# How can the expansion of space cause light to loose energy?

• whatdofisheat
In summary: The only reason that EM radiation can be modelled as a wave is because the quantum objects that it comprises are so numerous and interact so frequently that the underlying discreteness is smoothed out. In the same way that the surface of a liquid appears smooth and continuous when seen from a distance.MFIn summary, the conversation discusses the concept of how the distance between two objects can affect something traveling between them, specifically in terms of light. It is noted that from a relativistic standpoint, the idea of "traveling between" two objects is not applicable to light. Instead, as the space between the objects expands, so do the wavelengths of the light passing through it. This phenomenon does not
whatdofisheat
how can the area or distance between two objects getting larger cause something traveling between them to strech?

Note that from the light viewpoint it relativistically isn't "travelling between them" it just is with them together timelessly. So if the move apart it, and its wave lengths, must stretch. Notice that this is no longet true for forces carried by massive particles like the weak force. Also the expansion rate is proportional to the length, so each "proton width" hardly expands at all; therefore the confined gluons and the strong force are very little affected.

If I understand your question correctly: When two things stretch at the same rate and the space between the two objects stretch as well when something passes between them it is subjected to those same forces and it expands too.

As for the title of your thread (which is a good one; the subject too ) I don't think it is so much as light loses energy. Because energy can neither be created nor destroyed. The light travels through space (it's medium) and as its medium expands the distance between molecules of space is greater which makes it more difficult for the light to travel. Not by much though. Anyway, the energy is more spread out making it seem as though there is less of it, but there really isn't. I'm pretty sure that's how it works. My physics class is studying light and sound waves, so based on that information I'm pretty sure.

By the way: Welcome to PF! Glad to have you here.

I wish I could have read Self's post before I posted mine...I was still typing though .

Self, so let me see if this is right:the wave length the only portion of the light that would be affected?

misskitty said:
Self, so let me see if this is right:the wave length the only portion of the light that would be affected?

In the wave representation, what othe portion is there? This turns out to be a wave nature "observation', and the photon picture goes along for the ride, losing momentum and energy acording to Einstein's equation p = hf.

That makes sense. What part do the gluons you mentioned play in the senario?

I hope you don't mind me asking so many doltish questions .

thanks allot for all of your help

Your most welcome. Do you have an idea on how gluons play into this?

im pretty sure that the gluons do not have a cross sectional size that can be affected. and i would just like to bring your attention to your first post. there has been no experimental evedence that light travels on space. as i was just trying to write a paper of a simalar idea. yet a prof of mine told me to look at the experiment done my michelson morley. this is back when they thought aether made up the universe. the would then need to detect a diffrence in the speed of light if the aether or "space" as you call it was moving in some direction. so yes i think what you have to say was a very good idea and one i am almost convinced of i think you should look at this data it might give you a hand with your idea.

misskitty said:
I don't think it is so much as light loses energy. Because energy can neither be created nor destroyed. The light travels through space (it's medium) and as its medium expands the distance between molecules of space is greater which makes it more difficult for the light to travel. Not by much though. Anyway, the energy is more spread out making it seem as though there is less of it, but there really isn't. I'm pretty sure that's how it works. My physics class is studying light and sound waves, so based on that information I'm pretty sure.
Nope. The individual photons do indeed lose energy as space expands.

MF

I'm not sold on that. Do sound waves lose energy when the source is receeding?

Chronos said:
I'm not sold on that. Do sound waves lose energy when the source is receeding?
firsty - sound waves are quite different to photons. Why would you expect them to behave the same way? What makes you think that photons are in any way analagous to sound waves?

secondly - where photons are concerned the doppler redshift (caused by the relative motions of the source and detector in a given frame of reference) is not the same as the cosmological redshift (caused by the expansion of the reference frame between source and detector).

MF

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moving finger said:
firsty - sound waves are quite different to photons. Why would you expect them to behave the same way? What makes you think that photons are in any way analagous to sound waves?
Light is EM waves propagating through EM fields. When light is redshifted, the frequencies of the light waves are reduced and the effective wavelengths increase.

