“A cosmic mystery of immense proportions, once seemingly on the verge of solution, has deepened and left astronomers and astrophysicists more baffled than ever. The crux of the riddle is that the vast majority of the mass of the Universe seems to be missing.”
— William J. Broad, “If Theory is Right, Most of Universe is Still ‘Missing,'” New York Times (11 Sept. 1984)
Despite all the great strides that have taken place during the last century in our understanding of the Cosmos, astrophysicists are still left with a nudging scientific mystery of immense proportions: Most of the energy-mass content of the Universe seems to be unaccounted for. What makes up all the planets, stars, and galaxies, as well as all the energy and electromagnetic radiation that we can observe with our instruments, constitutes just under 5 percent of the Universe’s total—everything else, euphemismly called “dark matter” and “dark energy” respectively, is just anybody’s guess. Scientists have been on the hunt for all this missing cosmic content for decades with little success to date, which has led many members of the scientific community to question its very existence, while casting doubts on our current understanding of the way gravity works on cosmic scales. Nevertheless, the results from various astronomical surveys and theoretical studies that have been conducted through the years have indicated that even though we can’t see it, dark matter is definitely there. Two such new studies that were recently published independently by two international research teams come to give more strength to this hypothesis by presenting further indirect evidence for the existence of dark matter in the central and outer regions of the Milky Way galaxy respectively.
It may sound like a nefarious plot device for a science fiction movie but “dark matter” is a real scientific term devised by astronomers as one of the possible explanations for the strangeness of stellar motions observed inside galaxies. More specifically, while studying how stars and interstellar gas move within galaxies and galaxy clusters during the 20th century, astronomers soon realised that the velocities they were observing were too high for the structures of galaxies to remain stable. If all the matter being observed and accounted for was all there was, then galaxies should have flown apart—its contents should have been hurled into the intergalactic void. Either there was something really flawed with the current understanding of how gravity worked on cosmic scales, or there was something else whose gravity was holding galaxies together, something that was evading observations. Thus the “missing mass problem” was born, leading to a fevered debate within the scientific community that rages on to this day.
Of all the proposed explanations for the nature of dark matter, the leading one between astronomers is that it’s composed of non-baryonic Weakly Interacting Massive Particles, or WIMPS. Whereas all the visible matter in the Universe, like planets, stars, and galaxies, is composed of ordinary baryonic matter (protons and neutrons, as well as their corresponding anti-matter counterparts), non-baryonic WIMPS are hypothesized particles that don’t emit or absorb any electromagnetic radiation, thus making themselves invisible to observations. Yet, according to theory, dark matter’s presence can be inferred by the effects of its gravity on the rest of the Universe, giving it a dominant role in the formation and evolution of the latter’s large-scale structure.
A long list of observational and theoretical evidence that have been accumulated through the years, as well as various detailed computer simulations, have created an overwhelming consensus among astronomers in favor of the dark matter hypothesis. For instance, the study of the cosmic microwave background radiation that permeates the Universe has given results that show clear (albeit indirect) evidence for the existence of dark matter. Furthermore, the effects of gravitational lensing observed in galaxy clusters, which is the bending of the light from distant background cosmic objects by the gravity of the foreground clusters themselves, can be best explained only if dark matter is the dominant substance in the Universe. Nevertheless, an alternative hypothesis that has also gained some traction within the scientific community posits that there’s ultimately no need to invoke dark matter in order to explain how the Universe works—it all comes down to the way gravity works on the largest of scales. Called Modified Newtonian Dynamics, or MOND for short, this hypothesis proposes that Newton’s second law of motion behaves differently below a given acceleration at very long cosmic distances, like for instance the orbits of stars around the centers of galaxies, as opposed to how it works in the comparatively small scale of our Solar System, thus helping to explain the inexplicably high velocities of stars inside galaxies and galaxy clusters without the need for dark matter whatsoever.
While the dark matter hypothesis has historically proven to be much more successful in accounting for the observed properties of galaxies and galaxy clusters, proponents of MOND argue that the latter has enough successes of its own so as to be considered a viable alternative. Now, a couple of new studies recently published by two independent teams of astronomers give an interesting twist to the debate, by presenting new evidence in favor of dark matter. In the first such study, which was undertaken by a team led by Fabio Iocco, a postdoc researcher at the Institute for Theoretical Physics in Madrid, Spain and appeared at the online edition of the Nature Physics journal on 9 February, researchers conducted a comprehensive comparison between the observed rotation curve (the magnitude of stellar velocities) of the Milky Way and what would be expected if the latter consisted of visible matter alone. Even though this method has also been used countless times in the past, Iocco’s team specifically focused on the inner parts of the Milky Way galaxy as well as in the vicinity of the Sun where our Solar System resides, where the density and distribution of dark and visible matter alike were poorly mapped.
