Cosmological Calculations (astropy.cosmology
)¶
Introduction¶
The astropy.cosmology
subpackage contains classes for representing
cosmologies and utility functions for calculating commonly used
quantities that depend on a cosmological model. This includes
distances, ages, and lookback times corresponding to a measured
redshift or the transverse separation corresponding to a measured
angular separation.
Getting Started¶
Cosmological quantities are calculated using methods of a
Cosmology
object.
Examples¶
To calculate the Hubble constant at z=0 (i.e., H0
) and the number of
transverse proper kiloparsecs (kpc) corresponding to an arcminute at z=3:
>>> from astropy.cosmology import WMAP9 as cosmo
>>> cosmo.H(0)
<Quantity 69.32 km / (Mpc s)>
>>> cosmo.kpc_proper_per_arcmin(3)
<Quantity 472.97709620405266 kpc / arcmin>
Here WMAP9 is a builtin object describing a cosmology with the
parameters from the nineyear WMAP results. Several other builtin
cosmologies are also available (see Builtin Cosmologies). The
available methods of the cosmology object are listed in the methods
summary for the FLRW
class. If you are using
IPython you can also use tab completion to print a list of the
available methods. To do this, after importing the cosmology as in the
above example, type cosmo.
at the IPython prompt and then press
the tab key.
All of these methods also accept an arbitrarilyshaped array of redshifts as input:
>>> from astropy.cosmology import WMAP9 as cosmo
>>> cosmo.comoving_distance([0.5, 1.0, 1.5])
<Quantity [1916.06941724, 3363.07062107, 4451.7475201 ] Mpc>
You can create your own FLRWlike cosmology using one of the cosmology classes:
>>> from astropy.cosmology import FlatLambdaCDM
>>> cosmo = FlatLambdaCDM(H0=70, Om0=0.3, Tcmb0=2.725)
>>> cosmo
FlatLambdaCDM(H0=70 km / (Mpc s), Om0=0.3, Tcmb0=2.725 K,
Neff=3.04, m_nu=[0. 0. 0.] eV, Ob0=None)
Note the presence of additional cosmological parameters (e.g., Neff
,
the number of effective neutrino species) with default values; these
can also be specified explicitly in the call to the constructor.
The cosmology subpackage makes use of units
, so in many
cases returns values with units attached. Consult the documentation
for that subpackage for more details, but briefly here we will show how to
access the floating point or array values:
>>> from astropy.cosmology import WMAP9 as cosmo
>>> H0 = cosmo.H(0)
>>> H0.value, H0.unit
(69.32, Unit("km / (Mpc s)"))
Using astropy.cosmology
¶
Most of the functionality is enabled by the FLRW
object. This represents a homogeneous and isotropic cosmology
(characterized by the FriedmannLemaitreRobertsonWalker metric,
named after the people who solved Einstein’s field equation for this
special case). However, you cannot work with this class directly, as
you must specify a dark energy model by using one of its subclasses
instead, such as FlatLambdaCDM
.
Examples¶
You can create a new FlatLambdaCDM
object with
arguments giving the Hubble parameter and Omega matter (both at z=0):
>>> from astropy.cosmology import FlatLambdaCDM
>>> cosmo = FlatLambdaCDM(H0=70, Om0=0.3)
>>> cosmo
FlatLambdaCDM(H0=70 km / (Mpc s), Om0=0.3, Tcmb0=0 K,
Neff=3.04, m_nu=None, Ob0=None)
This can also be done more explicitly using units, which is recommended:
>>> from astropy.cosmology import FlatLambdaCDM
>>> import astropy.units as u
>>> cosmo = FlatLambdaCDM(H0=70 * u.km / u.s / u.Mpc, Tcmb0=2.725 * u.K, Om0=0.3)
However, most of the parameters that accept units (H0
, Tcmb0
)
have default units, so unit quantities do not have to be used (with the
exception of neutrino masses, where you must supply a unit if you want massive
neutrinos).
The predefined cosmologies described in the Getting Started
section are instances of FlatLambdaCDM
, and have
the same methods. So we can find the luminosity distance to
redshift 4 by:
>>> cosmo.luminosity_distance(4)
<Quantity 35842.353618623194 Mpc>
Or the age of the universe at z = 0:
>>> cosmo.age(0)
<Quantity 13.461701658024014 Gyr>
They also accept arrays of redshifts:
>>> cosmo.age([0.5, 1, 1.5]).value
array([8.42128013, 5.74698021, 4.19645373])
See the FLRW
and
FlatLambdaCDM
object docstring for all of the
methods and attributes available.
