Dark energy is one of the biggest mysteries in physics. It dominates the universe, making up 68 percent of its contents and pushing matter apart with its repulsive force. We often say dark energy because cosmologists are kind of in the dark – we really don't know what it is.
About 100 years ago, Einstein added a variable to his equations of general relativity to represent the energy infused in space itself. He called it lambda, or the cosmological constant. In 1998, teams led by Adam Riess, Saul Perlmutter, and Brian Schmidt confirmed the presence of dark energy, winning them a Nobel Prize.
The discovery matched Einstein’s hypothesis about an accelerating universe and a cosmological constant. But in April 2024, data from a new telescope instrument produced the biggest map of the cosmos yet. The results throw the cosmological constant into question, suggesting that dark energy might be changing.
That would not be what a cosmological constant would have been predicting. This opens the floodgates for new theories. To understand what the cosmological constant is, we have to sort of start at the beginning, which is the Big Bang.
There's this initial kick of energy, and the universe starts expanding outwards. The dominant force is, generally speaking, gravity. We would expect that because it’s filled with matter and dark matter, the universe would eventually slow down because of this attractive force due to gravity.
But not only is it not slowing down –- it's accelerating. That's akin to me on Earth throwing up a tennis ball, expecting it to come back down due to gravity. And it doesn't.
It keeps going up and it's accelerating upwards. The simplest explanation for this acceleration is Einstein’s cosmological constant, an unchanging dark energy that permeates empty space. The universe is expanding and as it expands, we're essentially getting more and more empty space between galaxies and consequently more and more dark energy.
This hypothesis is the basis of Lambda-CDM, cosmologists’ current model of the universe. Physicists and cosmologists are between a rock and a hard place. 95% of the energy and matter in that model we cannot fully explain.
There are many physicists who would be very eager to see deviations from the predictions made by that model, which would indicate some new kind of physics that would lead us to insights about dark energy and dark matter. In 2021, the Dark Energy Spectroscopic Instrument, known as DESI, kicked off a five-year effort to measure the positions and velocities of 40 million galaxies. The goal of DESI is to build the largest 3D map of the universe that we've ever achieved.
If you want to measure and understand dark energy, you want to look at the universe at its largest size. DESI has 5000 optical fibers that allows the instrument to collect 5000 galaxies at a time. As DESI’s fibers collect light from galaxies, a spectrograph splits the light into its individual wavelengths.
This reveals how quickly a galaxy is moving away from Earth, which the researchers use to infer its distance. But to measure the expansion rate of the universe, DESI scientists turn to baryon acoustic oscillations, or BAOs – the leftover imprints of pressure waves from the Big Bang. Each of these unique features is imprinted on the universe like a ripple frozen onto a pond.
And the only thing that changes its size is the expansion of the universe. So with DESI by measuring this 3D map of the universe over time, we can actually see how BAO has evolved over time and therefore the impact of dark energy on that expansion. In April 2024, an international collaboration of more than 500 scientists published a rigorous analysis of the data from DESI’s first year of observations.
The results revealed something startling. If dark energy were the cosmological constant, galaxies would appear to be accelerating apart at an unchanging rate. But DESI showed that in the universe’s most recent history, many galaxies appeared closer together than expected – suggesting that dark energy is weakening over time.
This is not at all what we were thinking that we would be obtaining with this first year results. So that was an amazing surprise. The DESI data reveals the expansion rate of the universe by measuring the size of the BAO feature as a function of time.
If dark energy is the cosmological constant, the rate should hold steady at a value of 1. That's where we started to see hints of a possible deviation from Lambda CDM. To claim a discovery, physicists must prove a five-sigma level of confidence – about a one-in-a-million probability that the findings are a result of random chance.
When the DESI scientists pooled their data with other sets of astronomical observations, they achieved a probability of 3. 5 sigma – about a one-in-300 chance that it's a statistical fluke. We’re not there yet.
