When thinking of outer space, stars, galaxies, planets and nebulae spring to mind in lieu of what in fact constitutes approximately 80% of the universe itself: dark matter. Dark matter is undetectable due to the fact that it does not emit, absorb or reflect light. Nonetheless, we can infer that it exists by observing some rather curious gravitational activity within a galaxy. The presence of dark matter has become widely accepted amongst the scientific community and many believe that it constitutes most of the matter present within our universe.
Evidence in support of dark matter is provided by a mathematical anomaly. The mass of a galaxy can be estimated by the mass of the objects it contains or via the speed and mass of gaseous clouds orbiting along its outer edges. According to a team of researchers from the University of Harvard, a confounding paradox exists within the observed and actual masses of many spiral galaxies: accounting for all the observed stars and gases, and the gravitational pull that the galaxies exert, the actual mass can sometimes be as much as five times larger than perceived. The visible matter alone would not be able to ensure that the galaxy remains intact, thus leading many to the conclusion of this ‘missing mass’ being undetectable.
Because of the mathematical relationship between orbital speed and mass, we can calculate the size of a particular body that is in orbit. Thus, for example, it is known that our planet Earth, which orbits the sun at about 30km/s, is in closer proximity to the sun than the moon, whose orbital velocity stands at a meagre 1km/s — their orbital speed is dependent on their mass and spatial distance from the sun, which exerts a stronger gravitational force on nearer objects. Yet, if we apply these laws to galaxies, contradictions occur. Newton’s law of universal gravitation dictates that the further away a planet is from the sun (and its gravitational pull) the slower its orbit would be. Surprisingly, the same does not run true for stars within galaxies.
In addition, when applying Kepler’s Third Law of planetary motion, one would stipulate that stars further away from a galactic centre should orbit more slowly than stars closer to the centre. Nonetheless, these outmost stars are in fact seen to orbit faster, which can only be explained in Newtonian terms when accounting for dark matter’s gravitational pull on them. In fact, the speed with which they travel when further away from their galactic centre is enormous and should actually tear the galaxy apart, because there does not appear to be sufficient visible mass available to supply all the gravity needed to maintain stability.
Through a natural phenomenon known as the Doppler Shift, the speed at which different galaxies are travelling toward or away from us can be determined photometrically — the light emitted by these galaxies “shifts” the further or closer they approach us and so, using a spectroscope, scientists can measure the extent of this Doppler shift to calculate how fast the light source is moving. By these means, a rotation curve (the orbital velocity of objects in tandem with their distance from the galactic centre) can be estimated. For any galaxy (assuming the nonexistence of dark matter and the prevalence of visible matter as the sole supplier of all gravity) we can calculate the rotational curve from the mass of all the objects inside of it and conclude that the majority of the galaxy’s mass would be at its centre. Hence, the inner section would rotate more rapidly than the sparsely populated outskirts of the galaxy. Yet, this is not the case, as the entire galaxy is shown to have a more constant orbital velocity; a flat rotational curve.
Whether or not dark matter is made up of baryonic (perceivable) or non-baryonic (non-perceivable) matter is the source of some conflict within the scientific community. On one hand, some astrophysicists believe that dark matter is constructed of Weakly Interacting Massive Particles (WIMPs). These can interact through gravity and weak nuclear forces, but are undetectable via electromagnetic radiation . It is believed that they regularly pass through normal matter, but make relatively little impact on the atomic nuclei that they move through. That is why they are said to be constituted by large and therefore slow moving particles.
Others believe that dark matter consists of Massive Astrophysical Compact Halo Objects (MACHOs) which emit no light and therefore are also undetectable. MACHOs can be in the form of black holes which do not emit or reflect light, only absorb it. Black holes, like dark matter, are detected by their gravitational effects on surrounding objects. A contentious issue with baryonic matter constituting the make-up of dark matter is that entities such as black holes mostly exist in isolation and the ‘missing matter’ identified within the universe in relation to the dark matter problem would have to be spread widely apart, so as to balance the gravity in the galaxy.
It has long been one of the great mysteries of science. Though, we may not be able to electromagnetically ascertain or entirely prove the presence of dark matter, its manifestations within the universe are evident. And until someone is able to empirically prove that it exists or come up with another credible theory to account for the missing mass in the universe, the concept of dark matter appears to be a more than sound theory.
(Image courtesy of Argonne National Laboratory)