DARK MATTER

Marco Vancini Photos

EVIDENCE

An overwhelming number of independent observations in the past decades from a cosmological to an astronomical scale lead to an image of our universe where it is dominated by two components whose exact nature is still widely unknown: dark energy and dark matter.

While dark energy resembles an almost uniformly distributed energy density responsible for the accelerated expansion of the universe, dark matter is a new kind of non-luminous matter that is so far not perceivable by any interaction except its gravitational one.

This section introduces evidence for dark matter from a large to a small scale, as well as the most popular candidates.

Cosmic Microwave Background

One of the most compelling pieces of evidence for the existence of dark matter arises from the precise measurements of the temperature anisotropies at the level of µK in the cosmic microwave background (CMB) by the satellite-based experiments WMAP and Planck.

These fluctuations are directly linked to the matter distribution at the point of recombination in the early universe about 380000 years after the big bang, and therefore give access to the energy density of the universe during this period of time. An angular power spectrum can be obtained.

The data is best described within the cosmological standard model, called Lambda (Λ) cold dark matter (ΛCDM) model, where Λ refers to the cosmological constant required to explain the current accelerated expansion of the universe. Cold dark matter density perturbations in the gravitational potential formed oscillations of the baryon-photon plasma before recombination. This lead to a characteristic oscillation pattern in the CMB power spectrum called acoustic peaks. The fractions of baryonic and dark matter can be computed with the relative height of these acoustic peaks. From these calculations, a flat universe was found with an energy density distribution of 4.9% normal (baryonic) matter, 25.9% dark matter, and 69.2% dark energy.

Structure Formation

Given the smallness of density fluctuations observed in the CMB and given the great amount of structure observed today, and even within the first billion years after the Big Bang, there is dark matter needed to form seeds of structure growth well before the release of the CMB.

Deep surveys of the galaxies and galaxy clusters in the universe revealed that they are forming so-called large scale structure: galaxies and clusters are forming chain-like structures, and the space between these structures is empty. It is thought that the cold dark mater (CDM, travelling much slower than light) made the seeds for these large scale structure, and then ordinary matter gathers by gravity and forms the galaxies and clusters.

A number of numerical simulations based on the CDM model succeeded in reproducing the large scale structure in our computers.

Gravitational Lensing

Another phenomenon pointing to the existence of dark matter is gravitational lensing as predicted by Albert Einstein in general relativity. Spacetime can be curved around a massive object referred to as lens. Light that is emitted by a source is deflected by the lens depending on its mass. The observer on Earth in the line of sight will see a distorted image of the source under study due to the bent light trajectories. A two-dimensional basic scheme is shown below.

With the help of known sources, the gravitational potential of the lens, for example a galaxy cluster, can be reconstructed from the degree of light deflection giving access to the amount of matter inside the bending object. When determining the mass of the luminous matter within several clusters with X-ray measurements, discrepancies in the quantity of mass were found. These can be resolved with the existence of dark matter within the objects.

Merging Galaxy Cluster

Gravitational lensing is also applied to galaxy-cluster collisions in order to reconstruct the center of mass after intermixing of the objects. 

Classically, the diffuse gas clouds, which represent the bulk amount of ordinary matter within the two clusters, would slow down due to friction. The point-like stellar components pass each other without collision and only interact gravitationally. From this assumption, the center of mass would follow the gas distribution (colored clouds on the picture below).

However, it was still found in the center of the respective cluster, clearly displaced from the location of the ordinary matter (Green contours on the picture below).

This paradox can be solved by including the existence of dark matter halos around both clusters following their original trajectories independent of the collision. Furthermore, an upper limit for the strength of the self-interaction of dark matter can be obtained from these studies.

The most prominent example is the Bullet Cluster, a system of two galaxy clusters collided about 150 million years ago.

Galaxy Rotation Curves

Additional evidence can be found on smaller scale structures by measuring the rotation velocity of spiral galaxies. The rotation velocity v(r) of stars with large radii r orbiting the galactic center is expected to follow Newtonian dynamics with

v(r)  ~ 1 / √r

as most of the luminous mass is located there.

However, the measured velocities stay approximately constant for increasing distances from the galactic center. This can be explained by adding a uniformly distributed dark matter halo to the given model.

Consequently, the observed velocity distribution can be represented by a composition of the galactic disk, the galactic gas, and the dark matter halo, as shown on the bottom for the galaxy Messier 33.

CANDIDATES

A number of hypothetical new primary particles and other astronomical objects are proposed as the candidates of the dark matter. The mass range spans 90 orders of magnitude.

Here, two  candidates that are most promising and relevant to XENON experiment are explained: WIMPs and axions. Each candidate is well-motivated and thus a great number of experimental efforts have been made so far.

Further top candidates are summarized in this article by the Discover Magazine.

WIMPs

The Weakly Interacting Massive Particle (WIMP) is a class of hypothetical particles that have been the leading candidate of the dark matter DM for decades. The main reason for its popularity is that it can naturally explain the correct amount of the dark matter in the universe.

The standard assumption of WIMP production is that it is thermally produced in the early universe at high temperature.
WIMPs can annihilate with each other into normal particles and the “weak” annihilation rate explains the amount of the dark matter of today.

XENONnT is searching for the WIMP dark matter in the parameter space where many theories predict its existence.

Axions

The Axion is a hypothetical particle arising from the Peccei-Quinn theory which is proposed to solve the so-called strong CP problem. The Axion is named by Frank Wilczek after a laundry detergent since it can clean up the strong CP problem with an axial current.

Generally, axions can be categorized into quantum chromodynamics (QCD) axions (with original motivations) and axion-like particles (ALPs).

Axions could be created via thermal mechanism or non-thermal mechanisms in the early universe. For QCD axions, the preferred mass region is micro electron Volts to milli electron Volts.