Cosmology¶

class astropy.cosmology.Cosmology(name=None, meta=None)[source]¶

Bases: object

Base-class for all Cosmologies.

Parameters:

*args: Arguments into the cosmology; used by subclasses, not this base class.
namepython:str or python:None (optional, keyword-only): The name of the cosmology.
metapython:dict or python:None (optional, keyword-only): Metadata for the cosmology, e.g., a reference.
**kwargs: Arguments into the cosmology; used by subclasses, not this base class.

Notes

Class instances are static – you cannot (and should not) change the values of the parameters. That is, all of the above attributes (except meta) are read only.

For details on how to create performant custom subclasses, see the documentation on Speeding up Integrals in Custom Cosmologies.

Attributes Summary

`from_format`	Transform object to a `Cosmology`.
`is_flat`	Return bool; `True` if the cosmology is flat.
`meta`
`name`	The name of the Cosmology instance.
`read`	Read and parse data to a `Cosmology`.
`to_format`	Transform this Cosmology to another format.
`write`	Write this Cosmology object out in the specified format.

Methods Summary

`clone`(*[, meta])	Returns a copy of this object with updated parameters, as specified.
`is_equivalent`(other, /, *[, format])	Check equivalence between Cosmologies.

Attributes Documentation

from_format¶

Transform object to a Cosmology.

This function provides the Cosmology interface to the Astropy unified I/O layer. This allows easily parsing supported data formats using syntax such as:

>>> from astropy.cosmology import Cosmology
>>> cosmo1 = Cosmology.from_format(cosmo_mapping, format='mapping')

When the from_format method is called from a subclass the subclass will provide a keyword argument cosmology=<class> to the registered parser. The method uses this cosmology class, regardless of the class indicated in the data, and sets parameters’ default values from the class’ signature.

Get help on the available readers using the help() method:

>>> Cosmology.from_format.help()  # Get help and list supported formats
>>> Cosmology.from_format.help('<format>')  # Get detailed help on a format
>>> Cosmology.from_format.list_formats()  # Print list of available formats

Parameters:

objobject

The object to parse according to ‘format’

*args

Positional arguments passed through to data parser.

formatpython:str or python:None, optional keyword-only

Object format specifier. For None (default) CosmologyFromFormat tries to identify the correct format.

**kwargs

Keyword arguments passed through to data parser. Parsers should accept the following keyword arguments:

cosmologythe class (or string name thereof) to use / check when
constructing the cosmology instance.

Returns:

outCosmology subclass instance: Cosmology corresponding to obj contents.

is_flat¶: Return bool; True if the cosmology is flat. This is abstract and must be defined in subclasses.

meta¶

name¶: The name of the Cosmology instance.

read¶

Read and parse data to a Cosmology.

This function provides the Cosmology interface to the Astropy unified I/O layer. This allows easily reading a file in supported data formats using syntax such as:

>>> from astropy.cosmology import Cosmology
>>> cosmo1 = Cosmology.read('<file name>')

When the read method is called from a subclass the subclass will provide a keyword argument cosmology=<class> to the registered read method. The method uses this cosmology class, regardless of the class indicated in the file, and sets parameters’ default values from the class’ signature.

Get help on the available readers using the help() method:

>>> Cosmology.read.help()  # Get help reading and list supported formats
>>> Cosmology.read.help(format='<format>')  # Get detailed help on a format
>>> Cosmology.read.list_formats()  # Print list of available formats

Parameters:

*args: Positional arguments passed through to data reader. If supplied the first argument is typically the input filename.
formatpython:str (optional, keyword-only): File format specifier.
**kwargs: Keyword arguments passed through to data reader.

Returns:

outCosmology subclass instance: Cosmology corresponding to file contents.

to_format¶

Transform this Cosmology to another format.

This function provides the Cosmology interface to the astropy unified I/O layer. This allows easily transforming to supported data formats using syntax such as:

>>> from astropy.cosmology import Planck18
>>> Planck18.to_format("mapping")
{'cosmology': astropy.cosmology.core.FlatLambdaCDM,
 'name': 'Planck18',
 'H0': <Quantity 67.66 km / (Mpc s)>,
 'Om0': 0.30966,
 ...

Get help on the available representations for Cosmology using the help() method:

>>> Cosmology.to_format.help()  # Get help and list supported formats
>>> Cosmology.to_format.help('<format>')  # Get detailed help on format
>>> Cosmology.to_format.list_formats()  # Print list of available formats

Parameters:

formatpython:str: Format specifier.
*args: Positional arguments passed through to data writer. If supplied the first argument is the output filename.
**kwargs: Keyword arguments passed through to data writer.

write¶

Write this Cosmology object out in the specified format.

