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By observing rotation rates of galaxies and clusters of galaxies, Fritz Zwicky and then Vera Rubin fifty years later discovered that most of the matter in the universe is invisible dark matter. Shortly thereafter, three Nobel Laureates used the brightness of type 1a supernovae as a standard candle to discover that the expansion rate of the universe is slightly increasing, and, thus, most of the energy in the universe is dark energy.
Galaxies and clusters of galaxies rotate as if there is a massive amount of extra matter and gravity within them. Scientists now realize that dark matter was essential to the early formation of stars and galaxies and the present rotation of galaxies. Similarly, dark energy was essential to the steady expansion of the universe over 14 billion years.
In the 1930s, Cal Tech Professor Fritz Zwicky observed that galaxies in the Coma Cluster of Galaxies were orbiting each other at 700 km/sec. The orbital velocity was surprising because gravity from the observed mass in the Coma Cluster should not have allowed galaxies to orbit each other at more than 200 km/sec. Their momentum should have flung the galaxies out of the clusters and into deep space. Zwicky hypothesized that the explanation might be unobservable dark matter, which would add extra gravity to the galaxies. At the time, the idea of dark matter did not catch on with the scientific community. Scientists did not accept the concept of dark matter until Vera Rubin observed its effects on individual galaxy rotations in the 1970s. While there is indirect evidence of dark matter, it has never been directly observed. Normal scientific instruments that detect electromagnetic radiation cannot detect it.
After her PhD, Rubin became an astronomer at the Carnegie Institute, where she collected images (plates) with a spectrograph attached to Kitt Peak Observatory’s 4- meter diameter Mayall Telescope (banner above) in Arizona and the 4 m Cerro Tololo telescope in Chile.[1] The Carnegie image tube (Figure 1‑41), developed by her collaborator William Ford, detected red and blue shifts in these plates. She was able to measure the rate of rotation of galaxies as a function of distance from the center by measuring the red or blue shift in hydrogen gas in the clouds in the outer parts of galaxies.
Figure 1‑41. Vera Rubin analyzing photographic plates with Carnegie image tube. Courtesy of Carnegie Institute.
Rubin found that the rotation rates of 11 galaxies were just as high in the outer parts of the galaxies as in the inner galaxies, which was odd. Outer objects (for example, Pluto) in a rotating galaxy or solar system should have much slower velocities than objects near the center. Rubin figured out that there must be a halo of matter around the galaxies, causing the galaxies to rotate faster. Rubin calculated that dark matter is up to 90% of the mass of the universe. She determined that stars and gas are in the range of 10% of the mass of the universe.[2]
Refined measurements in recent decades indicate that the amount of dark matter in the universe is 5 times greater than the amount of observable matter. It is likely that dark matter only interacts with gravitational force and does not interact with the electromagnetic force, gas pressure, or other forces. For this reason, scientists refer to dark matter as Cold Dark Matter (CDM).
In the early universe, dark matter gathered by gravity and concentrated in small regions. The gravity of these concentrated regions attracted hydrogen and helium gas, which then collapsed and formed the first stars. The gas could not have gathered and formed the first stars without these concentrated dark matter regions with intense gravity. The early giant stars exploded and became black holes, which then gathered matter around themselves and formed galaxies. Thus, galaxies formed in clusters around the early concentrations of dark matter, which led to the large-scale structure (clusters) of the universe (Figure 1‑42). In addition to causing the formation of the first stars and galaxies, dark matter is also essential to the continued rotation of galaxies.
Figure 1‑42. Large-scale structure of the current universe, which formed due to clumps and strings of dark matter in the early universe. Credit: IPAC/Caltech.
Einstein first conceived of dark energy when he derived the general theory of relativity. He inserted it in the equation to prevent the universe from collapsing upon itself. This concept was rejected when Lemaitre conceived of the expanding universe, which would have prevented collapse by the kinetic energy of expansion. Dark energy was no longer needed. Although people forgot about dark energy for many years, scientists observed that the universe expansion rate is slightly increasing, which can only be explained by dark energy.
Scientists detected the increased expansion rate of the universe, and thus dark energy, with type 1a supernovae. These supernovae explosions last for several weeks in ancient galaxies. They are so bright (5 billion times brighter than the sun) that they are seen in galaxies with redshifts greater than 1 (when the universe was 7 billion years old). Type 1a supernova explosions always have the same luminosity and light curve so they are standard candles that can be used to find the distances to galaxies. Type 1a supernova explosions in extremely redshifted ancient galaxies recently revealed that the rate of expansion of the universe is increasing because they show that the galaxies are further than expected with steady expansion. Scientists think dark energy is an inherent property of space. Thus, as space expands, the total amount of dark energy in the universe increases. Dark energy has become the largest expansive force in the universe during the last 5 billion years. Scientists calculate that the universe is 73% dark energy, 23% dark matter, and 4% normal matter. Of the normal matter, most is gas (4%), with only 0.5% in stars, and 0.05% in planets.
This paragraph is the text for the video at https://www.instagram.com/p/CVn-S8sLIJJ/ Not only was dark matter essential to the expansion of the universe, but it also led to the formation of the first stars. The early universe had slight variations in density. Dark matter collapsed toward higher density regions by gravity. Here is the dark matter collapsing in a region of space. Gravity from the dark matter then attracts the hydrogen and helium gas. When the red circle gets small enough it collapses and becomes a giant blue star. These stars exploded as supernova, which gathered back together and formed a black hole, which then attracted gas and caused the formation of billions of stars in a galaxy that swirled around it. https://www.instagram.com/p/CVn-S8sLIJJ/
References
References
[1] Van den Heuvel. 2016. Chapter 13: What is dark matter? In The Amazing Unity of the Universe. pg. 199.
[2] Rubin, Vera C., N. Thonnard, and W. K. Ford Jr. "Extended rotation curves of high-luminosity spiral galaxies. IV-Systematic dynamical properties, SA through SC." The Astrophysical Journal 225 (1978): L107-L111.
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Mayall telescope on Kitt Peak near Tucson, Arizona, where Vera Rubin collected many of her photographic plates of hydrogen gas velocity in galaxies.
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