Basically the title. The wavelength of light from the CMBR has expanded as the universe expanded and so it has lost energy since it is inversely proportional to the wavelength. Where did this energy go? It may have been asked before but I couldnt find it...
2102
So I have learned in class that light can get red-shifted as it travels through space. As I understand it, space itself expands and stretches out the wavelength of the light. This results in the light having a lower frequency which equates to lowering its energy.  My question is, where does the energy of the light go? Energy must go somewhere!   Does the energy the light had before go into the mechanism that's expanding the space? I'm imagining that light is being stretched out when its being red-shifted. So would this mean that the energy is still there and that it is just spread out over more space?
65559
I keep hearing that universe expands. Since all is relative, the expansion must be relative to something. And since universe is all there is ... You got my point.  The only points of reference are inside the universe itself. Bonds forming, their length measured in wavelengths, etc. Red shifting of light tells us things fly away. So our point of reference is only light. (am I missing something?)  Here is my question. In a thought experiment, would it be valid to say that properties of light are somehow changing around us if we keep the size of the universe constant? If yes, what are the consequences of this line of thought? Would it give us any additional insight into what's going on?
6407
According to de Broglie's wave-particle duality, the relation between electron's wavelength and momentum is $\lambda =h/mv$.  The proof of this is given in my textbook as follows:   De Broglie first used Einstein's famous equation relating matter and energy, $$E=mc^2,$$ where $E=$ energy, $m =$ mass, $c =$ speed of light. Using Planck's theory which states every quantum of a wave has a discrete amount of energy given by Planck's equation, $$E=h\nu,$$ where $E =$ energy, $h =$ Plank's constant ($6.62607 \times 10^{-34}\:\mathrm{ J\:s}$), $\nu =$ frequency. Since de Broglie believes particles and wave have the same traits, the two energies would be the same: $$mc^2=h\nu.$$ Because real particles do not travel at the speed of light, de Broglie substituted $v$, velocity, for $c$, the speed of light: $$mv^2=h\nu.$$   I want a direct proof without substituting $v$ for $c$. Is it possible to prove directly $\lambda=h/mv$ without substituting $v$ for $c$ in the equation $\lambda =h/mc$?
37464
Could someone judge my (stoner) hypothesis that the speed of light has changed over time -- i.e. as the universe has expanded in volume light has slowed down, perhaps going so far as back to the big bang when it was infinitely fast and there was no time because everything happened at once etc. Thinking that the speed at which information can propagate through the universe is linked to the size of it seems intuitive to me. My question -- is there an easy disproof of this? Would Einstein have to be wrong? Does it violate anything supposedly more fundamental such as quantum or string theories? Do any current experiments invalidate it? If not can you show me in any case why you think its unlikely.   Edit 8/24  I'm accepting Mark M.'s answer but will post this here because there is a character limit on comments  @Mark M thanks, good answer but as someone who has only read some popular physics, and should leave this to the experts, I'm still muddled in my personal theory. I don't see why you should need two units to measure the speed of light. The thing I have a hard time wrapping my head around is the relation of time and distance. They seem like they could fundamentally be the same thing. If you say time is measured fundamentally by the vibration of so and so quantum object in space... why can't we just measure that vibration distance as the constant... I'll repeat myself to try to be clear... there is a certain minimum distance that particles have to go to interact with each other... if it wasn't vibrating there wouldn't be time,its what creates the illusion of time...so instead of talking about speed or c as distance/time.... can't we simply talk about that distance a quantum object vibrates....I'll lead up to my point.....perhaps there can need be only one constant here, and that is the physical size of the universe. a tiny metal tuning fork doesn't appear to be vibrating at all but if you blew it up to the size of the empire state building the metal rods would move from window to window. perhaps as our universe expanded in size the length of that minimum vibration (perhaps infinite at point zero) would have expanded, and therefore created the illusion of time and the speed of light, which, as the universe expands, will continue slowing down. perhaps we are like a big balloon and we have been blown up and all the fields/particles-without-size are vibrating more and more in that space.. am i missing something obvious here?
51952
I have a problem with energy conservation in case of interfering waves.  Imagine two harmonic waves with amplitudes $A$. They both carry energy that is proportional to $A^2$, so the total energy is proportional to $2A^2$. When they interfere, the amplitude raises to $2A$, so energy is now proportional to $4A^2$ and bigger than before.  The equivalent question is what happens to the energy with the superposition of two waves that interfere destructively.  Also, if someone could comment on the statement about this problem in my physics book (Bykow, Butikow, Kondratiew):      The sources of the waves work with increased power during the interference because they feel the wave from the other source.
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I read that it makes a difference whether you calculate $\frac{dE(\lambda) }{d \lambda}=0$ or $\frac{dE(\omega)}{d \omega}=0$ in the sense that the maximum energy density with respect to the wavelength does not coincide with the frequency maximum that one would assume to be at $\omega_{max} = \frac{c}{\lambda_{max}}$. Actually, I do not understand why this is so ( Now, I am only interested in a pure physical explanation, mathematically this is clear and due to the chain rule). Does anybody know how to explain this odd thing?  Somehow I feel that the core of the question has not been completely answered. Although I can easily look up wikipedia, where the need for the chain rule is explained, I am rather interested in understanding where the following argument breaks down, which seems to be why this question has caused some confusion in the past(as you can see by googling this question):  So we have $E(\omega)$ the energy radiated at a given frequency. Now this function has a maximum somewhere, so there is a frequency where a maximal amount of energy is emitted. In other words: If you add the sum of the photon's energies at each frequency that are emitted, you will notice that the maximum is reached at this frquency. Now $E(\lambda)$ tells you basically the same for the wavelength, but again: We know where at which frequency the maximal amount of energy is radiated, so we know the corresponding wavelength.   I think there is a need to explain this.   If anything is unclear, please let me know.
