Recent content by Allday

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    Were the "Dark Ages" really dark?

    To help you answer your last question, the temperature of the CMB scales with redshift like so ##T = T_0 (1+z),\, T_0=2.725 {\rm K}##
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    Was the Professor's Big O Notation Example Correct?

    thanks everyone. thinking of the "=" sign not as a true equality but as indicating membership in a set makes it clearer for me.
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    Was the Professor's Big O Notation Example Correct?

    I decided to watch an MIT OpenCourseWare class on algorithms and the second lecture has me checking my understanding. A link to the lecture is here...
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    Fortran [Fortran 90/95] Error: function has no IMPLICIT type

    This is a little late, but it's worth mentioning that you can have the module entries to be private as default and then explicitly list the thing you want to expose publically. So you could have, PRIVATE ! module members are private by default PUBLIC :: BMatScal ! I want BMatScal to be...
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    CMB polarization: the E and B modes

    For anything related to the CMB I usually check out something by Wayne Hu. This will probably be useful ... http://astro.berkeley.edu/~mwhite/polar I'm interested in your question so I'll scan through and see if I can find your answer
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    CMB polarization: the E and B modes

    hi fairy._.queen, I'm not an expert on CMB polarization but I think I can clear a few things up. When folks refer to the "E" mode and "B" mode of the CMB polarization field they aren't referring directly to electric and magnetic fields. They are simply decomposing the vector field that...
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    Multi-Species Ideal Gas Law and Mean Molecular Mass

    thanks gadong. I'm modeling the cooling curve for astrophysical plasmas (i.e. low density and globally charge neutral) so I'm just interested in that portion of energy which comes from kinetic motion of the particles.
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    Multi-Species Ideal Gas Law and Mean Molecular Mass

    Hi forums. I have what I think is a simple question but I'm making myself confused. I'm trying to work out the relationship between energy density ( u = energy per unit volume ) and temperature in a multi-species ideal gas (no molecules just different mass ions). The simplest example of...
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    Stress Energy Tensor - Confusion about signs and metrics

    Thanks yenchin, those notes do help. It seems I want to use the form $$ T_{\alpha \beta} = (\rho + P) u_{\alpha} u_{\beta} + P g_{\alpha \beta} $$ along with the fact that the spatial components of ##u_{\alpha} ## are zero so that with my choice of metric sign convention $$ T_{i i} = P a^2 $$
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    Stress Energy Tensor - Confusion about signs and metrics

    Hi everyone. I'm working on deriving Friedmanns Equations from the Einstein Field Equations. I've got the '00' components worked out but I'm having some trouble with the spatial indices 'ii' of the stress energy tensor ## T_{ii} ##. I'm the FLRW metric with c=1 and signature (-,+,+,+) so that...
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    When can I use inverse differentials to calculate derivatives?

    thanks Halls and arildno, that makes it clear.
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    When can I use inverse differentials to calculate derivatives?

    This is a simple dummy question. What are the conditions under which the following relationship holds, dx/dt = Inverse(dt/dx) = 1/(dt/dx) meaning if I want to do a derivative and I know t(x) but not x(t) when can I just calculate dt/dx and put it over 1 to get dx/dt. I see this in...
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    I don't like Dark Matter and Dark Energy

    Before WIMPs some people thought that massive compact halo objects (MACHOs) could be the source of the dark matter. This is the idea that the dark matter is just non luminous regular matter (Jupiters, dim stars, black holes ...) but has fallen out of favor since surveys looking for their...
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    Can a 3-Space Be Isotropic About Two Distinct Points Without Being Homogeneous?

    One trivial way is to constrain the space to be flat, but that's not what your getting at here :wink:. I'm still trying to visualize your example but failing. I can see a 2-sphere being isotropic around two poles and not homogeneous, but I'm having trouble with the 3-sphere. Maybe it would be...
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    How Many Elements Formed from Primal Plasma?

    According to big bang nucleosynthesis all elements heavier than Lithium were formed in stars.
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