The universe is getting bigger. Every moment, the vast distances that separate galaxies stretch a little more, carrying them farther apart in an expansion that began with the Big Bang. For nearly a century, astronomers have sought to measure the precise rate of this cosmic growth. The number they are chasing is the Hubble constant, one of the most fundamental parameters in all of cosmology. It’s a single value that tells us how fast the universe is expanding, which in turn helps us determine its age, its size, and its ultimate fate.
For decades, the quest to pin down the Hubble constant was a story of steady progress. Telescopes grew more powerful, measurement techniques became more refined, and the value slowly converged. But today, the story has taken a dramatic turn. The two best methods for measuring this cosmic expansion rate now give two different answers. This disagreement, known to cosmologists as the “Hubble tension,” isn’t a minor statistical fluke. It’s a significant discrepancy that has resisted all attempts at reconciliation. The tension suggests one of two possibilities: either there is a hidden, systematic error in the measurements that has eluded some of the world’s sharpest scientific minds, or the universe is governed by physics more exotic and stranger than we ever imagined. This is the story of the Hubble constant and the cosmic crisis it has created.
A Universe in Motion
At the dawn of the 20th century, the prevailing view was that the universe was static and eternal. It was assumed to be a serene, unchanging sea of stars, confined largely within our own Milky Way galaxy. The fuzzy, spiral-shaped patches of light seen through telescopes, then called “spiral nebulae,” were thought to be nascent solar systems forming within our own cosmic backyard. This comfortable picture began to unravel with the work of American astronomer Vesto Slipher.
Working at the Lowell Observatory in Arizona, Slipher spent years meticulously studying the light from these spiral nebulae. He used a technique called spectroscopy, which splits light into its constituent colors, creating a spectrum. Embedded within this spectrum are dark lines, which act like barcodes, indicating the presence of specific chemical elements. Slipher noticed something odd about these barcodes. For the vast majority of the nebulae he observed, the spectral lines were shifted toward the red end of the spectrum.
This phenomenon, known as redshift, is analogous to the Doppler effect for sound. We’re all familiar with how the pitch of an ambulance siren sounds higher as it approaches and lower as it recedes. The same principle applies to light. Light waves from an object moving away from an observer are stretched, increasing their wavelength and shifting them toward the red part of the spectrum. A shift toward the blue, or blueshift, would indicate an object is moving closer. Slipher’s discovery that nearly all spiral nebulae were redshifted meant that they were almost all moving away from us at incredible speeds. The static universe was an illusion.
Slipher had discovered the motion, but it was astronomer Edwin Hubble who provided the scale. Working at the Mount Wilson Observatory in California with its then world-leading 100-inch Hooker telescope, Hubble embarked on a mission to determine the nature of these spiral nebulae. He did this by hunting for a special type of star known as a Cepheid variable.
Earlier, at the Harvard College Observatory, astronomer Henrietta Swan Leavitt had made a momentous discovery. While studying Cepheid variable stars, she found a direct relationship between their pulsation period – the time it took for them to brighten, dim, and brighten again – and their intrinsic luminosity, or their true brightness. This meant that by simply timing the pulse of a Cepheid, one could determine how bright it really was. By comparing this intrinsic brightness to its apparent brightness as seen from Earth, astronomers could calculate its distance with remarkable accuracy. Cepheids became the first reliable “standard candle” for measuring cosmic distances.
In 1924, Hubble identified a Cepheid variable in the Andromeda Nebula. His calculations showed it was located nearly a million light-years away, far outside the bounds of the Milky Way. This single observation shattered the old view of the universe. Andromeda wasn’t a nearby gas cloud; it was an entire galaxy, a sprawling “island universe” just like our own. The universe was suddenly vastly larger and more populated than anyone had believed.
Hubble, along with his colleague Milton Humason, continued this work, measuring the distances to dozens of other galaxies using Cepheids. When Hubble plotted the distances to these galaxies against the recessional velocities measured by Slipher, he uncovered a stunningly simple relationship: the farther away a galaxy is, the faster it is moving away from us. This became known as Hubble’s Law. It’s the signature of a uniformly expanding universe. It’s important to understand that the galaxies themselves aren’t rocketing through space. Rather, the fabric of space itself is stretching, carrying the galaxies along with it. An analogy is to imagine baking a raisin loaf. As the dough rises, every raisin moves away from every other raisin. A raisin twice as far away will appear to move away twice as fast.