122988
I'm making several assumptions, not sure if any are correct:   there is a black hole at the center of a galaxy the black hole is eating the galaxy   Eventually the galaxy will be gone, right?  Has this been observed? Do we know what happens afterwards?  Posting here since astronomy got merged into physics
45535
I know that the  is the leftover radiation from the "".   However, at every instant the surface is changing (at the rate of flow of time). So how constant is the CMBR?
81054
Is this a postulate? I get the expansion of the universe, the addition of discrete bits of space time between me and a distant galaxy, until very distant parts of the universe are moving relative to me, faster than the speed of light. But the other sure seems like  (giant steaming load of convenience) I think I get reasons why, because they don't have any other way to explain the size of the universe and so on. Just seems it would be much easier to  say the universe is eternal and cyclical in nature, and call it a day!
113464
Ok, my understanding of physics is still limited, hoping to change that in the following years, but I have a question.  Dark energy is what provokes the expansion of the universe, so, in theory, it must be exerting some kind of "anti-gravity", definitely an energy as I see it.  Would it be possible to turn dark energy into usable, electric energy? For example, using a piezoelectric sensor?  I think if we found a way to do that, we would provide the world with infinite, free energy. Forever.
88584
Folks, I have a naive question regarding the subject of dark energy and an accelerating universe:  From what I understand/read, it seems that the further we look out into deep space, the faster the objects are moving away from us - in all directions.   Is this basically what is meant by "accelerating universe"?  Because it seems that this situation is exactly what we should expect to see (according to Big Bang theory). The further out we peer into space, the further back in time we are "seeing", so wouldn't we expect to see higher rates of acceleration/expansion the further back in time we peer?
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Is there any lower or upper limit on the energy of a photon? i.e. does the mathematical framework we currently use to study photons blow up when a photon surpasses a certain upper limit of energy? (or the same on the opposite side?)  My thoughts: If I let the energy of a photon tend to infinity, its wavelength would be tending to zero, and since it is thought that we cannot really distinguish things when they are on the scale of a Planck Length, would the photon have its maximum energy when its wavelength is equal to the Planck length? (Feel free to correct me, I feel I might not be.) and for the opposite end of the spectrum a photon with the least energy would have zero wavelength, implying no photon, which is a trivial case.
104712
This is what I commonly read:  The CMB came to existence when atoms where formed and photons weren't constantly absorbed anymore. In other words, the universe became "transparent". Because of the expansion of the universe that radiation experienced red-shift so that now its temperature is slightly below 3 Kelvin.  So far, so good. But what does it mean for the CMB to have a temperature? Temperature usually means the jiggling about of atoms, but the CMB is radiation. So what is meant when it is said that the temperature of the CMB is about 3 K today?
63732
I am reading Berkeley Physics Course vol. 4 (Quantum Mechanics) , chapter 4 (photons). (1)  Section 46: book says: consider a typical photon emitted by  the source. It can be regarded as a a wave train of finite duration, spreading out in all directions in space and carrying a total energy $\hbar\omega$. If this is true, a photon emitted by a star which is few light years away, would have spread over a sphere of radius of few light years. Then it is not possible to &quot;detect&quot; that photon using a detector.  Then  why does the book say like this?  In section 47, the book gives same example of photon spreading over light years. But in this section, it says that how the people can use this as a paradox; and clarifies that paradox by saying that classical expression for &quot;energy density&quot; refers to a large number of photons, and not for single photon.  So does photon really spread in all directions?   (2)  In Section 38 book says: almost monochromatic photons can not be split into 2 photons of the same frequency which carry only a fraction of the energy of the original photon. While in section 48, it says (with reference to double-slit diffraction experiment) photon came through BOTH the slits.  Does a photon REALLY goes through both the slits? Doesn't it indicate that a single photon can be split into two photons of same frequency ?   Any help will be appreciated. Thanks
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For a given black body radiation curve, would the changes to the spectrum resulting from cosmological expansion and those from Doppler effects be distinguishable on the basis of the shapes of the resulting curves alone? Or, put another way, starting from the same spectrum, can both processes produce the same observation (for suitably chosen magnitudes of expansion or velocity)?
2134
Yesterday I read that we can affect the path and the 'form' (particle or wave) of a photon after the fact ().  Part of what is puzzling me is the beam-splitter. Are the individual photons actually being split into two new photons of lesser energy?  This  implies that you cannot split a photon but it seems that beam splitters do exactly that.
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Planck quantised the energy of light in order to solve the black body radiation problem. However, I am confused as to exactly what he quantised. On one hand, I have seen that he quantised the energy of oscillators in the black body (source below).     However, the second version of the story I've heard is that it is simply that light energy is given off in discrete chunks (where every photon itself can have any value of energy it likes depending on frequency and is not quantised).   To me, the first idea doesn't make sense because $E=hf$ and as $f$ can be any real number that means the energy of each photon can be any real number. I would believe the second version, however I don't understand how this version of quantisation leads to reducing the number of high frequency waves emitted and thus solving the UV catastrophe. Could you please explain exactly how releasing energy in chunks (of arbitrary size themselves) means less will be released in the higher frequencies?