Duke University

Understanding the Story of the Universe1920s to Present Day

Christine O’ConnellMath in the Universe- 89S

Hubert Bray28 March 2016

1

People have a tendency to want to understand phenomena in the Universe. For example,

religions were created in order to explain the unexplainable, and math was created in order to

understand the properties of the Universe. This paper will explain when scientists realized that

the Universe was much more complicated than they initially thought as well as their struggle to

determine the history of the Universe with a special emphasis on dark matter.

Galaxies

Beginning in the early 1920s, the Shapley-Curtis Debate split astronomers’ perceptions of

the Universe. Harlow Shapely declared that the size of the Milky Way galaxy was 300,000 light

years in diameter and encompassed the entire Universe. He based this theory on the assumption

that all global star clusters were about the same size. The distance to the global star cluster,

M13, had been calculated, and Shapley calculated the distance to other globular clusters by using

the apparent size and distance of the M13 as a reference. Astronomer Herbert Curtis argued that

the sun was the center of a galaxy only 30,000 light years in diameter. Furthermore, he stated

that the gas clouds found in the night sky or the spiral nebulae were island galaxies of similar

appearance to the Milky Way and did not lie within the Milky Way.1 Edwin Hubble sought to

put an end to this debate. In 1919, he used the Hooker telescope to take photographs of the then

called Andromeda nebula, exposing stars within it.

He measured the distance to this galaxy by using the technique of “standard candles”.

This technique involves the idea that objects have a fixed brightness. The perceived light is the

square of the distance, and Hubble used the Hooker telescope to measure the perceived light

from the Cepheid variable, a classification stars located with the Andromeda. Using these

Cephids, he determined that the Andromeda was around 900,000 light years away; however,

1 "The Shapley-Curtis Debate." The Milky Way. Web. 24 Mar. 2016.<http://astronomy.nmsu.edu/geas/lectures/lecture27/slide01.html>

2

improved technology now has the Andromeda at a little over two and a half million light years

away.2

Hubble concluded that the Andromeda was not a part of the Milky Way galaxy. His

analysis of Andromeda published in the “Extragalactic Nature of Nebulae” caused other

astronomers to look at other galaxies. The issue with calling something as a galaxy is that there

is not a concrete definition of a galaxy. Astronomers label a system of five stars a “group” and

hundreds of stars become a “cluster”. However, the question about what distinguishes a galaxy

from a cluster arises. According to the Oxford English Dictionary, a system of millions of stars

allows for the label “galaxy”. Furthermore, Forbes and Kroupa state that galaxies contain dark

matter, have a radius longer than 300 light-years, and lack star collisions due to their stability.

Galaxies also rotate around black holes similar to the Earth rotating around the sun. Despite the

ambiguous term “galaxy”, Andromeda is considered a galaxy and today, astronomers have

predicted over 100 billion galaxies in this Universe.3

The missing Universe: Predicting Dark Matter

Before the Andromeda was discovered, scientists like Harlow Shapley thought they knew

everything there was to know about the Universe. However, as scientists discover the Universe

more, they begin to know less. In 1974, two Princeton physicists Jeremiah Ostiker and James

Peebles researched how much matter was in the entire Universe. These two physicists had been

creating computer models of the Universe, but failed because the galaxies began to fall apart.

For example, if a disc of stars was entered into the computer, after one rotation period the stars

became blobs. This suggested that the galaxies were very unstable, which could be accounted by

2 "Hubble Space Telescope." Nasa. Web. 24 Mar. 2016. <http://www.nasa.gov/mission_pages/hubble/science/star-v1.html>.

3 Cartwright, John. "What Is a Galaxy?" Science. AAAS, 27 Jan. 2011. Web. 24 Mar. 2016. <http://www.sciencemag.org/news/2011/01/what-galaxy>.

3

the mass in the Universe creating gravitational pulls on other galaxies. Despite the fact that

galaxies contain millions, billions, or even trillions of stars, this mass is not large enough to

account for the gravitational pull exerted on other galaxies to prevent them from falling apart. In

order to account for this gravitational pull, the physicists input dark matter into their computer

simulations.