To that end, the researchers created the most complete compilation of rotation curve data to date that had ever appeared in the scientific literature, consisting of detailed measurements of the orbital motions of 2,780 objects in the Milky Way, coupled with data from state-of-the-art computer simulations of the distribution of matter in the galaxy. The results of Iocco’s team’s study showed that dark matter must be the dominant component of the Milky Way. “Our findings demonstrate the existence of dark matter in the inner Galaxy while making no assumptions on its distribution,” write the researchers in their study. “The comparison of the Milky Way observed rotation curve with the predictions of a wide array of baryonic models points strongly to the existence of a contribution to the gravitational potential of the Galaxy from an unseen, diffuse component. The statistical evidence is very strong already at small galactocentric distances, and it is robust against uncertainties on galactic morphology and kinematics. Without any assumption about the nature of this dark component of matter, our results open a new avenue for the determination of its distribution inside the Galaxy.”
Even though the statistical nature of Iocco’s team’s study can’t shed much light on the possible nature of dark matter, it could nevertheless be used as a starting point by future searches toward that goal. “This has powerful implications both on studies aimed at understanding the structure and evolution of the Milky Way in a cosmological context, and on direct and indirect dark matter searches, aimed at understanding the very nature of dark matter,” conclude the researchers.
A second study, which was conducted by a an international research team led by Dr. Sukanya Chakrabarti, an assistant professor of astrophysics at the Rochester Institute of Technology in New York and published at the Astrophysical Journal Letters, focused on the outer reaches of the Milky Way instead, while analysing archived data that had been collected with the VISTA Variables in the Via Lactea survey, or VVV VISTA for short, which is an ongoing detailed survey of the Milky Way’s central regions in five near-infrared bands, conducted with the European Southern Observatory’s 4.1 m-wide Visible and Infrared Survey Telescope for Astronomy in Chile. The goal of Chakrabarti’s team was to search for a type of variable stars called Cepheids in the distant regions of the galaxy. Cepheids are young superluminous stars that pulsate regularly at well-defined periods, which makes them invaluable tools to astronomers for measuring cosmic distances with great accuracy. The search in near-infrared wavelengths allowed the researchers to peer through all the gas and dust in the galaxy that obstruct observations in visible light. This way, by shifting through the data of the VVV VISTA survey, Chakrabarti’s team discovered four Cepheid stars which were located at the far side of the Milky Way, located very close to the galactic plane no more than one degree apart in the direction of the southern constellation Norma. The detailed spectroscopic study of the Cepheids’ light curves allowed researchers to determine that their distance was approximately 90 kpc, or 290,000 light-years away, putting them well beyond the realm of the Milky Way. “I decided to see if I could actually find the thing,” says Chakrabarti. “It was a difficult prediction to test because it was close to the plane, and therefore difficult to see in the optical. This new survey, VISTA, was able to help us to lift the veil and see these young pulsating stars.”
Furthermore, the location of the newly found Cepheids was also in very good agreement with the results of a previous study by Chakrabarti of a series of perturbations that had been observed in the movements of atomic hydrogen in the Milky Way’s disk, which Chakrabarti had argued were due to recent gravitational interactions with a dark matter-dominated dwarf satellite galaxy, in an area where none had been previously detected. “These are the most distant Cepheid variables close to the plane of our Galaxy discovered to date,” write the researchers in their study. “The fact that the Cepheids that we detect are at an average distance of 90 kpc, highly clustered in angle (within one degree) and in distance (within 20 percent of the mean value of 90 kpc), is difficult to explain without invoking the hypothesis of these stars being associated with a dwarf galaxy, which may be more extended in latitude than can be determined from the VVV survey alone.”
“These young stars are likely the signature of this predicted galaxy,” adds Chakrabarti. “They can’t be part of our galaxy because the disk of the Milky Way terminates at 48,000 light years.” In addition, the discovery of these Cepheids might also help to solve the “missing satellites” problem, which is the apparent lack of satellite galaxies around the Milky Way compared to the predictions of computer models which indicate that there should be many more than what have already been observed. But more importantly, the method employed by Chakrabarti’s team could be used in similar searches in the future, in order to provide more constraints to the nature of dark matter. “The discovery of the Cepheid variables shows that our method of finding the location of dark-matter dominated dwarf galaxies works,” comments Chakrabarti. “It may help us ultimately understand what dark matter is made up of. It also shows that Newton’s theory of gravity can be used out to the farthest reaches of a galaxy, and that there is no need to modify our theory of gravity.”
It is no exaggeration to state that the question of what the Universe is composed of may be the most fundamental one in all of science. The time when astronomers are able to finally determine the exact types of matter and energy that make up the entire Cosmos will constitute not only a monumental scientific achievement, but might even lead to the discovery of a unified Theory of Everything, which is the holy grail of modern physics. “The situation in cosmology today is no less profound than in Galileo’s time,” comments Dr. Stacy McGaugh, a professor of astronomy at the Case Western Reserve University in Cleveland, Ohio. “Is our Universe an unfamiliar darkness, filled with invisible mass, with the ‘normal’ matter of which we are composed no more than a bit of queer flotsam in a vast sea of dark matter and dark energy? Or is our inference of these dark components just a hint of our ignorance of some deeper theory? … Both possibilities have profound implications.”
Indeed, studies like those from both Dr. Iocco’s and Chakrabarti’s teams might just allow us in the future to provide a more definitive answer to these burning scientific questions. No one can be sure today what that answer will be exactly, but what we can be sure of is that whatever the case, it is bound to be a monumental one.
Video Credit: TED-Ed