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

Cosmic Background Radiation & Big Bang

  1. Dec 16, 2004 #1
    Dear All,

    I am trying to understand SPECIFICALLY the physics HOW and WHY the discovery of the 2 Bell scientists is hailed as something like the cosmological equivalent to the "missing link" as regard the Big Bang Theory ?

    I am looking for any and all links that explain this connection between the cosmic background radiation and the Big Bang so i can understand this.

    Thanks in advance for all responses,

    Celal Berker
    London England
     
  2. jcsd
  3. Dec 16, 2004 #2

    Phobos

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

  4. Dec 17, 2004 #3
    OK. so George Gamow predicted it. Can anyone in layman's terms explain the physics behind it ? Why does that radiation that is observed HAVE to be a remant of the Bing Bang ? Could it not just as easily mean something else ?

    Celal
     
  5. Dec 17, 2004 #4

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    The physics are easy. Gamow, and many others, said the early, hot big bang universe would leave a heat trace in the background. It was found to be there. Others also predicted the early universe would be composed of about %75 elemental hydrogen and about %25 helium.. that was later found to be true as well. An anology: I predict [using my theory] the next two cards you roll will be the queen of diamonds and two of spades. You might say I was just lucky when the queen of diamonds roll, but, what do you say when the two of spades follows?
     
  6. Dec 17, 2004 #5

    hellfire

    User Avatar
    Science Advisor

    I think the key point is the nearly perfect blackbody spectrum at 2.73° K (the temperature deviations are of 10-4), which cannot be reasonably explained by other alternative theories.
     
  7. Dec 17, 2004 #6
    Hello Chronos,

    a few questions please if you don't mind :

    1)you say :"the early, hot big bang universe would leave a heat trace in the background. It was found to be there."

    Why SHOULD there be some left over "heat trace" ?

    2) you also wrote :" Others also predicted the early universe would be composed of about %75 elemental hydrogen and about %25 helium.. that was later found to be true as well."

    The "early universe" ? I presume we are not now living in the "early universe". In which case can you please say a few words about this can be observed please ?

    Thank you,

    Celal
     
  8. Dec 17, 2004 #7
    Hello H.F.

    What does the B.Bang have to do with a black body ?

    Thanks,

    Celal
     
  9. Dec 17, 2004 #8

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Chronos and hellfire have (started) to answer your first and second questions (all three of which are excellent!), I'm sure there will be more to come.

    Your last one is either very easy to answer :smile:, or takes quite a lot of explaining :cry:

    First, what's observed fits well what's predicted by the BBT (Big Bang Theory), so like what happens in much of science, we have a theory that is consistent with a great many good observations and experiments - of several different kinds - and another set of good observations match yet more predictions, so more kudos for the BBT.

    Second, when something 'new' is observed, once several good observations are taken by independent teams (to ensure verification), scientists want to 'explain' the new phenomenon. This means seeing how well it is consistent with established theories (the observations are always - or should always - be primary; theory must account for observations). In the case of the CMBR, all kinds of ideas have been suggested as to what might lead to the observed results, but none (AFAIK) have come even close.

    The 'best' alternatives are rather contrived anyway (e.g. spinning iron whiskers, a component of the interstellar dust), but no matter how contrived, are quite inconsistent with the data (e.g. degree of isotropy, polarisation, detection of the SZE, close-to-ideal black body spectrum).
     
  10. Dec 17, 2004 #9

    hellfire

    User Avatar
    Science Advisor

    All the radiation emitted by a blackbody is of thermal nature, since it does not reflect light. A blackbody emits according to Planck's law, which gives the intensity as a function of the wavelength. This function is uniquely determined by a value of the temperature. It is assumed that the universe was in a nearly perfect thermal equilibrium at the time the cosmic background was generated (strictly speaking at the time the universe got transparent to the background). Since this radiation does not leave the universe and since there is also no other radiation coming from outside, the background should have a nearly perfect blackbody spectrum (it would be a perfect spectrum if the thermal equilibrium had been perfect). The amazing thing is that the big-bang model does predict not only that the CMB should have a blackbody spectrum, but it does also predict that this blackbody spectrum cannot be perfect (although it is nearly perfect).
     