Dark matter had not been a new idea by the time that Ostiker and Peebles predicted it. In

the 1930s, Fritz Zwicky, observed the rotation of galaxies within the Coma Cluster. This cluster

of galaxies is held together by gravity, but these galaxies are not revolving around a massive

object. The Doppler shift was used to determine the velocity of the galaxies. This technique

measured the shift in the wavelength of light from the Coma Cluster, which is dependent on the

motion of both the Earth (the observer) and the Coma Cluster (the source). Zwicky then

calculated the mass of the Coma Cluster using the viral theorem, which relates velocity of

objects to their experienced gravitational force. Since the gravitational force is related to mass,

the mass of the Coma Cluster could be calculated. This calculation provided the theoretical mass

of the galaxy, but Zwicky determined the visible mass by making a ratio of the total light output

to the mass and compared it with the Kapteyn stellar system. However, there were discrepancies

between these two calculations, so Zwicky theorized that there must be other mass that was not

from stars.4

While Ostier and Peebles predicted dark matter, and Zwicky had also theorized it,

astronomers were reluctant to accept this idea. Zwicky was a very difficult guy to get along

with, so his work was not taken seriously. Furthermore, Ostier and Peebles had no logical

4 "Section 2: Initial Evidence of Dark Matter." Physics. Annenberg Learner, Web. 26 Mar. 2016. <https://www.learner.org/courses/physics/unit/text.html?unit=10&secNum=2>.

4

explanation for the presence of dark matter other than the fact that it fit with their computer

models. Finally, astronomers began to accept dark matter after the research of Vera Rubin. Vera

Rubin used the model of planets orbiting around the sun, and realized that the velocities of

planets further away from the sun decreased exponentially because they experience less of a

gravitational force. However, when stars rotate around the center of galaxies, each of the stars

had around the same velocities. Therefore, dark matter explained how stars were able to rotate

around the center of galaxies at such a high velocity. 5

The discrepancy Rubin pointed out was too important for other physicist to ignore. As a

result, Professor of astronomy, Ricardo Giovanelli began to search for dark matter. Using a

telescope Arecibo telescope that collects radio waves, Giovanelli was able to reach the outskirts

of galaxies, where hydrogen, but not stars exist. He was able to determine that the hydrogen gas

orbits around the galaxies as fast as stars. Therefore, dark matter must also be influencing

hydrogen gas. Giovanelli’s research lead to the idea that dark matter is like a halo,

encompassing the entire galaxy. 6

5 "Evidence for Dark Matter." Dark Matter. Web. 26 Mar. 2016. <http://cdms.phy.queensu.ca/Public_Docs/DM_Intro.html>.

6 "BBC - Horizon - 2006 - Most of Our Universe Is Missing (Dark Matter) - Video Dailymotion." Dailymotion. BBC, 2006. Web. 26 Mar. 2016. <http://www.dailymotion.com/video/x226e38_bbc-horizon-2006-most-of-our-universe-is-missing-dark-matter_shortfilms>.

5

At the same time that these scientists were discovering that there must be dark matter in

the Universe, other scientists were studying the Big Bang. In 1948, George Gamow came up

with the theory of Big Bang nucleosynthesis, which considers the formation of nuclei excluding

the element of hydrogen during the beginning of the universe. According to this theory,

immediately after the Big Bang, the Universe cooled from 10^32 Kelvin to around 10^9 Kelvin.

During this time, nucleosynthesis, or the formation of light elements, occurred. Through the

collision of protons and neutrons, deutrerium (a hydrogen with a neutron) formed. This element

collided with other protons and neutrons to form element such as helium and lithium as well as

other isotopes.

This image shows the relative abundances of each element. Baryonic matter or matter of

protons and neutrons is considered normal matter since it is made up of atoms. It consists of

about 25% helium, .001% deuterium, and even smaller amounts of lithium. About 75% of mass

is hydrogen itself.7

Through observations, astronomers realized that the abundance of baryonic particles are

proportional to each other. By studying the amount of different baryonic particles in the

7 "Big Bang Nucleosynthesis." Web. 26 Mar. 2016. <http://w.astro.berkeley.edu/~mwhite/darkmatter/bbn.html>.

6

Universe, cosmologists realized that not all matter must be baryonic. Therefore, cosmologists

believed that dark matter was not made out atoms.8 The question still lingered, what caused the

strong gravitational pull exerted on other galaxies?

