Properties of the Universe from Big Bang to now --- Documentation for the cosmological calculator
Calculate various properties of the Universe at a given time t.
The advantage of
timeline over other cosmological calculators on the internet
Ned Wright's famous CosmoCalc,
it's Python wrapper,
is that it goes all the way back to inflation, 0.000...[31 zeros]..1 seconds
after Big Bang.
Furthermore, where all these calculators calculate the properties for an input
timeline calculates for an input age of the Universe, which is
more intuitive for non-astronomers.
timeline makes use of Python's Astropy, but for the
early epochs it "calculates backward" from radation-matter equality, assuming a
radiation-dominated Universe during that time.
The code was written in conjunction with the (Danish) popular science article "Big Bang --- en øjenvidneberetning" ("Big Bang --- an eyewitness account"), in order to calculate various properties of the Universe. It is meant for being run from the command line, since this is more tractable to non-experts, but can also be run from a Python environment is a slightly different way.
Prerequisites (mostly for non-experts)
First and foremost, you'll need to use a "terminal" which is a program that allows you to give commands to your computer. On most computers, you'll have an app called something like "Terminal".
A Python installation
The code is written in Python, so you'll need an installation of Python. If you don't have that, you can get it here (click "Downloads" and choose the one matching your computer).
In addition to the standard Python installation, you will need the
library. When you have Python installed, you can install
typing, in the terminal
$ pip install astropy
$ is just a way of showing that here comes a command; it shouldn't be
included in the command. If you get an error when trying to install, try
writing the word
sudo in front of the above command, and then type your
In a terminal, type the following command (in the same directory where you put
$ python timeline.py time unit [-Runit distance_unit] [-cosmo cosmology]
Arguments (for command line):
In the above command,
python is the command to make Python run the program,
timeline.py is the name of the progam. Additionally, there are two mandatory
arguments (i.e. words that must be written):
time Time quantity, i.e. a number unit Unit of time. Allowed values are s: Seconds min: Minutes h: Hours day: Days yr: Years kyr: kilo-years (i.e. 1000 years) Myr: mega-years Gyr: giga-years
Optional arguments are words that may be written. There are two; one for outputting the result in your preferred distance units, and one for using your preferred set of cosmological parameters. The syntax is
-Runit my_dist_unit Units for output distances. Allowed values include: angstrom (or AA), nm, mm, cm, m, km, AU, lightyears (or lyr), parsec (or pc), klyr, kpc, Mlyr, Mpc, Glyr, and Gpc. -cosmo my_cosmology Set of cosmological parameters. Allowed values are: Planck15 (default value), Planck13, WMAP9, WMAP7, WMAP5.
-cosmo (don't forget the dash "
-") are written
after the command, followed by your preferred value.
That is, if you want distances to be written in, say, parsec, you write
-Runit pc after your command, and if you prefer a WMAP 2009 cosmology rather
than a Planck 2015 cosmology, you append your command with
(again, the "
$" in the examples shouldn't be included)
Calculate the properties just after inflation:
$ python timeline.py 1e-32 s
Calculate the properties today:
$ python timeline.py 13.79 Gyr
Calculate the properties 500 million years after Big Bang, but use Gpc (giga-parsec, i.e. billion parsec) for distances:
$ python timeline.py 500 Myr -Runit Gpc
Calculate properties a microsecond after Big Bang, with distances written in cm, using a Planck 2013 cosmology:
$ python timeline.py 1e-6 s -Runit cm -cosmo Planck13
The following values are written out for the Universe at the chosen time t
- Scale factor (a, size of the Universe relative to today)
- Redshift (z, how much light emitted from a source is "stretched" before it reaches us)
- Hubble parameter (H(t), expansion rate of the Universe)
- Radius of observable Universe at t (dP; how far away could an observer at t theoretically see. This is called the particle horizon, and this calculation involves an integral that takes several seconds for late epochs, so if you're impatient, you may want to delete this line from the code)
- Radius of today's obs. Universe at t (a dP,0; how big was the part of the Universe that we can can see today at that time)
- Hubble distance (c / H(t); distance at which the expansion makes stuff recede faster than the speed of light)
- Gas and radiation:
- Temperature (T; average temperature of stuff in the Universe)
- Energy (E = kBT; the corresponding energy of particles)
- Energy density (the total, average energy of atoms, radiation, and everything else per volume)
- Ionized fraction (xe; fraction of hydrogen atoms that are ionized)
- Photon mean free path (how far can a photon travel before it hits an electron)
- mfp / dP (if this ratio is < 1, radation is coupled to matter; if it is > 1, photons free-stream through the entire Universe)
- Photon no. density (Number of photons per cubic centimeter)
- Baryon no. density (Number of atoms per cubic centimeter)
- Photon pressure
- Baryon pressure