As I pointed out in another post recently, Einstein never modeled light as photons, but as EM waves propagating through the aether. He was unable to reconcile the aether with GR, so he claimed that it had NO properties sensible to us. He knew that it had to exist to allow the propagation of light through space, but he couldn't get it to make sense in GR. The Unruh Effect is an example of interaction of matter with the aether (vacuum energy fields, ZPE), and should provide us with an instructive model about the properties of "empty" space and the nature of light.

turbo-1 said:
Light is EM waves propagating through EM fields. When light is redshifted, the frequencies of the light waves are reduced and the effective wavelengths increase.
I could equally well argue that light is comprised of quantum objects which we call photons, which propagate according to the configuration of mass/energy in their vicinity.

show me an experiment which you think demonstrates light behaving as a wave and I'll show you an alternative interpretation based on the motion of discrete quantum objects.

when space expands the energy of individual photons in that space is reduced, this is interpreted as an increase in wavelength of the light (ie redshifting).

turbo-1 said:
As I pointed out in another post recently, Einstein never modeled light as photons, but as EM waves propagating through the aether. He was unable to reconcile the aether with GR, so he claimed that it had NO properties sensible to us.
Please can you provide me with the details of this post?

turbo-1 said:
He knew that it had to exist to allow the propagation of light through space, but he couldn't get it to make sense in GR.
Einstein was awarded the Nobel prize for explaining the photoelectric effect on the basis of light being composed of discrete quantum objects.

MF

misskitty said:
...I don't think it is so much as light loses energy. Because energy can neither be created nor destroyed. The light travels through space (it's medium) and as its medium expands the distance between molecules of space is greater which makes it more difficult for the light to travel. Not by much though. Anyway, the energy is more spread out making it seem as though there is less of it, but there really isn't. I'm pretty sure that's how it works. My physics class is studying light and sound waves, so based on that information I'm pretty sure.
...

Hi misskitty. I think your confusion can be traced back to this misconception. You are assuming a simple form of energy conservation that does not apply in cosmology.

In cosmology you cannot assume that "energy can neither be created nor destroyed."

Indeed over 99 percent of the original energy in the cosmic microwave background has been destroyed by the expansion of space since the time the CMB was emitted.

the exact figure is more than 99 percent---according to the best current estimate by cosmologists (z = 1100) it is the fraction 1100/1101.

It is customary to give high school physics students, and also in some places freshman college physics students, incorrect ideas and then correct things for those who stick around as sophomores and juniors and become physics majors. Correct information, it is thought, would confuse the beginning students and slow down their learning basic laws like the conventional Energy Conservation law which DOES WORK FINE if you apply it to physics homework problems where it is appropriate. Engineer and chem majors are never going to need to know situations in which conservation does not apply. Only physics majors ever get told the straight dope.

If your teacher is a high school teacher then even tho he or she might be a wonderful teacher they may well not know this.

Some people have ideas about "where does the lost energy go?" but there is no straightforward answer. As far as the light knows, over 99 percent of its energy has been destroyed. I am NOT talking about "thinning out" which happens too (fewer photons per square kilometer) but about the loss of over 99 percent of an individual photon's energy due to spatial expansion.

marcus said:
Some people have ideas about "where does the lost energy go?" but there is no straightforward answer. As far as the light knows, over 99 percent of its energy has been destroyed. I am NOT talking about "thinning out" which happens too (fewer photons per square kilometer) but about the loss of over 99 percent of an individual photon's energy due to spatial expansion.

Plenty more discussion on this subject in :
Where did the energy in the CMB go to?

MF

moving finger said:
Plenty more discussion on this subject in :
Where did the energy in the CMB go to?

MF

hi MF, thanks for the link
I was thinking there should be some cosmology FAQ on the web that handles the limitations of energy conservation law in Gen Rel. I just don't happen to have a link. Perhaps the Usenet sci.physics.research FAQ, or Ned Wright's UCLA cosmology site, or something at John Baez site? I think I should be equipped with a link to some standard authority about this, but I don't know one, and it keeps coming up.

moving finger said:
I could equally well argue that light is comprised of quantum objects which we call photons, which propagate according to the configuration of mass/energy in their vicinity.

show me an experiment which you think demonstrates light behaving as a wave and I'll show you an alternative interpretation based on the motion of discrete quantum objects.

