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I know that light is considered as energy, and as such has a mass.

So it is certainly taken into account in universes models and calculations.

So I'm wondering: How much of the total mass of the universe does light and electromagnetic waves accounts for? And how much does it contributes to its expansion?

Is it positive or negative? Since it is considered as mass only, it should decrease expansion somehow. Am I right about this?

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  • $\begingroup$ The total weight of radiation is a small part of universe I think around $0.01\%$ this is because universe expanding makes radiation redshift and lose energy, it loses energy faster than normal matter. For more info en.m.wikipedia.org/wiki/Friedmann_equations $\endgroup$
    – kingW3
    May 6, 2017 at 17:58
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    $\begingroup$ Possible duplicate of How does light affect the universe? Is light dark matter is another related question $\endgroup$
    – James K
    May 6, 2017 at 18:52
  • $\begingroup$ Light is energy, but that does not mean it has mass. Light (i.e. photons) is massless (and massive particles do not travel at the speed of light). Energy (i.e. non-mass energy) can contribute to the distortion of space-time, but that's not the same as it having mass. $\endgroup$
    – StephenG - Help Ukraine
    May 7, 2017 at 3:41
  • $\begingroup$ @StephenG True, photons have no (rest) mass, but they don't have a rest frame either. A box of photons has more mass than an otherwise identical box that contains no photons. $\endgroup$
    – PM 2Ring
    May 7, 2017 at 15:38
  • $\begingroup$ I think you're referring to Invariant Mass, and while I'd normally link to e.g. Wikipedia my understanding is the Wikipedia page on this subject is far from ideal and I haven't found a good alternative. All I'll suggest is that the "box of photons has mass" argument is a delicate one that's easily misleading - I've never found a clear and unambiguous discussion of it ( hint : I'd like a link to one :-) ). $\endgroup$
    – StephenG - Help Ukraine
    May 7, 2017 at 19:05

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First, some clarifications:

I know that light is considered as energy

No, light is a group of photons and those have energy

So it is certainly taken into account in universes models and calculations.

The energy of radiation is minimal, reaching about $0.01\%$ (yes, what you said is correct, kingW3) of the energy of the Universe, and that's because of two phenomena causing it to:

  • Decrease in absolute amounts: because of redshift.

  • Decrease in relative amounts: because of more dark energy, because it doesn't "dilute" with the expansion:

    • Think of it in this way: even if there is more space (because of expansion) the ratio between the dark energy and volume of the observable universe is constant (almost, this is more like a metaphor).

    • Another way to think about (and was one of the possibilities of what is dark energy) is to think of it as a property of space. So you would see that the ratio remains constant.

Is it positive or negative?

  • If it were negative, we would be testing some kind of light propulsion engine or using it for building an Alcubierre warp drive. You don't see that happening. Also, light would be a tachyon (which are bad: if you ever see a theory produces a tachyon, throw the theory out of the window, as it's equivalent to saying it doesn't work)
  • If it were positive it would have mass, we wouldn't have discovered a lot of things, such as Special Relativity because of it traveling at subluminal speeds, and possibly it would hurt we could "feel it" (because of touch) it depending on the exact value of its mass.

it should decrease expansion somehow

It does, but $0.01\%$ of the energy of the Universe clearly is negligible, and as I said, that number decreases over time.

The mass of light is equal to...?

Light is massless, meaning that its rest mass ($m_0$) is zero. But that doesn't mean it can't interact gravitationally: the reason objects have gravity is that they have mass (and momentum, if not you would have what I think is about $92\%$ less mass), and in this case, mass is equivalent to energy in the sense that it curves Spacetime similarly.

Other way to phrase that last sentence is as StephenG put it:

[...] Energy (i.e. non-mass energy) can contribute to the distortion of space-time, but that's not the same as it having mass.

Summary of what I said, answering other questions:

How much of the total mass of the universe does light and electromagnetic waves accounts for?

About $0.01\%$.

And how much does it [energy of radiation] contributes to its [the Universe's] expansion?

Almost nothing.

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