Have you ever looked up at the night sky and wondered not just how big the universe is, but how fast it's growing? It’s one of the most fundamental questions we can ask about our cosmos. For decades, we thought we were getting closer to a definitive answer. But right now, as of June 2025, cosmology is facing a profound and thrilling predicament—a deep disagreement that some top scientists are calling a "cosmic conflict" and even a "crisis."
Welcome! This article was written especially for you by us here at FreeAstroScience.com, where we believe in making the grand, complex ideas of the universe accessible to everyone. We're going to unpack a puzzle known as the "Hubble Tension." It’s a high-stakes scientific detective story where the clues are written in the light of distant stars and the afterglow of the Big Bang itself. The world's most powerful telescopes, including the magnificent James Webb Space Telescope (JWST), are at the heart of this investigation, and the latest data is fanning the flames of the debate.
We invite you to join us on this journey. By the end, you'll understand why this isn't just an argument over numbers, but a debate that could fundamentally change our understanding of the universe.
Hubble constant contradictions Scientists have made new calculations of the speed at which the universe is expanding, using data taken by the James Webb Space Telescope (JWST) on multiple galaxies. Above, JWST’s image of one such galaxy, known as NGC 1365. (Courtesy: NASA, ESA, CSA, Janice Lee (NOIRLab), Alyssa Pagan (STScI))
What Exactly Is the "Hubble Tension"?
Imagine you have two of your most trusted friends, both expert navigators, giving you the speed limit for the universe.
- One friend studies the "early universe." They've analyzed the faint, ancient light left over from the Big Bang—the Cosmic Microwave Background (CMB). Based on this primordial echo and our best theory of the cosmos, the Standard Model of Cosmology (ΛCDM), they confidently tell you the universe is expanding at a rate of about 67.4 kilometers per second per megaparsec.
- Your other friend studies the "local universe." They build what's called the cosmic distance ladder, measuring the distances to stars and galaxies closer to us. Using pulsating stars and stellar explosions as mile markers, they are just as confident the expansion rate is faster, with the vast majority of measurements clustering around 73 km/s/Mpc.
Both are brilliant. Both have used the best tools available. Yet, their answers don't match. This bewildering discrepancy is the Hubble Tension.
That strange unit, km/s/Mpc, simply means that for every megaparsec (about 3.26 million light-years) of distance from us, the fabric of space itself is stretching faster by that many kilometers every second. The gap between 67.4 and 73 might not seem huge, but in the precise world of cosmology, it's a chasm. It tells us that either someone's measurements are off due to a hidden error, what we call a systematic uncertainty, or our entire map of the cosmos—the ΛCDM model—is wrong and we need new physics.
How Is the James Webb Telescope Changing the Game?
Enter the game-changer: the James Webb Space Telescope (JWST). With its incredible infrared vision and unparalleled resolution, we can see stars with a clarity we've only dreamed of. The hope was that JWST could refine our measurements and finally tell us which navigator is right. However, as a flood of new data in 2025 has shown, JWST hasn't ended the debate; it has made it even more interesting by providing sharp, compelling evidence for both sides.
Different teams of scientists are using JWST to look at different types of "standard candles"—celestial objects with a known intrinsic brightness that allow us to calculate their distance.
Team "Settle Down": The TRGB and JAGB Methods
One of the most prominent voices for a calmer resolution comes from the Chicago-Carnegie Hubble Program (CCHP), led by the renowned astronomer Wendy Freedman. Her team has been using JWST to perfect two powerful distance measurement techniques.
- The Tip of the Red Giant Branch (TRGB): This method uses old, red giant stars. There's a point in their life cycle where they reach a peak brightness before rapidly dimming. This maximum brightness is incredibly consistent. By measuring this "tip," we can calculate a galaxy's distance with amazing precision.
- J-region Asymptotic Giant Branch (JAGB) Stars: A newer, promising method uses carbon-rich stars that have a very consistent brightness in a specific shade of infrared light that JWST is perfectly tuned to see.
