The puzzling conundrum of dark matter’s existence
Most astronomers deem mysterious dark matter as the universe’s missing matter, while some offer other explanations
You may have heard the term dark matter or read about it in a scientific article and wondered just what the heck is dark matter, and why are astronomers and astrophysicists so interested in it?
Just to add a twist to the whole question regarding dark matter, let me begin by stating that not all astronomers and astrophysicists are convinced that dark matter actually exists. It is a highly controversial topic.
Some astronomers and astrophysicists view dark matter as the smoking gun — solid evidence proving what they believe or at least theorize; that it is the missing matter of the universe, while others remain skeptical, proposing other explanations. As we shall see, proving the existence of dark matter has not been an easy task.
Dark matter is a hypothetical form of matter and is believed to comprise 85 per cent of all the matter in the cosmos — that’s a lot of matter to be missing.
The first indications regarding dark matter’s existence came as a result of observations by the Dutch astronomer Jan Oort (190092), who, in 1927, was studying the Milky Way Galaxy and the velocity of its stars. The first astronomer to demonstrate that the Milky Way rotated, Oort discovered that the Milky Way stars appeared to be moving faster than the laws of physics should allow. They were moving too fast for the observed gravity to keep them within the galaxy.
Oort’s measurements indicated that, in order for the stars to remain within the galaxy, there needed to be twice as much mass as was observed.
In 1933, Swiss American astronomer Fritz Zwicky (1898-1974) was measuring the velocity of the galaxies in the Coma Cluster (in the constellation of Coma Berenices — Berenice’s Hair). After calculating the total mass of all the observed galaxies in the cluster and determining their velocities, he compared the amounts of motion-energy of the galaxies to the gravitational-energy holding them together, whereupon he discovered that the galaxies, like Oort’s Milky Way stars, were moving much faster than the force of gravity should have permitted.
He determined that the total mass of all the stars in the Coma Cluster was only about one per cent of the mass needed to keep the galaxies from escaping the cluster’s gravitational hold. He concluded that the cluster must have a greater mass than the cluster’s observable light suggested and that there must be an unknown, unseen mass within the cluster, something he called dunkle material, Dutch for dark matter.
After Oort’s observations, interest in this new, unknown mass waned, until, in the 1970s, American astronomer Vera Rubin (1928-2016), studied the speed at which galaxies rotate, measuring the velocity of their stars as a function of their distance from the galactic centre versus their predicted speed based on how the galaxy’s mass was distributed.
After examining numerous galaxies, her research indicated that the stars in the periphery of all galaxies move faster than the laws of gravity should permit. Her observations implied that all galaxies are surrounded by a vast cloud of invisible matter, thus furthering the hypothesis that dark matter did, in fact, exist, and is ubiquitous throughout the cosmos.
In addition to strengthening Oort’s and Zwicky’s hypotheses, Rubin’s studies also demonstrated that single galaxies, not just galaxy clusters, have more mass than suggested by their observable light.
FINDING DARK MATTER
Dark matter gets its name from the fact that it doesn’t interact with the electromagnetic field and, thereby, does not absorb, reflect or emit electromagnetic radiation, essentially making it dark, and, as a consequence, extremely difficult to detect by direct means.
One method that astronomers and astrophysicists use to prove the existence of dark matter is discerning its gravitational attraction rather than its luminosity, primarily by utilizing the phenomenon of gravitational lensing, first demonstrated by British astronomer Sir Arthur Eddington (18821944), the first person to demonstrate that light from distant stars can be deflected by gravity.
When light from a distant star or galaxy passes through another galaxy or cluster of galaxies, it is bent by the gravity of the matter present in that galaxy or galaxy cluster, causing the light from the origin source to appear to be coming from some other point or points in space. The greater the mass of the nearby galaxy or galaxy cluster, the greater the distortion of the background light source.
Dark matter is also inferred from the motion and heat of gas that produces observed X-rays, something that often occurs when two galaxy clusters merge. A prime example of this is the Bullet Cluster (in the southern constellation of Carina — the Ship), where, billions of years ago, two massive galaxy clusters collided.
