# Cast of Characters in the Big Bang

 redshift z shift in observed wavelength, divided by rest wavelength, minus 1 Hubble constant rate at which universe is expanding matter density average density of matter in the universe density parameter ratio of matter density to critical density cosmological constant extra negative curvature of space

Notes on the characters:

1. Redshift is directly observed, when we take the spectrum of a distant galaxy or quasar. We need to identify a line in the spectrum with a particular element and atomic transition, and compare the observed wavelength of that line with its rest wavelength. Objects at redshift z = 1 are really far away, very roughly half way across the observable universe. The current record holder is a quasar at z = 5.82, although there is a galaxy which might have z = 6.68.

2. Given the the redshifts to a bunch of galaxies, we can calculate their radial velocities; if we also know their distances, we can use them to calculate the Hubble constant, which is simply the ratio
```                   radial velocity     km/sec
H  = -----------------    ------
distance            Mpc
```
Current measurements indicate that H is somewhere between 60 and 80 km/sec/Mpc. The larger H, the faster the universe is expanding.

3. The mutual gravitational forces between stars, galaxies, and other matter in the universe tends to slow down its expansion. The larger the density of matter, the stronger the gravitional forces. Matter also acts to warp the fabric of space, pushing it towards positive curvature.

4. If the density of matter is large enough, gravitional forces might eventually (almost) halt the expansion of the universe. The amount of matter needed to (almost) halt the expansion is called the critical density, and the density parameter is the ratio
```                    actual density of matter
omega  =  --------------------------
critical density
```
Current observations indicate that omega is much less than one, perhaps 0.1 to 0.2. Hence, matter cannot halt the expansion.

5. The cosmological constant also affects the evolution of the universe, but in the sense opposite to that of matter: it encourages space to expand. On the other hand, it warps space in the positive direction, just as matter does. Current observations indicate that lambda probably lies somewhere between 0.5 and 1.0, making it more influential than matter.