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Thread: Expansion of the Universe.

  1. #1 Expansion of the Universe. 
    Forum Radioactive Isotope
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    This is a "simple" question that I hope someone can clarify for me.

    As everything expands (at the present moment) is the space -time fabric being stretched or is that a separate phenomenon to the distortion of said fabric by matter?

    Actually I have a second question (hopefully related)
    When matter distorts the space-time fabric is this phenomenon considered to apply at the nuclear level? (does the proton distort the space-time of the neutron?-I don't think they are massless objects

    3rdly (it is all yes/no! )

    Are the distances between the components of ,say a hydrogen atoms ( the proton ,neutron and electrons) affected by ther ongoing expansion of the universe ?

    I am on a roll so here is Question 4

    Does angular momentum increase the mass of an object ?(I suspect it cannot impart mass to a massless object such as the electron can it?)

    Thanks


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    Moderator Moderator Markus Hanke's Avatar
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    As everything expands (at the present moment) is the space -time fabric being stretched or is that a separate phenomenon to the distortion of said fabric by matter?
    Metric expansion is not really the same phenomenon as local curvature due to the presence of energy-momentum; this can most clearly be seen by the fact that even a universe completely devoid of any energy-momentum would still metrically expand : see Kasner metric. Having said that, it is important to understand that both metric expansion and local curvature are the result of the same underlying physical law, being the Einstein field equations.

    When matter distorts the space-time fabric is this phenomenon considered to apply at the nuclear level?
    Yes, it applies to all forms of energy-momentum, also on atomic scales.

    Are the distances between the components of ,say a hydrogen atoms ( the proton ,neutron and electrons) affected by ther ongoing expansion of the universe ?
    No. Metric expansion becomes apparent only on the very largest scales, where the universe can be considered as homogeneous and isotropic. It does not affect gravitationally ( or otherwise ) bound systems.

    Does angular momentum increase the mass of an object ?
    It doesn't increase a system's rest mass, but it does affect the total energy, also called relativistic mass.

    (I suspect it cannot impart mass to a massless object such as the electron can it?)
    The electron is not massless.


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    Quote Originally Posted by Markus Hanke View Post
    No. Metric expansion becomes apparent only on the very largest scales, where the universe can be considered as homogeneous and isotropic. It does not affect gravitationally ( or otherwise ) bound systems.
    So essentially all orbital systems will remain constant and it is impossible to physically directly measure any expansion?
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    Quote Originally Posted by van erst View Post
    So essentially all orbital systems will remain constant and it is impossible to physically directly measure any expansion?
    Yes, orbits are not affected; remember that metric expansion relies on homogeneity and isotropy, neither one of which is present on small scales, such as the solar system.
    As for measurements - that depends what you mean by that. It can be measured more or less directly through the redshift of light from very distant sources.
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    Quote Originally Posted by Markus Hanke View Post

    (I suspect it cannot impart mass to a massless object such as the electron can it?)
    The electron is not massless.
    thanks.Actually I was thinking of the photon when I wrote "electron" . What would be the answer to that -if it was a spinning photon?
    Still no mass? (do photons spin as a matter of course? Can the spin be stopped if that is the case?)
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    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by geordief View Post
    thanks.Actually I was thinking of the photon when I wrote "electron" . What would be the answer to that -if it was a spinning photon?
    Still no mass? (do photons spin as a matter of course? Can the spin be stopped if that is the case?)
    Photons have exactly one unit of intrinsic angular momentum ( which cannot be changed ); they don't have rest mass, but they do carry momentum and energy.
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