- Katherine Freese, cosmologist specialized in dark matter, visits the IFT this week to discuss with its experts the main challenges in this research field.
- Dark matter, which we can’t see but makes up 85% of the matter of the universe, could be made of a new type of particle, or even so-called primordial black holes.
- One of Freese’s most important predictions are dark stars, powered partially by dark matter, which could already have been observed by our instruments.
- Dark energy is almost 70% of everything in the universe and it could explain its accelerated expansion. Another one of Freese’s most interesting proposals is an expansion model that could require modifications to Einstein’s equations.
Written by: Joaquín Lázaro Souverville
Madrid, may 23rd 2024.- On the past May 21st, cosmologist and dark matter expert Katherine Freese was welcomed by the Institute of Theoretical Physics IFT UAM/CSIC to give a colloquium to share the latest research on dark matter. During her conference, she explained how we discovered it and are trying to detect it, what it could be made of and how it’s studied. Speaking of her own research, she highlighted her ‘dark stars’, those formed partially of dark matter as a heat source. In an interview granted to the IFT, Freese shared some of the topics she mentioned on her colloquium, she expanded some details on her work and ended with some advice for physics students.
Katherine Freese is Professor of Physics at the University of Texas in Austin and professor at Stockholm University. She describes her work as the study of “the smallest particles in the universe explaining the very largest structures”. She covers cosmology, particle physics and what the universe is made of, focusing particularly on dark matter.
What is dark matter?
Freese described dark matter as a strange substance that we can’t see directly. Only its influence on nearby objects. “There is 5 times as much dark matter as there is ordinary matter inside our own Milky Way galaxy. If we take everything we know about your body, the air, the walls, the Sun, the planets, everything we know, all made of atoms, all of that adds up to only 5% of the universe. This is unbelievable” the researcher explained. “Then we have 25% dark matter and 70% dark energy”.
The dark matter problem has a history of 90 years of study. It started with Knut Lundmark and Fritz Zwicky in the 30s. Zwicky observed galaxy clusters, and some appeared to move too fast if the only gravitational pull they felt was due to the observable stellar matter. It was Zwicky who coined the term ‘dark matter’ as a proposal to explain this phenomenon: a new kind of matter that we can’t see which interacts with its surroundings through gravity. Thus there would be some additional pull that would explain the movements of galaxies.
In the 70s, the idea of dark matter was more widely accepted by the scientific community thanks to the work of Vera Rubin. She observed that stars orbit too fast around the centre of the galaxy, a problem similar to that found by Zwicky 40 years prior. And she proposed the same solution.
These were key evidence to the discovery of dark matter, but Freese pointed out that today we have many more to confirm its existence. Even if dark matter doesn’t emit light, it does bend it, as any body with mass does. Thus we observe what we call gravitational lenses, distorted images of celestial bodies due to this bending of light.
She also mentioned the so-called Bullet Cluster, two galaxy clusters colliding, where you can see how the gas from both of them has clashed in the centre, but dark matter goes through with no impact, passing through both ordinary and other dark matter. Since it has no electromagnetic interactions with anything, even itself, it suffers no collision. Freese highlighted this cluster as the best proof we have that we are observing two types of matter that behave differently. Ordinary matter that we can observe, and dark matter.
Dark matter candidates: particles or black holes
Given that we don’t know the nature of this matter, the work of scientists like Freese is focused on finding out what it’s made of. The researcher pointed out three as the best candidates: WIMPs, axions and primordial black holes.
WIMPs are Weakly Interactive Massive Particles. As their name implies, they are particles that, besides experimenting gravity, also feel the weak force, the fundamental interaction responsible for some types of radioactivity. Freese explained the reason these are promising candidates: “In the early universe, we can calculate how many particles there were of all different types. And if these things annihilated among themselves with the weak force then you can ask how many are left in the universe today. It comes out just right to solve the dark matter problem”.
On the other side, axions are hypothetical elemental particles that were proposed to solve a problem with quantum mechanics. However, in studying the propertied they would have, it was found they would behave like the dark matter we have observed, and they would have a mass that matches with the expected amount for dark matter. Freese highlighted WIMPs and axions as good options because they weren’t invented only to solve the dark matter problem. “There are problems in particle theory that in solving them you automatically get these particles”.
She also mentioned primordial black holes. She explained how in the early universe there were density fluctuations, regions where mass would accumulate more than others. If this mass was enough, it could collapse into a black hole. Primordial black hole detection is something that has been brought up because of the results offered by LIGO (Laser Interferometry Gravitational-waves Observatory), a gravitational wave observatory that could detect these dark matter candidates.
When asked which candidate was her favourite, Freese said “I’m not able to choose one myself because this would require faith. In science we just have to wait and see what is reality”. Even though she has worked a lot on WIMPs, she pointed out just the same how promising axions or primordial black holes are as dark matter candidates.