when space expands the energy of individual photons in that space is reduced, this is interpreted as an increase in wavelength of the light (ie redshifting).
I respect your acceptance of the particle/wave duality. I suggest to you that you might look at this duality as an interesting convergence of classical and quantum physics. Classical physics tells us (quite appropriately) that energy travels as waves propagating through fields. Quantum physics tells us that EM waves collapse and can act as if they are point sources, and the likelihood that a point will be expressed at any location is a function of the sum of the magnitudes of the probabilities involved. Feynman gave some great lectures on this at a non-technical level. you'll need a few spare hours to view these, but they are priceless!

http://www.vega.org.uk/series/lectures/feynman/

This tendency of waves to collapse to a point-like function and the way that atoms on the receiving end must exhibit quantized (fixed-energy) reactions have made us comfortable with the concept that light actually travels in quantum-energy packets. This is not true. I think that light exists as waves propagating through EM fields, and that the waves decrease in energy as they propagate through the field, and that our detectors can only respond in quantum steps, leading us to think (erroneously) that light can only impinge upon our receptors with fixed energy levels and in point sources.

moving finger said:
Please can you provide me with the details of this post?
I can't locate the post right now, but here is the lecture that Einstein gave in 1920, after struggling for years to get rid of the aether. He finally reconciled himself with the aether because it is essential for EM wave propogation. Because he couldn't accept the idea that the aether represented a reference frame against which proper motion might be sensed, he claimed that it had NO properties other than the ability to propogate EM waves. This was probably not the right assumption, as demonstrated by the Unruh Effect. The assumption did not accord with the Machian view of space, either, although that is a bit more theoretical, modeling rotation and proper motion as if they had to be measured relative to the reference frames of every piece of matter in all the universe. I think Mach's aether failed only because he did not have an idea of the enormity of space, and neglected to treat the aether as a local reference fram.

http://www.tu-harburg.de/rzt/rzt/it/Ether.html

moving finger said:
Einstein was awarded the Nobel prize for explaining the photoelectric effect on the basis of light being composed of discrete quantum objects.
Yes, but Einstein had a great deal of difficulty accepting the particle/wave duality of light that many seem to credit to his work.

Einstein is often cited as the man who killed the aether and established the particle nature of photons, but this is in direct opposition to the views he expressed in his later published works. (see link above)

Hubble is often cited as the man who proved the reality of the Big Bang (cosmological expansion extrapolated back to a point source) although he publicly resisted and /or seriously doubted that explanation until the end of his life.

These guys were pretty serious scientists. It might be time to rein in the cosmologists riding on their work and ask them to explain how their extrapolations and assumptions can be justified. Just a thought.

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This is my favorite myth buster episode regarding tired light:
http://www.astro.ucla.edu/~wright/tiredlit.htm
Please consider that cosmologically redshifted photons are also time dilated. Photons from a z=2 supernova are only half as energetic as those from a z=1 supernova - but you receive twice as many of them due to time dilation effects. I don't see any missing energy.

Chronos said:
This is my favorite myth buster episode regarding tired light:
http://www.astro.ucla.edu/~wright/tiredlit.htm
Please consider that cosmologically redshifted photons are also time dilated. Photons from a z=2 supernova are only half as energetic as those from a z=1 supernova - but you receive twice as many of them due to time dilation effects. I don't see any missing energy.
Hi Chronos
I agree re tired light, however I am not sure that I understand your comment "Photons from a z=2 supernova are only half as energetic as those from a z=1 supernova - but you receive twice as many of them due to time dilation effects."

You are not suggesting (I hope) that given two similar supernovae (A and B), one (A) at z=1 and the other (B) at z=2, we receive twice as many photons from B as we do from A?

