Astronomers around the world are in a bit of a tizzy because they can’t seem to agree about how fast the universe is expanding.
Ever since our universe emerged from an explosion of a tiny speck of infinite density and gravity, it has been ballooning, and not at a steady rate, either — the expansion of the universe keeps getting faster.
But how quickly it’s expanding has been up for a dizzying debate. Measurements of this expansion rate from nearby sources seem to be in conflict with the same measurement taken from distant sources. One possible explanation is that, basically, something funky is going on in the universe, changing the expansion rate.
And one theorist has proposed that a brand-new particle has emerged and is altering the future destiny of our entire cosmos.
Hubble, Hubble, toil and trouble
Astronomers have devised multiple clever ways of measuring what they call the Hubble parameter, or Hubble constant (denoted for the folks with busy lives as H0). This number represents the expansion rate of the universe today.
One way to measure the expansion rate today is to look at nearby supernovas, the explosion of gas and dust launched from the universe’s largest stars upon their death. There’s a particular kind of supernova that has a very specific brightness, so we can compare how bright they look to how bright we know they’re supposed to be and calculate the distance. Then, by looking at the light from the supernova’s host galaxy, astrophysicists can also calculate how fast they are moving away from us. By putting all the pieces together, we then can calculate the universe’s expansion rate.
But there’s more to the universe than exploding stars. There’s also something called the cosmic microwave background, which is the leftover light from just after the Big Bang, when our universe was a mere baby, only 380,000 years old. With missions like the Planck satellite tasked with mapping this remnant radiation, scientists have incredibly precise maps of this background, which can be used to get a very accurate picture of the contents of the universe. And from there, we can take those ingredients and run the clock forward with computer models and be able to say what the expansion rate should be today — assuming that the fundamental ingredients of the universe haven’t changed since then.
These two estimates disagree by enough to make people a little bit worried that we’re missing something.
Look to the dark side
Perhaps, one or both measurements are incorrect or incomplete; plenty of scientists on either side of the debate are slinging the appropriate amount of mud at their opponents. But if we assume that both measurements are accurate, then we need something else to explain the different measurements. Since one measurement comes from the very early universe, and another comes from more relatively recent time, the thinking is that maybe some new ingredient in the cosmos is altering the expansion rate of the universe in a way that we didn’t already capture in our models.
And what’s dominating the expansion of the universe today is a mysterious phenomenon that we call dark energy. It’s an awesome name for something we basically don’t understand. All we know is that the expansion rate of the universe today is accelerating, and we call the force driving this acceleration “dark energy.”
In our comparisons from the young universe to the present-day universe, physicists assume that dark energy (whatever it is) is constant. But with this assumption, we have the present disagreement, so maybe dark energy is changing.
I guess it’s worth a shot. Let’s assume that dark energy is changing.
Scientists have a sneaking suspicion that dark energy has something to do with the energy that’s locked into the vacuum of space-time itself. This energy comes from all of the “quantum fields” that permeate the universe.
In modern quantum physics, every single kind of particle is tied to its own particular field. These fields wash through all of space-time, and sometimes bits of the fields get really excited in places, becoming the particles that we know and love — like electrons and quarks and neutrinos. So all the electrons belong to the electron field, all the neutrinos belong to the neutrino field, and so on. The interaction of these fields form the fundamental basis for our understanding of the quantum world.
And no matter where you go in the universe, you can’t escape the quantum fields. Even when they’re not vibrating enough in a particular location to make a particle, they’re still there, wiggling and vibrating and doing their normal quantum thing. So these quantum fields have a fundamental amount of energy associated with them, even in the bare empty vacuum itself.
If we want to use the exotic quantum energy of the vacuum of space-time to explain dark energy, we immediately run into problems. When we perform some very simple, very naive calculations of how much energy there is in the vacuum due to all the quantum fields, we end up with a number that is about 120 orders of magnitude stronger than what we observe dark energy to be. Whoops.
On the other hand, when we try some more sophisticated calculations, we end up with a number that is zero. Which also disagrees with the measured amount of dark energy. Whoops again.
So no matter what, we have a really hard time trying to understand dark energy through the language of the vacuum energy of space-time (the energy created by those quantum fields). But if these measurements of the expansion rate are accurate and dark energy really is changing, then this might give us a clue into the nature of those quantum fields. Specifically, if dark energy is changing, that means that the quantum fields themselves have changed.
A new enemy appears
In a recent paper published online in the preprint journal arXiv, theoretical physicist Massimo Cerdonio at the University of Padova has calculated the amount of change in the quantum fields needed to account for the change in dark energy.
If there is a new quantum field that’s responsible for the change in dark energy, that means there is a new particle out there in the universe.
And the amount of change in dark energy that Cerdonio calculated requires a certain kind of particle mass, which turns out to be roughly the same mass of a new kind of particle that’s already been predicted: the so-called axion. Physicists invented this theoretical particle to solve some problems with our quantum understanding of the strong nuclear force.
This particle presumably appeared in the very early universe, but has been “lurking” in the background while other forces and particles controlled the direction of the universe. And now it’s the axion’s turn …
Even so, we’ve never detected an axion, but if these calculations are correct, then that means that the axion is out there, filling up the universe and its quantum field. Also, this hypothetical axion is already making itself noticeable by changing the amount of dark energy in the cosmos. So it could be that even though we’ve never seen this particle in the laboratory, it’s already altering our universe at the very largest of scales.