<ul><li>The spin of a particle refers to, essentially, how it responds to magnetic fields.</li><li>In a hydrogen atom, the electron and proton can either have the same direction of spin (both up or both down) or they can have opposite spins.</li><li>When the proton and electron have the exact same spin, that configuration has ever so slightly more energy than when they’re the opposite.</li><li>Hydrogen atoms in the universe emit energy and glow in a very specific kind of light, known as 21 cm electromagnetic radiation.</li></ul>

How Humble Hydrogen Lights Up the Universe

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