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It's unclear for me how observers can be sure that the differences they see are actually originated far far away, and not result of local interference: cloud or

by Myrth 13y ago

It's unclear for me how observers can be sure that the differences they see are actually originated far far away, and not result of local interference: cloud or dark matter or whatever else. As far as I know, we have only 1 point of perspective, our Earth. Of course I have no idea how exactly they observe stuff, but that's the question I immediately have.

bladedtoys 13y ago
Very roughly speaking there are at least three ways to measure distances. For "near by "objects on can use parallax: look at the object on a given date then again 6 months later and see how much it shifts against the back ground stars. Simply triangulate using the known distance from earth to sun as half the side of a triangle. For "moderate" distances one can rely on certain variable stars that always have a known brightness. Then when you locate such a star, note how dim it is and calculate how far away it would have to be to have that brightness level. For "far" distances one can measure the red shift. That is to say, on the large scale everything in the universe appears to be moving away from us. Further away object move faster than nearby objects. Thus if you can measure how fast something is receding from us you can tell how far it is. The technique for measuring speed like this is to measure the Doppler red shift.
- Steuard 13y ago
  But none of those techniques is applicable to variations in the CMB (the subject of this article), which is a more or less uniform glow at constant (and in some sense, maximal) redshift (and by definition, behind the "background" stars). I suppose that if this "axis of evil" were due to a pattern in nearby dust clouds we might be able to see some hints of paralax, but paralax would probably be difficult to measure for something as diffuse and broad as the pattern visible in this data. It's my impression, for the record, that cosmologists are pretty confident that these variations aren't due to intervening dust. My sense is that the spectrum of the CMB in any given direction is very clearly a blackbody spectrum (the most precise blackbody spectrum in nature, in fact), so any absorption due to foreground matter should be pretty recognizable (and possibly even something they could correct for).
jerf 13y ago
The CMB is "so far away" that the largest structures that you can think of are still incredibly tiny points against the CMB. We can tell it's not being blocked by normal matter, because we know what that looks like. If you want to hypothesize other possible effects, well, join the club; that's a popular cosmologist past time, but bear in mind that in addition to explaining the lumps, your new hypothesis must actually explain the non-lumps, and everything else as well. This turns out to be a pretty big hurdle. Furthermore, we're not measuring "things", we're measuring temperature. If a "thing" was blocking the CMB, it would do so by being a hotter thing. Thus, a "thing" can't create a cold spot just by obscuring the CMB... it can only create a hot spot, relative to the CMB. Another place you can see this is in "dark matter" replacement theories. As unsatisfying as an invisible, undetectable particle is to answer the questions about the universe, it isn't enough to just explain one or two subquestions; you need to provide a better explanation of everything we see, including the places where it really, really looks like dark matter is the only feasible explanation (where galaxies collide and the visible matter appears to have gotten detacted from the dark matter halos, as seen by gravitational distortions, which is really hard to explain with any of the alternate theories). It's "easy" to beat the consensus on one detail; it's proved pretty hard to beat the consensus across the board, even as the consensus is known by everybody to be unsatisfactory in some ways.

by Myrth 13y ago

bladedtoys 13y ago

Very roughly speaking there are at least three ways to measure distances. For "near by "objects on can use parallax: look at the object on a given date then again 6 months later and see how much it shifts against the back ground stars. Simply triangulate using the known distance from earth to sun as half the side of a triangle. For "moderate" distances one can rely on certain variable stars that always have a known brightness. Then when you locate such a star, note how dim it is and calculate how far away it would have to be to have that brightness level. For "far" distances one can measure the red shift. That is to say, on the large scale everything in the universe appears to be moving away from us. Further away object move faster than nearby objects. Thus if you can measure how fast something is receding from us you can tell how far it is. The technique for measuring speed like this is to measure the Doppler red shift.

Steuard 13y ago
But none of those techniques is applicable to variations in the CMB (the subject of this article), which is a more or less uniform glow at constant (and in some sense, maximal) redshift (and by definition, behind the "background" stars). I suppose that if this "axis of evil" were due to a pattern in nearby dust clouds we might be able to see some hints of paralax, but paralax would probably be difficult to measure for something as diffuse and broad as the pattern visible in this data. It's my impression, for the record, that cosmologists are pretty confident that these variations aren't due to intervening dust. My sense is that the spectrum of the CMB in any given direction is very clearly a blackbody spectrum (the most precise blackbody spectrum in nature, in fact), so any absorption due to foreground matter should be pretty recognizable (and possibly even something they could correct for).

jerf 13y ago

The CMB is "so far away" that the largest structures that you can think of are still incredibly tiny points against the CMB. We can tell it's not being blocked by normal matter, because we know what that looks like. If you want to hypothesize other possible effects, well, join the club; that's a popular cosmologist past time, but bear in mind that in addition to explaining the lumps, your new hypothesis must actually explain the non-lumps, and everything else as well. This turns out to be a pretty big hurdle. Furthermore, we're not measuring "things", we're measuring temperature. If a "thing" was blocking the CMB, it would do so by being a hotter thing. Thus, a "thing" can't create a cold spot just by obscuring the CMB... it can only create a hot spot, relative to the CMB. Another place you can see this is in "dark matter" replacement theories. As unsatisfying as an invisible, undetectable particle is to answer the questions about the universe, it isn't enough to just explain one or two subquestions; you need to provide a better explanation of everything we see, including the places where it really, really looks like dark matter is the only feasible explanation (where galaxies collide and the visible matter appears to have gotten detacted from the dark matter halos, as seen by gravitational distortions, which is really hard to explain with any of the alternate theories). It's "easy" to beat the consensus on one detail; it's proved pretty hard to beat the consensus across the board, even as the consensus is known by everybody to be unsatisfactory in some ways.