In addition to flat universes, nonflat varieties are supported, such as
LambdaCDM
. A variety of standard cosmologies with the
parameters already defined are also available
(see Builtin Cosmologies):
>>> from astropy.cosmology import WMAP7 # WMAP 7year cosmology
>>> WMAP7.critical_density(0) # critical density at z = 0
<Quantity 9.31000324385361e30 g / cm3>
You can see how the density parameters evolve with redshift as well:
>>> from astropy.cosmology import WMAP7 # WMAP 7year cosmology
>>> WMAP7.Om([0, 1.0, 2.0]), WMAP7.Ode([0., 1.0, 2.0])
(array([0.272 , 0.74898522, 0.90905234]),
array([0.72791572, 0.2505506 , 0.0901026 ]))
Note that these do not quite add up to one, even though WMAP7 assumes a
flat universe, because photons and neutrinos are included. Also note
that the density parameters are unitless and so are not
Quantity
objects.
It is possible to specify the baryonic matter density at redshift zero
at class instantiation by passing the keyword argument Ob0
:
>>> from astropy.cosmology import FlatLambdaCDM
>>> cosmo = FlatLambdaCDM(H0=70, Om0=0.3, Ob0=0.05)
>>> cosmo
FlatLambdaCDM(H0=70 km / (Mpc s), Om0=0.3, Tcmb0=0 K,
Neff=3.04, m_nu=None, Ob0=0.05)
In this case the dark matteronly density at redshift 0 is
available as class attribute Odm0
and the redshift evolution of
dark and baryonic matter densities can be computed using the methods
Odm
and Ob
, respectively. If Ob0
is not specified at class
instantiation, it defaults to None
and any method relying on it
being specified will raise a ValueError
:
>>> from astropy.cosmology import FlatLambdaCDM
>>> cosmo = FlatLambdaCDM(H0=70, Om0=0.3)
>>> cosmo.Odm(1)
Traceback (most recent call last):
...
ValueError: Baryonic density not set for this cosmology, unclear
meaning of dark matter density
Cosmological instances have an optional name
attribute which can be
used to describe the cosmology:
>>> from astropy.cosmology import FlatwCDM
>>> cosmo = FlatwCDM(name='SNLS3+WMAP7', H0=71.58, Om0=0.262, w0=1.016)
>>> cosmo
FlatwCDM(name="SNLS3+WMAP7", H0=71.6 km / (Mpc s), Om0=0.262,
w0=1.02, Tcmb0=0 K, Neff=3.04, m_nu=None, Ob0=None)
This is also an example with a different model for dark energy: a flat universe with a constant dark energy equation of state, but not necessarily a cosmological constant. A variety of additional dark energy models are also supported (see Specifying a dark energy model).
An important point is that the cosmological parameters of each
instance are immutable — that is, if you want to change, say,
Om
, you need to make a new instance of the class. To make
this more convenient, a clone
operation is provided, which
allows you to make a copy with specified values changed.
Note that you cannot change the type of cosmology with this operation
(e.g., flat to nonflat).
To make a copy of a cosmological instance using the clone
operation:
>>> from astropy.cosmology import WMAP9
>>> newcosmo = WMAP9.clone(name='WMAP9 modified', Om0=0.3141)
>>> WMAP9.H0, newcosmo.H0 # some values unchanged
(<Quantity 69.32 km / (Mpc s)>, <Quantity 69.32 km / (Mpc s)>)
>>> WMAP9.Om0, newcosmo.Om0 # some changed
(0.2865, 0.3141)
>>> WMAP9.Ode0, newcosmo.Ode0 # Indirectly changed since this is flat
(0.7134130719051658, 0.6858130719051657)
Finding the Redshift at a Given Value of a Cosmological Quantity¶
If you know a cosmological quantity and you want to know the
redshift which it corresponds to, you can use z_at_value
.
Example¶
To find the redshift using z_at_value
:
>>> import astropy.units as u
>>> from astropy.cosmology import Planck13, z_at_value
>>> z_at_value(Planck13.age, 2 * u.Gyr)
3.1981226843560968
For some quantities, there can be more than one redshift that satisfies
a value. In this case you can use the zmin
and zmax
keywords
to restrict the search range. See the z_at_value
docstring for more
detailed usage examples.
Builtin Cosmologies¶
A number of preloaded cosmologies are available from analyses using the WMAP and Planck satellite data. For example:
>>> from astropy.cosmology import Planck13 # Planck 2013
>>> Planck13.lookback_time(2) # lookback time in Gyr at z=2
<Quantity 10.51184138 Gyr>
A full list of the predefined cosmologies is given by
cosmology.parameters.available
and summarized below:
Name 
Source 
H0 
Om 
Flat 

