If the Andromeda galaxy is 2,500,000 light-years away, when was the light we observe emitted?

The correct response is that if the Andromeda galaxy is 2,500,000 light-years away, the light we currently observe from it was emitted 2,500,000 years ago. This is based on the fundamental nature of light travel; light travels at a finite speed of approximately 299,792 kilometers per second (or about 186,282 miles per second).

When we say an object is a certain distance away in light-years, we are essentially stating how many years it takes for light to travel from that object to us. Thus, if Andromeda is 2,500,000 light-years distant, it means that the light we see today left Andromeda 2,500,000 years ago. This time scale encompasses the entire duration it takes for light to traverse that distance, providing a direct correlation between distance in light-years and the time in years since the light was emitted. Therefore, the observation we make today reflects the state of the Andromeda galaxy as it was 2.5 million years in the past.