Visualizing the Invisible: The Quest to Map the Dark Universe

MIDN 1/C Geoffrey DeSenaMIDN 1/C Raquel Smith

DeSena & Smith 1

EA486GProfessor Fahey

22APR2013The most difficult questions of the 21st century most likely

will continue to be the ones that have plagued mankind since the

dawn of civilization. Questions of origins, destinations, and

purpose may never be answered, but there remains one question of

scientific origin that is on the cusp of great understanding:

what is the Universe made of? Any astrophysicist can point to the

remnant of a supernova and show how its enriched guts have been

blown across the universe, seeding future solar systems with the

ingredients for life. However, it is much more difficult to look

at the night sky, bathed in a sheet of countless stars and

recognize that all that can be seen is but a small piece of all

that is. The hot, glowing gasses of distant stars, galaxies, and

nebulae – the baryonic matter – form the matter that mankind has

always known, but recent scientific discoveries have given us

reason to believe that there is a great deal more matter and

energy in the universe than we first suspected. One tool that may

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help in the understanding of this “dark matter” is the ability to

map its location and shape, but to paint a picture of the

invisible requires the understanding of what it does to the

visible.

The first step to studying the stuff of the cosmos is to

understand the principles upon which study of the cosmos is

founded. The fundamental principle is that the universe is

relatively homogeneous. With the local exceptions of stars, or

even galaxies, the universe is the same everywhere. Astronomers

have verified this for decades through the examination of light

spectra that can show unambiguously from what elements light is

being emitted and through which elements it is passing.1 This

applies not only the makeup of the universe, but the laws that

govern its behavior. Theories that bind the data collected in our

infinitesimal corner of the universe hold when predictions are

made for distant objects. Gravitation, derived from Einstein’s

general relativity, explains the motions of distant celestial

bodies with remarkable precision. If the instances of verified

prediction have been simple coincidence, and each part of the

1 Harrison, 76.

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universe abides by a different set of rules, trying to understand

anything beyond the horizon is an exercise in futility. Taking

this to not be the case, it will be assumed (with good reason)

that the laws of the physics can be applied throughout the

cosmos.

Credit for being the first to take note of the missing mass

of the universe is given to Fritz Zwicky who, in 1933, spent a

considerable amount of time studying the rotation of the Coma

galaxy cluster. He noticed that the galaxies were moving far

faster than theory predicted. By measuring the amount of visible

light from the galaxies, he determined that the luminous matter

only comprised approximately 10% of the total matter in the

cluster.2 Most likely due to his abrasive nature, Zwicky’s peers

largely ignored him.3 In the end, he would prove to be far ahead

of his time. The one to get the attention of the astrophysics

community was Vera Rubin, who found the same phenomena occurring

within galaxies.

Given that the vast majority of a galaxy’s mass is

concentrated at the center, the stars orbiting the galaxy should

2 Soter and Tyson.3 Ibid.

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have behaved much like the planets around Sun in our solar

system. With the luminosity and temperature of the stars in the

galaxy, Rubin could estimate the mass of the galaxy and the

required rotational velocities of the stars in order to remain in

orbit around the galaxy’s center. The speed should decrease with

the square of the distance from the center, but the observed

speeds remained constant as shown in Figure 1.

Figure 1: Rotational Velocities of Stars in the M33 Galaxy4

Rubin explains, “The lack of any decrease in stellar

velocities for stars at large distances from the nucleus in a

disk galaxy tells us that the mass density is falling very

slowly.”5 The same dark matter that Zwicky had theorized was

4 Image Courtesy of the University of Sheffield5 Rubin, 47.

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shown to be permeating galaxies. Further studies showed that the

dark matter formed a spherical halo, increasing linearly with

radius and extending beyond the galaxy approximately three times

its radius.6

Even though observations showed that the orbital mechanics

that we had mastered were not accurately describing the motions

of distant stars, the findings did little to offer solutions to

the mass disparity. A discovery that came about by coincidence

would lead to promising theories on the missing mass from

Zwicky’s clusters and Rubin’s galaxies. Two young researchers,

Arno Penzias and Robert Wilson, had gotten access a highly

directional radio telescope for pure research in 1965. Despite

all of their precautions, from removing interference from large

transmitting sources to cleaning the antennae of bird droppings,

the two could not remove the background noise that persisted

throughout all of their data. As luck would have it, Robert Dicke

of Princeton University had been looking for a way to collect

data to support his theory of an afterglow of the big bang. When

Wilson (ironically trained in steady state theory) contacted

6 Ibid, 120.

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Dicke’s lab, the theoretician begrudgingly passed on the

information.7 It turns out that the noise that Penzias and Wilson

had been trying to remove was the cosmic background radiation

Dicke had predicted. The three published their findings

concurrently8, but in the end, Penzias and Wilson received the

Nobel Prize.