WMAP5 
Komatsu et al. 2009 
70.2 
0.277 
Yes 
WMAP7 
Komatsu et al. 2011 
70.4 
0.272 
Yes 
WMAP9 
Hinshaw et al. 2013 
69.3 
0.287 
Yes 
Planck13 
Planck Collab 2013, Paper XVI 
67.8 
0.307 
Yes 
Planck15 
Planck Collab 2015, Paper XIII 
67.7 
0.307 
Yes 
Currently, all are instances of FlatLambdaCDM
.
More details about exactly where each set of parameters comes from
are available in the docstring for each object:
>>> from astropy.cosmology import WMAP7
>>> print(WMAP7.__doc__)
WMAP7 instance of FlatLambdaCDM cosmology
(from Komatsu et al. 2011, ApJS, 192, 18, doi: 10.1088/00670049/192/2/18.
Table 1 (WMAP + BAO + H0 ML).)
Specifying a Dark Energy Model¶
Along with the standard FlatLambdaCDM
model
described above, a number of additional dark energy models are
provided. FlatLambdaCDM
and LambdaCDM
assume that dark
energy is a cosmological constant, and should be the most commonly
used cases; the former assumes a flat universe, the latter allows
for spatial curvature. FlatwCDM
and
wCDM
assume a constant dark
energy equation of state parameterized by \(w_{0}\).
Two forms of a variable dark energy equation of state are provided: the simple
first order linear expansion \(w(z) = w_{0} + w_{z} z\) by
w0wzCDM
, as well as the common CPL form by
w0waCDM
: \(w(z) = w_{0} + w_{a} (1  a) =
w_{0} + w_{a} z / (1 + z)\) and its generalization to include a pivot
redshift by wpwaCDM
: \(w(z) = w_{p} + w_{a}
(a_{p}  a)\).
Users can specify their own equation of state by subclassing
FLRW
. See the provided subclasses for
examples. It is recommended, but not required, that all arguments to the
constructor of a new subclass be available as properties, since the
clone
method assumes this is the case. It is also advisable
to stick to subclassing FLRW
rather than one of
its subclasses, since some of them use internal optimizations that
also need to be propagated to any subclasses. Users wishing to
use similar tricks (which can make distance calculations much faster)
should consult the cosmology module source code for details.
Photons and Neutrinos¶
The cosmology classes (can) include the contribution to the energy density
from both photons and neutrinos. By default, the latter are assumed
massless. The three parameters controlling the properties of these
species, which are arguments to the initializers of all of the
cosmological classes, are Tcmb0
(the temperature of the cosmic microwave
background at z=0), Neff
(the effective number of neutrino species), and
m_nu
(the rest mass of the neutrino species). Tcmb0
and m_nu
should
be expressed as unit Quantities. All three have standard default values — 0 K,
3.04, and 0 eV, respectively. (The reason that Neff
is not 3 has to do
primarily with a small bump in the neutrino energy spectrum due to electron
positron annihilation, but is also affected by weak interaction physics.)
Setting the CMB temperature to 0 removes the contribution of both neutrinos and
photons. This is the default to ensure these components are excluded unless the
user explicitly requests them.
Massive neutrinos are treated using the approach described in the WMAP sevenyear cosmology paper (Komatsu et al. 2011, ApJS, 192, 18, section 3.3). This is not the simple \(\Omega_{\nu 0} h^2 = \sum_i m_{\nu\, i} / 93.04\,\mathrm{eV}\) approximation. Also note that the values of \(\Omega_{\nu}(z)\) include both the kinetic energy and the rest mass energy components, and that the Planck13 and Planck15 cosmologies include a single species of neutrinos with nonzero mass (which is not included in \(\Omega_{m0}\)).
