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Seminar presentation 18th May 2021 Iida Kostamo

1/20 Contents

• Background and history - How did we end up with the dark matter hypothesis? • Hot and - Candidates for cold dark matter • The halo density profile • Simulations (Millennium and Bolshoi) • Summary

Seminar presentation 18th May 2021 Iida Kostamo 2/20 Background

• The total mass-energy density of the universe (approximately): 1. 5% ordinary baryonic matter 2. 25% dark matter 3. 70% • Originally dark matter was referred to as "the missing mass" - 1930's: Fritz Zwicky did research on clusters • ...The problem is the missing light, not the missing mass, hence "dark matter"

Seminar presentation 18th May 2021 Iida Kostamo 3/20 The rotation curves of

• The rotation curve describes how the rotation velocity of an object depends on the distance from the center of the galaxy • Assumption: Kepler's III law, i.e. the rotation velocities decrease with increasing distance • In the 1970's Vera Rubin and her colleagues did research on the rotation curves of various spiral galaxies

Seminar presentation 18th May 2021 Iida Kostamo 4/20 The rotation curves of galaxies

• H = Hubble constant • The rotation curves become flat when the radius is large enough • The same result for all galaxies: the rotation curves are not descending → There must be non-luminous mass in galaxies

Seminar presentation 18th May 2021 Iida Kostamo 5/20 MACHOs (Massive Astrophysical Compact Halo Object)

• Objects that emit extremely little or no light → Difficult to observe • Planets, very faint stars, black holes, etc. • Could MACHOs form a significant fraction of dark matter? • Possible to observe through gravitational lensing

Seminar presentation 18th May 2021 Iida Kostamo 6/20 • If a MACHO passes in front of a bright star, the star appears brighter Gravitational • Results: Objects with masses in the range 0,3 Lunar masses – 100 Solar (micro)lensing masses can't make up a significant fraction of dark matter → (Most of) dark matter must be composed of unknown particles

Seminar presentation 18th May 2021 Iida Kostamo 7/20 Cosmic microwave background

• The universe as it was 400 000 years after the Big Bang • Information about the density perturbations in the early universe • If the universe was made of only baryonic matter, the early density perturbations would have to be larger than what we can observe from the CMB • Thus, (cold) dark matter forms potential wells in the early universe

Seminar presentation 18th May 2021 Iida Kostamo 8/20 Hot and cold dark matter

• "Hot" and "cold" refer to the average Hot Warm Cold velocities of particles in the early universe: • Cold → Nonrelativistic particles • Hot → Relativistic particles • The structure formation mechanism depends on whether the dark matter is hot or cold • In the picture, the top row describes the early universe, and the bottom row describes the present

Seminar presentation 18th May 2021 Iida Kostamo 9/20 Hot and cold dark matter

Hot Warm Cold • → Smaller structures fragment from larger ones - Not enough time to form the present structures • Cold dark matter → Smaller structures combine into larger ones - Fits theories better

→ CDM is the dominant form of DM

Seminar presentation 18th May 2021 Iida Kostamo 10/20 Candidates for CDM WIMPs (Weakly Interacting Massive Particle)

• As the universe cools and expands, the number density of WIMPs approaches a constant value, and this thermal relic density is naturally in the same range as the demanded density of dark matter → "WIMP-miracle" • Mass range 10 GeV – 1 TeV (electron: 0,5 MeV, proton: 940 MeV) • Interaction with W and Z gauge bosons, i.e. weak interaction

Seminar presentation 18th May 2021 Iida Kostamo 11/20 Candidates for CDM

• Peccei-Quinn solution to the strong CP (Charge-Parity) problem in particle physics - The neutron electric dipole moment? • Very low mass, but can be born nonrelativistic • Various boundary conditions for the mass - To make sure the lifetime of axions is longer than the age of the universe, the mass must be more than 20 eV

Seminar presentation 18th May 2021 Iida Kostamo 12/20 Halo density profile

• Density profile → Mass distribution • NFW profile (Navarro-Frenk- White) and Einasto profile • Two parameters in the NFW profile, three in the Einasto profile (α being one of them) → The latter can give better results

Seminar presentation 18th May 2021 Iida Kostamo 13/20 NFW profile vs. Einasto profile

• In the graph, the radius r and density ρ have been normalized by the scale radius R_s and scale density ρ_0 • For small radii, the Einasto profile isn't as steep as the NFW profile • ...However, compared to observations, the Einasto profile is also too "cuspy" → Core-cusp problem

Seminar presentation 18th May 2021 Iida Kostamo 14/20 N-body simulations

• Dynamical system of particles under the influence of physical forces - Not actual particles, masses are much larger • Simulations allow us to model the large-scale structure and evolution of the universe • As the small perturbations of the early universe grow, finding analytical solutions becomes impossible → Simulations

Seminar presentation 18th May 2021 Iida Kostamo 15/20 • More than 10 billion "particles" with a billion Solar masses each The Millennium • The mass function, the large-scale structure of the universe ("the cosmic web") simulation • The largest simulation of the growth of dark matter structure at the time (2005)

Seminar presentation 18th May 2021 Iida Kostamo 16/20 Bolshoi simulation

• Updated values for cosmological parameters • Better resolution • 8,6 billion "particles" with 200 million Solar masses each

Seminar presentation 18th May 2021 Iida Kostamo 17/20 Universal density profile

• In 2019, the haloes were simulated over a very wide mass range (galaxy clusters – Earth mass) • The L0 level is similar to the Millennium simulation • Conclusion: The structure of dark matter haloes is universal in all scales

Seminar presentation 18th May 2021 Iida Kostamo 18/20 Summary

• There is observational evidence for dark matter (Rotation curves, CMB, etc.) • The dark matter haloes are made of "cold" particles • The structure of the haloes is universal

Seminar presentation 18th May 2021 Iida Kostamo 19/20 Sources of the pictures

h ttp s://www.eso.org/public/images/eso1217a/ Slides 1 and 19 https://www.iac.es/en/projects/anisotropy-cosmic-microwave- background Slide 8 https://en.wikipedia.org/wiki/File:Dark_matter_halo.png Slide 2 http://burro.case.edu/Academics/Astr222/Cosmo/Structure/darkmatt er.html Slides 9 and 10 https://esahubble.org/images/potw1849a/ Slide 3 https://commons.wikimedia.org/wiki/File:Comparison_of_NFW_and_ Einasto_profiles.svg Slides 13 and 14 https://apnews.com/article/7db1ff28bca84797bcf0200861d74090 Slide 4 http://hipacc.ucsc.edu/Bolshoi/Images.html#img5 Slide 15 http://articles.adsabs.harvard.edu/pdf/1980ApJ...238..471R Slide 5 https://www.cfa.harvard.edu/imagelist/2012-12 Slide 6 top https://wwwmpa.mpa- garching.mpg.de/galform/virgo/millennium/ Slide 16 https://commons.wikimedia.org/wiki/File:AU_MIc_M- dwarf_artist%27s_conception.jpg Slide 6 bottom http://hipacc.ucsc.edu/Bolshoi/Images.html#bb4 Slide 17 https://www.nasa.gov/content/hubble-sees-a-smiling-lens Slide 7 https://www.nature.com/articles/s41586-020-2642-9 Slide 18

Seminar presentation 18th May 2021 Iida Kostamo 20/20