This Week in the Universe: July 13th – July 19th

Why this week was so dense in astrophysics and cosmology, I have no idea.

Astrophysics and Gravitation:

Strong Gravitational Lensing

Credit: Courbin, Meylan, Djorgovski, et al., EPFL/Caltech/WMKO.

F. Courbin, M. Tewes, S. G. Djorgovski, D. Sluse, A. Mahabal, F. Rerat, & G. Meylan (2010). First case of strong gravitational lensing by a QSO : SDSS J0013+1523 at z = 0.120 Astronomy & Astrophysics arXiv: 1002.4991v2

Observations from the W. M. Keck Observatory in Hawaii have lead to the first identification of a distant galaxy seen with the aid of gravitational lensing caused by a quasar.  Previously, gravitational lensing has been used to identify massive galaxies in front of quasars, not behind, so this is something new for astronomy.  Being able to “see past” the bright light of quasars to look at objects “behind” them, could be an important new technique for astrophysics.

For more, see ScienceShot: A Quasi-Stellar Looking Glass, Astronomers Discover an Unusual Cosmic Lens, See A Quasar Gravitationally Lens a Galaxy (for the First Time!)

Cosmic Ray Sources, Not So Well Understood

Butt, Y. (2009). Beyond the myth of the supernova-remnant origin of cosmic rays Nature, 460 (7256), 701-704 DOI: 10.1038/nature08127

An interesting letter in Physics Today this month by Yousaf Butt, from the Harvard–Smithsonian Center for Astrophysics, draws attention to the fact that supernova remnants may not actually be the most likely cause for cosmic rays reaching earth (as he pointed out in his Nature paper last year that was apparently somewhat forgotten by a few of his colleagues).  Butt argues that it’s not a trivial matter to distinguish between isolated supernova remnants and superbubbles, as cosmic ray sources, and that astronomers and astrophysicists shouldn’t be so quick to label a cause when they’re lacking in confirmation.  This is an excellent thing to keep in mind, regardless of field.

For more, see Cosmic rays’ origins unclear (the Physics Today letter).

WIMPy Sun? Really?

Daniel T. Cumberbatch, Joyce. A. Guzik, Joseph Silk, L. Scott Watson, & Stephen M. West (2010). Light WIMPs in the Sun: Constraints from Helioseismology arXiv arXiv: 1005.5102v1

From the introduction:

The recently revised solar abundances result in solar models that cannot reproduce currently observed helioseismic data. In this letter we explore the role of WIMPs in modifying the thermal gradient of the Sun.

Credit: B. Rakouskas - Dark Matter ruins emission/absorption picture?

When we realise we have an observation that doesn’t fit our current theories we have two options: find something wrong with the observation or its interpretation, or, create a new theory.  Why you’d chose WIMPs to be an element in your new theory, I have no idea, but that is what they did.  Is our sun actually, in any way, made up of dark matter? I would seriously, seriously doubt it… New Scientist, on the other hand, is not as credulous as I am.  The authors weren’t able to fully model the sun with a WIMPy core (there are a huge number of parameters, you can’t blame them), but they do suggest that if it were possible to come up with a model for our sun that could contain dark matter:

…both direct detection and accelerator probes may be complemented by using the Sun as a probe of dark matter.

That’s nice, however incredibly improbable it may be.  In conclusion, the sun is more confusing than we thought (but it still probably has nothing to do with dark matter).

For more, see Heart of darkness could explain sun mysteries (New Scientist), Beneath that blazing facade.

High Hopes for LISA

Miller, J. (2010). Laboratory experiment shows that noise can be lessened for LISA Physics Today, 63 (7) DOI: 10.1063/1.3463616

Turns out, if LISA actually ever gets built, it might work.  Maybe.

Dark Energy Debate

Eugenio Bianchi, & Carlo Rovelli (2010). Why all these prejudices against a constant? arXiv arXiv: 1002.3966v3

Nature had a great piece this week on the dark energy debate – ie. do we even need dark energy?  Bianchi and Rovelli argue, quite well, that no, we do not (why add in dark energy without anything to explain it when you can get the same physics by keeping the non-vanishing cosmological constant that general relativity came with in the first place?) .  They have a very accessible arXiv piece which is well worth a read, although the Cosmology Forum in Nature, that gives Rocky Kolb’s pro dark energy stance too, is certainly better than any summary I could give here.

For more, see Cosmology forum: Is dark energy really a mystery?.

