Dark Matter

Dark Matter’s Emergence in Astronomical Research:

In the fields of cosmology and astronomy, dark matter is truly a fascinating topic. The discovery of dark matter redefined astronomers’ notions of galaxies, and the mass distribution within those galaxies. It significantly shifted mankind’s perception of the cosmos, and what makes up the vast outer void of impenetrable darkness, which engulfs moons, planets, stars and galaxies, as if they were small vessels adrift in a tumultuous sea.

The initial theory of dark matter first emerged in the19th century. Images captured by telescopes, of faraway places, deep in the stillness of outer space, showed sections of the sky that were dark. These photographs led astronomers to postulate the existence of dark matter, which would explain the scattered distribution of stars, in these particular areas of the sky (Bucklin, 2017). “Stars did not appear to be evenly distributed, and scientists wondered if this was because dark regions lacked stars altogether, or if some absorbing matter was blocking their view of other stars,” (Bucklin, 2017).

A Scots-Irish physicist, by the name of Lord Kelvin, conducted research on these areas of the sky. His groundbreaking work involved figuring out the number of dark bodies within our own galaxy. As a part of Kelvin’s galaxy-based analyzation of dark bodies, he primarily focused on the varying speeds of stars, as they orbit around the center of the Milky Way. Through measuring the differing speeds of stars, making their lengthy journey around the center of our galaxy, he was able to determine the overall galactic mass. But a major discrepancy quickly became discernible in his detailed research (Bucklin, 2017). “There was a difference between that mass and the stars we can see. In one of his Baltimore Lectures on molecular dynamics and the wave theory of light, he concluded that ‘many of our stars, perhaps a great majority of them, may be dark bodies,’ “ (Bucklin, 2017).

At this point in time, the term “dark matter” hadn’t been used yet, to describe what was taking place in the astronomical observations, as well as in Lord Kelvin’s research. This would quickly change, as Henri Poincare became prominent in the astronomy scene in 1906. Poincare was the first person to coin the term “dark matter,” in his important astronomical work, titled “The Milky Way and Theory of Gases.” But despite using this term, Poincare discarded Kelvin’s assertions, by disagreeing with him and his conclusions, regarding the existence of dark matter (Bucklin, 2017). “Poincare wrote that ‘since his number is comparable to that which the telescope gives, then there is no dark matter, or at least not so much as there is of shining matter,’ ” (Bucklin, 2017).

As history moved forward, the case for dark matter extended even further. It wasn’t until some time later that significant evidence illustrated the existence of dark matter. Major evidence for dark matter emerged through the research of Fritz Zwicky, a Swiss-American astronomer, whose groundbreaking work embedded his unique name into the annals of history. To further understand and analyze dark matter, Zwicky closely studied 800 galaxies. His research revealed important insights, in relation to the velocity dispersion of these galaxies (Bucklin, 2017). “While the 800 galaxies he studied should have a velocity dispersions of 80 kilometers per second, he found that the real value was closer to 1,000 kilometers per second. This meant stars were traveling so fast that they should escape their mutual gravitational pull,” (Bucklin, 2017). The fascinating aspect of this observation was that the stars should have left their galaxies. But this wasn’t the case. There was only one plausible explanation. There clearly was more mass in galaxies than astronomers had previously believed (Bucklin, 2017). “ ‘If this would be confirmed,’ Zwicky wrote, in his 1933 paper. ‘We could get the surprising result that dark matter is present in much greater amount than luminous matter.’ The additional mass from this theorized dark matter would help explain how the galaxy cluster is able to hold together via gravitational attraction,” (Bucklin, 2017). It was later discovered that Zwicky’s estimation was off, when it came to the postulated amount of dark matter within galaxies. He actually overestimated this number, “by a factor of about 8,” (Bucklin, 2017). But despite this, Zwicky’s contribution to astronomical research was truly immense (Bucklin, 2017).

