We live in a universe consisting of galaxies which consist of stars which have planets revolving around them. But one can imagine totally different universes with completely different physical layouts. We can also imagine other universes with laws completely different from the laws of our universe. Let us imagine what it might be like if you had been born in such a universe, one very, very different from ours.
A Simple Oceanic Universe
One interesting possibility is a universe that is one huge ocean. In this universe there are no stars or planets, and no empty space. This universe consists entirely of warm water, and objects and life floating around in the warm water. There might be absolutely no gravity in this universe.
If you had been born in such a universe, you would have no concepts such as the idea of a day or the idea of a year. You probably also would have no home. You would not need one, since you would have no need to protect yourself from bad weather. You would pretty much be like a fish floating around in the water, although you might have arms allowing you to grab things. You would have no idea at all of up or down, and no concept of the surface of the ocean or the bottom of the ocean. It would simply be that no matter how far you traveled in one direction, there would always still be more ocean for you to swim in. If the water in such a universe moved around slowly, you might grow up with your family of oceanic creatures. But if the water moved quickly, you might be separated from your parents very soon after birth, and forced to fend for yourself from a very early age.
Living in such a universe, it is rather doubtful that you would be part of any very complicated society. But if you were part of some society, and the society had villages or cities, such centers would probably be sphere-shaped rather than the almost flat cities we see on our planet.
Such a universe might be totally dark, requiring evolution to produce alternate senses such as sonar for the various species that evolved. Or perhaps such a universe might be dimly lit by phosphorescent plants and phosphorescent rocks.
An Oceanic Universe of Bubble Habitats
Now let us imagine an oceanic universe similar to the one just imagined, except for one big difference: rather than consisting only of living things and objects floating around in water, this universe would be only about 90% water, with one tenth of it being air. The air portions would consist of big bubbles of air existing within the ocean stretching endlessly onward in all directions. In such a case there might evolve creatures designed to live within such bubbles. Imagine yourself as one of those creatures.
You might live in a small ten-meter wide bubble, where there lived only you or your family. Or you might live in a much larger bubble, along with your family and many other families. Your entire city might be in a bubble.
Living in such a bubble, you might be able to jump or fly to any part of the bubble (assuming this universe had little or no gravity). To gather food, you would probably have to dive into the water that surrounded the bubble. Venturing out into the water that surrounded the bubble, you would have to be careful to remember how to get back to your bubble, or you might never see your family again.
This type of universe would be very different from anything in human experience, but humans might actually one day be able to experience such an environment. Space visionaries have imagined a space hotel with a zero gravity center consisting of a spherical pool with a big air bubble inside it. Space tourists would swim halfway through the sphere to get to the waterless center.
A Universe With Countless Inhabited Micro-Planets
In our universe planets (having diameters of thousands of kilometers) revolve around stars, and stars are separated by huge distances (trillions of kilometers). But it is easy to imagine a very different type of universe in which beings live on tiny little micro-planets, spheres that are only a few dozen meters or a few hundred meters or a few thousand meters in size. Such a universe might have millions of such micro-planets in every unit of space the size of our solar system.
Someone might argue that such a universe couldn't exist, because spheres of that size would not have enough gravity to hold an atmosphere. But that objection is not sound, for two reasons. First, we can imagine a universe in which the space between planets and stars is filled with air like the air on our planet. We can also imagine a universe with an entirely different gravitational constant, and in such a universe small spheres might generate enough gravity to hold an atmosphere, and prevent inhabitants from floating off into space. Such planets might get their heat either from regular sunlike stars, or from very small mini-stars, or perhaps just from the background temperature of space (which could be between 60 and 80 degrees in an alternate universe).
