Dark matter... finally an evidence! Or not?

On August 21 there was an announcement regarding Dark Matter (DM) detection evidence from weak lensing observations of a known cluster merger (astro-ph/0608407). Douglas Clowe et al claim that the collision of two clusters, at z=0296, separates the dissipationless stellar and the X-ray emitting plasma components. The gravitational lensing maps shows clearly that the gravitational potential does not trace the plasma distribution, as can be seen from the picture. The X-ray images were made from Chandra data and the optical ones are a combination of VLT and HST/ACS in different bands (R, B, V).

What are the consequences of this claim? First of all, it is interesting to pay attention to the claim that the deformed structure detected does not depend on the (standard) cosmology adopted: Omega_M = 0.3, Omega_Lambda=0.7 and H0=70 km/s.Mpc. If the cosmology is changed, the distance and absolute masses will change, but not the observed structures, since the relative mass of the various structures do not change. The second claim is that the lensing cannot be explained by any alternative gravity model where the force scales with baryonic matter. In their words, "The lensing peaks require unseen matter concentrations that are more massive than and offset from the plasma."

So, what is measured is the spatial separation of baryonic matter from hypothesized DM during a cluster merger. Then, the hypothesis of DM is compared to visible matter+modified gravity (e.g., MOND), and the conclusion is that the observed displacement between the baryonic bulk and the gravitational well proves the presence of dark matter, under the assumption of "usual" gravity behavior.

For tomorrow, I'll comment the possibility of absence of cosmic acceleration (Middleditch, astro-ph/0608386) due to a flaw in the SN Ia explosion model paradigm (explosion of a degenerate core of a white dwarf under accretion of matter in a binary system).

I'll call for the day!

What is a planet, after all?

It's been a few months since I posted, actually almost exactly 7 months... Let's see if I can keep it going now. Meanwhile, my science life has changed quite a while, with a much greater drive to astrobiology than for cosmology. And this, particularly the discussion about planets, is what brought me back.

You all may have heard that Pluto is risking to be disconsidered being a planet after the IAU Meeting that is taking place in Praga right now. In August 22 there will be a voting session, conducted by an IAU Commission, called the Planet Definition Committee, to decide, among other things, the statuts of Pluto as a planet, after the discovery of some other cousins with comparable dimensions and as large a period as Pluto.

Naturally, such discussions involve not only scientific but also social, historical and even philosophical aspects. For now, we accept the following criteria to define a planet:

1. Planets must have masses above 5E20 kg
2. Planets must have diameters above 800 km
3. Planets must be spheroids (coming as a consequence of hidrostatic equilibrium in the presence of self-gravity)
4. Planets orbit stars, being neither another star or a satellite of another object

What about binary or multiple planets and objects? Are they different from a planet and its satellites? If the primary (most massive) object satisfies the above criteria, it will be a planet. For the secondary or others, if the baricenter of the considered objects reside outside the primary, the secondary will be also considered as a planet and it will be a binary (or multiple) system. Otherwise, the secondary (less massive) will be a satellite.

We recognize now the eight classical planets, which move in almost circular orbits close to the ecliptic plane, and other planetary objects, such as asteroids, comets and the so-called TransNetunian Objects (TNO). From the above 4 definitions, the asteroid Ceres, for instance, can be considered a planet but, for historical reasons, it may be referred to as a "dwarf planet", to distinguish it from the classical ones. So, calling Ceres an asteroid is not technically correct any longer...

The TNOs are now being called "plutons" and include, besides Pluto and Charon, candidates such as Sedna, Quaoar, Orcus, Varuna, 2003 EL61 and 2003 UB313, all of them with dimensions similar (0.5 to 2 times) to Pluto, round and orbiting the Sun.

To close this post, all non-planet objects orbiting the Sun are now called "Small Solar System Bodies", as opposed to the old nomenclature: "Minor Planets".

Good to be back to this place.