Gravitational Lensing Signature of Long Cosmic Strings

Abstract

The gravitational lensing by long, wiggly cosmic strings is shown to produce a large number of lensed images of a background source. In addition to pairs of images on either side of the string, a number of small images outline the string due to small-scale structure on the string. This image pattern could provide a highly distinctive signature of cosmic strings. Since the optical depth for multiple imaging of distant quasar sources by long strings may be comparable to that by galaxies, these image patterns should be clearly observable in the next generation of redshift surveys such as the Sloan digital sky survey. [S0031-9007(97)04026-X] PACS numbers: 98.62. Sb, 98.80.Cq An exciting outcome of the interplay between particle physics and cosmology is the realization that topological defects may be present in our Universe and may help to resolve some long-standing puzzles such as the origin of structure formation. A particular scenario which has been investigated over the past two decades is one where the relativistic motion of cosmic strings induces large-scale structure formation in the wakes that trail behind them [1]. Such strings would be present even now and, if observed, would be a culminating point for many exciting ideas in particle physics and cosmology. Past research has mostly focused on the observation of cosmic strings by the anisotropies they produce in the radiation background, and by the simple gravitational lensing characteristics of long straight strings. In this paper we reconsider this second approach and present a new signature in the lensing of galaxies or quasars resulting from realistic long cosmic strings. While the signature we describe is striking, because very few long strings are expected to span our horizon volume, one requires a survey over a significant fraction of the sky in order to place reliable constraints on cosmic string scenarios for the formation of large scale structure. The Sloan digital sky survey (SDDS) should be able to do just this.Cosmic strings are lineal gravitating sources with tension along the string equal to the mass per unit length m. In the case of strings produced at the grand unified epoch (cosmic time t ϳ 10 235 sec), m ϳ 10 22 gm͞cm. The gravitational effects of such strings are characterized by the dimensionless parameter 8pGm ϳ 10 25 , implying they are strong enough gravitational sources to seed structure formation in the Universe. A string network consists of two distinct components: closed loops of string, and long (infinite) strings. Loops are typically of size #GGmt ϳ 10 24 t and live for about one Hubble time. (G ϳ 100 is a numerical factor associated with the rate of gravitational radiation.) For infinite strings, numerical simulations of string networks show that there are order 10 long strings within a horizon volume at any epoch. These long strings sweep across the horizon at relativistic speeds and collide and reconnect with other strings. A very important fact, which has emerged after intensive numerical study of the string network evolut...

Gravitational lensing by cosmic strings in the era of wide-field surveys

“…[6] also indicate that the string should be straight on scales, ξ > ∼ ∆α ∼ 2 ′′ ; if this were not true, the images would be distorted as in Ref. [12]. Note that, here and in the rest of this paper, we quote distances (such as l, R and ξ) as angles projected on the sky.…”

“…We build on previous work (e.g. [7,8,9,10,11,12,13]) and revisit estimates of probability of lensing by a cosmic string. In particular, we identify simple yet effective strategies for confirming or refuting the string hypothesis, given one putative lens.…”

Probing cosmic superstrings with gravitational waves

“…Although cosmic strings and cosmic superstrings are created in the early universe, they are expected to persist and survive throughout cosmic history. However -in spite of their characteristic imprints on the cosmic microwave background (CMB) [10][11][12][13], small structure formation [14][15][16], reionization history [17][18][19] and gravitational lensing observations [20][21][22][23] -both cosmic strings and cosmic * Electronic address: Lara.Sousa@astro.up.pt † Electronic address: pedro.avelino@astro.up.pt superstrings have as of yet evaded detection.…”

Cosmic structure formation with topological defects

“…For a straight cosmic string lensing is very simple [148]. For a more realistic network of strings, characteristic caustics and cusps in the lensing signal are very generically expected [94,144,13].…”