Understanding the Universe: An Introduction to Astronomy, 2nd Edition

Overview

Examine the key concepts of an expanding universe from Albert Einstein to Edwin Hubble, led by a theoretical physicist. Did you know that everything we can see—even with our most powerful instruments—is only five percent of what we know exists? The other 95 percent of the cosmos is invisible to us, and yet it’s there, holding galaxies and galaxy clusters together… or causing space itself to fly apart.

Dark matter and dark energy are two of the most exciting subjects in astronomy and particle physics. Immerse yourself in the study of these mysterious phenomena in 24 lectures taught by an award-winning theoretical physicist with a gift for explaining difficult concepts in an engaging and easy-to-grasp way. Gain an understanding of the key concepts of the 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.

You’ll begin with Einstein’s theoretical physics work in the early 20th century, move to Hubble’s work in observational astronomy, and finally to the cutting edge of science today. Investigate the development of our universe, explore the standard model of particle physics, and come to understand how dark matter and dark energy fit into the big picture. On your way, you’ll study the motion of the stars in galaxies and galaxies within clusters; see the echoes of the Big Bang; witness exploding stars; and explore the geometry of space. Whether you’re a scientist, a student, or just a space enthusiast, your view of the universe will be forever changed. 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.

Course Structure

01: 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 whose density does not change as the universe expands.

Duration: 33 min

02: 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.

Duration: 30 min

03: 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.

Duration: 30 min

04: 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.

Duration: 30 min

05: 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.

Duration: 31 min

06: 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.

Duration: 31 min

07: 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.

Duration: 31 min

08: 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.

Duration: 31 min

09: 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.

Duration: 31 min

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.

Duration: 31 min

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.

Duration: 31 min

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.

Duration: 31 min

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.

Duration: 32 min

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.

Duration: 30 min

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.

Duration: 30 min

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.

Duration: 31 min

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—a constant amount of energy in every cubic centimeter of space itself. Vacuum energy is equivalent to Einstein’s idea of the cosmological constant.

Duration: 33 min

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.

Duration: 30 min

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.

Duration: 32 min

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.

Duration: 31 min

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.

Duration: 32 min

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.

Duration: 32 min

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.

Duration: 32 min

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.

Duration: 34 min