Can there be some kind of photon emission caused by space expansion?

In summary, the conversation discusses the concept of spacetime expansion and whether it can directly cause photon emission. The conclusion is that spacetime expansion is just a change in geometry and cannot cause any physical processes. The concept of "distance apart" is also discussed, with the suggestion that the expansion scalar is a better way to measure the expanding universe.
  • #1
Suekdccia
351
27
TL;DR Summary
Can there be some kind of photon emission caused by cosmological expansion?
Are there any kind of observed and experimentally verified processes or mechanisms where photon emission occurs and which are directly cause by spacetime expansion in some way?
 
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  • #2
No. Why would there be? Spacetime expansion is just geometry changing. Nothing is happening to the things that are moving apart. In their own frames of reference they are not even moving (other than whatever motion they have aside from expansion).

Put another way, expansion is coordinate motion, not proper motion.
 
  • #3
Suekdccia said:
spacetime expansion
Spacetime is not expanding. Spacetime just is.

"Space" is expanding if we choose standard FRW coordinates for our universe, but as @phinds says, this is really a coordinate effect and can't cause anything.

There is a physical invariant that can also be described as "expansion" in our universe, namely the positive expansion scalar of the congruence of comoving observers, but these observers are in free fall and free fall can't cause any emission process either.
 
  • #4
PeterDonis said:
scalar of the congruence
Just a sanity check please: Is this what an ordinary person would call "distance apart"? I really need to learn this stuff...
 
  • #5
hutchphd said:
Just a sanity check please: Is this what an ordinary person would call "distance apart"? I need to learn this stuff...
The expansion scalar is the coordinate-free generalisation of the naive "how fast the volume this stuff occupies is increasing".
 
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FAQ: Can there be some kind of photon emission caused by space expansion?

What is photon emission caused by space expansion?

Photon emission caused by space expansion refers to the phenomenon where photons (particles of light) are emitted as a result of the expansion of the universe. This expansion causes the wavelengths of photons to stretch, leading to a decrease in their energy and an increase in their wavelength.

How does space expansion affect photon emission?

As space expands, the wavelength of photons also increases, causing their energy to decrease. This results in a redshift of the photons, meaning they appear more red than their original color. This is known as cosmological redshift and is a key piece of evidence for the expansion of the universe.

Can space expansion cause the emission of new photons?

No, space expansion does not create new photons. It only affects the existing photons by stretching their wavelength and decreasing their energy. Photons are not created or destroyed, they simply exist and travel through space.

Is photon emission caused by space expansion the same as the Doppler effect?

No, photon emission caused by space expansion is not the same as the Doppler effect. The Doppler effect refers to the change in frequency and wavelength of a wave (such as sound or light) due to the relative motion between the source and observer. In contrast, photon emission caused by space expansion is a result of the expansion of the universe, not relative motion.

Can we observe photon emission caused by space expansion?

Yes, we can observe photon emission caused by space expansion through the redshift of light from distant galaxies. The further a galaxy is from us, the more its light is redshifted due to the expansion of space. This is one of the key pieces of evidence for the Big Bang theory and the ongoing expansion of the universe.

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