Watch out! It’s coming our way! This is our neighbour, Andromeda, the only other big island of stars in our galaxy cluster, the Local Group. Currently, it is plunging towards us 300 times faster than a speeding bullet. In 2.3 billion years it will fly by, its gravity ruffling the stars in the Milky Way. But this will be merely a dress rehearsal for the big event. Like a pendulum overshooting its lowest point and swinging back, in 5 billion years Andromeda will return. And this time it will collide catastrophically with the Milky Way.
Stars in galaxies are spread tremendously sparsely so very few of the Milky Way’s will actually collide with Andromeda’s. Nevertheless, the mutual gravity of the two galaxies will stir together the stars to create a “giant elliptical” galaxy, already christened Milkomeda.
Currently, the cosmic repulsion of empty space – the dark energy – is speeding up the expansion of the universe. This will drive ever more galaxies over the cosmic “light horizon”, the farthest distance from which light can reach us. By AD 100 billion, Milkomeda will be the only galaxy in the observable universe – a single island of stars floating in an unutterably vast, unutterably empty ocean of space. If any creatures survive on planets around long-lived red dwarf stars, they will look up at a night sky of total blackness. When you go to bed tonight, be thankful you have been born at the bright dawn of time when the stars are still burning and the universe is still filled with possibilities.
