How can we see the oldest light with stars and planets acting as obstacles on the path of light
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We can’t see light if it’s being blocked by a star.
Luckily only a very very tiny percentage of our view of space is blocked by stars. Stars are very small, and space is very big.
Because space is very, very big. That 1cm gap you see in the sky between stars? That could be billions of light years. If a star is being blocked, we can’t see it, but space is big enough that there are gaps for us to see all the way through
Gravitational lensing occurs when a massive celestial body — such as a galaxy cluster — causes a sufficient curvature of spacetime for the path of light around it to be visibly bent, as if by a lens. The body causing the light to curve is accordingly called a gravitational lens.
There are actually two ideas involved in this situation. The first is tha there are plenty of “gaps” in every direction where light travelling in a straight line from the moment of its creation encounters nothing but empty space on its journey to our eyeballs (or telescopes). In fact, there’s far more gap than obstacle.
Second, even where there are obstacles, a lot of the light travelling from the far side of such obstacles to Earth is bent around the obstacles through a phenomena known as gravitational lensing. It’s a bit beyond ELI5, but any sufficiently massive object or collection of objects warps space around it changing the apparent path of light travelling near it. Under the right circumstances, beams of light are bent around the obstacle rather than being blocked by it.
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