astrophysics

Physics often resembles the construction of a giant ship — the Universe. The physicist-engineers have yet to agree on the basic blueprints.

Imagine a tiny coin hidden inside a perfect safe, but the walls are so thin that the coin sometimes slips right through, leaving a barely audible sound. Axions are like those coins — except instead of metal, they're particles, and instead of a safe, they're ordinary matter. They barely notice obstacles, making them incredibly hard to catch directly.

If you photograph the same mountain with a film camera and a modern smartphone, you get different images: colors are brighter in one, details are blurrier in the other. The same goes for telescopes: each instrument sees the sky in its own way, adding its own distortions to the true light of galaxies, like a filter through which we look.

For centuries, three branches of physics (cosmology, particle physics, and nuclear physics) worked like isolated shores.

The clock on the kitchen wall ticks, and each tick relentlessly carries us from breakfast to lunch. This is so familiar that it seems like a law of nature, but physics says otherwise. In Einstein's equations, time is reversible: you can rewind the film, and no frame will give the trick away.

At night, an astronomer points a telescope at the edge of the visible universe and sees not stars but ripples — the imprint of the first moments. He knows these ripples should obey statistics, like the heights of people in a crowd. But he has only one crowd, and nothing to compare it with. This is cosmic variance.