Timeline for Would cosmological redshift be present in the following situation?
Current License: CC BY-SA 3.0
14 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jan 20, 2015 at 9:00 | comment | added | John Rennie | Let us continue this discussion in chat. | |
| Jan 20, 2015 at 8:58 | comment | added | Ehryk | What's a better way? I was hoping to pin it down to this (preposterous) example to nail down specifically what I'm having a hard time understanding; which is the two planets are fully stationary with respect to each other. | |
| Jan 20, 2015 at 8:57 | comment | added | John Rennie | @Ehryk: the trouble is that comments are a poor way to introduce soeone to how the FLRW metric works! | |
| Jan 20, 2015 at 8:57 | comment | added | Ehryk | If it's viewed that way, then the cable stays perfectly taut and doesn't flex during said 'movement'? I'm not sure I'd consider that movement. | |
| Jan 20, 2015 at 8:56 | comment | added | John Rennie | @Ehryk: Because: "Viewed this way the planets in your example are moving towards each other so there is a velocity induced blue shift." | |
| Jan 20, 2015 at 8:55 | comment | added | Ehryk | That last part confused me more. Which is it? If it is actually a property of expansion of space, then why wouldn't the situation pictured above experience redshifted planetary light? | |
| Jan 20, 2015 at 8:55 | comment | added | John Rennie | @Ehryk: I don't think it's possible to feally understand what's going on without understanding the GR description of the expansion. And that's hard! The way GR describes it is that the stars, galaxies etc are not moving relative to each other. More precisely, if we use comoving coordinates the velocities are all zero. Therefore there's no velocity induced redshift - it's all down to the expansion of spacetime. Viewed this way the planets in your example are moving towards each other so there is a velocity induced blue shift. | |
| Jan 20, 2015 at 8:46 | comment | added | pela | Its a really great question: Actually, if you calculate the observed wavelength of photons received from a distant source in a hypothetical universe that at first is static, then — after emission — expands for a while, and finally — before receiving the photons — stop expanding, you would measure a redshift, even though the source is at rest wrt. you both at the time of emission and detection. That implies that the redshift is in fact cosmological and not just due to peculiar velocities. | |
| Jan 20, 2015 at 8:35 | comment | added | Ehryk | I guess the fundamental part I'm asking is whether or not the cosmological redshift happens as the light travels through expanding space, or just due to observer velocities, peculiar or proper. | |
| Jan 20, 2015 at 8:33 | comment | added | Ehryk | But... the universe HAS expanded (even if the planets have not receded), and thus $a(t_0)$ would be != 1, and thus redshift, would it not? Let's say they're REALLY far apart, and the universe has doubled in size since the light was emitted from one planet. Would that not mean $\frac {\lambda} {\lambda_0} = \frac {1} {2} $ ? | |
| Jan 20, 2015 at 8:28 | comment | added | Ehryk | Why would they have 'peculiar velocity' toward each other, if they are at rest to each other? By the definition of peculiar velocity in the wiki article, would that not mean a peculiar velocity of $0\ m/s$? | |
| Jan 20, 2015 at 8:24 | comment | added | John Rennie | @Ehryk: the distance the light is spread over doesn't change because the planets are stationary wrt each other. So there is no change in the energy density and therefore no change in the wavelength. In effect the redshift due to expansion is balanced by the blue shift due to the planets peculiar velocity towards each other. | |
| Jan 20, 2015 at 8:21 | comment | added | Ehryk | So according to this, then, the planets would see redshifted light, as the energy density of the space between the planets has diminished from the expansion of the space between them. So - the planets would see somewhat redshifted light from the other? | |
| Jan 20, 2015 at 8:18 | history | answered | John Rennie | CC BY-SA 3.0 |