The cuspy halo problem (also known as the core-cusp problem) refers to a discrepancy between the inferred
dark matter
Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...
density profiles of low-mass galaxies and the density profiles predicted by cosmological
N-body simulation
In physics and astronomy, an ''N''-body simulation is a simulation of a dynamical system of particles, usually under the influence of physical forces, such as gravity (see ''n''-body problem for other applications). ''N''-body simulations ...
s. Nearly all simulations form dark matter halos which have "cuspy" dark matter distributions, with density increasing steeply at small radii, while the rotation curves of most observed dwarf galaxies suggest that they have flat central dark matter density profiles ("cores").
Several possible solutions to the core-cusp problem have been proposed. Many recent studies have shown that including
baryonic
In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classified ...
feedback (particularly feedback from
supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
e and
active galactic nuclei) can "flatten out" the core of a galaxy's dark matter profile, since feedback-driven gas outflows produce a time-varying
gravitational potential
In classical mechanics, the gravitational potential at a location is equal to the work (energy transferred) per unit mass that would be needed to move an object to that location from a fixed reference location. It is analogous to the electric p ...
that transfers energy to the orbits of the collisionless dark matter particles. Other works have shown that the core-cusp problem can be solved outside of the most widely accepted Cold Dark Matter (CDM) paradigm: simulations with
warm or
self-interacting dark matter also produce dark matter cores in low-mass galaxies. It is also possible that the distribution of dark matter that minimizes the system energy has a flat central dark matter density profile.
Simulation results
According to W.J.G. de Blok "The presence of a cusp in the centers of CDM halos is one of the earliest and strongest results derived from N-body cosmological simulations." Numerical simulations for CDM structure formation predict some structure properties that conflict with astronomical observations.
Observations
The discrepancies range from galaxies to clusters of galaxies. "The main one that has attracted a lot of attention is the cuspy halo problem, namely that CDM models predict halos that have a high density core or have an inner profile that is too steep compared to observations."
Potential solutions
The conflict between numerical simulations and
astronomical observation
Observational astronomy is a division of astronomy that is concerned with recording data about the observable universe, in contrast with theoretical astronomy, which is mainly concerned with calculating the measurable implications of physical ...
s creates numerical constraints related to the core/cusp problem. Observational constraints on halo concentrations imply the existence of theoretical constraints on cosmological parameters. According to
McGaugh, Barker, and de Blok, there might be 3 basic possibilities for interpreting the halo concentration limits stated by them or anyone else:
#"CDM halos must have cusps, so the stated limits hold and provide new constraints on cosmological parameters."
#"Something (e.g. feedback, modifications of the nature of dark matter) eliminates cusps and thus the constraints on cosmology."
# "The picture of halo formation suggested by CDM simulations is wrong."
One approach to solving the cusp-core problem in galactic halos is to consider models that modify the nature of dark matter; theorists have considered
warm,
fuzzy,
self-interacting, and
meta-
Meta (from the Greek μετά, ''meta'', meaning "after" or "beyond") is a prefix meaning "more comprehensive" or "transcending".
In modern nomenclature, ''meta''- can also serve as a prefix meaning self-referential, as a field of study or endea ...
cold dark matter, among other possibilities.
One straightforward solution could be that the distribution of dark matter that minimizes the system energy has a flat central dark matter density profile.
See also
*
Dwarf galaxy problem (also known as "the missing satellites problem")
*
List of unsolved problems in physics
The following is a list of notable unsolved problems grouped into broad areas of physics.
Some of the major unsolved problems in physics are theoretical, meaning that existing theories seem incapable of explaining a certain observed phenomenon ...
References
{{Dark matter
Dark matter
Unsolved problems in physics