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Physicist, Startup Founder, Blogger, Dad

Monday, January 26, 2015

Astrophysical Repulsion from Dark Energy

The manifestation of dark energy on cosmological scales is well known: gravitational repulsion which leads to the accelerating expansion of the universe. Perhaps surprisingly, there are potentially observable effects on galactic length scales as well.
The Dark Force: Astrophysical Repulsion from Dark Energy (http://arxiv.org/abs/1501.05952)

Chiu Man Ho, Stephen D. H. Hsu

Dark energy (i.e., a cosmological constant) leads, in the Newtonian approximation, to a repulsive force which grows linearly with distance. We discuss possible astrophysical effects of this "dark" force. For example, the dark force overcomes the gravitational attraction from an object (e.g., dwarf galaxy) of mass $10^7 M_\odot$ at a distance of $~ 23$ kpc. It seems possible that observable velocities of bound satellites (rotation curves) could be significantly affected, and therefore used to measure the dark energy density.

2 comments:

Rastus Odinga-Odinga said...

I'm afraid I don't get it. The dark energy [if it is due to a cosmological constant] is perfectly isotropic, it cannot exert a "force" in any direction. People sometimes informally picture it as a repulsive "force" that "forces" the universe to accelerate, but that is not correct; any three-vector in an isotropic system clearly has to be zero everywhere; one can have non-trivial cosmologies in general relativity only because the field is a second-rank tensor, not a vector. One can set up an accelerating cosmology in Newtonian mechanics only by picturing the universe as a finite ball of galaxies which is *not* isotropic about any point other than the center of the ball.

Rastus Odinga-Odinga said...

Right.... in Newtonian cosmology. For example, you say: "For example, the dark force overcomes the gravitational attraction from an object (e.g., dwarf galaxy) of mass $10^7 M_\odot$ at a distance of $~ 23$ kpc. "
So what you are saying is that, at 23 kpc, there is an outward force *vector* which cancels the inward force *vector*. That is perfectly sensible in Newtonian cosmology, but not in relativistic cosmology. A perfectly homogeneous and isotropic substance cannot exert a "force" in this sense. Actually what you are proposing is a new test of GR: the effect you are describing would exist if Newtonian gravity [or something like it] were correct. So the non-observation of the effect would confirm GR....... conversely, you have a chance to disconfirm GR. Good luck :-)


I am always puzzled when people say that the discovery of the cc was a surprise, because one would expect gravitational attraction to slow down the expansion. Yes, one would expect that if one believed in Newton. One would not have any expectation either way if one understood GR....

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