In my kindergarten quantum field theory class they told me that since "gravity" is really the exchange of spin 2 particles (tensor bosons aka gravitons, which incidentally have never been experimentally observed), interactions between charges (masses) is universally attractive. that's opposed to spin 1 fields --like photons-- that give rise to attractive and repulsive forces.
I had an interesting discussion about this with some of my fellow grad students, except we were talking about where the energy goes from the cosmological redshift.
It turns out that in general relativity (which as far as we can tell does a bang-up job of describing the large-scale dynamics of the universe), energy is not conserved.
I thought that might be where this was coming from. When you look at the CMB, and ask what's behind it, the answer is: more ionized gas. Not only that, but the gas is thick.. you can't see very far into it (off the top of my head, I think the density is about a billion times higher than the "ambeint" density today). The gas is also in a special state of equilibrium, a state whos spectrum is called blackbody.
This is like looking at the Sun. The surface temperature is 6500K, but at the center it's very hot, almost 15 million degrees. But you only see a uniform 6500K glow, and this is because the photons traveling from the inside of the sun have plenty of time to come into equilibrium with their local surroundings (lots of scattering), so by the time they reach the surface, they're down to 6500K.
Today everything is much less dense, the "ambient" stuff is highly ionized, and since the density is low, recombinations don't happen as often. not only that, but if you kept the density the same, the probability of a photon being scattered in an ionized gas is much lower than in a neutral gas, so we get a clear line of sight.
The "ambient" hydrogen in the universe is ionized, this is because there are more than enough UV sources (O and B stars, AGNs, quasars, etc. ) to ionize all of the gas not collected into small (several light years across) clouds, such as those in our galaxy.
The article is correct. the "dark ages" aren't really dark.. this is the period at which the gas in the universe was too cool to emit visible radiation.
It isn't until the enough UV sources turn on that all of the gas ionizes. At this point, visible light (which isn't in some strong resonance line) can escape to infinity.
Like H, only at certain frequencies. Since it's a diatomic molecule, there are not just electronic transitions, but rotational and vibrational "modes". these modes are also discrete energy levels, but it takes much less energy to excite them than the electronic levels and so H_2 absorbs in the IR as well (from ground).
In general, ionized and neutral gas are neither opaque or transparent. It depends on the density, temperature, the details of the radiation field, and also where an observer is located.
-note- although I'm a grad student in astrophysics, I still don't know jack. That being said, here we go.
If you were to watch an individual proton in an ionized gas, it wouldn't be a bare proton all of the time.. depending on the density of radiation and protons+electrons, at some average rate a lucky electron would recombine with it and go almost immediately* to hydrogen's ground state (emitting light corresponding to allowed transitions along the way, which isn't capable of exciting ground state electrons -- it escapes the plasma, although it will random walk through elastic scattering w/ p's and e's).
after some more time, a photon with energy above 13.6 eV will get sufficiently close to the hydrogen atom, and ionize it. There are cooling processes that allow the electon to dissipate its excess kinetic energy, and it will soon find another proton to dance with. The process repeats. A similar story holds for photons that merely excite H's energy levels, ie. they can't escape either. So, long story short, only photons with energy above the ionization potential of Hydrogen, and photons that will take you from the ground state of H to some other excited state, are absorbed by the plasma. What's left over is what we see. You can see how this will change once you add in different types of atoms, with their own unique spectra, but there will still be "windows" in the spectrum. So in that sense, and ionized plasma is transparent.
*this requires a quantum mechanical calculation to show
Actually they don't make the same assumption. the dynamics of the primoridial perturbation spectrum is determined by GR, whereas the dynamics of galaxies and galaxy clusters is well described by newtonian gravity.
And --speaking generally here-- scientists don't assume there's something wrong with the universe, they assume there's something they don't know about it yet.
For instance, in the 19th century two guys independently discovered something funny with Uranus' orbit.. that it wasn't quite moving the way it should given Newton's law of gravity and what they knew of the motions of Saturn and Jupiter. So each concluded there must be another planet out there perturbing Uranus' orbit. They each wrote to their observatories, and one of the observatories said "look at the balls on this guy scribbling some notes on paper saying he discovered a planet" and ignored it. The other observatory didn't ignore it and they found Neptune.
