Where/what is emitting cosmic background radiation, and when did it come into existance, was immediately after the big bang?

I know that the universe isn't 3D in the traditional sense, and I don't pretend to understand it. However if the big bang originated from a single point, and the cosmic radiation was emitted from that point at the very start, then unless he explosion/expansion/inflation (whatever happened) wasn't faster the speed of light, then the CMB would have reached the edge of the universe and gotten out of it.

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    $\begingroup$ The CMB (cosmic microwave background) was not emitted at the Big Bang, but much later when there was a macroscopic geometry, rather than a "single point". Also he Universe doesn't have an "edge" where things can fall off just as the Earth doesn't have an edge, ships can fall off. $\endgroup$
    – user346
    Commented Feb 3, 2011 at 2:54
  • $\begingroup$ See Luboš Motl's excellent answer to the question physics.stackexchange.com/questions/4680/… $\endgroup$
    – nibot
    Commented Feb 7, 2011 at 3:53

4 Answers 4


When the Universe was very young, the matter in it was very hot. That matter emitted thermal radiation, in the same way that other hot things do. For quite a while, the radiation was in equilibrium with the matter: photons of radiation were constantly being both emitted and absorbed by the matter (mostly the electrons that were zipping around).

At a certain point, the matter "decoupled" from the radiation, meaning that interactions between photons and other particles became very rare. The main reason for this is that most of the matter formed neutral atoms, rather than consisting of free charged particles (electrons and nuclei). Neutral atoms don't interact with radiation nearly as strongly as charged particles.

From that time on, most of the photons that were in existence simply flew through space, not interacting with anything. Those are the CMB photons that we see today.


The CMB was decoupled from the "hot bath" approximately 380000 years after the Big Bang. The moment was the end of the Photon epoch and called "the recombination".

Your question about the speed of light is ok, while it deals with different topic. There is nothing wrong about two objects moving with speeds higher than the speed of light avay from each other. It is just forbidden to transmit signals with the higher than light speed. Just get to the usual analogy with "ants on a ballon": two ants running away from each other with maximal speed 1cm/s and the baloon expansion adds some extra speed.



The big bang did not happen at a single point and explode into some existing space. Instead, the big bang happened "everywhere"; since then, space itself has expanded from the initial singularity into the present size.

At first the universe was incredibly hot and dense, so hot and dense that elementary particles could not yet combine to form neutral atoms, and particles, including photons, could only travel a short distance before hitting and interacting with other particles.

As the universe expanded, it cooled and and became less dense. Eventually the density crossed beneath a threshold allowing photons to travel essentially forever without running into something; we say that the universe became transparent to photons at this time.

Nearly all of the photons that were in the cosmic soup at that time are still here. Because the expansion of the universe is an expansion of all space, the expansion carried the photons with it, so they are everywhere.

These photons, with their wavelength stretched by the intervening expansion of space, form the cosmic microwave background radiation.

A good book on the early universe is The First Three Minutes by Steven Weinberg.


To understand this we can start with spacetime cosmology with matter and radiation. The expansion of the universe is based on pretty simple logic. Without laying down arguments I just state the Friedman-Lemaitre-Robertson-Walker FLRW energy (so called energy) equation for the evolution of a scale parameter of spatial distance a, $$ \Big(\frac{\dot a}{a}\Big)^2~=~\frac{8\pi G}{3}\rho~-~\frac{k}{a^2} $$ for $\rho$ the energy density. The Hubble parameter or constant with space at each time is $H~=~{\dot a}/a$. We set $k~=~0$ for a flat space ${\mathbb R}^3$ to match observations. Now the reasoning is that energy density for photons scales inversely with the length of the box. The box is thought of as a resonance cavity that is equivalent to a situation where the number of photons that leave is approximately equal to the number of photons that enter. During the radiation dominated period things were in a near equilibrium, so this is not out of line with some physical reasoning. In a stat-mech course an elementary problem of N-photons in a box uses the same logic, the energy of the photons scales inversely with the size of the box. So the energy of photons $E~=~hc/\lambda$, and the wave length scales with the scale factor a. So the density scales as $\rho~=~hc/a^4$.

So with this et up let us propose a time dependency on the scale factor a with time $a~\sim~t^n$. Put this into the "energy equation" and turn the crank and you find that $n~=~1/2$. The scale factor grows as the square root of time. This is an energy equation, and the balance tells us that the loss of energy in photons is equal to the gain in gravitational potential energy. This connects well with Newtonian analysis and the Pound-Rebka experiment.

We may continue further, for the photons in a box exert a pressure on the sides of the box $p~=~F/a^2$, and the force induces an increment of change in the size of the box $dE~=~Fdx$. The force is distributed on 3 different directions and so $p~=~\rho/3$. This may then be used in the equation $pV~=~NkT$ to find that for $p~\sim~a^{-4}$ and $V~\sim~ a^3$ with the above $E~\sim~1/\lambda$ that $\lambda~\sim~ 1/T$, which is Wein's law for the wavelength as the peak of the BB curve. The proportionality of the energy density with scale factor and temperature also gives $E~\sim~T^4$. So this physics is remarkably in line with laboratory understanding of the basic thermodynamics of radiation.


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