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Dark Matter, Dark Energy: The Dark Side of the Universe

Dark Matter, Dark Energy: The Dark Side of the Universe

Professor Sean Carroll Ph.D.
California Institute of Technology
Course No.  1272
Course No.  1272
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Course Overview

About This Course

24 lectures  |  31 minutes per lecture

There's more to the universe than meets the eye—a lot more. In recent years, scientists have discovered that 95% of the contents of the cosmos are invisible to our current methods of direct detection. Yet something is holding galaxies and galaxy clusters together, and something else is causing space to fly apart.

Scientists call these invisible components dark matter and dark energy; "dark" because these phenomena do not emit light, not because we are not learning more and more about them. In fact, dark matter and dark energy are the most eagerly studied subjects in astronomy and particle physics today.

If and when we discover this matter, it will further validate the "standard model" of physics which, so far, is the best description of how our universe works; if we cannot find this matter, or if it does not exist, then we will completely need to rethink the current "standard model" theory.

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There's more to the universe than meets the eye—a lot more. In recent years, scientists have discovered that 95% of the contents of the cosmos are invisible to our current methods of direct detection. Yet something is holding galaxies and galaxy clusters together, and something else is causing space to fly apart.

Scientists call these invisible components dark matter and dark energy; "dark" because these phenomena do not emit light, not because we are not learning more and more about them. In fact, dark matter and dark energy are the most eagerly studied subjects in astronomy and particle physics today.

If and when we discover this matter, it will further validate the "standard model" of physics which, so far, is the best description of how our universe works; if we cannot find this matter, or if it does not exist, then we will completely need to rethink the current "standard model" theory.

Join the exciting search for these mysterious phenomena in Dark Matter, Dark Energy: The Dark Side of the Universe, a mind-expanding, 24-lecture course taught by Dr. Sean Carroll, a theoretical physicist with a profound knowledge of the field. Starting with the early 20th-century work of Albert Einstein in theoretical physics and Edwin Hubble in observational astronomy, Dr. Carroll takes you through the key concepts of this revolutionary view of an expanding universe, concepts which have brought us—for the first time in history—to the brink of knowing what the universe is made of.

Welcome to the Dark Side

Everything you see with your eyes and with powerful instruments—stars, planets, galaxies, dust, and gas—and everything that you think of as atom-based matter is only 5% of what we now know exists. The rest is what Dr. Carroll calls the "dark sector," which consists of the following:

  • Dark matter: First proposed in the 1930s, the idea that there is missing mass influencing the behavior of galaxies began to look more and more likely from the 1970s on. We know that it is matter because we can detect its gravitational influence on visible matter, but we cannot see it. An inventory of the distribution of dark matter throughout space shows that it constitutes 25% of the energy density of the universe.
  • Dark energy: The greatest discoveries are the unexpected ones, which was the case in the late 1990s when two teams of astronomers competing to measure the rate at which the expansion of the universe is slowing down (as virtually everyone thought it must be) discovered that it is speeding up instead. A previously unknown, all-pervasive dark energy must be at work, representing 70% of the energy density of the universe.

Together, dark matter and dark energy account for all but a tiny fraction of everything there is; the ordinary matter that is left over is like the seasoning on the main dish. The story of how we arrived at this startling cosmic recipe is an absorbing drama that takes you through the breakthrough discoveries in astronomy and physics since the turn of the 20th century.

Concept by concept, Dark Matter, Dark Energy gives you the tools to appreciate this subject in depth. Dr. Carroll explains why scientists believe we live in a smooth, expanding universe that originated in a hot, dense state called the big bang.

You investigate the features of the infant universe that led to the large-scale structure we observe today, explore the standard model of particle physics and see how it provides the framework for understanding the interaction of all matter and radiation, and come to understand why dark matter and dark energy are logical consequences of a range of scientific theories and observations and how together they complete a grand picture of the universe.

Deduce the Existence of the Dark Sector

Several significant clues disclose the existence of dark matter and dark energy. In the case of dark matter, we have the evidence of:

  • Galaxy dynamics: The motions of the stars in galaxies and galaxies within clusters indicate that there is far more matter than is implied by visible stars and gas.
  • Echoes of the big bang: Variations in the leftover radiation from the big bang demonstrate that there must be dark matter pulling the ordinary matter we see.

