When you lie down on your back on a clear, dark, moonless night, what is it that you see? If your vision is outstanding and observing conditions are just right, you’re likely to see not only a few planets and thousands of stars, but also star clusters, some faint nebulae, the plane of the Milky Way, and maybe even a distant galaxy or two. But when you start to look more deeply — beyond what you can see with your naked eye — you start to find that there’s an amazing Universe past our own galaxy, past the stars, clusters and nebulae of the Milky Way out there.
* What once seemed like faint, fuzzy, inconsequential smudges have since revealed themselves to be distant galaxies, or island Universes not so different than our own, consisting of anywhere from hundreds of millions to many trillions of stars. And the Universe is full of them, with roughly as many galaxies in the part observable to us as there are stars in the entire galaxy we inhabit.
* The Universe originated from a hot, dense, very rapidly expanding state. It was full of matter and radiation, and over time it expanded, cooled, and the expansion rate began to slow. In addition, gravitational imperfections grew into galaxies and clusters of galaxies, or shrank into great cosmic voids.
* From a time billions of years ago when the Universe was almost perfectly uniform, with no life, planets, stars or galaxies in it, we now have — on average — hundreds of billions of stars in each of hundreds of billions of galaxies, populating an observable Universe some 92 billion light years across.
* As the Universe continues on in time, more and more galaxies are redding out as the Universe continues to accelerate. With each second that goes by (on average) thousands of stars and their planetary systems cross that horizon forever, and leave our ability to reach them for all eternity.
* Of the hundreds of billions of galaxies (maybe even as many as a trillion) in our Universe today, only about 3% of them are still reachable. And every time a mere three years goes by, another one fades from our present reach-ability.
We can always hope that some type of controlled wormhole, or spacetime-bending faster-than-light travel can save us, but there’s no evidence that such an innovation — despite our best science fiction dreams — can ever be practically realized. Until then, we’d better plan on starting our journey sooner rather than later.
Because the expansion marches on.[More.](https://medium.com/starts-with-a-bang/the-disappearing-universe-d7447467c63a)