When scientists talk about the expanding universe, they mean that it has been growing ever since its beginning with the Big Bang.
The galaxies outside of our own are moving away from us, and the ones that are farthest away are moving the fastest. This means that no matter what galaxy you happen to be in, all the other galaxies are moving away from you.
However, the galaxies are not moving through space, they are moving in space, because space is also moving. In other words, the universe has no center; everything is moving away from everything else. If you imagine a grid of space with a galaxy every million light years or so, after enough time passes this grid will stretch out so that the galaxies are spread to every two million light years, and so on, possibly into infinity.
The universe encompasses everything in existence, from the smallest atom to the largest galaxy; since forming some 13.7 billion years ago in the Big Bang, it has been expanding and may be infinite in its scope. The part of the universe of which we have knowledge is called the observable universe, the region around Earth from which light has had time to reach us.
One famous analogy to explain the expanding universe is imagining the universe like a loaf of raisin bread dough. As the bread rises and expands, the raisins move farther away from each other, but they are still stuck in the dough. In the case of the universe, there may be raisins out there that we can’t see any more because they have moved away so fast that their light has never reached Earth. Fortunately, gravity is in control of things at the local level and keeps our raisins together.
Who Figured This Out?
The American astronomer Edwin Hubble made the observations in 1925, proving that there is a direct relationship between the speeds of distant galaxies and their distances from Earth. The observation that galaxies are moving away from the Earth at speeds proportional to their distance has traditionally been known as Hubble’s Law, although it should be noted that, in 2018, the International Astronomical Union (IAU) voted to recommend amending the name to the Hubble–Lemaître law, in recognition of the contributions of both Hubble and the Belgian astronomer Georges Lemaître to the development of modern cosmology.
The Hubble Space Telescope was named after Edwin Hubble, and the single number that describes the rate of the cosmic expansion, relating the apparent recession velocities of external galaxies to their distance, is called the Hubble Constant.
Is the Universe Infinite?
It might be easier to explain about the beginning of the universe and the Big Bang Theory, than to talk about how it will end. It is possible that the universe will last forever, or it may be crushed out of existence in a reverse of the Big Bang scenario, but that would be so far in the future that it might as well be infinite. Until recently, cosmologists (the scientists who study the universe) assumed that the rate of the universe’s expansion was slowing because of the effects of gravity. However, current research indicates that the universe may expand to eternity. But research continues and new studies of supernovae in remote galaxies and a force called dark energy may modify the possible fates of the universe.