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

Singularity or Planckdensity

  1. Apr 24, 2006 #1
    Did our Big Bang start from a singularity or with a Planck density?
    What does the standard model say?
    Under the Planck density I understand the Planck mass divided by the volume of a sphere wirh the Planck radius.
    If the standard model has no answer, what is(are) the opinion(s) of our specialists?
  2. jcsd
  3. Apr 25, 2006 #2


    User Avatar
    Science Advisor

    I would say that for the standard model it makes no difference. Physics starts after the Planck era. However, for a theory of quantum cosmology there should be a difference. Singularities are believed to disappear in such a theory.
  4. Apr 25, 2006 #3
    Many thanks for your reaction. But what then, if singularities disappeared in q.c.?
  5. Apr 30, 2006 #4
    I'm not the specialist, but Planck density would have been created at the initial "moment" of the big bang (at t=0+). The moment before the big bang (t=0-) there was only a singularity. A singularity has no dimension or mass, thus before the big bang there was no Planck density.

    It would be interesting to know at the moment of the big bang, if that is a correct use of terms, which came first. Was it time? Or did all the attributes of the physical universe come into existence at once?

    I believe the Greek histories have Chronos (time) the first to be born from Chaos (the singularity).
  6. May 1, 2006 #5


    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    Planck density.
  7. May 2, 2006 #6
    Singularity Planckdensity & reation.

    Do you really believe our universe has been created?
    In physics we have never observed something coming from nothing. My physical intuition tells me that there must have been a transformation from some kind of energy into what we observe from our BB.
    Consequently I am not in a position to become mad because of a presumed end of our universe where for the same reason there will be no real end. :smile:
  8. May 2, 2006 #7


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    At least until recently conventional cosmology has been based on a pre-quantum vintage 1915 theory which fails at a certain point as you extrapolate backwards.

    You are coming into the discussion during a period of transition in cosmology when QUANTUM cosmology is gaining visibility and people are looking for ways to test it.

    People mean different things by "standard model" in this context. The famous "Standard Model" of particle physics is built on flat space with no gravity---it does not directly apply to early-universe cosmology at least in any comprehensive way.
    A cosmologist might consider the "standard model" to be the classical CDM-Lambda model based on 1915 Gen Relativity.
    that is a very beautiful and successful model of the universe, which however is PRE-QUANTUM and which FAILS at a certain point as you go back.
    It simply stops computing----it gives meaningless infinities.

    Quantum cosmology, as practiced by leading people like Bojowald, slightly changes the model so that IT GIVES NEARLY THE SAME ANSWERS almost immediately after the start of expansion, and from then onwards-----but fixes the classical pre-quantum failure right at the start of expansion.

    Any new theory is required to agree with observation----perhaps predicting very slight differences that can be used to test it for verisimilitude---but unlike CDM-Lambda, it should not fail as you extrapolate backwards in time.

    Right now I would say the situation is confused and in flux. there have been a number of attempts to build Quantum Cosmology models. Many or most have been abandoned. I would say that the leading contender is Bojowald's LQC.

    I have been watching how this plays out by seeing who gives the major invited talks at the top conferences and workshops. Increasingly it is Bojowald and other LQC.

    they were hardly known in 2001, when Bojowald published the key paper "Absence of Singularity in LQC"----have gained a lot of prominence since 2001.

    One way to find out about the situation is to read the ENCYCLOPEDIA OF MATHEMATICAL PHYSICS published by Ensevelier (a major science publisher). they had Bojowald write the article on QUANTUM COSMOLOGY for their encyclopedia.

    that itself is a kind of straw in the wind----an indicator that Bojo's LQC is pointing in the direction of a new standard.

    But in his encyclopedia article he does not merely advertise his QC model----he talks about the other attempts at QC, like earlier ones by Hawkings and by Wheeler. I won't try to mention them all. So he gives you an overview----which is confusing because the QC situation has been in flux for some 30 years or more.


    I think one can say this: there IS NO EVIDENCE OF SINGULARITIES IN NATURE. A singularity is just a place where a human-built model breaks down and gives meaningless infinities or just fails to compute. So a singularity is a product of our minds.

    In the past SINGULARTIES IN other PRE-QUANTUM THEORIES HAVE BEEN ELIMINATED by improving the models and quantizing them. So this is what one can reasonably expect to happen with the bang singularity in classical pre-quantum cosmology models.

    THERE IS NO SCIENTIFIC EVIDENCE THAT THE UNIVERSE BEGAN at the moment that the expansion phase began. Some scientists TALK as if it did because they are used to using a model that stops working when you reach that point going back in time. BUT OTHER MODELS DON'T STOP WORKING. So there is no reason to suppose that things began when expansion did---that is an artifact of a certain classical model.

    However since things are in flux model-wise, the closest thing we have to a STANDARD is the old pre-quantum model, which really was standard as recently as 5 years back (before LQC emerged on the scene or any other strong competitor). ACCORDING TO THAT pre-quantum STANDARD, the beginning of expansion is at a point where the model fails to compute. That is, it occurs at a singularity.

    This is a long way to answer your question
    The "standard model" (to the extent there is one at the moment) has a singularity (i.e. a model failure) at the start of the bang.

    the LQC model has a transition from contraction to expansion (it predicts gravity repels at very very high density----so contraction leads to very high density and pressure but never quite goes off the chart---also these things are quantum observables: density and pressure do not have definite classical values)

    the authoritative (100-page) review of LQC is here

    and also here

    the 10-page encyclopedia article is here

    Sorry that there are no exact up-to-date answers to your question right now. (at least I dont know any!)
    Last edited: May 2, 2006
  9. May 2, 2006 #8


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    BTW if I had to hazard a SHORT answer to your question, without any qualification. I would say the same as Chronos:smile:
    the Planck density being at the transition from contraction to expansion predicted by LQC.
    Maybe that short answer is the best one! (But really needs a lot of long-winded qualification because it is not yet the standard official story.)
    Last edited: May 2, 2006
  10. May 2, 2006 #9


    User Avatar
    Science Advisor
    Gold Member

    As an alternative there is always Abdel-Rahman's 1992 paper: Singularity-free self-creation cosmology. :smile:
    (Note you sometimes have to access this link twice for some reason.)

    Last edited: May 2, 2006
  11. May 3, 2006 #10


    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    hurk4, I was curious so I gave the [very] short answer. I'm intrigued by your objection to 'a universe from nothing'. At some point in the history of the universe, assuming causality is preserved, I see no way to avoid this issue. I find it unsettling and unsatisfactory, but, unavoidable without introducing circular logic - which further complicates the issue.

    I do, however, welcome your input. I am a big fan of causality and the laws of thermodynamics. I see no way to escape this prison without violating one or the other. I favor Einsteins opinion . . . the laws of themodynamics. Food for thought:

    Birth of the Universe

    A Universe from Nothing

    Simple quantum cosmology: Vacuum energy and initial state
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Singularity or Planckdensity
  1. Size of a Singularity (Replies: 46)

  2. Singularity Question. (Replies: 3)

  3. Singularity paradoxes (Replies: 4)