A Strange Alternate Universe of Micro-Planets
Now imagine living in such an alternate universe. Your micro-planet might be only 50 meters in diameter. Living on it might be only your family and a few other families. Looking up at the sky, you would not see a vast expanse of empty blue. You would see many other micro-planets. You could probably also see many other people living on those micro-planets. But you might have no way of getting to any of those micro-planets, even though they were only a few hundred meters away. However, if the space between micro-planets was not a vacuum but was instead filled with air, you could probably travel to other micro-planets by using some kind of device resembling wings or a plane.
On the other hand, if the space between micro-planets was a vacuum, you might have no way of ever visiting any of the nearby micro-planets. You would see lots of other people in the sky, but would never be able to touch them. It would be most tragic if you fell in love with someone on a nearby micro-planet, because you would never be able to kiss that person or touch that person. But even if you could not visit anyone on a nearby micro-planet, you would probably have some way of communicating with them, possibly by flashing hand-signals, or using lights or fire as signals.
A Universe With Densely Packed Stars
In our universe stars are very far away, but it is easy to imagine a universe in which the distance between stars is a thousand times smaller. What would it be like living in such a universe?
For one thing, the night sky would be far more interesting, with starlight providing as much illumination as the sun. There would be no darkness at night, and the sky at night would look rather like a huge collection of jewels resting on a background of black velvet. If you lived in such a universe, alien visitations would probably be ridiculously common. You might hardly pay any attention when you read in your morning newspaper that another alien spaceship had arrived in orbit around your planet. Your planet would probably already be packed with visitors from other planets, and when you went to a bar it might well resemble the bar in the first Star Wars movie, with a huge variety of alien creatures.
A Universe With a Compact Spherical Geometry
In our universe, space is flat or almost flat. But another possible spatial geometry is for a universe to be spatially spherical. If you lived in a spatially spherical universe which was relatively compact, you might experience something incredibly strange as an astronaut. You might set off in a spaceship to explore the deepest reaches of space. After traveling for a long time, you might see ahead of you a planet that looked rather like your own planet. You would be very excited, and say to yourself: what a discovery – another planet like my own planet! As you approached nearer and nearer the planet, you would notice more and more resemblances between this planet and your own planet. Finally, as your ship drew closer, you would at last realize the truth: the planet ahead was actually your own planet, not some distant planet. Because of the spherical geometry of your universe, you would have traveled exactly to where you had started out from.
A Universe With Lots of Antimatter
In our universe there seems to be virtually no antimatter, but it is easy to imagine a universe in which there is a great deal of antimatter scattered around. When the tiniest bit of antimatter touches the tiniest bit of matter, both are converted to an enormous amount of energy.
Therefore, in an alternate universe containing lots of antimatter, you would not worry much about your career, your bank account, your love life, war, or the next election. What you would worry about above all is something in the sky blowing up and killing you. You would know that every year there would be a significant chance of an explosion in the sky that showered your planet with deadly radiation. So you would probably spend most of your life deep underground, waiting for the end of the radiation shower from the latest matter and antimatter explosion out in space.
A Universe With Very High Radioactivity
The previous hypothetical universes are very different from our own. But let us imagine a universe exactly like ours, with one difference: the strong nuclear force is significantly weaker, and the weak nuclear force is stronger. As a result, there is much more radioactivity. All this radioactivity causes everyone to be exposed to high radiation throughout their lives. As a result, almost everyone dies of cancer by the time they get to be sixteen.
If you lived in such a universe, you would probably not remember your mother and father, as they both would have died of cancer a few years after you were born. You probably would have been raised by one of your older brothers or sisters. In your society people would encourage marriage between people who just recently reached puberty. It would be the only way to keep your species alive, with almost everyone dying of cancer while they are still teenagers.
You would probably only go to school for a few years, and start working at around age 8. If you announced to your boss as a 12-year old that you were pregnant, your boss (who would probably be only one or two years older than you) would congratulate you for following the norms of your society. But you would know that a few years after giving birth, you would also be dead of cancer, along with all of your friends. You, like all mothers on your planet, would weep about the fact that you would never be able to see your child grow up even halfway. Be thankful you do not live in such a universe.