Further explorations into the subject of the cosmic

background radiation (CBR) have led to the launch of three

satellites mapping this radiation in every direction. The latest

results provided by European Space Agency’s Planck space

telescope show the minute anisotropy of the CBR to unprecedented

precision, showing the state of the universe a mere 380,000 years

after its birth. 9 The results, however, have given researchers

reason to be skeptical of earlier predictions. The details of the

CBR are beyond the scope of this essay, but the findings revealed

that clusters of energy mapped by Planck indicate that there was

a great deal more mass present than expected.

7 PBS, 1998.8 Penzias and Wilson, 1965.9 European Space Agency, 2013.

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All of these experiments have shown that the visible stuff

in the universe does not account for what is to be expected, but

they have shown little in the way of providing details of the

missing mass. Another form of energy not discussed, which may be

responsible for the accelerating expansion of the universe is

currently known as “dark energy.” The most recent estimate of

breakdown of matter in the universe is shown in Figure 2.

Figure 2: Mass Estimates of the Universe10

At present, it is estimated that all of the stuff that is

visible (interacts with the electromagnetic spectrum) comprises

only about 5% of the known universe. We will not address dark

energy. Our focus will remain on the 27% currently known as dark

matter.

10 Image courtesy of ESA/Planck

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At this point, it is clear that something is missing from

our calculations, but in order to give this mystery mass a shape,

we need to find it. The one place dark matter can consistently be

found is in and around galaxies. Just as Rubin did in the 1970s,

astronomers are observing the rotation of galaxies to estimate

the amount of matter needed to hold it in its current state. The

shape and total mass of the dark matter halo can be more

accurately estimated using a phenomenon predicted by Einstein’s

general relativity: gravitational lensing. By general relativity,

the gravity of massive objects bends the space around it. Objects

passing near a very massive object continue to travel straight

through its own relative space, but that space has been warped by

the larger mass, and its path will bend as if being pulled toward

the center of the large mass. This phenomenon was observed during

a solar eclipse in 1919. Stars that were known to be behind the

Sun were visible around the edges of the eclipse because the

light had curved around the sun to find the observers on Earth.

At great distances, when light from galaxies is bent around

massive objects like galaxy clusters, the galaxy appears smeared

in a ring (known as an Einstein ring) around the near object. It

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will make a full circle if the two objects lie on a line with the

Earth (strong lensing), and ring segments if they are at some

angle (weak lensing). Examples can be seen in Figure 3.

Figure 3. Gravitational Lensing Examples11

Based on the size and shape of the Einstein ring,

astrophysicists can infer the mass of the lensing object. This

phenomenon becomes extremely important when the lensing object is

not visible. By analyzing the thousands of lensing cases,

researchers at the California Institute of Technology have formed

a detailed image of a dark matter cluster. Figure 4 shows the

path of travel of a light source behind the dark matter.

11 Image courtesy of NASA/ESA

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Figure 4. Light Ray through a Dark Matter Cluster12

The dark matter filaments in Figure 4 are gradually forming

a larger group, pulled together by its own gravity. Intergalactic

collisions may shed light on what will happen as the dark matter

joins into this larger cluster.

In what has been named the Bullet Cluster, the collision of

two galaxy clusters offers the ability to view the differing

motions of the baryonic matter and the dark matter. The baryonic

gas clouds and dark matter of the colliding galaxy clusters are

shown in Figure 5.

