Andrew Lawson
WRIT 340
26 February 2013
Dark Matter and Dark Energy: Not Just a Star Wars Trope
Besides making you look smart at dinner parties, knowledge of dark energy and dark
matter holds the key to the fate of the universe. After getting over the initial shock of that
statement, the first question that comes to mind, obviously, is how? Well, it turns out that,
depending on the nature of dark energy, the universe will either collapse back in on itself or rip
itself apart. Moving past the initial shock, you think to yourself: What makes these entities
‘dark’? These substances are referred to as ‘dark’ simply because they cannot be observed by
conventional methods and have avoided detection by even the complex sensors scientists can
create. Don’t let the trivial nomenclature mislead you though, for the theories behind dark matter
and dark energy are extremely complex. And although both dark energy and dark matter have the
word ‘dark’ in the description, there are entirely different concepts at play. The reason they are
generally lumped together is because they are two great, unknown forces modern science is still
trying to comprehend. By analyzing the history behind each and understanding the basic
implications, one can gain a greater appreciation for the eventual fate of the universe.
A Briefer History of Time via Dark Energy
Dark matter and dark energy were first conceived as a solution to an observational
discrepancy. Up until the 1990s, with what little information was available, scientists had
theorized that the universe was constantly expanding at a decreasing rate due to the effects of
gravity. The laws of gravity dictate that every object exerts an attractive force on everything else,
so it would be logical to assume that eventually all these attractive forces would slow down the
universe’s expansion, since the gravitational forces would act to ‘pull’ everything back in to
some central point, overcoming the initial universal inflation imparted by the Big Bang. Yet in
1998, by comparing the changing length scales over which the universe expanded, the Hubble
Space Telescope found evidence that the rate of universal expansion was actually increasing
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[1].1 The only conclusion that could be reached was that some invisible force, much stronger
than the existing gravitational forces, was acting to push the universe apart faster and faster. This
phenomenon was attributed to dark energy, a concept made up shortly thereafter to explain the
findings and labeled as dark due to its invisibility on all wavelengths of the electromagnetic
spectrum.
Such a concept, invisible energy in space, was not entirely unheard of; in the 1920s
Einstein’s theory of general relativity predicted that the rate of the universe’s expansion was
determined by the average density of the empty space. That’s right –
space, conventionally thought to be empty, is not entirely empty. It’s
theorized that the empty space can possess energy. But not just any
amount of energy; there is a constant, uniform density of energy
contained within all space – appropriately named the ‘cosmological
constant’ (Fig 1) [2]. Granted, scientists have no idea the value of
E
4E
Fig 1: Due to the proportionality of
the constant, a space with 4 times as
much area has 4 times as much
energy E
the constant or even why it exists in the first place, but many
theorize its existence.
In Einstein’s theory, this energy is an inherent property of space – as new space is created
by the established universal expansion, new energy is also created in proportion with the
cosmological constant density to fill that space. And you can see how the cycle continues: since
there is more energy, the rate of expansion increases, which makes more space, which creates
more energy, which increases expansion rate and so on (Fig 2). And of course, there are many
theories2 as to the increasing rate of universal expansion, but this specific model has the largest
backing in the scientific community [3].
For such a poorly understood concept, dark energy plays a significant role in the
universe. In fact, roughly 70% of the universe is thought to be dark energy. That’s right, a clear
majority of the universe is solely energy. The traditional matter that you and I know? Everything
from massive planets and stars all the way down to the smartphones in our pockets – all of that
1
Suggested multimedia: animation on red shift, the pattern used to observe universal expansion
2
For additional information, see: Theoretical Models of Dark Energy. Jaewon Yoo , Yuki Watanabe
. Dec 2012. 61 pp. Int.J.Mod.Phys.
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‘normal’ matter only makes up 5% of the universe. So then what is that other 20% of the
universe made up of? Another cryptic entity – dark matter.
