Friday, June 22, 2018

Lensing And Rotation Curve Data Consistent At One Sigma In Distant Galaxy

A new study has compared the amount of gravitational lensing observed in a galaxy 500 million light years away, with an estimate of its mass (including dark matter in a dark matter hypothesis) based upon the velocity with which stars rotate around the galaxy, and found the two measurements of galactic gravitational mass to be consistent within a one sigma margin of error (i.e. one standard deviation).

This is not inconsistent with a general relativity plus dark matter model if the distribution of the dark matter particles is not significantly constrained. But, it is also consistent with any modified gravity model in which the modification to gravity affects photons and ordinary matter in the same way (most such models do, although "massive gravity", which was already ruled out with other data, does not even in the limit as graviton mass approaches zero). The paper states the restriction on modified gravity theories as follows:
Our result implies that significant deviations from γ = 1 can only occur on scales greater than ∼2 kiloparsecs, thereby excluding alternative gravity models that produce the observed accelerated expansion of the Universe but predict γ not equal to 1 on galactic scales.
So, it doesn't actually prove that general relativity is correct at the galactic scale relative to gravity modifications as the press release report on the study claims.

Notably, this paper also contradicts a prior study from July of 2017 by Wang, et al., that concluded that rotation curve and lensing data for galaxies are inconsistent, which I recap below the fold. The contradictory paper, however, relies upon the NFW dark matter halo shape model, which many prior observations have determined is a poor description of inferred dark matter distributions actually measured (which are inferred to have an "isothermal" distribution instead, see, e.g. sources cited here), even though the NFW halo shape is what a collisionless dark matter particle model naively predicts. Indeed, reaffirming Wang (2017) the paper in Science states in the body text that:
Our current data cannot distinguish between highly concentrated dark matter, a steep stellar mass-to-light gradient or an intermediate solution, but E325 is definitely not consistent with an NFW dark matter halo and constant stellar mass-to-light ratio.
This important finding is unfortunately not mentioned in the abstract to the paper.

The editorially supplied significance statement and abstract from the new article from the journal Science are as follows:
Testing General Relativity on galaxy scales 
Einstein's theory of gravity, General Relativity (GR), has been tested precisely within the Solar System. However, it has been difficult to test GR on the scale of an individual galaxy. Collett et al. exploited a nearby gravitational lens system, in which light from a distant galaxy (the source) is bent by a foreground galaxy (the lens). Mass distribution in the lens was compared with the curvature of space-time around the lens, independently determined from the distorted image of the source. The result supports GR and eliminates some alternative theories of gravity. 
Einstein’s theory of gravity, General Relativity, has been precisely tested on Solar System scales, but the long-range nature of gravity is still poorly constrained. The nearby strong gravitational lens ESO 325-G004 provides a laboratory to probe the weak-field regime of gravity and measure the spatial curvature generated per unit mass, γ. By reconstructing the observed light profile of the lensed arcs and the observed spatially resolved stellar kinematics with a single self-consistent model, we conclude that γ = 0.97 ± 0.09 at 68% confidence. Our result is consistent with the prediction of 1 from General Relativity and provides a strong extragalactic constraint on the weak-field metric of gravity.
Thomas E. Collett, et al., "A precise extragalactic test of General Relativity." 360 (6395) Science 1342-1346 (2018) DOI: 10.1126/science.aao2469 (pay per view). Preprint available here.

Meanwhile, as the Triton Station blog points out, the Radial Acceleration Relation still holds with a single universal constant, for all galaxies, to a precision consistent with all scatter being due to errors in astronomy measurements, while a recent claim to the contrary is fundamentally flawed.

The paper from a year ago contradicting the result just reported in the journal Science is as follows: 
In cosmological N-body simulations, the baryon effects on the cold dark matter (CDM) halos can be used to solve the small scale problems in ΛCDM cosmology, such as cusp-core problem and missing satellites problem. It turns out that the resultant total density profiles (baryons plus CDM), for halos with mass ranges from dwarf galaxies to galaxy clusters, can match the observations of the rotation curves better than NFW profile. In our previous work, however, we found that such density profiles fail to match the most recent strong gravitational lensing observations. In this paper, we do the converse: we fit the most recent strong lensing observations with the predicted lensing probabilities based on the so-called (α,β,γ) double power-law profile, and use the best-fit parameters (α=3.04,β=1.39,γ=1.88) to calculate the rotation curves. We find that, at outer parts for a typical galaxy, the rotation curve calculated with our fitted density profile is much lower than observations and those based on simulations, including the NFW profile. This again verifies and strengthen the conclusions in our previous works: in ΛCDM paradigm, it is difficult to reconcile the contradictions between the observations for rotation curves and strong gravitational lensing.
Lin Wang, Da-Ming Chen, Ran Li "The total density profile of DM halos fitted from strong lensing" (July 31, 2017).

As the body text in Wang (2017) explains:
It is now well established that, whatever the manners the baryon effects are included in the collisionless CDM N-body cosmological simulations, if the resultant density profiles can match the observations of rotation curves, they cannot simultaneously predict the observations of strong gravitational lensing (under- or over-predict). And for the case of typical galaxies, the reverse is also true, namely, the SIS profile preferred by strong lensing cannot be supported by the observations of rotation curves near the centers of galaxies.

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