  11. Dec 17, 2004 #10
    Here's a model that may help you understand the CMBR a little bit better. Take a chunk of iron and throw it in a furnace and it will begin to glow -red at first, then yellow, and finally a bluish-white. The chunk of iron is now radiating like a blackbody. The radiation released by the chunk of iron will follow a plank curve. By measuring the strongest wavelength (lambda max), we can determine the temperature of the iron.

    If we remove the iron from the furnace, it will lose it's heat and stop glowing after a while. If, instead, we could place the chunk of iron in a closed system such that no heat can enter or exit, it would retain it's heat forever. This is the model for our early Universe: a glowing chunk of iron that heat cannot enter or leave.

    If left alone, the chunk of iron will remain at the same temperature forever. If, instead, we double the volume of the iron (keeping the mass the same and thus lowering the density appropriately,) it will still have the same amount of thermal energy, but it will be spread over a greater volume => therefore the average temperature of the iron will drop => The wavelength of lambda max will increase.

    This is what our Universe has been doing since the Big Bang. If we can estimate what the temperature and average density of the Universe at some previous point in time, along with the current density of the Universe, we can calculate what the temperature should be now. Now, this temperature is down to around 3 degrees Kelvin, which has a lambda max in the microwave region of the EM spectrum.
     
  12. Dec 17, 2004 #11

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    The early universe is the first 500,000 or so years following the big bang that preceded formation of structures such as galaxies. The universe was basically a huge gas cloud composed strictly of matter formed during big bang nucleosynthesis. Having calculated the temperature, conditions and duration of this phase, scientists predicted this cloud would be about 75%H, 25% with a small amount of light elements such as lithium. When we look at light passing through intergalactic gas clouds [which are remnants of the original huge cloud] we see they are composed of elements at the ratios predicted.
     
  13. Dec 17, 2004 #12

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    One other observation - that's not often mentioned - which supports the CMBR being the highly redshifted glow of 'the surface of last scattering' (from the time when matter and radiation 'decoupled') was done earlier this year (or was it late last year?).

    A team of astronomers working with one of the European VLTs (I don't remember which one) estimated the temperature of the CMBR at a time billions of years ago - by observing the spectral lines from a molecular transition (or was it an atomic one?) from gas in some distant galaxy or cluster. While the error bars were relatively large, the value for the temperature (~10K, IIRC) is quite consistent with what the BBT predicts it would be, that number of billions of years ago. :cool:
     
  14. Dec 17, 2004 #13

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

  15. Dec 17, 2004 #14
    CMBR Physics on internet:-
    1.As I know the best place to learn CMBR Physics is
    Hu's page. He gives a long introduction of CMBR physics. Hu has done really a great job in CMBR physics this is clear from his publications.

    http://background.uchicago.edu/
    2. Max Tegmark's CMB analysis center is also a good place to learn CMBR physics.
    This page provides links of all important webpages related to CMBR physics around the globe.
    http://www.hep.upenn.edu/~max/cmb/experiments.html
    3. Want to know quickely about the relation between CMBR and Big bang, visit here
    http://www.astro.ubc.ca/people/scott/cmb_intro.html
     
  16. Dec 18, 2004 #15
    Hello HF,

    First of all many thanks to everyone who has contributed to this thread and to my greatly increased understanding of this phenomenon as a result.

    Just one more (perhaps) final question to HF : You use the term "background". How is this to be understood ? Is there some "foreground" in this scheme ?

    Thanks again,

    Celal
     
  17. Dec 18, 2004 #16

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    Background is meant in the sense that the CMB permeates the entire universe.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Cosmic Background Radiation & Big Bang
Loading...