Final answer:
Observing an object that is 15 light years away means we are looking at it as it was 15 years ago, not as it is presently. Astronomical observations are essentially views into the past, and for distant galaxies, can show us conditions from millions to billions of years ago. Such observations are critical in the field of cosmology for studying the evolution of the universe.
Step-by-step explanation:
To observe an object that is 15 light years away means you are seeing the object as it was 15 years ago because the light from that object has taken 15 years to reach us here on Earth. Since light travels at a constant speed of about 186,000 miles per second (which is roughly 300,000 kilometers per second), the light-year is a measure of distance in astronomy, which describes the distance light travels in one year. Hence, astronomical observations are like looking through a time machine; in this case, the light we are seeing now left the object 15 years in the past. The vast distances in space imply that light from celestial objects often takes a very long time to reach us, be it from the Moon, which we see with about a 1-second delay, or much further objects like galaxies, which we observe as they were millions to billions of years ago, allowing us to study the universe's evolution over time.
However, it's important to note that the further we look into space, the 'older' the light is that we receive, which introduces large uncertainties as we peer back towards the early universe. These uncertainties necessitate caution when drawing conclusions from observational data in the field of cosmology, a science characterized by continual discoveries and competing theories. We must consider these factors to understand and interpret the light from distant objects in the context of astronomy.