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Describe & explain the significance of the 3K CMB

  1. Apr 6, 2014 #1
    Ok so I'm a bit confused as to how to attempt to answer this question? Describing its significance vs explaining its significance?

    Here is my attempt, please be ruthless when correcting me as I feel like there are some gaps in my knowledge and misconceptions I have:

    Where I put '/' I wasn't sure which phrase or word was better to use or correct, please can you tell me which one?

    The 3K cosmic microwave background radiation is electromagnetic radiation in the microwave region of the electromagnetic spectrum and can be detected from all directions. It is the predominant electromagnetic radiation emitted from space. It correspond to the temperature at which space/ the universe is now at-3K. Finally it is uniform and isotropic/ its intensity is the same in all directions.

    The 3K CMB provides evidence for the hot Big Bang model of the universe.

    According to the hot Big Bang model of the universe;
    - the universe began from a singularity called the Big Bang from which space and time evolved
    -initially the universe was very hot and emitted electromagnetic radiation mainly of shorter wavelengths, this electromagnetic radiation was almost perfectly uniform/isotropic and was given out intensely/emitted in all directions
    - 3x10^5 years after the Big Bang the universe became transparent to electromagnetic radiation and it was able to move freely through the universe
    -after the Big Bang the universe began to expand and cool
    -as a result these shorter wavelengths of electromagnetic radiation were stretched out so that they were of a longer wavelength
    -the current temperature of universe is 3K

    The fact that the CMB is in the microwave region of the em spectrum supports the idea of the expansion of the universe , and that the universe must have originally begun from a single point- the Big Bang. Secondly the presence of the CMB corresponds to the temperature of the universe/space-3K, supporting the idea of the cooling of the universe and the fact that initially the universe must have been very hot. Finally the fact that the CMB is almost perfectly uniform/isotropic supports the idea that the universe was initially isotropic, however there were some irregularities in the density of the universe, which were the seedlings for the formation of galaxies and stars that were to come after the Big Bang.
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  3. Apr 6, 2014 #2

    Simon Bridge

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    You have described what the CMB is - well done.

    What is the significance of it: what is signifcant about it?
    i.e. what would it mean if there were no CMB at all?

    Probably you'll be fine just talking about what you know about the CMB ... in a pinch, you can look up the words that puzzle you: "describe" and "explain" have quite different uses.

    3K and -3K are quite different ... did you intend hyphenation?
    How can "the Universe" have a temperature ... it's not 3K everywhere is it?
    How can radiation be assigned a precise temperature like that?

    Maybe you will do well to start out concentrating just on the existence of CMB, then decide what is significant about it?

    Otherwise: decent effort.
  4. Apr 6, 2014 #3


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    You should also, most emphatically, NOT say that the universe started at a point. That IS the statement you always here in the popular press and on TV but it is absolutely not true. Think about how that statement comports with your (correct) statement that the universe is isotropic.
  5. Apr 6, 2014 #4


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    Why are you asking? Are you writing an essay for school? If a grade is on the line, don't get too scattered, stay organized and focused. But if YOU YOURSELF are asking the question then you can afford to be as scattered as possible, grow and stretch your ideas.

    I assume this is not for school. In that case the way to phrase the question (since you were in doubt between "describe" vs "explain") might be
    Explain why the CMB is so interesting.

    That would involve giving at least a brief description of the microwave sky as it is actually observed and a brief account of HOW we think the background radiation arose.

    But the main curiosity would be why it is so darned interesting to so many varied types of researchers. It's interesting for a variety of reasons.

    What kind of cake do you like, or live on (if cake is life)?
    Angel food is good and reasonably healthy since low fat, only the whites of the eggs etc. I like it a lot. There is also princess cake and a type of layer cake made of thick chocolate whip between crunchy disks of meringue. That's good too. What's your favorite?
    Last edited: Apr 6, 2014
  6. Apr 6, 2014 #5
    @Simon Bridge

    Ok, I understand that the whole universe is not at that temperature, its the average temperature right?! :S
    I still don't understand the concept of the temperature though? I understand hot objects give out radiation of short wavelengths, which have very high energies and cool objects emit radiation with long wavelengths, which have low energies. The same logic can be applied to the expanding universe ? Which would mean that the 3K is the average temperature of the universe and not of the microwave radiation? The microwave radiation is emitted at cooler temperatures though? :S

    Well if there was no CMB at all then it would mean that in fact the universe is not expanding, since the original em radiation of short wavelengths would have not been stretched. Therefore the universe should be saturated with em radiation of short wavelengths. Shorter wavelengths of em radiation correspond to a higher temperature , therefore the universe would not have an average temperature of 2.7K, it would be much higher. But all of the above isn't true, we do in fact detect a high intensity of CMB and the universe is at an average temperature of 2.7K, therefore the CMB provides evidence for the Big Bang model?
    Last edited: Apr 6, 2014
  7. Apr 6, 2014 #6
    @phinds I'm confused, where did it start from then?