Adding massive neutrinos can have significant performance implications. In particular, the computation of distance measures and lookback times are factors of three to four times slower than in the massless neutrino case. Therefore, if you need to compute many distances in such a cosmology and performance is critical, it is particularly useful to calculate them on a grid and use interpolation.
Examples¶
The contribution of photons and neutrinos to the total massenergy density can be found as a function of redshift:
>>> from astropy.cosmology import WMAP7 # WMAP 7year cosmology
>>> WMAP7.Ogamma0, WMAP7.Onu0 # Current epoch values
(4.985694972799396e05, 3.442154948307989e05)
>>> z = [0, 1.0, 2.0]
>>> WMAP7.Ogamma(z), WMAP7.Onu(z)
(array([4.98603986e05, 2.74593395e04, 4.99915942e04]),
array([3.44239306e05, 1.89580995e04, 3.45145089e04]))
If you want to exclude photons and neutrinos from your calculations, you can
set Tcmb0
to 0 (which is also the default):
>>> from astropy.cosmology import FlatLambdaCDM
>>> import astropy.units as u
>>> cos = FlatLambdaCDM(70.4 * u.km / u.s / u.Mpc, 0.272, Tcmb0 = 0.0 * u.K)
>>> cos.Ogamma0, cos.Onu0
(0.0, 0.0)
You can include photons but exclude any contributions from neutrinos by
setting Tcmb0
to be nonzero (2.725 K is the standard value for our
Universe) but setting Neff
to 0:
>>> from astropy.cosmology import FlatLambdaCDM
>>> cos = FlatLambdaCDM(70.4, 0.272, Tcmb0=2.725, Neff=0)
>>> cos.Ogamma([0, 1, 2]) # Photons are still present
array([4.98603986e05, 2.74642208e04, 5.00086413e04])
>>> cos.Onu([0, 1, 2]) # But not neutrinos
array([0., 0., 0.])
The number of neutrino species is assumed to be the floor of Neff
,
which in the default case is Neff=3
. Therefore, if nonzero neutrino masses
are desired, then three masses should be provided. However, if only one
value is provided, all of the species are assumed to have the same mass.
Neff
is assumed to be shared equally between each species.
>>> from astropy.cosmology import FlatLambdaCDM
>>> import astropy.units as u
>>> H0 = 70.4 * u.km / u.s / u.Mpc
>>> m_nu = 0 * u.eV
>>> cosmo = FlatLambdaCDM(H0, 0.272, Tcmb0=2.725, m_nu=m_nu)
>>> cosmo.has_massive_nu
False
>>> cosmo.m_nu
<Quantity [0., 0., 0.] eV>
>>> m_nu = [0.0, 0.05, 0.10] * u.eV
>>> cosmo = FlatLambdaCDM(H0, 0.272, Tcmb0=2.725, m_nu=m_nu)
>>> cosmo.has_massive_nu
True
>>> cosmo.m_nu
<Quantity [0. , 0.05, 0.1 ] eV>
>>> cosmo.Onu([0, 1.0, 15.0])
array([0.00327011, 0.00896845, 0.01257946])
>>> cosmo.Onu(1) * cosmo.critical_density(1)
<Quantity 2.444380380370406e31 g / cm3>
While these examples used FlatLambdaCDM
,
the above examples also apply for all of the other cosmology classes.
For Developers: Using astropy.cosmology
Inside astropy
¶
If you are writing code for the astropy
core or an affiliated package,
it is often useful to assume a default cosmology so that the exact
cosmology does not have to be specified every time a function or method
is called. In this case, it is possible to specify a “default”
cosmology.
You can set the default cosmology to a predefined value by using the
“default_cosmology” option in the [cosmology.core]
section of the
configuration file (see Configuration System (astropy.config)). Alternatively, you can
use the set
function of default_cosmology
to
set a cosmology for the current Python session. If you have not set a
default cosmology using one of the methods described above, then the
cosmology module will default to using the nineyear WMAP parameters.
