November 19, 2015
As technology drives our understanding of the Universe to new heights each year, there remain topics and hypotheses that leave astronomers and physicists in wonder. Dark matter, matter that cannot be observed directly, is one of the most mysterious subjects of analysis and research in modern astronomy, and, as said by popular astrophysicist Neil deGrasse Tyson, dark matter “is the longest-standing unsolved problem in modern astrophysics.”1 Its origin as a subject of study date back to the early 1930s2, and its nature has yet to be fully explained by scientists. However, by studying galaxy rotation curves, velocity dispersions of galaxies, and gravitational lensing, one can support the “Dark Matter” hypothesis as a more plausible explanation to the mysteries of the universe rather than attempting to redefine our understanding of gravity itself.
The “Dark Matter” hyppothesis states that there is another kind of matter, “Dark Matter”, that comprises much of the mass in the universe and it is hypothesized that dark matter constitutes up to 84.5% of all matter in the universe3. This matter, however, differs from the matter we are familiar with, as we cannot observe dark matter with telescopes. Dark matter does not absorb or emit light and electromagnetic radiation, and does not consist of any particles that we can observe3. Understandably, this makes studying dark matter difficult, leaving scientists with little information about it other than its likely existence.
The idea of dark matter dates back to 1932, when Jan Oort found there was “missing mass” in the orbital velocities of stars and galaxies in clusters. When observing objects in the universe, the gravitational effects of visible matter did not correspond to their expected values, suggesting there was something else that existed that astronomers were incapable of seeing.2
Since 1932, there have been many observations that support the notion that the universe consists of matter that we cannot see, also known as dark matter. Supporting evidence for dark matter involve studies of galaxy rotation curves, velocity dispersions of galaxies, and gravitational lensing.
Much of the evidence supporting the existence of dark matter stems from the study of galaxy rotation curves, which help astronomers deduce how much, and where, mass exists in a galaxy that rotates. Galaxy rotation curves measure the velocity of visible stars or gas relative compared to their distance from the galaxy’s center. When observing stars and gas further away from the center of a galaxy, without dark matter, it would be expected that the velocity of stars further from the center would be lower than those closer to the center of the galaxy, according to modern physics and Kepler’s Laws. However, in many instances this expected behavior is not observed in rotating galaxies, and observing stars further from the center exhibit orbit velocities which do not coincide with the matter of the galaxy, has led to the hypothesis of dark matter.4
In 1939, astronomer Horace Babcock’s studied rotation curves of the Andromeda Galaxy and found that the ratio between mass and luminosity increased radially. When Babcock made observations of matter further away from the center of a galaxy, he calculated that there was more mass present than can be seen. Babcock, however, attributed this observation to factors unrelated to dark or “missing” matter, such as the absorption of light within Andromeda.5
Fast forwarding to the late 1960s and early 1970s, Vera Rubin was the first astronomer to research evidence that suggested the existence of dark matter. Using galaxy rotation curves Rubin’s research showed that the majority of galaxies contained about six times as much dark mass as could be accounted for by luminous matter. As more astronomers supported her work, it became accepted that dark matter made up much of the matter in galaxies.6
After Vera Rubin’s widely supported work that measured galaxy rotation curves, research calculating velocity dispersions of elliptical galaxies was used to further provide evidence of dark matter.7 Velocity dispersion is used to find the mass of an elliptical galaxy by measuring the spread in velocities of the stars in a galaxy. This dispersion can then be used to derive the mass of the galaxy using the virial theorem. Applying this technique, astronomers not only found more supporting evidence of dark matter, but also found that galaxies were orbiting each other at velocities up to ten times their visible radii, further suggesting evidence of dark matter.8
Studying galaxy clusters is another vital source of evidence supporting dark matter theories, and the methods of estimating their masses suggest that these clusters contain more mass than is indicated by a galaxy’s visible body. One of the methods of estimating a galaxy cluster’s mass is by observing the gravitational lensing it exhibits. Gravitational lensing is when the force of an object’s gravity bends the light around it from the perspective of the observer, as predicted by Albert Einstein’s general theory of relativity9. By measuring the distortion due to gravitational lensing, the mass of the object that causes the lensing can be obtained. When this analysis has been performed on galaxy clusters, the total mass of the cluster exceeded what it should be according to the light it projects, which corresponds to the expected amount of dark matter according to dark matter theory.10
As we study distant galaxies and apply methods of estimating their masses, we find evidence to suggest that dark matter exists due to how galaxies orbit each other and bend light via gravity. This behavior is inconsistent with what would be expected of these objects if based solely on their visible matter, and therefore strengthens the support for dark matter theory. It is, however, also argued that our understanding of gravity may still be limited and the framework in which we view the forces in our universe is therefore inappropriate. It could be argued that gravity simply behaves differently in different contexts within the universe.
I, along with many scientists reject this notion and believe that the gravitational constant is universal. All observations of gravity, and those that have led to discoveries of stars, galaxies and planets, have used a consistent framework and understanding of gravitational laws, Newtonian physics, and General Relativity. The gravitational lensing that was observed by Hubble has also been predicted by Einstein’s general theory of relativity. Our understanding of these laws has consistently produced results and validated many theories over time. There are frequent studies that question whether gravity’s laws are universal or whether our application of Newtonian physics and General Relativity are incorrect, but each leads to the same conclusion- that as far as we know, our understanding is correct.
Through the analysis of distant galaxies and the velocities in which their stars orbit within them, the way in which galaxies orbit each other, and the way that massive galaxy clusters bend light- all due to gravity- we observe that the mass required to create these gravitational pulls are significantly higher than what they should be given the mass that is observed through telescopes and spectrometers. While it may seem strange for something to exist without us able to observe it directly, it seems that dark matter is currently the only explanation for this observed phenomena within our universe. Perhaps one day, in our lifetime, we will better understand this dark matter and even be able to observe its properties- until then, however, we’ll have to continue solving this mystery.
I'm Patrick El-Hage and I live and work in San Francisco. I'm also on Twitter.