How is it known that the CMB is everywhere in the Universe when

[Paulsrocket]

Under one trillionth or less percent of the Universe was examined?

[pzkpfw]

It comes to us, from all directions. So it's coming from everywhere.

It's not like finding a specific thing in one location and thinking that's everywhere.

Bear in mind it was predicted to exist, and was later found. That'd be a good coincidence if it's actually something else.

Edited January 4 by pzkpfw

[Paulsrocket]

11 hours ago, pzkpfw said:

It comes to us, from all directions. So it's coming from everywhere.

It's not like finding a specific thing in one location and thinking that's everywhere.

Bear in mind it was predicted to exist, and was later found. That'd be a good coincidence if it's actually something else.

And why would the remnants from an explosion be traveling in all directions instead of just away from the bang?  How does physics predict this behavior?

[exchemist]

31 minutes ago, Paulsrocket said:

And why would the remnants from an explosion be traveling in all directions instead of just away from the bang?  How does physics predict this behavior?

It wasn't an explosion at a location within the universe. It was radiation that filled the whole universe as it expanded. Since it would have filled the whole universe initially and no process is envisaged that could confine it later to a limited region, the prediction of the theory is that it should come from everywhere. As it apparently does.

[swansont]

40 minutes ago, Paulsrocket said:

And why would the remnants from an explosion be traveling in all directions instead of just away from the bang?  How does physics predict this behavior?

The CMB is not a remnant of the big bang itself, even though some descriptions say or suggest this. It's from the recombination, which happened about 380,000 years after the big bang, when the universe was cool enough to form atoms without them immediately ionizing again. That happened everywhere, so the radiation is from everywhere in the universe. The universe has expanded since then, and so the radiation has cooled

https://www.cfa.harvard.edu/research/topic/cosmic-microwave-background#:~:text=Light from recombination was very,as the cosmic microwave background. 

[Paulsrocket]

1 hour ago, swansont said:

The CMB is not a remnant of the big bang itself, even though some descriptions say or suggest this. It's from the recombination, which happened about 380,000 years after the big bang, when the universe was cool enough to form atoms without them immediately ionizing again. That happened everywhere, so the radiation is from everywhere in the universe. The universe has expanded since then, and so the radiation has cooled

https://www.cfa.harvard.edu/research/topic/cosmic-microwave-background#:~:text=Light from recombination was very,as the cosmic microwave background. 

Can you present the math which yielded the 380,000 year time frame?

[exchemist]

11 minutes ago, Paulsrocket said:

Can you present the math which yielded the 380,000 year time frame?

Not sure about the maths but I presume the principle will be an extrapolation backward from the measured black body temperature of the radiation today, using the measured expansion rate, and seeing how much time is required for the wavelengths to shorten, i.e. for temperature to go up, as you "recompress" the universe, to the point that you get to a plasma. But I don't pretend to be a cosmologist.

[Paulsrocket]

27 minutes ago, exchemist said:

Not sure about the maths but I presume the principle will be an extrapolation backward from the measured black body temperature of the radiation today, using the measured expansion rate, and seeing how much time is required for the wavelengths to shorten, i.e. for temperature to go up, as you "recompress" the universe, to the point that you get to a plasma. But I don't pretend to be a cosmologist.

You can't say that if as claimed the CMB is not a remnant of the big bang itself.  Also you present mathematical variables without an equation tying them together which yields 380,000 years, so where does the number come from.

[exchemist]

2 minutes ago, Paulsrocket said:

You can't say that if as claimed the CMB is not a remnant of the big bang itself.  Also you present mathematical variables without an equation tying them together which yields 380,000 years, so where does the number come from.

The CMBR could, conceivably, be due to something else. But nothing suggests itself and the observed uniformity of the CMBR is consistent with the big bang  theory of expansion from a small, hot start, as evidenced by the expansion measurement itself. So the two observations complement one another and applying Ockham’s Razor, the big bang accounts for both so why look further?

As for the maths, as I say, not being a cosmologist and not being trained in GR, I have to leave that to those that have done the calculation. Maybe someone else here will be able to help you with that.

[swansont]

4 hours ago, Paulsrocket said:

Can you present the math which yielded the 380,000 year time frame?

Here’s math that shows what the temperature needs to be for the universe to not be opaque; it’s around 3000 K

https://thecuriousastronomer.wordpress.com/2016/06/13/the-temperature-of-the-universe-at-recombination-decoupling/

https://ned.ipac.caltech.edu/level5/Sept02/Kinney/Kinney3.html

That happens at a redshift of around z = 1100

[MigL]

The temperature when the universe becomes transparent, and electrons can 'stick' to hydrogen nucleii, is basically the ionization temperature ( energy ) of Hydrogen.
At higher temperatures atoms cannot bind, and you have a radiation filled plasma that is opaque like the Sun.

As the present CMB temperature is 2.7^oK , that indicates ( according to gas laws ) that the universe has expanded to over 1000 times its size since the recombination era.