What is Cosmic Background Radiation ?


I have been googling Cosmic Background Radiation, but am still confused as to the location of its source. Is it just very old light finally arriving from very distant sources? Or is earth also surrounded by nearby CBR sources that in the fullness of time will arrive at very distant galaxies?

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To understand, you need to know that back, long long ago, the universe was opaque. Light could not go anywhere without being absorbed into the dense cloud of plasma that was everywhere.

But, there came a point in time where the universe had expanded enough that it stopped being opaque. So when that happened, the light that had been getting absorbed could now just go. This light is the cosmic background radiation. The actual acronym is the “CMB” for cosmic microwave background.

It happened everywhere, there were no specific CMB sources, as everything was a source. We are constantly being bombarded by the CMB radiation coming from places that are farther and farther away.

So there’s “Cosmic Radiation” and “Cosmic Background Radiation” – The CBR, the background radiation is leftover from the big bang – it’s [source is everywhere](https://profoundphysics.com/did-the-big-bang-happen-everywhere-at-once/) – we detect it absolutely everywhere we look, in every direction.
The big bang happened when the universe was essentially one point, and the expansion happened everywhere, and is still happening everywhere. It’s basically just left-over energy from billions of years ago. There isn’t “A source” it’s just kinda, there, in the background, all the time.

Then there’s other sources of “Cosmic Radiation” – massive scary space stuff like nearby stars, supernovae, quasars, black holes.
Obviously this type of radiation is localized and directional, when we get hit with a solar flare from the sun, it’s a pretty obvious bit of Cosmic Radiation.

The source is all the matter in the universe. When matter formed after the big bang it was still very hot and was plasma, which is an ionized gas. Plasma is not transparent to light so the light the warm material emitted got absorbed again.

The matter was called down when the universe expand and approximately 379,000 years after the big bang was cold enough and no longer be ionized. The temperature was around 3000 kelvin. So now the universe is filled with primary Hydrogen and helium gas and it is transparent to light. So the glow of the hot matter was not absorbed and continued to travel.

The expansion of the universe has changed the wavelength of light from the visible spectrum to microwaves where we find it today.

The travel time of light to use depend on the distance to the source when it was emitted and the expansion of the universe. So the source for what we observe now is matter was at a distance so the travel time of the light is age of the universe minus approximately 379,000 years.

>Or is earth also surrounded by nearby CBR sources that in the fullness of time will arrive at very distant galaxies?

This is closer to the truth. CBR is everywhere. “The fullness of time” is every moment since the cosmos “cooled” enough for photons to travel through empty space between other substance. Every point in space (including every distant galaxy, no matter how far away) are being pummeled by these photons. The “distance” to where the apparent sphere of photons from that long ago moment seem to be “coming from” is equally distant from every point at the same time. No matter where you are in the universe, “the CBR” looks equally far away, because it is a measure of how long ago that radiation first started, plus the speed of light (and the expansion of space).

According to the Big Bang model, the entire universe originated from a single point. That is, at age t=0, all the energy content of the universe was packed into a single point. Then as we progress in time, the volume of the universe expanded (and is continuing to expand).

In this model, the early universe was still so dense that most light would bounce off other objects. So in effect light was everywhere and the universe was opaque dense plasma. After a certain point, the universe expanded enough and cooled enough (same thing really) that it became transparent – that bouncing light now stopped bouncing and is free to travel straight through space.

So the CMB we perceive today is exactly the light that happened to be traveling this way from exactly the right distance from us when that transition from opaque to transparent happened. Notice we are not special in this – everywhere you go in the universe you will also observe the same thing (but the shell of light will just be a different one); similarly as time progresses we will just observe CMB from a farther and farther shell.

So just imagine a deflated balloon with some colored fog inside. This fog is the CMB and the balloon is the universe (starting out with low volume). Imagine the balloon is filled with regular air now (this doesn’t represent anything – just the expansion of space). And imagine we are at any given point inside the balloon – in all directions we will see a bit of that colored fog.