Anisotropy May Cause Breakdown of the Cosmological Principle

Attila Meszaros, Lajos G. Balazs, Zsolt Bagoly, & Peter Veres (2010). Impact on cosmology of the celestial anisotropy of the short gamma-ray bursts Baltic Astronomy, Vol.18, 293-296 (2009) arXiv: 1005.1558v1

The abstract, short and sweet:

Recently the anisotropy of the short gamma-ray bursts detected by BATSE was announced (Vavrek et al. 2008). The impact of this discovery on cosmology is discussed. It is shown that the anisotropy found may cause the breakdown of the cosmological principle.

This is actually a rather neat little paper (typesetting aside).  The authors argue that the Cosmological Principle requires the universe to be homogeneous and isotropic (in a broad sense) and recent observations of anisotropy thus suggest a breakdown in this principle.  This honestly sounds quite reasonable, except for the fact that the Cosmological Principle (ie. the fact that their are no preferred reference frames) is pretty fundamental to general relativity.  To me, the problem doesn’t quite lie with the Cosmological Principle, but lies with the interpretation of this homogeneity and isotropy requirement.  We know the universe is not homogeneous and isotropic on all levels (the existence of animals, planets, galaxies, etc. easily demonstrate this), so what is so fundamental about it being homogeneous and isotropic on a larger scale?  The authors claim this requirement only needs to hold for spacetime “larger than the size of any structure (void, filament, supercluster, etc.)”, but what about that scale is so fundamental?  As far a general relativity is concerned, nothing.  The universe is *not* homogeneous and isotropic (sorry, FLRW cosmology), so we shouldn’t feel the need to bundle that up with Cosmological Principle (no preferred reference frames).

For more, see June 2010 Notes of the Alternative Cosmology Group [pdf].

Homogeneity of the Universe  ⇒ Dark Energy?

Chris Clarkson, & Roy Maartens (2010). Inhomogeneity and the foundations of concordance cosmology Class. Quantum Grav. 27 124008 (2010) arXiv: 1005.2165v2

From the introduction:

The unresolved nature of and of alternative forms of dark energy throws into sharp focus the foundations of the standard model, in particular, the spatial homogeneity assumption.

As cosmology goes, I think this is actually pretty exciting. The authors do a great job throwing a few wrenches into the standard concordance model.  Based on the usual assumptions in modern cosmology, of a homogeneous, unexciting, spacetime, it appears that, given our current observations, dark energy is a must (different interpretation than Bianchi and Rovelli, of course).  However, and this relates to the previous article, who really says our spacetime needs to be homogeneous?  It appears that by just making our spacetime a tiny bit more complicated, adding in allowance for spatial inhomogeneity, we may be able to escape the dark energy problem entirely.

Until we demonstrate observationally that the Universe is homogeneous on large scales, we should consider inhomogeneous spacetimes even if they are philosophically uncomfortable, particularly in light of the fact that in their simplest incarnation they can explain away the dark energy problem through inhomogeneity, without apparently causing other problems.

I hope to see more along these lines in cosmology very soon.

For more, see June 2010 Notes of the Alternative Cosmology Group [pdf].

CERN Neutron Time-of-Flight to Age Galaxy

Mosconi, M., et al . (2010). Neutron physics of the Re/Os clock. I. Measurement of the (n,γ) cross sections of ^{186,187,188}Os at the CERN n_TOF facility Physical Review C, 82 (1) DOI: 10.1103/PhysRevC.82.015802

Rhenium-187, believed to be produced in the first stellar explosions in our young galaxy, and its decay parent, Osmium-187, are currently being studied by CERN to help get a better feel for the age of our galaxy.  By looking at the relative concentrations of Osmium-187 and Rhenium-187 in our galaxy (and knowing how long it really takes Osmium-187 to decay, thanks to CERN and Karlsruhe experiments), astrophysicists should be able to get a much more accurate date for the formation of our Milky Way.

For more, see Refining a Cosmic Clock: Particle Accelerator Research Helps Narrow Down the Age of Our Galaxy.

Nested Black Hole Universes?

Nikodem J. Poplawski (2010). Cosmology with torsion – an alternative to cosmic inflation arXiv arXiv: 1007.0587v1

A snippet of the abstract:

The Einstein-Cartan-Kibble-Sciama theory of gravity provides a simple scenario in early cosmology which is alternative to standard cosmic inflation and does not require scalar fields… This scenario also suggests that the contraction of our Universe preceding the state of minimum radius could correspond to the dynamics of matter inside the event horizon of a newly formed black hole existing in another universe.

