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Dark Matter in the Solar System, Galaxy, and Beyond
Stockholm University, Faculty of Science, Department of Physics.ORCID iD: 0000-0001-5686-3743
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

There is evidence that dark matter constitutes a majority of the Universe's matter content. Yet, we are ignorant about its nature. Understanding dark matter requires new physics, possibly in the form of a new species of fundamental particles. So far, the evidence supporting the existence of dark matter is purely gravitational, ranging from mass measurements on galactic scales, to cosmological probes such as the cosmic microwave background radiation. For many proposed models of particle dark matter, the strongest constraints to its properties do not come from particle collider or direct detection experiments on Earth, but from the vast laboratory of space. This thesis focuses on such extra-terrestrial probes, and discusses three different indirect signatures of dark matter.

(1) A first part of this thesis is about the process of dark matter capture by the Sun, whereby dark matter annihilating in the Sun's core could give rise to an observable flux of high-energy neutrinos. In this work, I was the first to thoroughly test the common assumption that captured dark matter particles thermalise to the Sun's core temperature in negligible time. I found that the thermalisation process is short with respect to current age of the Sun, for most cases of interest. (2) A second part concerns a radio signal associated with the epoch when the first stars were born. A measurement of this signal indicated an unexpectedly low hydrogen gas temperature, which was speculated to be explained by cooling via dark matter interactions. In my work, I proposed an alternative and qualitatively different cooling mechanism via spin-dependent dark matter interactions. While bounds coming from stellar cooling excluded significant cooling for the simple model I considered, perhaps the same cooling mechanism is allowed in an alternative dark matter model. (3) Thirdly, a significant part of this thesis is about the mass distribution of the Galactic disk, which can be measured by analysing the dynamics of stars under the assumption of equilibrium. Although most of the matter in the Galactic disk is made up of stars and hydrogen gas, exact measurements can still constrain the amount of dark matter. Potentially, dark matter could form a dark disk that is co-planar with the stellar disk, arising either from the Galactic accretion of in-falling satellites or by a strongly self-interacting dark matter subcomponent. Together with my collaborators, I made significant progress in terms of the statistical modelling of stellar dynamics. I measured the matter density of the solar neighbourhood using Galactic disk stars and data from the Gaia mission. I found a surplus matter density close to the Galactic mid-plane, with respect to the observed baryonic and extrapolated dark matter halo densities. This result could be due to a dark disk structure, a misunderstood density of baryons, or due to systematics related to the data or equilibrium assumption. I also developed an alternative method for weighing the Galactic disk using stellar streams. This method does not rely on the same equilibrium assumption for stars in the Galactic disk, and will be used to provide a complementary mass measurement in future work.

The different indirect probes of dark matter discussed in this thesis span a great range of spatial scales − from stellar interactions relevant to our own solar system, to the matter distribution of the Milky Way, and even cosmological signals from the dawn of the first stars. Through the macroscopic phenomenology of dark matter, the microscopic particle nature of dark matter can be constrained. Doing so is a window into new physics and a deeper understanding of the Universe we live in.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2020. , p. 65
Keywords [en]
dark matter: phenomenology, dark matter: indirect detection, dark matter: particle nature, Galactic dynamics, Galactic composition
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Physics
Identifiers
URN: urn:nbn:se:su:diva-180534ISBN: 978-91-7911-120-5 (print)ISBN: 978-91-7911-121-2 (electronic)OAI: oai:DiVA.org:su-180534DiVA, id: diva2:1420883
Public defence
2020-05-25, sal FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 14:15 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 5: Manuscript.

