Guest speaker Jackson O’Donnell from UC Santa Cruz shares the ongoing research on dark matter studies
By EKATERINA MEDVEDEVA — science@theaggie.org
You, me and everything that we are used to seeing around us is made up of matter. So is the gas and celestial bodies in outer space, like stars; In fact, we are partly made up of “star stuff” as a result of the long processes of star life cycles and the formation of the universe as we know it.
However, all of this regular matter, also called baryonic matter, that we can “see” (or rather detect directly) makes up only 5% of the content of the universe. A much larger portion — about 27%, according to the European Organization for Nuclear Research (CERN) — consists of so-called dark matter, the nature of which is still mostly shrouded in mystery. Unlike baryonic matter, dark matter does not emit electromagnetic radiation.
“Researchers have been able to infer [its] existence […] only from the gravitational effect it seems to have on visible matter,” the study reads.
While the standard cosmological model (also known as ΛCDM) that postulates the existence of cold dark matter (CDM) has been successful in making predictions about our universe on large scales, there remains a lot of discrepancies on the small scale level. This has spurred additional extensions of dark matter studies that consider different variations on its properties.
In late February, a guest speaker from UC Santa Cruz — Jackson O’Donnell, who is a physics Ph.D. student — gave a talk at a UC Davis cosmology seminar about ongoing work in self-interacting dark matter (SIDM) research.
“In ΛCDM, we say that cold dark matter is an ideal gas,” O’Donnell said. “So if [it is made up of] particles, those particles just fly by each other and they only interact through gravity. With the self-interacting dark matter, [we are] saying that if two dark matter particles fly close enough to each other, they can bounce off of each other. So, the large scale structure of the universe stays exactly the same, because in most of the universe the dark matter is thin enough that [its particles] are never going to interact with each other anyway. [Instead, SIDM] really [shows effects] inside of Halos of dark matter, where there’s enough of it [for them] to possibly bounce off of each other.”
One of the things that the ΛCDM model fails to explain, which SIDM can make up for, is the existence of supermassive black holes very early on in the universe. These have been confirmed by observations from several telescopes, including the James Webb Space Telescope (JWST). This problem is aggravated by the fact that intermediate-mass black holes are scarcely observed in the universe, which creates a gap in our knowledge regarding how lightweight black holes (which are relatively well-studied) and how supermassive black holes form.
A study led by M. Grant Roberts, a doctoral student at UC Santa Cruz, explored “the possibility that a fraction of the cosmological dark matter could be ultra-strongly self-interacting, [which] would imply that gravothermal collapse [occurred] at early times in the cores of dark matter halos, followed by accretion,” resulting in formation of supermassive black holes.
“The dark matter self-interaction is a necessary component because the dark matter particles need a way to scatter off one another, much stronger than just gravitational interactions,” Roberts said. “This scatter causes the dark matter to bunch up in the very inner central regions of the galaxy, which allows them to collapse into supermassive black hole seeds.”
While this study considers SIDM on dwarf galaxy scales, on the other side of the spectrum, O’Donnell’s talk at UC Davis primarily was concerned about constraints on dark matter self-interaction in galaxy clusters. The quantity of interest that is constrained is the cross section of self-interaction, measured in centimeters squared per gram, which in simple terms describes how close the dark matter particles have to be to interact with each other.
“The current constraints are very different on these small scales and big scales, [which] naively seems inconsistent, but could actually make sense if there is a simple velocity dependence, [where if] dark matter particles are flying by each other slowly, they’re likely to interact and they don’t need to be quite so close to bounce off of each other,” O’Donnell said. “But if they’re moving fast, it could be that they have a lower cross section, [so] they need to almost exactly hit each other to interact.”
For these types of studies, strong gravitational lensing (where light gets bent due to a curvature of spacetime created by a massive object on its path) turns out to be an extremely powerful tool, as it allows researchers to measure the mass enclosed in a particular lensing region, allowing them to learn about the amount of dark matter contained within it.
“We have more [Integral Field Units] (IFU) data, [which is a kind of] spectroscopy where you get spatial and spectral information,” O’Donnell said when describing the future prospect of his work. “My advisor and I had a night of observing on one of the Keck telescopes back in September and we used an instrument called [Keck Cosmic Web Imager] (KCWI), which [UC Davis Department of Physics Professor] Tucker Jones’ group […] helped us out a lot with that. So, we have data on a few more strong lensing galaxy clusters sitting around waiting for us to do a similar analysis on.”
Besides SIDM, there are many other extensions — such as axion dark matter, warm dark matter and others — that offer unique potential explanations for many phenomena in our universe that for now remain unknown, which nevertheless offer an exciting outlook on the future of astrophysics.
Written by: Ekaterina Medvedeva — science@theaggie.org
