When astronomers peer at the heavens through telescopes, they see distant objects not as they are now, but as they were when the light reaching the telescopes embarked on its journey across space. In this respect, a telescope is also a “timescope.” For example, if a nearby star exploded yesterday, we would be blissfully unaware of this cataclysm for years, until the pulse of light announcing the star’s demise arrived on Earth. Looking further afield, we see stars in the neighboring Andromeda galaxy as they looked about 2.5 million years ago. More distant galaxies appear correspondingly older. The Hubble Space Telescope routinely records images of galaxies as they appeared long before Earth even existed. The oldest galaxies can actually be seen still in the process of formation, more that 12 billion years ago. So by penetrating farther and farther into space, astronomers can watch the history of the universe unfolding in reverse.
— Cosmic Jackpot by Paul Davies