* Astronomy

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RE: Cosmic Strings

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Title: On the Number of Cosmic Strings in Dark Energy Cosmologies
Authors: Rosa Consiglio, Olga Sazhina, Giuseppe Longo, Mikhail Sazhin, Franco Pezzella

The number of cosmic strings in the observable universe is relevant in determining the probability to detect the presence of cosmic strings through their gravitational signatures. In particular, we refer to the observation of gravitational lensing events and anisotropy in the CMB radiation induced by cosmic strings. In this paper a simple method is adopted to obtain an approximate estimate of the number of segments of cosmic strings, crossing the particle horizon, which fall inside the visible (i. e. observed) part of the universe. We show that a different choice of the CDM cosmological model, which affects volumes slightly, does actually have only a small weight on the expected number of cosmic string segments

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Title: Extremely High Energy Neutrinos from Cosmic Strings
Authors: Veniamin Berezinsky, Eray Sabancilar, Alexander Vilenkin

Superstring theory and other supersymmetric theories predict the existence of relatively light, weakly interacting scalar particles, called moduli, with a universal form of coupling to matter. Such particles can be emitted from cusps of cosmic strings, where extremely large Lorentz factors are achieved momentarily. Highly boosted modulus bursts emanating from cusps subsequently decay into gluons, they generate parton cascades which in turn produce large numbers of pions and then neutrinos. Due to very large Lorentz factors, extremely high energy neutrinos, up to the Planck scale and above, are produced. For some model parameters, the predicted flux of neutrinos with energies \gtrsim 10^{21} eV is observable by JEM-EUSO and by the future large radio detectors LOFAR and SKA.

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Title: Are GRB 090423-like bursts from the cosmic superconducting strings?
Authors: Yu Wang, Yi-Zhong Fan, Da-Ming Wei

Superconducting cosmic strings may be able to power energetic gamma-ray bursts, in particular at high-redshifts (z>6). The ultra-relativistic outflows driven by the superconducting cosmic strings, however, seem too narrow to be consistent with the afterglow data of GRB 090423 and GRB 080913.

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Title: Can Primordial Magnetic Fields Seeded by Electroweak Strings Cause an Alignment of Quasar Axes on Cosmological Scales?
Authors: Robert Poltis and Dejan Stojkovic

The decay of nontopological electroweak strings may leave an observable imprint in the Universe today in the form of primordial magnetic fields. Protogalaxies preferentially tend to form with their axis of rotation parallel to an external magnetic field, and, moreover, an external magnetic field produces torque which tends to align the galaxy axis with the magnetic field. We demonstrate that the shape of a magnetic field left over from two looped electroweak strings can explain the observed nontrivial alignment of quasar polarization vectors and make predictions for future observations.

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Title: The 21 cm Signature of Cosmic String Wakes
Authors: Robert H. Brandenberger, Rebecca J. Danos, Oscar F Hernandez, Gilbert P. Holder (McGill University)

We discuss the signature of a cosmic string wake in 21cm redshift surveys. Since 21cm surveys probe higher redshifts than optical large-scale structure surveys, the signatures of cosmic strings are more manifest in 21cm maps than they are in optical galaxy surveys. We find that, provided the tension of the cosmic string exceeds a critical value (which depends on both the redshift when the string wake is created and the redshift of observation), a cosmic string wake will generate an emission signal with a brightness temperature which approaches a limiting value which at a redshift of z + 1 = 30 is close to 400 mK in the limit of large string tension. The signal will have a specific signature in position space: the excess 21cm radiation will be confined to a wedge-shaped region whose tip corresponds to the position of the string, whose planar dimensions are set by the planar dimensions of the string wake, and whose thickness (in redshift direction) depends on the string tension. For wakes created at z_i + 1 = 10^3, then at a redshift of z + 1 = 30 the critical value of the string tension \mu is G \mu = 6 x 10^{-7}, and it decreases linearly with redshift (for wakes created at the time of equal matter and radiation, the critical value is a factor of two lower at the same redshift). For smaller tensions, cosmic strings lead to an observable absorption signal with the same wedge geometry.

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Did wobbly cosmic strings create huge explosions?

Cosmic strings - imperfections in space-time - may be to blame for two unusual bursts of gamma rays in the early universe.
Gamma-ray bursts come in two types. Long-lived, high-intensity bursts are thought to originate from collapsing stars, while shorter, dimmer ones are emitted when neutron stars merge with one another or with black holes.
In 2008 and 2009 two unusual bursts were seen, both short-lived but with the brightness of long bursts. Both occurred within a billion years of the big bang.
K. S. Cheng of the University of Hong Kong and colleagues say the culprit could be cosmic strings - defects in space-time, like streaks in clear plastic, that formed as the early universe cooled.
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Title: High Redshift Gamma-Ray Bursts: Observational Signatures of Superconducting Cosmic Strings?
Authors: K. S. Cheng, Yun-Wei Yu, T. Harko

The high-redshift gamma-ray bursts (GRBs), GRBs 080913 and 090423, challenge the conventional GRB progenitor models by their short durations, typical for short GRBs, and their high energy releases, typical for long GRBs. Meanwhile, the GRB rate inferred from high-redshift GRBs also remarkably exceeds the prediction of the collapsar model, with an ordinary star formation history. We show that all these contradictions could be eliminated naturally, if we ascribe some high-redshift GRBs to electromagnetic bursts of superconducting cosmic strings. High-redshift GRBs could become a reasonable way to test the superconducting cosmic string model, because the event rate of cosmic string bursts increases rapidly with increasing redshifts, whereas the collapsar rate decreases.

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Baffling quasar alignment hints at cosmic strings

Something has made neighbouring quasars in the distant universe point in a similar direction when their orientations ought to be random. Could this be the signature of cosmic strings - gigantic kinks in the fabric of space-time?
In 2005, Damien Hutsemekers at the University of Liège, Belgium, and colleagues reported an unusual effect in observations of 355 quasars. They found that light from these quasars tended to be polarised, with the electromagnetic oscillations confined to a particular plane that can be described by a polarisation vector. Though there is no obvious reason to think these vectors should be oriented in a special way from one quasar to the next, Hutsemekers's team found that the orientations were not random. If they took any two adjacent quasars, the polarisation vectors pointed in much the same direction.
What's more, as the team looked at ever more distant quasars, they saw this vector rotate by about 30 degrees with every 3.26 billion light years from Earth. The vector turned clockwise when they looked in the direction of the north galactic pole of the Milky Way and anticlockwise looking towards the south pole.
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Space-time should have universe-sized snags called cosmic strings running across it, but none have yet been found. That could be because they broke into a tangle of smaller strings and beads soon after the big bang. The good news is that this would have created gravitational waves that could be detected on Earth.
Many theories predict the existence of cosmic strings. These topological defects in space-time, which can be larger than the observable universe, should have formed as the cosmos cooled from its hot beginnings. The imprint of their extremely high gravity was expected to be seen in the cosmic microwave background - the radiation left over from the big bang - or as gravitational lenses that bend distant light towards us. But no convincing evidence has been seen.

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