The rate of this expansion is encapsulated in a single number: the Hubble constant (H₀). It is the slope of the line on Hubble’s plot of distance versus velocity. Measuring its value with precision has been a central goal of cosmology ever since, as it is key to understanding the universe’s past and future.
Building the Cosmic Distance Ladder
Measuring the Hubble constant sounds straightforward in principle. You measure the distance to a galaxy and its redshift, and you divide the velocity by the distance. The difficulty lies in that first step: measuring cosmic distances. Beyond our immediate galactic neighborhood, distances are immensely difficult to determine. Astronomers have overcome this challenge by constructing what is known as the cosmic distance ladder, a series of overlapping measurement techniques where each “rung” is used to calibrate the next, more distant one.
The foundation of the entire ladder, its first and most important rung, is based on a simple geometric principle called parallax. You can see parallax for yourself by holding a finger out at arm’s length and closing one eye, then the other. Your finger will appear to shift its position against the distant background. By measuring this apparent shift and knowing the distance between your eyes, you can calculate the distance to your finger. Astronomers do the same thing with stars. They measure a star’s position against very distant background galaxies and then measure it again six months later, when the Earth is on the opposite side of its orbit around the Sun. The tiny shift in the star’s apparent position allows for a direct calculation of its distance.
For many years, parallax could only be used for the very nearest stars. The launch of the European Space Agency’s (ESA) Hipparcos satellite in 1989 and, more recently, its successor Gaia, has revolutionized this process. From their vantage point in space, free from the blurring effects of Earth’s atmosphere, these satellites have measured the parallaxes of over a billion stars with unprecedented precision, building a highly accurate 3D map of our local cosmic environment. This robust foundation is essential for calibrating the next rung of the ladder.
The second rung belongs to the Cepheid variables, the same standard candles Hubble used. The Gaia satellite has provided precise parallax distances to many Cepheids within our own Milky Way. This allows astronomers to fine-tune the period-luminosity relationship discovered by Leavitt. With this refined calibration, they can then look for Cepheids in nearby galaxies. The Hubble Space Telescope has been instrumental in this effort. Its sharp vision allows it to pick out individual Cepheid stars in galaxies tens of millions of light-years away. By observing their pulsation periods, astronomers can determine their distances and, in doing so, extend the reach of the cosmic distance ladder.
But even with the Hubble telescope, Cepheids become too faint to see in very distant galaxies. To probe the far reaches of the cosmos, astronomers need an even brighter standard candle. This brings us to the third and highest rung of the ladder: Type Ia supernovae. These are not just stellar explosions; they are cataclysmic, thermonuclear detonations of a specific kind of star called a white dwarf. A Type Ia supernova occurs when a white dwarf in a binary star system siphons material from its companion star. When the white dwarf’s mass reaches a precise, critical limit – about 1.4 times the mass of our Sun – it triggers a runaway nuclear fusion reaction that blows the star apart.
Because they all detonate at this same critical mass, Type Ia supernovae have a remarkably consistent peak luminosity. They are like cosmic light bulbs of a standard wattage. An astronomer seeing one of these explosions, even in a galaxy billions of light-years away, can measure its apparent brightness, compare it to its known intrinsic brightness, and calculate the distance to its host galaxy. They are so bright they can briefly outshine their entire host galaxy, making them visible across enormous cosmic distances.
The distance ladder is assembled by linking these rungs. First, parallax measurements from Gaia provide accurate distances to Cepheids in the Milky Way. Second, the Hubble Space Telescope observes these calibrated Cepheids in nearby galaxies that have also, by chance, hosted a Type Ia supernova. This allows for a precise calibration of the true brightness of Type Ia supernovae. Finally, astronomers use ground-based and space-based telescopes to scan the skies for these supernovae in the distant universe. By measuring the redshift of the supernova’s host galaxy and knowing its distance, they can calculate the Hubble constant.
