Dark matter revealed in the collision of galactic clusters

The biggest recent news in astronomy is not Pluto but the fact that dark matter is clearly revealed in collisions of galactic clusters. This point is emphasized by Sean Carroll in Cosmic Variance who gives a nice exposition of how the a combination of information from gravitational lensing and X-ray pictures of the bullet cluster helped to nail down the elusive dark matter.


These pictures depict the collisions of the bullet cluster
(1E 0657-56), moving to the right colliding with a much large cluster of galaxies. When clusters of galaxies collide, the dark matter in each shown in blue moves on unaffected while the normal matter shown in pink gets deflected due to electromagnetic interactions.

This illustration was put together by combining the optical pictures from Magellan and Hubble telescopes with the x-ray observation with the Chandra space telescope.

This is picture in optical wavelegths of the galaxies which lie behind the bullet cluster.


To the expert 'eye' there is an evident duplication and distortion in the shape of these galaxies which can only be due to the lensing of these galaxies by a unseen distribution of mass in the foreground.





Using the lensing theory one can reconstruct the distribution of the unseen matter in the foreground which turns out to be two sperical mass distribution close to the bullet cluster and its companion.



Most of the normal mattter in the galactic clusters is not in stars but in the interstellar and intergalatic gas (mainly hydrogen and helium). This gas scatters light in the form of X-rays due to its high velocity. After 140 hours of observations with the Chandra X-ray telescope this is the picture that emerges of the visible matter near the bullet cluster



Now superimposing the distribution of normal matter as seen in the X-ray observations with the dark matter distribution
as inferred from lensing, the following startling picture
emerges
(Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.)


The normal visible matter in each cluster is offset from the actual mass distribution! The dominant mass in each cluster must be in some weakly interacting form of matter which did not get deflected in a collision which is inferred to have occured 150 million years earlier.

There has long been evidence of dark matter in the galaxies- as the rotation speed of the gas in the periphery is much larger than what would be supported by the gravitational attraction of the visible matter.
One theory for explaning this anomaly was MOND- modifying Newtonian dynamics- or the theory of gravity at the distance of galaxies
instead of introducing dark matter.
With this observation MOND is ruled out.
Details can be found in this paper. They give an upper limit on the self-interaction cross section of dark mattter to be
sigma/m < 1 cm^2/gm
which also rules out the strongly self-interacting dark matter theory .

The problem is with Jupiter not Pluto

(Picture credit:IAU-Martin Kornmesser)




Lord Raleigh is supposed to have said "Physics is the only science, all the rest is stamp collecting". This normally not-to-be-taken-seriously type of remark seems to apply to the august gathering (oh! there's a pun here) of the members of IAU in Prague whose sole purpose -or so it seems- is to classify the bodies in our solar system as planets or non-planets.

One camp wants to promote Ceres (currently an asteroid but which used to be a planet in 19th century) , the newly discovered UB313 (a.k.a. Xena) and Pluto's moon Charon to the status of planets. The other camp would like to demote Pluto and lump it with these three bodies and call them all dwarf planets.

Meanwhile there is real long standing question about planets which remains unanswered. It is known that Jupiter emits 70% more light than what it gets from the sun.
What is the source of this energy? A gasseous planet must have minimum mass of 1.8 times Jupiter's mass before the hydrogen fusion can take place in its core (the planet then turns into a star). So where does Jupiter get this excess energy since it is too puny to be a star? Thats a real physics question.

The news from the IAU gathering is that the division between the two camps is right down the midddle and the issue will be settled by votes later today and Pluto's fate hangs by a thread.
Yawn...

Off the beaten track from Benasque to Eriste

The view (of the Universe) from Benasque

August is a slack season for research in southern Europe. The stagnant heat makes it difficult to sleep -even on the job, and the tony crowd heads for the mountains.
This August a conference held in the town of Benasque on the Spanish Pyrenees on Cosmology, Inflation and the CMB packed in the glitterati, not least because these subjects are hot.

Mukhanov one of the pioneers of the idea that- perturbations of the zero-point energy amplified during the exponential expansion of the universe (inflation) are responsible for the formation of structure in the universe - claimed quite forcefully that now could be proved as some sort of a mathematical theorem that quantum-gravity fluctuations need an exponential expansion to have observational consequences. I wanted to ask him why everyone blindly assumes that quantum qravity fluctuations have to be the source of structure formation in the universe. There is a well developed theory called "warm inflation" by Berera and co. where they show that thermal fluctuations during inflation can give rise to the type of perturbations which lead to formation of galaxies. So whether the primordial perturbations are of quantum or thermal origin is still an open question that should be decided by experiments. A similar question in foundational quantum mechanics -are the probabilities which we observe in quantum mechanics just classical probabilities associated with coarse graining over unobserved parameters - was resolved by a nifty experimental test proposed by John Bell (called Bell's inequality) and the experiments settled this question decisively in favour of quantum mechanics. A similar test is needed in cosmology which will enable experimentalists one day to
prove conclusively that the observed structure microwave anisotropy in the universe is of quantum mechanical origin or otherwise.
I wanted to ask Mukhanov all this but in the face of forceful authority I just nodded meekly while MEGO (My Eyes Glazed Over).

Dick Bond the guru of CMB data analysis informed the unwashed masses that the cognoscenti have jetissoned the frequentist interpretation of probabilities and switched to the Bayesian philosophy in deciding which theories fit best with experimental results. David Lyth, the grand old man of Inflation and cosmology who (literally) wrote the book, asked if this was an intermediate step when one is trying to test a large number of unmotivated theories on the basis of preliminary experimental data, and once we have a well founded theory it should be easy to check if its predictions pass through the experimental data points in the old fashioned way. The answer he got in return was a earful of statistical noise.

And finally here's a calm view of the universe from Benasque.