I’m just going to leave this one, but I will point out that “could correspond” and “having evidence to suggest a correspondence” are two very different things.

For more, see Why Our Universe Must Have Been Born Inside a Black Hole (MIT Tech Review arXiv Blog).

High Energy Physics and Particles:

Quark Speed from the CDF

The CDF Collaboration, & T. Aaltonen (2009). Measurement of $d\sigma/dy$ of Drell-Yan $e^+e^-$ pairs in the $Z$ Mass Region from $p\bar{p}$ Collisions at $\sqrt{s}=1.96$ TeV Physics Letters arXiv: 0908.3914v4

From Fermilab:

Members of the CDF Rochester group have extracted the rapidity distribution from a sample of approximately 170,000 Z bosons decaying into positrons and electrons. The measurement confirmed that the most recent parton distribution functions accurately describe the fractional momentum distribution of quarks in the proton.

Honestly, I don’t know why this is exciting.  Everyone likes validation, I guess.

For more, see Result of the Week: Quark speed, Jiyeon Han’s PhD thesis, “The Dierential Cross Section Distribution of Drell-Yan Dielectron Pairs in the Z Boson Mass Region” [pdf].

ILC Excitement Builds

For more, see Forget the Large Hadron Collider. All hail Cern’s new, straight-line atom smasher, World lays groundwork for future linear collider, CERN: ILC to be build 2012-2019.

General Relativity, Quantum Gravity, et al.:

Hořava-Lifshitz isn’t Dead After All?

Petr Hořava & Charles M. Melby-Thompson (2010). General Covariance in Quantum Gravity at a Lifshitz Point arXiv arXiv: 1007.2410v1

Back in 2009, Hořava-Lifshitz gravity was exceptionally exciting.  Unfortunately for Petr Hořava, it didn’t take long before people found some strange inconsistencies in the theory that didn’t match up with reality.  Were these inconsistencies ever really resolved?  Somewhat (see: New formulation of Horava-Lifshitz quantum gravity as a master constraint theory and Projectable Version of Horava-Lifshitz Gravity), but doubters certainly still remained.  Two weeks ago, Petr Hořava presented his latest, “General Covariance in Quantum Gravity at a Lifshitz Point” at GR19 to mixed reviews (the paper is now out on the arXiv).  The newest formulation of Hořava-Lifshitz gravity exhibits anisotropic scaling at short distances and reproduces much of (but not all) general relativity at long distances.  Has it resolved all of the issues of the original Hořava gravity theory? It doesn’t appear so, although detailed work is still pending.

The Gravitational Self-Force is Turning Heads

Samuel E. Gralla, & Robert M. Wald (2009). Derivation of Gravitational Self-Force arXiv arXiv: 0907.0414v1

The Gravitational Self-Force, the force on a body moving through a gravitational field caused by the mass/energy of the body itself, seems to be becoming a little bit of a hot topic lately amongst some relativists (see: High-Accuracy Comparison between the Post-Newtonian and Self-Force Dynamics of Black-Hole Binaries).  Wald presented his latest (from last year) on how to address corrections to geodesic motion due to the “gravitational self-force” of a particle two weeks ago and, while honestly, it isn’t exceptionally exciting, it is very important to have figured out in order to put GR predictions on an even more accurate level.  We know that massive bodies curve spacetime and that has to apply to even our test particles, so it’s worthwhile to make sure we actually know how to handle those corrections, analytically.

For more, see Wald’s Introduction to Gravitational Self-Force.

Verlinde Style Gravity Meets Causal Dynamical Triangulation with Some Hořava-Lifshitz Thrown In?

J. Ambjorn, A. Goerlich, J. Jurkiewicz, & R. Loll (2010). CDT—an Entropic Theory of Quantum Gravity arXiv arXiv: 1007.2560v1

The article (a lecture series, really) starts off explaining why a lattice formulation, based off of causal dynamical triangulation, could lead to a promising theory of quantum gravity (a theory of the background – like CDT gives – is always a good start if you want to recover GR… maybe not if you want QFT, but that’s a different matter) and then sort of progresses briefly into an Erik Verlinde style entropic interpretation of CDT with some tie ins to Hořava-Lifshitz gravity in the end.  Other than being an interesting combination of trendy theories, I’m not sure what else this paper has to offer in terms of new insights (although if does give a good discussion on CDT).

-Sarah Kavassalis