Science
Science
[A graph with a curve that begins at zero, then peaks at a given frequency, indicated via a thin vertical line, and then fades down towards zero. It is possible to see the data points, which fit the curve perfectly. The y-axis is labelled. Along the x-axis, the zero point and the frequency where the peak has its maximum are labelled and close to the arrow the unit of this axis is written.] y-axis: Energy Density Along the x-axis: 0 160.4 GHz [Above the graph to the right is the following formula, with the last inner parentheses only included to make the formula clear, since in the drawing the fractions are written above and below horizontal lines:] I(f) = (2hf 3 /c 2 )(1/(e hf/kT -1)) [Below the graph is written the following:] Science. It works, bitches.

The solid line represents the theoretical radiation for a blackbody at 2.73 K according to Planck's Law (derived as early as 1900 by Max Planck ). The formula, almost as written in the graph, can be found here . The only changes are that on Wikipedia, the frequency f is represented by the Greek letter ν (nu) and the temperature T is included as an independent variable, so I ( f ) becomes I ( v , T ). However, I ( v , T ) still represents the spectral radiance (similar to energy density). In this formula, h is the Planck constant, c is the speed of light in a vacuum, and k is the Boltzmann constant. The frequency ( f or v ) along the x -axis is measured in gigahertz . The curve peaks at 160.4 GHz. There is no scale or unit on the energy density on the y -axis.
The theory is that the blackbody in question was the universe at the point when it had cooled down enough to allow photons to escape , 0.38 million years into its 13.8 billion years history. The photons that reach us today are the ones that have been travelling to us at lightspeed since then. As the light from astronomical objects suffers from redshift due to the expansion of the universe, and this shift becomes more pronounced with distance from the observer, this light displays in the infrared range.
The title text praises viewers who can identify where this equation and corresponding graph come from (without consulting this wiki, of course).
This comic was made into a T-shirt, but is no longer available.
On the xkcd store, there was both an explanation for the title:
And specifically an explanation for the graph:
The above is a direct copy paste, with errors. The current wiki page of the COBE mission can be found at Cosmic Background Explorer on Wikipedia .
[A graph with a curve that begins at zero, then peaks at a given frequency, indicated via a thin vertical line, and then fades down towards zero. It is possible to see the data points, which fit the curve perfectly. The y-axis is labelled. Along the x-axis, the zero point and the frequency where the peak has its maximum are labelled and close to the arrow the unit of this axis is written.] y-axis: Energy Density Along the x-axis: 0 160.4 GHz [Above the graph to the right is the following formula, with the last inner parentheses only included to make the formula clear, since in the drawing the fractions are written above and below horizontal lines:] I(f) = (2hf 3 /c 2 )(1/(e hf/kT -1)) [Below the graph is written the following:] Science. It works, bitches.