There's a serious hint, and that's why we're so excited about it. And we want to continue taking data to see if that hint is confirmed or if it goes away. If we confirm that dark energy is varying with time, then it just is not compatible with Lambda-CDM.
If some theorist comes along and can explain the observations, this is a Nobel Prize. It's a very tall task. But perhaps someone out there is up to it.
Imagine a phase of matter that’s at once a solid and a liquid. Like a solid, it has a rigid, spatially-ordered structure. But it flows like a superfluid – entirely without friction.
This is a supersolid. Since the 1950s, condensed matter physicists have debated whether this exotic phase of matter exists. But this year, scientists at the University of Innsbruck in Austria created a supersolid in the lab for the first time.
A supersolid state of matter is one of these unique situations in which you can really see directly the impact of quantum mechanics on measurable properties of a system. The results may help explain how matter behaves in some of the most extreme conditions in the universe. In 1937, scientists cooled liquid helium-4 to just above absolute zero.
They observed a new phase of matter: the superfluid, which flows like a fluid with zero viscosity. Here you see the liquid cooled down and passing into the superfluid phase. In 1949, the future Nobel Laureate Lars Onsager predicted that this frictionless behavior could be partly explained by a bizarre phenomenon: the formation of vortices, or microscopic quantum tornadoes.
That's really the fingerprint, I would say – even the hallmark – of superfluidity. So there's a very simple picture to understand this. Imagine that you have a bucket.
If you put a superfluid inside this bucket, if you rotate the bucket very slow, the superfluid would just stay at rest. It would not move at all. Then you exceed a critical velocity.
And then the system has to respond in some way. The rotation frequency is so large that the fluid inside cannot ignore anymore that something is happening. There is all this angular momentum that we are imposing, so instead of globally rotating, the superfluid will create a number of very small tornado.
These quantum tornadoes were also predicted to exist in the supersolid – an exotic phase of matter with both superfluid properties and a rigid solid structure. You need to detect the two natures of the system. So you need to detect the solid part and the superfluid part.
We knew very well the importance of observing vortices. To create these vortices you need to put your system under rotation. But our supersolid state is quite fragile.
And so we needed to find a good way to rotate the system without destroying it. The Innsbruck team harnessed the magnetic properties of dysprosium atoms to steer their system. Using lasers, we cool these atoms down to very close to the absolute zero.
You can trap these atoms and the system behaves as a whole single quantum wave. The team steered the atoms’ internal magnetic field about 50 times a second – fast enough to generate vortices while preserving the delicate quantum phase. And we saw the presence of these little holes – these were the vortices.
For the first time, they observed the dual nature of a supersolid. It was producing exactly the same images as the simulation. They were really able to observe it.
And this was a big, big result. In the long term, studying the dynamics of supersolids could help us understand mysterious quantum behaviors like superconductivity. But the Innsbruck team already discovered a surprising link to astrophysics.
We recently found a possible application of this rotating super solid in nature, for a system that is actually very far away from us, which is a neutron star. The inner crust of this neutron star is predicted to be a modulated superfluid. This is exactly what we usually call a supersolid state.
Some astrophysicists now suspect that a neutron star’s supersolid interior may explain a strange phenomenon known as a glitch, where the star briefly changes its rotation speed. Sometimes they spin up for a very short moment and then they slow down. And so one theory says that the vortex, which is created in the super solid at some point during the slowing down of the star, starts to escape and they crash into the wall of the star, transferring angular momentum and making the star spinning up.
Of course, we cannot have access to neutron stars because they are too far away. But we can have access to rotating supersolids in a lab with vortices inside of it. We had no idea that we could make this connection.
And that's the beauty to, you know, science and exchange of ideas and going and speaking to people away from your domain of investigation. It’s a very nice project. I’m very excited.
This is the amplituhedron. Hidden within its geometry is the answer to one of the most fundamental questions in physics: what happens when particles collide? In the decade since the discovery of the amplituhedron, physicists have looked for other shapes that predict the outcomes of particle interactions.
