B Dark Energy, Dark Matter and the Speed of Light

[mpross]

TL;DR Summary

Does "c" change as light travels through dark matter and dark energy?

Quote from NASA:
My understanding of dark energy is based on NASA's report: https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy; were NASA state as follows: "It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the universe.

My question is: If dark energy and dark matter compose 95% of the universe, and photons travel at c in vacuum, has anyone measured the change in the speed of light traveling through space, that according to the latest theory is 95% dark matter and dark energy and is far from a vacuum?

 

[Orodruin]

Whatever dark energy is it does not significantly interact with photons. It is dark. The effect on light propagation is not relevant.

I also think you fail to understand exactly how little matter there is in the universe. Even if there was significant interaction of light with dark energy or dark matter, the average density would be very very very small and not "far from a vacuum" as you state.

 

[mpross]

I agree with you.

My next question is about dark energy, I understand that is solves certain equations that calculate the rate of acceleration of the universe. My question is, how do we know that this particular solution matches reality, at this point it is just theoretical physics without any experimental data, is that correct, or am I missing something?

 

[Orodruin]

The entire point of introducing dark energy in the first place is that it is needed to match experimental data…

 

[Ibix]

My question is, how do we know that this particular solution matches reality,

The whole reason for invoking dark energy is that our models don't match our measurements of reality without it. We don't know much more about it than that, though.

 

[PeroK]

at this point it is just theoretical physics without any experimental data, is that correct, or am I missing something?

It's the other way round. It's the measurable experimental data of accelerated expansion without a full theoretical explanation.

 

[mpross]

Thanks PeroK, the question still remains, why latch onto "dark energy" as the only solution when there is no data supporting the existence of "dark energy" the fact that in theory it solves an equation is not experimental data proving the existence of "dark matter". For example the acceleration of the universe could be the result of an external force pulling the universe apart at an accelerated rate, and there is no "dark matter". Has anyone tried using an "outside the universe" gravity force causing the acceleration calculation?

 

[mpross]

This could be done by simply measuring the acceleration in different directions, if there is a variation, then that disproves "dark energy" which is assumed to be uniform, in favor of an outside force that would not be uniform.

 

[PeroK]

Thanks PeroK, the question still remains, why latch onto "dark energy" as the only solution

it's not the only solution, but it's the most obvious!

when there is no data supporting the existence of "dark energy"

there is data. There is just no way to relate the data to known physics.

the fact that in theory it solves an equation is not experimental data proving the existence of "dark matter".

Dark matter is not the same as dark energy. Dark matter is a theory. No one claims its existence is proved.

 

[PeroK]

This could be done by simply measuring the acceleration in different directions, if there is a variation, then that disproves "dark energy" which is assumed to be uniform, in favor of an outside force that would not be uniform.

Believe it or not, but physicists base their theories on precisely that sort of experimental data! The expansion generally appears homogeneous and isotropic ( same in every direction). So, the data does generally rule out things like "external force".

The expansion is very different from the acceleration of objects under an external force.

 

[mpross]

Thanks for that PeroK. My understanding from the physics I studied and read now, is that the explanation for acceleration is due to dark energy, not the expansion, and that this acceleration is recent in the history of the universe, and that raises more questions, like why suddenly dark energy shows up, why wasn't it there in the first few billion years of the universe. This again makes me think of external forces showing up as an explanation, thoughts?

 

[mpross]

What explains the acceleration of the expansion of the universe?Observations of the explosions of white dwarf stars in binary systems, so-called Type Ia supernovae, in the 1990s then led scientists to the conclusion that a third component, dark energy, made up 68% of the cosmos, and is responsible for driving an acceleration in the expansion of the universe.Mar 30, 2017

 

[Ibix]

why wasn't it there in the first few billion years of the universe.

It was.

The point about dark energy is that its density doesn't decrease as a function of the expansion, while the density of matter does. Dark energy was always there - it just becomes more important over time.

 

[Orodruin]

For example the acceleration of the universe could be the result of an external force pulling the universe apart at an accelerated rate, and there is no "dark matter".

First of all, you are confusing dark matter and dark energy here. Apart from the word "dark" in the name there is not much they have in common.

Second, your assertion that the acceleration could be caused by an external force unfortunately goes to show only that you have not understood how cosmology - or indeed classical gravity - works. If you have a reference to a reputable source discussing this, feel free to provide it. Otherwise it is just a personal speculation, and that is off-limits in this forum.

why latch onto "dark energy" as the only solution

This is simply a false assertion. Physicists are continuously testing out different models and considering whether they fit data or not. The hypothesis of dark energy is just the simplest theory to match observations. This does not mean that others have not been considered. That you have not heard of them does not mean they do not exist. You have labeled this thread with "A" level and thus you should be expected to search the literature and understand that this is the case.

This could be done by simply measuring the acceleration in different directions, if there is a variation, then that disproves "dark energy" which is assumed to be uniform, in favor of an outside force that would not be uniform.

