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There is now a substantial observational basis for estimates of the cosmic mean densities of all the known and more significant forms of matter and energy in the present-day universe. The compilation of the energy inventory offers an overview of the integrated effects of the energy transfers involved in all the physical processes of cosmic evolution operating on scales ranging from the Hubble length to black holes and atomic nuclei. The compilation also offers a way to assess how well we understand the physics of cosmic evolution, by the degree of consistency among related entries. Very significant observational advances, particularly from large-scale surveys including the Two Degree Field Galaxy Redshift Survey (2dF: Colless et al. 2001), the Sloan Digital Sky Survey (SDSS: York et al. 2000; Abazajian et al. 2003), the Two Micron All-Sky Survey (2MASS: Huchra et al. 2003), the HI Parkes All Sky Survey (HIPASS: Zwaan et al. 2003), and the Wilkinson Microwave Anisotropy Probe (WMAP: Bennett et al. 2003a), make it timely to compile what is known about the entire energy inventory.

We present here our choices for the categories and estimates of the entries in the inventory. Many of the arguments in this exercise are updated versions of what is in the literature. Some arguments are new, as is the adoption of a single universal unit - the density parameter - that makes comparisons across a broad variety of forms of energy immediate, but the central new development is that the considerable range of consistency checks demonstrates that many of the entries in the inventory are meaningful and believable.

People have been making inventories for a long time: the medieval Domesday Book (1086-7) is an impressive example. Einstein's (1917) physical picture for a homogeneous universe led to a new kind of inventory: under the assumption that one can reach a fair sample of the universe, one can estimate the cosmic mean mass density and space curvature, and test the predicted relation between the two. De Sitter (1917) commenced this tradition. Hubble's (1929) redshift-distance relation led to a revision of the relation between the mass density and space curvature, and his use of redshifts to convert galaxy counts into number densities greatly improved the estimate of the mean mass density (Hubble 1934). 1 One may also consider the relations among the mean luminosity density of the galaxies, the production of the heavy elements, and the surface brightness of the night sky (Partridge & Peebles 1967; Peebles & Partridge 1967); the relation between galaxy colors, the initial mass function, and the star formation history (Searle, Sargent, & Bagnuolo 1973); the relation between the last two sets of considerations (Tinsley 1973); the relation between the light element abundances and the baryon mass density (Gott et al. 1974); and the relation between the luminosity density of the quasars and the mean mass density in quasars and their remnants (Soltan 1982). Basu and Lynden-Bell (1990) show how one can analyze what is learned from this rich set of considerations in terms of an entropy inventory. We have chosen instead to base this discussion on an energy inventory.

Our inventory includes the mass densities in the various states of baryons. This is an updated version of the baryon budget of Fukugita, Hogan & Peebles (1998; FHP). Most entries in this part of the inventory have not changed much in the past half decade, while substantial advances in the observational constraints have considerably reduced the uncertainties. It appears that most of the baryonic components are observationally well constrained, apart from the largest entry, for warm plasma, which still is driven by the need to balance the budget rather than more directly by the observations.

The largest entries, for dark matter and the cosmological constant, or dark energy, are well constrained within a cosmological theory that is reasonably well tested, but the physical natures of these entries remain quite hypothetical. We understand the physical natures of magnetic fields and cosmic rays, but the theories of the evolution of these components, and the estimates of their contributions to the present energy inventory, are quite uncertain. The situation for most of the other entries tends to be between these extremes: the physical natures of the entries are adequately characterized, for the most part, and our estimates of their energy densities, while generally not very precise, seem to be meaningfully constrained by the observations.

Several cautionary remarks are in order. First, some types of energy are not readily expressed as sums of simple components; we must adopt conventions. Second, there is no arrangement of categories that offers a uniquely natural place for each component; we must again adopt conventions. Perhaps further advances in the understanding of cosmic evolution will lead to a more logically ordered inventory. Third, it is arguably artificial to represent binding energies as very small negative density parameters. The advantage that it simplifies comparisons across the entire inventory. Fourth, it is a task for future work to make some of our estimates more accurate by using data and computations that exist but are difficult to assemble. We mention the main examples in Section 3.