Galaxy clusters are typically surrounded by a huge cloud of hydrogen gas, the mass of which is normally greater than the combined mass of the galaxies in the cluster. When the two giant galaxy clusters collided, the galaxies passed by one another unimpeded, with their respective masses unaffected. However, when the two hydrogen gas clouds surrounding the clusters smashed into each other, they, slowed by the drag effect, stopped and merged into one large cloud of heated hydrogen gas that emits high levels of X-rays.
Studies of the Bullet Cluster have demonstrated that gravitational lensing is strongest in the separated regions of the two galaxy clusters on either side of the central gas cloud. This indicates that the majority of the gravitation of the galaxy pair is likely the result of dark matter, which passed through the gas cloud during the collision.
THE SKEPTICS
As mentioned, not all astronomers and astrophysicists are convinced, at least not convincingly, of the existence of dark matter. Some suggest that the case of the universe’s missing matter can be resolved by modifications to Isaac Newton’s Law of Motion and/or Albert Einstein’s General Theory of Relativity — in particular, his Theory of Gravity.
Others postulate that the missing matter in the galaxies could possibly be brown dwarf stars (failed stars that never ignited because they lacked sufficient mass to start burning), white dwarf stars (the remnant cores of dead, small-to-medium mass sized stars) or neutron stars or black holes (remnants of more massive stars that have exploded).
Current evidence, however, suggests that there aren’t enough brown and/or white dwarf stars to account for all the implied dark matter and, also, that neutron stars and black holes are relatively rare and, therefore, not a factor.
While the preponderance of positive evidence suggests something like a consensus in favour of dark matter among astronomers and astrophysicists, there is no similar consensus as to exactly what particles (if any) make up dark matter and what its properties are. It has been suggested that dark matter could be composed of a large number of low-mass particles, such as axioms, or a smaller number of heavier-mass particles, like neutralinos, or an even smaller number of ultra-massive particles that arose from gravitational interactions.
Other theories postulate that it might be a particle that originated from an aspect of physics of which astrophysicists are not yet fully aware or it might actually be a non-particle-like fluid that permeates the cosmos and gravitates.
Whatever the nature of dark matter, it will, no doubt, continue to pose a puzzling conundrum for astronomers and astrophysicists for decades to come.
IN THE SKY THIS WEEK
• Venus (mag. -4.2, in Gemini — the Twins) becomes visible by about 9:10 p.m., 29 degrees above the western horizon, before sinking towards the horizon and disappearing from view around 12:20 a.m.
• Mars (mag. +1.5, in Gemini) appears to the upper left of Venus, 32 degrees above the western horizon around 9:45 p.m., before it, too, drops towards the horizon and sets by 1:05 a.m.
• Venus and Mars play tag with the waxing, crescent moon on the evenings of May 22 to 24, with the Moon to the lower right of Venus on the 22nd, between Mars and Venus on the 23rd (with Mars to its upper left and Venus to its lower right), and to the upper right of Mars on the evening of the 24th.
• Saturn (mag. +1.0, in Aquarius — the Waterbearer) rises in the southeast around 2:30 a.m., reaching 19 degrees above the horizon before fading as dawn breaks around 4:45 a.m.
• Mercury, two degrees below the eastern horizon at dawn, and Jupiter, only seven degrees above the eastern horizon at dawn, are not observable this week.
• The star, Arcturus (Alpha Bootis) in the constellation of Bootes — the Herdsman, has an anniversary of sorts this week. On May 27, 1933, the light from Arcturus was sent to a photoelectric switch to turn on the lights of the World’s Fair in Chicago, Ill. Arcturus is around 37 light years from Earth and the light that switched on the 1933 fair had left its host star at about the time of the previous Chicago World’s Fair in 1893.
(Note: In last week’s article, I mislabelled Procyon as Alpha Orionis, when, in fact, it is Alpha Canis Minoris, in Canis Minor — the Little Dog. My apologies.)
Until next week, clear skies.
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. His column, Atlantic Skies, appears every two weeks.