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Starts With A Bang

Space Wasn’t Always A Big Place

The expanding Universe, full of galaxies and the complex structure we observe today, arose from a smaller, hotter, denser, more uniform state. But even that initial state had its origins, with cosmic inflation as the leading candidate for where that all came from. (C. FAUCHER-GIGUÈRE, A. LIDZ, AND L. HERNQUIST, SCIENCE 319, 5859 (47))

Today, our observable Universe extends for 46 billion light-years in all directions. But early on, things were much smaller.


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 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.

The distant Universe, as viewed here through the plane of the Milky Way, consists of stars and galaxies, as well as opaque gas and dust, going back as far as we can see. But we know we aren’t seeing it all, no matter how we look. (TWO MICRON ALL SKY SURVEY (2MASS))

If we look at the Universe today, there’s no denying the enormity of its scale. Containing somewhere in the neighborhood of 400 billion stars, our Milky Way galaxy stretches for over 100,000 light-years in diameter. The distances between the stars is enormous, with the closest star to our Sun (Proxima Centauri) located some 4.24 light-years away: over 40 trillion kilometers distant.

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.

The Universe is an amazing place, and the way it came to be today is something very much worth being thankful for. Although our most spectacular pictures of space are rich in galaxies, the majority of the volume of the Universe is devoid of matter, galaxies, and light entirely. (NASA, ESA, HUBBLE HERITAGE TEAM (STSCI / AURA); J. BLAKESLEE)

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.

The growth of the cosmic web and the large-scale structure in the Universe, shown here with the expansion itself scaled out, results in the Universe becoming more clustered and clumpier as time goes on. Initially small density fluctuations will grow to form a cosmic web with great voids separating them, but what appear to be the largest wall-like and supercluster-like structures may not be true, bound structures after all. (VOLKER SPRINGEL)

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.

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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.

A map of more than one milion galaxies in the Universe, where each dot is its own galaxy. The various colors represent distances, with redder representing farther away. Despite what you might assume from this image, most of the Universe is empty, intergalactic space. (DANIEL EISENSTEIN AND THE SDSS-III COLLABORATION)

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, 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).

A selection of some of the most distant galaxies in the observable Universe, from the Hubble Ultra Deep Field. When we observe the Universe at great distances, we’re seeing it as it was in the distant past: smaller, denser, hotter, and less evolved. (NASA, ESA, AND N. PIRZKAL (EUROPEAN SPACE AGENCY/STSCI))

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.

The first stars in the Universe will be surrounded by neutral atoms of (mostly) hydrogen gas, which absorbs the starlight. The hydrogen makes the Universe opaque to visible, ultraviolet, and a large fraction of near-infrared light, but longer wavelengths may yet be observable and visible to near-future observatories. The temperature during this time was not 3K, but hot enough to boil liquid nitrogen, and the Universe was tens of thousands of times denser than it is today on the large-scale average. (NICOLE RAGER FULLER / NATIONAL SCIENCE FOUNDATION)

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.

At the high temperatures achieved in the very young Universe, not only can particles and photons be spontaneously created, given enough energy, but also antiparticles and unstable particles as well, resulting in a primordial particle-and-antiparticle soup. Yet even with these conditions, only a few specific states, or particles, can emerge, and by the time a few seconds have passed, the Universe is much larger than it was in the earliest stages. (BROOKHAVEN NATIONAL LABORATORY)

It means that if we went back to a time where the Universe was approximately a decade old, ten years after the Big Bang 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, 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, 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’s early stages, was smaller than the size of our Solar System.

The size of the Universe, in light years, versus the amount of time that’s passed since the Big Bang. This is presented on a logarithmic scale, with a number of momentous events annotated for clarity. This only applies to the observable Universe. (E. SIEGEL)

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.

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: 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: around 20 centimeters (8″), or around the size of a soccer ball.

Blue and red lines represent a “traditional” Big Bang scenario, where everything starts at time t=0, including spacetime itself. But in an inflationary scenario (yellow), we never reach a singularity, where space goes to a singular state; instead, it can only get arbitrarily small in the past, while time continues to go backwards forever. Only the last minuscule fraction of a second, from the end of inflation, imprints itself on our observable Universe today. The size of our now-observable Universe at the end of inflation must have been at least the size of a soccer ball, no smaller. (E. SIEGEL)

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 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.


Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.

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