distance the light has traveled....or so I've read.
-
--"Are we sure the universe is expanding?"
No.
We are sure that SPACE is expanding.
The universe? We don't know.
--"The red-shift of the galaxies could be due to..."
There are four possible causes for red-shift. All four are well understood and none of them give any support to the "tired-light theory"
1) Doppler-Fizeau effect: If a galaxy has a radial component to its radial velocity (relative to us) then the perceived frequency of its light will be modified. Blue-shifted if the galaxy is moving towards us (e.g., the Andromeda galaxy), red-shifted if the galaxy is actually moving away from us.
2) Cosmic expansion (a.k.a. cosmological red-shift): Space is expanding everywhere, including inside ourselves (and even inside each atom). The rate is extremely slow, so that over a distance of 2 m (a tall human), who lives for a century, the total amount of expansion would be a bit less than one micrometre. Obviously, chemical bonds are strong enough to counter this.
Because space expands everywhere, it expands the photons themselves as they travel from there to here. THIS is the red-shift that comes from the expansion of space.
In the beginning (a century ago) and in popular books, numbers 1 and 2 are often combined... and confused.
3) gravitational red-shift (or blue-shift). Relative to the local space, light maintains a constant speed. Yet, as it "climbs" out of a gravitational well (coming out of a very massive galaxy, for example) it must lose energy. Since it cannot lose this energy as speed, then it must lose it as "wavelength" (the energy content of a single photon is inversely proportional to its wavelength).
There is a way to tell this form of red-shift from the other ones.
4) Relativistic red-shift. Even if a galaxy has no radial velocity, if it is moving sideways at relativistic speed, we will perceive its time flow as being slowed. In such a case, an event that for us produces a photon of a very specific wavelength, because the event takes place in a very specific time interval (an electron dropping from one level to the next, for example), will appear to take longer for us, therefore resulting in a photon with a longer wavelength.
No.
We are sure that SPACE is expanding.
The universe? We don't know.
--"The red-shift of the galaxies could be due to..."
There are four possible causes for red-shift. All four are well understood and none of them give any support to the "tired-light theory"
1) Doppler-Fizeau effect: If a galaxy has a radial component to its radial velocity (relative to us) then the perceived frequency of its light will be modified. Blue-shifted if the galaxy is moving towards us (e.g., the Andromeda galaxy), red-shifted if the galaxy is actually moving away from us.
2) Cosmic expansion (a.k.a. cosmological red-shift): Space is expanding everywhere, including inside ourselves (and even inside each atom). The rate is extremely slow, so that over a distance of 2 m (a tall human), who lives for a century, the total amount of expansion would be a bit less than one micrometre. Obviously, chemical bonds are strong enough to counter this.
Because space expands everywhere, it expands the photons themselves as they travel from there to here. THIS is the red-shift that comes from the expansion of space.
In the beginning (a century ago) and in popular books, numbers 1 and 2 are often combined... and confused.
3) gravitational red-shift (or blue-shift). Relative to the local space, light maintains a constant speed. Yet, as it "climbs" out of a gravitational well (coming out of a very massive galaxy, for example) it must lose energy. Since it cannot lose this energy as speed, then it must lose it as "wavelength" (the energy content of a single photon is inversely proportional to its wavelength).
There is a way to tell this form of red-shift from the other ones.
4) Relativistic red-shift. Even if a galaxy has no radial velocity, if it is moving sideways at relativistic speed, we will perceive its time flow as being slowed. In such a case, an event that for us produces a photon of a very specific wavelength, because the event takes place in a very specific time interval (an electron dropping from one level to the next, for example), will appear to take longer for us, therefore resulting in a photon with a longer wavelength.
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