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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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
