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Marco Roncadelli: Observational Evidence for Dark Matter

March 3, 2015

(Note of the editor: my stenographic skills failed me on this talk, because the speaker sped up as he found out he was close to finishing his time, while still halfway. I offer below a rather didactical “first part” of Roncadelli’s talk, which I think is still useful as a general introduction to the topic.)

The standard way to detect dark matter is to see the influence of dark matter on luminous matter. It was realized only relatively recently that dark matter must be fundamentally different from luminous matter. It must be non baryonic.

The hypothesis of non baryonic dark matter finds candidates from high-energy particle physics. Also remarkable is the fact that it is in agreement with observational evidence for it in galaxies and clusters.

Observations have come to the conclusion that there is also a significant amount of dark energy in the universe, and we do not know what this is. Only something with negative pressure, creating accelerated cosmic expansion. Here particle physics offer no natural candidate to explain it.

There are two methods to discover DM in galaxies and clusters: a dynamical analysis, which rests on gravitational effects produced on luminous matter; and gravitational lensing.

In 1844 Bessel studied the orbit of Sirius. He discovered that its motion was not a straight line but it obscillated. It was due to Sirius B, a dark companion. As a clever mathematician Bessel was able to tell where the companion was, and a pointed observation soon found the companion. Later Adams and Le Verrier found anomalies in the orbit of Uranus, predicting the existence of Neptune.
In 1932 Oort used stars near the Sun to find local dark matter. Zwicky one year later used as tracer galaxies in Coma, and dark matter was hypothesized within that cluster. Three year later the same story happened with galaxies in the Virgo cluster.

Gravitational lensing is more recent. The curvature of space can be detected by distortions in light propagation. The strategy is to derive the lens mass from the observed properties of the images.

Strong lensing is a caustic effect. If you suppose that the lens is axially symmetric along the optical axis, then Einstein caustic is the point on the optical axis beyond the lens. You can observe a full circle around the lens mass from an object behind it.

Weak lensing is similar but here you have a cluster, and you look at galaxies close to the observational plane, at arbitrary distances. One then sees that elliptical galaxies get distorted, stretched along one direction and squeezed along the line from the center of the cluster to the galaxy. The overall effect is a net tangential polarization of the images. Since one does not know the original shape of the galaxies, one needs to perform a statistical analysis to extract the shape of the lens mass.

An important quantity is the mass to light ratio. For galaxies and clusters, Q is the total mass divided by the optical luminosity, and q is The luminous mass to light ratio. The latter can be determined from stellar evolution models, Q can be determined by observations only. Since the total mass divided by luminoyus mass M/m is measured by Q/q, the knowledge of Q yields the amount of dark matter.

Inside galaxies instead the evidence of dark matter comes from the study of spiral galaxies, where a central bulge is surrounded by a disk. A dynamical analysis studies the rotational curve. The keplerian prediction is that the rotation speed should decrease with radius; but we see a constant speed at large radii.

In April 1998 using a sample of type-1 supernovae at large redshift it was shown that the present Universe is accelerating. It was believed that cosmic deceleration would be produced by gravitational attraction, but the data showed the opposite. The negative pressure is diffuse and negligible inside galaxy clusters, so this does not invalidate the analyses leading to DM estimates.

So what is the universe made of ? We do not know. We are however led to believe that luminous matter is a very small part of it.

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