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How did "Visible light" come into existence?

  1. Jan 17, 2015 #1
    As i read when the big Bang happened,it was in a hot dense state for the first million years. Then the gases started to collect together and cool down,to form stars. (Sorry if i`m being wrong).
    So when did and how did the first ray of visible light came into existence?
  2. jcsd
  3. Jan 17, 2015 #2


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    I'm no expert nor have studied formally cosmology, but I guess the very first photon was generated through atomic transition, again may be, it could be wrong.
    I also suggest that the word visible light be replaced by more general electromagnetic field, because people define visible relative to what human's eye are capable of sensing. Human's visible differs from dog's visible.
  4. Jan 17, 2015 #3


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  5. Jan 17, 2015 #4
    Again, I'm not a cosmologist either, but my understanding is that light of some sort (i.e. electromagnetic radiation) was present almost from the beginning - the very, very early Universe underwent a very brief but very rapid phase of exponential expansion called inflation, driven by a type of field filling the Universe called a scalar field - read up on scalar fields and inflation for more! The scalar field was unstable however and very rapidly it decayed and its energy was converted into photons of electromagnetic radiation. Inflation ended, and the Universe was filled with a blaze of light - its first light. At this point the Universe was something like 10-32 seconds old, and all the matter and radiation in the observable Universe today was contained in a space about the size of a grapefruit.

    Because of the Universe's extremely high temperature, most of this radiation would have been incredibly high energy/frequency photons, i.e. gamma rays, but some would have been visible light right from the start. (Under such incredibly high energy densities as well, enough radiation would have been concentrated into one place to give rise to particles of matter and antimatter - remember E=mc2! It would have been the high-energy gamma ray photons that gave rise to matter. One pretty interesting and mindblowing consequence of this is that the particles that make up your body - and everything else around you - began life as gamma radiation, high-energy light!). As the Universe expanded and cooled, however, its peak wavelength would have become longer, redshifting and cooling into x-rays, ultraviolet rays and visible light. So after a few tens of thousands of years the Universe would have been dominated by visible light, produced at the end of inflation.

    The Universe at the time would have been incredibly bright, as lots of high-energy photons were knocking about, but you wouldn't have been able to see very far. The Universe was opaque, as it was too hot for atoms to form. Nuclei and electrons roamed the Universe freely, absorbing and emitting photons and scattering them in all directions - for the photons, it'd be like being inside a huge cosmic pinball machine. Light couldn't have travelled more than a fraction of a millimetre before being absorbed or scattered by a stray electron or nucleus. The whole universe was a hot, opaque, glowing fog. Then, at around 380,000 years, electrons and nuclei finally combined to form the first atoms of hydrogen and helium, and the Universe turned from a hot opaque plasma into transparent hydrogen and helium gas. As the Universe became transparent, the "fog" lifted, and light was finally free to travel through the Universe.

    This light continued to get redshifted by the expansion of space, stretching and cooling further into infrared and microwave radiation. That microwave radiation is still with us today, after 13.8 billlion years - it's the cosmic microwave background radiation, and is the most convincing evidence we have for the Big Bang - that original light that was created at the end of inflation, was coupled to matter and then released when the first atoms formed, and has subsequently been redshifted into microwaves. Turn a TV or radio to a channel it doesn't receive, and a small percentage of the dancing static you see or hear is that microwave radiation left over from the end of inflation - the first light in the Universe.

    Getting back to 380,000 years after the Big Bang, the Universe was now a cooling, expanding, dark cloud of hydrogen and helium gas. The cosmic background radiation no longer interacted strongly with it, and it was now becoming redshifted beyond visible light into the infrared anyway. The Universe had entered the dark ages - a time when there was essentially no source of light anywhere. Matter, however, was invisibly clumping together under the force of gravity, forming the seeds for the first stars and galaxies. After 200 million years or so, the first stars formed, and the dark ages ended. And since then, most of the visible light in the Universe has come from stars!
    Last edited: Jan 17, 2015
  6. Jan 17, 2015 #5
    Thanks to all.. I`ll read up on the topics suggested.
    Amaterasu21- Thanks for explaining it in such a simple way. I`ll get back to you all if i have any more doubts..
  7. Jan 17, 2015 #6


    Staff: Mentor

    It was also converted into quarks and leptons, at least according to inflation theory.

    Not necessarily; as above, quarks and leptons were created directly from the decay of the scalar field. (There was certainly pair production by gamma rays as well, but that reaction ran in both directions at about the same rate because the temperature was so high--quark-antiquark and lepton-antilepton pairs were colliding and annihilating each other, turning into gamma rays, about as fast as they were being created from gamma rays.)
  8. Jan 17, 2015 #7
    Wasn't particle creation and annihilation going on at roughly the same rate, though? That would mean the quarks and leptons produced by the decay of the scalar field would have almost instantly been annihilated and converted into radiation and back again many, many times before stabilising as the quarks and electrons that make up our bodies today. (I guess that means they didn't necessarily "begin life" as gamma radiation though, even though their energy has almost certainly been gamma radiation before.)

    Timelord88 - no problem! Just bear in mind I'm not a cosmologist, so do look into the things I've said and listen to some of the other members here who might have a better idea of what they're talking about than I do! ;)
  9. Jan 17, 2015 #8


    Staff: Mentor

    Yes. See the second part of my post.

    Yes; but the energy contained in photons today has almost certainly been in quarks and leptons before too, for the same reason.
  10. Jan 19, 2015 #9
    Last edited: Jan 19, 2015
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