how the parts within the universe work gives us an idea.

    Also a few scientific papers add weight.

    The Fueling and Evolution of AGN: Internal and External Triggers

    Authors: Shardha Jogee (Space Telescope Science Institute)
    (Submitted on 20 Aug 2004 (v1), last revised 10 Jul 2008 (this version, v2))


    [0809.0537] Cosmographic Hubble fits to the supernova data
    Cosmographic Hubble fits to the supernova data

    Authors: Celine Cattoen (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
    (Submitted on 3 Sep 2008)

  2. jcsd
  3. Jan 17, 2009 #2
    Now that almost everyone are saying that most of the universe (~75%) is Dark Energy theese guys are robbing all that content ?
    As the model says nothing on the contents of DE (>0,=0,<0) any value will be ok.
    But a good model should say something.
  4. Jan 19, 2009 #3

    Heldervelez said

    What is dark energy and what is dark matter?

    Is there a difference?

    In what context are they refering to.

    Most of the matter/energy are found as degenerate matter/energy in and around compact objects and since we cannot see this matter , but for the gravity influence we call these dark matter/energy.
  5. Jan 19, 2009 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    (bold added)

    This (bold part) seems to be a common theme in your posts, Sundance.

    However, as stated it also seems quite inconsistent with relevant astronomical observations.

    For example, the mass of a typical rich cluster of galaxies is estimated to be distributed widely throughout the cluster, over ~several Mpc, and almost none of this mass is "degenerate", much less "in and around compact objects".

    But perhaps it's just that your words are insufficiently precise; would you mind clarifying please?


    * how is the "degenerate matter/energy" in a typical rich cluster of galaxies distributed?

    * what do you mean by "in and around" (compact objects)? as in what distance is a typical "around" one?
  6. Jan 20, 2009 #5


    User Avatar
    Science Advisor

    Much of the time this is true. All we had to do was look hard enough until we found a situation for which it wasn't. Behold the Bullet Cluster. Scientific paper here.

    The basic idea here is that since dark matter doesn't interact, or interacts very weakly, during a collision it should pass straight through. If we're talking about a pair of galaxy clusters that are colliding, the galaxies should do the same, as they will mostly just miss one another.

    But most of the normal matter in a galaxy cluster is not in its galaxies. Most of the normal matter is in this very hot, diffuse gas that is visible in x-rays. So what Clowe et. al. have done here is they compared the x-ray image of the cluster (where you can clearly see the results of the collision: one cluster has "punched through" the other) to the gravity lensing mass reconstruction of the cluster. They find that the mass is around the galaxies (which means around the dark matter), and not around the hot x-ray gas that got stuck in the middle after the collision.

    Dark energy is a different beast entirely, and we don't expect it to be associated with normal matter by much, if at all.
  7. Jan 20, 2009 #6
    how our universe startsa expanding ? what force cause it to expand?
  8. Jan 20, 2009 #7


    User Avatar
    Science Advisor

    Answering that question would require understanding of cosmic inflation began. Since we don't yet even know the specifics of what cosmic inflation was, it's a bit premature to talk about how it began. So the short answer is: we don't yet know.
  9. Jan 20, 2009 #8


    User Avatar
    Science Advisor

    Well, our region of the universe had a start. But the entire universe? We don't know. We also don't know what our own region started from.

    The Jeans' Length describes this. Basically, the expansion is a large-scale phenomenon. For stuff that is close together, the local density outweighs the overall expansion, and stuff collapses. The length scale between collapse and expansion is called the Jeans' Length, and it depends upon just how overdense the region is.

    Er, plasma and matter in stars and other compact objects is considered normal matter. This is much of the stuff that makes up that 4%.
  10. Jan 20, 2009 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Interesting work, thanks.

    What, may I ask, does this paper have to do with the origin of the universe?

    And irrespective of that answer, what does it have to do with degenerate matter?

    There are several answers to these questions, in addition to those provided by Chalnoth.

    If you make models from GR and the Standard Model (of particle physics; SM), add CDM and DE and inflation, you can show these models make predictions that are consistent with almost all relevant astronomical observations*.

    In these models the currently observable universe 'starts' with physical conditions at the extreme edge of the domain of applicability of the SM; the physical state is one of high temperature and density, with fairly well constrained composition (quarks, anti-quarks, gluons, photons, leptons, etc).