Dark matter is clear to see compared to dark energy, which reveals itself subtly but unmistakably through:

  • Exploding stars: Type Ia supernovae provide a standard candle to measure the distances to faraway galaxies. By combining this information with redshift (which measures how fast a galaxy recedes), astronomers conclude that something is causing galaxies to recede at a faster and faster velocity.
  • Geometry of space: Observations that space is "flat" (with neither positive nor negative curvature) imply a total energy density for the universe that is stunningly consistent with the dark energy hypothesis.

Each of these techniques deduces the existence of dark matter or dark energy from the gravitational fields they cause. But what if our theory of gravity is faulty? Could adjustments to Einstein's general theory of relativity, which forms our modern understanding of gravity, do away with the need for the dark sector?

You explore a theory called Modified Newtonian Dynamics, which successfully dispenses with dark matter in individual galaxies. This theory fails, however, when applied to clusters and has nothing to say about the expansion of the universe.

"It is impossible, in principle, to think of a theory in this day and age that will completely do away with dark matter," says Dr. Carroll, pointing in particular to a convincing piece of evidence from the aftermath of the collision of two galaxies.

Known as the Bullet Cluster, it shows a central region of ordinary matter (evident through telltale x-ray emissions), on either side of which are far more extensive clouds of what can only be dark matter, disclosed by gravitational lensing.

Explaining away dark energy is similarly difficult, because it requires revising the fundamental equation of general relativity. "The problem is that this equation of Einstein's is actually quite remarkable," says Dr. Carroll. "If you try to mess with it just a little bit, you break it."

The overriding question remains: What are dark matter and dark energy? We do not yet know for certain, but physicists have come up with an array of creative ideas and ways to test them. Dark Matter, Dark Energy covers the most promising proposals and looks ahead to experiments that will dramatically improve our understanding of the dark sector.

Take a Voyage of Scientific Discovery

Dr. Carroll has a knack for explaining the latest complex picture of the universe in easy-to-follow terms—a skill honed by his more than 250 scientific seminars, colloquia, educational discussions, and popular talks. Relaxed, eloquent, wryly funny, and brimming with ideas, he has received the Graduate Student Council Teaching Award from MIT for his course on general relativity, as well as research grants from NASA, the U.S. Department of Energy, and the National Science Foundation.

With his expert guidance, your previously held ideas about the fate (and possibly the origin) of the universe will be altered permanently. A rich voyage of scientific discovery, Dark Matter, Dark Energy provides you with a comprehensive look at these two mysterious phenomena—and their startling implications for our understanding of the universe.

