There are (at least) two ways that I can interpret your question.

First, "is the photon always traveling as a massless particle, on null geodesics?"

Second, "is the velocity associated with null geodesics always the same as measured locally?"

Within the confines of standard physics, the first question can be answered by noting that if the photon veered off null geodesics, i.e., if it acquired an effective mass, its velocity would be energy-dependent. This would have a particular effect for gravitational lensing: photons of different wavelength would follow slightly different trajectories, leading to a "rainbow effect" when observing distant lensed objects. The fact that no such effect is observed can put an upper limit on the effective mass of the photon.

The second question can be anwsered by noting that any such deviation of the "invariant speed" from its locally measured value can be absorbed into the metric tensor; conversely, and consequently, such variations amount to significant variations in the gravitational potential. Such variations would have observable consequences, affecting the dynamics of star clusters and galaxies.

Dear Peter: The value of 299792458 m/s is not experimental. You remember correctly: it is the definition of the meter. That is, one meter equals 1/299792458 times the distance traveled by light in a vacuum during 9192631770 hyperfine transitions of Cs-133 (i.e., 1 second). So by definition, the speed of light is always this value. To explore the possibility that the speed of light is variable, we need to compare it to something else, e.g., an independent definition of a length or velocity; or alternatively, we need to explore the consequences of a variable speed of light. This is why I was so circumspect with my answer: I wanted to identify clearly what the observable consequences of a spatially varying velocity of light would be.

Again, there are two possible meanings (within the confines of relativity theory): either light doesn't travel on null geodesics, or the velocity associated with null geodesics is itself variable. However, the second of these two possibilities is really something that can be absorbed into the metric tensor, so it becomes a matter of semantics, not a real physical effect. The first possibility endows photons with an effective mass, with observable consequences, and observational limits on the upper limit of the photon mass would have to be taken into account.

Dear Peter, why should it be different inside and outside the Solar System? If it is different depending on outside circumstances, why would the "Solar System" be the border? And if so, what is our solar system? Is it defined by gravitational influence of the Sun or the Heliopause?

If the idea of a constant velocity of light would be wrong, many things could not be explained or would not fit as they apparently do fit in our observations, Dr. Toth has already explained it very well.

Well this is common knowledge, of course the speed of light will be different in each medium - it has even been accomplished that light has been "stopped" completely. But this is not a function of place but condition. Of course light passing through a nebular might have a different velocity than that traveling through complete vacuum, but mostly space is almost perfect vacuum.

Dear Peter: If your concern is "media effects" on the speed of light (not something more fundamental, namely variability of the speed of light in a pure vacuum) then I think this is quite a valid question, but the topic is fairly well understood. Inside the solar system, the speed of electromagnetic waves indeed varies because of their interaction with the charged particles of the solar wind. This variability is also frequency-dependent. When radio-metric methods (Doppler, range) are used to track distant spacecraft, it is essential to have a good idea of solar activity, to correctly model the propagation delay of the radio signal due to the presence of charged particles. But the further from the Sun, or the higher the frequency, the less the signal is affected by the interplanetary medium. So when it comes to visible light traveling in interstellar space, the deviation from the vacuum speed of light is exceedingly tiny.

Dear Peter: Not sure why you are saying that "the 'stellar' wind from all other stars is much stronger than we think". In addition to models and indirect measurements, we now have direct measurements of the interstellar medium courtesy of Voyager 1. Strong or not strong, the interstellar medium is still orders of magnitude less dense than the best vacuum terrestrial labs can produce... and even in those vacua, the speed of light is not distinguishable from its vacuum value.

I am not aware of stellar wind from other stars having an effect on the climate. Cosmic radiation, yes, but that is a different animal: it is high energy ionizing radiation that is deflected by the solar wind, so the amount of it that reaches the atmosphere is dependent on solar activity, and because it is ionizing radiation, it affects cloud formation rate. (Well, this is my understanding from what I read years ago about the topic anyway.)

A much more variable speed of light would have very visible effects that would show up in astronomical observations. That is because the moment light no longer travels on null geodesics, its response to the interstellar medium is frequency-dependent: this means that light from distant stars, for instance, would be refracted into pretty rainbows. This is not something we observe. Also, light from very distant objects would take different amounts of time to travel to the Earth depending on frequency; this could have a very demonstrable effect for, e.g., supernova observations at cosmological distances. The bottom line is that these observations all suggest that the actual speed of electromagnetic waves in the interstellar/intergalactic medium is indistinguishable from the theoretical, vacuum speed, which in turn is constant.

