This is an embarrassing clarification to request, but I keep getting confused, even when sort of following the hypersine model thread and others like it.(adsbygoogle = window.adsbygoogle || []).push({});

Is this correct?

[itex]a(t)[/itex] "Scale Factor" - length multiplicative factor, a function of time as experienced by an observer co-moving with the CMB

[itex]\dot { a } (t)\quad =\quad \frac { { d }^{ ' } }{ dt } a(t)[/itex] "rate of change of length" or "rate of expansion"

[itex]\ddot { a } (t)\quad =\quad \frac { { d }^{ '' } }{ dt } a(t)[/itex] Rate of change of "rate of expansion", aka [itex]\lambda [/itex], aka the "cosmological constant".

When someone says, in the context of cosmology that "space-time" looks flat they mean that [itex]\lambda [/itex] is very close to one, that the rate of expansion or the rate of change in length seems nearly constant.

When someone talks about positive cosmological curvature they mean [itex]\lambda [/itex] is positive and the rate of expansion is increasing. This is associated with [itex]\Omega [/itex].

too many rates of, then giving them new names.... oof

**Physics Forums - The Fusion of Science and Community**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Embarrasingly Basic Clarification

Loading...

Similar Threads - Embarrasingly Basic Clarification | Date |
---|---|

I A basic question about units | Dec 31, 2016 |

B Some more basic questions on Dark Energy | Aug 3, 2016 |

Spin Foam Model clarification | Jan 27, 2016 |

I'm not sure where to start with very basic cosmology | Aug 31, 2015 |

Comments - Inflationary Misconceptions and the Basics of Cosmological Horizons | Aug 28, 2015 |

**Physics Forums - The Fusion of Science and Community**