970x125
On a crisp night in the late 1920s, Edwin Hubble stood in the dome of the 100-inch Hooker telescope at Mount Wilson Observatory, high above the smog and streetlamps of Los Angeles. Through that giant eye, he measured the light from distant “spiral nebulae” — what we now call galaxies — and found something remarkable. Their light was shifted toward the red end of the spectrum, a sign that they were racing away from us. It was as if the universe itself were stretching.
970x125
When light from a moving source is stretched to longer wavelengths, we call it redshift — much like the way a passing train’s whistle drops in pitch as it moves away. Hubble discovered that the farther a galaxy was, the greater its redshift — meaning the faster it was receding. This became the Hubble–Lemaître law, a simple but revolutionary equation showing that the universe is expanding.
But here’s the subtlety: the galaxies are not flying through space as bullets through the air. Instead, the space between them is stretching.
A common analogy is raisin bread dough rising in the oven — as the dough expands, every raisin moves away from every other raisin, and the farther apart two raisins start, the faster they separate. Crucially, the bread isn’t expanding into the kitchen; the dough itself is the “space.” In the same way, the universe isn’t expanding into some empty void — it’s the distance scale itself that’s growing. This is why galaxies farther away show greater redshift: they’re not just distant in space, they’re distant in time, and the intervening space has been stretching for billions of years.
The implication was staggering: if the galaxies are all moving apart today, then in the distant past, they must have been much closer together. Follow this logic far enough back and you arrive at a moment when all the matter, energy, space, and time we know were compressed into a single, unimaginably dense point.
The birth of the Big Bang idea
The first to put this into words was Georges Lemaître, a Belgian priest and physicist. In 1931, he proposed that the universe began from a “primeval atom” — an idea that would later be nicknamed the Big Bang. At the time, the name was meant to be dismissive; British astronomer Fred Hoyle, along with his student and celebrated Indian astrophysicist Jayant Narlikar, champions of the rival Steady State theory, coined it in a radio broadcast to mock the idea of a cosmic explosion. Ironically, the label stuck and became the most famous phrase in cosmology.
The smoking gun in New Jersey
For decades, the debate raged: was the universe eternal and unchanging, or did it have a beginning? The tie was broken not in an ivory tower, but in a New Jersey field. In 1964, Arno Penzias and Robert Wilson, two engineers at Bell Labs, were testing a radio antenna for satellite communications when they picked up a persistent hiss of microwave noise. They cleaned the antenna, even shooed away nesting pigeons — but the signal stayed.
Story continues below this ad
Unbeknownst to them, just 50 km away, Princeton physicist Robert Dicke and his team were preparing to search for the faint afterglow of the Big Bang. When the groups connected, the truth emerged: Penzias and Wilson had stumbled upon the cosmic microwave background (CMB), the fossil light from the universe’s infancy, released about 380,000 years after the Big Bang.
A timeline of everything
The CMB confirmed that the universe had indeed begun in a hot, dense state and has been cooling and expanding ever since. In the first fraction of a second, an incredible burst of inflation stretched space faster than the speed of light. This expansion wasn’t into anything — rather, the very fabric of space itself was stretching, carrying galaxies along with it. As space grows, so does the distance scale we use to measure it: a galaxy whose light left billions of years ago was much closer then than it is today. That’s why the farther away we look, the greater the redshift we see — we are peering not just across space, but back in time, to when the universe was smaller.
The CMB is the afterglow from a time about 380,000 years after the Big Bang, when the universe had cooled enough for electrons and protons to form neutral atoms, letting light travel freely for the first time. That light has been on a 13.8-billion-year journey to us, its wavelength stretched by cosmic expansion from the fierce glare of the early universe into the faint microwave glow we detect today.
Over the next minutes after the Big Bang, nuclear fusion forged the first elements: hydrogen, helium, and traces of lithium. Hundreds of millions of years later, the first stars and galaxies ignited, manufacturing heavier elements in their cores and seeding the cosmos with the building blocks of planets and life. Billions of years on, our Sun and Earth formed from recycled stardust, and here we are — creatures of carbon, contemplating the birth of time.
Story continues below this ad
Why it matters
The Big Bang theory is not just an origin story; it’s a framework that explains everything from the cosmic web of galaxies to the faintest ripples in the CMB. It predicts the abundance of light elements, the distribution of galaxies, and the universe’s large-scale geometry. Without it, we’d have no coherent picture of cosmic history.
Our expanding perspective
Today, the expansion first seen by Hubble is still ongoing — in fact, it’s accelerating, driven by the mysterious dark energy. The latest measurements from telescopes like Hubble’s successor, the James Webb Space Telescope, and surveys like the Sloan Digital Sky Survey continue to refine our understanding of the early universe, probing the first galaxies that emerged from cosmic darkness.
The journey from a lone astronomer squinting at galaxies to a global scientific collaboration mapping the cosmos is a reminder that big ideas often start with small clues. As Carl Sagan once put it, “We are a way for the cosmos to know itself.” The Big Bang is not just about how the universe began — it’s about how we began, too.
Shravan Hanasoge is an astrophysicist at the Tata Institute of Fundamental Research.
970x125
