So this is the first “This Week in the Universe” post for the PLoG which means that it is a slightly longer “week” than normal (I lost some days in the switch). This series aims to look at the papers and results that are being discussed by researchers in astrophysics, gravitation, cosmology, high energy and particle physics, general relativity and quantum gravity.
Astrophysics and Gravitation:
Lisa J. Kewley, David Rupke, H. Jabran Zahid, Margaret J. Geller, & Elizabeth J. Barton (2010). Metallicity Gradients and Gas Flows in Galaxy Pairs arXiv DOI: 1008.2204
Astrophysicists had suspected that galaxies must have some mechanism for mixing metal-poor gas from their outskirts with metal-rich gas in their centres to explain the amount of star formation that seemed to occurs. Now, a team at the University of Hawaii, Honolulu, has observed this process in eight spiral galaxies that have a close “neighbour” galaxy. It appears that interacting galaxies have an easier time of sharing the heavy metals around than galaxies that are alone.
From the authors:
These observations provide the first solid evidence that metallicity gradients in interacting galaxies are systematically different from metallicity gradients in isolated spiral galaxies. Our results suggest that there is a strong relationship between metallicity gradients and the gas dynamics in galaxy interactions and mergers.
Why it occurs, is still unknown, but it is interesting.
For more, see Galactic Collisions Spread the Wealth.
Cooking with Supermassive Black Holes
Mayer L, Kazantzidis S, Escala A, & Callegari S (2010). Direct formation of supermassive black holes via multi-scale gas inflows in galaxy mergers. Nature, 466 (7310), 1082-4 PMID: 20740009
From the abstract:
[W]e report simulations showing that mergers between massive protogalaxies naturally produce the conditions for direct collapse into a supermassive black hole with no need to suppress cooling and star formation. Merger-driven gas inflows give rise to an unstable, massive nuclear gas disk of a few billion solar masses, which funnels more than 108 solar masses of gas to a sub-parsec-scale gas cloud in only 100,000 years. The cloud undergoes gravitational collapse, which eventually leads to the formation of a massive black hole.
If observations of super massive black holes at the centres of very old galaxies are correct, it would imply that they are able to form faster than current theory allows. The authors of this paper have preformed a simulation, under different initial conditions than previously considered, that does in fact predict supermassive black hole formation directly from the collapse of dense gas clouds. While this fits with observation (sort of), it is somewhat unexpected as it hadn’t previously been thought that gas clouds could be dense/massive enough to lead to a supermassive black hole.
Probing Dark Matter with Black Holes?
Mikhail Gorchtein, Stefano Profumo, & Lorenzo Ubaldi (2010). Probing Dark Matter with AGN Jets arXiv arXiv: 1008.2230v1
From the abstract:
We study the possibility of detecting a signature of particle dark matter in the spectrum of gamma-ray photons from active galactic nuclei (AGNs) resulting from the scattering of high-energy particles in the AGN jet off of dark matter particles.
In summary, if dark matter exists as a certain type of particle and if we become able to observe and predict active galactic nuclei (really, the particle jets from supermassive black holes) accurately, we might be able to infer the existence of said dark matter particles from the behaviour of the black hole particle jets.
For more, see Black Holes + Dark Matter = Light (New Scientist).
High Energy Physics and Particles:
A Changing Fine-Structure Constant?
J. K. Webb, J. A. King, M. T. Murphy, V. V. Flambaum, R. F. Carswell, & M. B. Bainbridge (2010). Evidence for spatial variation of the fine structure constant arXiv arXiv: 1008.3907v1
From the abstract:
We previously reported observations of quasar spectra from the Keck telescope suggesting a smaller value of the fine structure constant, alpha, at high redshift. A new sample of 153 measurements from the ESO Very Large Telescope (VLT), probing a different direction in the universe, also depends on redshift, but in the opposite sense, that is, alpha appears on average to be larger in the past.
This is really the hot topic for the week (and the previous week): further observations of a changing fine-structure constant (α)! I probably should have included it with “astrophysics”, but its implications for high energy physics are too important to ignore, as it characterizes the strength of electromagnetic interactions. Over the past decade, there has been a body of work building up arguing for and against, both observational and theoretical, suggestions that the fine-structure constant is not really constant.
The fine-structure constant helps to quantify how tightly electrons can hold on to atoms, so if it had been different earlier in the life of our universe, we should be able to see it in the frequencies that atoms were emitting/absorbing light. In 1999, Webb and the Keck Telescope team in Hawaii noted the first observation that α might not be so constant. Since then, many contradictory results have appeared. The latest results from Keck are quite the puzzle though. While previous observations have suggested α may change temporally, Webb’s latest observations suggest that it also has a spatial variation – ie. looking back in time in one direction sees a smaller fine-structure constant and looking back in time in the opposite direction sees a larger fine-structure constant. This is just baffling. While there currently isn’t any evidence to suggest some systematic error leading to these results, there is also no good theoretical reason to explain them either.
It is possible that we just happen to be in this happy minima in our universe where the fine-structure constant works just perfect for EM-interactions leading to matter and life… but it’s also possible that this is just all wrong.
For more, see Ye cannae change the laws of physics.
General Relativity, Quantum Gravity, et al.:
What is the Strength of Gravity?
Harold V. Parks, & James E. Faller (2010). A Simple Pendulum Determination of the Gravitational Constant Phys. Rev. Let arXiv: 1008.3203v2
Getting a lot of discussion this week is the latest “G” measurement from Parks and Faller. They measured the gravitational constant to be 6.67234 × 10−11 m3 kg−1 s−2, with an uncertainty of 21 p.p.m., which happens to be 10 standard deviations lower than the last good measurement in 2000 by Jens Gundlach and Stephen Merkowitz who obtained G = 6.674215 × 10−11 m3 kg−1 s−2, with an uncertainty of 14 p.p.m.. How on earth can this happen? No one believes that G is changing, but why can’t we get a consistent measurement? This is a serious and strange question, and one that I don’t have a good answer to.
String Theory, Experimentally Testable Today?
L. Borsten, D. Dahanayake, M. J. Duff, A. Marrani, & W. Rubens (2010). Four-qubit entanglement from string theory Physical Review Letters arXiv: 1005.4915v2
The abstract, short and sweet:
We invoke the black hole/qubit correspondence to derive the classification of four-qubit entanglement. The U-duality orbits resulting from timelike reduction of string theory from D=4 to D=3 yield 31 entanglement families, which reduce to nine up to permutation of the four qubits.
Basically, the authors are able to make some new predictions about four-qubit entangled systems using black hole/qubit correspondence (via string theory), that can be experimentally tested. From author Mike Duff:
If experiments prove that our predictions about quantum entanglement are correct, this will demonstrate that string theory ‘works’ to predict the behaviour of entangled quantum systems.
This isn’t a “if our predictions are correct about entanglement, string theory is correct” statement, this is a “if our predictions are correct about entanglement, black hole/qubit correspondence from string theory is useful” statement.