Here's why:
* Light travels at a finite speed: Light takes time to travel from distant objects to Earth. So, when we look at something very far away, we are actually seeing it as it was in the past.
* Hubble's limits: Hubble can see objects that are so far away, the light from them has been traveling for billions of years. This means we are seeing them as they were billions of years ago, when the universe was much younger.
* The "deep field" images: Hubble's deep field images are some of the most famous examples. They capture light from galaxies that existed when the universe was only a few billion years old!
So, how far is that? It's difficult to give a specific distance. Astronomers often refer to the redshift of objects, which indicates how much the light has been stretched due to the expansion of the universe. Hubble's farthest observations have redshifts of around z=10. This corresponds to a distance of approximately 13.4 billion light-years.
Important note: The observable universe is estimated to be about 46.5 billion light-years in radius. This means that there are objects beyond Hubble's current reach, even though they are within the observable universe.
Future telescopes, like the James Webb Space Telescope, will be able to see even further back in time and provide us with an even more detailed picture of the early universe.
