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

Why big bang?

  1. Sep 17, 2008 #1
    Hey there

    Why did the big bang happened? what made it happen? what is the cause of the big bang?

    Could be vacuum fluctuations?
    Or a collision between different branes or universes?

    Could be some sort of a creator?

    What is the most proved and most appropriate theory that talks about the cause of big bang?


    thnx
     
  2. jcsd
  3. Sep 17, 2008 #2

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Perhaps you need to re read our site guide lines. Anyone who attempts to address your 2nd sentence will be in violation of the guidelines.
     
  4. Sep 17, 2008 #3
    You are absolutely right, I am sorry. I removed my previous message. Let me rephrase what I meant to say:

    There is currently no scientific theory on what may have caused the Big Bang.

    With our current observing power we cannot obtain enough information not even for the first fractions of seconds after the Big Bang (called 'the Dark Ages' among cosmologists), so anything now attempting to address those times can only be considered as pure speculation.
     
  5. Sep 17, 2008 #4

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    It is probably a meaningless question. There being no time before the big bang, casuality is a bit tricky. There are lots of areas in science (especially relativity and QM) where it is meaningless to ask some obvious questions such as which event happend first or where a particle is.

    Not necessarily - we don't have direct observational evidence for the early universe but we can make experiments which lead to theories of what would happen at those energies.
    We don't have observational evidence of the flight of a cannon ball at the battle of Gettysburg but we have a theory about the path the ball followed.
     
  6. Sep 17, 2008 #5
    I agree that, if you define time begining with the Big Bang, then yes, there is no point in asking what happened "before" and causality does become an issue. But I find asking what the Big Bang was and how it might have happened quite a meaningful question, at least in the context of addressing the Big Bang theory itself. I think that the Big Bang only covers our ignorance of what could create an expanding Universe.

    I also agree that we can do calculations and try to see what could lead to the things we actually can see and measure. But as far as I know, with the surveys done so far and the ones planned for the next few years, we can only test some of the thousands of models on inflation, for example.

    I find this area very interesting and admire the people working on it, but I think we still have a long way to go before having an elegant explanation of how the Universe started.
     
  7. Sep 17, 2008 #6

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Are you actually claiming that time does not continue back before the start of expansion?
    I'm curious, mgb_phys, what scientific basis do you think there is for a statement like that?
     
  8. Sep 17, 2008 #7

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    Space-time continues before expansion but it doesn't exist before the initial singularity, at least not in science!
     
  9. Sep 17, 2008 #8

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    I don't understand why you say that. Many reputable scientists work with quantum cosmology models which reproduce the classic FRW picture but do not suffer from an initial singularity.

    What do you mean by "science", if it excludes these people? They publish in the best peer-reviewed journals. Their work is highly cited. One was elected president of the General Relativity and Gravitation society---the main professional society relevant to cosmology---last year. Another was awarded the GRG's main prize for work in cosmology.

    Last year's GRG meeting had about 600 participants---in all kinds of cosmology-related science. They have these big international meetings every three years.
    This is observational (gravitation'l wave, neutrino, gamma, CMB....) theory (black hole, structure formation, classical cosmology, quantum cosmology...). Everything.
    And in that context the quantum cosmologists stood out prominently.

    You are saying that they are not scientists?

    You are saying that what they do is not science, simply because their models don't break down at the big bang (like classical 1915 GR does) but keep on running back in time?

    The field of cosmology has changed a lot in recent years---is it possible that you are not keeping up?

    I'm curious as to why you imply those models are not "science" which continue back before the point where the classic model breaks down. Running models like that has, in fact, become science (but if you haven't been keeping up you may not have noticed.)

    As it stands, you appear to set your own opinion up in defiance the main body of professionals, conference organizers, the editorial and peer-review boards of the main professional journals, and it looks kind of, well, odd. :wink:
     
  10. Sep 17, 2008 #9

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    I would say they are mathematicians.
    In maths you can invent a universe with any geometry you want, with any rules of arithmatic and make consistent statements about it. But it doesn't mean it's real.

    Probably - I'm not a comsologist

    I would say that big bang time=0 model is the orthodoxy, it might be wrong but outside a subset of quantum comologists I think it is the standard model.
    quote- Professor Carroll urged cosmologists to broaden their horizons: "We're trained to say there was no time before the Big Bang, when we should say that we don't know whether there was anything - or if there was, what it was."

    There are hints of structure in the CMB which suggests an ordered state in the early universe ( see Prof Carroll) 'before' the big bang - of course you could argue then that 'THIS' big bang wasn't the original start of time , but that just pushes back the start.
     
    Last edited: Sep 17, 2008
  11. Sep 30, 2008 #10
    This is one of those places where science enters the faith realm, which is why it's fun to talk about.

    Really, all the ideas and theories place a 'creator' as the initial cause. I've read some really 'fluffy' papers where the author was going with some kind of eternal grouping of energy that has always existed. Considering how 'God' is defined in most modern culture these days, atheists may be hard to come by. :P
     
  12. Oct 1, 2008 #11
    I would hardly consider a grouping of energy as a "creator" in the deistic sense.

    I have not read these papers, but I guess if one is trying to explain the Big Bang, it makes sense to go for some kind of energy source.
     
  13. Oct 1, 2008 #12

    vanesch

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I would like to remind the participants here not to derive in religious debates, this is against the guidelines. Please try to make meaningful contributions, and if you are joking, use smileys :smile:
     
  14. Oct 1, 2008 #13

    vanesch

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I guess it is a matter of different standards in different disciplines. Of course, speculative theoretical physics is somehow part of science - I guess that what mgb_phys is after, is that one shouldn't forget that all this is highly speculative - especially to experimentalists who are used to quite stronger requirements for proof. It doesn't mean it isn't science of course.

