During the 1930’s, Mt. Wilson Observatory was famous for the revolutionary work of Edwin Hubble. Galaxies were discovered to exist outside the Milky Way and the universe was found to be expanding. In 1931, Einstein would visit the observatory and at Caltech, listen to a seminar by Fr. Georges Lemaitre on the theory of the origins of the universe, later dubbed the Big Bang. The founder of observatory, George Ellery Hale, was busy working on the successor to the 100-inch Mt. Wilson telescope. In 1934, the 200-inch mirror was cast, with a great amount of public fanfare for the future observatory at Mt. Palomar. Meanwhile, at the same facility, under the radar of the media, Fritz Zwicky was unearthing one of the great mysteries of astronomy today.
The Coma Cluster consists of some 1,000 galaxies at a distance of 320 million light years from Earth. The cluster itself is about 20 million light years wide. In 1933, Zwicky published a study of the cluster which indicated its mass was much greater than its visible content could account for. Had the optical mass of the cluster was all that existed, the velocities of the galaxies would have exceeded the escape velocity of the cluster, meaning there would not have been a cluster at all. Zwicky realized there must have been mass in the cluster that could not be seen. It was this extra mass that increased the escape velocity of the cluster keeping it intact. The results were originally published in a Swiss journal Helvetica Physica Atca (Zwicky was originally from Switzerland). Zwicky dubbed the invisible mass dunkle materie, or dark matter.
Acknowledgment: D. Carter (Liverpool John Moores University) and the Coma HST ACS Treasury Team.
It would take a few decades for the concept of dark matter to gain traction in astronomy. Part of it was new technology needed to be developed before it could be researched further. Part of it was Zwicky was ahead of his time. Besides dark matter, Zwicky developed the groundwork for ideas such as supernovae, neutron stars, and galactic gravitational lenses. Familiar to astronomers today, these were radically advanced concepts during the 1930’s. Zwicky also had, even by the standards of academia, a contentious personality towards other astronomers. Most famously, Zwicky referred to his fellow astronomers as “spherical bastards”. Why? They were, according to Zwicky, bastards any way you looked at them. Needless to say, this did not endear Zwicky to other astronomers, who were reluctant to promote Zwicky’s ideas.
Those issues aside, one by one, Zwicky’s theories received observational confirmation. Zwicky would discover 120 supernovae, the most by any astronomer. The first neutron star was discovered in 1967 with the radio detection of a pulsar in the Crab Nebula. The first image of gravitational lensing of a quasar occurred in 1979. As for dark matter, it would take the efforts of Vera Rubin, who faced bias against her work, to provide verification of its existence.
Unlike Zwicky, Vera Rubin did not give cause for astronomers to disdain her work. Astronomy has been traditionally a male dominated field. Many top graduate astronomy departments did not admit women until the 1970’s, and it was Vera Rubin who became the first women to observe at Mt. Palomar in 1967, nearly two decades after it had seen first light. The overall bias against women caused astronomers to greet Rubin’s early work with skepticism, and in some cases, downright hostility. In the end, the high quality of Rubin’s work would break through these barriers.
During the 1970’s, Rubin and her colleague Kent Ford, embarked on a study of galactic rotation curves. Kent Ford was responsible for building spectrographs sensitive enough to detect the Doppler shifts of stars as they orbited around a galactic core. Rubin decided upon this program for among other things, it would not require as much telescope time to complete as she had to balance her career with her family life. The expected result was that the farther out a star was from the center of mass in a galaxy, the slower its velocity would be. When it came time to bump the data against the model, the results came out like this.
This mirrored Zwicky’s study of the Coma cluster four decades earlier. If the optical mass was all there was to the galaxy, the stars at the outer edge were going so fast they would escape the gravity of the galaxy. However, that was not the case and there was much more dark matter holding these galaxies together than luminous matter. In fact, Rubin’s measurements indicated that only 10% of galactic mass was of the visible variety, and 90% was dark matter. Thus, 90% of galaxies were made of stuff that astronomers had no idea what it was. This was a staggering revelation.
When a discovery such as this that runs so counter-intuitive to the expected result, it will typically come under very critical review. That’s a good thing and a necessary part of the scientific process as long as it is the data being scrutinized and it does not become a personal matter. The best way to rebut criticism of a discovery is to provide replication. In 1970, Rubin and Ford published the flat galactic rotation curve for the Andromeda Galaxy. Throughout the decade, astronomers sought out a solution for the rotational curve without dark matter. However, by 1978, dozens of rotational curves reproduced the original result and hundreds more would follow the next decade. The dam had broke and by the time I was an undergrad student in the early 1980’s, dark matter had become standard material in galactic astronomy courses.
I distinctly remember the shock of learning 90% of the universe was made of dark matter whose nature was not known, but whose gravitational effects were clearly observed. In 1998, astronomy was rocked again by the discovery that matter, both luminous and dark, itself comprised only 20-25% of the universe. And this discovery would again trace its roots to Fritz Zwicky.