In a landmark May 2025 paper in The Astrophysical Journal, Freedman's team, using a painstaking analysis that included a blinding procedure to prevent bias, reported their results. Their best estimate, combining new JWST data with older Hubble observations for the TRGB method, landed on a value of:
H₀ = 70.39 km/s/Mpc
This result is fascinating because it falls almost perfectly in the middle of the two warring camps. The CCHP team argues this could resolve the tension, suggesting the true value isn't 67 or 73, but 70. If so, it would mean our Standard Model of Cosmology is safe, and the discrepancy was likely due to small, unaccounted-for errors in previous measurements. Their JWST-only JAGB result was even lower, at 67.80 km/s/Mpc, remarkably close to the CMB value.
Team "Crisis": Why the Debate Rages On
This middle-ground value is far from the final word. Other leading astronomers are pushing back, arguing the tension is stronger than ever. Dan Scolnic of Duke University declared in January 2025 that the Hubble Tension is now a "crisis," and he isn't convinced by the lower value. He and his collaborators argue that the CCHP analysis may have excluded certain supernovae that, if included, would push the result back up toward 73.
Furthermore, other independent teams using completely different methods are still finding high values.
- Surface Brightness Fluctuations (SBF): A team led by Joe Jensen of Utah Valley University used JWST to measure statistical fluctuations in the light of old elliptical galaxies. Their result? 73.8 km/s/Mpc.
- Time-Delay Cosmography: Perhaps the most convincing evidence for a real tension comes from the TDCOSMO consortium. They use the mind-bending physics of gravitational lensing—where the gravity of a massive galaxy bends the light from a quasar behind it into multiple images. By measuring the time delay as the quasar's light travels along these different paths, they can calculate a distance completely independently of the stellar distance ladder. Their latest result is 72.1 km/s/Mpc.
These results, using entirely different physics, strongly support a higher value for the Hubble constant and suggest the tension is very real.
So, Is It a Crisis or a Misunderstanding?
We are at a genuine crossroads in science, and it boils down to two exhilarating possibilities.
Possibility 1: It's a Misunderstanding (Systematic Errors)
This is the more conservative, but no less important, explanation. It suggests that somewhere in our complex chain of observations and calculations, there's a subtle, hidden error. Think of it like trying to measure a vast room with a tape measure that's just a tiny bit stretched. Over a short distance, the error is negligible. But when you measure the whole room, that tiny error adds up to a significant mistake.
These systematic errors in astronomy could be anything:
- Not perfectly accounting for the dimming effect of cosmic dust.
- The light from our target star being contaminated by a fainter, unseen neighbor (an effect called crowding or blending).
- The photometric zero-points, or the absolute calibration of our instruments, being off by a fraction of a percent.
The CCHP team, with its result of ~70 km/s/Mpc, argues that these systematics are the most likely culprit.
Possibility 2: It's a Crisis (New Physics)
This is the possibility that makes scientists' hearts beat a little faster. What if all the measurements are correct? What if the universe really was expanding at one rate long ago, and is expanding at a different, faster rate now, in a way our current theories can't explain?
If this is true, it means our Standard Model of Cosmology (ΛCDM) is incomplete. It would be like discovering a new, unknown force of nature or a new, exotic particle that influenced the early universe. Scientists have proposed ideas like "early dark energy" that would have given the universe an extra "kick" shortly after the Big Bang. Confirming this would be a Nobel Prize-worthy discovery, launching a new era of cosmology as we hunt for the missing piece of the cosmic puzzle.
Conclusion: A Beautiful Moment of Uncertainty
Right now, we stand in a moment of beautiful scientific uncertainty. The universe has presented us with a profound riddle. The latest 2025 data from JWST have not provided a simple answer but have instead deepened the mystery and hardened the convictions of every side of this cosmic conflict.
This isn't a failure of science; it's science at its best. It's a passionate, data-driven, and honest search for the truth. The answer, whether it's a subtle correction to our methods or a revolution in our understanding of reality, will redefine our place in the cosmos.
Here at FreeAstroScience.com, we believe that these questions are for everyone. We seek to educate you and inspire you never to turn off your mind and to keep it active at all times, because the sleep of reason breeds monsters.
Thank you for reading. We hope you'll come back as we continue to explore the frontiers of knowledge together.
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