The discussion centers on the Cosmic Microwave Background (CMB) and its implications regarding the universe's structure and expansion. Participants argue that the CMB cannot be merely redshifted light from distant galaxies due to its near-perfect black-body spectrum and extreme uniformity, which contradicts the idea of an infinite universe filled with galaxies. The conversation highlights the limitations of steady-state models and the significance of Olbers' Paradox in understanding the night sky's brightness in relation to cosmic expansion.
PREREQUISITESAstronomers, cosmologists, and physics students interested in the fundamental properties of the universe and the implications of cosmic background radiation.
That and its extreme uniformity: the CMB is uniform to one part in 100,000.nicksauce said:If that was the case, there would be no way to explain why the CMB is (almost) a perfect black body.
really said:If the universe is infinitely large and the microwave background is actually light from extremely distant galaxies then wouldn't we expect it to be pretty uniform?
Not remotely. Basically, if you were to propose a steady-state model where this sort of thing sort of makes sense, then the far-away universe would be every bit as inhomogeneous as the nearby universe. So you wouldn't be looking out to the same distance in every direction: every place you look on the sky, you'd see a galaxy at a different distance. The temperatures would vary hugely.really said:If the universe is infinitely large and the microwave background is actually light from extremely distant galaxies then wouldn't we expect it to be pretty uniform?
If the universe was infinite the light from those galaxies would have been thermalised by an infinite number of collisions with an infinite amount of gas/dust.Chalnoth said:Nicksauce also has a very good point about the spectrum: galaxies are very poor black bodies. Their spectra are chock full of all sorts of interesting features. By contrast, the CMB is almost a perfect black-body. So the CMB cannot simply be made up of lots of redshifted galaxies anyway.
really said:What temperature would you expect?
mgb_phys said:If the universe was infinite the light from those galaxies would have been thermalised by an infinite number of collisions with an infinite amount of gas/dust.
Although if the universe was infinite, you wouldn't have 3K temperature for the dust.
Well, no, because it would still have to interact with one last galaxy along the line of sight, which would always produce some combination of emission and absorption lines depending on said galaxy's chemistry.mgb_phys said:If the universe was infinite the light from those galaxies would have been thermalised by an infinite number of collisions with an infinite amount of gas/dust.
That effect is extremely small at the frequencies where the CMB is strongest. Even our own galaxy isn't that bright at around 90-100GHz.mgb_phys said:That's also true of the CMB - you have to remove the effects of foreground galaxies.
If the column were uniform, sure, that would work. But the problem is that the universe is expanding and sparsely-populated, which means that the radiation won't get a chance to thermalize before it redshifts away on its trip between galaxies.mgb_phys said:I'm not convinced that you can't produce a uniform thermal spectrum by putting any source through an infinite column on absorbing material.
Expansion counters this. I was assuming an expanding universe, along the lines of a steady state universe idea, where you have an exponential expansion where hydrogen is continually produced out of the vacuum (completely unrealistic, but at least it isn't quite as obviously false as a non-expanding eternal universe).Chronos said:See Olber's paradox. If the universe was infinitely large, infinitely old, and populated by an infinite number of galaxies, the night sky would be as bright as the surface of an average star.
Chronos said:See Olber's paradox. If the universe was infinitely large, infinitely old, and populated by an infinite number of galaxies, the night sky would be as bright as the surface of an average star.
The Olbers' Paradox assumes a static universe, with no expansion.really said:How would it be as bright as the surface of a star if the light redshifts out of the visible spectrum?
Chalnoth said:The Olbers' Paradox assumes a static universe, with no expansion.
That's not really possible. The only way to get redshifts of atomic emission/absorption lines is to have some sort of doppler or gravitational redshift. When applied to the universe as a whole, that means expansion.really said:So do I, a static universe with redshift caused by something other than expansion.
Chalnoth said:That's not really possible. The only way to get redshifts of atomic emission/absorption lines is to have some sort of doppler or gravitational redshift. When applied to the universe as a whole, that means expansion.
Furthermore, a static universe is unstable: any slight perturbation will cause either collapse or eternal expansion.
As near as I can tell, there is no evidence that the Pioneer anomaly has anything whatsoever to do with basic physics. It's more likely something to do with the spacecraft itself.Calimero said:Photonic clocks? Time running slower in the past. I know its been refuted by gravity probe B, but nevertheless it is interesting. At least it is in the good agreement with pioneer anomaly.