# Modification of gravity

1. Jun 26, 2015

### wolram

The only modification theory i know of is Mond, this paper suggests that a modification of gravity is needed to explain unexpected radial velocities, so other than Mond what other theories are there?

arXiv:1506.07569 [pdf, other]
Dynamical History of the Local Group in LCDM
Indranil Banik, Hongsheng Zhao
Comments: 17 pages, 12 figures, 5 tables. Submitted to Monthly Notices of the Royal Astronomical Society in this form
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

The positions and velocities of galaxies in the Local Group (LG) measure the gravitational field within it. This is mostly due to the Milky Way (MW) and Andromeda (M31). We constrain their masses using a sample of 32 galaxies with measured distances and radial velocities (RVs). To do this, we follow the trajectories of several thousand simulated particles on a pure Hubble flow from redshift 9. For each observed galaxy, we obtain a trajectory which today is at the same position. Its final velocity is the model prediction for the velocity of that galaxy. We carefully consider the impact of tides raised by objects outside the LG. We directly include Centaurus A and try to account for IC 342 and M81. With our analysis, the total LG mass is $4.33^{+0.37}_{-0.32} \times {10}^{12} M_\odot$, with $0.20^{+0.05}_{-0}$ of this being in the MW. However, no plausible set of initial conditions yields a good match to the RVs of our sample of LG galaxies. We introduce a parameter $\sigma_{extra}$ to quantify the typical disagreement between observed RVs and those predicted by the best-fitting model. We find that $\sigma_{extra} \approx 45^{+7}_{-5}$ km/s. This seems too high to explain as a result of interactions between LG dwarf galaxies. We suggest that the observations may be explained by a past close flyby of the MW and M31, which arises in some modified gravity theories due to a shorter orbital period. Gravitational slingshot encounters of material in the LG with either of these massive fast-moving galaxies could plausibly explain why some non-satellite LG galaxies are racing away from the LG even faster than a pure Hubble flow (e.g. DDO 99, 125 and 190). A modification to gravity might also explain why some galaxies have RVs substantially below our model predictions.

2. Jun 26, 2015

### Garth

I'll read and consider this paper in detail; but my first question would be, "Can this anomaly be explained by inter-galactic DM?" (i.e. in the LG cluster halo)

Garth

3. Jun 26, 2015

### Chalnoth

From a quick once-over, I don't see any discussion of the density profile of the galaxies.

4. Jun 27, 2015

### haushofer

You could also look for models of Massive Gravity, in which the graviton acquires a mass. Cosmologically this can be interesting (although highly constrained), because with a mass the graviton would obey a 1/r-law, which causes the gravitational atraction to become smaller with distance. This opens up the possibility of accelerated expansion, as I understand it.

But modifications of GR are highly constrained by both theory and experiment. See e.g. Hinterbichler's notes on massive gravity.

5. Jun 27, 2015

### wolram

Your opinion will be highly appreciated, report back to me when you have considered this paper

6. Jul 4, 2015

### Garth

Hi wolram, I should have replied sooner.

They do model for the DM halos of each galaxy but I understand that each galactic cluster is gravitationally bound by a DM cluster halo, so there might be much DM between each galaxies that could perturb their trajectories - which might be the explanation of this anomaly.

Garth