WMAP Science on a Sphere Microwave Sky Images Seven Year Maps
This page provides access to a number of images that have been formatted to
be compatible with "Science On A Sphere", a 3D visualization system developed by
the National Oceanic and Atmospheric
Administration (NOAA) and implemented at NASA's
Goddard
Space Flight Center (GSFC) (among other places). These images are provided as a courtesy
to our users; they do not constitute an endorsement of any product or service.
The Cosmic Microwave Background (CMB) radiation is the remnant heat from the
Big Bang. This radiation pervades the universe and, if we could see in
microwaves, it would appear as a nearly uniform glow across the entire sky.
However, when we measure this radiation very carefully we can discern extremely
faint variations in the brightness from point to point across the sky, called
"anisotropy". These variations encode a great deal of information about the
properties of our universe, such as its age and content. The
"Wilkinson Microwave Anisotropy Probe"
(WMAP) mission has measured these variations and
found that the universe is 13.7 billion years old, and it consists of 5%
atoms, 23% dark matter, and 72% dark energy.
The images here represent maps of the full sky constructed with data from the Seven Year WMAP Data
Release. Click on the images below to retrieve high-resolution (4096x2048) versions. All of the images
may be downloaded at once through the files
wmap_sos_images_v4.tar.gz (105.6 MB) and
wmap_sos_images_v4.zip (105.6 MB).
The first image shows the CMB fluctuations from the Seven Year WMAP
survey. The average brightness corresponds to a temperature of 2.725 Kelvins
(degrees above absolute zero; equivalent to -270 C or -455 F). The colors
represent temperature variations, as in a weather map: red regions are warmer
and blue regions are colder than average by 0.0002 degrees. This map was formed from the five frequency bands
shown below in such a way as to suppress the signal from our own Milky Way Galaxy.
WMAP Seven Year Frequency Band Maps (Linear Color Scale)
K Band -- 23 GHz
Ka Band -- 33 GHz
Q Band -- 41 GHz
V Band -- 61 GHz
W Band -- 94 GHz
In addition to the CMB, our own Milky Way Galaxy is a source of microwave radiation.
Fortunately, the two sources have a different frequency spectrum (or "color"), so they
can be separated using multifrequency observations. WMAP uses 5 frequency bands to
discern CMB emission from Galactic emission: 23, 33, 41, 61, and 94 GHz. These five
images show the microwave brightness measured in each frequency band. The signal is
measured in units of Kelvins, and the color scale goes from blue at -0.0002 Kelvins
below average (-200 microKelvins) to red at 0.0002 Kelvins above average (+200
microKelvins). The red band running through the center of the image is the emission
from our Milky Way, which is much brighter than the CMB signal. By combining these
five images in a particular way (shown above), we can suppress the signal from the
Milky Way.
WMAP Seven Year Frequency Band Maps (Nonlinear Color Scale)
K Band -- 23 GHz
Ka Band -- 33 GHz
Q Band -- 41 GHz
V Band -- 61 GHz
W Band -- 94 GHz
These are the same five images as above, except the color scale is distorted to show both the faint
variations in the CMB and the much brighter variations in the Milky Way signal.
WMAP Seven Year Polarization Maps by Frequency Band
Without Polarization Vectors
K Band -- 23 GHz
Ka Band -- 33 GHz
Q Band -- 41 GHz
V Band -- 61 GHz
W Band -- 94 GHz
With Polarization Vectors
K Band -- 23 GHz
Ka Band -- 33 GHz
Q Band -- 41 GHz
V Band -- 61 GHz
W Band -- 94 GHz
In addition to measuring brightness variations, the WMAP mission is also
capable of measuring a more specialized property of the microwaves called
polarization. CMB polarization can provide information about when the first
stars turned on and whether there were gravity waves in the very early
universe.
These images show the polarized portion of the microwave signal at two of the
five frequency bands: 23 and 33 GHz. The color represents the strength of the
polarization: blue is no polarization while red is relatively strong; the color
scale ranges between 0 to 50 μK for K band and between 0 to 35 μK for the
other bands. The white
vector lines indicate the direction of polarization. (The segment lengths are
logarithmically proportional to the strength of the polarization, and they are
not drawn where the polarization is weak enough that it cannot be distinguished
from instrument noise.)
The signal seen in the polarization maps arises almost entirely from our own
Milky Way Galaxy. Specifically it is mostly due to "synchrotron radiation" that
is produced by high energy electrons spiraling around magnetic field lines in
our Galaxy. As with the brightness variations, the polarized signal can be
largely suppressed by combining multifrequency data. Once this is done, the CMB
polarization left behind tells us that the first stars in the universe first
formed when the universe was about 400 million years old. As of yet, the
polarization provides no evidence for gravity waves in the early universe.
Projection Conventions
The Science on a Sphere (SOS) exhibit was originally designed to show
the Earth and various other planets, which are easily represented as spheres
we view from the outside. When using the Science on a Sphere to
represent the sky, one has to pick some nontrivial visualization
conventions. This is because it is much more natural to think of the
sky as a sphere that we view from the inside.
One possible approach is to map the sky onto the sphere as directly as
possible. When we look in some direction on the sky, we can specify
that direction as a vector, and color the sphere at the tip of that
vector according to what we see. This means that the sky would look
correct when viewed from the center of the sphere. However, when
viewed from outside the sphere, all of the features on the sky
(constellations, for example) would appear to be backwards---mirror
images of how we usually see them.
To rectify this problem, one can do a parity transform of the
projection. This can be thought of as mapping each point on the
sphere to the point directly opposite (and then doing some rotations
so the North Pole is on top again, if desired). This will turn the
constellations right side out, for a viewer outside the sphere,
although it becomes slightly more difficult to explain how the image on the
sphere corresponds to the visible sky.
The Science on a Sphere images here use this convention, where the
constellations are right-side-out when seen from the outside. Note
that if the other convention is desired, this can be achieved by
flipping the images left-to-right in an image editing program.
The Science on a Sphere projection is a simple latitude-longitude
grid, also known as an Equatorial Cylindrical Equidistant projection
(ECE). See the SOS web site for details and sample images:
http://sos.noaa.gov/index.html
The images projected here have the galactic center in the center of
the image. The Galactic North pole corresponds to the entire top edge
of the image, and the Galactic South pole corresponds to the entire
bottom edge of the image. The left and right edges have galactic
longitude l=180 degrees. This orientation was chosen because the
galactic center is usually the most recognizable, and so it was
put in the center of the rectangular image.