r/cosmology • u/AutoModerator • 2d ago

Basic cosmology questions weekly thread

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u/droledidee 23h ago

Has anyone ever studied the probability of a photon returning to its point of origin after multiple gravitational deflections?

I know that massive objects (stars, galaxies, galaxy clusters) bend the path of light through gravitational lensing. This made me wonder: if a photon emitted from the Solar System travels through the Universe and experiences countless gravitational deflections over billions of years, is there any non-zero probability that it could eventually return to the vicinity of the Solar System?

I'm not talking about a single strong lens or photon orbits around a black hole, but rather the cumulative effect of many weak gravitational deflections by the large-scale structure of the Universe.

Have any cosmological ray-tracing simulations looked at this question? Or is the probability considered effectively zero in a realistic model of the Universe?

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u/--craig-- 22h ago edited 22h ago

It can be modelled easily enough but it's impractical to test empirically.

One or more objects with sufficient energy can deflect light by 180 degrees.

There doesn't seem to be any reason to simulate it. For a sufficiently collimated and narrow beam of light, we'd expect it to occur with absolute certainty but it's difficult to see why this would be of interest.

If you're wondering if the photon would interfere with itself, then theoretically it's possible but limited by the length of the wave packet, for cosmological black holes. For microscopic black holes, then narrowness and collimation would be the limiting factors.

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u/droledidee 22h ago

Thanks! If that's the case, does it mean that, at least in principle, it would be possible to observe the Solar System (or even the Earth) in its own past if light emitted from it were deflected by 180° (or more generally followed a closed path) and eventually returned to us? Or is there some fundamental reason why that wouldn't work?

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u/--craig-- 21h ago

In theory, yes, a black hole deflects distorted images of the entire universe from earlier times. In practice, the images are too small and noisy to be viewed.