Years later, further cosmological research added to what was already known about dark matter. In the 1960s and 1970s, Vera Rubin emerged into the astronomy scene. Her work, just like Fritz Zwicky’s, would have truly a massive impact. Rubin’s research primarily related to the rotation curves of galaxies. A rotation curve examines how the orbital speeds of stars correlate with their distances from galactic centers. The goal of using a rotation curve is to figure out mass distribution within galaxies. Theoretically speaking, the farther away stars are from galactic centers, the slower their orbital velocities should be. This notion connects to the idea that mass falls away at significant distances from the galactic center. But in stark contrast to what velocity curves should exhibit, Rubin’s work illustrated the opposite (Bucklin, 2017). “But Rubin found something different in the spiral galaxies she studied: instead of sloping down, their rotation curves seemed to level off. This implied that the stars in the far outer regions of these spiral galaxies were moving just as quickly as stars near the center. The observed, visible mass of a galaxy did not have enough gravity to hold these fast-moving stars together,” (Bucklin, 2017). Using this key information, Rubin came to the conclusion that there are significant amounts of dark matter in galaxies. Dark matter vastly outnumbers the amount of visible matter within the universe, as well as explaining the observed velocities of stars, at varying distances from the centers of galaxies (Bucklin, 2017). “Over the course of the 1970s, other scientists, despite some initial skepticism, confirmed these findings. A large ‘halo’ of dark matter surrounds each galaxy,” (Bucklin, 2017).

Further Evidence for Dark Matter’s Existence:

In addition to the previously discussed historical research, which supports the existence of dark matter, further evidence exists. One such piece of evidence is based on the fact that astronomers have measured the amount of visible mass in the universe. The visible mass within the universe is actually quite small (Siegel, 2015). Based on this information, astronomers have come to the conclusion that “the normal matter is only about 5% of what’s required to be responsible for all the energy in the Universe,” (Siegel, 2015). It can be inferred that something else must be contributing to the rest of the energy in the universe. This other thing is dark matter (Siegel, 2015).

In addition to the measured visible mass of the universe, galaxies cluster together. This is yet another indication that dark matter exists. What’s crazy about the clumping of galaxies is that this takes place on truly a massive scale, with hundreds, and even thousands of galaxies binding together. Using the velocities of galaxies, as well as the laws of gravity, the total mass can be determined, to figure out how much mass is needed to hold together galaxy clusters. Further observations help astronomers to measure the amount of matter in galaxy clusters (Siegel, 2015). “There’s a lot! But it’s not enough. It’s only about 13-17% of the total mass necessary to keep the clusters bound. There has to be some other form of matter in there to account for the mass: some form of dark matter,” (Siegel, 2015).

A significant piece of evidence, which illustrates the existence of dark matter, is gravitational lensing. Gravitational lensing functions exactly how it sounds. Mass essentially serves as a lens. As light from extremely distant objects passes through it, mass bends and curves that light, changing the appearance of those faraway objects. Depending on the amount of mass the light passes through, gravitational lensing can either be strong or weak. Through gravitational lensing, astronomers have been able to measure the mass creating that lensing. This mass lies between the observer and a distant background object, which emits light that ends up becoming distorted, by the time it reaches the observer (Siegel, 2015). “Through every observation ever made, we’ve measured a ‘total mass’ that’s consistent with being approximately six times as great as the amount of mass that we expect from the normal matter alone,” (Siegel, 2015).

The Cosmic Microwave Background adds to the pile of evidence, concerning the existence of dark matter. This specific piece of evidence is quite significant. The Big Bang created a leftover glow, known as the Cosmic Microwave Background. There are specific fluctuations within the CMB, and these fluctuations exhibit an interesting pattern. This pattern is directly related to both matter and radiation, and the ways in which they interact with each other. When matter and radiation interact, “waves” are created in the Cosmic Microwave Background (Siegel, 2015). “If there’s dark matter present, it affects the radiation and normal matter due to gravity, but doesn’t interact the way normal matter does with itself or radiation. So we reconstruct this pattern of fluctuations, and find that it’s only consistent with a Universe that’s made up of 5% normal matter, 27% dark matter and 68% dark energy,” (Siegel, 2015).

One Proposed Detection Method - A Billion Tiny Pendulums:

