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cosmo

set the cosmology

Set the cosmology used (i.e., $H_0$, $q_0$, and $\Lambda_0$).

Syntax: cosmo <$H_0$><$q_0$><$\Lambda_0$>

where <$H_0$> is the Hubble constant in km/s/Mpc, <$q_0$> is the deceleration parameter, and <$\Lambda_0$> is the cosmological constant. If the cosmological constant is non-zero then at present XSPEC requires that the universe is flat. In this case the value of <$q_0$> will be ignored and XSPEC will assume that $\Omega_{matter} = 1-\Lambda_0$; the comoving distance is then obtained by exact numerical integration. <$\Lambda_0$> may be negative – $\Omega_{matter}$ is then greater than 1 – but must be less than 1, since $\Omega_{matter}$ has to be positive. With <$\Lambda_0$> = 0 the Mattig formula in <$q_0$> is used instead. Note that this is a change of branch rather than a limit: Mattig with <$q_0$> = 0 describes an empty universe, whereas the flat branch as $\Lambda_0$ tends to 0 describes an Einstein-de Sitter one, so the two differ at $\Lambda_0 = 0$. The default values are <$H_0$> = 70, <$q_0$> = 0.0, and <$\Lambda_0$> = 0.73

Examples:

XSPEC>cosmo 100
// Set <$H_0$>= 100 km/s/Mpc
XSPEC>cosmo ,0
// Set <$q_0$>= 0
XSPEC>cosmo „0.7
// Set a flat universe with <$\Lambda_0$>= 0.7

next up previous contents
Next: method Up: Setting Commands Previous: abund   Contents