I watched a video lecture from Alan Guth on inflation (undergrad level), and there is something in it I don't understand.(adsbygoogle = window.adsbygoogle || []).push({});

He first presents the inflation scalar field, Φ, which has an energy density associated with it, V(Φ). V(Φ) is stuck at a local minimum, at Φ = 0 (what is called a "false vaccum") :

He then invoke an equation, derived earlier in the course, which relates the derivative with respect to time of the mass density of the Universe, ρ, to its pressure p :

Then, at around 9:43 prof. Guth says that the left-hand side of the equation, d(ρ)/dt, is 0, because the scalar field is stuck at the false vaccum value. Leading to :

Where u is the energy density of the Universe.

Here is what bugs me : ρ represents the mass density of the Universe, not the energy density of the false vaccum, V(Φ). I get that V(Φ) is constant, but why does that implies that the mass density ρ is also constant ?

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

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

# I Inflation lecture from Guth : why does d(ρ)/dt = 0 ?

Have something to add?

Draft saved
Draft deleted

Loading...

Similar Threads - Inflation lecture Guth | Date |
---|---|

I Inflation and Omega | Yesterday at 1:53 PM |

B The Universe without Cosmic Inflation? | Feb 3, 2018 |

I What is the universe expanding into ? Eternal inflation | Nov 20, 2017 |

B Lectures on inflation | Sep 5, 2016 |

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