Imagine peering into the night sky with a powerful telescope and seeing galaxies scattered across the cosmic landscape. Some are so distant that their light has been travelling for over 13 billion years just to reach us.
This incredible fact tells us something profound about our universe: it began as a very small, unimaginably hot and dense region, and has been expanding and cooling ever since. This article aims to explain why our universe expands, how we observe its past and present simultaneously, and why it’s not simply growing “into” something else.
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Space is expanding throughout the universe - (Credit: Dark Energy Survey/DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA - R. Colombari & M. Zamani via NOIRLab)
A Universe in a Grapefruit
It might sound astonishing, but the visible universe—now so big that its farthest galaxies lie more than 13 billion light-years away—was once packed into a volume roughly the size of a grapefruit. At the very beginning, all matter and energy were crammed together, resulting in extremely high temperatures and densities.
Related article: How big is the universe? (Universal-Sci)
Over the past 13.8 billion years (approximately), the universe has expanded, becoming vast and “thin.” Today, if you take any average cubic meter of space in the universe, it contains only about six atoms—a far emptier space than any vacuum we can create here on Earth. By contrast, a cubic meter of air on Earth contains on the order of 102510^{25}1025 atoms. This gives us a sense of how rarefied the cosmos has become.
Expansion Means Cooling
When you compress a gas, it heats up; when it expands, it cools. We experience a small version of this when using a bicycle pump: rapid pumping warms the base of the cylinder due to the compression of air. In the early universe, when everything was extremely compressed, temperatures were unimaginably high.
As space expanded, everything spread out and cooled. Nowadays, the universe is not only huge but also cold in its average temperature (just a few degrees above absolute zero), though stars and galaxies form “clumps” of matter pulled together by gravity.
Why There Are Clumps and Clusters
Gravity has played a crucial role in shaping the large-scale structure of the cosmos. As the universe expanded, tiny variations in density became amplified by gravitational attraction.
A detailed image of the entire sky, made using nine years of WMAP data, shows temperature changes from 13.77 billion years ago (seen as color differences). These small changes were the starting points that eventually grew into galaxies.
(Image Credit: NASA / WMAP Science Team via Wikimedia Commons)
Matter eventually clumped together in certain regions, forming stars, galaxies, and galaxy clusters, while other regions grew emptier. This is why the universe is not uniformly smooth but instead shows vast cosmic structures, like web-like patterns of galaxies separated by enormous voids.
Interesting article: How long has gravity existed? (Universal-Sci)
The Balloon Analogy: Expanding “Into” Nothing
It’s tempting to imagine the universe as an explosion within a pre-existing space, with galaxies as “shrapnel” flying outward. But that picture is misleading. In fact, the universe creates more space as it expands—it’s not exploding into something else. An often-used analogy is to picture the surface of a balloon.
If you draw dots on the balloon (representing galaxies) and then inflate it, you’ll notice the dots move away from each other as the balloon expands.
Crucially, they are not moving through space on the balloon’s surface; rather, the surface itself is stretching.
In the real universe—which has three dimensions of space, plus time—new space is continuously being “made” between galaxies.
Because more space appears between galaxies, those that lie far apart move away from each other faster than those that are relatively close. This explains why we see more distant galaxies receding at higher velocities, a relationship famously described by Hubble’s Law.
Seeing the Past and the Present Together
When we observe distant galaxies, we look back in time. Light takes time to travel, so a galaxy 10 billion light-years away appears to us as it was 10 billion years ago, not how it is “right now.” Meanwhile, galaxies much closer to us are seen more or less as they are today.
As an example, this is the Southern Pinwheel Galaxy, located about 15 million light-years away. We are seeing it today as it appeared 15 million years ago
(Image Credit: ESO/IDA/Danish 1.5 m/R. Gendler, S. Guisard and C. Thöne)
When astronomers use powerful telescopes—like the Hubble Space Telescope—to photograph thousands of galaxies in a single field of view, they capture galaxies at many different distances and hence many different stages of cosmic history, all in one image. Larger, brighter galaxies in the image are typically closer and more “current,” whereas faint smudges could be galaxies as they looked billions of years ago.
In other words, an image of the distant universe is a complex tapestry that includes both the ancient past and more recent epochs simultaneously.
Interesting article: How do astronomers know the age of the planets and stars? (Universal-Sci)
The Big Bang: A Moment, Not a Place
A key insight is that the Big Bang was not an explosion at a single point in space; rather, it was the moment in time when the universe began. Every point in today’s cosmos, including the place where you are now, was once part of that incredibly dense, hot beginning. It wasn’t “somewhere else”; it was everywhere, all at once.
Since that moment—some 13.8 billion years ago—the universe has been expanding through time, creating more and more space in the process. That is why modern cosmology describes the Big Bang as the start of both time and the expansion of space, rather than an explosion in a pre-existing void.
Today, astronomers continue to probe deeper into the universe’s early history, capturing images that reveal how structures formed over billions of years. Each new discovery brings us closer to understanding our cosmic origins—and, by extension, our own place in a universe that is still growing, moment by moment, into the future.
Sources and further reading:
How big is the universe? (Universal-Sci)
Scientists precisely measure total amount of matter in the universe (University of California, Riverside)
Prof. Dr. Ralph Wijers (Universiteit van Nederland)
It's cold in the cosmos! This is how astronomers arrive at determining the temperature of space (Sky At Night/BBC)
How long has gravity existed? (Universal-Sci)
How do astronomers know the age of the planets and stars? (Universal-Sci)
Let there be light: What happened in the Early Universe? (ESO)
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