The Mystery of Dark Energy

Feels like everywhere I turn in the last couple of weeks I’m seeing an article about dark energy. It’s having a moment, so I thought I should probably try and write a blog post about it. The thing is, when I write blog posts I like to either write about things we’re pretty sure we know quite a bit about (like last week’s on star deaths) or about something new that we’re just finding out but are pretty sure about.

Dark energy is…neither of those things. It’s mostly defined by the fact that we have no idea what it is and are trying, puzzle piece by puzzle piece, to put together a picture of how it works. I’ve covered dark energy in bits before, but here I will attempt to get a little deeper into what we know and how we know it. This field is changing quickly, so it’s entirely possible that this post will be out of date very fast, but I’ll do my best.

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Dark Forces

Going back to the very beginning of things, the universe exploded into existence about 13.8 billion years ago. We call this the Big Bang. Everything has been spreading outward ever since.

In a universe with no dark energy this outward expansion would be entirely driven by the leftover energy of the Big Bang. That energy would be dissipating over time and the gravity of all the stuff in the universe would be pulling inward on all the other stuff and the expansion of the universe would be slowing down. Eventually the force of gravity would overcome the leftover Bang force and start pulling everything back, resulting eventually in the universe ending in a Big Crunch. 

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In 1998 a study was published that used observations of Type 1a supernovae, which is when a white dwarf steals a bunch of matter from a companion star and then explodes. These explosions have very predictable brightnesses. Since we know how bright they actually are, we can figure out how far away they must be and how fast they’re moving by how bright they appear to be and how their light appears to be red or blueshifted due to their motion (more on that later).

Anyway in 1998 observations of these supernovae were used to make the bombshell claim that the expansion of the universe is not slowing down at all, but rather is accelerating. This acceleration couldn’t be the push from the Big Bang still, it had to be something else. And this something else is counteracting the pull of gravity of everything in the universe on everything else. 

Having no idea what this something else actually was beyond a mysterious outward force, astronomers gave it the appropriately mysterious name of dark energy. Since 1998 we’ve found piles of additional evidence that it’s there. And it’s been confusing us ever since.

The Hubble Wars

So once we accepted that the universe was expanding at an accelerated rate, we wanted to know just how much things were speeding up. We’ve actually been measuring the expansion of the universe since the 1920s when Edwin Hubble figured out that the universe doesn’t hold still (the big discovery in 1998 was that this expansion wasn’t happening at a steady pace or slowing down).

Hubble determined that the rate at which things in the universe are moving away from us depends on their distance (i.e. a more distant galaxy would appear to be receding from us faster than a closer-in galaxy would). He figured out the relationship between this distance and rate of movement. It involves something we now call the Hubble constant (H₀). Knowing H₀ you can figure out how fast things specific things are moving away from us and therefore how much the expansion of the universe is accelerating. Of course, that assumes things are accelerating at a steady rate. 

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According to our accepted model for how things work in the universe, which we call the standard model of physics or sometimes the Lambda Cold Dark Matter model, dark energy is a constant force. In fact, the Lambda represents constant dark energy. This means that, while it is pushing the expansion of the universe to accelerate, it’s doing so at a steady rate. It says that the universe is a car on an empty highway smoothly accelerating constantly instead of occasionally pumping the brake or slamming the gas harder.

And there’s where the Hubble Wars start (okay fine, nobody but me calls it that. Everyone else calls it the Hubble tension. I like my name better). Basically if you try to measure H₀ using things that are far away in the distant universe you get a different number than if you try to measure things that are close in. The standard model of physics says H₀ should be the same everywhere because dark energy is a constant, steady force.

Some proposed solutions to the Hubble Wars (fine, the Hubble tension) involve errors in the measurements themselves, that some as-yet unknown aspect of the local universe (such as our Local Group of galaxies living in an unusually large void) is affecting the closer-in measurements, or that our uncertainties about the measurements have been underestimated. All of these would preserve the lovely idea of dark energy being a steady, reliable force.

But of course, there’s the other possibility: that we’re totally wrong about dark energy and it’s not constant at all.

Enter DESI

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In 2021 a new instrument attached to the Mayall 4-meter Telescope at Kitt Peak National Observatory in Arizona opened its eyes for the first time. This was the Dark Energy Spectroscopic Instrument, aka DESI. And it’s been a game changer. 

Designed for a five-year survey of the universe, DESI is measuring the position and spectra of 40 million galaxies. Measuring spectra, the light fingerprints of objects, allows us to measure how fast they’re moving towards or away from us.

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Things moving towards us will have the lines in their spectra shifted towards the blue end of the electromagnetic spectrum (blueshifting). Things moving away will have their spectral lines shifted towards the red end (redshifting). The faster the object is moving, the greater the shift. Essentially this means DESI can measure current position and movement of all these galaxies. Taken altogether it can form a picture of the history of the expansion of the universe and therefore the effect of dark energy over time.

Just in November DESI was in the news for showing a strong link between dark energy and black holes. I blogged about it at the time (tried to anyway…it’s a complicated topic). And that was based on early data from DESI—it only had its first big data release on March 19^th, and it spelled bad news for the standard model of physics. 

Slowing Down

The first DESI data release, which contains data on 18.7 million objects, actually does not go against the standard model—at least, not on its own. But when combined with measurements from the Cosmic Microwave Background, supernovae, and gravitational lensing, there’s a problem. All those measurements together paint a picture of dark energy that is weakening.

Here’s where I am going to insert a very Big and Important Point of Order: there is still quite a bit of uncertainty with this finding. There almost always is with new discoveries, but physics has a gold standard that it calls “5 sigma”. Basically a finding that reaches 5 sigma statistical significance means that points of interest in the data are real things and not background noise.

Combining the DESI data with the different other measurements (CMB, lensing, etc.) produces statistical significances of 2.8-4.2. None of those are 5, and nobody is accepting this as firmly as they would a 5-sigma result. But 4.2 is close enough to make everyone pay attention.

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If it is the case that dark energy is weakening (again, not a firmly accepted idea without that 5-sigma result), what does this mean for the universe? Well, it doesn’t mean that the expansion is slowing down. The car is still accelerating down the highway, still gaining speed. It’s just not gaining speed as quickly as it used to. 

It also means that our standard model of physics needs tweaking, something we’ve been getting hints at for a while now. It means we know even less about dark energy than we thought we did—if it’s something that can change dramatically over time, then there’s something fundamental that we don’t understand about it. And it means we have no idea how the universe will end.

So Long, Farewell

Prior to the discovery of dark energy, we assumed the universe would end with the Big Crunch. Once we realized that gravity was losing a battle to dark energy, we assumed that the universe would just keep on expanding forever, and just kind of fizzle out, sometimes called the heat death of the universe. But if dark energy is weakening? Who knows!

If it is weakening and keeps weakening, then that would give gravity a chance to eventually gain the upper hand again. It would take a stupidly extraordinarily huge amount of time, but if gravity can eventually win then the Big Crunch is back on the table.

But clearly there is something fundamental about dark energy that we simply don’t understand. If it’s weakening now, would that mean it can strengthen again in the future? If so, then the heat death version of the universe’s end could still happen.

The TL;DR of it all is that we apparently really have no idea what dark energy is or how it works. And considering the very fate of the universe will be determined by what dark energy is doing, that is kind of frustrating. But DESI (not to mention astronomers and cosmologists across the world) is on the case. Can we solve the Mystery of Dark Energy? Not yet…but don’t count us out!