Tell about Galaxies ?

One example of the cosmological principle at work is the cosmic microwave

Using instruments such as NASA's Wilkinson Microwave Anisotropy Probe, astronomers have found the CMB is virtually identical wherever one looks.

Measurements of the universe's expansion — through watching galaxies race away from us

show that it is about 13.82 billion years old.

As the universe gets older and bigger, however, galaxies will recede farther and farther from Earth. This will make them more difficult to see in telescopes.

The universe is expanding faster than the speed of light

Also, the universe is accelerating in its expansion.

This is where the concept of the "observable universe"

In 1 trillion to 2 trillion years, Livio said, this means that there will be galaxies that are beyond what we can see from Earth.

We can only see light from galaxies whose light had enough time to reach us," Livio said. "It doesn't mean that that's all there is in the universe. Hence, the definition of the observable universe."

Galaxies also change over time.

The Milky Way is on a collision course with the nearby Andromeda Galaxy, and both will merge in about 4 billion years.

Later on, other galaxies in our Local Group

the galaxies closest to us — will eventually combine. Residents of that future galaxy would have a much darker universe to observe

Civilizations started then, they would have no evidence that there was a universe with 100 billion galaxies

"They would not see the expansion. They would probably not be able to tell there was a Big Bang.