AstroPy 距离红移转换的精度

Question

AstroPy.coordinates.Distance函数的红移转换精度如何？

它似乎只对千分位有用（远不如浮点数精度问题）：

from astropy import units as u
from astropy.coordinates import SkyCoord, Distance
from astropy.cosmology import Planck15

z1 = 0.05598
z2 = 0.31427

dist1 = Distance(unit=u.pc, z = z1, cosmology = Planck15)
dist2 = Distance(unit=u.pc, z = z2, cosmology = Planck15)

dist1.z    #prints 0.05718
dist2.z    #prints 0.31916

我正在使用它来计算河外源之间的 3D 距离，这些差异在 Mpc 的数量级上，这对于我正在研究的内容来说非常大。这是 AstroPy 不可避免的局限吗？

Answer 1

这适用于 python 3.7 的 astropy 3.2.1。

from astropy import units as u
from astropy.coordinates import SkyCoord, Distance
from astropy.cosmology import Planck15

z1 = 0.05598
z2 = 0.31427

dist1 = Distance(unit=u.pc, z = z1, cosmology = Planck15)
dist2 = Distance(unit=u.pc, z = z2, cosmology = Planck15)

dist1.z
Out[9]: 0.055979999974738834

dist2.z
Out[10]: 0.31427000077974493

看起来计算精确到大约 7 位有效数字。

z3 = 1.31427987654321

dist3 = Distance(unit=u.pc, z = z3, cosmology = Planck15)

dist3.z
Out[23]: 1.3142798808605372

z4 = 900.31427987654321

dist4 = Distance(unit=u.pc, z = z4, cosmology = Planck15)

dist4.z
Out[29]: 900.3142861453044

在接近 z=1000 的地方，这将返回一个错误，指出该值已达到最大值，因为此时您已接近 CMB 区域。