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

Big bang can be disproved?

  1. Oct 10, 2011 #1
    If we can find a galaxy(or anything at all) that is more than 13.7 billion light years away can we say that the big bang theory is wrong?

    Hubble has already found a galaxy that is 13.2 billion light years away.http://www.spacetelescope.org/news/heic1103
    Last edited by a moderator: Oct 10, 2011
  2. jcsd
  3. Oct 10, 2011 #2


    User Avatar
    Gold Member


    The universe is significantly larger than 13.7 billion light years in radius.

    Universal expansion is not limited to the speed of light.
    Last edited by a moderator: May 5, 2017
  4. Oct 10, 2011 #3
    Absolutely unbelievable, isn't it?
  5. Oct 10, 2011 #4

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Yeah, but that's not what that 13.2 billion figure in the article referenced OP was about. In fact, the article referenced by the OP never once used the term "light year".

    Last point first: Hubble discovered a galaxy whose light took 13.2 billion years to reach us.

    Note that the article never said anything about light years. While it is valid to equate (for example) the 4.3 years it takes light emitted by a nearby star to reach us with a distance of 4.3 light years, this concept loses validity when time spans are long. The use of velocity*time=distance in that calculation ignores the expansion of the universe.

    13.2 billion years ago, when that galaxy emitted that light that we are seeing just now, the distance between that galaxy and ours was much less than 13.2 billion light years. Moreover, that galaxy is "now" much further than 13.2 billion light years away from us.

    So what would falsify the big bang? Seeing stars with high metallicity in those very remote galaxies, for one.
  6. Oct 10, 2011 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Hardly :biggrin:

    Would you care to explain why you think it is "absolutely unbelievable"?

    Relativity (the 1915 theory of gravity, by now extremely well-tested) tells us in our type of universe to expect large enough distances to be increasing at rates greater than c.

    And there is nothing in relativity that says they can't.

    In another type of universe---modeled by a different solution to the Einstein equation---large enough distances might be decreasing at rates greater than c.

    Again this is not against the rules, it is just a different case of the same thing. A lot of people seem to think that relativity forbids distances to change at rates exceeding c. It certainly does not! They are just confused by what they were told about motion in a local frame of reference. With that conventional idea, motion in a local frame, somebody gets somewhere. Some point of reference, like a destination, is approached.

    In a pattern of uniform longrange distance increase nobody goes anywhere. In the absence of a gravitation bond holding stuff together, distances simply increase by some small fraction of a percent every million years or so. No destination is approached. Nature does as she pleases with geometry. The current rate that is observed at present is an increase of 1/140 of one percent per million years. It seems very slow, if you try to picture it. But for very large distances it can amount to a rate of increase which is > c.

    This is required by realistic solutions of the 1915 equation. And that relativity equation is our law of gravity---no one has been able to find a better, simpler, or more accurate one. The 1915 equation has been checked repeatedly for 90 some years in every way people have been able to think of, and is still being checked.

    So I would say, just get over it. You have no scientific reason to object to distances increasing in that way. You have only a confused prejudice based on hearsay, or perhaps on misunderstanding something told you that was actually correct.

    It is admittedly quite curious! Geometry can be dynamic, rather than static. From the standpoint of us primates living on a small rock----with our experience of geometry being mainly static and anchored to that rock---it is unintuitive that over very large distances goemetry might not be static. Spatial relationships might change---might have curvature too---three angles might not add up "right". But we just have to accept Nature on her own terms, let her be what she wants to be instead of what seems intuitive to us monkeys :biggrin:

    Suggestion: google "wright balloon model" and watch the animation for a few minutes. The galaxies stay at the same place (same lat. and long.) but the distances between them increase. For larger distances the rate of increase is greater than the speed the little wiggler photons are traveling.
    Last edited: Oct 10, 2011
  7. Oct 10, 2011 #6


    User Avatar
    Science Advisor
    Gold Member

    Finding a galaxy behind the surface of last scattering would, to put it mildly, be unexpected and play havoc with BBT. Years ago it was claimed that some stars appear older than the putative age of the universe. Of course we knew our stellar evolution models were imperfect, and the age of the universe was not known with much precision. Most scientists took a wait and see attitude. This is usually the smart thing to do when you have observations that conflict with a mainstream model. If the model is defective, there will inevitably be other corroborating observations. The temptation to pull a 'chicken little' is certainly there in such cases. But, you don't get do overs in science when you champion a startling new 'discovery' that turns out badly. Think of the recent uproar created by the OPERA group, for example. If wrong, these poor guys will be forever remembered for this one colossal blunder, regardless of whatever other great work they may have done, or do.
  8. Oct 10, 2011 #7


    User Avatar
    Gold Member

    As has already been pointed out, the article you referenced never used the term "light year". When using terms in physics, one of the things I came to realize early on is that sloppiness in terminology leads to and/or is a reflection of sloppiness in thinking. Just getting a handle on what basic terms MEAN takes you a long way towards understanding a lot of physics.

    So, i can easily understand how you read "13.2 billion years" and just slipped in the "light", especially since I've done the same thing when not paying attention, but I'd suggest that you pay careful attention to terminology and fundamental units.