In 1937, Zwicky discovered a supernova that was distinctly different from what he had observed before. This supernova was brighter, and faded at a slower rate. Unlike the other supernovae, this was not the death throes of a high mass star. This was an explosion of a white dwarf that was siphoning mass from a companion star. Six years earlier, Subrahmanyan Chandrasekhar determined the maximum mass a white dwarf can obtain is 1.4 Suns. Once a white dwarf tops this amount of mass, its dense (a teaspoon of a white dwarf weighs 15 tons) carbon rich core ignites and creates a supernova. Since white dwarfs are the same mass when they explode, the brightness of these events are roughly identical. This gives astronomers a “standard candle” to calibrate distance. These events were eventually referred to as a Type 1a supernova.
During the late 1990’s, two teams of astronomers were competing to measure how gravity slowed the expansion of the universe since the Big Bang. The expectation was over time, gravity would rein in the rate of expansion. The way to determine this is to measure the red shift of Type 1a supernovae. As an object races away from Earth, its spectrum is shifted towards the red. The faster it is moving away, the greater the red shift. As these supernovae serve as a standard candle, their distance could be determined. The farther away these events were, the older they occurred as it would take longer for their light to reach Earth. Thus, the goal was to utilize Type 1a supernovae to measure the expansion of the universe throughout its history.
In 1998, the High-Z Supernova Search Team and the Supernova Cosmology Project independently released their results. What they found was not only did gravity fail to slow the expansion of the universe, but the expansion was accelerating. Gravity, obviously, was still around, but there was a mysterious force in the universe that not only counteracted gravity, but was of increasing presence as the universe expands. This force was referred to as dark energy and makes up some 70% of the universe.
What was discovered as well is the universe, like stars and galaxies, has evolved over time. Up to 5 billion years ago, gravity did slow down the expansion of the universe. After that epoch, dark energy became a stronger force and the expansion began to speed up. To put that time into perspective, the Earth and the Solar System is 4.5 billion years old. The timeline of the expansion is shown below.
As the expansion accelerates, some hypothesize that the universe will end in what is called the Big Rip. This refers to a state where the universe expands to the point where stars, planets, and even atomic particles are shredded apart. To be sure, life could not exist in such a universe. However, there is no need to worry, the latest estimate is a Big Rip would take place 22 billion years from now. And at any rate, we’ll need to learn more about what exactly dark energy is before we arrive at a definitive theory on the ultimate fate of the universe.
Currently, the South Pole Telescope is dedicated to researching dark energy. Built by the University of Chicago, the 10-meter telescope is dedicated to locating galaxy clusters for the purpose of mapping cluster formation throughout the life of the universe. It is hoped this effort will provide further answers on how both gravity and dark energy has shaped the expansion of the universe. The future of dark energy research remains in the Southern Hemisphere but in Chile.
The Vera Rubin Observatory is currently under construction in Northern Chile and is expected to be operational by 2024. The cost of the Rubin Observatory is estimated at $465 million, about half the cost of a new NFL stadium. Funded by a combination of public and private sources, this 8.4 meter wide angle telescope is designed to have the ability to survey the entire sky in three nights. The wide field of view will enable the Rubin Observatory to map large scale galactic structures and survey Type 1a supernovae over a ten year period. This will provide a more comprehensive map of how dark energy and dark matter have influenced the overall structure of the universe throughout time.
Given the dramatic nature of the discovery of dark energy, dark matter has been a bit overlooked since 1998. However, there have been some key advancements in the study of dark matter during that time. The Hubble Space Telescope was able to map dark matter by measuring gravitational lensing. Gravity bends light rays and as a result, distorts galactic images. While dark matter cannot be seen its gravitational effects can be observed. What was found is that dark matter had a smoother distribution during the early universe but is more clumpy now. This clumpiness creates a scaffolding for which galactic clusters form upon.
In 2015, dark matter was detected interacting with itself. In a galactic collision in the cluster Abell 3827, dark matter was detected to lag behind normal matter. This could provide a key clue as to what dark matter is as previously it was only detected to interact gravitationally. If “dark forces” among dark matter exist causing this interaction, then this discovery may help physicists model dark matter in a more complex and accurate manner. Ideally, this will help determine what exactly dark matter is.
So where does our knowledge of the universe stand now? The recent ESA Planck mission mapped out the cosmic microwave background (CMB). The CMB is the remnant of the Big Bang and shows us the state of the universe when it was 380,000 years old. By measuring the fluctuations in the CMB, astronomers were able to breakdown the universe as follows:
Dark Energy: 68.3%
Dark Matter: 26.8%
Ordinary Matter: 4.9%
So the matter we see around us, the Earth, Sun, stars and galaxies, only comprise slightly less than five percent of the universe. The remaining 95% is made of stuff that is unknown to us. That, along with the question of life beyond Earth, represents the most important mystery for astronomers to solve in the upcoming decade.
*Image on top of post is the galactic cluster Abell 2218. Where there is a galactic cluster, there is dark matter. The gravity of both ordinary and dark matter deflects light just like a lens does. The result is galaxies behind the cluster are magnified and distorted. The magnification effect allows astronomers to image galaxies far away behind the cluster that ordinarily would not visible. Credits: NASA, Andrew Fruchter and the ERO Team [Sylvia Baggett (STScI), Richard Hook (ST-ECF), Zoltan Levay (STScI)] (STScI).