Why did the universe get transparent 380,000 years after the Big Bang?

[old_eyes]

The early universe was a hot dense(ish) cloud of protons, neutrons, electrons, photons, neutrinos and probably a bunch of other stuff. This soup is opaque. Light doesn’t get through it. Then suddenly (well relatively suddenly), after 380,000 years neutral atoms form and the universe becomes transparent. When asked why, I have always resorted to waving my arms and saying that before that time the universe was too hot, and that is why 380k years is the closest we can hope to get to the Big Bang with photon based astronomy.

but I never really understood why 380k years.

Along comes a Medium article that explains it all simply and clearly. Worth a read and a chance to marvel at the Goldilocks universe that supports us. Just the right physical parameters.

https://medium.com/starts-with-a-bang/the-quantum-reason-why-neutral-atoms-first-formed-9b6eae87dfbc

[michael.h.f.wilkinson]

The short form (with some arms waving) for me has always been: When the universe cooled enough for ionized hydrogen to recombine into neutral hydrogen, visible light could travel largely unhindered. By contrast, the universe became opaque to the 21 cm wavelength of neutral hydrogen. Thus, transparency is wavelength dependent. Only when stars and galaxies formed, and the neutral hydrogen got partly ionized, and concentrated in regions of higher density around galaxies and clusters did the universe become (largely) transparent in the 21 cm wavelength.

[old_eyes]

You are of course right about re-ionisation. The bit that I found fascinating was the explanation about why, from one perspective, neutral atoms appeared so quickly, and from another, so slowly. All buried in the subtleties of quantum mechanics.

[900SL]

Can somebody explain why that article states that the photons on the early universe had high kinetic energy and momentum? I always thought (A level physics) that photons were massless and KE was 1/2mV2. Is this a quantum thing? 

[vlaiv]

1 minute ago, 900SL said:

Can somebody explain why that article states that the photons on the early universe had high kinetic energy and momentum? I always thought (A level physics) that photons were massless and KE was 1/2mV2. Is this a quantum thing? 

Yep - quantum thing.

Momentum of photon is h / lambda  or h * f

(depends on frequency - higher frequency - higher momentum - that is why x-rays and gamma rays are dangerous - high energy / momentum)

Energy of photon is given by h * c / lambda or h * c * f

Energy and momentum of photon are related as E = p * c

 

[900SL]

Thanks Vlaiv ! 

(I'm an Engineer .. equations and concepts are simpler in my world)

[DaveS]

.

 

Edited February 16, 2023 by DaveS
Not the intended video.

[jjohnson3803]

Physics joke:

What's new (nu)?  Energy divided by Planck's Constant.  

🤣

 

[George Jones]

On 16/02/2023 at 12:13, 900SL said:

Can somebody explain why that article states that the photons on the early universe had high kinetic energy and momentum? I always thought (A level physics) that photons were massless

Yes, the rest mass of a photon is zeoro

 

On 16/02/2023 at 12:13, 900SL said:

and KE was 1/2mV2.

This is (approx) true only for non-relativistic physics.

Another way to @vlaiv's final equation. The world's most well-known equation, E =mc^2, is a special case of a more general equation (my favourite physics equation),

E^2 - (cp)^2 = (mc^2)^2,

where E is the total energy of the "particle", m is the rest mass of he particle, and p is the momentum of the particle.

Suppose that a particle with non-zero rest mass m is at rest, so that its momentum p is zero. The equation then is E^2  = (mc^2)^2, which, after taking square roots on both sides, gives the famous E =mc^2. E =mc^2 is true for particles that have non-zero m, and that are at rest. E =mc^2 is not true for any moving particles.

Now consider the case of a particle that has zero energy E, and has zero rest mass m, e.g., a photon. Then E^2 - (cp)^2 = (mc^2)^2 becomes E = cp.