Basically, the "cold spot" just isn't far enough away from the expected variations for there to be much reason to believe it's anything different than normal variation.Gravitons said:Whats this all about the cold spot found on the WMAP satellite's photo of cosmic microwave background radiation? To explain this there's even been a possibility of a parallel universe's gravitational effect causing this cold spot?
No, there are definitely photons reaching the detector from the location of the big cold spot, and their temperature is of the same order of magnitude as the other cold spots.Tanelorn said:Can the presence of a large body of matter block the CBMR from reaching us and thus produce a cold spot?
If you look at the way it works, the difference in temperature is approximately linearly-related to the amount of flux you get in anyone detector. So first they calibrate the detectors off of the dipole (which is much brighter than the galaxy across most of the sky), and since the CMB dipole is a fluctuation with CMB spectral scaling, anything else which has CMB spectral scaling will also raise/lower its temperature proportional to how much it is raised/lowered by the dipole.Tanelorn said:Ok so the intensity (ie. amplitude) is constant in every direction (is this correct?), but the center frequency of the black body radiation varies by a tiny percentage. It must be difficult to measure the center frequency so accurately when the shape of the radiation has such a rounded top?
Each detector only sees the flux in one particular frequency band. Increasing the temperature increases the flux an individual detector sees. And the amount it increases the flux is approximately proportional to the amount of temperature increase (as long as the temperature increase is small compared to the average temperature level...which it is).Tanelorn said:I feel like I should be getting this, but I am not. It should be right up my alley, so to speak, which is a bit disconcerting.
Forgetting the detectors for the moment, we are trying to measure the black body temperature variation in different directions of the sky. This is equivlent to variation in the frequency of the black body radiation. Is this correct?
Now the detectors appear to comparing intensity from different directions, how does this tell us the frequency and thus temperature?
Well, no, each detector is a single detector. They have horns which only allow photons within a certain range of wavelength to pass. Then all of the photons that are able to make it to the bottom of the horn dump their energy into the detector, which is read off as a voltage.Tanelorn said:So each detector is actually a bank of several narrowband detectors, each tuned to detect the amplitude of a particularly wavelength. How many different wavelengths are being detected simultaneously?
I believe that's how it works. That's the basic idea with Planck, though one detector in Planck is looking at the sky, whereas the other is looking at a 4K reference source. With WMAP, it actually has two back-to-back telescopes looking at different parts of the sky.Tanelorn said:Is it correct that for the detector to be able to make very accurate relative measurements of different points in the sky that there are two banks of these detectors each looking in different directions simultaneously? Do they switch these rapidly into a common receiver perhaps with a double down conversion before being sampled?
It varies based upon frequency. With WMAP, the beam varies from just under a degree to around a quarter of a degree in resolution.Tanelorn said:How much of the sky is each detector measuring ie. beam width? Presumably they use some kind of high gain, very highly directional horn antenna?
Oh, I think you misunderstood me. There are different sets of detectors for different wave bands, at 23GHz, 33GHz, 41GHz, 61GHz, and 94GHz. The 23GHz detector, for instance, doesn't only measure photons at exactly 23GHz, but actually captures photons from about 19GHz to about 25GHz. That's what I've been talking about.Tanelorn said:Thanks Chalnoth,
Re: "Then all of the photons that are able to make it to the bottom of the horn dump their energy into the detector, which is read off as a voltage."
What I am not getting is if the 5 different wavelength photons are all summed at the same detector producing a single voltage then the frequency information is lost.
See law[/url]. Temperature impacts not only the frequency of radiation, but also the amount that is emitted.Tanelorn said:Also it seems like I may have been under a misapprehension for a very long time.
I thought that the CMBR image was showing different temperatures, which I thought meant different photon wavelengths.
How can temperature be determined from the number or intensity of the photons?
It is showing different temperatures. A gas of photons in equilibrium at a certain temperature T have wavelengths distributed according to a black body: http://en.wikipedia.org/wiki/Black_body" . The CMB is the closest thing that nature has to a black body, and so even a perfectly uniform CMB with temperature T would consist of photons across a range of frequencies.Tanelorn said:I thought that the CMBR image was showing different temperatures, which I thought meant different photon wavelengths.