A leading effort using this method is a project named SH0ES, led by Nobel laureate Adam Riess. Over years of meticulous work, the SH0ES team has refined every step of this process, hunting for errors and improving precision. Their result has consistently converged on a value of around 73 kilometers per second per megaparsec. This means that for every 3.26 million light-years farther a galaxy is from us, it appears to recede about 73 kilometers per second faster.
A Voice from the Early Universe
For many years, the distance ladder was the primary way to measure the Hubble constant. But in recent decades, a completely independent method has emerged, one that looks not at the “local” universe of today but at the “early” universe, just after the Big Bang. This method relies on analyzing the oldest light in the cosmos, the afterglow of creation itself: the Cosmic Microwave Background (CMB).
According to the Big Bang theory, for the first 380,000 years of its existence, the universe was an incredibly hot, dense, and opaque soup of subatomic particles and radiation. Light was trapped in this primordial plasma, unable to travel freely. As the universe expanded, it cooled. Once the temperature dropped sufficiently, protons and electrons could combine to form neutral hydrogen atoms in an event called recombination. Suddenly, the universe became transparent, and the light that had been trapped was released to travel unimpeded through space.
Today, billions of years later, that ancient light fills the entire sky. The expansion of the universe has stretched its wavelengths so much that it is no longer visible light but is instead detected in the microwave portion of the electromagnetic spectrum. This is the Cosmic Microwave Background. It is an astonishingly uniform faint glow, with a temperature of just 2.73 degrees above absolute zero.
But it’s not perfectly uniform. In the 1990s, NASA’s COBE satellite discovered minuscule temperature variations, or anisotropies, in the CMB. These are tiny ripples, differences in temperature of only about one part in 100,000. Subsequent missions, including NASA’s WMAP and ESA’s Planck satellite, have mapped these ripples with exquisite detail.
These tiny temperature fluctuations are incredibly important. They are the imprints of slight density variations in the primordial soup of the early universe. Regions that were slightly denser had slightly stronger gravity, making them slightly hotter. These primordial seeds would later grow, through gravitational collapse, into the vast clusters of galaxies we see today. The pattern of these hot and cold spots in the CMB contains a wealth of information about the conditions of the early universe.
Scientists can analyze this pattern using what is known as the Lambda-CDM model (ΛCDM), the standard model of cosmology. This model is built on Albert Einstein‘s theory of General Relativity and posits that the universe is made of just a few key ingredients: normal matter (the stuff that makes up stars, planets, and us), dark matter (an unseen substance whose gravity holds galaxies together), and dark energy (a mysterious force causing the expansion of the universe to accelerate).
By feeding the precise pattern of fluctuations seen in the Planck data into the ΛCDM model, cosmologists can run the clock forward. The model, governed by the known laws of physics, predicts how a universe with those specific initial conditions should have evolved over 13.8 billion years. This simulation produces a picture of the universe today, including a prediction for what the Hubble constant should be. The value derived from this method is stunningly precise: about 67.4 kilometers per second per megaparsec.
A Tale of Two Numbers
Herein lies the crisis. The SH0ES team’s measurement from the local, “late” universe, using the cosmic distance ladder, gives a value of about 73. The Planck satellite’s prediction from the “early” universe, using the CMB and our best model of cosmology, gives a value of about 67.
At first, this might not seem like a huge difference. But the precision of both measurements is now so high that their uncertainty ranges do not overlap. This isn’t just a case of measurement error that might be resolved with more data. The two most reliable methods for determining the expansion rate of the universe give fundamentally different answers. The probability that this is a random statistical fluke is less than one in a million. The universe appears to be expanding about 9% faster today than our best model of the cosmos, based on its initial conditions, predicts it should be.
This discrepancy, the Hubble tension, has forced the scientific community to reconsider its assumptions. The first and most mundane possibility is that there is a subtle, unknown systematic error in one or both of the measurements. Every rung of the distance ladder is being scrutinized. Could our understanding of Cepheid variable physics be incomplete? Do they behave differently in different galactic environments? Is there some undiscovered variation in Type Ia supernovae? Astronomers are searching for these potential flaws, but so far, the distance ladder appears solid. Similarly, cosmologists are re-examining the Planck data and the complex analysis used to interpret it. It is an incredibly intricate process, and a hidden effect could be throwing off the result. But to date, no one has found a convincing error.