There are similar stories like this (Dirac eq. predicting the first order correction to the electron magnetic moment, or hell, predicting the existence of what we now call positrons, GR predicting a universe in motion (expanding, contracting, although Einstein wasn't too happy about this at first.)
The point is you don't go around modifying you're theories every time you run into something you can't explain.
armchair physicists and real working physicists are distinguishable particles. Unlucky for us, you need an expensive detector (like a grad. degree in physics) to tell them apart.
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In my kindergarten quantum field theory class they told me that since "gravity" is really the exchange of spin 2 particles (tensor bosons aka gravitons, which incidentally have never been experimentally observed), interactions between charges (masses) is universally attractive. that's opposed to spin 1 fields --like photons-- that give rise to attractive and repulsive forces.
I had an interesting discussion about this with some of my fellow grad students, except we were talking about where the energy goes from the cosmological redshift.
It turns out that in general relativity (which as far as we can tell does a bang-up job of describing the large-scale dynamics of the universe), energy is not conserved.
I thought that might be where this was coming from. When you look at the CMB, and ask what's behind it, the answer is: more ionized gas. Not only that, but the gas is thick.. you can't see very far into it (off the top of my head, I think the density is about a billion times higher than the "ambeint" density today). The gas is also in a special state of equilibrium, a state whos spectrum is called blackbody.
This is like looking at the Sun. The surface temperature is 6500K, but at the center it's very hot, almost 15 million degrees. But you only see a uniform 6500K glow, and this is because the photons traveling from the inside of the sun have plenty of time to come into equilibrium with their local surroundings (lots of scattering), so by the time they reach the surface, they're down to 6500K.
Today everything is much less dense, the "ambient" stuff is highly ionized, and since the density is low, recombinations don't happen as often. not only that, but if you kept the density the same, the probability of a photon being scattered in an ionized gas is much lower than in a neutral gas, so we get a clear line of sight.
The article is correct. the "dark ages" aren't really dark.. this is the period at which the gas in the universe was too cool to emit visible radiation. It isn't until the enough UV sources turn on that all of the gas ionizes. At this point, visible light (which isn't in some strong resonance line) can escape to infinity.
In general, ionized and neutral gas are neither opaque or transparent. It depends on the density, temperature, the details of the radiation field, and also where an observer is located.
If you were to watch an individual proton in an ionized gas, it wouldn't be a bare proton all of the time.. depending on the density of radiation and protons+electrons, at some average rate a lucky electron would recombine with it and go almost immediately* to hydrogen's ground state (emitting light corresponding to allowed transitions along the way, which isn't capable of exciting ground state electrons -- it escapes the plasma, although it will random walk through elastic scattering w/ p's and e's).
after some more time, a photon with energy above 13.6 eV will get sufficiently close to the hydrogen atom, and ionize it. There are cooling processes that allow the electon to dissipate its excess kinetic energy, and it will soon find another proton to dance with. The process repeats. A similar story holds for photons that merely excite H's energy levels, ie. they can't escape either. So, long story short, only photons with energy above the ionization potential of Hydrogen, and photons that will take you from the ground state of H to some other excited state, are absorbed by the plasma. What's left over is what we see. You can see how this will change once you add in different types of atoms, with their own unique spectra, but there will still be "windows" in the spectrum. So in that sense, and ionized plasma is transparent.
*this requires a quantum mechanical calculation to show
And --speaking generally here-- scientists don't assume there's something wrong with the universe, they assume there's something they don't know about it yet.
For instance, in the 19th century two guys independently discovered something funny with Uranus' orbit.. that it wasn't quite moving the way it should given Newton's law of gravity and what they knew of the motions of Saturn and Jupiter. So each concluded there must be another planet out there perturbing Uranus' orbit. They each wrote to their observatories, and one of the observatories said "look at the balls on this guy scribbling some notes on paper saying he discovered a planet" and ignored it. The other observatory didn't ignore it and they found Neptune.
There are similar stories like this (Dirac eq. predicting the first order correction to the electron magnetic moment, or hell, predicting the existence of what we now call positrons, GR predicting a universe in motion (expanding, contracting, although Einstein wasn't too happy about this at first.)
The point is you don't go around modifying you're theories every time you run into something you can't explain.
armchair physicists and real working physicists are distinguishable particles. Unlucky for us, you need an expensive detector (like a grad. degree in physics) to tell them apart.