@Peter: Well if there is something and no vaccuum, why can't we measure it? Why can we not detect whatever there may be? so what is your idea of what we miss?

So do you have published peer reviewed papers about this unified physics? As far as I know, this approach is of course the goal of all scientific work in the field of physics, but it has not been successfully implemented, because there are always errors.

I cannot find any peer reviewed papers there. Where have you published?

I looked at your article about extending New horizons to Andrea - you make many claims, but provide no clue at all, where you get indications of a former companion star/ brown dwarf of our Sun. What indications are there?

Why do you claim that the Sun is 7 billion years old, when current data shows it is only 4.6 billion years old. you make many claims, without providing proof.

Dear Peter,

I read some of the chapters of your book and already stumbled upon several things, which are odd. First of all, I have problems with your style of argumentation as you make a lot of claims in your book, but do not provide proof, calculations or anything to base these claims on. You also ignore facts and consequences of your universe model, which contradict what we actually see in the universe.

Just one example. You provide no proof whatsoever for your "Dark Companion". How do you know it formed? I am not saying it is entirely impossible that it did, but there is no commonly known indication that it did. So what indication do you have?

You further claim that its remnants are still orbiting the common barycenter of the solar system and you claim it is further circled by Pluto, which is actually a satellite of that companion. You further claim that Pluto and the remains of the dark companion (which also is the source for the Kuiper Belt, if I understood you correclty) are circling around the Sun in the same time (so that Pluto actually circles around it). There are several errors in this estimate, which you ignore or might not have seen:

1) You claim Pluto and the central remains are oribiting the solar system barycenter at the same orbital period - yet the rest of the remains, which constitute the Kuiper Belt in your opinion, have different orbital periods. Why? Only for convenience? Also you miss one very important thing: you mention the observations of Tycho Brahe, but you skip on the results of them. Kepler used Brahe's observation data to formulate three laws, one clearly shows that the orbital period is a function of its semimajor axis (which of course can also be derived using Newton's law of gravity, resp. the elliptical geometry). Since you claim that the Dark Companion has a position 10 AU behind Pluto, it is not possible that it shares the same orbital period. It is actually a very easy idea: Orbits are the consequence of a balance between centrifugal and gravity force. So if a celestial object is further out, gravity becomes smaller, therefore its velocity must be smaller too, otherwise it would not be in a stable orbit. It is not possible that two objects have the same orbital period but not the same semi-major axis. This is a direct consequence of Brahe's observations, which you use as basis for your argumentation. Meaning there is a huge contradiction in your argumentation.

2) you further claim there has been a catastrophic event that propelled a proto-Mars into Earth 3.5 billion years ago (interestingly you fail to explain where now-Mars then would come from) and this event formed the Moon and today's Earth. However we have geological, resp. geophysical evidence that Earth is 4.5 billion years old, resp. the Moon. On Earth we found rocks, which are 4.4 billion years old, on Moon 4.5 billion years old. Even the most pessimistic estimates say it is 3.8 billion years, still approx. 10% off your demand. You can review some details here and probably find sources for that:

https://en.wikipedia.org/wiki/Oldest_dated_rocks

Also we know that life on Earth began 3.5 billion years ago, possibly even 3.7 billion years.

Schopf, J. William; Kudryavtsev, Anatoliy B.; Czaja, Andrew D.; Tripathi, Abhishek B. (5 October 2007). "Evidence of Archean life: Stromatolites and microfossils". Precambrian Research (Amsterdam, the Netherlands: Elsevier) 158 (3–4): 141–155

https://en.wikipedia.org/wiki/Abiogenesis

So exactly at the time of that catastrophic impact, you claim happened, life emerged, or possibly 200 million years before.

How do you explain this? How do you explain that there is a material on Earth which formed 1 billion years before that catastrophic event? How did it survive this event on the surface?

I find your style of argumentation, i.e. not providing proof of your model, only making claims and not dealing with contradictions your model causes with found evidence of the formation of Earth and the solar system, very unprofessional. I strongly suggest you remember scientific principles and how to argue a hypothesis correctly. If you make a hypothesis, try to disprove it - if you can't, you might be right. Also of course, discuss your ideas with other scientists, to find holes in your argumentation and close them. Right now your "book" reads like a fantasy novel and not a scientific explanation.