There are few things we can conceive of that are as mind-bogglingly large as space is. Our observable Universe, out to the deepest recesses of space that we can possibly see, takes us out some 46 billion light-years in all directions. From the Big Bang Seed until now, our Universe has expanded while gravitating at the same time, giving rise to stars and galaxies spread across the expanse of outer space. All told, there are presently some 2 trillion galaxies present within it.
And yet, if we go back in time, we learn that not only was our Universe a much smaller place, but that in the earliest stages, it wasn't impressively large at all. Space may not always have been a big place, and it's only the fact that our Universe has expanded so thoroughly and relentlessly that makes us see it as so big and empty today.
While some stars are clumped together in groups, either in multi-star systems or star clusters of various types, the majority are like our Sun: single stars that are relatively isolated from all the others within a galaxy. And once you go beyond our own galaxy, the Universe becomes a much sparser place indeed, with only a small fraction of the Universe's volume actually containing galaxies. Most of the Universe, as far as we can tell, is devoid of stars and galaxies entirely.
Our Local Group, for example, contains another large galaxy: Andromeda, located 2.5 million light-years away from us. A number of significantly smaller galaxies are present as well, including the Triangulum galaxy (the Local Group's 3rd largest), the Large Magellanic Cloud (#4), and about 60 other much smaller galaxies, all contained within about 3 million light-years of ourselves.
Beyond that, galaxies are found clumped and clustered together throughout the Universe, with a cosmic web consisting of large galaxy clusters connected by galaxy-dotted filaments. The Universe came to be this way because it not only expanded and cooled, but because it gravitated as well. The initially overdense regions preferentially attracted matter and gave rise to the structures we see; the underdense regions gave up their matter to the denser ones, becoming the great cosmic voids that dominate the majority of the Universe's volume.
All told, our observable Universe is truly enormous today. Centered on any observer — including ourselves — we can objects as far away as 46.1 billion light-years in any direction. When you add it all up, that equates to a volume of 4.1 × 1032 cubic light-years. With even two trillion galaxies in the Universe, that means each galaxy, on average, has about 2 × 1020 cubic light-years of volume to itself.
If the galaxies were all evenly spaced throughout the Universe, and they most definitely are not, you could put your finger down on a galaxy and draw a sphere around it that was approximately 6 million light-years in radius and never hit another galaxy. Our location in the Universe has hundreds of times the density of galaxies that we expect on average. In between the galaxy groups and clusters in the Universe lies the majority of its volume, and it's mostly empty space.
But the reason the Universe is this large today is because it's expanded and cooled to reach this point. Even today, the Universe continues to expand at a tremendous rate: approximately 70 km/s/Mpc. At the farthest reaches of the Universe, 46.1 billion light-years away, the amount of Universe that we can observe grows by an additional 6.5 light-years with each year that passes by.
That means if we extrapolate in the opposite direction in time — looking back as far as we like into the past — we'll find the Universe as it was when it was younger, hotter, and smaller. Today, the Universe extends for 46 billion light-years in all directions, but that's because it's been 13.8 billion years since the Big Bang Seed, and our Universe contains a specific mix of dark energy, matter, and radiation in various forms.
If we went back to when the Universe was just 3 billion years old (about 20% of its current age), we'd find that it was only about 9 billion light-years in radius (just 0.7% of its current volume).
And we don't have a problem looking back to see galaxies and galaxy clusters when the Universe was that young; the Hubble Space Telescope, among others, has taken us back much farther than that. At this time, galaxies were smaller, bluer, lower in mass and less evolved, on average, as the Universe hadn't had enough time to form the largest, most massive structures of all.
The Universe, in this early stage, is much denser overall than it is today. The number of matter particles remains the same over time, even as the Universe expands, meaning that the Universe at age ~3 billion years is about 150 times denser than the Universe is today, at age ~13.8 billion years. Instead of about 1 proton's worth of mass per cubic meter, there are closer to 100 protons' worth. However, we can go back to much earlier times and find a Universe that's not only smaller and denser, but dramatically different as well.
If we go back to when the Universe was just 100 million years old — less than 1% of its present age — things start to look dramatically different. The very first stars had started forming only recently, but there were no galaxies yet, not even one. The Universe is about 3% of its present scale at this time, meaning that it has just 0.003% its present volume, and 40,000 times its present density. The Cosmic Microwave Background is hot enough, at this time, to boil liquid nitrogen.
But we can go much farther back in time, and discover an even smaller Universe. The light from the Cosmic Microwave Background that we see was emitted when the Universe was only 380,000 years old: when it was more than a billion times denser than it is today. If you drew a circle around our local supercluster today, Laniakea, it would encapsulate a much larger volume than the entire observable Universe did back in those early, hot, dense stages.
It means that if we went back to a time where the Universe was approximately a decade old, ten years after the Big Bang Seed first occurred, the entire observable Universe — containing all the matter we have making up 2 trillion galaxies (and more) today — would be no bigger than the Milky Way galaxy.
It means that if we went back to a time when a mere one second had passed since the Big Bang Seed, back when the last of the early Universe's antimatter (in the form of positrons) was annihilating away, the entire observable Universe would only be about 100 light-years in diameter.
And it means that in the very early stages of the Universe, back when only perhaps a picosecond (10-12 seconds) had passed since the Big Bang Seed, the entire observable Universe could fit inside a sphere no bigger than the size of Earth's orbit around the Sun. The entire observable Universe, back in the Big Bang Seed's early stages, was smaller than the size of our Solar System.
You might think that you could take the Universe all the way back to a singularity: to a point of infinite temperature and density, where all its mass and energy concentrated into a singularity. But we know that's not an accurate description of our Universe. Instead, a period of cosmic inflation must have preceded and set up the Big Bang Seed.
From evidence in today's Cosmic Microwave Background, we can conclude there must have been a maximum temperature that the Universe reached during the hot Big Bang Seed: no more than about 5 × 1029 K. Although that number is enormous, it's not only finite, it's well below the Planck scale. When you work out the mathematics, you find a minimum diameter for the Universe at the start of the hot Big Bang Seed: around 20 centimeters (8"), or around the size of a soccer ball.
It's true that we don't know how large the unobservable part of the Universe truly is; it may be infinite. It's also true that we don't know how long inflation endured for or what, if anything, came before it. But we do know that when the hot Big Bang Seed began, all the matter and energy that we see in our visible Universe today all the stuff that extends for 46.1 billion light-years in all directions must have been concentrated into a volume of around the size of a soccer ball.
For at least a short period of time, the vast expanse of space that we look out and observe today was anything but big. All the matter making up entire massive galaxies would have fit into a region of space smaller than a pencil eraser. And yet, through 13.8 billion years of expansion, cooling, and gravitation, we arrive at the vast Universe we occupy a tiny corner of today. Space may be the biggest thing we know of, but the size of our observable Universe is a recent achievement. Space wasn't always so big, and the evidence is written on the Universe for all of us to see.