Available from: 2020-04-28 Created: 2020-04-01 Last updated: 2020-05-26Bibliographically approved
List of papers
1. Thermalization time scales for WIMP capture by the Sun in effective theories
Open this publication in new window or tab >>Thermalization time scales for WIMP capture by the Sun in effective theories
2017 (English)In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 5, article id 046Article in journal (Refereed) Published
Abstract [en]

I study the process of dark matter capture by the Sun, under the assumption of a Weakly Interacting Massive Particle (WIMP), in the framework of non-relativistic effective field theory. Hypothetically, WIMPs from the galactic halo can scatter against atomic nuclei in the solar interior, settle to thermal equilibrium with the solar core and annihilate to produce an observable flux of neutrinos. In particular, I examine the thermalization process using Monte-Carlo integration of WIMP trajectories. I consider WIMPs in a mass range of 10{1000 GeV and WIMP-nucleon interaction operators with different dependence on spin and transferred momentum. I find that the density profiles of captured WIMPs are in accordance with a thermal profile described by the Sun's gravitational potential and core temperature. Depending on the operator that governs the interaction, the majority of the thermalization time is spent in either the solar interior or exterior. If normalizing the WIMP-nuclei interaction strength to a specific capture rate, I find that the thermalization time differs at most by 3 orders of magnitude between operators. In most cases of interest, the thermalization time is many orders of magnitude shorter than the age of the solar system.

Keywords
dark matter detectors, dark matter experiments, neutrino astronomy, neutrino detectors
National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-145274 (URN)10.1088/1475-7516/2017/05/046 (DOI)000402878200046 ()
Available from: 2017-07-26 Created: 2017-07-26 Last updated: 2020-04-14Bibliographically approved
2. 21 cm cosmology and spin temperature reduction via spin-dependent dark matter interactions
Open this publication in new window or tab >>21 cm cosmology and spin temperature reduction via spin-dependent dark matter interactions
2019 (English)In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 6, article id 014Article in journal (Refereed) Published
Abstract [en]

The EDGES low-band experiment has measured an absorption feature in the cosmic microwave background radiation (CMB), corresponding to the 21 cm hyperfine transition of hydrogen at redshift z similar or equal to 17, before the era of cosmic reionization. The amplitude of this absorption is connected to the ratio of singlet and triplet hyperfine states in the hydrogen gas, which can be parametrized by a spin temperature. The EDGES result suggests that the spin temperature is lower than the expected temperatures of both the CMB and the hydrogen gas. A variety of mechanisms have been proposed in order to explain this signal, for example by lowering the kinetic temperature of the hydrogen gas via dark matter interactions. We introduce an alternative mechanism, by which a sub-GeV dark matter particle with spin-dependent coupling to nucleons or electrons can cause hyperfine transitions and lower the spin temperature directly, with negligible reduction of the kinetic temperature of the hydrogen gas. We consider a model with an asymmetric dark matter fermion and a light pseudo-vector mediator. Significant reduction of the spin temperature by this simple model is excluded, most strongly by coupling constant bounds coming from stellar cooling. Perhaps an alternative dark sector model, subject to different sets of constraints, can lower the spin temperature by the same mechanism.

Keywords
cosmology of theories beyond the SM, particle physics - cosmology connection
National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-170096 (URN)10.1088/1475-7516/2019/06/014 (DOI)000470866600003 ()
Available from: 2019-07-03 Created: 2019-07-03 Last updated: 2020-04-14Bibliographically approved
3. The dynamical matter density in the solar neighbourhood inferred from Gaia DR1
Open this publication in new window or tab >>The dynamical matter density in the solar neighbourhood inferred from Gaia DR1
2019 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 482, no 1, p. 262-277Article in journal (Refereed) Published
Abstract [en]

We determine the total dynamical density in the solar neighbourhood using the Tycho-Gaia Astrometric Solution catalogue. Astrometric measurements of proper motion and parallax of stars inform us of both the stellar number density distribution and the velocity distribution of stars close to the plane. Assuming equilibrium, these distributions are interrelated through the local dynamical density. For the first time, we do a full joint fit of the velocity and stellar number density distributions while accounting for the astrometric error of individual stars, in the framework of Bayesian hierarchical modelling. We use a sample of stars whose distance extends to approximately 160 pc from the Sun. We find a local matter density of rho(0) = 0.119(-0.012)(+0.015) M-circle dot pc(-3), where the result is presented as the median to the posterior distribution, plus/ minus its 16th and 84th percentiles. We find the Sun's position above the Galactic plane to be z(circle dot) = 15.29(-2.16)(+2.24) pc, and the Sun's velocity perpendicular to the Galactic plane to be W-circle dot = 7.19(-0.18)(+0.18) km s(-1).

