In fact, my above estimate of 15 billion light years is roughly equal to the age of the universe itself. My second oversimplification was to neglect the fact that the expansion of the universe has slowed down. According to Friedmann's solutions, galaxies that are moving away from us at a given speed today were moving faster when the universe was younger. Decelerating Universe Coasting Universe To understand the horizon of the universe better, consider the two figures above, which represent the motions of distant galaxies. In each case, the horizontal axis represents the position of the Milky Way.
Distances are in light years and times in years.
45 o slope, while a light signal moving toward the Milky Way is represented by a yellow line with a negative 45 o slope. Therefore, it is beyond our event horizon today. For example, the figures suggest that you will see a galaxy near the event horizon as it was when the universe was roughly half its present age. But that is not correct. Because we perceive time running slower for a rapidly receding galaxy, that galaxy will actually be much younger than half the age of the universe.