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

A couple of questions about The Big Bang Theory

  1. Jun 16, 2013 #1
    I understand the concept used here, but am unsure about how scientists can be so sure of the actual age of our universe. They explain it by saying it's like pressing rewind, based on the speed that galaxies are moving away from our own, but I don't understand how you can press rewind to find out how long ago "the beginning" really was, when we don't know where the universe ends, if there is an end.

    Since scientists will admit they have no idea how big the universe actually is, and like to use the term "The observable universe" how can they identify an end point to "rewind" from?

    Also, I understand the concept that "nothing" can move faster than the speed of light, with empty space being nothing, but is the light itself still not traveling faster than the speed of light in this instance? If light itself travels faster than the speed of itself based on the laws of physics, is that not a contradiction to that particular law of physics in itself?

    Anyways, I would appreciate some feedback on these two questions, and you don't have to "dumb" it down for me. I am not a physicist nor scientist, but will have no problem understanding any formula's that can help explain how these experts are making calculations that contradict their own "rules".

    So to sum it up, here are the 2 questions once again.

    1: How can we press rewind to calculate the age of our universe when we don't know how big the universe is?

    2: How can light travel faster than the speed of itself under any circumstances?
  2. jcsd
  3. Jun 16, 2013 #2


    User Avatar
    Gold Member

    1: It doesn't matter how big it is, it just matters how long things have been going on. Redshift of distant objects, and other information (the CBM for example) provide plenty of evidence for the age.

    2) I doesn't and no one has said that it does (if they know what they are talking about). You may be getting confused here because the expansion of the universe, under the accelerating effect of "dark energy" IS happening faster than c, but no speeding tickets are issued because nothing is moving faster than c in any inertial frame of reference. Google "metric expansion" and take a look at the link in my signature.
  4. Jun 16, 2013 #3
    Thank you :)
  5. Jun 19, 2013 #4
    For a discussion on the age of the universe try here...and particularly the PLANCK section...


    Just last year, 2013, new data suggests the universe is a bit older than was thought...now about 13.8 billion years old.

    So in one sense nobody knows the actual age of the universe; we have no way to prove it...but there is lots of good evidence as outlined in the link if one assumes things started from a big bang. The FLRW λ-CDM cosmological model appears to show a good fit with observations....so it is the mainstream, consensus, [most popular] model so far. Of course most 'consensus science of past generations have been superseded by better models, so we will have to hope for now we are pretty close.

    That's not really the best way to think of it....there is no "empty space"....all space has vacuum energy of various types....and distant space IS receding away from us!!!..and we from it!!!!

    A good rule when thinking about the speed of light is to remember "locally, light always travels at speed 'c' ". That means right where you are , regardless of your speed relative to anything you'd like to pick, earth, sun, me, whatever, you will measure light at 'c'. That's because right where you are spacetime is flat close to you.....If I measure light right where I am, I will also measure 'c'; but if you try to observe my light from some distance, or me yours, we will in general observe different speeds....

    When distances are involved, and spacetime is curved, like in cosmology and spacetime with gravity [curvature], observations of distant light may appear to show varying speeds...

    Here is an example: At the Hubble radius, a distance of about 13.2 billion light years all around us, space [and distant galaxies there] is receding from us at a recession velocity 'c'.....according to to our FLRW cosmological model...using the distance 'metric' mentioned in a post already. So right ' there', right 'now', light appears to us to be 'standing still'.....[right there any beings will measure their local light at 'c' and our light as 'standing still']....in the future that distant light will start to approach us because of the nature of the expansion of the universe...and will eventually arrive here on earth....Other measures, other models, will show something different.
    Last edited: Jun 19, 2013
  6. Jun 19, 2013 #5


    User Avatar
    Gold Member

    Actually, I've heard it said on this forum numerous times that it's about 3c at that distance. I've been quoting that myself for at least a year so if it's wrong, I'd like to know about it. Where did the 'c" come from? I'm sure it was one of the senior folks on the forum who stated it as 3c but I don't remember who.
  7. Jun 19, 2013 #6
    You can relax Phinds its not in error. The c is simply a good multiplication factor. Recessive velocity is non inertia based for the OP. The galaxies are not moving at 3c. Recessive velocity is distance dependant. The greater the distance the greater the recessive velocity. I honestly wish they used a better term than velocity which implies momentum but were stuck with it. My sig contains a calculator called lightcone. Here is a sample printout. There is some guides including the metrics for the OP it will help with understanding how age is derived