12 Image courtesy of NASA/ESA, R. Massey (California Institute of Technology)

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Figure 5. Bullet Cluster Galaxy Collision13

Figure 5 shows a photograph of the galaxy cluster with an

overlay of x-ray radiation from the baryonic gas (in pink) and

locations of the dark matter based on gravitational lensing

measurements (in blue). As the gas of the two galaxies pass

through each other, the particles collide. The resulting energy

release is detected in the form of x-rays on Earth. The shape of

the gas indicates that the clouds have been slowed by friction,

even forming a visible bow shock in the right-hand gas cloud. The

dark matter, however, continues on with no sign of interaction

with the baryonic matter or its counterparts.14 The aftermath of

such a collision has been found near the cluster Cl0024+1652.

This is the first instance we have found of dark matter being 13 Image courtesy of NASA/CXC, CfA, STScI, ESWFI, Magellan, U. Arizona, M. Markevitch et al., D. Clowe et al.14 Thaller, Michelle. TEDx Talks. Binghamton University. Feb. 2013.

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entirely separated from a baryonic mass. Theories show that the

gravitational forces could have stretched the dark matter of the

clusters, forming a ring as viewed from Earth.15 The dark matter

has been superimposed on the cluster in Figure 6.

Figure 6: Dark Matter Ring from Galaxy Cluster Collision16

Discoveries like this rule out dark matter theories that

propose cold baryonic matter. The bullet cluster and others like

it indicate dark matter does not interact with known particles in

any way other than gravity.

The distribution of dark matter throughout galaxies and

galaxy clusters may indicate how baryonic and dark matter formed

the universe as we see it today. Not only did Planck’s findings

reveal that there was a great deal more matter in the universe

than expected, but it appeared to dampen the vibrations during

15 European Space Agency, Hubble Sees Dark Matter Ring in Galaxy Cluster, 2007.16 Images courtesy of NASA/ESA

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the time when the universe was dense enough to transmit sound.17

With the boundaries of the variations in temperature of the early

universe, researchers at the Millennium Simulation Project have

been able to reconstruct the location of dark matter. The result

is a sinuous web stretching throughout the cosmos. The two

dimensional representation can be seen in Figure 7.

Figure 7. The Dark Matter Web18

The Millenium Run, as the animation is called, uses more than

10 billion particles contained in an eight cubic light year

volume of space in the early universe. The simulation shows the

history of approximately 20 million galaxies and took Planck’s

supercomputing center more than a month to run.19 The presence of

this structure shows how the baryonic matter that formed stars

and galaxies might have been able to come together.17 Ibid.18 Image courtesy The Millennium Simulation Project19 The Millennium Simulation Project, 2005

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Another Planck project, The Aquarius Project, has developed

a similar simulation focused on a single structure. The animation

shows the development of the Aq-A-1 Aquarius halo of similar mass

as the Milky Way galaxy from a redshift over 13 billion years.20

Figure 8 shows four frames of this simulation.

Figure 8. Dark Matter Scaffolding Simulation21

Beginning from a homogeneous distribution of particles, the

simulation shows the interaction of over 200 million baryonic

particles with the dark matter scaffolding. The center of the

cluster can be seen gathering at the intersection of the dark

matter strands. Over billions of years, it is possible that this

scaffolding of dark matter pulled together the matter that makes

20 The Aquarius Project, 200821 Image courtesy of The Aquarius Project

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up the entire visible universe, allowing it to clump together,

forming gas clouds that would become stars and galaxies.

The ability to create an image of the matter that does not

interact with light has given us a new tool to help understanding

what this mysterious matter is and what it is not. The evidence

that visible objects make up only a small piece of the mass of

the universe is overwhelming. The next step is to identify it. By

showing its location and shape, we can more intuitively examine

its effects and patterns. This field of study will require

approaches from the quantum scale to the universal scale.

Visualization provides an understandable and approachable manner

to learning about the hidden facets of the universe. The

incredible images flooding the internet will hopefully catch the

eye of young scientists searching for answers. The future of dark

matter research will need their help in answering the fundamental

questions of what makes up our Universe. New findings show the

underlying structure of dark matter throughout the universe. This

scaffolding has made possible, through its ability to attract,

but not interact with, the stuff of stars and galaxies. Without

it, the matter of the early universe may never have come together

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in such a way to form the planets and elements that make possible

the life that is now trying to understand it. The impending

discovery of the true nature of dark matter may be next step in

the discovery of our true origins.

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