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In astronomy, dark matter is a hypothetical form of matter that appears not to interact with light or the electromagnetic field. Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be seen. Such effects occur in the context of formation and evolution of galaxies, gravitational lensing, the observable universe's current structure, mass position in galactic collisions, the motion of galaxies within galaxy clusters, and cosmic microwave background anisotropies.
The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. Galaxy formation is hypothesized to occur from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. The simplest model in general agreement with observed phenomena is the Lambda-CDM model—that is, clustering and merging allows galaxies to accumulate mass, determining both their shape and structure. Hydrodynamics simulation, which simulates both baryons and dark matter, is widely used to study galaxy formation and evolution.
The rotation curve of a disc galaxy is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. A significant discrepancy exists between the experimental curves observed, and a curve derived by applying gravity theory to the matter observed in a galaxy. Theories involving dark matter are the main postulated solutions to account for the variance.
In cosmology and physics, cold dark matter (CDM) is a hypothetical type of dark matter. According to the current standard model of cosmology, Lambda-CDM model, approximately 27% of the universe is dark matter and 68% is dark energy, with only a small fraction being the ordinary baryonic matter that composes stars, planets, and living organisms. Cold refers to the fact that the dark matter moves slowly compared to the speed of light, giving it a vanishing equation of state. Dark indicates that it interacts very weakly with ordinary matter and electromagnetic radiation. Proposed candidates for CDM include weakly interacting massive particles, primordial black holes, and axions.
A MAssive Compact Halo Object (MACHO) is a kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body that emits little or no radiation and drifts through interstellar space unassociated with any planetary system. Since MACHOs are not luminous, they are hard to detect. MACHO candidates include black holes or neutron stars as well as brown dwarfs and unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs. The term was coined by astrophysicist Kim Griest.
A galactic halo is an extended, roughly spherical component of a galaxy which extends beyond the main, visible component. Several distinct components of a galaxy comprise its halo:
The cuspy halo problem is a discrepancy between the inferred dark matter density profiles of low-mass galaxies and the density profiles predicted by cosmological N-body simulations. 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").
In modern models of physical cosmology, a dark matter halo is a basic unit of cosmological structure. It is a hypothetical region that has decoupled from cosmic expansion and contains gravitationally bound matter. A single dark matter halo may contain multiple virialized clumps of dark matter bound together by gravity, known as subhalos. Modern cosmological models, such as ΛCDM, propose that dark matter halos and subhalos may contain galaxies. The dark matter halo of a galaxy envelops the galactic disc and extends well beyond the edge of the visible galaxy. Thought to consist of dark matter, halos have not been observed directly. Their existence is inferred through observations of their effects on the motions of stars and gas in galaxies and gravitational lensing. Dark matter halos play a key role in current models of galaxy formation and evolution. Theories that attempt to explain the nature of dark matter halos with varying degrees of success include cold dark matter (CDM), warm dark matter, and massive compact halo objects (MACHOs).
In physical cosmology, structure formation describes the creation of galaxies, galaxy clusters, and larger structures starting from small fluctuations in mass density resulting from processes that created matter. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.8 billion years ago. However, looking at the night sky today, structures on all scales can be seen, from stars and planets to galaxies. On even larger scales, galaxy clusters and sheet-like structures of galaxies are separated by enormous voids containing few galaxies. Structure formation models gravitational instability of small ripples in mass density to predict these shapes, confirming the consistency of the physical model.
A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and luminous host galaxy. Satellite galaxies and their constituents are bound to their host galaxy, in the same way that planets within the Solar System are gravitationally bound to the Sun. While most satellite galaxies are dwarf galaxies, satellite galaxies of large galaxy clusters can be much more massive. The Milky Way is orbited by about fifty satellite galaxies, the largest of which is the Large Magellanic Cloud.
The Navarro–Frenk–White (NFW) profile is a spatial mass distribution of dark matter fitted to dark matter halos identified in N-body simulations by Julio Navarro, Carlos Frenk and Simon White. The NFW profile is one of the most commonly used model profiles for dark matter halos.
The halo occupation distribution (HOD) is a parameter of the halo model of galaxy clustering. The halo model provides one view of the large scale structure of the universe as clumps of dark matter, while the HOD provides a view of how galactic matter is distributed within each of the dark matter clumps. The HOD is used to describe three related properties of the halo model: the probability distribution relating the mass of a dark matter halo to the number of galaxies that form within that halo; the distribution in space of galactic matter within a dark matter halo; the distribution of velocities of galactic matter relative to dark matter within a dark matter halo.
While the presence of any mass bends the path of light passing near it, this effect rarely produces the giant arcs and multiple images associated with strong gravitational lensing. Most lines of sight in the universe are thoroughly in the weak lensing regime, in which the deflection is impossible to detect in a single background source. However, even in these cases, the presence of the foreground mass can be detected, by way of a systematic alignment of background sources around the lensing mass. Weak gravitational lensing is thus an intrinsically statistical measurement, but it provides a way to measure the masses of astronomical objects without requiring assumptions about their composition or dynamical state.
Modified Newtonian dynamics (MOND) is a theory that proposes a modification of Newton's second law to account for observed properties of galaxies. Its primary motivation is to explain galaxy rotation curves without invoking dark matter, and is one of the most well-known theories of this class. However, it has not gained widespread acceptance, with the majority of astrophysicists supporting the Lambda-CDM model as providing the better fit to observations.
The Press–Schechter formalism is a mathematical model for predicting the number of objects of a certain mass within a given volume of the Universe. It was described in an academic paper by William H. Press and Paul Schechter in 1974.
In astrophysics, the virial mass is the mass of a gravitationally bound astrophysical system, assuming the virial theorem applies. In the context of galaxy formation and dark matter halos, the virial mass is defined as the mass enclosed within the virial radius of a gravitationally bound system, a radius within which the system obeys the virial theorem. The virial radius is determined using a "top-hat" model. A spherical "top hat" density perturbation destined to become a galaxy begins to expand, but the expansion is halted and reversed due to the mass collapsing under gravity until the sphere reaches equilibrium – it is said to be virialized. Within this radius, the sphere obeys the virial theorem which says that the average kinetic energy is equal to minus one half times the average potential energy, , and this radius defines the virial radius.
In astrophysics and cosmology scalar field dark matter is a classical, minimally coupled, scalar field postulated to account for the inferred dark matter.
Ma Chung-pei is an astrophysicist and cosmologist. She is the Judy Chandler Webb Professor of Astronomy and Physics at the University of California, Berkeley. She led the teams that discovered several of largest known black holes from 2011 to 2016.
Marc S. Seigar is an astrophysicist, academic and author. He is the Dean of the College of Natural Sciences and Mathematics, and a Professor of Physics and Astronomy at the University of Toledo.
Jenny Wagner is a German physicist, cosmologist, and book author.
References
- ↑ Wu, Hao-Yi; Zentner, Andrew R.; Wechsler, Risa H. (2010-04-20). "The Impact of Theoretical Uncertainties in the Halo Mass Function and Halo Bias on Precision Cosmology". The Astrophysical Journal. 713 (2): 856–864. arXiv: 0910.3668 . Bibcode:2010ApJ...713..856W. doi: 10.1088/0004-637X/713/2/856 .
