For decades, scientists have used two different measurements to analyze the behavior of the universe. Now, the meaning of these models is being thrown into question — by one of their pioneers.

Dark Energy Universe Expansion

NASAThe Tarantula Nebula, a section of the universe where new stars are forming.

Historically, experts have used two separate approaches to measure how the universe is expanding. While the measurements have produced two different results, they were both generally thought to be reliable, with the variance explained away by the imprecision that comes with analyzing such a complicated matter. Over time, it was thought, instruments would get more accurate, and this discrepancy would disappear.

However, this has not been the case. Instead, increasingly precise measurements have kept landing near the same incompatible values, making the discrepancy harder to dismiss as an error. Now, some of the scientists involved in pioneering these measurements are sounding the alarm.

But what was the inciting incident that caused this chaos? The answer lies in the changing state of dark energy itself.

How Fast Is The Universe Really Expanding?

In the late 1990s, one team featuring astrophysicist Adam Riess and another led by Saul Perlmutter studied distant exploding stars called Type Ia supernovae. Models at the time predicted that the gravitational pull of all the matter in the universe would slow the expansion of the universe, which would be reflected in their measurements of the exploding stars.

But these teams noticed that, strangely, the opposite was occurring. Rather than slowing down, the universe’s expansion appeared to be accelerating.

This discovery earned Riess, Perlmutter, and another researcher named Brian Schmidt the 2011 Nobel Prize in Physics. Their research would eventually be incorporated into calculations through the “Lambda cold dark matter” model, or ΛCDM, with Λ representing the cosmological constant of dark energy that, somehow, was causing the universe’s expansion to accelerate.

Dark Energy Universe Expansion

National Optical-Infrared Astronomy Research LaboratoryDark energy is a mysterious force that drives the expansion of the universe.

The problem was that scientists could not agree on exactly how fast the universe is expanding. The Planck satellite used ΛCDM and cosmic microwave background radiation — light left over from early in the universe’s history — to estimate the expansion speed at 67 kilometers per second per megaparsec. Riess’ SH0ES team, in contrast, measured distances between nearby galaxies to determine that the expansion was closer to 73 kilometers per second per megaparsec. This discrepancy became known as the “Hubble tension.”

While it was believed that these numbers would one day align as measurements became more exact, the opposite has occurred. There’s also another problem: the findings of the Dark Energy Spectroscopic Instrument.

Is Dark Energy Changing?

The Dark Energy Spectroscopic Instrument, or DESI, maps millions of galaxies and quasars to reconstruct how the universe has expanded over billions of years.

Taken by themselves, DESI’s measurements appeared to be compatible with ΛCDM. But in March 2025, researchers announced that, when they combined them with the observations of the cosmic microwave background and supernovae, they determined that dark energy might not be as constant as they thought. Instead, the very nature of dark energy may itself be evolving over time.

Dark Energy Spectroscopic Instrument

NOIRLab/KPNO/NSF/AURA/P. MarenfeldThe Dark Energy Spectroscopic Instrument at Kitt Peak National Observatory near Tucson, Arizona.

This is striking because, in the old model of ΛCDM, the “Λ” was to be treated as a constant. By demonstrating that this might not actually be the case, the researchers revealed that the simplest version of the standard cosmological model may not tell the whole story.

Riess himself has noted the remarkability of this finding. In a paper published in August 2025, he and fellow astronomer Alexie Leauthaud wrote, “Widening cracks are appearing in the Λ cold dark matter (ΛCDM) model.”

“It is becoming increasingly clear that the standard cosmological model struggles to describe the full expansion history of the Universe as revealed by the cosmic microwave background, baryon acoustic oscillation measurements, and locally calibrated type Ia supernovae,” the paper reads.

What Does This Mean?

Once this news became public, the internet was awash with theories about what this meant for our understanding of the universe. One video on the topic posted by Dr. Alyssa Sokol (@alyssa.sokol.phd), which currently has over 266,000 views, declared, “We may very well be on the verge of throwing out the standard model of cosmology.”

The Pillars Of Creation

NASAThe “Pillars of Creation,” massive trunks of interstellar gas and dust nearly 7,000 light-years from Earth.

This is a real possibility — but researchers still aren’t sure about the true consequences of these findings.

For example, in July 2026, DESI researchers released a new analysis using the “Lyman-alpha forest.” This generates measurements by looking at patterns produced when ancient quasar light passes through clouds of hydrogen across the universe. They found this measurement to be more precise — and its central value moved toward ΛCDM. This could further complicate the idea that dark energy is changing.

In short, astronomers aren’t totally sure about the current state of our universe or how fast it is expanding, but it’s certainly an exciting time in the field, and the nature of our understanding of the universe may dramatically change again soon.

@alyssa.sokol.phd

What a time to be alive and curious 😀🤯 amirite? “The Hubble Tension” refers to the fact that we have two independent estimates for the expansion rate of the universe that largely DISAGREE! So which one is it, what’s the truth?!? And what does it mean for physics? Well it has to do with dark energy. Dark Energy is the name we give to whatever is causing the universe to expand and accelerate. We say space-time itself is stretching out and dark energy is the driving force (it’s not a force but words are hard; who knows lol) That being said! Dark energy, and specifically the extent to which it *dominates* the matter energy density of the universe, is MOSTLY RESPONSIBLE for the fate of the universe. Meaning, does the expansion proceed to accelerate and drive everything apart for all time forever? Or, will this dark energy eventually tire out or become less dominant somehow, causing matter to dominate? Well what happens then 👀 maybe the Big Crunch? A reversal of sorts where the universe gets too heavy and falls back on itself, is pretty much what that is 🤔 It’s all very exciting and really brushes up on my personal favorite biggest open question of the universe: What is empty space? What is space-time? Why is it stretching? What else is it doing? What happens below Planck-scales and beyond black hole event horizons 👀

♬ original sound – Dr. Alyssa Sokol


For more space oddities, learn about how black holes may be the source of dark energy. After that, explore a detailed 3D map of our galaxy.