nbodykit.cosmology.linearnbody¶

Classes

LinearNbody(background[, c_b, c_ncdm_1ev_z0])

Perturbations of matter under matter only interaction on an expanding cosmology background.

class nbodykit.cosmology.linearnbody.LinearNbody(background, c_b=0, c_ncdm_1ev_z0=134.423)[source]¶

Perturbations of matter under matter only interaction on an expanding cosmology background. Ignoring interaction with radiation.

This can be considered the linear order perturbative model of hydro nbody-simulations.

Parameters

c_b (float) – baryon velocity in km/s. set to zero to evolve as cdm. Related to the thermal history of baryons.
background (object) –

an object with attributes:
efunc(z), Omega_b(z), Omega_cdm(z), Omega_ncdm(z) and m_ncdm

For example, a Cosmology object from nbodykit will work here.
c_ncdm_1ev_z0 (float) – neutrino velocity in km/s at z=0 for 1ev. The default number 134.423 is widely used. set to zero to evolve as cdm. c_ncdm = cncdm_1ev_z0 / a / m_ncdm

Attributes

Methods

`J`(k, a)	The potential term.
`integrate`(k, q0, p0, a[, rtol])	Solve the 3 fluid model from initial position and momentum q0, and p0, at times a.
`seed_from_synchronous`(cosmology, a0[, Tk])	Convert synchronuous gauge velocity (h_prime) to the momentum in n-body gauge.

integrate(k, q0, p0, a, rtol=0.0001)[source]¶

Solve the 3 fluid model from initial position and momentum q0, and p0, at times a.

This can be slow if neutrino velocity and baryon velocity are non-zero. (200,000+ function evaluations with RK45, when rtol=1e-7)

Parameters

a (array_like) – scale factor requesting the output.
k (array_like,) – k values to compute the solution, in h/Mpc. From classylss’s transfer function.
q0 (array_like (Nk, 3)) – initial position, -d, produced by seed from CLASSylss transfer function three species are cdm, baryon and ncdm.
p0 (array_like (Nk, 3)) – initial momentum, a * v, from seed. three species are cdm, baryon and ncdm.
rtol (float) – relative accuracy. It appears 1e-4 is good for k ~ 10 with reasonable velocities.

Returns

a (array_like (Na), very close but not in general identical to input a.)
q (array_like (Na, Nk, 3)) – position (-d) at different requested a’s
p (array_like (Na, Nk, 3)) – momentum (a * v) at different requested a’s. v is peculiar velocity.

static seed_from_synchronous(cosmology, a0, Tk=None)[source]¶

Convert synchronuous gauge velocity (h_prime) to the momentum in n-body gauge.

This function repacks the columns to cdm, baryon and ncdm for both q and p, such that at a = a0,

q = - d

p = a v = - a dd / dt

Parameters

cosmology (object, Cosmology.) – the cosmology object to obtain hubble (with dimension of time unit)
a0 (float) – the scaling factor to seed, 0.01 for z=99
Tk (structured array) – use a precomputed transfer function, must be the same format as the output of Cosmology.get_transfer(), with ‘k’ in h/Mpc units, ‘d_cdm’, ‘d_b’, ‘d_ncdm[0]’ and ‘h_prime’.

Returns