Vcurrent =
Vnew
vcurrent
→ ΔV
Ecurrent =
Enew
Vnew =
Vcurrent +
ΔV
ΔV→ ΔE
Enew =
Ecurrent +
ΔE
Fig 2: The cycle of universal expansion. Starting at Vcurrent, new space Δv is created, which forms Δe energy. This new
amount of energy and space is then tabulated as the new value, which is now one unit greater. Because the new values for
E and V now spawn a proportional amount of E and V, this new iteration’s worth of ΔV and ΔE is now larger than the
previous cycles’ ΔV and ΔE. Thus, the total volume of the universe increases exponentially.
Dark Matter
Dark matter (similarly named for its invisible nature) is a little better understood than
dark energy, primarily because we can rationalize it in a more traditional sense. We’re all
familiar with all of the particles in an atom – all those fun little protons, electrons, and neutrons
we learned about it high school chemistry. Physicists figure that dark matter is a new, low energy
particle that is just difficult to detect during regular particle accelerator experiments. This low
energy nature of the particle means that it does not produce very many ‘light’ (in the electromagnetic sense) emitting reactions. But these dark matter particles don’t reside in regular atoms;
they just float through space freely. And because they usually clump up with each other, similar
to how planets are lumps of normal mass, they were originally confused as other cosmological
entities, such as black holes.
Dark matter was initially thought to be the same thing as black holes – the celestial
bodies Steven Hawking made famous in the 70s. Black holes, zones in which the local gravity is
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so intense that nothing escapes its pull (not even light, hence the name), have been shown to
have a lensing effect. Due to the fact that they ‘bend’ the space-time fabric around them (Fig 3),
similar to how a bowling ball or any other heavy object placed on a bed creates a divot around it,
light passing near a black hole curves around it such that an object behind the black hole has
multiple, distorted copies visible around the front side of the black hole [4]. However, in some
situations where there was thought to be a black hole, there was no gravitational lensing, proving
that some other source of mass was affecting the gravitational field. Hence, dark matter was
conceptualized as the solution to this discrepancy.
False Image
Observer
Object
‘Line of Sight’
False Image
Fig 3: Gravitational Lensing occurs as the blocked image’s line of sight is bent, striking the observer, who linearly
extrapolates two false images around the original object.
And although it might sound spooky, odds are that you or I interact with dark matter on a
daily basis, since dark matter is present in our lives, even if we might not realize it. It is
estimated that throughout the entire universe, including our home planet, there are approximately
105 of these particles per cubic meter [5]. And while that might seem like a lot, this is one of the
factors that make dark matter so hard to spot. This is actually an extremely small concentration
of particles, considering that air at atmospheric conditions has nearly 2.68x1022 molecules per
cubic meter, a number nearly one million, billion times larger. Although the odds are against
them, scientists hope to build more precise instrumentation that will allow us observe these
particles passing through the earth.
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Completing the Theoretical Puzzle
It’s as if modern theoretical physicists are trying to complete a jigsaw puzzle after having
lost the lid. They have no idea how all of the pieces should fit together, and to make things even
harder, they have no idea if they even have all of the pieces. Sometimes, they are able to place a
couple of pieces next to each other and guess what kind of shape should go there to in order to
complete the chunk, but they just can’t find it (how dark energy, dark matter, or the famed Higgs
Boson are theorized in order to complete existing theories). Sometimes, they randomly spot a
piece on the floor and have to find where it belongs (how electrons or other high energy particles
were originally discovered). And it gets even harder when you consider that the ‘baryonic’
(sometimes called atomic) matter that you and I can interact with takes up less than 5% (Fig 4)!
In total, scientists have been able to calculate that about ¾ of the entire universe’s composition is
dark energy, with dark matter taking up 22%. So it’s as if there are also invisible pieces in the
puzzle, and the scientists have to guess where they are.
