What is the universe made of

The world is full of many things, and we surely have discovered only a small fraction of them. But we do have credible evidence about what things are made of and about the amounts of the types of mass involved. Table 2.1 lists contributions to the total mass of the universe by some of the more interesting and important types of matter and radiation.7 The numbers in the middle column, which usually are termed "density parameters," are fractions of the total. The last column lists the mean mass density in each component.8

People, planets, and stars are made of baryons - the neutrons and protons in atomic nuclei of the chemical elements - with enough electrons to keep the electric charge neutral. The mass in the inner parts of our Milky Way Galaxy is largely in baryons in stars. The same is true of the central parts of the other large galaxies. The outer regions of the galaxies contain baryons, mostly in the form of plasma, but there is more mass in dark matter, which is not baryonic. In the average over much larger scales the biggest contribution is shown as the entry for the first component in the table, dark energy. This is the new name for Einstein's cosmological constant, A.

The gravitational action of dark energy is illustrated in Figure 2.1. In general relativity theory the positive pressure of a fluid adds to the gravitational

7 Fukugita and Peebles (2004) discuss the observational basis for these mass estimates and their uncertainties and also give estimates of the masses in a considerable variety of other components.

8 The total mass density summed over all components is such that, in general relativity theory, space sections at constant world time are not curved. Spacetime is curved, but space sections at constant time have close to Euclidean geometry.

2.3 What is the universe made of? Table 2.1. Cosmic mass inventory

Mass Mass density,

2.3 What is the universe made of? Table 2.1. Cosmic mass inventory

Mass Mass density,

Category

fraction"

gcm

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