MF

turbo-1 said:
I respect your acceptance of the particle/wave duality.
I do not accept it the way you perhaps think I do.
IMHO, we have internalized concepts of what we think waves and particles are, and we try to impose these macroscopic intuitions onto the quantum world. Thus, for example, we often try to think of photons as analogous to waves like sound waves, or water waves (as Chronos was suggesting in post #12). This can lead to misconceptions, and I believe one of those misconceptions is “wave/particle duality”. We try to force a quantum object to fit into our picture of being either a wave or a particle when in fact it is neither wave nor particle. The consequence is that sometimes we see properties which seem to fit the wave picture, sometimes we see properties that fit the particle picture – hence we invoke “wave/particle duality” and the “collapse”. What this really tells us (IMHO) is not that quantum objects have a dualistic nature, but that they have a nature which is not compatible with either the wave or particle picture.

turbo-1 said:
I suggest to you that you might look at this duality as an interesting convergence of classical and quantum physics. Classical physics tells us (quite appropriately) that energy travels as waves propagating through fields.
I disagree – I think the opposite! IMHO it is precisely the continued attempt to force QM to fit the classical paradigm of “either wave or particle” that prevents us from truly understanding what is going on at the quantum level.

turbo-1 said:
Quantum physics tells us that EM waves collapse and can act as if they are point sources, and the likelihood that a point will be expressed at any location is a function of the sum of the magnitudes of the probabilities involved.
This is what I mean by “misconceptions”. There is (IMHO) in fact no “EM wave” as you call it, because quantum objects are not waves, and there is no “collapse” and it is not the case that they “can act as if they are point sources” – these are all consequences of trying to force quantum objects to fit your intuitive macroscopic paradigm of “either wave or particle”, when in fact they are neither wave nor particle.

turbo-1 said:
This tendency of waves to collapse to a point-like function and the way that atoms on the receiving end must exhibit quantized (fixed-energy) reactions have made us comfortable with the concept that light actually travels in quantum-energy packets. This is not true.
IMHO, the problem is the one I have outlined above. Quantum objects are not “waves” which “collapse to point-like functions”, this is our distorted attempt to understand QM assuming the macroscopic “either wave or particle” paradigm.

turbo-1 said:
I think that light exists as waves propagating through EM fields, and that the waves decrease in energy as they propagate through the field, and that our detectors can only respond in quantum steps, leading us to think (erroneously) that light can only impinge upon our receptors with fixed energy levels and in point sources.
I respect your belief, but IMHO I do not think we can ever hope to properly understand quantum objects as long as we continue to try and shoehorn them into an “either wave or particle” paradigm. Your picture carries with it all of the ugly “collapse” implications as the “wave” (spread out over the entire universe I might add!) suddenly stops being a wave and all of it’s energy is suddenly collected and focused into a single “particle” at the moment of detection.

turbo-1 said:
I can't locate the post right now, but here is the lecture that Einstein gave in 1920, after struggling for years to get rid of the aether. He finally reconciled himself with the aether because it is essential for EM wave propogation.
Another reason for rejecting the “wave” picture of quantum objects.

turbo-1 said:
Because he couldn't accept the idea that the aether represented a reference frame against which proper motion might be sensed, he claimed that it had NO properties other than the ability to propogate EM waves. This was probably not the right assumption, as demonstrated by the Unruh Effect. The assumption did not accord with the Machian view of space, either, although that is a bit more theoretical, modeling rotation and proper motion as if they had to be measured relative to the reference frames of every piece of matter in all the universe. I think Mach's aether failed only because he did not have an idea of the enormity of space, and neglected to treat the aether as a local reference fram.
IMHO Mach’s principle is alive and kicking, and I believe will eventually need to be accounted for in a quantum-mechanical theory of gravity, whereas I don’t see any hope for the aether (except insofar as Mach’s principle suggests a reference frame dictated by mass-energy which could be called the “aether”).

turbo-1 said:
http://www.tu-harburg.de/rzt/rzt/it/Ether.html[/QUOTE]
Thanks, I shall read with pleasure.

moving finger said:
Einstein was awarded the Nobel prize for explaining the photoelectric effect on the basis of light being composed of discrete quantum objects.
turbo-1 said:
Yes, but Einstein had a great deal of difficulty accepting the particle/wave duality of light that many seem to credit to his work.
As many of us still do!