This function provides the Cosmology interface to the astropy unified I/O layer. This allows easily writing a file in supported data formats using syntax such as:

>>> from astropy.cosmology import Planck18
>>> Planck18.write('<file name>')

Get help on the available writers for Cosmology using the help() method:

>>> Cosmology.write.help()  # Get help writing and list supported formats
>>> Cosmology.write.help(format='<format>')  # Get detailed help on format
>>> Cosmology.write.list_formats()  # Print list of available formats

Parameters:

*args: Positional arguments passed through to data writer. If supplied the first argument is the output filename.
formatpython:str (optional, keyword-only): File format specifier.
**kwargs: Keyword arguments passed through to data writer.

Methods Documentation

clone(*, meta=None, **kwargs)[source]¶

Returns a copy of this object with updated parameters, as specified.

This cannot be used to change the type of the cosmology, so clone() cannot be used to change between flat and non-flat cosmologies.

Parameters:

metapython:mapping or python:None (optional, keyword-only): Metadata that will update the current metadata.
**kwargs: Cosmology parameter (and name) modifications. If any parameter is changed and a new name is not given, the name will be set to “[old name] (modified)”.

Returns:

newcosmoCosmology subclass instance: A new instance of this class with updated parameters as specified. If no arguments are given, then a reference to this object is returned instead of copy.

Examples

To make a copy of the Planck13 cosmology with a different matter density (Om0), and a new name:

>>> from astropy.cosmology import Planck13
>>> Planck13.clone(name="Modified Planck 2013", Om0=0.35)
FlatLambdaCDM(name="Modified Planck 2013", H0=67.77 km / (Mpc s),
              Om0=0.35, ...

If no name is specified, the new name will note the modification.

>>> Planck13.clone(Om0=0.35).name
'Planck13 (modified)'

is_equivalent(other: Any, /, *, format: _FormatType = False) → bool[source]¶

Check equivalence between Cosmologies.

Two cosmologies may be equivalent even if not the same class. For example, an instance of LambdaCDM might have \(\Omega_0=1\) and \(\Omega_k=0\) and therefore be flat, like FlatLambdaCDM.

Parameters:

otherCosmology subclass instance, positional-only: The object to which to compare.
formatbool or python:None or python:str, optional keyword-only: Whether to allow, before equivalence is checked, the object to be converted to a Cosmology. This allows, e.g. a Table to be equivalent to a Cosmology. False (default) will not allow conversion. True or None will, and will use the auto-identification to try to infer the correct format. A str is assumed to be the correct format to use when converting. format is broadcast to match the shape of other. Note that the cosmology arguments are not broadcast against format, so it cannot determine the output shape.

Returns:

bool: True if cosmologies are equivalent, False otherwise.

Examples

Two cosmologies may be equivalent even if not of the same class. In this examples the LambdaCDM has Ode0 set to the same value calculated in FlatLambdaCDM.

>>> import astropy.units as u
>>> from astropy.cosmology import LambdaCDM, FlatLambdaCDM
>>> cosmo1 = LambdaCDM(70 * (u.km/u.s/u.Mpc), 0.3, 0.7)
>>> cosmo2 = FlatLambdaCDM(70 * (u.km/u.s/u.Mpc), 0.3)
>>> cosmo1.is_equivalent(cosmo2)
True

While in this example, the cosmologies are not equivalent.

>>> cosmo3 = FlatLambdaCDM(70 * (u.km/u.s/u.Mpc), 0.3, Tcmb0=3 * u.K)
>>> cosmo3.is_equivalent(cosmo2)
False

Also, using the keyword argument, the notion of equivalence is extended to any Python object that can be converted to a Cosmology.

>>> from astropy.cosmology import Planck18
>>> tbl = Planck18.to_format("astropy.table")
>>> Planck18.is_equivalent(tbl, format=True)
True

The list of valid formats, e.g. the Table in this example, may be checked with Cosmology.from_format.list_formats().

As can be seen in the list of formats, not all formats can be auto-identified by Cosmology.from_format.registry. Objects of these kinds can still be checked for equivalence, but the correct format string must be used.

>>> tbl = Planck18.to_format("yaml")
>>> Planck18.is_equivalent(tbl, format="yaml")
True

Navigation

Cosmology¶