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A novel theory suggests that the current expansion rate of the universe is not fixed, but was in fact quite different in the early universe—and that the universe could start contracting at some point.If that happens, the current understanding that the universe will end with a great expansion of space that pushes even atoms apart, could be turned on its head.This theory has the potential to encompass the entire expansion history of our universe within one single, unified model.
Imagine you’re trying to push two magnets together. But instead of resisting, they suddenly repel each other with growing force, sending them flying apart at an ever-increasing speed. That’s a bit like our universe.
For the last few billion years, something mysterious has been pushing everything apart faster and faster, a cosmic accelerator we call dark energy. The simplest explanation for this accelerating push has always been the cosmological constant, a steady, unchanging energy tucked into the fabric of space itself. We’ve always thought of it as, well, constant. A bedrock number in the cosmic equations.
But what if our universe is a little more mischievous than that? A new idea is brewing in the minds of some cosmic architects, a truly wild notion: what if this constant isn’t constant at all? According to a new study, the universe may not continue in its expansion forever, with matter getting more and more spread out. Instead, it may someday collapse back in on itself, a terrifying possibility known as the Big Crunch.
Now, imagine a cosmic thermostat that not only adjusts the temperature but can flip from freezing cold to scorching hot. This is the heart of a novel framework: a cosmological constant that changes its sign, where the universe’s fundamental vacuum energy density could have actually transitioned from a negative value in the very early universe to the positive one we observe today. That’s according to a new paper appearing in the preprint journal arXiv.
It’s a bold move, but it offers a unified way to perhaps smooth out some persistent wrinkles in our cosmic story—like the nagging difference between how fast we think the universe is expanding based on early cosmos data, and how fast we measure it expanding locally (a discrepancy physicists sometimes call Hubble tension).
So, how would this cosmic thermostat actually work? Imagine a field, not unlike the electromagnetic field we’re familiar with, but one that permeates all of spacetime and carries the energy that acts as our cosmological constant. In these dynamic dark energy models, this field has an effective potential, like a rolling hill for a tiny marble.
In the very early universe, this marble was sitting in a dip that corresponded to a negative cosmological constant. Think of it as a brief, intense period where gravity—instead of pulling things together—was almost repelling itself, but in a way that eventually led to contraction or a different kind of evolution. Then, at some critical point in cosmic history, this field transitioned, rolling into a new dip that gave us the positive cosmological constant we see today.
This isn’t just about patching up a problem here or there; it’s a grand vision that aims to describe the entire expansion history of our universe within one single, unified model, embracing this dramatic phase shift.
So, how do you even begin to test such a daring idea? Scientists model these dynamic dark energy scenarios by stretching our standard cosmic rulebook, the ΛCDM model, which describes a flat universe with a cosmological constant, cold dark matter, and ordinary matter. This baseline model, the one we generally agree on, includes familiar ingredients like ordinary matter, radiation, and our usual, static cosmological constant.
But these new models add a twist: a scalar field, which acts a bit like a hidden dimension of influence. Its energy potential dictates when and how the cosmological constant flips its sign.
These cosmic blueprints, then, are defined by key numbers: the time when the big flip happened, the initial value of that constant in the baby universe, and how steeply its energy potential changes. By tweaking these dials, researchers compare the model’s predictions for things like the Hubble parameter (which is just a fancy name for the universe’s expansion rate at any given time) and how the dark energy itself behaves over time, against what our telescopes actually see. They also look at cosmographic parameters, which are basically independent measurements of the universe’s expansion history that don’t care why it’s expanding the way it is.
If this sign-changing cosmological constant is real, it wouldn’t just be an abstract mathematical curiosity. It would leave its fingerprints all over the cosmos, especially in how galaxies and giant clusters of galaxies formed and grew. Think of it: a different kind of push and pull in the early universe would have directly affected the seeds of cosmic structure. Surprisingly, these models manage to play nice with what we currently observe, even with their radical proposal of a vacuum energy density that flips from negative to positive. They offer a plausible alternative that doesn’t immediately crash and burn when confronted with our most precise cosmic measurements.
But let’s be honest, any idea this daring comes with its own set of puzzles.
⬇️ More Cosmic Mysteries
One big hurdle is making sure this dynamic dark energy field doesn’t throw a cosmic tantrum, leading to nasty mathematical divergences or sudden, unrealistic jumps in its behavior. We’re often modeling these sign changes as crisp, abrupt flips, like a light switch. But in the real, messy universe, any such transition would probably be more like a dimmer switch, a smoother, more gradual evolution. And honestly, getting the math to play nice without needing some serious fudging of the numbers (or as physicists say, fine-tuning of parameters) is always a tough ask. Plus, exactly how this scalar field makes the cosmological constant change its spots, and how it plays with all the other particles and forces—well, that’s still very much a work in progress.
Despite these technical bumps, the sheer elegance of a sign-changing cosmological constant is pretty compelling. It doesn’t just offer another way to describe the universe; it offers a better way, potentially smoothing over some of those cosmic wrinkles we talked about earlier, like the Hubble tension and even the cosmic coincidence problem. This is just a fancy way of saying: why are dark energy and matter densities roughly equal now, in our particular epoch, when they’ve been so different for most of cosmic history? These dynamic models give us a unified story for the entire life of the universe, from its earliest moments to its accelerating future, all under one theoretical roof.
Our universe, it seems, is far more dynamic and surprising than we ever dared to imagine. What once seemed like an unshakeable constant might just be a fleeting phase in a grander cosmic dance. As we keep probing the depths of space and crunching the numbers, new ideas will always bubble up, challenging our assumptions and pushing the boundaries of what’s possible. After all, isn’t that the most thrilling part of the journey? Who knows what other fundamental constants might turn out to be anything but.
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Paul M. Sutter
Contributor, science educator and theoretical cosmologist
Paul M. Sutter is a science educator and a cosmologist at Johns Hopkins University and the author of How to Die in Space: A Journey Through Dangerous Astrophysical Phenomena and Your Place in the Universe: Understanding Our Big, Messy Existence. Sutter is also the host of various science programs, and he’s on social media. Check out his Ask a Spaceman podcast and his YouTube page.