"Niels Bohr brainwashed a whole generation of physicists into believing that the problem had been solved"
Murray Gell-Mann

“It is unreasonable for a single photon to travel simultaneously two routes. Remove the half-silvered mirror, and one finds that one counter goes off or the other. Thus the photon has traveled only one route. It travels only one route, but it travels both routes; it travels both routes, but it travels only one route. What nonsense! How obvious is it that quantum theory is inconsistent! Bohr emphasised that there is no inconsistency. We are dealing with two different experiments. The one with the half-silvered mirror removed tells which route. The one with the half-silvered mirror in place provides evidence that the photon traveled both routes. But it is impossible to do both experiments at once”
Albert Einstein

turbo-1 said:
Einstein is often cited as the man who killed the aether and established the particle nature of photons, but this is in direct opposition to the views he expressed in his later published works. (see link above)
Further evidence (IMHO) that thinking in terms of a “duality” is a misconception.

turbo-1 said:
Hubble is often cited as the man who proved the reality of the Big Bang (cosmological expansion extrapolated back to a point source) although he publicly resisted and /or seriously doubted that explanation until the end of his life.
Yes, and Schroedinger (the man who came up with the wavefunction which bears his name) was opposed to any idea of “collapse” of the wavefunction :

"If we have to go on with these damned quantum jumps, then I'm sorry that I ever got involved."
Erwin Schroedinger

Both Einstein & Schroedinger were deeply dissatisfied with the notion of “wave-particle duality” and “collapse of the wavefunction”. Here is an extract from a letter sent by Albert to Erwin :

"You are the only person with whom I am actually willing to come to terms. Almost all the other fellows do not look from the facts to the theory but from the theory to the facts; they cannot extricate themselves from a once accepted conceptual net, but only flop about in it in a grotesque way."

It’s time that wave-particle duality was consigned to the dustbin, where it belongs.

MF

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moving finger said:
Hi Chronos
I agree re tired light, however I am not sure that I understand your comment "Photons from a z=2 supernova are only half as energetic as those from a z=1 supernova - but you receive twice as many of them due to time dilation effects."

You are not suggesting (I hope) that given two similar supernovae (A and B), one (A) at z=1 and the other (B) at z=2, we receive twice as many photons from B as we do from A?

MF
Yes, I am suggesting that. And I think I have a good reason to make that claim. The redshifted photons are lazy, but still get here.

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Chronos said:
Yes, I am suggesting that. And I think I have a good reason to make that claim. The redshifted photons are lazy, but still get here.
not sure we are talking about the same things.

are you suggesting that there are twice as many photons (in total) arriving from the z=2 supernova when compared to the z=1 supernova?
MF

Cosmological red shift is the 'Cosmological Solution' equivalent to the 'One Body Solution' and its gravitational red shift.

So an equivalent question to: "How can the expansion of space cause light to loose energy?" is: "How can a photon loose energy when it is gravitationally red shifted?"

Remember to be consistent in answering this question. Many textbooks will hand wave and talk about gravitational potential energy. However as GR reinterprets gravitational forces as inertial forces caused by curvature the concept of gravitational potential energy is radically changed, or even eliminated.

A photon is emitted at one frequency, climbs from one level to a higher level and is received at a lower frequency, and therefore lower energy. But where has its energy gone? No forces have acted on the photon, no work has been done on, or by, it; so why has its energy changed?

Garth

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moving finger said:
not sure we are talking about the same things.

are you suggesting that there are twice as many photons (in total) arriving from the z=2 supernova when compared to the z=1 supernova?
MF
Photons are unnecessarily obscuring things. A better analogy is in order: A Z=1 supernova shines at an intensity of say 100 watts per minute for 10 days [according to a Z=1 watch] before fading away. By the time the light gets to Z=0, the observer at Z=0 measures 50 watts per minute, but it shines for 20 days due to time dilation.

Chronos said:
Photons are unnecessarily obscuring things. A better analogy is in order: A Z=1 supernova shines at an intensity of say 100 watts per minute for 10 days [according to a Z=1 watch] before fading away. By the time the light gets to Z=0, the observer at Z=0 measures 50 watts per minute, but it shines for 20 days due to time dilation.
Huh? Light intensity falls off by an inverse square rule, so doubling the time of the light-curve falls WAY short of delivering equivalent energy.

turbo-1 said:
Huh? Light intensity falls off by an inverse square rule, so doubling the time of the light-curve falls WAY short of delivering equivalent energy.
The intensity is irrelevant. The wavelength of say sodium light from a Z=1 source is twice that of a Z=0 source, hence has only half the energy of the Z=0 emitter. The point is you receive those half-energy wavelengths for double the time it took the Z=1 source to emit them.

