If we don't have a continuous film of the event, we only capture a given moment in the expansion. If radiation travels at different rates, then depending on the frequency range we use in our 'camera', we in effect get at this point in time, here on earth, different snapshots in time, from there, millions, or billions, of years ago, where the expansion is occurring. The ring effect is not real there, where the event is happening; it is due to seeing, here and now, only that moment at that frequency in the distant past.
There is a NASA web site containing an image showing “a supernova remnant - the remains of a star that exploded in a nearby galaxy known as the Small Magellanic Cloud”,(see the next to the last image). The image shows, in false colors, what the supernova remnant looks like, at this point in time, here on earth, in the x-ray, visible, and radio regions of the spectrum. The outer circle is the radio region. Just a little smaller circle represents the x-ray region. Inside both of these circles is the visible region. Presumably the supernova was expanding from a very tight core over a period of hours, days, or weeks. The outer region corresponds to a later time than the inner region in the evolution of this supernova remnant. Thus we can conclude that the energy arriving now comes from different periods in the development of the supernova. Fortunately, for our ability to collect data, visible light seems to travel faster than either radio or x-ray ‘light' - and that apparently goes for gamma radiation as well. The image shows the visible region near the end of luminosity, not at the peak. The visible light from the central area, representing the initial time of creation of the supernova, has faded or passed. (Once we see the visible burst from a supernova, we can point to that location to pick up X-ray and Gamma Ray bursts that arrive later.)
The fact that the speed of light is not constant allows us to look more carefully at the question whether radiation has mass. As discussed in appendix 3, we can find good reasons that radiation has mass - visible light photons about one millionths the mass of an electron, x rays a thousand times more than visible light, gamma rays a million times more mass than visible photons. That I think will eventually be confirmed experimentally. For high-energy radiation the fact of a speed slower than c, can be conceived as connected to a greater mass than that of visible photons. On the other hand, with radio waves, the slowing down mechanism, in that case, is probably due to the imperfect vacuum through which the radio waves pass, and like radio waves generated here on earth, they bend and change 'frequency' and 'arrival time' at the slightest provocation.
Another indication that the speed of ‘light' is not constant can be found in even earlier empirical results. In looking at the time that light reaches us from a distant supernova, it was found that ultra violet energy peaks months before the x ray region shows a peak. [See Herbert Friedman, “The Astronomer's Universe”, 1990, p.175.] This is a further indication that most probably the limit c is reached only for relatively low frequencies (visible light) and that high frequency radiation has a lower speed limit. The velocity difference, between UV and X-rays, may only be found in the fifth decimal of c, or even in the sixths or seventh, but that would be enough for some radiation to reach us tens or hundreds of years later than visual radiation, when the origin of the light is millions or billions of light years away. What is interpreted to be a neutron star could well be the delayed radiation of a supernova explosion that originated at the same time as the visual radiation but at a slightly slower speed.
In classical physics, kinetic energy is a product that combines matter and motion, and potential energy is used for the amount of kinetic energy that could be gotten out of a particular, and well defined, physical situation in which motion and forces are implicit and capable of generating a specific amount of kinetic energy.
We can therefore question the meaning and interpretation of the formula E = mc^2, as derived from SRT. In a nuclear explosion, matter that supposedly disappears is obtained by adding up the neutrons, protons, and electrons, before and after - and the mass is less after. As an analogy, if we burn a wooden log and weigh only the ashes, we might conclude that mass has been lost. Einstein assumed that radiation has no mass, so nobody counts gamma rays, x rays, etc. It is the fact that nuclear fragments reach immense speeds, speeds close to the velocity of light that most probably gives atomic explosions such immense power - it is not because matter is converted to energy.
The consequences are startling. Visible light covers a miniscule portion of the radiation spectrum - less than one order of magnitude at about 10^14 to 10^15 cycles per second. Gamma rays are at about 10^20 (or more) - a million times greater; radio waves at about 10^8 - a million times smaller. We can try to imagine frequencies as low as one cycle per second. But at a speed of 3x10^10 cm per second, this represents a wavelength of 3x10^10 cm or about 200,000 miles. The idea of a ‘location' during one second, or within the space of one wavelength, loses all meaning. As we try to deal with extremely low frequencies, our ideas of separation and duration dissolve, and the concepts of mass, location, and time, can no longer be applied. Whether the speed of ‘light' at such a low frequency is c, or perhaps greater, is a permissible question. Is there something that is omnipresent, or traveling at infinite speed - gravity, for example?
If one metaphysical speculation is allowed: The lowest vibrations may be those that connect the universe in an immediacy, and in a sense that is beyond time and beyond space.