New observations from the Dark Energy Spectroscopic Instrument suggest this mysterious force is actually growing weaker – with potentially dramatic consequences for the cosmos

So does this mean the big crunch is back on the table?

It sounds like this is looking at the rate of change (derivative) of red shift, right? How does the instrument get the rate of change when we have such a small window of time for comparison?

Astronomer here! This is something I’ve been waiting for with great excitement… and good news, it was worth the wait! ([Here](https://www.desi.lbl.gov/2025/03/19/desi-dr2-results-march-19-guide/) is the summary of results from the team itself btw, far better than the linked article IMO.)

The [Dark Energy Spectroscopic Instrument (DESI)](https://www.desi.lbl.gov/) measures the effect of dark energy on the expansion of the universe. [Dark energy](https://en.wikipedia.org/wiki/Dark_energy) is a mysterious form of energy that makes up ~70% of the “stuff” in our universe- we know this because the expansion of the universe is *accelerating*- that is, it is getting bigger faster over time- and we have nowhere enough normal matter (made up of you and me, stars, gas, galaxies, etc) to explain this accelerating expansion. But we also don’t know what dark energy could *be*- it was discovered in the 1990s, but it’s such a huge problem we frankly haven’t been able to study it in detail until now.

So, enter DESI! They’re using a telescope on Kitt Peak in Arizona to gather data on *millions* of galaxies out to 11 billion light years away from us, and then create a 3D map of the universe. The idea is once you have all this detailed data, you can look carefully at the movement of these galaxies over the age of the universe and see whether there’s any changes in its expansion (and, thus, figure out what dark energy is doing, and then thus hopefully get a handle on what it *is*). [Here’s](https://web.ub.edu/documents/2710030/15588963/DESI4DEF.jpg/373ffc68-4530-0da7-0312-b3d1ebbeeff9?t=1712222700184) a nice cartoon by PhD student Claire Lamann (who works on DESI) illustrating this, and [a nice YouTube video](https://youtu.be/fQkFS5yot5I)!

Now, it should be emphasized that this is not the *first* data release from DESI- they did another one last year, which hinted that there *might* be a change over time in dark energy (and thus the expansion of the universe), but it wasn’t robust enough to know for sure. But today the new results are out, and they’re *really* getting convincing that dark energy evolves over time! Specifically, to date our “best” model to describe the universe, Lambda CDM, assumed that dark energy was *constant* over time. You can’t assume a giant thing like that is changing until you have good evidence of it, so you’d better get *really* good evidence like measurements from millions of galaxies, you know? And if you take the DESI data combine it with data from supernova explosions, the [Cosmic Microwave Background (CMB)](https://en.wikipedia.org/wiki/Cosmic_microwave_background), and others, the odds of what DESI is claiming has 2.8 to 4.2 sigma significance. (A 3-sigma event has a 0.3% chance of being a statistical fluke, but many 3-sigma events in physics have faded away with more data.) So, we are not yet at the “gold standard” in physics of 5 sigma… but damn, this is intriguing AF. [Here](https://www.desi.lbl.gov/wp-content/uploads/sites/8/2025/03/DR2-cosmo-annotation-600×513.png) is another great cartoon by Claire explaining this better than words could!

Ok, so that’s great, dark energy may well be changing- what does that mean for the fate of the universe? Well, as of right now, as we can measure it, the universe is *still* just accelerating in its expansion with no real changing, and these new results don’t indicate that is going to change in the immediate future. (Sorry, [Big Crunch](https://en.wikipedia.org/wiki/Big_Crunch) fans, but there’s still no real evidence this is going to happen.) But obviously, if dark energy *can* change over time, that has a helluva lot of interesting implications, and no one knows just how it’s going to play out yet. Personally, I’m just amazed that we are *finally* getting such interesting information at all on dark energy after spending literally decades not being able to make heads or tails on the problem- so exciting to see the DESI results! Can’t wait to the next data release!

Observe something for only a few years, see a change within this small window… PANIC HEADLINES!!!! Jesus, this could be fucking normal! ebbs and flows, we have no fucking clue.