    @marcus I'm not writing an essay, I have exams, so I'm revising the points from my specification, and I just happened to stumble on this point.
    Secondly , I don't live on cake, but cake always makes everything better, it's the light at the end of a dark tunnel. Bad exam-->cake, bad day-->cake, bad life-->cake. I'm kidding, but seriously cake is amazing. My favourite is chocolate fudge cake with vanilla ice cream on the side!
  8. Apr 6, 2014 #7
    @Simon Bridge, yes , it was meant to be a hyphen!
  9. Apr 6, 2014 #8


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    One reason the CMB is interesting is simply its age. It is the oldest light. It got loose with the temperature of the gas got down to 3000 kelvin (a temperature where things glow orange-color). So it is the oldest visual (light) picture of the universe we have.

    Another reason is because it establishes a universal REST FRAME. It defines for us what it means to be at rest with respect to the universe.
    People call that being "at CMB rest".

    If you are at CMB rest then the temperature of the microwave sky is the same in all directions (up to one part in 100 thousand, i.e. to within 1/1000 of one percent, which is the spicily blotchy variation that people make maps of and which seeded structure coagulation).

    But if instead of being essentially the same in all directions you see a spot in the sky that is 1% hotter than the rest, then you are moving at 1% of the speed of light in that direction.

    That called a doppler effect. Motion with respect to a uniform soup of light causes a doppler hot spot to appear ahead of you and a doppler cold spot to the rear.

    So that's one of several reasons CMB is interesting. It gives us a criterion of being at REST wrt the universe. ie. wrt the ancient hot gas that eventually everything coagulated from, i.e. wrt the ancient light which is the glow of that hot gas (just as it was becoming transparent).

    CMB is also interesting because it gives us a criterion of ABSOLUTE TIME. That is sometimes called "Universe time". It is the time presumably measured by observers all over the universe who are AT REST. It is also the time that the cosmologists MODEL of the universe runs on, but that's too technical a direction to go in right now. Anyway the the ancient light gives us a really important reference frame (ideas of stillness and the time the universe itself is running on) so that's one of several reasons it is so interesting.

    EDIT: I just saw that this question is related to EXAM REVIEW. So my answer is not so appropriate. You have to stay focused on accepted wisdom, my answer is more in a philosophical direction and stretches you out. So do your exams first and think about that later, maybe.
  10. Apr 6, 2014 #9
    @marcus, no, its fine, it was my fault for not posting this in the H/W section :/ is there any way I can move this thread?
  11. Apr 6, 2014 #10


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    Not to worry, in my inexpert opinion. It seems to me that preparing one's mind for exam is in the overlap between disinterested discussion and Homework. If you want I guess you could use the "report" button and say " wrong forum, should have started thread in HW section, sorry, please move". Or you could just start a new thread over there. Mod will move you anyway if it seems important. My completely unauthoritative reckoning is relax think about Gateau Diane and ask some more questions.

    Did you already ask some more questions and I didn't notice and they didn't get a response?

    Remind us. Or ask some new questions. The CMB is great.

    Tonight, about 10PM standard or 11PM daylight saving you can SEE the direction in the sky that the solar system is sailing, at around 370 km per second. That is where the doppler hotspot is.
    It is near the constellation called CORVUS around 7 degrees south of equator. So 37 degrees south of overhead if you live at 30 north latitude.

    CORVUS is a small diamond shape of 4 stars, or like the four corners of an old-fashioned kite. made of paper and two sticks.

    The direction we are going, relative to the universe, is not exactly that diamond shape, it is a little to the right, to the west. But if you can see Corvus (which the Babylonians called "The Raven") then you have roughly the correct direction.

    That is where the doppler hotspot in the CMB sky is.

    light goes 300,000 km/s so if you divide 370/300000 that will tell you by what fraction the microwaves are hotter in that direction. It is about a tenth of a percent, a little more than a tenth of a percent.

    What do you want to learn or understand that you don't already, about the CMB? It's the soup of ancient light that everything is swimming in.

    It is 1090 times cooler than when it was emitted and started traveling. the original temperature was around 3000 kelvin and now just divide that by 1090 to find roughly what it is today.
    And its wavelengths are 1090 times longer than when it started traveling. Longer wavelengths=cooler light.

    Gateau Diane is made of 3 large disks of meringue layered between cool creamy chocolate and also covered on the outside with the same. You put it in the fridge a couple of hours before you feast on it. This is a good moral lesson in self-control.
    Last edited: Apr 6, 2014
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