# How is the universe becoming increasingly unreachable for things traveling at speed of light?

I've seen a few articles like this that say the majority of the universe is practically unreachable to us, even if we were to travel at the speed of light. My understanding is that there is enough energy and matter in between us and bodies that are ~14-15 billion light years away that the bodies are moving away from us faster than the speed of light. Because we are limited by the speed of light, those bodies are now unreachable to us. Is this roughly right?

I am trying to build a mental model of what this would look like and came up with this:

Hopefully, I am not oversimplifying too much, but I imagine

• A being us
• D being the body we want to reach
• and B and C being representations of the "matter" that are causing space to expand.
• None of these are individually stronger than a black hole
• The distances AB, BC, and CD are not expanding faster than the speed of light,
• but the combination of them makes AD expand faster than the speed of light, thus rendering D unreachable.

That being said, if we were to shoot a photon from A, would that photon not eventually be able to reach B in a finite time? And if it can reach B, then it should eventually reach C? And if it can reach C, then it can reach D, making D actually reachable?

More specifically, any point in the universe would be reachable as long as no segment of our path to that point expands faster than the speed at which we're traveling.

So probably something is wrong with my mental model or my interpretation of the article, but I'm not sure what - can someone help me understand?

That being said, if we were to shoot a photon from A, would that photon not eventually be able to reach B in a finite time? And if it can reach B, then it should eventually reach C? And if it can reach C, then it can reach D, making D actually reachable?

A photon shot from C now can reach D. However, by the time the photon from A reaches C, it will no longer be possible for a photon from C to reach D. Keep in mind that these objects are separating at a continuously increasing rate.

• Does this imply that C will eventually become a black hole, seeing that the photon cannot ever reach D? Or has it always been a black hole? Commented Mar 4, 2023 at 21:13
• @SiddSingal The difficulty is not in escaping C but in reaching D. Indeed, from any starting point, you could escape to sufficient distance that you can never return. So these points are kind of like the opposite of black holes, in some sense. That can also be understood by realizing that dark energy creates a gravitational repulsion that becomes stronger at greater distances. When the separation becomes too great, the gravity cannot be overcome.
– Sten
Commented Mar 4, 2023 at 21:30
• So we accept that there is some moment in time T = t_0 that a photon shot from C could have reached D. And then according to this answer, at times T > t_ac (perhaps the time it hypothetically took a photon to travel from A to C), a photon will be unable to travel from C to D. And this is completely independent of A and B existing. Does this imply that, for example, the moon will be unreachable by us some day? If not, then what is fundamentally different between the earth and the moon VS. C and D that makes a photon eventually unable to travel from C to D Commented Mar 4, 2023 at 21:48
• "And this is completely independent of A and B existing." -> this is an assumption I made about your answer - let me know if I'm wrong Commented Mar 4, 2023 at 21:53
• I've been deliberately avoiding talking about speeds, but it's accurate to say that when measured using sequences of rulers that are comoving with the expansion of the universe, the photon's distance from C grows at an increasing rate. By the same measure, the C-D distance also grows at an increasing rate. The photon-D distance might grow from the outset (if at the emission time C and D are separating faster than the speed of light, by the above distance measure), or the distance might initially decrease but eventually turn around and start to grow.
– Sten
Commented Mar 5, 2023 at 11:21