Why back ground (cosmic )radiation is cool when big bang is hot phenomeno?

Question

if big bang had occured as praposed 12 billan years ago it was definatly very hot inceident ,now we say that back ground radiation which echoing in arround us is from big bang but why is cool radiation

Answer 1

Think of everyday experience, such as pumping up a bicycle tire with a hand-held pump. As you compress the air to force it into the tire, you'll find the barrel of the pump getting very hot --- carry on long enough, and you could actually burn yourself. This is an example of a pretty general phenomenon: compress things, and they heat up.

Correspondingly, pretty much everything COOLS with EXPANSION (that's how refrigerators work), and the Universe is expanding!

A detailed analysis shows that if R is a characteristic radius for a matter-dominated Universe (as it is now), then RT (where T is the radiation temperature) is constant. An expanding universe implies a dropping temperature.

And in any case, we don't actually see the stuff (or the prevailing temperatures) back at the big-bang itself --- we see the "opaque wall" after which the Universe became essentially transparent. (Before that, it was opaque.) What we do see now, had a temperature of ~ 3000 degrees back then. The Universe's radius then was ~ one-thousandth of what it is now. The expansion by a factor of ~ 1000 has cooled the radiation then emitted down to ~ 3 degrees.

One way of thinking about this (though it's not a rigorous proof) is to think of radiation as its constituent electromagnetic waves filling a box that is "expanding with the Universe." (If this seems strange, one way of deriving the Planck spectrum of radiation is to consider precisely the way in which such waves, with nodes at the end points, could fill a [stationary] box, taking account of how "energetically expensive" each wavelength or frequency is because of "E = h nu.")

If the radiation keeps its nodes riding along with the expanding walls of this new box, the WAVELENGTH will simply be proportional to the radius. THAT can be thought of as how the REDSHIFT comes about, with wavelength expanding proportional to the radius of the Universe.

Correspondingly, the frequency is dropping, and frequency, like temperature, is a measure of the characteristic energy around. So this analogy suggests (though without proper and careful analysis it doesn't prove) that the temperature will drop inversely proportional to the Universe's size.

Whatever one thinks of this analogy, what it suggests is fully confirmed by other more detailed arguments. And in fact, one of the remarkable results is that if you have a Planck spectrum of a given temperature at some stage in the Universe, it will REMAIN a Planck spectrum of ever-decreasing temperature (with T inversely proportional to R) as the Universe expands!

Live long and prosper.

Answer 2

The microwave history isn't from the time of the tremendous bang. The microwave history is from a time presently after the tremendous bang. there turned right into a time at the same time as the universe become so dense and warm that electromagnetic waves might want to no longer penetrate very far earlier encountering some thing that would want to dam the type of the wave/photon. at the same time as the Universe had accelerated sufficiently and sufficient numbers of particle anti-particle pairs had annihilated themselves the electromagnetic waves were extra often than not equivalent to a particular temperature. The waves were everywhere entering into all instructions. The Universe has persisted to improve over the numerous Billion years on the grounds that that aspect. The waves that are arriving now have had their wave length made longer as they traversed Billions of sunshine years of the increasing universe from someplace that become faraway from the position we presently are.

Answer 3

It's 'cool' in a relative sense. All of the energy is still there (minus what has been converted into matter) but it's spread out into a -huge- volume of space (namely, the entire Universe, which is pretty big ☺) That means that the energy density is a lot lower, so the equivalent black-body radiation temperature is much lower.


Doug

Answer 4

The big bang was not a hot phenomenon.
Heat never came along til matter came into existence.
This is the back ground radiation they are seeing.

Answer 5

It is not "cool" as you say. The temperature in space is 2-3 degrees above the absolute zero. In fact, the volume of space has absorbed the radiation created by big bang and that volume is bigger, so the density of heat is less. There is nothing as "cool", because cool is a relative term. Scientifically, there is either heat or no heat, so cool is used to do comparison between to different levels of heat.

Answer 6

Radiation isn't hot or cool in and of itself. It's a QUANTITY of radiation that can make something hot.

A small amount of sunlight comes in even on a cool day. But if you get a lot of sunlight, it seems hot to you.

Answer 7

first of all, pls improve your spelling!