Why look for a new theory of gravity if the big bang cosmology is correct?

Occasionally we read in the popular press, especially online, that someone has come up with a new theory of gravity. Why is that even necessary if the current theory describing the evolution of the universe is so correct?

The standard ΛCDM big bang cosmology is derived from an application of certain non-biblical boundary conditions to the physics of Einstein’s general relativity theory. But when that was applied to the universe as a whole, two problems developed for the secular model. One is the need to add in dark energy (or the cosmological constant, Λ (Lambda), to Einstein’s field equations) and the other is the need for a significant amount of invisible cold dark matter (CDM).

On the scale of galaxies and even clusters of galaxies Newtonian physics is used as it is the low gravity limit of general relativity. But without the addition of dark matter the resulting theory, using the known density of visible matter in galaxies (see Fig. 1) and clusters, does not match observations. But for more than 40 years now dark matter has been sought in various lab experiments with consistently negative results. This has developed into what is called the dark matter crisis.1


Figure 1: Typical rotation curve of a spiral galaxy: Speeds (V) in km/s units as a function of distance from the centre of the galaxy (R) in 1000 light-year (ly) units. The upper curve shows the speeds of the stars in disk region determined from their visible light and the gasses beyond that determined from radio frequency emissions. The lower curve shows what standard Newtonian physics predicts should be observed. The discrepancy is made up by positing the existence of invisible dark matter. Credit: Wikipedia

Occasionally a claim is made that a theorist has some inkling of what dark matter particles might be but the crisis remains.2 Dark matter particles have been sought without success in the Galaxy using very sensitive detectors deep in underground mines,3 or with the Large Hadron Collider (LHC) over 10 years of experiments looking for the lowest mass stable particle in a theorised class of as-yet-undiscovered supersymmetric particles.4

The observational data from thousands of galaxies together with the negative outcome of all the experiments searching for Dark Matter particles indicate that either something is wrong with the physics we use or that the expected dark matter is much more elusive than supposed, or, indeed, does not, in fact, exist—which gets us back to something being wrong with the physics. Continue reading

Why should baryons define where the dark matter is? Another dark matter problem

A research paper1 recently accepted for publication in Physical Review Letters titled “The radial acceleration relation in rotationally supported galaxies”2 highlights a discovery that is bad news for dark matter. It certainly does not strengthen the case for halo dark matter around spiral galaxies.

The research team, McGaugh et al, took data for 153 spiral disk galaxies from the Spitzer Photometry and Accurate Rotation Curves (SPARC) database that represents spiral galaxies of all types and morphologies, from very bright to very low surface brightness disks. It included representative spiral galaxies that would be assumed to contain a very high fraction of dark matter at very low orbital accelerations to those with very little dark matter at high orbital accelerations. These galaxies are all assumed to be rotationally supported, which means their disks are assumed to be gravitationally bound by the included matter inside any radial distance (R) from the centre of the galaxy. The speeds of the stars and gases (V) as a function of their measured radial distance (R) determines what is known as a rotation curve V(R). See Fig. 1.

In this paper the observed acceleration, gobs, at each radial distance R from the centre of the chosen galaxies, was calculated from the measured values of V(R) and R for each galaxy, totalling 2693 data points over the 153 galaxies. Also using infrared data the mass density was accurately measured at these same radial points, which permitted, via the Poisson equation, a direct calculation of the expected acceleration, gbar, due to the baryonic matter (protons and neutrons, i.e. normal matter) content within the same galaxies. No free fit parameters were used in these estimations, except a single fixed Mass-to-Light ratio of 0.5 was used across all galaxies.


Figure 1: Examples of mass models and rotation curves for individual galaxies. The points with error bars in the upper panels are the observed rotation curves V (R). The errors represent both random errors and systematic uncertainty in the circular velocity due to asymmetry in the velocity field. Each baryonic component is represented: dotted lines for the gas, dashed lines for the stellar disk, and dash-dotted lines for the bulge, when present. The sum of these components is the baryonic mass model (solid line). The lower panels illustrate the run of gbar and gobs for each galaxy, with the dashed line being the line of unity. Note that higher accelerations occur at smaller radii. From left to right each line is replotted in gray to illustrate how progressively fainter galaxies probe progressively lower regimes of acceleration.

Assuming standard Newtonian (or Keplerian) physics the acceleration due to the baryonic matter, gbar, is all we should need to correctly calculate the rotation curve of any galaxy. See Fig. 1 (which reproduces their Fig. 2). Some representative rotation curves are shown by the upper-most black circles with error bars. Quite obviously the solid blue lines—the expect rotation velocities due to the observed baryonic matter—do not follow the observed rotation curves, but fall well below, in most galaxies. This is the reason halo dark matter is invoked. See Fig. 2. Continue reading