Dark Matter

Marco Vancini Photos

EVIDENCE

An overwhelming number of independent observations paint an image of our Universe where its evolution is dominated by two components, the exact natures of which are still widely unknown: dark energy and dark matter.

While dark energy appears as 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) imperceivable via any interaction except gravity.

This section introduces evidence for dark matter from large to small scales, as well as the two most popular particle candidates.

Cosmic Microwave Background

One of the most compelling pieces of evidence for the existence of dark matter arises from 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. They enable access to the energy density of the Universe during this period of time, from which an angular power spectrum can be obtained.

The data is best described by the cosmological standard model, called the Lambda (Λ) cold dark matter (ΛCDM) model, where Λ refers to the cosmological constant required to explain the current accelerated expansion of the Universe. Gravitational effects from cold dark matter density fluctuations created oscillations in the baryon-photon plasma before recombination. This led to a characteristic oscillation pattern in the CMB power spectrum: acoustic peaks. The fractions of baryonic matter and dark matter can be computed from the relative heights of these acoustic peaks. From these calculations, the Universe was found to be flat, with an energy density distribution of 4.9% normal (baryonic) matter, 25.9% dark matter, and 69.2% dark energy.

Structure Formation

Given how small the density fluctuations observed in the CMB are and given the specific cosmological structures observed to exist since roughly a billion years after the Big Bang, dark matter is required to have existed even before the release of the CMB to seed the growth of these structures.

Deep surveys of galaxies and galaxy clusters in the Universe have revealed the formation of so-called large scale structure: galaxies and clusters are coalescing into chain-like structures and the space between these structures is empty. It is thought that the cold dark matter (CDM travelling much slower than light) makes the skeleton of this large-scale structure, and then ordinary matter gravitates toward them to form galaxies and clusters.

A number of numerical simulations based on the ΛCDM model have succeeded in reproducing the large scale structure with computers.

Gravitational Lensing

Another phenomenon pointing to the existence of dark matter is gravitational lensing as predicted by Albert Einstein with general relativity. Spacetime, and therefore light paths, can be curved around a massive object, creating an effect similar to a traditional glass lens. Light that is emitted by a source is deflected to some extent by the lensing object, depending on the lensing mass. An observer on Earth will see a distorted image of the source due to the bent light trajectories, despite it being behind the lensing object. The centers of mass creating gravitational lensing can be seen in blue in the Bullet Cluster image below. In this image is a three dimensional example of how mass densities might be reconstructed.

With the help of known sources, the gravitational potential of the lensing object (such as a galaxy cluster) can be reconstructed from the light’s degree of deflection, and therefore the amount of matter inside the bending object can be calculated. Comparing lensing mass measurements with the mass of the luminous matter via X-ray measurements, discrepancies in the quantity of mass in several galaxy clusters were found. These can be resolved with the existence of a large amount of dark matter.

Merging Galaxy Cluster

Gravitational lensing is also applied to galaxy cluster collisions in order to reconstruct the centers of mass after the objects intermix.

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

However, gravitational lensing reconstructs the centers of mass still within their respective clusters, clearly displaced from the location of the ordinary matter (blue-shaded regions in the picture).

This paradox can be solved by including the existence of dark matter halos around both clusters that follow their original trajectories without collisions. Furthermore, an upper limit for the strength of the self-interaction of dark matter can be obtained from these studies.

The most prominent example of merging galaxy clusters and shown in the picture is the Bullet Cluster, a system of two galaxy clusters that collided about 150 million years ago.

Galaxy Rotation Curves

Additional evidence for dark matter can be found with smaller-scale structures by measuring the rotation velocity of objects around 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,

since most of the luminous matter is in the center of the galaxy.

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 explained by a mass density profile composed of the galactic disk, the galactic gas, and the dark matter halo. For example, the image here shows the galaxy NGC 2008.

CANDIDATES

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

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

Other 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 has been the leading candidate for dark matter for decades. The main reason for its popularity is that it can naturally explain the correct amount of dark matter in the Universe.

The standard assumption of WIMP production is that WIMPs are thermally produced in the early Universe at high temperatures. WIMPs can annihilate with each other into normal particles and the “weak” annihilation rate explains the stable amount of the dark matter of today. If dark matter annihilates, that also indicates it would interact with regular matter in other ways beyond gravitationally.

XENONnT is searching for WIMP dark matter in the parameter space where it is predicted to exist by many theories.

Axions

The Axion is a hypothetical particle arising from the Peccei-Quinn theory, which is proposed to solve the so-called strong CP problem. It was named by Frank Wilczek after a dish soap brand 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). The expected interaction strengths and particle attributes are consistent with the known properties of dark matter.

Axions could be created via thermal or non-thermal mechanisms in the early Universe. They would be lighter and more plentiful than Weakly Interacting Massive Particles.

DARK MATTER