This Week in the Universe: September 2nd – September 8th

Astrophysics and Gravitation:

What Supernova 1987A Left Behind

France, K., McCray, R., Heng, K., Kirshner, R., Challis, P., Bouchet, P., Crotts, A., Dwek, E., Fransson, C., Garnavich, P., Larsson, J., Lawrence, S., Lundqvist, P., Panagia, N., Pun, C., Smith, N., Sollerman, J., Sonneborn, G., Stocke, J., Wang, L., & Wheeler, J. (2010). Observing Supernova 1987A with the Refurbished Hubble Space Telescope Science DOI: 10.1126/science.1192134

Hubble Video: Supernova 1987A Observed by Hubble from 1994 – 2006

Old data from the Hubble Telescope is shedding new light on the remnants of Supernova 1987A.  It appears that shock waves of gas that were/are being sent out by the supernova core are brightening the ring shaped cloud of dust surrounding it.  It may not add anything critical to our understanding of gravitational collapse, but it makes for pretty visuals.

For more, see ‘Lost years’ end for backyard supernova.

Seeding Magnetic Fields in Spacetime?

Mahajan, S., & Yoshida, Z. (2010). Twisting Space-Time: Relativistic Origin of Seed Magnetic Field and Vorticity Physical Review Letters, 105 (9) DOI: 10.1103/PhysRevLett.105.095005

The Abstract:

We demonstrate that a purely ideal mechanism, originating in the space-time distortion caused by the demands of special relativity, can break the topological constraint (leading to helicity conservation) that would forbid the emergence of a magnetic field (a generalized vorticity) in an ideal nonrelativistic dynamics. The new mechanism, arising from the interaction between the inhomogeneous flow fields and inhomogeneous entropy, is universal and can provide a finite seed even for mildly relativistic flows.

I have to say, this abstract doesn’t make a lot of sense to me, as written.  There is no “spacetime distortion” caused by special relativity, so I’m not entirely sure I understand their meaning here, but let’s address the subject at hand.  In 1979, E.N. Parker wrote a book called “Cosmical Magnetic Fields: Their Origin and Their Activity” in which addressed the issue of, as the title would suggest, cosmic magnetic fields, specifically the generation of said magnetic fields within astronomical bodies.  Then, in 2002, Lawrence Widrow wrote a brief follow up, of sorts, called “Origin of galactic and extragalactic magnetic fields“, where he talks about:

A variety of observations suggest that magnetic fields are present in all galaxies and galaxy clusters… However, fundamental questions concerning the nature of the dynamo as well as the origin of the seed fields necessary to prime it remain unclear.

Finally, in 2008, a fairly comprehensive review paper was written by Kulsrud and Zweibel called, “On the origin of cosmic magnetic fields“, which ends with,

Our conclusion as to the most likely origin of cosmic magnetic fields is that they are first produced at moderate field strengths by primordial mechanisms and then changed and their strength increased to their present value and structure by a galactic disc dynamo. The primordial mechanisms have not yet been seriously developed, and this preliminary amplification of the magnetic fields is still very open.

So this basically brings us to 2010.  In all of the early work on cosmic magnetic field origins, “nonideal mechanisms”, like the baroclinic effect (apparently…), were used to suggest plausible creation or “seed” scenarios for these magnetic fields.  However, the authors of this current paper take a different approach:

[Generalized vorticity] can be generated in strictly ideal dynamics, as long as the dynamics is explicitly embedded in the space-time dictated by the demands of special relativity. The generalized vorticity is, then, generated through a source term born out of the special relativistic ‘‘modifications’’ to the interaction of an inhomogeneous flow with inhomogeneous entropy.

The authors are talking about vorticity instead of magnetism directly because of the “mathematical and dynamical similarity between magnetic fields and fluid vorticity” (it might be a good strategy).  So what are these modifications? Basically, they’re just adding in special relativity to fluid dynamics and drawing an analogy with magnetism.  This has nothing to do with any spacetime distortion (because “spacetime” as a concept only exists in special and general relativity, anyway… so adding special relativity doesn’t “distort” it) or “topology” (the word does appear once in the body of the paper though).

For more, see Origin of magnetic fields may lie in special relativity’s spacetime distortions.

Static Universes Never Die

Well they don’t.

David F. Crawford (2010). Observational evidence favours a static universe arXiv : 1009.0953

Oh good, because enough people aren’t writing about “tired-light cosmology” and “static-universes” these days.  At 42 pages, this bad boy manages to reinterpret all cosmological observations that disprove tired-light and static universe models all the while solving the solar neutrino problem! Happy Day, turns out 1930s astrophysics was right! Oh wait… no, this is just rehashing things that were rejected a long, long time ago.

For more, see Non-Expanding Cosmology Attempts To Oust Big Bang Theory.

High Energy Physics and Particles:

I guess it was a slow week in high energy physics, so I’ll just direct you to high energy physicist Jon Butterworth’s  new blog, “Life and Physics“, hosted by The Guardian.  For more, see: Peter Higgs, UCL and the Right Honorable William Waldegrave.

General Relativity, Quantum Gravity, et al.:

Early Universe was Chaotic?

Katrin Gelfert, & Adilson E. Motter (2010). (Non)Invariance of Dynamical Quantities for Orbit Equivalent Flows Communications in Mathematical Physics : 10.1007/s00220-010-1120-x

Adilson Motter:

According to the classical theory of general relativity, the early universe experienced infinitely many oscillations between contracting and expanding directions… This could mean that the early evolution of the universe, though not necessarily its current state, depended very sensitively on the initial conditions set by the big bang… [W]e have established the conditions under which the indicators of chaos are relativistic invariants.

Sounds good.

For more, see Researchers show that the big bang was followed by chaos.

Spacetime is 2D… on Quantum Gravity Scales

Steven Carlip (2010). The Small Scale Structure of Spacetime arXiv arXiv: 1009.1136v1

It’s generally believed by theorists that, on small scales, spacetime is effectively two-dimensional.  Why this is, is still mystery, like much of quantum gravity, but the issues are starting to become better understood.

From the abstract:

I summarize the evidence for such “spontaneous dimensional reduction,” and suggest an additional argument coming from the strong-coupling limit of the Wheeler-DeWitt equation. If this description proves to be correct, it suggests a fascinating relationship between small-scale quantum spacetime and the behavior of cosmologies near an asymptotically silent singularity.

It’s a very good review paper (plus some new insights), so it’s worth the read.

For more, see Why Spacetime on the Tiniest Scale May Be Two-Dimensional.

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One Response to This Week in the Universe: September 2nd – September 8th

  1. John Rennie says:

    Sara,

    Thanks for the good summaries, and I have two naive questions concerning Carlip’s paper on 2-D spacetime.

    First, string theorists often talk about strings vibrating through larger numbers of dimensions than we commonly see—11 is the usual number, no? So is that conception of the universe at odds with what Carlip is writing about? Or do they reconcile in a way that’s non-obvious (to me, anyway).

    Second, if I’m not misremembering another lay discussion of Carlip’s paper, I think it was mentioned that of these two dimensions, one was presumably of space and one was of time. Is that the case? Do physicists talk about multiple dimensions of time these days?

    Thanks for any illumination you can offer!