Observational Cosmology and Time Delay
Physicist says when we look at the sky what we see is in past. We don't know the current state of universe cause we can't see it. How can they be so sure
It sounds almost like a sci-fi movie plot, doesn't it? The idea that we are literally surrounded by the ghosts of the universe's past, and that the present moment is completely hidden from our eyes.
But physicists are incredibly confident about this because of two simple, unbreakable facts: light has a maximum speed limit, and the universe is mind-bogglingly huge.
Think of light like a cosmic postal carrier. When a star "sends" you an image of itself, that image has to travel across space to reach your eyes. Even though light is the fastest thing in the universe, traveling at an astonishing , it still takes time to deliver the mail.
Because of this delay, when you look at the Sun, you aren't seeing it as it is right now. You are seeing it as it was about 8 minutes ago, because it took 8 minutes for those light rays to travel the 150 million kilometers from the Sun to Earth. If the Sun suddenly went out, we wouldn't know a thing about it for 8 whole minutes!
This delay is what astronomers call lookback time. The further out we look, the further back in time we see.
Before we dive deeper into how we measure these immense distances to prove this lookback effect, let's customize our journey. I want to make sure this course matches exactly what you are looking for.