When we observe galaxies from a telescope, we typically see them as they appeared in the past, not as they are at the present moment. This is due to the finite speed of light. Light takes time to travel across vast distances, including the vast distances between galaxies and our telescopes on Earth.
For example, let's say we observe a galaxy that is located 1 billion light-years away. The light we detect from that galaxy has traveled for 1 billion years to reach our telescope. Therefore, we see the galaxy as it was 1 billion years ago, not as it is right now.
This phenomenon is often referred to as "lookback time" or "cosmic time." The farther away a galaxy is, the longer it takes for its light to reach us, and thus, we observe it as it appeared further back in cosmic history.
By studying galaxies at various distances, astronomers can effectively peer into the past and gain insights into the evolution of the universe over different epochs. This concept has been instrumental in our understanding of the early universe and cosmic history.