The Surface of Last Screaming

The ‘surface of last screaming’ presented in Fig. 9 is a pedagogical analog for the CMB. If you're struggling to cope with the concept of this strange surface at circa 2.7 Degrees Kelvin; if the cosmic microwave background (microwave radiation that fills all space in the observable universe) don't stick into your head, you've gotta read this.

I've read about 'The Surface of Last Screaming' on a paper titled

I N F L A TION

AND THE

C O S M I C M I C R O W A V E B A C K G R O U N D

and "signed" arXiv:astro-ph/0305179 (v1, 12 May 2003)

Here we go. (The below text is actually the caption of Fig.9)

🆃🅷🅴 🆂🆄🆁🅵🅰🅲🅴 🅾🅵 🅻🅰🆂🆃 🆂🅲🆁🅴🅰🅼🅸🅽🅶.

Consider an infinite field full of people screaming.

The circles are their heads. You are screaming too. (Your head is the black dot.)

Now suppose everyone stops screaming at the same time. What will you hear?

Sound travels at 330 m/s. One second after everyone stops screaming you will be able to hear the screams from a ‘surface of last screaming’ 330 meters away from you in all directions.

After 3 seconds the faint screaming will be coming from 1 km away...etc.

No matter how long you wait, faint screaming will always be coming from the surface of last screaming – a surface that is receding from you at the speed of sound (‘vsound’).

The same can be said of any observer – each is the center of a surface of last screaming.

In particular, observers on your surface of last screaming are currently hearing you scream since you are on their surface of last screaming. The screams from the people closer to you than the surface of last screaming have passed you by – you hear nothing from them (gray heads).

When we observe the CMB in every direction we are seeing photons from the surface of last scattering. We are seeing back to a time soon after the big bang when the entire universe was opaque (screaming).

You can't see the CMB with your naked eye, but it is everywhere in the universe.