Why When scientists look at the very distant galaxies through powerful telescopes they see the galaxies as they were millions or billions of years ago?

Answer 1

Because light takes time to travel. Light travels at approximately 300,000km/second.

The Sun is about 150 million km away from Earth. Light takes 8 minutes to reach us from the Sun.. But for a galaxy 1 million light years away, the light takes 1 million years to reach us. So when it finally gets here, it shows that galaxy as it was 1 million years ago. It could explode tonight but we wouldn't know until the light reached us in 1 million years.

Answer 2

When you "look at" something, the light that left it and traveled from there to you is
now entering your eyes, and it takes time for light to get from one place to another.

-- When you look at the sun, you see the light that left it roughly 8 minutes ago.

-- When you look at the moon, you see the light that left it roughly 1.27 seconds ago.

-- When you look at the stove on the other end of your kitchen, you see the light
that left it roughly 0.0000000122 second ago.

-- When you look at the nearest star outside the solar system, you see the light
that left it roughly 4.2 years ago.

-- When you look at the nearest galaxy outside the Milky Way, you see the light
that left it roughly 2.5 million years ago.

-- When you look at the farthest galaxy that our present observational technology
can detect, you see the light that left it roughly 14 billion years ago.

Technically, you can never see anything as it is "now".

Answer 3

It takes light a while to reach us, from distant galaxies - light is fast, but it isn't instantaneous.