In 2020, researchers at the National Institute of Standards and Technology (NIST), came up with a new way to discover dark matter. This modern advanced proposal involves the utilization of billions of millimeter-sized pendulums. The goal of using these pendulums would be to sense dark matter, through the manner in which it interacts with visible manner. The scale of this experiment is truly mind-boggling, considering the size of the dark matter detector (as big as a grain of salt!) (Stein, 2020). “The experiment would be one of the few to search for dark matter particles with a mass as great as that of a grain of salt, a scale rarely explored and never studied by sensors capable of recording tiny gravitational forces,” (Stein, 2020). This method of detecting dark matter is based on the coupling that occurs between visible and dark matter. As previously discussed, the device used to make the measurements would be extremely small, at the minuscule size of a grain of salt. The extreme sensitivity of the device utilized would make this method of detection very precise (Stein, 2020). “The experiment would be sensitive to particles ranging from about 1/5,000 of a milligram to a few milligrams,” (Stein, 2020). The way in which this experiment would be carried out is quite involved. In fact, scientists at NIST have a few different ideas, when it comes to carrying out this experiment. There are two differing ways they have proposed the use of microscopic devices, to measure dark matter (Stein, 2020). “Both involve tiny, millimeter-size mechanical devices acting as exquisitely sensitive gravitational detectors. The sensors would be cooled to temperatures just above absolute zero to minimize heat-related electrical noise and shielded from cosmic rays and other sources of radioactivity. In one scenario, a myriad of highly sensitive pendulums would each deflect slightly in response to the tug of a passing dark matter particle,” (Stein, 2020).

Another Proposed Detection Method - GPS Satellites and Atomic Clock Networks:

This is yet another interesting proposal, when it comes to detecting and measuring dark matter. This method of detecting dark matter involves the utilization of GPS satellites and atomic clock networks. The astronomers who have come up with this idea, Andrei Derevianko and Maxim Pospelov, postulate that atomic clocks could be disrupted by dark matter. They believe that the effects of dark matter on atomic clocks could be measured. This would help astronomers to discover where there are pockets of dark matter. They have begun their research by examining data gathered by 30 GPS satellites (Futurism, 2014). “Correlated networks of atomic clocks such as the GPS and some ground networks already in existence, can be used as a powerful tool to search for the topological defect dark matter where initially synchronized clocks will become desynchronized,” (Futurism, 2014).

Dark Matter’s Role - Galactic and Cosmological Evolution:

Very early on in the history of the universe, a few hundred million years after the Big Bang had occurred, galaxies emerged out of dark matter halos. The immense amounts of dark matter pulled atoms together, creating extremely dense areas, allowing for nuclear fusion to take place. This ultimately led to the creation of stars, and eventually galaxies. As galaxies significantly grew in their size and scale, they attracted other galaxies, leading to the formulation of galaxy clusters, as well as superclusters. These galaxies moved around each other, either flying by one another, colliding into each other, or merging, and thus created larger galaxies in the process (Ash, 2014). “Some of them underwent traumatic processes such as galaxy harassment, stripping, strangulation, and even cannibalism—things that sound like they apply more to a violent criminal than to a galaxy,” (Ash, 2014). As is illustrated above, dark matter plays truly a key role in the formulation of galaxies within the universe. Galaxies begin as dark matter halos, progressing into regions where star creation can begin. Dark matter also influences the interactions between galaxies over time. This is based entirely on the galactic mass, and the amount of gravity in different galaxies. As a result of differing amounts of gravity in varying galaxies, over time galaxies have interacted with each other in unique ways. This has shaped their current locations; the clusters and regions of space they currently inhabit (Ash, 2014).

Conclusion:

Dark matter is truly a fascinating aspect of cosmology. It has been absolutely key in redefining astronomers’ notions of where mass is located within the universe. Previously, for quite a long period of history, the mass of galaxies was believed to be concentrated in visible matter. The theory for dark matter flipped this widespread idea upside down. The discovery of dark matter forever changed the way astronomers view mass distribution in the universe, and has restructured mankind’s perception of the vast outer void, extending for light years into the impenetrable stillness of interstellar space.

Works Cited:

Bucklin, S. (2017, February 3). A history of dark matter. Ars Technica. https://arstechnica.com/ science/2017/02/a-history-of-dark-matter/.

Siegel, E. (2015, July 30). 7 Independent Pieces Of Evidence For Dark Matter. Medium. https:// medium.com/starts-with-a-bang/7-independent-pieces-of-evidence-for-dark- matter-3692126a2283.

Stein, B. (2020, October 13). A Billion Tiny Pendulums Could Detect the Universe's Missing Mass. NIST. https://www.nist.gov/news-events/news/2020/10/billion-tiny-pendulums- could-detect-universes-missing-mass.

Futurism. (2014, November 20). Scientists Propose New Way To Detect Dark Matter. Futurism. https://futurism.com/scientists-propose-new-way-detect-dark-matter.

Ash, S. (2014, February 20). Galaxies Are Everywhere. Why Don't Astronomers Know Where They Come From? Slate Magazine. https://slate.com/technology/2014/02/galaxy- formation-dark-matter-ellipticals-spirals-harassment-stripping-strangulation-and- cannibalism.html.