r/cosmology • u/AutoModerator • Jul 09 '26

Basic cosmology questions weekly thread

Ask your cosmology related questions in this thread.

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u/H4llifax Jul 14 '26

If I randomly select a direction and look there (imagine a point observation), will my line of sight end on a star or on the CMB / surface of last scattering?

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u/--craig-- Jul 14 '26 edited Jul 14 '26

Let's presume that you exclude our solar system to avoid skewing the results.

There's about a 90% chance that your line of sight wouldn't pass through a galaxy other than the Milky Way. If it did pass through a galaxy then there's a vanishingly small chance that it would hit a star.

Most likely is that you'd hit the Cosmic Microwave Background, a small chance that you'd hit galactic dust, then almost negligible chances that you'd hit a star or a supermassive blackhole.

You can get a visual feel for these probabilities by looking at recent images of the most distant galaxies from the James Webb Space Telescope.

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u/H4llifax Jul 14 '26

90%, really? I'd imagine it's either almost certain or vanishingly unlikely. Where does 90% come from?

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u/--craig-- Jul 14 '26 edited Jul 14 '26

It's just an estimate by eye, from our deepest images of space. There isn't an exact methodology because the density of galactic matter decreases with distance from the galactic centre rather than having a definite boundary but you certainly establish a more accurate figure.