Example: On the page, the lookback time for z = 0.03 (snap 96) is 0.475 Gyr. However, I got 0.424 Gyr, 0.424 Gyr and 0.408 Gyr by using Planck15, Planck18 and WMAP7 cosmologies in astropy.

Note that values you see listed on that page are rounded. I imagine if you use the TNG cosmology, you should get to a very similar answer with astropy.

Rodrigo Freitas

13 Nov '23

Thank you Dylan.

I assume the difference in the times comes from the method. I am numerically integrating eq. 30 from Hogg's paper (https://arxiv.org/pdf/astro-ph/9905116.pdf) while the function you showed uses an analytic formula, which seems to be an approximation.

Anyway, the differences are only on de order of 50 Myr, not much larger.

Hi,

I noticed that the values of lookback time in the TNG50-1 page (https://www.tng-project.org/data/downloads/TNG50-1/) differ from what I obtained with astropy.cosmology classes.

Example: On the page, the lookback time for z = 0.03 (snap 96) is 0.475 Gyr. However, I got 0.424 Gyr, 0.424 Gyr and 0.408 Gyr by using Planck15, Planck18 and WMAP7 cosmologies in astropy.

Here is what I did:

Are you using a different method to compute the lookback times shown in the table of the mentioned page?

Thank you in advance.

Hi Rodrigo,

This is probably using the following simple function:

Note that values you see listed on that page are rounded. I imagine if you use the TNG cosmology, you should get to a very similar answer with astropy.

Thank you Dylan.

I assume the difference in the times comes from the method. I am numerically integrating eq. 30 from Hogg's paper (https://arxiv.org/pdf/astro-ph/9905116.pdf) while the function you showed uses an analytic formula, which seems to be an approximation.

Anyway, the differences are only on de order of 50 Myr, not much larger.

Obrigado!