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

How can empty space give off radiation from the Big Bang?

  1. Jul 4, 2005 #1
    From what I understand, empty space has a temperature of 2.7 degrees K? And that empty space still gives off micro-wave radiation from the big bang?

    Can someone help me visualize this? For me, the idea of empty space giving off radiation and having a temperature that isn't absolute zero is almost like trying to visualize a dry towel being used as an excellent conductor for electricity...hopefully someone catches what I mean by this.

    I just can't seem to grasp how an empty space can give off radiation. Surely someone understands what I mean :P any help would be appreciated :)
    Last edited: Jul 4, 2005
  2. jcsd
  3. Jul 4, 2005 #2
    It's not that empty space is giving off radiation, but what we see as microwave background radiation is radiation which was emitted by the "cosmic soup" 300.000 years after the big bang. And it is this radiation (which has cooled about 1000 times since it was emitted) that has a temperature of 2.7 K.
  4. Jul 4, 2005 #3
    Ah ok, I think i'm starting to get it.

    But "radiation" is such a vague word. What exactly IS this micro-wave radiation that is left over, made of? I think that's the bulk of what i'm mainly having a hard time understanding.
  5. Jul 4, 2005 #4
  6. Jul 4, 2005 #5
    That helped a lot! Thanks!

    But still...lol one more question:

    We can't see the microwave radiation because our eyes obviously were only meant to see the "visible" spectrum.

    But even though we aren't able to see the micro-wave radiation, it is still there, and shouldn't it interfere with me looking at the stars in the sky?

    For instance, during the day, I can't see a single star in the sky (usually), BUT if somehow my eyes had evolved to see a completely different portion of the electromagnetic spectrum, would I still be able to see the sky & space clearly during the day? Even though all the 'undetectable' rays of radiation from the sun are littering the sky?
    Last edited: Jul 4, 2005
  7. Jul 4, 2005 #6

    We are all constantly bombarded with electromagnetic radiation of all wave length. The CMBR (microwaves you are asking about) don't hit us with enough photons for the eye to see even if we could see them normally. However other sources emit microwaves, radio waves, infared, and all other types of EMR. Only light in the visible spectum is detected by the human eye regardless of what else is there.

    Look up into the sky at night and you will see many stars. Even more are out there that you can't see, but some photons from the things you can't see still gets through your eye. In fact every moment we are bathed in electromagnetic radiation from everything between us and the big bang. If one were out in space they would be being hit by light/EMR emitted from everything in the "visible" universe all at one time.
  8. Jul 4, 2005 #7
    Thanks! That makes a lot of sense that there isn't much to be seen anyways, since it is only 2.7 degrees K. man i'm learning a lot here and i don't wanna stop :D so...

    Now my question is: why is that particular micro-wave radiation EVERYWHERE in space? Naturally in my head, I think of the photons that make-up this micro-wave signature as constantly moving forward at the speed of light. And I'm sorta visualizing it like this:

    Say i have a flashlight, I turn on the flashlight (big bang), then shut it off (end of the big bang?). Now there is a beam of light traveling in a straightline across the universe, but that beam has an end behind it.

    Does the micro-wave radiation for the big bang not have an end? Why can we 'see' it from everywhere in empty space? Shouldn't a good portion of it, I dunno, gotten absorbed by other matter by now? Or atleast the "beam" of it should have already passed us and gone to the other side of the universe only to....(whatever happens when light reaches the "edge" of the universe)...and not be detectable from EVERY point in space anymore?

    Thanks so much guys for answering me so far!
  9. Jul 4, 2005 #8


    User Avatar
    Science Advisor
    Gold Member

    Like all radiation, CMB photons were emitted by matter. The matter which emitted these particular photons was all the matter in the universe! For the first ~380,000 years after the big bang, the atomic particles [i.e., protons, neutrons, electrons] that form the atoms which comprise our solar system, galaxy and observable universe were floating around loose in a dense, hot plasma cloud. Until the temperature dropped low enough for electrons to pair up with protons and form atoms, this cloud was opaque. Once atoms formed, the remaining photons [which greatly outnumbered atoms in the universe] were free to travel througout the universe. While the observable universe was 'only' 0.1% it's present size at that time, it was still mighty big and rapidly expanding. The more distant [at that time] photons are only just now catching up with us. Due to expansion of the universe, they have been 'stretched' [redshifted] to about 1100 times the wavelength they possessed at the moment of recombination. They will continue being stretched in the future as the universe continues to expand.

    A precautionary note is in order. None of this should be taken to mean the universe is, or ever was finite in size. What is finite, is how much of it is visible to an observer at our location at any given time in the history of the universe. This is due to the finite speed of light and expansion of the universe. The part that is viewable is called the Hubble volume.
  10. Jul 4, 2005 #9
    Ohhh, so they are still catching up to us! So, does that mean there's a space "behind" these CMBR photons in our universe where the CMBR is not detected? (like in my flashlight example)

    And the precautionary note about the universe, is that meant to say "The universe is infinite" or "We have no conclusive evidence that the universe is either finite or infinite yet." ? Cuz in my head I'm thinking "Ok, well if the hubble volume was only .1% of its current size back when the CMBR photons were free to travel, wouldn't that mean they would be long passed us by now and waaay waaay "out there" and not detectable in the space right outside our atmosphere?
  11. Jul 4, 2005 #10
    Our model of the big bang says that all space and matter were created in an instant. The matter was in a plasma state which is very hot and gives off gama radiation. This radiation was emitted from everywhere in space until matter got cool enough to clump together under the force of gravity. Until then the universe was basically opaque, meaning it looked the same everywhere.

    The big bang has an end to it also, every day we can see a little more of the universe. This is called the hubble sphere. It constantaly grows spherically around us.

    I kind of already answered this in the above paragraph, maybe doing some searching on google for "hubble sphere" might help you. The radiation from the big bang wasn't microwaves, it was made by gamma rays, the most energetic photons that exist. Over time and distance (as space expands) the gamma rays get shifted to a lower energy level, ie; microwaves. Eventually I guess it will be shifted so far that it would not be observable anymore, but it will always be there at the edge of our hubble sphere.
  12. Jul 4, 2005 #11

    I think that pretty much clears up most of my confusion!! :biggrin:

    The rest I can get from reading up more about the Hubble Sphere like you said.

    Thanks a ton guys for helpin me with this one :)
  13. Jul 5, 2005 #12


    User Avatar
    Science Advisor
    Gold Member

    A good and proper question CP! We don't know if the 'universe' is finite, or infinite. Neither possibility can be ruled out. It is not at all clear if we will ever be able to rule out either possibility. Also, well done by GOD_AM. We could haggle over some of the details, but that is the big picture.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook