Edwin Hubble and the Expanding Universe
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How Edwin Hubble Unlocked the Mind-Blowing Universe Beyond Ours

ByUtsav

In 1929, one astronomer looking through a telescope on a mountain outside Los Angeles changed what the universe was. Not what it looked like, or how big it seemed — what it fundamentally is. Before Edwin Hubble, most astronomers believed the Milky Way was the entire universe: a single island of stars floating in otherwise empty space. By the end of the 1920s, Hubble had shown that the Milky Way is just one galaxy among countless others, and that all of them are rushing away from each other as space itself stretches. The universe was not static and it was not alone. It was expanding, and it was enormous beyond anything astronomers had dared to calculate.

Also read: General Relativity and its Mind-Bending Predictions That Changed Our View of Space

That single realization — that space is expanding — is the thread connecting redshifted starlight, the Big Bang, and the modern picture of cosmic history. Here is how Hubble got there, what the evidence actually shows, and why astronomers are still refining his numbers a century later.

Edwin Hubble, 1931
Edwin Hubble, 1931

Edwin Hubble in 1931, two years after publishing the velocity-distance relation that now bears his name.

A Universe That Fit Inside One Galaxy

At the start of the twentieth century, the “spiral nebulae” visible through telescopes were a mystery. Were they nearby clouds of gas swirling within the Milky Way, or were they separate galaxies entirely, far outside it? The question mattered enormously, because the answer would determine the actual size of the universe. Through the 1910s and early 1920s, astronomers argued over it without a way to settle the debate — nobody could measure the distances involved.

Edwin Hubble was, on paper, an unlikely person to resolve it. Born in Missouri in 1889, he trained in mathematics and astronomy at the University of Chicago, then went to Oxford on a Rhodes Scholarship, where he earned a degree in jurisprudence to satisfy his father’s wishes. He briefly practiced law and taught high school before abandoning both to return to astronomy. After serving in World War I, Hubble joined the Mount Wilson Observatory in California, home to the 100-inch Hooker Telescope — at the time, the largest and most powerful telescope in the world. It was exactly the instrument the spiral-nebulae question needed.

The 100-inch Hooker Telescope at Mount Wilson Observatory
The 100-inch Hooker Telescope at Mount Wilson Observatory

The 100-inch telescope at Mount Wilson Observatory, the instrument Hubble used to resolve individual stars in the Andromeda nebula.

Finding a Yardstick Among the Stars

Hubble’s breakthrough depended on a discovery made over a decade earlier by Henrietta Leavitt, an astronomer at Harvard Observatory. Leavitt had studied a class of pulsating stars called Cepheid variables and found a precise relationship between how bright a Cepheid truly is and how fast it pulses. That relationship turned Cepheids into reliable “standard candles”: if you know a star’s true brightness and measure how dim it appears from Earth, you can calculate its distance.

Using the Hooker Telescope in 1923, Hubble spotted a faint, flickering point of light in the Andromeda nebula and identified it as a Cepheid variable. Applying Leavitt’s relationship, he calculated its distance — and the number was staggering. Andromeda was roughly a million light-years away, far beyond the known boundaries of the Milky Way. It was not a nearby gas cloud. It was an entire galaxy in its own right. Hubble found more Cepheids in other nebulae and reached the same conclusion each time: the universe was full of galaxies, and the Milky Way was just one of them.

The Clue Hidden in the Color of Light

Settling the size of the universe raised an even bigger question: were these galaxies just sitting there, or were they moving? The answer came from a technique called spectroscopy, which spreads a galaxy’s light into its component wavelengths, much like a prism splits sunlight into a rainbow. Within that spectrum, elements like hydrogen and calcium produce distinctive dark lines at very specific, well-known wavelengths.

When astronomers examined the spectra of distant galaxies, those familiar lines weren’t quite where they should have been. They were shifted toward the red end of the spectrum — toward longer wavelengths. This is called redshift, and it happens because space itself is stretching while light travels through it, elongating the light’s wavelength along the way. Vesto Slipher, working at Lowell Observatory, had already measured redshifts in dozens of spiral nebulae by the early 1910s and noticed that most seemed to be receding. What nobody had done yet was connect that recession to actual distance.

Diagram of cosmological redshift as light travels through expanding space
Diagram of cosmological redshift as light travels through expanding space

As light travels across an expanding universe, its wavelength stretches toward the red end of the spectrum — a phenomenon called cosmological redshift.

Hubble’s Law: Distance and Speed, Locked Together

Working with fellow Mount Wilson astronomer Milton Humason, Hubble combined two datasets: Slipher’s redshift measurements, extended and refined by Humason, and his own Cepheid-based distances to the same galaxies. When Hubble plotted velocity against distance for eighteen galaxies in his landmark 1929 paper, a clear pattern emerged — one so consistent it amounted to a law. The farther away a galaxy was, the faster it appeared to be receding from us, and the relationship was directly proportional. A galaxy twice as far away was moving away roughly twice as fast.

This is Hubble’s Law, and the constant of proportionality in it — the Hubble constant — became one of the most consequential numbers in all of science. It doesn’t just describe how fast individual galaxies are moving. It describes the rate at which space itself is expanding, everywhere, all the time. Crucially, this isn’t galaxies flying outward through some pre-existing void, the way debris flies from an explosion. It’s the space between galaxies stretching, carrying the galaxies along with it. Run that expansion backward in time, and everything in the universe converges toward a single point in the distant past.

