Dark Matter

Can dark matter be composed of more than just one type of particle or substance?

• Yes

• No

• We don't know

Answer explanation

The term “dark matter” broadly refers to missing mass in a cosmological system that cannot be observed directly, but whose gravitational effects can be measured. There are multiple candidates for dark matter that fit this criterion and contribute to its total energy density, even if most have almost negligible contributions.

Which particles from the Standard Model are dark matter candidates?

• Higgs boson

• 1. Weak Bosons (W±,Z0)

• Neutrinos

• None

Answer explanation

Despite their tiny masses, the vast amount of neutrinos produced in the early universe result in  a non-negligible gravitational effect on other matter. However, neutrinos contribute as “hot” dark matter and can only account for a minute proportion of the measured dark matter energy density.

What causes the dark matter “wind” perceived from the perspective of an Earth-bound detector?

• 1. The rotation of the dark matter halo around the Milky Way

• 1. The movement of the Sun around the galaxy centre

• 1. The movement of the Earth around the Sun

• 1. The Earth’s rotation

Answer explanation

The net rotation of a galactic dark matter halo is very small compared to that of the galaxy disk. The Sun has a radial velocity around the Milky Way centre of 220 km/s, passing through the dark matter halo. To an Earth-bound detector, the dark matter particles in the halo would appear as going at that speed in the opposite direction. Earth’s rotation around the Sun, at only 30 km/s and with a ~30º declination with respect to the plane of the galaxy,  only produces a minor annual modulation effect on the flux of dark matter particles on Earth.

Which of the following are viable candidates for cold dark matter? (Select all that apply)

• Neutrinos

• 1. Axions and Axion-like particles (ALPs)

• 1. Quarks and gluons

• 1. Weakly Interacting Massive Particles (WIMPs)

Answer explanation

WIMPs and Axions, if they exist, could explain the expected abundance of cold dark matter in the universe. Despite their small masses, Axions and ALPs would not be produced thermally and thus would behave as a “cold gas”. They have strong theoretical and phenomenological support, and are the focus of most experimental efforts in both direct and indirect searches. Neutrinos are considered “hot dark matter” and cannot produce the large matter structures in the universe on their own. Quarks and gluons experience the strong force and would not contribute to dark matter.

Why is dark matter an important ingredient of the universe? (Select all that apply)

• 1. It’s responsible for the reheating of the universe after inflation.

• 1. Holds galaxies and galaxy clusters together through its gravitational influence.

• 1. Shaped the “cosmic web” of large scale structures of the universe.

• 1. Affects the universe’s expansion rate and geometry.

• 1. Drives the accelerated expansion of the universe.

Answer explanation

Without a cold and abundant dark matter component, there would be no formation of large scale structures nor time to form galaxies given the current age of the universe. Galaxies themselves would rotate much slower and clusters of galaxies would dissipate without the gravitational influence of dark matter. The main contribution to the positive pressure (gravity) in the universe is dark matter, which counters the expansion of the universe driven by dark energy (even though it’s not enough to balance the expansion).