Carlos Frenk sought to discover the remaining composition of the Universe. He drew off

of Einstein’s theory that states that mass can distort the Universe. This distortion effects how

light hits the Universe because light is deflected by mass. The picture below shows arch-like

objects that are galaxies, whose light had been distorted by mass. Therefore, this picture allowed

him to figure out the weight of the galaxy and relate the normal mass to dark matter. However,

his calculations suggested that dark matter only accounted for 23% of the mass-energy

relationship of the Universe. What else in the Universe was there?9

Saul Perlmutter, an astrophysicist who was awarded a Nobel Prize in Physics had the

answer to this question. He realized that since everything in the Universe is attracted to each

8 "Big Bang Nucleosynthesis." Big Bang Nucleosynthesis. Web. 26 Mar. 2016. <http://cdms.berkeley.edu/Education/DMpages/science/essays/essays/BBN.shtml>.

9 "Professor Carlos S. Frenk - 'Our Implausible Universe'" Vimeo. Web. 26 Mar. 2016. <https://vimeo.com/3265677>.

7

other due to gravity, the Universe should be slowing down. However, he detected that the

Universe is speeding up as he measured the speed of exploding stars. The answer to this

problem lied in dark energy. Dark energy could provide enough energy for the Universe to

speed up, and since energy is proportional to mass, it could answer the gravitational problem of

the Universe.10

Efforts to Confirm the Presence of Dark Matter

Carlos Frenk used a computer simulation to create the Universe. The elements he inputs

in this simulation are from the Standard Model as well as dark matter. The computer simulation

of galaxies in the universe were very similar to actual galaxies in the Universe. Furthermore, the

simulation shows how ordinary matter formed from dark matter at the beginning of the Universe.

This suggested to Carlos Frenk that he knew the physics of the Universe. However, critics argue

that there can be biases and if one knows appearance of an actual galaxy, it can influence the

designer to manipulate the computer simulation of a galaxy such that it resembles and has the

same properties of a galaxy like the Milky Way. In response to the criticism, astrophysicist

David Spergel sought to prove the accuracy of Carlos Frenk. He designed a satellite in 2003

called WMAP (Wilkinson Microwave Anisotropy Probe), which took images of the beginning of

the Universe. 6

10 Sanders, Robert. "Saul Perlmutter Awarded 2011 Nobel Prize in Physics." Berkeley News. 4 Oct. 2011. Web. 26 Mar. 2016. <http://news.berkeley.edu/2011/10/04/saul-perlmutter-awarded-2011-nobel-prize-in-physics/>.

6

8

This satellite measured the microwave afterglow of the Big Bang. Calculations from this

picture confirmed that the Universe was 13.77 billion years old and suggested that the ratio of

baryonic matter to dark matter to dark energy was 4.6: 24: 71.4 respectively. Furthermore, this

picture of the Universe around the beginning of time had the ability to become the Universe

today because they had very similar patterns. This would only happen if the ratio of normal

matter to dark matter to dark energy were these exact ratios.11 This was further studied by the

European Space Agency using the Planck satellite to make observations of the cosmic

microwave background, which is the heat left over 400,000 years after the Big Bang when the

Universe cooled. Data gathered from August 2009 until October 2013 determined-mass ratio of

normal baryonic matter, dark matter, and dark energy in the Universe. These observations found

that Ωb*h²= .0226, where Ωb is fraction of total mass-energy in the Universe and h is the hubble

constant, which is related to the expansion of the Universe. Using this equation, researchers

found that normal matter consists of under 5% of the mass in the Universe. Replacing normal

matter with dark matter, allowed for the equation Ωc*h²  = .1186, and dark matter was

11 "A Bold Critic of Big Bang's 'Smoking Gun'" QuantaMagazine, 3 July 2014. Web. 26 Mar. 2016. <https://www.quantamagazine.org/20140703-a-bold-critic-of-the-big-bangs-smoking-gun/>.

9

discovered to be around 26% of the mass-energy relationship. Therefore, dark energy must

account for around 69% of the mass-energy relationship in the Universe. While these equations

predict that dark matter exists, they do not say anything about the identity of dark matter.12

Detecting Dark Matter

As mentioned above, the Big Bang suggests that dark matter is not baryonic mass.

