Gas Rich Galaxies Confirm Prediction of Modified Gravity Theory

[Jacques]

Gas Rich Galaxies Confirm Prediction of Modified Gravity Theory

Surprising comming from an astronomy professor!

What do you think about that.

Personnaly I prefer MOND, but I don't know if it explain the lensing by close galaxies of distant galaxies.

[imatfaal]

Jacques - I haven't read the article, but I know from past experience that sciencedaily can sometimes exaggerate and cherry-pick. I will try and find time, and I should recommend you do as well, to read the actual article; I won't understand half of it - perhaps you will do better, but its much better than a journo's take on the article.

[Jacques]

A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies here is a link to the paper.

Will read it also

[Widdekind]

The observed gravitational anomalies, seen in space (where theory & observation don't yet mesh), seem to vary, based on spatial scale, with one type of anomaly, on ultra-large scales (clusters, cosmic web); and, another on more middling scales (galaxies):

 

Almost everyone agrees that on scales of large galaxy clusters and up, the Universe is well described by dark matter -- dark energy theory. However, according to McGaugh this cosmology does not account well for what happens at the scales of galaxies and smaller.

 

 

 

 

If baryonic disk mass (M_b) is related to the disk velocity (V_f), then the disk material dominates its own dynamics. Ultimately, disk dynamics are not determined by, for example, the mass of the central bulge. As a disk is a highly non-spherical object, g = G M / R² may be highly inaccurate.

 

 

 

 

 

[math]\Lambda CDM[/math] predicts that [math]log(M_b) = 8.1 + 2.7 \, log(V_f)[/math], whilst observations show that [math]log(M_b) = 6 + 3.7 \, log(V_f)[/math]. These formula become equal, when [math]V_f = 10^{2.1} \approx 130 \, km/s[/math], corresponding to [math]M_b \approx 10^{13.8} \approx 60 \times 10^{12} M_{\odot}[/math]. Below that mass, the 'detection fraction', defined as the ratio of observation-to-theory, is:

 

[math]f_d \equiv \frac{10^6 \, V_f^{3.7}}{10^{8.1} \, V_f^{2.7}} \approx \frac{V_f}{130 \, km/s}[/math]

If the phenomena is not an artifact of detection, perhaps progressively more massive galaxies, with presumably stronger gravity, retain progressively more of their expected baryonic component, the lighter galaxies losing the same, to galactic winds, etc. ??

 

Edited February 26, 2011 by Widdekind

[Jacques]

As a disk is a highly non-spherical object, g = G M / R2 may be highly inaccurate.

I agree.Not sure but I suppose that [math] \Lambda CDM [/math] doesn't use that equation ?

[Widdekind]

I agree.Not sure but I suppose that [math] \Lambda CDM [/math] doesn't use that equation ?

 

Is not 'Virial Mass' calculated from [math]\frac{<v^2>}{R} \equiv \frac{G M_{vir}}{R^2}[/math] (where the velocity dispersion, and radius, are observed quantities) ?

[Jacques]

Is not 'Virial Mass' calculated from [math]\frac{<v^2>}{R} \equiv \frac{G M_{vir}}{R^2}[/math] (where the velocity dispersion, and radius, are observed quantities) ?

I cannot tell, I don't understand your question...

I am not very good in math like you seem to be, but I was thinking about how to calculate g

[math]\frac{G M}{R^2}[/math] but M is not constant if you model the galaxy disk by a disk. M will be proportionnal to [math] r^2[/math] so it will give a constant acceleration

Edited March 2, 2011 by Jacques

[Widdekind]

I cannot tell, I don't understand your question...

I am not very good in math like you seem to be, but I was thinking about how to calculate g

[math]\frac{G M}{R^2}[/math] but M is not constant if you model the galaxy disk by a disk. M will be proportionnal to [math] r^2[/math] so it will give a constant acceleration

 

Masses in circular motion, about a center of attraction, by pure geometry, are undergoing an acceleration [math]a = \frac{v^2}{r}[/math]. And, when observing distant clusters of galaxies, doppler shifts in the light from the moving galaxies gives a 'velocity dispersion', the spread in (radial, along LOS) velocities, which is assumed equivalent to the "v²" in the above equation. Then, assuming the same spherical symmetry, G M / R² is assumed to be the gravity force explaining the observed quasi-circular motions. That works well, for spheroidal, "elliptical" galaxies, but seems wildly inaccurate for a non-spherical, disk-dominated, spiral galaxy.

[swansont]

http://blogs.discovermagazine.com/cosmicvariance/2011/02/26/dark-matter-just-fine-thanks/

 

The basic problem (which we see all the time at SFN) is that the focus is too narrow. The theory can't just explain one phenomenon. It has to explain and fit over a much wider spectrum. If MOND explains gas-rich galaxies, it fails at other observations. It can't do both, which means it fails.

[FX]

Just like Dark Matter. It's such a strange Universe.

[Widdekind]

When observing distant galaxies, earth astronomers physically observe a Power Flux (W/m²), and an Angular Diameter, and a Velocity Dispersion (<v²>). Now, roughly speaking, if the distance to that distant galaxy is assumed to be D, then its actual physical radial diameter is [math]R = D \theta[/math], and its actual physical luminosity is [math]L = P \times 4 \pi D^2[/math]. So, since the inferred Virial Mass scales as the physical radius, [math]G M_{vir} \equiv \, <v^2> R \propto D[/math], then the inferred Mass-to-Light Ratio is [math]\Upsilon \equiv M/L \propto D/D^2 \propto D^{-1}[/math].

 

Now, for a given observed redshift, the inferred distance to the redshifted object scales as H^-1. So, simply assuming that the older, higher Hubble Constant values, reported in the early 20th century AD, were actually more accurate, would, superficially, seem to allow us to "concentrate" cosmic mass, and thereby "increase" the inferred cosmic matter density. But, since we see only a certain amount of light, simply assuming that "everything's closer to earth than we thought", would actually make dark matter more of a problem -- [math]\Upsilon \propto D^{-1} \propto H[/math]. So, if the Hubble Constant was 3x higher (say), Dark Matter would be 3x more prevalent, since everything would be 3x closer, and so 9x dimmer, on emission...

 

To 'solve' the DM problem, by artificially 'compacting' the cosmos, would require a HC that was 5x higher, so that the 'galactic avg mass density' [math]M_{vir} / D^3 \propto D^{-2}[/math] would be 25x higher, accounting for the difference, between observed baryonic densities, and the critical density. But, increasing the HC, increases the critical density, by the same amount -- you can't 'catch up' mathematically:

 

[math]\rho_{crit} \equiv \frac{3 H^2}{8 \pi G} \propto H^2[/math]

 

[math]\rho_{gal} \equiv \frac{M_{vir}}{D^3} \propto H^2[/math]

There seems no easy way, to 'solve' the DM problem, by adjusting the HC.