Big Crunches and Hubble Bubbles: A Dark Energy News Dive

The Hubble Tension is a problem, at least if you’re an astro nerd (I’m guessing the rest of the world drifts along perfectly unaware of this existential astronomical issue). I wrote a whole post about it back in the day, but to sum up real quick: astronomers get different values for how much the expansion of the universe is accelerating if they measure it using things closer to us (astronomically speaking) than they do measuring it using things far away.

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The reason this is a problem is that the expansion’s acceleration is being driven (theoretically) by dark energy and that is something that (theoretically) should have remained constant over time and therefore (theoretically) we should get the same values for the acceleration of universal expansion no matter where we measure. 

Two different stories came across my screen this past week, each claiming different potential solutions to the Hubble Tension. Seeing as the two solutions they offer are contradictory and more or less opposed, at least one of them is very incorrect. The thing is, at the moment we have no idea which one is wrong or if both are, because dark energy remains a gigantic mystery.

I thought it would be fun to do a little deep dive into these two theories and see what they each imply about dark energy—and the eventual fate of the universe!

TL;DR

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Okay, real quick if you don’t want to read the earlier post to find out the details: if we look at, say, the Cosmic Microwave Background, which is exceptionally far away from us, it says the universe’s expansion is accelerating at a rate of of 67.7 km/s/Mpc (yes, the units are odd, just focus on the number for now). If we look at nearby things like Type 1a supernova, we get a value of 73.5 km/s/Mpc.

So our measurements imply that the universe is expanding faster closer to us than it is far away. But dark energy is supposed to be constant everywhere, so something’s up. There are three possible solutions. 1. We are wrong about dark energy being constant. 2. There is something fundamentally incorrect about the measurements. 3. There is something that we’re missing that we don’t even know that we’re missing. The two news stories I’m going to talk about here cover options 1 and 2. As for option 3…well if we’re missing something important we don’t know it (yet).

Crunchy

What if we’re wrong about dark energy being a constant? This was once considered an absolute impossibility, but only in March a group of researchers working with data from the Dark Energy Spectroscopic Instrument (a wunderkind tool for mapping out dark energy) said their research suggests that dark energy is weakening. If dark energy can weaken, it cannot be constant.

If dark energy isn’t constant, that’s an easy solution to the Hubble Tension. We get different values for the universe’s acceleration because it has actually changed over time, and is changing still to this day. And a weakening dark energy force has implications for the universe at large. 

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It was a shock, back in 1929 when good ol’ Edwin Hubble (he of telescope fame) showed that the universe wasn’t just expanding, but that the expansion was getting faster. If the force driving the expansion was, in fact, constant, this implied an ever-growing universe in which individual galaxy clusters would get farther and farther apart from each other, even as the clusters themselves would eventually collapse into singular mega-galaxies.

In this model the end of the universe is a long, slow one, sometimes called the heat death of the universe. Over time spans with waaaaaay too many zeroes for me to write out, everything slowly and eventually dissolves into nothingness even as the distances between the last mega-galaxies expands so far and fast that light from one would never reach another. It’s a dark, lonely ending.

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But if dark energy is weakening, then this ever-expanding universe is called into question. If it weakens enough, the collective gravitational pull of everything in the universe may eventually overcome it, opening the possibility that the universe can end in a Big Crunch. Gravity would draw everything back together into a similar singularity from which it initially sprang (or Banged) 13.8 billion years ago.

One of the stories I read this week offers up a specific model for how this could happen. It is, admittedly, built up out of a whole bunch of uncertainties and ideas. It includes a hypothetical particle known as an axion and something called the negative cosmological constant (which is basically an anti-dark energy)…so it’s quite possibly more of an interesting thought experiment than a viable model. But technically at the moment we can’t say it’s not viable, because we don’t know what the heck dark energy is and what it’s doing, and if it is accurate it means our universe is going to be much shorter lived than current theories predict. 

Basically this axion/negative cosmological constant model suggests that the universe will reach its maximum expansion, about 70% larger than it is now, within 7 billion years. Then the graviational backslide begins as it starts to contract, all ending with a Big Crunch within 20 billion years. That’s a whole life span of roughly 34 billion years. That’s a number that’s easy to write out. It would be rather a short-lived universe, if that idea of dark energy turns out to be correct.

Bubbly

But let’s say we’re not wrong about dark energy, that it is a constant. Then the explanation for the Hubble Tension could be rooted in problems with the measurements we’re making of the universe’s expansion instead. One possibility is not so much that we’re measuring things incorrectly so much as the space around us is resulting in misleading measurements.

This particular theory suggests that our Milky Way (and, in fact, the entire Local Group of galaxies to which it belongs) sits in an unusual patch of space: a void, perhaps 2 billion light years across and at least 20% less dense than the universal average. This void is sometimes delightfully referred to as a Hubble Bubble. If you think of a normal soap bubble for a moment, it has a well-defined edge. The bubble isn’t empty (there’s air molecules in there) but there is a much denser membrane of material that makes up the edge of the bubble. 

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The same would be true of this theoretical Hubble Bubble—along its edge would be a denser shell of universe stuff: galaxies, gas clouds, dark matter, etc. All this extra stuff along the edge of the void would exert a gravitational pull on things in the void.

If the Milky Way was near the void’s center, astronomers would see everything around it being pulled away not just as a result of dark energy, but also the collective gravity of the Bubble’s edge. In other words things inside the Bubble with us, and therefore fairly close to the Milky Way  (astronomically speaking), would appear to be receding from us faster than things well outside the Bubble that are only being pushed by dark energy. Which lines up precisely with what we see.

The big problem with this idea is that the generally accepted theory says space is more or less the same in all directions. A 2-billion-light-year emptier patch shouldn’t exist. And yet the other news story that caught my attention this week highlights new research suggesting that (stop me if you’ve heard this one before) this assumption might be completely wrong.

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In a way that I admit I do not fully understand because I did not dive into all the details, this study suggests that baryonic acoustic oscillations support the idea that we are in a Hubble Bubble. These oscillations are little variations in the density of the matter of the universe that were caused by density waves in the primordial stuff of the universe shortly after the Big Bang. Something about the way these oscillations relate to redshift (which is a way we measure how fast things are moving away from us) is altered if we’re in a void, and this study says that’s what’s happening. 

So this study suggests that it’s not an incorrect assumption about the nature of dark energy causing the Hubble Tension. It’s an incorrect assumption about the structure of the universe that’s messing with our measurements. In this theory there is nothing wrong with the concept of dark energy as a constant force, and an ever-expanding universe that eventually suffers heat death in a number of years too terrifying for a puny human brain to contemplate is the natural outcome.

What’s Going On?

We’ve got one model saying our measurements and the way we think about the structure of space are fine and it’s the way we think about dark energy that’s wrong and we could all be heading for a Big Crunch. Then we’ve got the other model saying our ideas about dark energy are fine and it’s our concept of the structure of local space that’s wrong and is leading to incorrect measurements and the universe can happily keep expanding forever.

Clearly these cannot both be correct. In fact, it may turn out that neither is correct. But that’s the thing with dark energy right now—we have no idea which is more likely to be right, or if both concepts are completely wrong. Remember, there’s always option 3: something entirely unknown to us is going on and we just haven’t figured it out yet.

In conclusion the only thing I can honestly tell you for certain is that, while we’ve got some wild and wonderful notions that are epically fun to think about, we really have no idea what is going on with dark energy. Which feels like a heckuva thing to have to say about the stuff that makes up 68% of the known universe, but then the universe never asked for my opinion about anything.