Now of course, we might also wonder how this affects our daily lives (besides giving you
a greater appreciation for scientists next time you try to complete a puzzle). For dark matter,
finally discovering the origin will have little effect on most people. I wish I could make it
something consequential, but for how interesting and evasive it is, sadly it has no real
involvement in daily life; it is more
just a remnant of the past – a key to
Composition of the Universe
exploring the origins and processes
involved according to the big bang
theory. By this theory, which is the
Dark Matter
74%
4%
most accepted theory as to the origin
of the universe, the universe started off
as an infinitely small sphere, smaller
Dark Energy
22%
Matter We Can
Interact With
than the tip of a needle or even the
width of a hair. There was an initial
Fig 4: Composition of the Universe. Note that the baryonic matter (matter
we can interact with) comprises less than 5% of the total composition.
explosion (still unknown in origin)
that set off a rapid expansion. During the expansion, there was a set amount of matter created
during the so-called ‘production phase’. Looking at the amount of mass in the universe, scientists
found that only 20% of the originally calculated amount is visible. And due to a fundamental
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principle of physics, that mass is neither created nor destroyed, it is impossible for this matter to
have ‘disappeared’. Thus, it was concluded that this other 80% of the original matter exists as
dark matter. As dark matter particles have yet to be detected, the theory still stands unconfirmed.
Dark matter’s confirmation via observation would solely offer knowledge for knowledge’s sake.
Dark energy, on the other hand, has
more interesting consequences; it holds in its
hands the fate of the universe. Literally.
Depending on whether the dark energy proves
to be constant, as predicted by Einstein’s
theory, or some other, constantly changing
value, the universe will assume one of two
distinct end states. If constant density dark
energy proves to be false, then the universe will
most likely abide by the traditional theory that
universal expansion is slowing. If this is the
Fig 5: The Big Crunch ceases the expansion and brings the
universe back into an infinitesimally small point. The Big
Rip predicts infinite expansion. ©HowStuffWorks
case, then the universal expansion will
eventually reach a tipping point as the initial
energy imparted by the Big Bang dissipates. At
that point, the gravitational effects of the existing mass will start to reverse the expansion, and
the universe will collapse back in on itself. Such a scenario is colloquially referred to as ‘The Big
Crunch’ (See Fig 5). In the other scenario, in which the density constant proves to be true, the
universe will keep expanding indefinitely, with the rate of expansion increasing exponentially.
However, at a certain point, the force of the dark energy will dominate all other forces, tearing
apart the gravitational ‘bonds’ shared by bodies in orbit and eventually even the nuclear and
electric bonds holding atoms together. This scenario is colloquially referred to as ‘The Big Rip’.
So it turns out that the normal matter that you and I interact with or see every day is just a
small minority of the universe and that the eventual fate of our universe as we know it is dictated
by two barely understood entities. While dark matters give us some perspective as to the past,
dark energy gives us foresight into the future. Eventually, the dark energy in the universe will
either force the universe apart or make it converge and collapse in on itself. By these
implications, whether or not we as humans ever determine precisely what dark energy does, the
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universe it still of some finite timespan. It would just be nice to finally figure out which way
we’re going to go. But don’t get too worried, for neither of these scenarios is bound to happen
any time soon. You’ll have plenty more dinner parties to attend.
Author Bio
Andrew is a junior at USC studying mechanical engineering. When he’s not busy fulfilling nerdy
stereotypes, he occasionally finds time to read and gawk at modern theoretical physics.
Abstract
Dark matter and dark energy, both still unconfirmed entities, are discussed in their basic forms.
Historical background is given with regards to both subjects, then current areas of study are
examined. Finally, the effects that both have on society, even though they are very negligible, are
stated. And while dark matter has no real effect, understanding dark energy proves to hold within
it the fate of the universe.
References
[1] NASA. "Dark Energy, Dark Matter." NASA Science. NASA, Apr.-May 2012. Web. 18 Feb.
2013.
[2] LSST. "Dark Energy." Large Synoptic Survey Telescope. N.p., n.d. Web. 18 Feb. 2013.
[3] NASA. "Dark Energy, Dark Matter." NASA Science. NASA, Apr.-May 2012. Web. 18 Feb.
2013.
[4] Ibid
[5] Biello, David. "What Are Dark Matter and Dark Energy, and How Are They Affecting the
Universe?" Scientific American. N.p., 28 Aug. 2006. Web. 18 Feb. 2013
[6] Hubble Discoveries. "Dark Energy: FATE OF THE UNIVERSE." Hubble Site.. Web. 18
Feb. 2013.
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