CosmoFit.GCG¶

class CosmoFit.GCG(params)[source]¶

Bases: Cosmology

Generalized Chaplygin Gas (GCG).

Unlike LCDM/wCDM/CPL/JBP/BA – which all just reparametrize dark energy’s equation of state on top of an independent matter component – GCG replaces dark energy with a single exotic fluid with equation of state

p = -A / rho^alpha

that unifies dark matter and dark energy: it behaves like pressureless dust at early times (large rho, small |p/rho|) and like a cosmological constant at late times (rho -> const), interpolating between the two as the universe expands. The Omega_m parameter here is therefore ordinary baryonic + any additional matter on top of the GCG fluid, not the GCG fluid’s own dust-like contribution.

Density evolution (closed-form solution of the GCG continuity equation):

rho_GCG(z) / rho_GCG(0)
    = [ A_s + (1 - A_s) (1+z)^(3(1+alpha)) ]^(1/(1+alpha))

Effective equation of state:

w(z) = -A_s / [ A_s + (1-A_s)(1+z)^(3(1+alpha)) ]
       * (1+z)^(3(1+alpha)) ... equivalently
w(z) = -1 + (1-A_s)(1+z)^(3(1+alpha))
             / [A_s + (1-A_s)(1+z)^(3(1+alpha))]

so w(0) = -A_s and w(z) -> 0 (dust-like) as z -> infinity. A_s = 1 reduces GCG to flat/curved LCDM (rho_GCG(z) = rho_GCG(0) for any alpha); alpha = 1 is the original (“pure”) Chaplygin gas of Kamenshchik, Moschella & Pasquier (2001).

Notes

Uses the shared A_s and alpha fields of CosmologyParameters (ignored by every other model, the same way LCDM ignores w0/wa).

References

Kamenshchik, Moschella & Pasquier (2001), Phys. Lett. B 511, 265, arXiv:gr-qc/0103004. Bento, Bertolami & Sen (2002), Phys. Rev. D 66, 043507, arXiv:gr-qc/0202064.

Parameters:: params (CosmologyParameters)

__init__(params)¶

Parameters:: params (CosmologyParameters)

Methods

`E`(z)	Dimensionless Hubble parameter.
`H`(z)
`Omega_de`(z)	GCG effective density parameter.
`Omega_matter`(z)	Matter density at redshift `z`, in units of today's critical density -- i.e. the `Omega_m (1+z)^3` term as it appears inside `E(z)^2`, before dividing by `E(z)^2`.
`__init__`(params)
`dEdz`(z)	Derivative of E(z).
`fde`(z)	GCG density evolution factor.
`mu`(a[, k])	Effective-to-Newtonian gravitational coupling ratio, G_eff(a,k)/G_N, entering the linear growth equation solved by `GrowthCalculator` (see that module's docstring for the equation itself).
`plain_name`()	This model's name as plain text: `MODEL_NAME` if it declares one, else the class name.
`plot_label`()	This model's name as it should appear in a figure legend or title -- `MODEL_LABEL` (LaTeX) if it declares one, else `plain_name()`.
`refresh`()	Rebuild internal numerical tables after the underlying `params` object has been mutated in place (e.g. by `params.update(theta)` during an MCMC step).
`w`(z)	Effective dark-energy equation of state.

Attributes

`A_planck`
`A_s`
`EXTRA_PARAMS`	Extra parameters this model adds beyond CosmologyParameters.
`H0`
`MB`
`MODEL_LABEL`	a LaTeX math string where the plain name is really a set of symbols (`LCDM` -> `$\Lambda$CDM`), None where the plain name is already what a reader should see (an acronym like `CPL`).
`MODEL_NAME`	Plain-text name for this model, for tables, JSON and log lines.
`N_eff`
`Omega_b`
`Omega_de0`
`Omega_k`
`Omega_m`
`alpha`
`compute_rd`	Whether `rd` is computed from the physical densities (`rd_computed()`) rather than read from the free `rd` parameter.
`derive_sigma8`	Whether `sigma8` is derived from the Boltzmann code rather than read from the free `sigma8` parameter.
`h`	Reduced Hubble constant, `H0 / 100`.
`ln1e10As`
`m_nu`
`n_s`
`omega_b_h2`	Physical baryon density, `Omega_b h^2` -- the combination BBN and the CMB actually constrain.
`omega_cdm_h2`	Physical cold dark matter density, `Omega_c h^2` -- matter less baryons.
`omega_m_h2`	Physical matter density, `Omega_m h^2`.
`rd`	The sound horizon at the drag epoch [Mpc] that the BAO likelihoods divide by.
`sigma8`	Present-day `sigma_8`, the normalization every growth prediction is built on.
`tau_reio`
`w0`
`wa`