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" Our galaxy is surrounded by a cloud of about fifty nearby galaxies, known as our Local Group. Foremost among them is the Andromeda Galaxy, a beautiful spiral galaxy, and the only galaxy in our group larger than our own. Gravity is pulling the two toward each other, and in four billion years (before our Sun has died) they will collide and unite. With so much distance between the stars of each galaxy, this collision will do surprisingly little to upset the stars and their planets. Its main effect will be to disrupt the delicate spiral structures of the partners, probably merging into a more uniform elliptical galaxy about three times as large. Eventually (in hundreds of billions of years) all the other galaxies in our group will have merged in too, forming a single giant galaxy.28 Zooming further out, we see many more groups of galaxies, some with as many as a thousand members.29 Eventually these groups resolve into a larger structure: the cosmic web—long, thick threads of galaxies, called filaments. These filaments criss-cross space in a kind of three-dimensional network, as if someone took a random set of points in space and connected each to its nearest handful of neighbors. Where the filaments intersect, space is bright and rich with galaxies.30 Between such filaments are dark and empty expanses, known as cosmic voids. As far as we can tell, this cosmic web continues indefinitely. At the very least, it continues as far as we can see or go. It is these final limits on our knowledge and action that appear to set the ultimate scale in our universe. We have known for almost a century that our universe is expanding, pulling the groups of galaxies apart. And twenty years ago we discovered that this expansion is accelerating. Cosmologists believe this puts a hard limit on what we will ever be able to observe or affect.31 We can currently see a sphere around us extending out 46 billion light years in all directions, known as the observable universe. Light from galaxies beyond this sphere hasn’t yet had time to reach us.32 Next year we will see a little further. The observable universe will increase in radius by a single light year, and about 25 more galaxies will come into view. But on our leading cosmological theory, the rate at which new galaxies become visible will decline, and those currently more than 63 billion light years away will never become visible from the Earth. We could call the region within this distance the eventually observable universe.33 But much more importantly, accelerating expansion also puts a limit on what we can ever affect. If, today, you shine a ray of light out into space, it could reach any galaxy that is currently less than 16 billion light years away. But galaxies further than this are being pulled away so quickly that neither light, nor anything else we might send, could ever affect them.34 "
― Toby Ord , The Precipice: Existential Risk and the Future of Humanity
