New dark energy data could change our understanding of the universe

Are scientists wrong about dark energy? It looks like that could be the case.

Let’s dive into some recent findings from DESI, or the Dark Energy Spectroscopic Instrument, to talk about its weird findings and how that might change our understanding of the universe.

The basics: What is dark energy?

Dark energy is not related to dark matter, even though they have similar names. They’re both called “dark,” though, because they’re not directly observable. We infer their existence based on how we’ve observed our universe behaves.

Around ten billion years after the universe began, the expansion of the universe started to speed up when dark energy began to dominate our universe.

Breaking that down, it’s not just that the universe is expanding — it’s that the rate of expansion is increasing. Gravity tells us that this shouldn’t be happening. It’s as if you threw something in the air on Earth and it kept going up, which we know shouldn’t happen.

Some sort of energy has to be driving that acceleration, and scientists have given it the name “dark energy.” We think it comprises about 68.3 to 70 percent of the universe.

What do scientists think dark energy could be?

There are four prevailing theories for what dark energy might be.

First, it could be vacuum energy — this is the idea that there is a theoretical energy distributed throughout space. What if there were pairs of particles and antiparticles, and they cancelled each other out? The energy created by them coming into existence and then being cancelled out could be the force behind dark energy. But this twould create too much energy, to the point where stars and planets would never have formed.

Second, some scientists think that dark energy is just wrinkles in space — a defect in the fabric of space that formed in the early universe.

Third, it could be some sort of fluid or energy field that behaves opposite to normal matter.

Fourth, some think dark energy is a defect in Einstein’s theory of general relativity — that relativity and gravity work in a different way when you’re talking about the scale of the universe. Basically, this says dark energy doesn’t exist and you can modify our understanding of gravity in a way to account for the accelerating expansion of the universe.

Right now, the Standard Model of Cosmology is how we understand the universe as it exists. It says that the Big Bang created the universe, and the universe is about 5% regular matter, 27% dark matter, and 68% dark energy. It’s also called Lambda CDM, or the lambda cold dark matter model. Lambda, which represents dark energy, is constant over time in the Standard Model.

We have no confirmation that dark energy exists. It’s just the best way to explain the universe, as we have observed it.

People think because there are parts of the Standard Model we don’t understand that it’s some sort of cop out. It’s not. And more and more we’ve been discovering things that have challenged the Standard Model, which means that we may fundamentally misunderstand our universe and it’s time to go back to the drawing board.

So, then, what are these new dark energy results?

Let’s talk about DESI. This is an instrument installed in 2018 on a telescope at Kitt Peak Observatory in Arizona. DESI has a five-year observing lifetime, during which it will measure and analyze the light of over 30 million galaxies and quasars. The goal is to accurately measure the expansion history of the universe.

Well, we just got DESI’s first year results and…they basically fundamentally contradict what we knew about the universe. This new data tells us that dark energy may be weakening.

DESI created the largest 3D map of the universe ever that stretches back over 11 billion years. The way DESI works is to collect light from galaxies and quasars, which are extremely luminous active galactic nuclei that emit radiation across the electromagnetic spectrum. But to create an accurate map of the universe, you need to know how far apart the things in it are, which is why DESI worked to make redshift measurements that are as accurate as possible.

As the universe expands, everything within it stretches — including the wavelength of light. It shifts towards the longer, or redder, end of the spectrum — hence the name redshift. That’s the reason “high redshift” galaxies are also the ones that are furthest away from us. Scientists can use redshift to measure how the universe is expanding, at what speed it’s expanding, and to figure out the distance to the oldest objects in the universe — as well as the age of the universe.

DESI is measuring light, but also getting measurements on the expansion of the universe that areas accurate as possible by looking at BAO, or baryonic acoustic oscillations. In the early universe after the Big Bang, the universe was a thick, hot soup made up of light and matter. Scientists think matter was evenly distributed, but gravity was trying to pull clumps together to form the first stars and galaxies.

But as the galaxies formed, the matter heated up, and then that heat counteracted gravity and pushed objects apart — then it cooled off and gravity pulled it back together. This cycle created these baryonic acoustic oscillations, basically sound waves that spread outward in bubble like formations.

After the universe cooled, 380,000 years after the Big Bang, atoms formed, which cooled down matter enough for gravity to overcome the heat. As larger structures like galaxies began to form, these oscillations were imprinted and frozen in time. We can still read those frozen bubbles today, and because they’re all the same size, scientists can tell how far or close to us the galaxies they’re imprinted upon are. The way these BAO signatures have changed over time is one way we can measure the expansion of the universe.

And now DESI tells us that dark energy may not actually be constant over time. Dark energy appears to be weakening. We may, in fact, live in a variable universe.

Let’s not throw the Standard Model out just yet

Okay, so let’s caveat here. First, this is not enough evidence to disrupt the Standard Model of Cosmology. Remember that models aren’t sacred, and some scientists may be too attached to them, but most just want to describe the universe around them as accurately as possible — Lambda CDM does that.

The things here that differ from Lambda CDM are pretty small — small enough to just be error. DESI has four more years to collect data, so we’ll keep an eye on this.

But what makes this compelling, and worth it to me to do an entire video about, is that when you add the DESI data to other data we’ve collected — specifically Planck’s map of the cosmic microwave background and recent supernova maps — things get very interesting.

The fact is, this isn’t the first time we’ve gotten an indication that dark energy might be variable. A recent survey of 1,635 Type Ia supernovae, which have standard brightnesses and therefore can be used to accurately measure distance, found that the observed brightness didn’t line up with the idea of a constant lambda. This is just one of three recent supernova surveys that found the same thing.

And then there’s the issue of the Hubble constant. I went in depth on this a couple of months ago — JWST confirmed that Hubble’s measurements of the Hubble constant are accurate. That might not sound like a big deal.

Here’s the full analysis but basically the Hubble constant measures the rate of expansion of the universe. Measurement of cosmic microwave background, light left over from the Big Bang, gives us one value. Hubble’s surveys gave us a different value. There can’t be two values of Hubble’s constant — I said at the end of that video that if both values were true, and Hubble’s constant changed over time — which would mean that dark energy was variable not constant, it would break the Standard Model of Cosmology.

The model isn’t broken yet. There’s still a lot about it that describes our universe as we have seen it. But the fact that multiple sources of data are all pushing us to consider the fact that dark energy might not be constant — that’s pretty compelling to me.