The cosmic-ray energy spectrum is explained as a plot of flux against energy, both on log scales, forming a power law with two key features: the knee and the ankle. Below the knee (around 10^15 eV), cosmic rays are common enough to detect about one per square meter per second, and these likely originate within the Milky Way. Above the knee, flux drops sharply to about one particle per square meter per year, pointing to extragalactic sources, while above the ankle (around 10^18 to 10^19 eV) the flux falls to roughly one particle per square kilometer per year, marking the rarest, most extragalactic events like the Oh-My-God particle.
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The flux at the lowest energies (around 10^11 to 10^12 eV) is compared to the particle energies produced at CERN, arriving naturally at about one per second per square meter.
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The drop in flux at higher energies is attributed partly to geometric spreading: as cosmic rays travel farther from fewer sources, the chance of hitting a small detector on Earth shrinks.
465 words · auto-generated from the episode video
16:21Let's first talk about like the energy spectrum of cosmic rays, right? Because you could be like, oh, that's a fluke, right? It's like, I don't know. Well, the more and more that we detect, we can actually now trace what's the probability that we're gonna get a certain cosmic ray of a certain energy. And that's called the energy spectrum. On the left hand side, that's the plot. On the y-axis is the flux of like how much we're getting and on the x-axis is in the amount of energy. Both are in log scale, crucially. So you're getting a power law. Now, the ones that are low energy on the left hand side, those things are pretty common. You'll get like one particle per square meter per second.
17:01That's the flux. So if you had like a detector, those one square meter, every second you'd get a single particle that's like, I don't know, 10 to the 11, 10 to the 12, like the stuff at CERN. The universe is giving us the stuff at CERN at about one per second per square meter. Then there is a knee in this plot. The feature there is called a knee because up to this energy, which is about like 10 to the 15 electron volts, all of the stuff before then is stuff that probably originates within the Milky Way.
17:43There's almost a ceiling to how far it could have been from. Yeah, but then there's a little kink and that's telling us that there is another source. And that other source is probably extra galactic. This is where we're getting to like one particle per square meter per year. The probability of it coming straight at Earth in our little square meter detector is getting lower and lower. Partly because in every direction, the greater the distance, the easier it is as it travels over that long distance for it to miss Earth because we come in increasingly smaller. Yeah, the source itself is not, I mean, there's not that many sources that are doing this.
18:24So as it spreads out, the probability is very, very low. And then finally, there's an ankle and that's one particle per square kilometer per year. That's at around 10 to the 18, 10 to the 19. Those are the ones that we want. Those are extra, extra galactic. There's the ones that happen within our solar system, then there's the one that happens within the galaxy, and then there's the extra galactic, the ones that we don't like what, that's the oh my God particle. So there's these three sources that we're trying to make sense of. In order to find the ones that are really, really energetic,
19:05we need to do a lot of work.