    [tex]{\small\begin{array}{|c|c|c|c|c|c|}\hline R_{0} (Gly) & R_{\infty} (Gly) & S_{eq} & H_{0} & \Omega_\Lambda & \Omega_m\\ \hline 14.4&17.3&3400&67.9&0.693&0.307\\ \hline \end{array}}[/tex] [tex]{\small\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline a=1/S&S&z&T (Gy)&R (Gly)&D_{now} (Gly)&D_{then}(Gly)&D_{hor}(Gly)&V_{now} (c)&V_{then} (c) \\ \hline 0.001&1090.000&1089.000&0.0004&0.0006&45.332&0.042&0.057&3.15&66.18\\ \hline 0.003&339.773&338.773&0.0025&0.0040&44.184&0.130&0.179&3.07&32.87\\ \hline 0.009&105.913&104.913&0.0153&0.0235&42.012&0.397&0.552&2.92&16.90\\ \hline 0.030&33.015&32.015&0.0902&0.1363&38.052&1.153&1.652&2.64&8.45\\ \hline 0.097&10.291&9.291&0.5223&0.7851&30.918&3.004&4.606&2.15&3.83\\ \hline 0.312&3.208&2.208&2.9777&4.3736&18.248&5.688&10.827&1.27&1.30\\ \hline 1.000&1.000&0.000&13.7872&14.3999&0.000&0.000&16.472&0.00&0.00\\ \hline 3.208&0.312&-0.688&32.8849&17.1849&11.118&35.666&17.225&0.77&2.08\\ \hline 7.580&0.132&-0.868&47.7251&17.2911&14.219&107.786&17.291&0.99&6.23\\ \hline 17.911&0.056&-0.944&62.5981&17.2993&15.536&278.256&17.299&1.08&16.08\\ \hline 42.321&0.024&-0.976&77.4737&17.2998&16.093&681.061&17.300&1.12&39.37\\ \hline 100.000&0.010&-0.990&92.3494&17.2999&16.328&1632.838&17.300&1.13&94.38\\ \hline \end{array}}[/tex]
  8. Jun 19, 2013 #7
    At the Hubble radius, recession 'c' is correct...but my distance figure of 13.2 seems to be one of several I have in my notes....13.8 seems likely closer.....but that may be out of date also...

    Something interesting is at 3c recession velocity...but I have forgotten just what.

    edit: I see Mordred posted while I was composing....
  9. Jun 19, 2013 #8
    Mordred...those numbers look a little different than I have seen.....

    are those up to date??

    Or am I out of date because new 2013 Planck data changed the age of the universe a bit??
  10. Jun 19, 2013 #9
    Those are Planck numbers as of 2013. here is the Wmap as of 2013. Jorrie's latest version includes both as well as adding graph capabilities and sone curvature flexibility.

    Here is WMAp

    [tex]{\small\begin{array}{|c|c|c|c|c|c|}\hline R_{0} (Gly) & R_{\infty} (Gly) & S_{eq} & H_{0} & \Omega_\Lambda & \Omega_m\\ \hline 14&16.5&3300&69.8&0.72&0.28\\ \hline \end{array}}[/tex] [tex]{\small\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline a=1/S&S&z&T (Gy)&R (Gly)&D_{now} (Gly)&D_{then}(Gly)&D_{hor}(Gly)&V_{now} (c)&V_{then} (c) \\ \hline 0.001&1090.000&1089.000&0.0004&0.0006&45.732&0.042&0.056&3.27&65.82\\ \hline 0.003&339.773&338.773&0.0025&0.0040&44.566&0.131&0.178&3.18&32.61\\ \hline 0.009&105.913&104.913&0.0156&0.0239&42.357&0.400&0.549&3.03&16.74\\ \hline 0.030&33.015&32.015&0.0918&0.1388&38.325&1.161&1.640&2.74&8.36\\ \hline 0.097&10.291&9.291&0.5317&0.7992&31.063&3.018&4.554&2.22&3.78\\ \hline 0.312&3.208&2.208&3.0271&4.4332&18.181&5.667&10.595&1.30&1.28\\ \hline 1.000&1.000&0.000&13.7533&13.9999&0.000&0.000&15.792&0.00&0.00\\ \hline 3.208&0.312&-0.688&32.0781&16.4035&10.686&34.281&16.429&0.76&2.09\\ \hline 7.580&0.132&-0.868&46.2361&16.4926&13.645&103.433&16.493&0.97&6.27\\ \hline 17.911&0.056&-0.944&60.4216&16.4994&14.900&266.882&16.499&1.06&16.18\\ \hline 42.321&0.024&-0.976&74.6094&16.4998&15.432&653.095&16.500&1.10&39.58\\ \hline 100.000&0.010&-0.990&88.7972&16.4999&15.657&1565.669&16.500&1.12&94.89\\ \hline \end{array}}[/tex]
  11. Jun 19, 2013 #10


    User Avatar
    Gold Member

    I was not questioning whether any speed limits were being broken ... I understand recession velocity. My question is whether at the edge of the OU it is c or 3c.
  12. Jun 19, 2013 #11
    I know you know that phinds that was for the OP on recessive velocity. Thought I mentioned that. From what I see from the calculator Vnow the 3c crosses just prior to OU.
    around 42Gly. Its easy to finetune exactly where the 3c comes in. Although it would depend on which Hubble constant value you use. Planck or WMAP
    Last edited: Jun 19, 2013
  13. Jun 20, 2013 #12
    Mordred posts
    3c explanations:
    yes..it's in Mordred's table posted above....3c is about the speed at z = 1090 or so which is the 'surface of last scattering...as far as we can see into the past before glare blocks our view....or back to about 380,000 years after the big bang.....all different explanations of the same phenomena...
  14. Jun 20, 2013 #13
    Mordred posts:

    Oh good grief...must I 'recalibrate' what little I know because the universe is a smidgeon older....
    This is the third set of revised age data since I have been on the forums.....
  15. Jun 20, 2013 #14
    Roflmao, I wouldn't worry about recalibrating just yet. At least not until further verification evidence but thats just my opinion. Lol when I first started studying cosmology there have been numerous changes in the six parameters. I gotten so used to it I expect them to change slightly with each new dataset.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted
Similar Discussions: A couple of questions about The Big Bang Theory