    If I'm preaching to the choir on this and this was just a one-time slip-up, then ignore me. :smile:
  9. Oct 11, 2011 #8
    I think D H got it right about what was wrong in my OP ,we can measure distances between nearby stars and slip the word "light" in between "million..... years" without going wrong but if are talking about ancient galaxies we can't do that because the universal expansion will become very significant.
  10. Oct 11, 2011 #9
    sorry if i have used the term "light year" carelessly, actually i was not talking about size of the universe being equal to or greater than 13.7 billion light years ,what i meant was that if we can spot a galaxy that is more than 13.7 billion light years away ,that means ,the very fact that we are able to see it proves that the galaxy existed more than 13.7 billion years ago because even if the galaxy is not there in the same position now ,it has taken more than 13.7 billion years for the light from it to reach us but that's not possible because when the universe itself is thought to be 13.7 billion years old.Hence ......

    If the BBT is true we should never be able to see a galaxy which so far away ,right???
  11. Oct 11, 2011 #10


    User Avatar
    Gold Member

    You need to read up more on the history of the universe. You misunderstand the situation completely. The object which we see NOW emitted its light 13+billion years ago BUT IT WAS NOT AT THAT TIME 13+billion light years away. It was much closer, so there is no contradiction at all.

    It has taken 13+billion years for the light to REACH us, not because it was emitted that far away but because the photons have been getting dragged away from us by the expansion of the space between us and the point of emission. That's why it is red-shifted.
  12. Oct 11, 2011 #11


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Shashanka with all due respect I think Phinds is right. You do sound confused. One thing that might help is to get used to using one of the simple cosmology distance calculators.

    One I like is called "cosmos calculator". I have the link in my signature, it is the "morgan" link---Professor Morgan put it on line. Or you can just google "cosmos calculator" and you will get it.

    To start a session you type these 3 numbers into the boxes over on the left labeled Omega, Lambda, Hubble.
    .27 for Omega
    .73 for Lambda
    71 for Hubble parameter

    then you put in any redshift that might be observed for some galaxy or quasar, and it will tell you the distance now---and the distance back then when the thing emitted the light.

    You can keep putting in different observed redshifts and recalculating. for each new redshift you put in you get some new distances. It also tells recession speeds.

    If you try it, and have any trouble, please let us know. I or someone will help. It is pretty easy. You probably won't have any trouble.

    It also tells you what the Hubble expansion rate was back then when the light was emitted, which is kind of interesting. The 71 is just the value at present, it has been changing in the course of the universe's history. The calculator implements the standard model of the universe used in cosmology. So it is a hands-on way to become familiar with the standard model cosmos. Verbal descriptions can actually interfere with understanding.

    Good luck, if you try it!
  13. Oct 11, 2011 #12
    I apologise. :)

    I did not think the statement "absolutely unbelievable" required an explanation; the fact that there is a universe in existence rather than nothing at all, and that there are galaxies receding away from us at faster than c, which at some point, they will disappear from us forever. I am sorry that you don't find it as astonishing as I do... :p
  14. Oct 11, 2011 #13


    User Avatar
    Gold Member

    Do not apologize. The universe is indeed a wondrous place, full of all manner of miracles and delights.
  15. Oct 11, 2011 #14


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Wondrous indeed! That is well put. And full of all manner of miracles and delights.

    I like that turn of phrase and am apt to borrow it in future. But also, I find, quite believable (which makes it all the more wonderful.)

    Ealia, there is a neat trick whereby light emitted from a galaxy receding at rate > c can still reach us. You may know of it, and understand how it works already---I don't know how much cosmology you have read.

    If you aren't familiar with this you might like to pursue it by asking Dave or any of us questions. It's pretty simple, once explained. But also kind of wonderful.

    It's something you find out when you play around with that online "cosmos calculator" that I suggested Shashanka google and try.
  16. Oct 11, 2011 #15


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Most of the galaxies we know about and can observe with a telescope have recession rates greater than c.

    This is because the great majority we can see have redshifts > 1.7. Indeed 1.7 is not an especially big redshift for a galaxy to have.

    But using the calculator you find that something whose light comes in to us with redshift z = 1.7 was already receding > c when it emitted the light and today is receding even faster.
    All the more so for any galaxy whose redshift is greater than 1.7. Try plugging in 2, or 3, or 4

    For people who are not put off by simple online calculators, here is the recipe:

    Go to http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html

    On the left you see four boxes
    Matter density
    Cosmological constant
    Hubble rate

    At the start of every session you have to load the first three and then you can put in whatever redshift and press the "calculate" button. You put .27, .73, and 71 in the first three boxes. These are the parameters of the best-fit standard cosmo model.

    Matter density .27
    Cosmological constant .73
    Hubble rate 71
    Redshift 1.7

    Then press "calculate".

    You'll see the recession rate was a bit more than c. Anyone who is curious how the light could have gotten here (it would obviously at first have been swept back) can basically ask anybody who has been around this forum for a while, Dave, Brian Powell, Chalnoth and many more.