The inventory, which is presented in Table 1, is arranged by categories and components within categories. The explanations of conventions and sources for each entry are presented in Section 2, in subsections with numbers keyed to the category numbers in the first column of the table. Our discussion of checks of the entries is not so simply ordered, because the checks depend on relations among considerations of entries scattered through the table. A guide to the considerable variety of checks detailed in Section 2 is presented in Section 3.

Table 1. The Cosmic Energy Inventory

      Components Totals  

1 dark sector   0.954 ± 0.003
1.1   energy   0.72 ± 0.03  
1.2   dark matter   0.23 ± 0.03  
1.3   primeval gravitational waves ltapprox 10-10  

2 primeval thermal remnants 0.0010 ± 0.0005
2.1   electromagnetic radiation 10-4.3  
2.2   neutrinos 10-2.9±0.1  
2.3   prestellar nuclear binding energy -10-4.1  

3 baryon rest mass   0.045 ± 0.003
3.1   warm intergalactic plasma 0.040 ± 0.003
3.1a           virialized regions of galaxies             0.024 ± 0.005    
3.1b           intergalactic             0.016 ± 0.005    
3.2   intracluster plasma 0.0018 ± 0.0007
3.3   main sequence stars                 spheroids and bulges 0.0015 ± 0.0004
3.4     disks and irregulars 0.00055 ± 0.00014
3.5   white dwarfs   0.00036 ± 0.00008
3.6   neutron stars   0.00005 ± 0.00002
3.7   black holes   0.00007 ± 0.00002
3.8   substellar objects   0.00014 ± 0.00007
3.9   HI + HeI   0.00062 ± 0.00010
3.10   molecular gas   0.00016 ± 0.00006
3.11   planets 10-6  
3.12   condensed matter 10-5.6  
3.13   sequestered in massive black holes 10-5.4(1 + epsilonn)  

4 primeval gravitational binding energy -10-6.1±0.1
4.1   virialized halos of galaxies -10-7.2  
4.2   clusters   -10-6.9  
4.3   large-scale structure -10-6.2  

5 binding energy from dissipative gravitational settling   -10-4.9
5.1   baryon-dominated parts of galaxies -10-8.8  
5.2   main sequence stars and substellar objects -10-8.1  
5.3   white dwarfs   -10-7.4  
5.4   neutron stars   -10-5.2  
5.5   stellar mass black holes -10-4.2 epsilons  
5.6   galactic nuclei early type -10-5.6 epsilonn  
5.7     late type -10-5.8 epsilonn  

6. poststellar nuclear binding energy -10-5.2
6.1   main sequence stars and substellar objects -10-5.8  
6.2   diffuse material in galaxies -10-6.5  
6.3   white dwarfs -10-5.6  
6.4   clusters   -10-6.5  
6.5   intergalactic   -10-6.2±0.5  

7 poststellar radiation 10-5.7±0.1
7.1   resolved radio-microwave 10-10.3  
7.2   far infrared   10-6.1  
7.3   optical   10-5.8±0.2  
7.4   X-gamma ray   10-7.9  
7.5   gravitational waves 10-12  

8 stellar neutrinos 10-5.5
8.1   nuclear burning 10-6.8  
8.2   white dwarf formation 10-7.7  
8.3   core collapse 10-5.5  

9 cosmic rays and magnetic fields 10-8.4+0.6-0.3

10 kinetic energy in the intergalactic medium 10-8.0

1 Hubble's (1936) estimate based on galaxy masses derived from the velocity dispersion in the Virgo Cluster, which takes account of what is now termed nonbaryonic dark matter, translates to density parameter Omegam ~ 0.1, impressively close to the modern value. Back.

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