is there any theory or observational evidence that our universe is electrically neutral or not?

  • It seems our universe is neutral in large scale.

    There is CP violationwiki problem about matter and dark matter.

    Similarly is there any theory about whether our universe is electrically neutral?

    Cosmic rays are not neutral. However the atomic nucleus and electrons are just separated. There should be no charge excess in total.

    What is the largest scale that is severely non-neutral?

    OK. I find a paper. This can partly answer my question.

    Note that the paper by Düren (the one you quoted in your comment) has not appeared in any peer-reviewed journal. That is a serious indication that it didn't make it through the review process for some good reasons, e.g. it violates known observational constraints.

    Do you mean areas of large and stable static charges? Because otherwise anywhere there is a magnetic field there is some imbalance which is generating the current and hence the field - there is good evidence of magnetic fields at galactic scale. But I guess that is not what you mean.

    You mean it could be un-neutral in galactic scale? We do not care about time-scale here. Suppose there is an isolated galaxy which loses many of its electrons and can not get the electrons back for some kind of reason.

    That would still be a Universe that was electrically neutral of course - merely on a >galatic scale as the electrons had gone somewhere - unless we assuming that conservation of electric charge does not apply.

    Is there any hypothesis that some galaxy could be un-neutral? Or even larger structure could be un-nrutral?

    I think the question should be: "Are there exactly as many electrons as protons in the entire Universe?" Ignoring short-lived +or- particles that are probably balanced.

    @questionhang A significant charge imbalance on a large but localized scale within a galaxy would result in a correspondingly significant increase in the Compton scattering (and inverse compton scattering) effects. The mean free path of photons passing through the region would also go down. More to the point, individual parts (stars, namely) of the area would be expected to have a net charge, and the electromagnetic forces between them (and the rest of the balancing charges) would easily dominate over gravity, to an easily observed degree. This has not been observed.

  • By observation, gravity dominates the universe on large scales. If there were a significant disparity in positive and negative charges then we would expect electromagnetism—which is approximately $10^{39}$ times more powerful than gravity—to dominate. So by this observation we can conclude that the universe is approximiately electrically neutral. Exactly why this should be so is not well-understood, and ultimately ties into the matter-antimatter problem, aka the baryon asymmetry.

    Protons and electrons are produced through distinct processes. That we have many protons is not particularly remarkable, as this would come about by conservation of baryon numbers as other particles decayed. Why we have any electrons at all is a fair bit more mysterious.

    In the early universe, we understand the energy was well above the electron-positron pair production threshold, and so the early universe had large amounts of both electrons and positrons. When the universe cooled below this threshold, the production would have stopped and we would expect the electrons and positrons to then collide and annihilate each other, leaving essentially none of either. But we obviously see large amounts of electrons: approximately as many electrons as protons by the aforementioned observational evidence. And we also do not see large amounts of antimatter—positrons in particular. If there were large amounts of positrons out there, we would expect to see tell-tale signatures in space as they annihilate with regular matter (or in any weak force interactions, such as would be evident in supernovae), and we have no such observations.

    So somehow the early universe must have produced significantly more electrons than positrons. In other words, we have strong observational evidence that there must be an asymmetry in physics between matter and antimatter production. There are ways to work this into the theory, but the Standard Model by default does not support it, and experts have not really settled on any one particular modification.

    The universe is accelerating apart, which can't be accounted for by gravity. An abundance of one charge over the other could conceptually push the universe apart. There are probably other reasons to suspect this isn't the case since that's never mentioned as a possible explanation for cosmic inflation, but I don't think invoking gravity is sufficient justification to dismiss the possibility of the universe having a net electric charge.

    @EricJ. Such a push would be limited by the speed of light, and its effects would diminish with distance. Both of these are false for the expansion of the universe. Electromagnetism acts *within* space, whereas the expansion is of space itself (or, more mathematically, a change in the metric).

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