However, the composition of dark matter has yet to be determined. This is because dark matter

is very difficult to detect because it neither admits nor absorbs light. Additionally, dark matter

does not interact with other particles, although it is affected by gravity. Therefore, the key to

understanding dark matter lies within gravity and gravitational lensing.13

Gravitational lensing is a technique used to determine the size of galaxies. Einstein’s

theory of general relativity states that matter curves space time. Therefore, mass will bend light,

and even dark matter will bend light. Astronomers can determine the size of galaxies by

determining how much light is bent. This technique is important because the size of the galaxy

is important in order to make observations of galaxy clusters because astronomers need to

observe galaxies of similar sizes. Furthermore, gravitational lensing is important for analyzing

the collisions of galaxy clusters because it can outline where the mass of the clusters lie. 14

Astronomers are also looking at galaxy clusters to help determine the location of dark

matter. Researchers examined 9,000 galaxy clusters from the Sloan Digital Sky Survey DR8

12 "Planck Mission Full Results Confirm Canonical Cosmology Model." Dark Matter Dark Energy Dark Gravity. 06 Mar. 2015. Web. 26 Mar. 2016. <http://darkmatterdarkenergy.com/2015/03/07/planck-mission-full-results-confirm-canonical-cosmology-model/>.

13 "Dark Matter and Dark Energy - National Geographic." National Geographic. Web. 26 Mar. 2016. <http://science.nationalgeographic.com/science/space/dark-matter/>.

14 Grocut, Emma. "What Is Gravitational Lensing?" What Is Gravitational Lensing? CFHTLenS. Web. 26 Mar. 2016. <http://www.cfhtlens.org/public/what-gravitational-lensing>.

10

galaxy catalog and analyzed the clusters based on how clumped together the galaxies were to

each other. They discovered that most galaxy clusters were about 100 million light years away

from each other. However, crowded galaxy clusters were spaced out further apart from other

galaxy clusters. This discovery suggests that dark matter influences the crowding of galaxy

clusters. Researches say that it will be important to study galaxy clusters in the future in order to

understand dark matter.15

In addition to studying galaxy clusters themselves, astronomers have also researched the

collisions of galaxy clusters. It was discovered that the hot gas in the collision experienced

resistance from a drag force; however, the nature of dark matter prevented itself from

experiencing resistance. This allowed for the hot gas separated from the dark matter, further

confirming the presence of dark matter. The picture bellow is the Bullet Cluster and shows this

phenomenon with the red represented as hot gas and the blue represented as dark matter. 16

15 "Galaxy Clusters Reveal New Dark Matter Insights." NASA/JPL. N.p., n.d. Web. 26 Mar. 2016. <http://www.jpl.nasa.gov/news/news.php?feature=4829>.

16 "Chandra Press Room." Nasa Finds Proof of Dark Matter. N.p., 21 Aug. 2006. Web. 28 Mar.2016. <http://chandra.harvard.edu/press/06_releases/press_082106.html>.

11

In conclusion, the realization that the Universe is comprised of billions of galaxies has

opened many new areas of research. Physicists want to know the composition of the Universe,

and one of the mysteries revolves around the mass of the Universe. Due to the high gravitational

pull exerted on other galaxies, most physicists believe that the Universe is made up of baryonic

matter, dark matter, as well as dark energy. While neither dark matter nor dark energy has been

discovered, the only possible way that this strong gravitational pull could exist is by defying

Newton’s law of universal gravitation. This law requires a gravitational constant G. However,

some scientists believe that Newton’s equation is incorrect and that a universal gravitational

constant does not exist, and therefore, the laws of gravity are not understood. Most scientists

though are reluctant to accept this idea for two reasons. First of all, if Newton’s law of universal

gravitation is incorrect, people know way less about the Universe than expected.17 After all,

most researches who studied dark matter had the goal of figuring out the history of the Universe,

so thinking that Newton’s universal law of gravitation is incorrect complicates the problem much

further. Secondly, this paper shows that dark matter is likely to exist because of the physics of

17 "Is Dark Matter Just Us Being Wrong About Gravity?" Motherboard. Web. 26 Mar. 2016. <http://motherboard.vice.com/blog/is-dark-matter-just-us-being-wrong-about-gravity>.

12

galaxies. In the future, further investigations will need to be done to determine the composition

of dark matter as well as dark energy, which remains a large mystery today.

13