For the question on how to detect WIMPs or axions, Freese described two types of detection: direct and indirect. Direct detection consists of big underground detectors based on calculations done by Freese herself, among others. They have to be underground to avoid false positives detecting other particles going through them, like cosmic rays. Efforts are being made with this detectors so that in the following years an observation can be made, and Freese said that the DAMA detector in Italy is already showing promising results
Indirect detection through gamma rays and the Fermi-LAT experiment
On the other hand, Freese clarified that there are other innovative ways of detecting dark matter indirectly. A clear example she highlighted is the Large Area Telescope (LAT) aboard NASA’s Fermi satellite. With it, they search for signals of dark matter annihilation in the form of gamma rays from regions where there is a lot of dark matter, like the centre of our galaxy. Instead of the direct detection of dark matter particles, it’s the indirect detection of the by-products of their interactions. IFT’s researcher Miguel Ángel Sánchez Conde is Science Convener of this Fermi-LAT collaboration, participating in this international effort to discern if this gamma ray signals are other events, or if they actually are dark matter signals.
Stars powered by dark matter: dark stars
Part of Freese’s work on dark matter deals with a kind of object she suggested herself in 2007, along with Paolo Gondolo and Douglas Spolyar: dark stars. The first thing Freese insisted on as she explained what her dark stars are is that they are not made of dark matter. They are made of ordinary matter, hydrogen and helium, and a bit of dark matter. What this dark matter does is provide a heat source.
The Sun and other ordinary stars have fusion in their cores, they fuse hydrogen into helium to generate energy that keeps them bright and hot. This doesn’t happen in dark stars. According to Freese, they would be the first stars to be formed, in the centre of protogalactic objects, future galaxies in the early universe. Being in regions with a lot of dark matter, there would also be a lot of dark matter annihilation. This annihilation means that two dark particles interact, and two different particles come out of this interaction. This particles have energy, and as they get stuck in the clouds of hydrogen and helium, it turns the cloud into a star that Freese described as very strange.
They can be as wide as 10 times the distance from the Earth to the Sun. Their mass starts out similar to our Sun’s, but it can continue to grow until it’s a million times more massive, and a billion times brighter. This means they are observable, since they can be as bright as an entire galaxy.
The James Webb Space Telescope (JWST) could detect them, since it can observe far away objects, from the early universe. Freese pointed out that it has already found objects that could be dark stars. “We have identified some candidates that have the right spectra”. It still can’t be confirmed due to lack of information, like if they are point objects, which would be the case for dark stars, or extended objects, like a galaxy. But Freese expressed her optimism on future observations from James Webb. She considers that we will be able to observe these objects more carefully and determine if they really are dark stars.
Dark energy
Freese described dark energy as something that is causing the acceleration of the expansion of our universe, which makes up 70% of it. On contrast, she defined matter as objects interacting through gravity, pulling on each other. However, dark energy has some sort of repulsive behaviour.
Even if little is known with certainty, there are theories on it. Freese explained one of the more well-known theories: that it could be some kind of vacuum energy. In this case, vacuum doesn’t mean emptiness, there is something there. “In this room where we’re talking right now, we have particle-antiparticle pairs that come into existence very briefly and then they go away again” she said. “There are experiments that have proved it”. Vacuum energy can be calculated based on this. However, the theory gives results that are too high compared with observations. Freese pointed out that we know it’s not as high as the math says, but it isn’t zero, and we can’t explain it.
Freese worked on an alternative that doesn’t require vacuum energy, the Cardassian expansion. She poses that the universe is made only of matter, both dark and ordinary, without vacuum energy, but with different equations. In this model there is no dark energy, but Einstein’s equations might require modifications. There are many theoretical models such as this one trying to explain dark energy, even if they haven’t been proven yet. “This is a very hard problem” Freese concluded.
Working in physics according to Katherine
But even facing such difficult problems, Freese said it’s fun for her. She likes the creativity of science, to interact with other people throwing ideas at each other. “I remember my son used to come in the room saying ‘I wanna play too’ cause he saw I was having so much fun. I enjoy the process of understanding these fundamental puzzles of the universe”.
On the subject of possible applications, she pointed out that she doesn’t stop to think of the practical influence that her research could have, because she doesn’t know if it will have any. “Do I believe dark matter annihilation is going to power lightbulbs? No, well, I don’t know. But if you understand what most of the mass of the universe is you can’t tell me it’s not going to be important for our daily life”. She said that in the practical side, you can’t know which could be the consequences of a discovery.
Lastly, Freese finished with advice for students and future physicists. The first thing she mentioned is the importance of finding a good mentor, someone to take you in interesting directions and with whom you can have a nice relationship. But on the job in general, she said “Believe in yourself. If you have a passion for something, then follow it”.
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