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24 Lectures
  • 1
    Fundamental Building Blocks
    Scientists now have a complete inventory of the universe, which is composed of three basic constituents: Ordinary matter includes every kind of particle ever directly observed; dark matter consists of massive particles known only because of their gravitational effects; and dark energy is a smoothly distributed component that whose density does not change as the universe expands. x
  • 2
    The Smooth, Expanding Universe
    Imagine looking into a clear night sky with perfect vision. What would you see? This lecture surveys the visible universe—from the stars in our galaxy to the cloudy patches called nebulae that astronomer Edwin Hubble proved are galaxies in their own right—and Hubble's discovery that the universe is expanding. x
  • 3
    Space, Time, and Gravity
    Einstein taught us that space and time can be combined into spacetime, which has the ability to evolve and grow. Indeed, what we think of as gravity is just a manifestation of the curvature of spacetime. To find things in the universe—including dark matter and dark energy—all we have to do is to map out this curvature. x
  • 4
    Cosmology in Einstein's Universe
    The expansion of the universe is governed by its spatial curvature and energy density, both of which have specific ways of changing as the universe grows. These features are related to each other by Einstein's general theory of relativity, which can be used to model the past and possible future of the universe. x
  • 5
    Galaxies and Clusters
    Applying the laws of dynamics to galaxies and galaxy clusters, we find that more matter is required to account for their motions than can be observed. Some of the missing mass is hot gas; however, this is still not enough, and we need to invoke some new kind of particle in galaxies and clusters: dark matter. x
  • 6
    Gravitational Lensing
    Another way to detect invisible matter is to use light as a probe of the gravitational field. Passing through curved spacetime, the path of a light ray is deflected due to gravitational lensing. Lensing demonstrates the existence of gravitational fields where there is essentially no ordinary matter. x
  • 7
    Atoms and Particles
    We peer into the atom to discover the constituents of ordinary matter: nuclei and electrons. Nuclei are made of protons and neutrons, which in turn are made of quarks. Electrons and quarks are examples of fermions, or matter particles. There are also bosons, or force-carrying particles, such as photons and gluons. x
  • 8
    The Standard Model of Particle Physics
    In the 1960s and 1970s, physicists developed a comprehensive theory of known fermions and bosons. Now called the standard model, this theory fits an impressive amount of data, but it leaves two crucial puzzles: the hypothetical Higgs boson and the graviton, the carrier of the gravitational force. x
  • 9
    Relic Particles from the Big Bang
    Armed with the core principles of particle physics, we know enough about the early universe to predict how many of each type of particle should be left over from the Big Bang. These "relic abundances" are crucial to understanding the origin of dark matter and light elements. x
  • 10
    Primordial Nucleosynthesis
    The process of nucleosynthesis describes how protons and neutrons were assembled into light elements during the first few minutes after the Big Bang. We can observe these primordial elements today and check on Einsteinian cosmology and a stringent constraint on theories of dark matter. x
  • 11
    The Cosmic Microwave Background
    About 380,000 years after the Big Bang, the universe had cooled sufficiently for electrons and nuclei to combine into atoms allowing light to travel much more freely. The relic photons from this era are visible to us today as the cosmic microwave background, which holds clues to the composition and structure of the universe. x
  • 12
    Dark Stars and Black Holes
    Candidates for dark matter include small, dark stars called Massive Compact Halo Objects (MACHOs) and black holes. Such objects are ultimately composed of ordinary matter, of which there just isn't enough to account for the dark matter. We are forced to conclude that the dark matter is a new kind of particle. x
  • 13
    WIMPs and Supersymmetry
    Weakly interacting massive particles (WIMPs) are ideal candidates for what comprises dark matter. WIMPs may have their origins in supersymmetry, which posits a hidden symmetry between bosons and fermions, and predicts a host of new, as-yet-unobserved particles, including WIMPs. x
  • 14
    The Accelerating Universe
    In the late 1990s, two groups of astronomers found to their astonishment that the expansion of the universe is speeding up rather than slowing down. Such behavior can't be explained by any kind of matter and suggests the existence of an entirely new component: dark energy. x
  • 15
    The Geometry of Space
    Precise measurements of the cosmic microwave background let us measure the total energy density of the universe by observing the geometry of space. We find that the energy in matter alone is not enough, confirming the need for dark energy. x
  • 16
    Smooth Tension and Acceleration
    Dark energy is smoothly distributed throughout the universe and its density is nearly constant, even though the universe is expanding. Unlike gas under pressure in a container, dark energy is a kind of "negative pressure"—or tension—that imparts an accelerated expansion to the universe. x
  • 17
    Vacuum Energy
    The density and distribution of dark energy remain the same across all of space­time, but what exactly is dark energy? There are many possibilities, the simplest of which is vacuum energy—an constant amount of energy in every cubic centimeter of space itself. Vacuum energy is equivalent to Einstein's idea of the cosmological constant. x
  • 18
    Quintessence
    Another idea about dark energy is that it results from a new field in nature, analogous to the electromagnetic field but remaining persistent as the universe expands. This field is called quintessence. It would be observationally distinguishable from the cosmological constant. x
  • 19
    Was Einstein Right?
    We have inferred the existence of dark matter and dark energy from the gravitational fields they cause. In this lecture, we explore proposals that a modified theory of gravity might allow us to dispense with the need for invoking dark stuff. However, this turns out to be very difficult in practice. x
  • 20
    Inflation
    Before we had observational evidence that the universe is accelerating, cosmologists considered the possibility of a period of rapid acceleration at very early times—a scenario known as inflation. x
  • 21
    Strings and Extra Dimensions
    We know about the dark sector because of gravity, and string theory is an ambitious attempt to unify gravitation with the other forces of nature into a theory of everything. String theory promises a theory of quantum gravity, but it also predicts extra, unseen spatial dimensions that are difficult to test. x
  • 22
    Beyond the Observable Universe
    The speed of light and the age of the observable universe are finite. That means we can't see the whole universe because our vision can only stretch so far. The "multi­verse"—a hypothesis of regions where conditions are very different from those we see in our observable universe—may help explain properties of dark energy. x
  • 23
    Future Experiments
    Astronomers are designing new observatories to probe the acceleration of the universe and other cosmic phenomena. Physicists are also looking forward to new experiments that will dramatically improve our understanding of particles and forces, and how ordinary matter fits in with dark matter and dark energy. x
  • 24
    The Past and Future of the Dark Side
    The concordance cosmology is an excellent fit to a variety of data, but it presents us with deep puzzles: What are dark matter and dark energy? Why do they have the densities they do? Our own universe seems unnatural to us. That's good news, as it is a clue to the next level of understanding. x

