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COSMO_PARAM

COSMO_PARAM

## Purpose

Derive full set of cosmological density parameters from a partial set

## Explanation

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

COSMO_PARAM, Omega_m, Omega_lambda, Omega_k, q0

## Input-outputs

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

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

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

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

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