    The estimated total mass of all electron degenerate objects (white dwarfs), nuclear degenerate objects (including possibly exotic hadronic degenerate states; neutron stars), and black holes (of stellar mass and above) in the observable universe is trivial compared with the estimated mass of baryons in the IGM (and inter-cluster medium) in the form of low density, high temperature plasma (elemental composition mostly H with some He; metals make up a tiny fraction).

    * consistency with 'lab physics' - GR, QED, QCD, etc - is built-in
  11. Jan 21, 2009 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Maybe a mentor could split this post, and maybe the previous one of yours, from this thread and attach it to https://www.physicsforums.com/showthread.php?t=284178"?

    It seems a discussion of the astrophysics of jets is quite out of place in a thread entitled "Origins of the Universe" in the Cosmology section! :tongue:
    Last edited by a moderator: Apr 24, 2017
  12. Jan 21, 2009 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Last edited by a moderator: Apr 24, 2017
  13. Jan 23, 2009 #12
    about the OP question:

    What have been said is that space is expanding.

    Just because we are measuring an increase on the ratio space/matter, we can not say 'ad hoc' that it is an expansion. Other scenarios are possible.

    Not to mention the limiting philosophical concern, put in evidence in the http://en.wikipedia.org/wiki/Sphere-world" [Broken], that ultimately limits our knowledge.
    Last edited by a moderator: May 3, 2017
  14. Jan 23, 2009 #13


    User Avatar
    Science Advisor

    Er, no. Other scenarios are not possible. It is expansion. Now, it is conceivably possible that one may use some different coordinate system to describe what is going on in a different way, but this does not invalidate the picture of an expanding universe. The expanding universe picture is still going to be accurate, because it's been demonstrated to be accurate.

    Er, that wikipedia article shows precisely how you'd examine whether or not we were living in such a world: by measuring the curvature of space.
    Last edited by a moderator: May 3, 2017
  15. Jan 23, 2009 #14
    I did not invalidate the scenario of expansion, neither took an Emphasis on coordinate system.

    Inherently to the act of measuring, is the acceptance of a measuring rod as standard. Well, this is allways a material piece that we take from our local world, and assume to be constant. We must state all the assumptions and be aware of them.

    It seems to me that the assertion of expansion has an hidden assumption: matter is a constant.

    What I've said, not being biased and removing assumptions, is that we are "measuring an increase on the ratio space/matter".

    "measuring the curvature of space[" -- AFAIK it is globally flat, as we measure it. And I do not put GR in question.
    The experience of Poincaré is another way of saying (like a member of this forum remember us) "A fish is not aware of the water"
  16. Jan 23, 2009 #15


    User Avatar
    Science Advisor

    Well, no. It isn't a hidden assumption. It's a very explicit assumption: that of conservation of the stress-energy tensor. The conservation of this tensor, in fact, is required in General Relativity.
  17. Jan 23, 2009 #16
    GR was written by Einstein as we know, and as of the moment of writing we was not considering an expanding universe. So the "The conservation of this tensor, in fact, is required in General Relativity" is much more natural and easy to apply in an universe as the way Einstein thought. The expanding universe came later. And so, we can not advocate against Einstein easily.
    What you mean is, I think, that the overall mass-energy (or energy-momentum) content is conservative. In a expanding universe we have non conservative possibilities: photons loose energy, and a redefinition of 'energy' or 'conservation' is eventually needed. The overall 'temperature' of the universe is also lowering. 'Temperature' definition as a energy equivalent is in trouble.
    You say "...conservation of the stress-energy tensor..." but I do preffer to think about "..overall mass-energy content is conservative.." with the meaning that no matter-energy has been created/destroyed after the initial moment of the universe. I think that we have the same concept in mind but expressed differently.
  18. Jan 23, 2009 #17


    User Avatar
    Science Advisor

    While Einstein was the originator of General Relativity, the development of GR did not stop with him, and nor does the status of the theory rest upon Einstein's formulation. There is no reason whatsoever to spent one iota of time worrying about what sorts of things Einstein considered likely or not. What is important is what the evidence says.

    And you can prefer saying that overall mass-energy content is conservative all you like, but it's still wrong.
  19. Jan 23, 2009 #18
    Hello All

    Can you actually see exapansion of the universe?

    Chalnoth said

    Can yu supply an image that actually shows expansion?

    I know they speak of space/time expansion based on ad hoc ideas.
  20. Jan 23, 2009 #19


    User Avatar
    Science Advisor

    Can you see atoms? No? How do you know they are there?


    Saying that and backing it up are two entirely different things.
  21. Jan 23, 2009 #20

    An image from out there, would be nice.

    Maths and graphs are down here.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook