Looking at the night sky, the heavens appear to be utterly empty. Space draws its name from seeming to be just that – empty space. While seemingly obvious, this assumption does not hold true. Approximately 68% of the universe
i is composed of mysterious energy known as dark energy. Originally predicted by Einstein’s erroneous cosmological constant, dark energy may function as a reduced form of a constant in the equations for relativity. If this form of energy is indeed fixed,
ii dark energy functions as a constant term in equations. Considered in Friedman’s equation,
iii H
2 = (8 π G / 3) ρ - k c
2 / R
2 + Λ/3, the cosmological constant Λ/3 may be construed as dark energy. While Einstein may have inadvertently directed attention to the existence of dark energy, the discovery that the Universe is flat lends strength to an argument for the existence of dark energy’s counterpart, dark matter. In very general terms, dark matter is defined as a structure with mass and that does not reflect light, hence the title “dark." For the Universe to be flat it must contain a certain amount of mass to meet the required density so that gravitational waves exist also exist in a flat plane. As observable mass and energy alone, another mechanism must be at work. Dark matter has been inferred to be this mechanism, providing a large portion of the Universe’s mass without reflecting light. For the sake of understanding, this entry will focus primarily on dark matter, as its counterpart dark energy requires an understanding of dark matter.
Given the strange nature of dark matter a question remains: what is the function of dark matter? For dark matter to exist, such matter must have mass. Simulations of the Milky Way Galaxy from the Big Bang to the present predict a scattering of matteriv. As the Milky Way is not a particularly massive galaxy, no collection of identifiable objects is capable of providing the gravitational pull to hold the galaxy together. In the context of the simulations, identifiable objects are defined as objects who may reflect light. The mass of the visible objects alone is not sufficient for the Milky Way to maintain its spiral form – suggesting another source of mass. For the universe as it currently exists to make sense, mass that does not reflect light must exist. Thus it may be concluded that dark matter is responsible for mass but do not reflect light – lending it the prefix “dark.”
Despite rapid advancements in modern telescopes, both studying and observing dark matter is extremely difficult. As this phenomenon cannot be seen by conventional means, scientists must turn to creative and inventive methods of detection. Chief among the techniques used to detect this strange form of matter is gravitational lensing. Measuring the distortion and bending of far-away light
v, the potential effect of an object between the observer and the vent may be measured. When scientists observed the collision of the Bullet Cluster, they found that the majority of the mass after the collision was located on the periphery of the collision – proof that dark matter particles do not interact with one another. If the opposite held true – that dark matter particles do interact with one another, the gas in the center of the collision would have been slowed down. Thus, dark matter’s lack of interaction with itself allows for the distribution of mass along the periphery of the Bullet Cluster, as indicated by orange lines in the picture below.
Numerous astronomical objects have been proposed as candidates to be dark matter. When used in this sense, the term dark matter is applied to mean matter that at extremely low luminosities and temperatures. “Baryonic”vi dark matter, matter made from regular elements and compounds, may include black holes, dwarf stars and neutron stars. While it is tempting to accept both dwarf and neutron stars as dark matter, these may be ruled out due to the age of the universe. Put simply, the universe has not existed for a time period sufficient to achieve the creation of enough neutron stars to account for all of dark energy. Black holes seem to be a logical candidate due to their absorbance of light – yet are not common enough in the Universe to account for all of dark matter. This causes scientists to look to non-standard matter, also known as “Non-Baryonic Matter.” Such matter would be composed of unknown exotic particles high enough in mass to have an observable effect on galaxies. Finally, some scientists argue that dark matter does not exist. Equating the gravitational shift to differing properties of gravity on large scales, dark matter may be unknown gravitational properties of extremely high mass objects.vii
Sources:
