It's a very good question.
Most people imagine that the universe is overall neutral.
There would be a problem if the universe is finite (compact) with a net charge, since electric field lines would wind around the universe forever if the photon is truly massless. But the universe need not be finite - it seems that it is quite close to being flat, and may be infinite, too.
But a charged universe has been proposed on more than one occasion. An early attempt was made, I think by Herman Bondi, to explain the expansion of the universe by means of electrostatic repulsion.
He suggested this might be caused by a tiny difference in the charge on the electron and the proton, and then worked out some of the consequences, which others have followed up on.
Within a very short time, however, the charge difference between electron and proton was constrained to be extremely small by improved direct experiments of the Millikan oil drop type. The current limit on this charge difference is
Δqe−p<∼10−21e ,
which is sufficient to rule it out as a cause of the expansion of the universe.
There are other limits on similar charge differences between other particle species coming from experiment and astronomical observations, and any charge on the photon is very tightly constrained to be zero. For certain kinds of dark matter there are also very tight constraints on any charge that it may carry.
But baryon number is also shown by direct constraints to be conserved in all observed particle interactions.
And yet, by all observational evidence from cosmic rays, there is no sign of any large concentration of antimatter in the universe. So it appears that either the universe started with an excess of baryons, or that something in the basic interactions occurring in the early universe must have violated baryon number conservation.
There are actually imaginable mechanisms, even within the standard model of particle physics, to violate baryon number conservation. However none of these standard model mechanisms would seem to be sufficient to produce the observed matter-antimatter asymmetry in the best available models of the early universe. So something beyond the standard model would seem to be required.
So one may well question whether charge conservation might be violated, too. Possibly charge might have escaped into extra dimensions, if such exist, or there might be some other charge violating interaction that was active in the early universe.
It turns out that, if the distribution of the net charge were uniform and homogeneous, and if one models the universe as a medium with infinite conductivity, which seems as if it should be a reasonable assumption at most times during the hot Big Bang evolution, then there would be no detectible electric fields.
However there could be a magnetic field due to the motion of the net charge in a universe which is expanding. This field in turn would have created vorticity in the matter, and this vorticity, it turns out is sufficient to create anisotropy in the cosmic microwave background radiation.
This argument places a very strong cosmological constraint on any possible net charge of the universe. The excess charge per baryon would be bounded by
qe−p<10−26e .
http://arxiv.org/pdf/hep-
The constraint would be weaker for a non-uniform magnetic field, but still quite strong, and comparable with the best terrestrial bounds on the electron proton charge difference.
However this argument is quite obviously pretty strongly model dependent - one might very possibly be able to avoid it if one tried a non-standard cosmology of some stripe: but then other constraints naturally come into play of course.
The direct effects of the net charge on photons and other particles travelling through the universe, it turns out, would be expected to be quite small as long as the charge is small. The presence of the net charge will result in Compton scattering of photons, but this is a very small cross-section.
There would be some index of refraction, too, for the space between the galaxies due to the presence of the charge, if it is taken to be uniformly distributed as some sort of gas of charged particles. But on the basis of calculations of the plasma frequency for a given charge density this could be made quite small, except for very low frequencies.
If one assumed say, that the net charge was carried by a uniform gas of protons, there would very likely be observable effects on the spectrum of high energy cosmic rays. The interactions of protons have fairly large cross-sections by comparison with the electromagnetic interactions.
Most people imagine that the universe is overall neutral.
There would be a problem if the universe is finite (compact) with a net charge, since electric field lines would wind around the universe forever if the photon is truly massless. But the universe need not be finite - it seems that it is quite close to being flat, and may be infinite, too.
But a charged universe has been proposed on more than one occasion. An early attempt was made, I think by Herman Bondi, to explain the expansion of the universe by means of electrostatic repulsion.
He suggested this might be caused by a tiny difference in the charge on the electron and the proton, and then worked out some of the consequences, which others have followed up on.
Within a very short time, however, the charge difference between electron and proton was constrained to be extremely small by improved direct experiments of the Millikan oil drop type. The current limit on this charge difference is
which is sufficient to rule it out as a cause of the expansion of the universe.
There are other limits on similar charge differences between other particle species coming from experiment and astronomical observations, and any charge on the photon is very tightly constrained to be zero. For certain kinds of dark matter there are also very tight constraints on any charge that it may carry.
But baryon number is also shown by direct constraints to be conserved in all observed particle interactions.
And yet, by all observational evidence from cosmic rays, there is no sign of any large concentration of antimatter in the universe. So it appears that either the universe started with an excess of baryons, or that something in the basic interactions occurring in the early universe must have violated baryon number conservation.
There are actually imaginable mechanisms, even within the standard model of particle physics, to violate baryon number conservation. However none of these standard model mechanisms would seem to be sufficient to produce the observed matter-antimatter asymmetry in the best available models of the early universe. So something beyond the standard model would seem to be required.
So one may well question whether charge conservation might be violated, too. Possibly charge might have escaped into extra dimensions, if such exist, or there might be some other charge violating interaction that was active in the early universe.
It turns out that, if the distribution of the net charge were uniform and homogeneous, and if one models the universe as a medium with infinite conductivity, which seems as if it should be a reasonable assumption at most times during the hot Big Bang evolution, then there would be no detectible electric fields.
However there could be a magnetic field due to the motion of the net charge in a universe which is expanding. This field in turn would have created vorticity in the matter, and this vorticity, it turns out is sufficient to create anisotropy in the cosmic microwave background radiation.
This argument places a very strong cosmological constraint on any possible net charge of the universe. The excess charge per baryon would be bounded by
http://arxiv.org/pdf/hep-
The constraint would be weaker for a non-uniform magnetic field, but still quite strong, and comparable with the best terrestrial bounds on the electron proton charge difference.
However this argument is quite obviously pretty strongly model dependent - one might very possibly be able to avoid it if one tried a non-standard cosmology of some stripe: but then other constraints naturally come into play of course.
The direct effects of the net charge on photons and other particles travelling through the universe, it turns out, would be expected to be quite small as long as the charge is small. The presence of the net charge will result in Compton scattering of photons, but this is a very small cross-section.
There would be some index of refraction, too, for the space between the galaxies due to the presence of the charge, if it is taken to be uniformly distributed as some sort of gas of charged particles. But on the basis of calculations of the plasma frequency for a given charge density this could be made quite small, except for very low frequencies.
If one assumed say, that the net charge was carried by a uniform gas of protons, there would very likely be observable effects on the spectrum of high energy cosmic rays. The interactions of protons have fairly large cross-sections by comparison with the electromagnetic interactions.
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