Name

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    COSMO_PARAM

Purpose

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    Derive full set of cosmological density parameters from a partial set

Explanation

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    This procedure is called by LUMDIST and GALAGE to allow the user a choice
    in defining any two of four cosmological density parameters.
    Given any two of the four input parameters -- (1) the normalized matter
    density Omega_m (2) the normalized cosmological constant, Omega_lambda
    (3) the normalized curvature term, Omega_k and (4) the deceleration
    parameter q0 -- this program will derive the remaining two. Here
    "normalized" means divided by the closure density so that
    Omega_m + Omega_lambda + Omega_k = 1. For a more
    precise definition see Carroll, Press, & Turner (1992, ArAA, 30, 499).
    If less than two parameters are defined, this procedure sets default
    values of Omega_k=0 (flat space), Omega_lambda = 0.7, Omega_m = 0.3
    and q0 = -0.55

Calling Sequence

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      COSMO_PARAM, Omega_m, Omega_lambda, Omega_k, q0

Input-outputs

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    Omega_M - normalized matter energy density, non-negative numeric scalar
    Omega_Lambda - Normalized cosmological constant, numeric scalar
    Omega_k - normalized curvature parameter, numeric scalar. This is zero
              for a flat universe
    q0 - Deceleration parameter, numeric scalar = -R*(R'')/(R')^2
          = 0.5*Omega_m - Omega_lambda

Notes

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    If more than two parameters are defined upon input (overspecification),
    then the first two defined parameters in the ordered list Omega_m,
    Omega_lambda, Omega_k, q0 are used to define the cosmology.

Example

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    Suppose one has Omega_m = 0.3, and Omega_k = 0.5 then to determine
    Omega_lambda and q0
   
      IDL> cosmo_param, 0.3, omega_lambda, 0.5, q0
 
      which will return omega_lambda = 0.2 and q0 = -2.45

Revision History

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      W. Landsman Raytheon ITSS April 2000
      Better Error checking W. Landsman/D. Syphers October 2010