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another CMB question, from Hawkings book

  • fguihen
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Ive been reading stephen hawkings "the universe in a nutshell". In one chapter he describes how the light we see from older galaxies show they were much closer together. this makes sense, but he then mentions how by measuring the CMB we find it at a constant temp of 2.7 degrees above absolute zero and this proves the radation came from a region of space opaque to microwaves, and this proves that our light cone ( what we see from our point in space) must pass through a certain amount of matter, enough to curve spacetime. i dont understand how it proves that it comes from a region opaque to radiowaves. where does he get this deduction from
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16 years 2 months ago #35356

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Replied by pmgisme on topic Re: another CMB question, from Hawkings book

Imagine loads of footballers kicking a loads of balls all over the place in an enclosed stadium.

Every time a ball is kicked it hits another player who kicks it again to another player etc. etc.

This kicking goes on around and around endlessly.The balls never escape because they always hit a player and because the "stadium" is enclosed.

The "balls" are PHOTONS
The "players" are charged ELECTRONS and PROTONS which bounce PHOTONS between them.

This is "thermal equilibrium" and is "opaque" because a given Photon does not get far before it is deflected. (Kicked back)

But the stadium is expanding and the "players" are getting tired (The universe is getting cooler.)

Suddenly, (at temp about 3000 degrees) the Protons and Electrons join together to form neutral atoms and go on strike.

They stop playing ball with the Photons.

The Photons now burst out of the stadium.
(about 300k years after the Big Bang) and fly forever freely across the universe hitting nothing at all.(Eventually some hit our detectors).
The uniform temp and wavelength proves they all did this at the same time...when the universe stopped being "opaque".

Hope that helps.

Peter.
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Replied by michaeloconnell on topic Re: another CMB question, from Hawkings book

You've been watching too much Champions League me thinks...
:wink:
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Replied by fguihen on topic Re: another CMB question, from Hawkings book

excellent explination!! perfect. thanks!
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Replied by fguihen on topic Re: another CMB question, from Hawkings book

one question, how come this was not gradual? if the universe is expanding at a steady rate, then should some light not have escaped before the rest, and this keep happening until it expanded enough so that all the light escaped? how was it all trapped, then suddenly liberated? was there a big jump all of a sudden in the speed of expansion or something?
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Replied by fguihen on topic think i found my answer on wikk

found my answer on wikipedia. bless that site:

The Big Bang theory predicted the existence of the cosmic microwave background radiation or CMB which is composed of photons first emitted during baryogenesis. Because the early universe was in thermal equilibrium, the temperature of the radiation and the plasma were equal until the plasma recombined. Before atoms formed, radiation was constantly absorbed and re-emitted in a process called Compton scattering: the early universe was opaque to light. However, cooling due to the expansion of the universe allowed the temperature to eventually fall below 3,000 K at which point electrons and nuclei combined to form atoms and the primordial plasma turned into a neutral gas. This is known as photon decoupling. A universe with only neutral atoms allows radiation to travel largely unimpeded.


thanks for your help all
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Replied by fguihen on topic Re: another CMB question, from Hawkings book

another question!!! during which phase of the birth of the universe were electrons born? these are needed to create matter, and in the wiki article i can see where protons and neutrons came from during baryogenysis, and then nuclei during nucelosynthesys, but where/ when were the electrons formed?
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Replied by pmgisme on topic Re: another CMB question, from Hawkings book

Quick comment:

The word "Electron" was first coined by the Irish Physicist George Johnston Stoney at Galway University in 1874.

Where did electrons come from ?

They came from Galway of course.

Like myself.

Peter.
16 years 2 months ago #35398

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Replied by voyager on topic Re: another CMB question, from Hawkings book

another question!!! during which phase of the birth of the universe were electrons born? these are needed to create matter, and in the wiki article i can see where protons and neutrons came from during baryogenysis, and then nuclei during nucelosynthesys, but where/ when were the electrons formed?


I'm not sure but I think electrons came into being at the same time as quarks which I think was right at the start.
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Replied by Son Goku on topic Re: another CMB question, from Hawkings book

another question!!! during which phase of the birth of the universe were electrons born? these are needed to create matter, and in the wiki article i can see where protons and neutrons came from during baryogenysis, and then nuclei during nucelosynthesys, but where/ when were the electrons formed?

When the electronuclear force seperated into the strong and electroweak forces. Before then quarks and an electrons were one single particle.
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16 years 2 months ago #35468

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Replied by fguihen on topic Re: another CMB question, from Hawkings book

so am i to understand that quarks and electrons are much the same types of particles?i thought quarks made up the baryon particles ( protons,neutrons etc)?
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Replied by voyager on topic Re: another CMB question, from Hawkings book

so am i to understand that quarks and electrons are much the same types of particles?i thought quarks made up the baryon particles ( protons,neutrons etc)?


No, they are not the same particles now, but they were while the Nuclear forces were unified ... at least that's how I understand it.
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Replied by Son Goku on topic Re: another CMB question, from Hawkings book

so am i to understand that quarks and electrons are much the same types of particles?i thought quarks made up the baryon particles ( protons,neutrons etc)?

Quarks and electrons aren't the same particle and in fact share no similarities in general, except that they both have spin 1/2.
However when the strong and electroweak forces were unified they acted on only one type of particle, because (and this might sound weird) that force needed only one "representation".
When the forces seperated however the two new forces needed two seperate representations, giving rise to two classes of particles.

This is similar to how neutrinos and electrons became seperate particles after electroweak symmetry breaking.
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Replied by fguihen on topic Re: another CMB question, from Hawkings book

looks like i have a lot more reading to do! really interesting stuff, but theres a whole lot to get your head around! thanks guys
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