Title text: Bonus points if you can identify the science in question
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 GHz (Giga (or billion) Hertz). 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 in fact the universe at the point when it had cooled down enough to allow photons to escape, 0.38 million years into the universe's 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).
 Official T-shirt explanation
This comic was made into a T-shirt. xkcd stores.
On the xkcd store, there is both an explanation for the title:
Science: We finally figured out that you could separate fact from superstition by a completely radical method: observation. You can try things, measure them, and see how they work! Bitches.
And specifically an explanation for the graph:
The graph on the back of the shirt is data from the COBE mission which looked at the background microwave glow of the universe and found that it fit perfectly with the idea that the universe used to be really hot everywhere. This strongly reinforced the Big Bang theory and was one of the most dramatic examples of an experiment agreeing with a theory in history -- the data points fit perfectly, with error bars too small to draw on the graph. It's one of the most triumphant scientific results in history.
The above is a direct copy paste, with errors. The current wiki page of the COBE mission can be found at http://en.wikipedia.org/wiki/Cosmic_Background_Explorer
- [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 point, which the curve fits perfectly. The y-axis is labeled. Along the x-axis, the zero point and the frequency where the peak has its maximum are labeled and close to the arrow the unit of this axis is written.]
- y-axis: Energy Density
- Along the x-axis:
- [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) = (2hf3/c2)(1/(ehf/kT-1))
- [Below the graph is written the following:]
- It works, bitches.
- This was the 48th comic originally posted to LiveJournal.
- This comic kept its original title: "Science"
- It is part of the last six comics on LiveJournal that all had a title without the word "Drawing" in it.
- Five of these had exactly the same title on both sites.
- Only 11 comics have the same title on both sites.
- Apart from the thirteen first comics posted to LiveJournal, there were only three other comics without the word "Drawing" in the title before these last six.
- Original Randall quote: "Bonus points if you can identify the science in question."
- Only difference between this and the title text on xkcd is the last period: "."
- It was rare that these two texts were so similar.
- This comic was one of the last 11 comics posted on LiveJournal.
- For some reason, this comic was first posted a week later on xkcd (25 January 2006), on the day that 53: Hobby was released on LiveJournal.
- On the day 54: Science was released on LiveJournal (18 January 2006), another comic (51: Malaria) was released on xkcd.
- 51: Malaria also became the next comic released on LiveJournal, but this meant that three comics in a row were posted a release day earlier on xkcd than on LiveJournal.
- Only with the last comic released on LiveJournal, 55: Useless, did the two sites release the same comic on the same day again.
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