I'm reading Cosmology by Harrison and in Chapter 21 he discussed the horizon riddle, I understood the problem that he posed but his solution was confusing. "Consider two widely separated observers, A (for Albert) and B (for Bertha). We suppose they can see each other. Each has a horizon such that A cannot see things beyond his horizon and B cannot see things beyond her horizon. Each sees things the other cannot see. We ask: Can B communicate to A information that extends A’s knowledge of things beyond his horizon? If so, then a third observer C may communicate to B information that extends her horizon, which can then be communicated to A. Hence, an unlimited sequence of observers B, C, D, E, . . . may extend A’s knowledge of the universe to indefinite limits. According to this argument A has no true horizon. This is the horizon riddle. When we speak of things that are seen or not seen we usually have in mind those that endure (particle horizon) and are represented by world lines.Thus the horizon riddle applies to the particle horizon of the universe. We consider the particle horizon in a static universe (Figure 21.6) and show that the riddle has a simple solution. We have supposed that luminous galaxies originated 10 billion years ago and the particle horizon is therefore at distance 10 billion light years. Observers A and B see each other and have overlapping horizons. Suppose A and B are separated by a distance of 6 billion light years. B sends out information that travels at the speed of light and takes 6 billion years to reach A. Hence A receives from B information that was sent 6 billion years ago when the universe was 4 billion years old. But B’s particle horizon in the past at the time when the information was sent was only 4 billion light years distant. Thus B’s horizon at that time did not extend beyond A’s present horizon. " My confusion lies in the statement "Observers A and B see each other and have overlapping horizons. Suppose A and B are separated by a distance of 6 billion light years." How can A and B be 6 billion light years apart and still see each other given by the time B sent the signal to A the universe was just 4 billion years old. Each observer A and B has a particle horizon 4 billion light years in radius so by overlapping their horizon there is no way that they can have 6 billion light years in separation but still see each other, we are sure that A and B should lie outside of each others particle horizon if their particle horizon have a radius of 4 billion light years, although some of their horizon overlaps. Also, I don't understand "Thus B’s horizon at that time did not extend beyond A’s present horizon. " The first image is the confusion. The second image is what I think it should be.