    It's neat how it happens.
  17. Oct 11, 2011 #16
    That is very interesting marcus. If you could please, I would very much like to hear how that occurs - I have not heard of it before. Thanks. :)
  18. Oct 11, 2011 #17

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I wrote some stuff about this.
  19. Oct 12, 2011 #18


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Maybe you already know this: cosmo is a math science. Everything we say is about a model. The model is very simple and is built into calculators.
    With purely verbal explanation there is little gain in understanding. To learn you need hands-on experience with the calculator (or the Friedman equation that it is based on.)

    I don't know you. You may be smart on a math level (as well as on verbal level). You may have already tried Siobhan Morgan's calculator, that I was urging in post #11 and again in post #15.
    If not, my verbal explanation might not help. But if you listen and ALSO play with the calculator it might work.

    The Friedman eqn governs H(t) and you can see that H must always decrease.
    But if you played with the calculator you would have ALREADY SEEN that H decreases over time. As you plug in higher and higher redshift the age of the universe goes down and down and the calculator TELLS YOU what H is at each age of the U. H goes up as you go back in time.

    Take a pencil and make a table showing how as the age increases---1 billion 2 billion 3 billion....---the Hubble rate is decreasing. Quite sharply at first!

    Now there is some algebra which a 14 year old can do.
    The key equation is v = HD. What distance corresponds to v = c?
    That is the distance that is growing at exactly rate c. Larger distances grow faster. Smaller ones no so fast.

    The 14 year old child tells you the critical distance is c/H.

    If the photon aimed at us can ever get to within distance c/H of is, it will make it!

    But I told you H was decreasing. You can and should check it with the calculator.
    Therefore the critical distance is INCREASING.

    All the photon has to do is hang in there and stay roughly the same distance from us (or even be swept back a iittle), for long enough, and the critical distance will REACH OUT TO IT. And then it is safe. It will be within the sphere of distances that are NOT increasing faster than c. And it will begin to make real progress and reduce the distance it needs to go and actually gain ground.

    So say the distance to the galaxy is increasing at 1.1 c and it emits a photon in our direction. The distance to the photon will then be only be growing at 0.1 c
    It has a fighting chance. It keeps on struggling towards us and only gets swept back slowly, at 0.1 c.

    Now the critical distance c/H is reaching out very fast because (as you can see by experimenting with the calculator) the denominator H is decreasing rapidly. After a while if the photon keeps trying, it will be within the magic circle. So even though it has to fight the expanson of distances, the photon will eventually get to us.

    The whole thing depends on H decreasing and now it is decreasing much more slowly than it did earlier when expansion was only a few billion years old. So this opportunity is shrinking. Eventually the trick will not work or will do so only in a very marginal way. Too bad. But that is still far in the future.

    Our sky is still full of galaxies that emitted light to us when they were receding >c and the light is still getting to us.

    The time will come when there are not so many galaxies visible in the sky. But it is far in the future so not to worry.

    You should be able to calculate the critical distance c/H.

    c is 300,000 km per second
    H is 71 km per second per megaparsec (a unit of distance)
    So c/H would be some number of megaparsecs that you can calculate.

    I invite you to find that number of megaparsecs. Google will convert it to other units like meters or miles or lightyears, if you want.

    Whatever that distance is, distances bigger than it are increasing >c
    and distances smaller than it are increasing slower than c.
    Very important distance. It has a special name: Hubble radius.

    Maybe you knew all that. If you didn't, and have never calculated the Hubble radius, why not try?

    Confucius is alleged to have said: "Tell me and I will forget. Show me and I might remember. Involve me and I will understand." John Dewey the American philosopher said "learn by doing" or words to that effect :biggrin: I was just reading the Wikipedia on "experiential learning". Did you ever read anything about that?
    Last edited: Oct 12, 2011
  20. Oct 12, 2011 #19
    Thanks marcus and george, for all the information :).I am not able to open the cosmos calculator because some kind of plug-in is not installed , i installed it but i am still not able to open it.I use a very old PC , the modem goes into a 'coma' sometimes, i don't know what exactly is wrong.It is asking me to install it again and again.(forget it)

    Is the Hubble radius equal to this ?? 1.30218021 x 10^23km
    i did it in a hurry ,i might be wrong.

    Is the Hubble radius expanding faster than the Universal radius? if yes,
    will it catch up?
  21. Oct 12, 2011 #20


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I wanted to check your answer and happened to use the google calculator.

    when I type 300000/71 into the google search window I get 4225.
    (sometimes I have to hit "return" or type an equal sign to make it calculate. Do you use the google calculator?

    That answer means the Hubble radius is 4225 megaparsecs. Say I want that in lightyears, then I type in (without the quotes)
    "4225 megaparsecs in iight years"
    It says "1.378e10 iight years"
    That is the same as 13.8 billion light years. It is a proper distance, not to be confused with light travel time.

    On the way I saw that your answer was correct. Because when I had not finished yet and had just typed in "4225 megaparsec" it told me that this was "1.3037e26 meters". Essentially what you said.
    The google calculator tends to jump to the conclusion that I want the answer in standard SI units, unless I specify otherwise by saying "in light years" or something.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Big bang can be disproved?
  1. Can we see the big bang? (Replies: 12)