As I told you in part 16 that whenever we look out into the night and gaze at stars, we are actually experiencing the past. The stars we see in the night sky are very far away from us, so far the star light we see has taken a long time to travel across space to reach our eyes. Thus with powerful telescopes, we are actually looking billions of year in the past. But you can ask that then we should also be able to see the big bang which happened 13.8 billion years in the past. In fact between us and bing bang there is a wall, not a physical but a barrier to see further in the past.
Big Bang Timeline
Recombination Epoch: Photon Decoupling
This edge of the observable Universe is about 13.8 billion light-years away. The light from that point left when the universe was only a few hundred thousand years old, and only now has finally reached us. What’s even strange, the place that emitted that radiation is now 46 billion light-years away from us. First, think the universe is 13.8 billion years old so if we look far enough then we can see big bang. In principle this is true but about 100,000 years after the big bang, just after the universe born early universe temperature was very high 1 Billion Kelvin. And at that temp hydrogen broken a part every time proton and electron try to bind together so there was a charged plasma of these particles and that plasma was opaque for radiations. In practice, we can see the light only from as far back as the time of photon decoupling in the recombination epoch. That is when particles were first able to emit photons that were not quickly reabsorbed by other particles.
Surface Of The Last Scattering
Before then, the universe was filled with a plasma that was opaque to photons. After more 280,000 years passed after the bing bang, the temperature dropped to 4000K and electrons and atomic nuclei first become bound to form neutral atoms. Photons are no longer in thermal equilibrium with matter and the Universe first becomes transparent. Now as of today that overall temperature of the universe is dropped to 3 degrees Kelvin and this is the remanent, the afterglow of BigBang which we could see on our Old TV sets. The moment universe became neutral and transparent is called the surface of the last scattering.
We Can See The Actual Big Bang
If we could have microwave eyes, we can see every night big bang in the sky. We can see that explosion now as we have a way to see this cosmic glow with the naked eye. Switch your old television set and between channels, a significant portion of static you see on your tv is radiation from the big bang.