Chronos said:
Photons are unnecessarily obscuring things. A better analogy is in order: A Z=1 supernova shines at an intensity of say 100 watts per minute for 10 days [according to a Z=1 watch] before fading away. By the time the light gets to Z=0, the observer at Z=0 measures 50 watts per minute, but it shines for 20 days due to time dilation.
(sorry, please change that to watts, not "watts per minute" - watts is already a time-dependent unit, so 100 watts for 10 days then becomes 24kW hours, and 50 watts for 20 days is also 24kW hours)

in which case... ok...nothing controversial in that...and... (getting back to your original suggestion)...what about the light from the Z=2 supernova (which at source should also emit 24kW hours)? Are you suggesting that we again should see (compensating for the distance dispersion) 24kW hours from this source?

MF

moving finger said:
(sorry, please change that to watts, not "watts per minute" - watts is already a time-dependent unit, so 100 watts for 10 days then becomes 24kW hours, and 50 watts for 20 days is also 24kW hours)

in which case... ok...nothing controversial in that...and... (getting back to your original suggestion)...what about the light from the Z=2 supernova (which at source should also emit 24kW hours)? Are you suggesting that we again should see (compensating for the distance dispersion) 24kW hours from this source?

MF
Well, I would say 25kW hours [just being mathematically correct], but that aside, that is precisely what I am saying. There is no more missing energy than there is any missing time. Energy x Time works in every reference frame, in my opinion. I am attempting to eliminate the cowpies from the equation of state.

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Chronos said:
Well, I would say 25kW hours [just being mathematically correct],
oh my... 100 watts times 24 hours per day times 10 days equals...
how many kW hours did you say? (sorry)

Chronos said:
but that aside, that is precisely what I am saying. There is no more missing energy than there is any missing time. Energy x Time works out in every reference frame, in my opinion.+
But the whole point is that there IS missing energy. The total energy flux (integrating over time) received from the z=2 SN is less than we would expect after correcting for the extra distance dispersion and based on the assumption that it is the same standard candle as the z=1 SN, and the amount by which it is less is actually (roughly) proportional to distance (Hubble's "law"). This is because the photons from the z=2 SN are more redshifted (have lower energy) than the z=2 SN, thus there is less energy per photon.

MF

I admit I am clinging to a stubborn faith in the soundness of the laws of thermodynamics. I believe GR nicely accounts for the apparent missing energy as an illusion created by time dilation effects.

Chronos said:
I believe GR nicely accounts for the apparent missing energy as an illusion created by time dilation effects.
an illusion?
are you suggesting that what we think we measure as redshifted photons (redshifted = lower in energy) are not actually redshifted after all, but are in fact an illusion?
Can you elaborate?
MF

I am repeating my mantra, there is no missing energy... All the waves reach the destination, they merely take more time when forced to travel through stretched space. In the end, they all finish the journey. Would you agree energy density is time dependent?

Chronos said:
I am repeating my mantra, there is no missing energy... All the waves reach the destination, they merely take more time when forced to travel through stretched space. In the end, they all finish the journey. Would you agree energy density is time dependent?
My understanding is that what you suggest does not agree with the experimental data.

There are at least 2 effects going on here. I will deliberately try to describe them without referring either to waves or particles.

One effect is time-dilation, which is basically the "spreading out" of the intensity vs time distribution curve of energy emitted from an event (such as a SN) as the source of that event moves away from us. I agree this effect does not reduce the total energy received from the source, it simply "spreads it out" over a greater time distribution (simplistically, the SN would look dimmer but would be visible for longer - the total energy received integrating over time would not change).

(as an aside : Obviously (or at least I hope its obvious) we would see such time dilation effects only for "events" where energy-flux changes with time (eg a SN), we would never observe a time dilation effect on the energy from a continuously (constantly) emitting source moving at a constant speed relative to ourselves.)

The other effect is called "cosmological redshifting", which is a genuine reduction in energy of the light received from the SN caused by the expansion of space between the moment of emission and the moment of detection. Integrating over time makes no difference - the same cosmological redhsift effect can be observed from "continuous" sources (which emit continuously) as well as "event sources" such as supernovae

These are two very separate and cumulative effects, and one does not explain or cancel out the other.

MF

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