No, you are wrong. The universe does not consist solely of dark energy.

and that raises more questions, like why suddenly dark energy shows up, why wasn't it there in the first few billion years of the universe

This is well understood in the current cosmological models. It was always there, but in earlier times it was not dominating the dynamics of the expansion. However, unlike matter or radiation, dark energy does not dilute, meaning that as the universe expands it becomes more and more relevant. You really need to read up on how the current models work before you start questioning them.

This again makes me think of external forces showing up as an explanation, thoughts?

I think you are pushing a personal theory without any real substance in terms of actual mathematical predictions and/or published references and that this is against forum rules.

 

[PeroK]

why suddenly dark energy shows up, why wasn't it there in the first few billion years of the universe.

Dark energy has always been there. It started to dominate as the universe expanded. And, as the universe continues to expand it will dominate even more in the future.

This is covered in any introductory Cosmology textbook.

The big question is to explain the numerical value of dark energy per unit volume. That's the missing piece.

 

[mpross]

First of all, you are confusing dark matter and dark energy here. Apart from the word "dark" in the name there is not much they have in common.

Second, your assertion that the acceleration could be caused by an external force unfortunately goes to show only that you have not understood how cosmology - or indeed classical gravity - works. If you have a reference to a reputable source discussing this, feel free to provide it. Otherwise it is just a personal speculation, and that is off-limits in this forum.This is simply a false assertion. Physicists are continuously testing out different models and considering whether they fit data or not. The hypothesis of dark energy is just the simplest theory to match observations. This does not mean that others have not been considered. That you have not heard of them does not mean they do not exist. You have labeled this thread with "A" level and thus you should be expected to search the literature and understand that this is the case.No, you are wrong. The universe does not consist solely of dark energy.This is well understood in the current cosmological models. It was always there, but in earlier times it was not dominating the dynamics of the expansion. However, unlike matter or radiation, dark energy does not dilute, meaning that as the universe expands it becomes more and more relevant. You really need to read up on how the current models work before you start questioning them.I think you are pushing a personal theory without any real substance in terms of actual mathematical predictions and/or published references and that this is against forum rules.

You are right, sorry about that, I will be more careful in the future regarding personal theories.

 

[weirdoguy]

Dark matter is a theory. No one claims its existence is proved.

Well, e.g. quantum mechanics is also "a theory" I guess the word "hypothesis" would be less confusing.

 

[PeroK]

Well, e.g. quantum mechanics is also "a theory" I guess the word "hypothesis" would be less confusing.

Yes, hypothesis is better. It's also worth noting that there are several different hypotheses for what dark matter might be: black holes, neutrinos, a new type of noninteracting particle etc.

 

[berkeman]

You have labeled this thread with "A" level and thus you should be expected to search the literature and understand that this is the case.

[Mentor Note -- Thread prefix has been updated to "B" level]

 

[mpross]

Thanks to everyone for responding to my questions, I am new to this forum, and following your suggestions will be studying further before posting questions I should know the answers to, my apologies.

 

[bhobba]

Thanks to everyone for responding to my questions, I am new to this forum, and following your suggestions will be studying further before posting questions I should know the answers to, my apologies.

Just a little note about the speed of light. We know the speed of light from all sorts of experiments (eg Maxwell's- Faraday equation ∇XE = (-1/C) ∂B/∂t); many of which, on the surface, you may not think are measuring the speed of light. If dark energy were changing the speed of light actual measurement of the speed of light would not gell with the indirect measurements mentioned previously. I am unaware of any discrepancy, but others with greater knowledge may know some. As an aside, it is possible to derive all of Maxwell's Equations from Coulomb's law and an undetermined constant C in special relativity - see attached document. It is also interesting as an exercise to understand why the argument does not work for gravity. Many experiments confirming Maxwell's equations also confirm the undetermined constant C is the speed of light as do equations like E=MC^2.

Thanks
Bill

 

[bhobba]

It is also possible to derive the Lorentz Transformation with an undetermined constant C that you measure from experiments. Interestingly SR is a theory about the symmetries of an inertial frame and the geometry it implies. Mathematicians discovered in the 19th century a strong connection between symmetries and geometry. Again see attached.

 

[Orodruin]

Interestingly SR is a theory about the symmetries of an inertial frame and the geometry it implies.

Inertial frames as such have no particular special standing apart from being the spacetime equivalent of Cartesian coordinates. Relativity is a theory about the geometry and symmetries of spacetime (in the case of SR, Minkowski spacetime). What coordinates you use to describe that spacetime is of lesser importance.

 

[mpross]

Just a little note about the speed of light. We know the speed of light from all sorts of experiments (eg Maxwell's- Faraday equation ∇XE = (-1/C) ∂B/∂t); many of which, on the surface, you may not think are measuring the speed of light. If dark energy were changing the speed of light actual measurement of the speed of light would not gell with the indirect measurements mentioned previously. I am unaware of any discrepancy, but others with greater knowledge may know some. As an aside, it is possible to derive all of Maxwell's Equations from Coulomb's law and an undetermined constant C in special relativity - see attached document. It is also interesting as an exercise to understand why the argument does not work for gravity. Many experiments confirming Maxwell's equations also confirm the undetermined constant C is the speed of light as do equations like E=MC^2.

Thanks
Bill

Thanks - very interesting paper, I will give a deeper response once I have gone over it a few times.
Michael