Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: cosmological constant and gravity

Learn more physics!

Most recent answer: 02/24/2020
Q:
How are the cosmological constants affected by the expansion of space as accelerated by dark energy? If the gravity acting between any two objects is inversely proportional to the square of the separation distance between the object's centers and that separation distance "stretches", what effect does this have on gravity, if any?
- Drew Richards (age 34)
Deland, Florida, USA
A:

The ordinary gravity between massive things does get weaker as they get farther apart. The effect of ordinary mass on the expansion of space, slowing it down, thus gets weaker the more the space expands. 

A cosmological constant is different. It corresponds to a fixed density of mass, not a fixed amount. As space expands the density doesn't change. It causes an accelerating expansion, with the acceleration proportional to the size. That gives an exponentially growing size, once the ordinary mass becomes unimportant.

(If that sounds la bit ike it violates conservation of energy, that's ok because that principle ends up looking different in General Relativity.)

Based on observations, it seems that our universe crossed from having a high enough density of ordinary mass causing slowing expansion to a low enough density for the cosmological constant to be more important roughly seven billion years ago.

Mike W.


(published on 02/24/2020)

Follow-up on this answer.