Eight New Quadruple Lenses Aren’t Just Gorgeous, They Reveal Dark Matter’s Temperature
“Ever since astronomers first realized that the Universe required the existence of dark matter to explain the cosmos that we see, we’ve sought to understand its nature. While direct detection efforts have still failed to bear fruit, indirect detection through astronomical observations not only reveal the presence of dark matter, but this novel method of using quadruply lensed quasar systems has given us some very strong, meaningful constraints on just how cold dark matter needs to be.
Dark matter that’s too hot or energetic cannot form structures below a certain scale, and the observations of these ultra-distant, quadruple-lens systems show us that dark matter must form clumps on very small scales after all, consistent with them being born as arbitrarily cold as we can imagine. Dark matter’s not hot, nor can it even be very warm. As more of these systems come in and our instruments go beyond what even Hubble’s capabilities are, we might even discover what cosmologists have long suspected: dark matter must not only be cold today, but it must have been born cold.”
We might not yet know the nature of dark matter, as we’ve never been able to detect the particle responsible for it directly. But we know it clumps and gravitates together, with the exact way it would do so dependent on the amount of kinetic energy it had when it was born relative to its mass. Dark matter could have been extremely hot, such as a scenario where it was made from neutrinos, cold, such as from a very heavy WIMP particle (or a born-super-cold axion), or anywhere in between.