    Whether time (whatever it may mean) "continues" or not before "the start" of the expansion, whether the proposed quantum models are correct or not, whether inflation is true or not remains, from the P.o.v. of an experimentalist, still a very speculative working hypothesis, while for the theorist that works on these problems, it is "almost standard science". We're still on the level of "does the Higgs exist or not", you know :smile:

    To give you a feeling, for active string theorists, it is probably almost "standard science" that the universe is 10 or 11 dimensional :wink: while an experimentalist will consider that as almost wild speculation at this point.
     
  15. Oct 1, 2008 #14

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    I didn't say that - I said it is the 'standard model' that time doesn't exist before the singularity. No universe, no mass-energy, no space-time = no time!
     
  16. Oct 1, 2008 #15

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    There are two issues here. I would say that it is speculative to assert that time does not exist before the big bang. We don't know that.

    I personally am skeptical and reserve judgement (and I think many cosmologists do as well). Regular time evolution may continue back, or it may not. There are equally good models, as far as fitting the data.

    So I think it is speculation and liable to mislead readers, to assert that time does not exist before big bang.
    ======================

    The other issue is how one should regard cosmologists currently studying cosmology models that extend back before. Should one flatly declare them not to be scientists?

    Well the definition of a scientist is in part membership and status in a community. Who is recognized by peers and colleagues.

    I think it would be healthy to do an objective reality check at this point. The relevant professional association is the GRG society (General Relativity and Gravitation). It holds large international meetings every three years, elects officers, awards the Xanthopoulos prize for research in GR/gravitation/cosmology. A good check would be to look at who gets invited to give plenary talks, who gets honored for their work by special awards, who gets elected president of the association.

    I shouldn't have to point out the obvious, but if you look you will find that the scientists who are exploring nonsingular cosmic models (typically bounce models) are in very good standing indeed! :biggrin:

    BTW I just did a keyword search at arxiv and compared the number of bounce-related preprints in two four-year periods 2001-2004 versus 2005-present. There has been an order of magnitude increase. Jumped from 91 to 774. This is a very rough measure. There has been a comparable jump in citations, again a rough measure.

    One should be very skeptical about which models might or might not be correct. Several recent papers I've seen try to identify observable differences in what models predict---especially subtle effects on CMB and structure formation. But it's clearly too early to make distinctions empirically.

    I don't know of any professional cosmologist who would say at this point that time does NOT exist before big bang. Maybe there are---I simply don't know of anyone who says this. My guess is most professionals would reserve judgment at this point.

    So I don't think the poster I quoted, mgb, is presenting an orthodox or "standard" view, if he declares that time does not exist before bang.

    I would say that there is no scientific reason to declare that it exists and no scientific reason to declare that it does not exist. Should mention, though, that the models that don't break down have the advantage of simplicity---don't have to invent anything new, just run the model back. Occam would like that. But they still need to be tested.

    ===============
    in case anyone is interested in some of the details:
    http://www.grg18.com/
    about 600 attended--all branches of gravitation science (observational e.g. wave, as well as theory)
    Ashtekar was elected president of GRG at the last meeting
    Bojowald was awarded the Xanthopoulos (split with another quantum gravitist)
    http://www.science.psu.edu/alert/Bojowald5-2008.htm
    http://grg.maths.qmul.ac.uk/grgsoc/xanthrules.html
     
    Last edited: Oct 1, 2008
  17. Oct 1, 2008 #16

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    An interesting debate is wether theories about something that is fundementally unobservable are science.

    So structure in the CMB that suggests that the big bang wasn't a singulairty is astronomy. But theories about separate universes before or after (if even that's a meaningfull think to say) this one that can fundementally never be tested or observed is different.

    At which point theoretical physics becomes maths or philosophy is symantics or depends on the person in doing your university's phone directory! But I think it is legitimate to regard theories that cannot lead to testable hypothesis as not science.
     
  18. Oct 1, 2008 #17

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    I have quoted some things you said, mgb, in the preceding post. It is all right to say that the cosmology model based on vintage 1915 GR breaks down at the start (the intial singularity or failure of the model.) We can agree that this shows the familiar classical model has limits of applicability. That's well known.

    It is not all right to say that people studying the newer models which do not break down are not scientists. It is not all right to say that their work is not science, and in particular that it is not gravitational physics.

    Can we agree on that? I hope :smile:

    Hello? We are not talking about theories which cannot lead to testable hypotheses. Maybe you haven't been following current research closely enough for me to discuss this with you. For instance Bojowald's work is aimed at determining what can be learned from the Planck satellite mission. You might read my earlier post mentioning CMB and structure formation.
     
    Last edited: Oct 1, 2008
  19. Oct 1, 2008 #18

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    Observations of eg. the CMB that show structure in the pre-inflation universe and suggest that there wasn't a simple singularity are obviosuly obervational science.

    Models that propose multiple parralel or serial bounce universes with a singluarity and so no observable consequences are different.

    The OP's question was what happened before the big bang. The point is that in the standard model of a singularity that isn't answerable.
     
  20. Oct 1, 2008 #19
    Do we KNOW that there was no time before BB, or is it just a guess? Are you saying that BB didn't have a cause, it just happened? Isn't this just like saying "God did it"?
     
  21. Oct 1, 2008 #20
    Did the universe dwell in the singularity before suddenly starting to expand?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Why big bang?
  1. Why big bang (Replies: 6)

Loading...