Spectrum showing absorption lines shifted toward the red end
Spectrum showing absorption lines shifted toward the red end

The dark absorption lines in a galaxy’s spectrum (bottom) are shifted toward red relative to their normal position (top) — the signature astronomers use to measure cosmological redshift.

From an Expanding Universe to the Big Bang

Hubble’s observations landed in the middle of an already heated theoretical argument. When Albert Einstein applied general relativity to the universe as a whole in 1917, his equations implied the universe should be either expanding or contracting — a conclusion he disliked enough that he added a fudge factor, the cosmological constant, to force a static solution. Willem de Sitter, Alexander Friedmann, and Georges Lemaître each independently explored expanding-universe models anyway, treating Einstein’s original equations at face value. Lemaître in particular proposed, ahead of Hubble’s observational proof, that the universe had expanded from a highly concentrated initial state.

Hubble’s data gave that theoretical work an empirical foundation. If galaxies are receding from each other in every direction, and if the relationship between distance and speed is consistent, then the universe must have been smaller and denser in the past — all the way back to a single, extraordinarily hot and dense point roughly 13.8 billion years ago. That initial state, and its subsequent expansion, is what we now call the Big Bang. Around 380,000 years after that beginning, the universe cooled enough for electrons to bind to nuclei and form the first atoms, releasing the radiation we now detect as the cosmic microwave background — the oldest light in the universe, and one of the strongest pieces of evidence for the Big Bang model.

Illustration of the universe's expansion from the Big Bang to today
Illustration of the universe’s expansion from the Big Bang to today

The cosmic microwave background, mapped by the Planck satellite, shows tiny temperature variations from roughly 380,000 years after the Big Bang — the seeds from which galaxies eventually formed.

A Number Astronomers Have Spent a Century Refining

Hubble’s original 1929 estimate for the expansion rate was far too high — about 500 kilometers per second per megaparsec, which implied a universe younger than Earth itself, an obvious contradiction. The error traced back to miscalibrated Cepheid distances. Over the following decades, astronomers steadily revised the number downward as measurement techniques improved: Walter Baade’s recalibration in the 1950s cut it roughly in half, and by the 1970s and 1980s, rival teams were still divided between values near 50 and values near 100, unable to agree even on the size of the universe to within a factor of two.

The Hubble Space Telescope, launched in 1990 and named in Hubble’s honor, was built partly to resolve this exact dispute. One of its founding scientific goals was pinning down the Hubble constant with real precision. In 1999, after an eight-year effort observing Cepheid variables in eighteen galaxies out to 65 million light-years, the HST Key Project team announced a value of 70 kilometers per second per megaparsec, with an uncertainty of only about 10 percent — a dramatic improvement that placed the age of the universe at roughly 12 billion years, later refined further with additional data. Even now, different measurement methods — one based on the cosmic microwave background, another based on nearby Cepheids and supernovae — produce slightly different values for the Hubble constant, a persistent discrepancy astronomers call the “Hubble tension.” Resolving it may point toward new physics not yet accounted for in our models of the cosmos.

Spiral galaxy NGC 4603 with Cepheid variable stars used to measure cosmic distances
Spiral galaxy NGC 4603 with Cepheid variable stars used to measure cosmic distances

NGC 4603, the most distant galaxy in which Cepheid variables have been directly identified — one of the eighteen galaxies used to refine the Hubble constant in the HST Key Project.

Why This Still Matters

Hubble’s law didn’t just settle an argument about galaxy distances. It gave cosmology something it had never had before: a way to measure the universe rather than only theorize about it. Every subsequent advance in cosmology — the discovery that the universe’s expansion is accelerating, thanks to a mysterious force now called dark energy; the mapping of the cosmic microwave background; the use of gravitational lensing to weigh galaxy clusters; the James Webb Space Telescope’s ability to catch light from galaxies that formed just a few hundred million years after the Big Bang — builds on the basic method Hubble established: use redshift and distance together to reconstruct cosmic history.

It also reframed humanity’s place in the universe twice over in a single decade. First by showing that our galaxy is not unique, and then by showing that the universe itself has a beginning, an age, and a story that can, in principle, be measured.

What’s Still Unsettled

The Hubble tension remains genuinely unresolved — it isn’t yet clear whether it reflects subtle measurement errors, an incomplete cosmological model, or an entirely new ingredient in the universe’s makeup. Dark energy, the force apparently driving the acceleration Hubble’s method helped uncover decades later, still has no confirmed physical explanation. And while the Big Bang model is supported by multiple independent lines of evidence, the very first fraction of a second — the era of cosmic inflation, before the universe was even a second old — remains theoretical, constrained by indirect evidence rather than direct observation.

The Takeaway

Edwin Hubble didn’t set out to discover the Big Bang. He set out to answer a narrower question: are those faint spiral smudges in the sky part of our galaxy, or something else entirely? The answer he found — that they are separate galaxies, and that they are all fleeing from each other as space expands — turned out to be the single observation that made modern cosmology possible. A century later, astronomers are still pointing telescopes at the same basic question Hubble asked, just with far better instruments and far higher stakes: how fast is the universe expanding, and what does that tell us about how it began.

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  1. Pingback: The Discovery of Cosmic Microwave Background - Celestial Inquiry

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