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:

- 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**. - 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. - 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.
- 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 ratioactual 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. - 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.

Copyright © Michael Richmond. This work is licensed under a Creative Commons License.