Cosmic string: Difference between revisions

Revision as of 16:01, 30 November 2023

Revision as of 15:18, 3 December 2023

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A piece of evidence supporting cosmic string theory is a phenomenon noticed in observations of the "double [[quasar]]" called [[Twin Quasar|Q0957+561A,B]]. Originally discovered by [[Dennis Walsh]], Bob Carswell, and [[Ray Weymann]] in 1979, the double image of this quasar is caused by a galaxy positioned between it and the Earth. The [[gravitational lens]] effect of this intermediate galaxy bends the quasar's light so that it follows two paths of different lengths to Earth. The result is that we see two images of the same quasar, one arriving a short time after the other (about 417.1 days later). However, a team of astronomers at the [[Harvard-Smithsonian Center for Astrophysics]] led by [[Rudolph Schild]] studied the quasar and found that during the period between September 1994 and July 1995 the two images appeared to have no time delay; changes in the brightness of the two images occurred simultaneously on four separate occasions. Schild and his team believe that the only explanation for this observation is that a cosmic string passed between the Earth and the quasar during that time period traveling at very high speed and oscillating with a period of about 100 days.<ref>{{cite journal |arxiv=astro-ph/0406434 |bibcode=2004A&A...422..477S|doi=10.1051/0004-6361:20040274 |title=Anomalous fluctuations in observations of Q0957+561 A,B: Smoking gun of a cosmic string? |year=2004 |last1=Schild |first1=R. |last2=Masnyak |first2=I. S. |last3=Hnatyk |first3=B. I. |last4=Zhdanov |first4=V. I. |journal=Astronomy and Astrophysics |volume=422 |issue=2 |pages=477–482|s2cid=16939392}}</ref>

Currently the most sensitive bounds on cosmic string parameters come from the non-detection of gravitational waves by [[pulsar timing array]] data.<ref>{{Cite journal|arxiv=1508.03024 |title=The NANOGrav Nine-year Data Set: Limits on the Isotropic Stochastic Gravitational Wave Background |journal=The Astrophysical Journal |volume=821 |issue=1 |pages=13 |year=2015|last1=Arzoumanian |first1=Zaven |last2=Brazier |first2=Adam |last3=Burke-Spolaor |first3=Sarah |last4=Chamberlin |first4=Sydney |last5=Chatterjee |first5=Shami |last6=Christy |first6=Brian |last7=Cordes |first7=Jim |last8=Cornish |first8=Neil |last9=Demorest |first9=Paul |last10=Deng |first10=Xihao |last11=Dolch |first11=Tim |last12=Ellis |first12=Justin |last13=Ferdman |first13=Rob |last14=Fonseca |first14=Emmanuel |last15=Garver-Daniels |first15=Nate |last16=Jenet |first16=Fredrick |last17=Jones |first17=Glenn |last18=Kaspi |first18=Vicky |last19=Koop |first19=Michael |last20=Lam |first20=Michael |last21=Lazio |first21=Joseph |last22=Levin |first22=Lina |last23=Lommen |first23=Andrea |last24=Lorimer |first24=Duncan |last25=Luo |first25=Jin |last26=Lynch |first26=Ryan |last27=Madison |first27=Dustin |last28=McLaughlin |first28=Maura |last29=McWilliams |first29=Sean |last30=Mingarelli |first30=Chiara |display-authors=29 |doi=10.3847/0004-637X/821/1/13 |bibcode = 2016ApJ...821...13A |s2cid=34191834 |doi-access=free }}</ref> The earthbound [[LIGO|Laser Interferometer Gravitational-Wave Observatory]] (LIGO) and especially the space-based gravitational wave detector [[Laser Interferometer Space Antenna]] (LISA) will search for gravitational waves and are likely to be sensitive enough to detect signals from cosmic strings, provided the relevant cosmic string tensions are not too small.

==String theory and cosmic strings==

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During the early days of string theory both string theorists and cosmic string theorists believed that there was no direct connection between [[superstrings]] and cosmic strings (the names were chosen independently by analogy with [[twine|ordinary string]]). The possibility of cosmic strings being produced in the early universe was first envisioned by quantum field theorist [[Tom Kibble]] in 1976,<ref name="Kibble 1976" /> and this sprouted the first flurry of interest in the field.

In 1985, during the [[first superstring revolution]], [[Edward Witten]] contemplated on the possibility of fundamental superstrings having been produced in the early universe and stretched to macroscopic scales, in which case (following the nomenclature of Tom Kibble) they would then be referred to as cosmic superstrings.<ref name="witten-cosmic-superstrings">{{cite journal |last1=Witten |first1=Edward |title=Cosmic Superstrings |journal=Phys. Lett. B |date=1985 |volume=153 |issue=4–5 |pages=243-246243–246 |doi=10.1016/0370-2693(85)90540-4}}</ref> He concluded that had they been produced they would have either disintegrated into smaller strings before ever reaching macroscopic scales (in the case of [[Type I superstring]] theory), they would always appear as boundaries of [[Domain wall (string theory)|___domain walls]] whose tension would force the strings to collapse rather than grow to cosmic scales (in the context of [[Heterotic string|heterotic superstring]] theory), or having a characteristic energy scale close to the [[Planck energy]] they would be produced before [[cosmological inflation]] and hence be diluted away with the expansion of the universe and not be observable.

Much has changed since these early days, primarily due to the [[second superstring revolution]]. It is now known that string theory contains, in addition to the fundamental strings which define the theory perturbatively, other one-dimensional objects, such as D-strings, and higher-dimensional objects such as D-branes, NS-branes and M-branes partially wrapped on compact internal spacetime dimensions, while being spatially extended in one non-compact dimension. The possibility of [[Large extra dimension|large compact dimensions]] and large [[Randall–Sundrum model|warp factors]] allows strings with tension much lower than the Planck scale.