The Cosmology Crisis Just Got Even Worse

Sabine Hossenfelder • 2025-07-15 • 7:05 minutes • YouTube

🤖 AI-Generated Summary:

Is Dark Energy Changing Over Time? New Evidence Challenges a Fundamental Cosmic Constant

One of the most intriguing scientific developments in recent years revolves around a profound question in cosmology: Is the cosmological constant truly constant? This constant, often associated with dark energy, has long been thought to drive the accelerated expansion of our universe at a steady rate. However, recent evidence is challenging this notion, suggesting that dark energy might actually be changing over time—specifically, that it was stronger in the past and is now weakening.

What is Dark Energy and the Cosmological Constant?

Dark energy is the mysterious force propelling the universe's expansion to speed up rather than slow down. The simplest explanation for dark energy is the cosmological constant—a fixed amount of energy inherent to empty space itself. Imagine that even "nothing" has energy, more than enough to push galaxies apart faster and faster as time goes on.

New Data Challenges the Constant Nature of Dark Energy

Over the past year, new results from several cutting-edge experiments have hinted that dark energy is not fixed. The Dark Energy Survey (DES) analyzed light from distant supernovae to study how the universe's expansion has changed over time. Simultaneously, the DESI (Dark Energy Spectroscopic Instrument) project studied the distribution of galaxies and galaxy clusters, focusing on patterns called baryon acoustic oscillations—relics of sound waves from the early universe.

Both DES and DESI found tentative evidence (around a three sigma confidence level) that dark energy is weakening. While this is not definitive proof, it corresponds to roughly a 3 in 1,000 chance that the signal is a fluke.

New Support from the South Pole Telescope

Adding weight to these findings, the South Pole Telescope (SPT) recently released precise measurements of the cosmic microwave background (CMB)—the afterglow radiation from the Big Bang. Unlike telescopes scanning the entire sky, the SPT focuses on a small patch with high precision, measuring temperature and polarization fluctuations.

SPT data confirms the ongoing "Hubble tension"—a discrepancy between local and early universe measurements of the universe's expansion rate—with a striking 6.2 sigma significance. More importantly for dark energy, their data also supports the idea that dark energy was stronger in the past, though their results are slightly closer to the classic cosmological constant model. When combined with DESI data, the evidence for weakening dark energy becomes even stronger.

Why Does This Matter?

If dark energy changes over time, it could help resolve the Hubble tension, an outstanding puzzle in cosmology concerning differing measurements of the universe’s expansion rate. More fundamentally, a time-varying dark energy implies it is not just a constant property of space but a dynamic field permeating the cosmos.

In physics, fields are linked to particles. This means dark energy might be related to a new, undiscovered particle or connected to known particles like the Higgs boson. Various theories have proposed such links, though none have been conclusively validated. These new findings may encourage physicists to revisit these ideas.

Implications for the Fate of the Universe

A constant dark energy predicts that cosmic expansion will accelerate forever, leading to a cold, ever-expanding cosmos. But if dark energy weakens—or even disappears—this could slow or reverse expansion, potentially leading to a cosmic collapse or cyclic universe with recurring big bangs.

Caution and Open Questions

While these results are exciting, they come with caveats. The analyses assume a specific cosmological model, and it’s possible the real solution lies elsewhere—not in dark energy’s constancy, but in our understanding of the universe's overall framework.

Staying Informed with Balanced News

In a world where news often blends facts with opinion, platforms like Ground News offer a valuable way to get a balanced perspective. Ground News aggregates coverage from across the political spectrum, rates factuality, and highlights blind spots in media coverage. This helps readers break out of information bubbles and understand complex issues like these new cosmological findings with context and clarity.

The story of dark energy is still unfolding, and these latest findings invite us to rethink some of our deepest assumptions about the universe. Whether dark energy is truly fading or not, the quest to understand it pushes the boundaries of science—and our imagination.

Stay tuned for more updates on this cosmic mystery, and keep questioning the universe around you.

📝 Transcript (162 entries):

One of the biggest science news of the last year was tentative evidence that the cosmological constant is not constant. A perfect case of a coonian crisis in science, a clash between theory and data. And the crisis just got worse. The data say that rather than being constant, dark energy was stronger in the past and is now getting weaker.

