Final answer:
From studying electromagnetic radiation, we're able to understand the past state of celestial bodies and regions of space, as the light we observe corresponds to different times in the history of the universe due to its finite speed.
Step-by-step explanation:
The ability to observe the universe using electromagnetic radiation is pivotal in understanding the cosmos. Since electromagnetic waves travel at a finite speed, the information they carry reflects the state of the universe in the past, when that radiation was emitted. Because the speed of light is finite, we cannot see what the universe is like at the present moment; we only see what it was like at the time the light was emitted. This means that when we receive light from a star located 1,000 light-years away, we are seeing that star as it was 1,000 years ago.
As we look at Cosmic Microwave Background radiation, we are seeing the universe as it was shortly after the Big Bang, which is as close as we can get to observing the infancy of the cosmos. Similarly, when we study radiation from different objects in space, we are essentially peering back in time, getting a snapshot of that celestial body's past state. However, because this information is finite and travels toward us over vast distances, it provides us with a window into the history of the universe rather than its current status.
Therefore, when the question asks whether we can tell what the universe is like at the present moment or at any selected time in the past, the correct answer would be (b) Yes, at any selected time in the past because the electromagnetic waves we observe today have traveled to us over years and show us the state of the universe at the time they were emitted.