Keywords
Galaxy: disc, Galaxy: kinematics and dynamics, Galaxy: structure
National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-165794 (URN)10.1093/mnras/sty2400 (DOI)000454575300020 ()
Available from: 2019-02-19 Created: 2019-02-19 Last updated: 2020-04-14Bibliographically approved
4. Measuring the local matter density using Gaia DR2
Open this publication in new window or tab >>Measuring the local matter density using Gaia DR2
2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 623, article id A30Article in journal (Refereed) Published
Abstract [en]

Aims. We determine the total dynamical matter density in the solar neighbourhood using the second Gaia data release (DR2).

Methods. The dynamical matter density distribution is inferred in a framework of a Bayesian hierarchical model, which accounts for position and velocity of all individual stars, as well as the full error covariance matrix of astrometric observables, in a joint fit of the vertical velocity distribution and stellar number density distribution. This was done for eight separate data samples, with different cuts in observed absolute magnitude, each containing about 25 000 stars. The model for the total matter density does not rely on any underlying baryonic model, although we assumed that it is symmetrical, smooth, and monotonically decreasing with distance from the mid-plane.

Results. We infer a density distribution which is strongly peaked in the region close to the Galactic plane (less than or similar to 60 pc), for all eight stellar samples. Assuming a baryonic model and a dark matter halo of constant density, this corresponds to a surplus surface density of approximately 5-9 M-circle dot pc(-2). For the Sun's position and vertical velocity with respect to the Galactic plane, we infer Z(circle dot) = 4.76 +/- 2.27 pc and W-circle dot = 7.24 +/- 0.19 km s(-1).

Conclusions. These results suggest a surplus of matter close to the Galactic plane, possibly explained by an underestimated density of cold gas. We discuss possible systematic effects that could bias our result, for example unmodelled non-equilibrium effects, and how to account for such effects in future extensions of this work.

Keywords
Galaxy: kinematics and dynamics, Galaxy: disk, solar neighborhood, astrometry
National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-167662 (URN)10.1051/0004-6361/201834718 (DOI)000459752300002 ()
Available from: 2019-04-03 Created: 2019-04-03 Last updated: 2020-04-14Bibliographically approved
5. Measuring the Matter Density of the Galactic Disk Using Stellar Streams
Open this publication in new window or tab >>Measuring the Matter Density of the Galactic Disk Using Stellar Streams
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We present a novel method for determining the total matter surface density of the Galactic disk by analysing the kinematics of a dynamically cold stellar stream that passes through or close to the Galactic plane. The method relies on the fact that the vertical component of energy for such stream stars is approximately constant, such that their vertical positions and vertical velocities are interrelated via the matter density of the Galactic disk. By testing our method on mock data stellar streams, with realistic phase-space dispersions and Gaia uncertainties, we demonstrate that it is applicable to small streams out to a distance of a few kilo-parsec, and that the surface density of the disk can be determined to a precision of 6 %. This method is complementary to other mass measurements. In particular, it does not rely on any equilibrium assumption for stars in the Galactic disk, and also makes it possible to measure the surface density to good precision at large distances from the Sun. Such measurements would inform us of the matter composition of the Galactic disk and its spatial variation, place stronger constraints on dark disk sub-structure, and even diagnose possible non-equilibrium effects that bias other types of dynamical mass measurements.

Keywords
stars: kinematics and dynamics, Galaxy: fundamental parameters, Galaxy: structure
National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-180533 (URN)
Available from: 2020-04-01 Created: 2020-04-01 Last updated: 2020-04-14

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