It is strongly recommended that you use the default cosmology through
the default_cosmology
science state object. An
override option can then be provided using something like the
following:
def myfunc(..., cosmo=None):
from astropy.cosmology import default_cosmology
if cosmo is None:
cosmo = default_cosmology.get()
... your code here ...
This ensures that all code consistently uses the default cosmology unless explicitly overridden.
Note
In general it is better to use an explicit cosmology (for example
WMAP9.H(0)
instead of
cosmology.default_cosmology.get().H(0)
). Use of the default
cosmology should generally be reserved for code that will be
included in the astropy
core or an affiliated package.
See Also¶
Hogg, “Distance measures in cosmology”, https://arxiv.org/abs/astroph/9905116
Linder, “Exploring the Expansion History of the Universe”, https://arxiv.org/abs/astroph/0208512
NASA’s Legacy Archive for Microwave Background Data Analysis, https://lambda.gsfc.nasa.gov/
Range of Validity and Reliability¶
The code in this subpackage is tested against several widely used
online cosmology calculators and has been used to perform many
calculations in refereed papers. You can check the range of redshifts
over which the code is regularly tested in the module
astropy.cosmology.tests.test_cosmology
. If you find any bugs,
please let us know by opening an issue at the GitHub repository!
A more difficult question is the range of redshifts over which
the code is expected to return valid results. This is necessarily
modeldependent, but in general you should not expect the numeric
results to be well behaved for redshifts more than a few times
larger than the epoch of matterradiation equality (so, for typical
models, not above z = 56,000, but for some models much lower redshifts
may be illbehaved). In particular, you should pay attention to warnings
from the scipy
integration package about integrals failing to converge
(which may only be issued once per session).
The builtin cosmologies use the parameters as listed in the respective papers. These provide only a limited range of precision, and so you should not expect derived quantities to match beyond that precision. For example, the Planck 2013 and 2015 results only provide the Hubble constant to four digits. Therefore, they should not be expected to match the age quoted by the Planck team to better than that, despite the fact that five digits are quoted in the papers.
Reference/API¶
astropy.cosmology Package¶
astropy.cosmology contains classes and functions for cosmological distance measures and other cosmologyrelated calculations.
See the Astropy documentation for more detailed usage examples and references.
Functions¶

Find the redshift 
Classes¶

A class describing an isotropic and homogeneous (FriedmannLemaitreRobertsonWalker) cosmology. 

FLRW cosmology with a cosmological constant and no curvature. 

FLRW cosmology with a CPL dark energy equation of state and no curvature. 

FLRW cosmology with a constant dark energy equation of state and no spatial curvature. 

FLRW cosmology with a cosmological constant and curvature. 
The default cosmology to use. To change it::. 


FLRW cosmology with a CPL dark energy equation of state and curvature. 

FLRW cosmology with a variable dark energy equation of state and curvature. 

FLRW cosmology with a constant dark energy equation of state and curvature. 

FLRW cosmology with a CPL dark energy equation of state, a pivot redshift, and curvature. 