
Chromoscope: the Milky Way in different wavelengths - mashgin
http://www.chromoscope.net/
======
pizza
Whoa.. this is amazing.. Anyone know what's up with the jaggedness in the
x-ray band? Also, I thought that the cosmic background radiation was
isotropic, i.e. that the universe looks homogenous far enough away/back in
time - is that what it means for the radial horizon/line to look, more or
less, all blown out?

edit: holy shit you can zoom in by scrolling

~~~
raattgift
After subtracting out foregrounds and known redshifts (mostly from our own
peculiar motion) the cosmic _microwave_ background (CMB) is very nearly
uniform, with a spectrum like a blackbody radiator with a temperature of 2.73
kelvins, to within one part in a hundred thousand. To see the cosmological hot
and cold spots requires serious observatory engineering (starting with
BOOMERanG, 1998 and compare
[http://www.physics.princeton.edu/cosmology/sk/](http://www.physics.princeton.edu/cosmology/sk/)
1993 which really didn't see them well).

The galaxy as a whole is a substantial foreground. :-) Other more distant
galaxies and clusters are also "foreground", essentially because they are
newer than the surface of last scattering[1] which produced the CMB. Having
all-sky views in different wavelengths lets us find and remove foregrounds. As
you can see, in different wavelengths, the sky can be far from uniform.

As to X-Rays, the greenish Chromoscope X-Ray image was produced by the ROSAT
team (RÖntgen SATellite, operating in the 1990s). Here's another ROSAT image
with a paragraph of explanation:
[http://www.mpe.mpg.de/xray/wave/rosat/gallery/five_years/ima...](http://www.mpe.mpg.de/xray/wave/rosat/gallery/five_years/images/bild_16.php)

The curved black features in the greenish Chromoscope X-Ray image are almost
certainly unobserved parts of the sky. Here
[http://sci.esa.int/planck/46740-planck-sky-coverage-
mollweid...](http://sci.esa.int/planck/46740-planck-sky-coverage-mollweide-
projection/) are some animations of how the Planck satellite scans the sky,
recording details of the cosmic microwave background. ROSAT's scanning was
likely pretty similar, but I don't know the details.

\- --

[1] When the universe was hotter and denser, atomic nuclei would be entirely
stripped of their electrons. Free electrons and totally ionized atoms are
extremely good at scattering light into a fog, which in turn inversely
scattered the charged particles into thermal equilibrium. As the cooled, the
charged particles could recombine without being blown apart by high-energy
photons. The charged matter went on to collapse gravitationally into dense
clouds and galaxies, while the light (and neutrinos) as massless (or extremely
low mass) particles were mostly free to zip about as a cooling "ghost" of the
last scattering surface. So we have decent reasons to expect a cosmic neutrino
background (CvB, v like the greek letter \nu), with features roughly similar
to what we see in the cosmic microwave background. Additionally, there's a
tenuous background (or set of backgrounds) of more massive particles that
managed to avoid being drawn into structures that evolved into galaxy
clusters. Like the CMB and CvB, it will have grown very cold and tenuous,
making it a bit harder to spot than most normal matter (being weak and
extremely diffuse emissions, and due to sparseness not much absorption of
faraway light on its way to us).

The backgrounds are interesting because they tell us about overdensities at
the surface of last scattering, and we have decent reasons to expect that
those evolved from overdensities in the even earlier universe. The Cosmic
Gravitational Wave background, which hopefully we will eventually be able to
study in some detail, is likely to tell us a lot about what could and couldn't
have happened before the surface of last scattering (and ideally we can trace
the observables back to the _extremely_ hot dense phase we call the "big
bang").

~~~
raattgift
Oh, hey, on the CMB front Peter Coles
([http://telescoper.wordpress.com/](http://telescoper.wordpress.com/)) of
Cardiff University says it better here
[https://ned.ipac.caltech.edu/level5/Glossary/Essay_lss.html](https://ned.ipac.caltech.edu/level5/Glossary/Essay_lss.html)
(1999). I strongly recommend the second paragraph, starting, "To
visualise...", (with the tiny caveat that there is an unstated choice of gauge
in the "about twice the speed of light"), and the fourth paragraph starting,
"Imagine...".

