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SUMMARY OF KEY IDEAS

The Big Bang

Astronomers believe that the universe began as an exceedingly dense cosmic singularity that expanded explosively in an event called the Big Bang. The Hubble law describes the ongoing expansion of the universe and the rate at which superclusters of galaxies move apart.
The observable universe extends about 13.8 billion light-years in every direction from Earth to what is called the cosmic light horizon. We cannot see any objects that may exist beyond the cosmic light horizon because light from these objects has not had enough time to reach us.
According to the theory of inflation, early in its existence, the universe expanded very rapidly for a short period, spreading matter that was originally far from our location (and hence at different temperatures and densities) throughout a volume of the universe so large that we cannot yet observe it. The observable universe today is thus a growing volume of space containing matter and radiation that were in close contact with our matter and radiation during the first instant after the Big Bang (and hence at the same temperature, pressure, and density). Inflation explains the isotropic and homogeneous appearance of the universe.

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DISCOVERY 14-1: Superstring Theory and M-Theory

To combine gravitation and the other three fundamental forces in nature into one comprehensive Theory of Everything, scientists have had to consider a universe that contains more than the four dimensions we know about today (three of space and one of time). The theories that mathematically describe this new formulation of the universe are called superstring theories. There are basically five such theories, which allow all of the particles we have been studying—such as quarks, protons, neutrons, electrons, and photons—to exist.

Spacetime in superstring theories has 10 dimensions, of which 6 are everywhere rolled up into such tiny volumes that we cannot detect them directly. The other 4 dimensions are our normal spacetime. Superstring’s more general spacetime carries with it properties that allow scientists to combine all four forces in nature into one set of equations.

The difficulty in reconciling quantum mechanics (describing the weak, strong, and electromagnetic forces) and general relativity (describing gravitation) is that the three forces in quantum mechanics are quantized, whereas general relativity is not. In other words, the weak, strong, and electromagnetic forces are transmitted by particles. For example, the quanta of electromagnetism are photons. Gravity, as described by general relativity, is based on a smooth and continuous, rather than quantized, force. Specifically, the distortion of spacetime by matter and energy creates the gravitational force.

Superstring theories begin with a different assumption about all particles and their interactions than do either quantum mechanics or general relativity. The new theories assert that each particle is actually a tiny vibrating string, with different types of particles vibrating at different rates, like different strings on a guitar. Gravitation has its own energy-sharing particle, the graviton, analogous to photons for electromagnetism. The interactions between the strings create all of the properties of matter and energy.

The predictions made by superstring theories begin with the assumption that general relativity is the correct “classical” theory for describing the gravitational interaction between matter and energy. This may seem trivial, but, because general relativity today correctly predicts everything in its realm of validity, a more comprehensive (superstring) theory needs to keep that accuracy, or the larger theory is wrong. Some additional predictions of superstring theories include the following:

The universe cannot have positive curvature (which it does not, as we have seen).
Some of the clumpy structure that we see as superclusters of galaxies could have been caused by the effects of superstring activity during inflation (with the rest due to the expansion of quantum fluctuations).
Spacetime may not be entirely smooth. It may have structural defects, like a flawed diamond or ice that has broken into abutting chunks. These defects in spacetime would appear as one-dimensional “cosmic strings” with great density. Their attraction would pull normal matter around them, creating strings of galaxies. These cosmic strings have not yet been found.
Some of the dark matter may be particles predicted by string theory.
Most versions of superstring theory include a cosmological constant, but there is no underlying reason yet known for the value of the cosmological constant that may exist today.
The speed of light is the same for all photons. If different wavelengths of light from the same event arrived at different times, then superstring (and relativity) theories would be wrong.

You may have found the idea of five superstring theories of the universe to be four too many. So do scientists who study string theory. American physicist Ed Witten has shown that in 11 dimensions, all five string theories are equivalent. The one 11-dimensional theory of strings is called M-theory.

Superstring and M-theories are, so far, consistent with observations, but it remains to be seen if they will continue to maintain consistency with future observations and, very importantly, if they will make predictions that can be tested. Without being able to do that, they will remain truly elegant mathematical formalisms, but not science.

A Brief History of Spacetime, Matter, Energy, and Everything

Four basic forces—gravity, electromagnetism, the strong nuclear force, and the weak nuclear force—explain the interactions observed in the universe.
According to current theory, all four forces were identical just after the Big Bang (although our understanding of the nature of matter and energy during this time is extremely limited). At the end of the Planck time (about 10^–43 seconds after the Big Bang), gravity became a separate force. A short time later, the strong nuclear force became a distinct force. A final separation created the electromagnetic force and the weak nuclear force.
Observations show that the universe is flat and that the cosmic microwave background is almost perfectly isotropic, resulting from a brief period of very rapid expansion (the inflationary epoch) in the very early universe.
In its first 30,000 years, the universe was radiation-dominated, during which time photons prevented matter from forming clumps. Then it was matter-dominated, during which time superclusters and smaller clumps of matter formed. Today it is dark energy–dominated. Dark energy of some sort supplies a repulsive gravitational force that causes superclusters to accelerate away from each other.
During the first 380,000 years of the universe, matter and energy formed an opaque plasma, called the primordial fireball. Cosmic microwave background radiation is the greatly redshifted remnant of the universe as it existed about 380,000 years after the Big Bang.
About 380,000 years after the Big Bang, spacetime expansion caused the temperature of the universe to fall below 3000 K, allowing protons and electrons to combine and thereby form neutral hydrogen atoms. This period is called the era of recombination. The universe became transparent during the era of recombination, with the photons that existed back then still traveling through space today. In other words, the microwave background radiation is composed of the oldest photons in the universe.
Clusters of galaxies and individual galaxies formed from pieces of enormous hydrogen and helium clouds, each of which became a separate supercluster of galaxies.
All of the superclusters and some of the clusters of galaxies within each supercluster are moving away from one another.
Supermassive black holes appear to have “seeded” the formation of most galaxies.
During the matter-dominated era, structure formed in the universe. As the universe goes farther into the dark energy–dominated era, the large-scale structure of superclusters of galaxies will fade away.

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The Fate of the Universe

The average density of matter and dark energy in the universe determines the curvature of space and the ultimate fate of the universe.
The universe is accelerating outward and it will expand forever.

WHAT DID YOU THINK?

What is the universe? It is all of the matter, energy, and spacetime that will ever be detectable from Earth or that will ever affect us.
Did the universe have a beginning? Yes. It occurred about 13.8 billion years ago, in an event called the Big Bang.
Into what is the universe expanding? Nothing. The Big Bang created space and time (spacetime), as well as all matter and energy in the universe. Spacetime is expanding to accommodate the expansion of the universe.
How strong is gravity compared to the other forces in nature? Gravity is by far the weakest force.
Will the universe last forever? Current observations support the belief that the universe will last forever.