Distance

100 200 300 400 500

1500

1000

Fig. 4.4 Distance-velocity relationships for different objects of the Universe. Circles, data for distances < 25 Mpc; diamonds, distances > 25 Mpc. (1 Mpc = 3 x 1019 km.) The good correlation between the two parameters (grey line) obtained for SNe Ia gives the Hubble parameter H0 = 72 ± 2 km s-1 Mpc-1. Less distant objects give H0 = 75 ± 8 km s-1 Mpc-1, identical to the former value within error limits. From these and other observations and models the best-fit value, 71 ± 4 km s-1 Mpc-1, and the age of the Universe, 13.7 ± 0.2 Gyr, are inferred. After Freedman et al. (2001) and Freedman and Turner (2003).

1500

1000

Distance, Mpc

Fig. 4.4 Distance-velocity relationships for different objects of the Universe. Circles, data for distances < 25 Mpc; diamonds, distances > 25 Mpc. (1 Mpc = 3 x 1019 km.) The good correlation between the two parameters (grey line) obtained for SNe Ia gives the Hubble parameter H0 = 72 ± 2 km s-1 Mpc-1. Less distant objects give H0 = 75 ± 8 km s-1 Mpc-1, identical to the former value within error limits. From these and other observations and models the best-fit value, 71 ± 4 km s-1 Mpc-1, and the age of the Universe, 13.7 ± 0.2 Gyr, are inferred. After Freedman et al. (2001) and Freedman and Turner (2003).

(1929) discovered a linear relationship between the distance from distant objects and redshift in their light spectra, which supported this model. Subsequent precise measurements of redshift and distance, together with evaluations of the cosmic microwave background, have allowed the age of the Universe to be determined with great precision and confidence at 13.7 ± 0.2 Gyr.

The Big Bang provides a setting for the formation of baryonic matter. Models predict that the first baryonic particles, protons and neutrons, were formed at a late stage of the explosion, when the temperature dropped below 1011 K. During a time interval of ~ 30 min the temperature decreased from 1011 to 109 K and the density from 10 to 10-5 g cm-3, and H, D, 3He and 4He were formed. The BBN models predict relative abundances for these nuclides close to those directly observed in primitive prestellar matter. Hydrogen and 4He were dominant, and the 4He mass fraction (~ 0.28) is determined by the neutron/proton ratio, which became fixed at ~ one-sixth when weak interactions were terminated, at ~ 5 x 109 K.

The fragile nuclides D and 3He are less abundant at present. Modelling of the decline of D during late stages of the nucleosynthesis process allows the baryonic density of the Universe to be estimated in the range (1-5) x 10-31 g cm-3. This is 20 times less than is required for the Universe to be "closed" and is much lower than the total matter density of the Universe derived from the "emerging standard cosmological model" (Spergel et al., 2003).

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