If the measurements are correct, then the problem must lie with the theory that connects them: the ΛCDM model. The Hubble tension could be the first significant crack in the standard model of cosmology, a hint that it is incomplete. This is a tantalizing prospect, as it could point the way toward “new physics.”
What kind of new physics might resolve the tension? The discrepancy is between a prediction based on the early universe and a measurement of the late universe. This suggests that something may have happened in the intervening cosmic history that isn’t accounted for in the standard model. One popular idea is the existence of “early dark energy.” This would be a new form of energy that was present in the first few hundred thousand years after the Big Bang. It would have given the universe an extra “kick,” causing it to expand faster than expected. This early dark energy would then have needed to decay away quickly, leaving no trace today other than a mismatch in the Hubble constant.
Other possibilities are even more exotic. Perhaps dark matter or dark energy have properties we don’t understand. Maybe they interact with each other or with normal matter in some unknown way. Some have even proposed that our theory of gravity, General Relativity, may need to be modified on cosmological scales. Another possibility is the existence of a new, undiscovered subatomic particle, such as a “sterile neutrino,” that would have affected the expansion rate in the universe’s youth.
The Tie-Breakers
With the two leading methods at an impasse, scientists are turning to a host of independent techniques to measure the Hubble constant, hoping one can act as a tie-breaker.
One of the most promising new methods involves using gravitational lensing. According to General Relativity, the mass of a large galaxy can warp the spacetime around it, acting like a giant cosmic lens. If a distant, bright object like a quasar lies directly behind this galaxy from our point of view, its light can be bent and magnified, creating multiple images of the same quasar. Because the light for each of these images travels a slightly different path through the warped spacetime, they arrive at Earth at slightly different times. Quasars flicker in brightness, so by monitoring these multiple images, astronomers can measure the time delay between the flickers. This time delay, combined with a model of how mass is distributed in the lensing galaxy, allows for a direct geometric calculation of the distance, which can then be used to determine the Hubble constant. Projects like H0LiCOW have used this technique, and their results have tended to align more closely with the distance ladder value of ~73, adding to the mystery.
Another completely novel approach has been made possible by the dawn of gravitational wave astronomy. In 2017, the LIGO and Virgo observatories detected gravitational waves – ripples in spacetime – from the collision of two neutron stars. Crucially, telescopes also saw the flash of light from this cataclysmic event. The gravitational waves themselves provided a direct measurement of the distance to the collision, independent of any standard candles. The light from the explosion, meanwhile, gave astronomers the galaxy’s redshift. With both distance and velocity, they could calculate the Hubble constant. These events, dubbed “standard sirens,” are still rare, so the measurement is not yet as precise as the other methods. But as more are detected, they promise to provide a powerful and entirely independent check.
Other methods are also being developed. One technique, called the Tip of the Red Giant Branch (TRGB), uses a different type of star as a standard candle. Red giant stars, at a certain point in their evolution, reach a very specific and consistent peak brightness. This method is being used by some teams and it has yielded a value for the Hubble constant that falls somewhere in between the Planck and SH0ES results, further complicating the picture.
Summary
The Hubble constant is a measure of how fast our universe is expanding. For decades, astronomers have refined their techniques to measure it, seeking a single, definitive value. But today, we are faced with a significant puzzle. Measurements of the expansion rate today, using a meticulously constructed cosmic distance ladder of stars and supernovae, give a value of about 73. A prediction of the expansion rate, based on the afterglow of the Big Bang and our standard model of cosmology, give a value of about 67. The two results are incompatible.
This Hubble tension represents one of the biggest challenges in cosmology. It forces us to question our methods and our models. The resolution could come from the discovery of a subtle error in our observations of the cosmos. Or, more excitingly, it could be telling us that our fundamental understanding of the universe is incomplete. The quest to solve this cosmic conundrum is pushing astronomers and physicists to develop new observational techniques and to imagine new theories of the universe’s evolution. Finding the answer will undoubtedly mark a major milestone in our long journey to understand the cosmos and our place within it.
