What Do We Really Mean When We Say the Universe Is Expanding?

Hi Hello! I know it’s been a while since I wrote anything meaningful, but better late than never.

Well then, “What Do We Really Mean When We Say the Universe Is Expanding?” is our question of the day and I will try to put my understandings based on various sources in simple terms.

Abh Shuru se Shuruwat karte hai!

A long, long time ago, around 13.8 billion years ago, the universe began to exist- as a singularity. A point of infinite density and temperature. In a fraction of a second, it expanded and cooled, allowing energy to become matter: first subatomic particles, then atoms. Gravity pulled this matter together to form stars, planets, galaxies- everything we see in the skies today. Over a period of time, the expansion slowed down but never stopped, and even today, the universe is expanding.

Studies believe the “driving force” behind this big bang, and now the expansion of the universe, is a mysterious force we call dark matter—but that is another topic for another day.

But how did we decide it’s expanding in the first place, right? Edwin Hubble, the man after whom the famous NASA telescope was named, had discovered something, something important. The lights of the celestial bodies as observed from the telescope were dim-bright at specific timings. That helped in calculating their distance from us, the first cue towards the expansion of the universe.

Not just that, he observed the spectral lines were symmetrically shifted- the light from the distant galaxies was stretched towards the red end of the spectrum. Greater the distance, greater the shift- The Redshift, as we call it today.

This sounds like the Doppler effect, doesn’t it? Like how an ambulance sounds higher pitched as it approaches and lower as it moves away? But that’s due to relative motion. In our case, it’s not just the source moving- The Space itself is expanding, like a stretched rubber band.

Now that we know it is expanding and all, what does it expand into anyway? You blow air into a balloon, it occupies surrounding space and expands. So similarly, what does the universe “devour”?

(Spoiler: The answer is quite underwhelming)
But for now, imagine yourself as an ant walking on the surface of a balloon, marked with tiny dots. As air is blown in, the balloon expands, and from the ant’s 2D perspective, all the dots seem to drift farther and farther away. The poor ant is left wondering, “Where is everything going?!”
Only when it steps out and sees the balloon from our point of view does it realize it’s not just a flat world- it’s part of a 3D object expanding in a higher dimension. So, is our universe doing the same? Expanding into a fourth dimension? Maybe even time, as suggested by movies?

Well, sadly for you and me both, that’s just speculation without any backing for now.

To put it simply: the universe “expands” into itself- weird, right? I know. But that’s genuinely how scientists describe it. Think of a yeast dough rising: as it ferments, it grows. Not by moving into empty space, but by stretching and expanding within itself, changing the distances between particles of dough. That’s the best comparison I can think of. Dough, or even slime- that’s just how the universe keeps expanding. Not into anything, yet somehow, growing all the same.

In fact, space can even bend, twist, and turn within itself; the spacetime fabric. We have The Schwarzschild solution which describes how space curves around still, non-rotating objects, while the Kerr metric adds rotation and twists in spacetime -especially near black holes.

But remember, these describe local situations and not the expansion of space itself.

When we talk about expansion, it’s more like rolling out a patterned blanket on a bed. As the blanket stretches, the printed dots or flowers move away from each other- not because they’re moving, but because the fabric beneath them is growing. That is what’s happening to us, and to galaxies across the universe. We’re not flying apart through space- The Space itself is stretching between us.

There’s no edge, no center, nothing outside it. The fabric of space, once tightly packed, continues to stretch with time- becoming more of what it already is.

It is what there always was, and always will be.