This year, physicists Carolina Figueiredo and Nima Arkani-Hamed had a breakthrough, linking geometry with real-world physics and significantly advancing the efforts of quantum geometry. Particle interactions are difficult to predict. Because quantum physics forbids these events from being observed directly, physicists can learn only the initial conditions and the outcome.
You have a bunch of particles coming in and a bunch of particles coming out, and you don't know what happened in the middle. But the way we have to compute what happens is to pretend that we're actually kind of following the path of the particles. So you're filling in a history of what happened to each particle.
That's what Feynman diagrams are about. Physicists use Feynman diagrams to track what might happen in a collision using lines to represent particle trajectories. A Feynman diagram is one such possibility.
But because of quantum mechanics, the final answer is given by the sum over all such possibilities. When you add up terms for all possible Feynman diagrams for a collision, you get a number called the scattering amplitude – the probability of a certain outcome. But that method has a big drawback.
There can be many, many, many, many pictures that contribute to a given process. The Feynman rules, while very correct, really make it computationally intractable to make predictions for experiments. Every single time we work very hard to compute some prediction for some experiment, the prediction itself mocks us by how simple it is.
This famous prediction that got Feynman his Nobel Prize – it’s many pages of algebra, and when all of the dust settles, you find the number one. There's been kind of an effort to try to reveal a certain simplicity that was hidden in the standard way that you compute it. We're trying to find different structures–different ideas–that land you on the answer kind of directly.
Physicist Nima Arkani-Hamed and his team discovered the amplituhedron while exploring a connection between Feynman diagrams and mathematical objects called polygon triangulations. These triangulations of polygons are literally just a set of edges that meet on a certain vertex. Which is exactly what we use to describe particle interactions.
The Feynman diagram is basically a graph. The amplituhedron in some way was encoding the information of all these Feynman diagrams that were found for a particular theory. The amplituhedron was a shortcut.
Its volume gave the sum of these Feynman diagrams – the scattering amplitude for the particle interaction’s outcome. I could entirely describe how to make predictions in this theory without ever referring to even a particle, honestly. But there was a catch: it didn’t work for real-world particles.
Arkani-Hamed’s team soon identified another shape, the associahedron, which encoded Feynman diagrams for a theory of different particles. This was when Carolina Figueiredo came across a striking coincidence. Scattering amplitudes take the form of fractions.
Physicists often focus on collisions where the denominator becomes small, which represent likely outcomes for particle interactions. But Figueiredo began looking for situations where the numerator becomes small, representing unlikely interactions. There was also something sitting upstairs that was encoding some extra information.
First, Figueiredo used the associahedron to look at collisions involving simple particles known as “colored scalars. ” She found that certain collisions flattened its volume to zero, meaning that they represented collisions forbidden for these particles. The amplitude is something which is related with the volume of this geometry.
Okay. And so from there, it was easy to understand that if I found limits where these geometries collapsed – like, reduced in dimension – then the volume would go to zero. So the amplitude would also have to vanish.
Figueiredo also looked at collisions involving pions and gluons, two other real-world particles. She found that all the theories shared the same zeros – meaning that they outlawed exactly the same particle collisions. What we observed is that for three different theories, a similar feature was there – not just similar, but like, remarkably similar – almost identical.
This was very striking at the time just because there was no obvious reason for these zeros to be located at precisely the same spots. These hidden zeros revealed that the three different theories were described by the same function, deriving from curves on geometric surfaces. There’s kind of a master object that encoded all the three different theories.
The reason why we understood that there were these zeros is precisely because we have the formulation in terms of the geometry. This is the first time that amplitudes for real-world particles have been calculated in a geometric way. It builds on the dream of the amplituhedron, suggesting that the right shapes can lead to new and more efficient ways of understanding the quantum world.
The complexity of the computation – it's highly reduced precisely because you're packaging everything in an extremely nice way. You're exploiting all the structure. It's tremendously powerful to have not just a machinery for generating scattering amplitudes, but to have a geometric interpretation behind it.
I'm very excited that there's an alternative.