Now, if you've read the news recently, it might not feel like dark energy is getting weaker, but another experiment just strengthened the case. Let's have a look. Dark energy is the name we've given to whatever keeps cosmic expansion speeding up. The easiest way to explain it is with a cosmological constant that one can interpret as the energy of empty space itself which if true means that even nothing has more energy than I do before coffee. Last year we've seen new results most importantly from the dark energy survey and from desi. The dark energy survey looked at how the light from supernovi differs with their distance to us. The Desi experiment looks at patterns in galaxies and galaxy clusters, the so-called berion acoustic oscillations that are left over from sound waves in the plasma in the early universe. Both have found tentative evidence that dark energy is getting weaker in the range of three sigma.

That's about a three in a,000 chance that it's a coincidental fluke in the data. And these findings are now backed up by a precise measurement of the cosmic microwave background CMBB for short coming from the South Pole telescope. This is a telescope with a 10 m dish and rather than scanning the entire sky, it focuses on the details in a small part of the sky where they measure both the temperature and the polarization of the CMBB. In galactic coordinates, it's this green patch down here. To put this into relation to the other measurements, the dark energy survey measures supernovi. So these are fairly near to us and quite late in the history of the universe. The baron acoustic oscillations happen in the plasma in the early universe from around 10,000 to 350,000 or so years after the big bang. These patterns then get imprinted into the distribution of matter in the universe. This is what Desi looks for. The CMBB is created when the barrier on acoustic oscillations end. So these are three separate pieces of evidence. The new paper's a summary of the data that the South Pole telescope collected in 2019 and 2020 and it just appeared on the archive. For one thing, they confirm the tension in the hover rate at a stunning 6.2 sigma away from local measurements. But they also confirm the same preference for a weakening of dark energy. that is it was stronger in the past, possibly even stronger than a cosmological constant could ever be. The result has a smaller error bar. However, the significance of the tension with a constant arc energy is about the same as with the earlier measurements in the range of three sigma. This is because their best fit value is somewhat closer to the vanilla cosmological constant. They also combine their data with Desi and say that then the significance increases. Again, it's not a huge improvement, but that said, honestly, I was expecting that I'd have to tell you, sorry, it went away, but it doesn't seem to want to go away. What does this mean? Well, for one thing, if dark energy indeed depends on time, that would also alleviate the Hubble tension.

But maybe more importantly, if dark energy is not a constant, this is a clue that it's sort of a field that permeates the universe. And in physics, fields are always linked to particles. This means that either dark energy is made of a new type of particle or it's actually linked to an existing particle, for example, the hix boson. This has been proposed a couple of times in various ways and none of these theories have been very convincing so far. But maybe physicists will now revisit this idea. If this finding turns out to hold and dark energy indeed weakens, this also changes the outlook for the future of our universe. Because if dark energy is constant, then the expansion will just keep on getting faster and faster. But if dark energy weakens or maybe even goes away entirely, this makes it possible that the expansion slows down, the universe might even collapse, making ideas of cyclic universes with recurring big banks more plausible. On my meter, I give this a 2 out of 10. This is because all these parameter fits already assume a particular model for the universe, and maybe the issue isn't dark energy after all, but that we shouldn't be using this model. But if you felt a disturbance in the force recently, it might not be your anxiety.

It could just be the vacuum energy dropping or rising, or maybe it's just gas. One of the biggest problems with news today is that you don't just get the facts, you get someone's opinion about the facts. If you find that as annoying as I do, there's a solution to this problem. Have a look at Ground News. Ground News is a news platform that compares how a story is being reported by different outlets across the political spectrum. They collect and summarize news from all over the world and give you a quick summary together with a lot of extra information that you don't find in the standard media. A curious example is this recent news about how the Trump administration lifted some technology export restrictions on China, which basically wasn't covered by the political right.

Ground news also gives you a factuality rating for each news item, and it tells you whom the media outlets are owned by and where the news has appeared. And it's not like they do only US news. You can, for example, switch to EU news here. They also help you get out of your information bubble with this feature they call blind spot. It shows you news that has been almost exclusively covered only by one side of the political spectrum. Ground News has really improved my news reading a lot. If you want to try it too, use my link or QR code because that'll give you a 40% discount on the Vantage plan which gives you access to all their features. It's an easy way to do something for yourself and for this channel. Thanks for watching. See you tomorrow.

YouTube Deep Summary