How can temperature be determined from the number or intensity of the photons?
Good!Tanelorn said:Ok thanks guys the WMAP detectors are a lot clearer now as well as a better understanding of the way temperature is determined.
Well, it's the combination of the number of photons, the energy per photon, and the area over the sky from which those photons came.Tanelorn said:Just to be sure, Flux is another way of saying intensity, or numbers of photons, correct?
Well, right. This is why CMB experiments make use of data at multiple frequencies: other sources of radiation have very different dependence upon frequency, and so with multiple frequencies we can distinguish between them.Tanelorn said:Flux can also be affected by other things though correct? eg. a galaxy between us and the CMBR. Could we be seeing variation due to this and falsely attributing this to temperature variation?
What dispersion? The speed of light is identical for all wavelengths, so I'm not sure what you're talking about.Tanelorn said:Also wouldn't dispersion over 13.7B years of traveling tend to want to flatten out any initial photon temperature variation?
Ah, well, that's not an issue here because the CMB isn't a coherent beam of light. It's light that was emitted everywhere in every direction. So there's no divergence of the beam to speak of, because there's no beam in the first place.Tanelorn said:Re: Dispersion: This is the bending of different frequencies of light where you might think one frequency came from one direction, different to the original direction. However I suspect something like this isn't going to effect the temperature map of the sky very much.
What I really meant was divergence, where a light beam spreads or disperses over distance. Here the distances are truly vast, so could something like dispersion cause an error in the temperatures seen across the sky? Perhaps making the measured temperatures across the sky look less variable than they really were when the photons were first emitted?
Yeah, that's what I was referring to in my first statement. There is no sense of divergence, because there is no beam in the first place. Divergence is only a sensible thing to talk about when you have a focused beam of light. In this case, however, the CMB is better thought of as a nearly-uniform gas of photons that expands along with the universe.Tanelorn said:Chalnoth, as I said I meant divergence, where a light beam spreads or disperses over distance.
In this case the distances are truly vast, so could something like dispersion cause an error in the temperatures seen across the sky?
Perhaps making the measured temperatures across the sky look less variable than they really were when the photons were first emitted?
I don't understand what you're thinking here. The "cold spot" is simply consistent with our current models of the early universe. There isn't any good reason to believe there is anything special going on here.Tanelorn said:Chalnoth, but doesn't that then provide some relief from having to explain the temperature uniformity with a rapid phase of inflation? Or is the near uniform gas of photons not sufficient alone?
Chalnoth said:The "cold spot" is simply consistent with our current models of the early universe.
Because it's a slight statistical outlier. It's well within bounds for current models, but many people are attracted to such small signals, in the hopes of finding something.Dotini said:Exactly how is it simple that the cold spot is consistent with early universe models? If it were consistent, why would it be the subject of various articles and papers?
Chalnoth said:I don't understand what you're thinking here. The "cold spot" is simply consistent with our current models of the early universe. There isn't any good reason to believe there is anything special going on here.
That's not really the case. What you need during inflation is a small region of space-time dominated by an inflaton with a nearly uniform distribution. Inflation makes the temperature uniform. But it does need to start with a nearly-uniform field value for the inflaton.Tanelorn said:Chalnoth, I wasnt referring to the cold spot here, just how there was a need during inflation for all parts of the universe to be in thermal equilibrium with each other in order to explain the uniform CMBR temperatures that we see today. I recall the speed of sound was mentioned. So what I was asking was if divergence of the CMBR radiation also equalises the CMBR temperatures anyway, then maybe that helps relax or reduce the need for the temperature to be so equal or flat or uniform during the inflation stage. Maybe I am not using the right terms.
Huh? It's no closer or further away from the big bang, in any sense, than the rest of the CMB.Imax said:Maybe the Coldspot is at or near the Big Bang :)?
