# If dark energy is accelerating the expansion of the universe, why can't I feel it?

Based on the Lambda-CDM cosmological model, our universe is not only expanding, but is accelerating in its expansion. However, the Equivalence Principle would suggest that inertia manifests itself in non-inertial reference frames as a pseudo-force, a body force similar to gravity but anti-parallel to the direction of acceleration. If this is in fact the case—with seemingly no reference frame being truly un-accelerated (due to the expansion of the universe)—why can't I feel it? Is it because the acceleration is too weak? Or is it because I have never not known the presence of this pseudo-force (so I am just used to it)? And if there is a small apparent force, what direction is it in given the isotropy of the expansion?

• You need to be patient, in the future you may feel it: arxiv.org/abs/astro-ph/0302506 "The phantom energy rips apart the Milky Way, solar system, Earth, and ultimately the molecules, atoms, nuclei, and nucleons of which we are composed, before the death of the Universe in a Big Rip''." – Count Iblis Jul 16 '14 at 3:43
• @CountIblis So are you saying that even the acceleration is not constant (jerk $\not=0$)? – Bryson S. Jul 16 '14 at 3:46
• You know, I was about to flag that comment as offensive, until I realized you meant the time rate of change of acceleration :) – 299792458 Jul 16 '14 at 6:28
• @New_new_newbie Thank you for not reflexively flagging it!!! Our language definitely provides ample opportunity for misunderstanding – Bryson S. Jul 16 '14 at 11:09

Is it because the acceleration is too weak?

It is too weak with respect to the four forces we measure. The fact that the four known forces are so much stronger means that agglomerates of particles, up to the scale of galaxies are not internally affected, they keep their structure intact, like the famous raisins in the rising bread. It is only at the level of clusters of galaxies that the expansion and the acceleration can be observed.

And if there is a small apparent force, what direction is it in?

A cluster of galaxies sees expansion in all three space dimensions. The balloon surface analogy, blowing up the balloon with a gnat on it, might give an insight. The gnat sees the surface expanding away from it in both surface directions.

• This addresses the expansion, but not the effects of the acceleration. Does the equivalence principle no longer apply? – Bryson S. Jul 16 '14 at 3:43
• @BrysonS. The equivalence principle still applies. If the expansion were large enough for you to feel it, but not large enough to kill you, the effect would be feeling yourself uniformly stretched out in every direction from your center of mass- you would not feel a net acceleration in one direction. – Logan R. Kearsley Jul 16 '14 at 5:54
• @Logan R. Kearsley But when I am in a car or airplane that is accelerating I do feel it and can sense its direction, so why should the universe as a whole be any different? Remember, I am focusing on the acceleration, not just the expansion. – Bryson S. Jul 16 '14 at 11:06
• The acceleration comes from all around each point, not from one direction as when you are in a plane. It is weak enough so that it does not pull clusters apart or galaxies or planets or atoms. – anna v Jul 16 '14 at 12:17
• Yes, from each point (x,y,z,t) in every direction. Please note that if it were not for the cohesive forces (gravity, nuclear electromagnetic) we would not be able to measure expansion anyway! It is the fact that our local surroundings do not change that we can see everything else expanding and an accelerated expansion in our time frame. – anna v Jul 16 '14 at 13:57