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Sean Carroll
Ph.D. Sean Carroll
California Institute of Technology

Professor Sean Carroll is a Senior Research Associate in Physics at the California Institute of Technology. He earned his undergraduate degree from Villanova University and his Ph.D. in Astrophysics from Harvard in 1993. Before arriving at Caltech, Professor Carroll taught in the Physics Department and the Enrico Fermi Institute at the University of Chicago, and did postdoctoral research at the Massachusetts Institute of Technology and at the Institute for Theoretical Physics at the University of California, Santa Barbara. Professor Carroll is the author of Spacetime and Geometry: An Introduction to General Relativity, published in 2003. He has taught more than 200 scientific seminars and colloquia and given more than 50 educational and popular talks. In addition, he has written for numerous publications including Nature, New Scientist, The American Scientist, and Physics Today. Professor Carroll has received research grants from NASA, the U.S. Department of Energy, and the National Science Foundation, as well as fellowships from the Sloan and Packard foundations. He has been the Malmstrom Lecturer at Hamline University, the Resnick Lecturer at Rensselaer Polytechnic Institute, and a National Science Foundation Distinguished Lecturer. While at MIT, Carroll won the Graduate Student Council Teaching Award for his course on general relativity. In 2006 he received the Arts and Sciences Alumni Medallion from Villanova University.

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Reviews

Rated 4.7 out of 5 by 83 reviewers.
Rated 4 out of 5 by Good summary of current astrophysical thought A good overview of current astrophysical thought and thought about sub-nuclear physics. Necessarily handwaving, but a good summary. You will know the difference between fermions and bosons at the end, and lots of other interesting things. One thing I didn't like was the obvious arrogance and anti-religious mindset of the instructor, particularly in the first two lectures where he said that any discussion of the universe being less than a billion years old was not worth considering. He is snubbing creationists, and he didn't need to do that -- he could have pointed out that there are creationist astrophysicists, but he was not one and would not be covering those topics. In the second lecture he talked about the expanding universe, but did not cover the debate about the "cosmological principle", which explains what we see from a creationist perspective and doesn't require the concept of an expanding universe to match the observable data. Given all the things he covered that "we do not understand" and are "inconsistent with theory", rejecting creationist ideas by not even acknowledging that they exist seemed inappropriate in several places. For those interested in the other points of view, I would recommend "Starlight and Time" by Russell Humphreys and Ken Ham, and "The Anthropic Cosmological Principle" by John D. Barrow and Frank J. Tippler. October 25, 2014
Rated 5 out of 5 by A Superb Lecturer If it were a book, I'd say "I couldn't put it down!". I'm looking forward to more books and courses from him. June 19, 2014
Rated 5 out of 5 by Wishing for More This course was an amazing and accessible presentation of these exciting phenomena for the lay mind. As a lifelong liberal arts scholar and only occasional science fan, I long to understand concepts in cosmology as well as in physics, but am stymied by the technicalities in most presentations. Dr. Carroll presents in a way that is understandable and comprehensive without appearing to have dumbed down the material. I WISH he would be invited to present a mini-course to his students on the ramifications of the recent findings re gravity waves in the early universe as well as of the discovery of the Higgs Boson. --I also wish there were a Q&A online component to the courses. -Thanks, Sean Carroll. April 24, 2014
Rated 5 out of 5 by Great Course - Great Lecturer Professor Sean Carroll is an excellent lecturer. He has a love for the subject and it shows. He proceeds in a very organized and systematic manner, and he is very clear. He does have a sense of humor, but he always remains on subject. I highly recommend his courses. April 12, 2014
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