It is often stated that the scientists who discovered CMBR in the mid-1960s "heard" the radiation. Did they actually "hear" it (as in sound)? If so, what was it they "heard"?
[Physics] hearing Cosmic Microwave Background Radiation
radiation
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You are assuming the Big Bang happened at a point, so the CMB is a shell of radiation expanding outwards from that point. However the Big Bang happened everywhere so every point in the universe is a source of the CMB. The CMB radiation we are detecting today comes from regions of the universe that were about 13.8 billion light years away at the moment the CMB was emitted (those points are a lot farther away now).
The fact that the Big Bang happened everywhere is a difficult conceptual issue for non-physicists. See my answer to the question Was the singularity at Big Bang perfectly uniform and if so, why did the universe lose its uniformity? for a non-physicist friendly discussion of this.
Is the luminosity indeed dependent on the velocity of the particle interacting with the substance?
Luminosity is the amount of electromagnetic energy a body radiates per unit of time.
I answered a related question on this a while back. Cherenkov Radiation. If you take a minute to read through that, it might persuade you that flux and medium are more important than velocity, (obviously as long as the velocity is above the speed of light in the medium).
I don't think the luminosity depends on the velocity of the particle so much it does on the flux density and the particular atomic structure of the medium through which it passes.
Image Source: Nuclear Reactor Glow
In terms of luminosity, putting more charged particles through a given medium would seem to increase the luminosity, as it is defined above, more than any other factor. I don't rule out the role of velocity in increasing luminosity, I just feel that there are more important factors.
Best Answer
Yes, you can indeed hear it, and you do so by essentially the same technique you use to listen to an ordinary (non-digital!) radio: in outline what you do is listen (in the sense of 'having your system detect') to a rather narrow range of frequencies and then shift that range down to audio frequencies, which you can then listen (in the sense of 'making a sound to which your ears are sensitive') to. The very simplest way of doing this (which is pretty much what primitive AM radios do) is to listen (first sense) to a narrow range of frequencies and then essentially rectify what you hear, which gives you audio directly.
I believe that Robert Wilson and Arno Penzias did indeed actually listen (second sense) to what their antenna was hearing (first sense), not least so they could rule out obvious bogons -- if the thing was hearing something like a radar or some other artificial source this would have been immediately apparent.
There are a couple of references to this, behind some of which are audio which you can listen to!
There is a rather famous thing related to this. If you have an old analogue TV and you tune it between channels you see 'snow' -- random noise on the screen. A significant fraction of that snow (I remember 10% but I think it is less -- perhaps 1%?) is the CMB.
I think it's just amazing that you can both hear and see the CMB with really very mundane equipment.
I apologise for the two senses I have used for 'listen' and 'hear': I wrote this without realising I was doing that as these two uses are common, at least among people I worked with.