Gravitational component of light and neutrinos in the universe

[BernieM]

Given that a photon or neutrino creates a gravitational effect, and given that the CMB, microwave photons emitted 13.8 billion years ago are still around in the universe, then it would follow that ALL photons or neutrinos that have not since been captured should also still be present in the universe, like the CMB. What is the overall universal net gravitational value contributed by all similar particles emitted since the big bang?

It would appear that over the last 13 or so billion years the universe is gaining 'gravitional weight' so to speak.

 

[Nabeshin]

Given that a photon or neutrino creates a gravitational effect, and given that the CMB, microwave photons emitted 13.8 billion years ago are still around in the universe, then it would follow that ALL photons or neutrinos that have not since been captured should also still be present in the universe, like the CMB. What is the overall universal net gravitational value contributed by all similar particles emitted since the big bang?

It would appear that over the last 13 or so billion years the universe is gaining 'gravitional weight' so to speak.

Well, it's a bit unclear what you mean by gravitational value. We can define how much of the mass/energy density of the universe is made up on photons and neutrinos, and the answer turns out to be something like 0.001%. So in comparison to the gravitational contribution of baryonic matter, and certainly dark matter and dark energy, these photons are irrelevant.

Does that answer your question? I'm not entirely sure what you mean when you speak of net gravitational value...

 

[BernieM]

By net gravitational effect I mean given all the photons that exist that have never been re-captured, the quantity of gravity contributed by them. Does the .001% take into consideration all the photons ever produced in the universe since the big bang? Or does it assume the quantity of photons being emitted in the universe at the present time only?

 

[Nabeshin]

Does the .001% take into consideration all the photons ever produced in the universe since the big bang? Or does it assume the quantity of photons being emitted in the universe at the present time only?

Let me clarify here and say that it doesn't really matter. Either way, the net gravitational contribution to the overall expansion of the universe is negligible. Furthermore, when we cite a figure like the 0.001% we're assuming the universe to be homogeneous, i.e. it's basically just a sea of matter, photons, and dark energy with no clumping. Obviously this isn't the case, except for the CMB. As far as local overdensities go, again, the contributions of radiation and neutrinos are completely negligible.

At any rate, the universe is not "gaining gravitational weight", as for an object to emit a photon the energy must come from somewhere. In the case of a star, the star loses mass. If anything, you could say the universe would be losing "gravitational weight" since photons are diluted quicker by the expansion of the universe than normal baryonic matter is, so over time they contribute less and less.