The organization of this entry is slightly chaotic because I started to write it before I've seen the full version of that text.
As far as I understand the article, a quirky quasar combined with double images of galaxies that look like if they originate from a cosmic string actually lead some astronomers to believe that
- a huge cosmic string - possibly a macroscopic heterotic or type II string - is stretched across our galaxy
OK, let me now pretend that I believe this stuff. The cosmic superstrings were recently studied - and "predicted" :-) - by Joe Polchinski et al., see his following papers
- hep-th/0410082 and hep-th/0412244 (new!)
I also recommend you a recent review of the cosmic string issues by T. Kibble who is not a string theorist but who has a lot to say about the cosmic strings especially because he is the #1 founding father of "cosmic string theory":
Those who have read The Elegant Universe by Brian Greene also know that a string observed in the telescope is also Edward Witten's favorite scenario how string theory will eventually be proved - because nothing would settle the question whether the strings exist so cleanly.
News - gravitational lensing of CSL-1:
Before I read the whole article, Joe Polchinski wrote me that he had been interviewed, and he believes that the discovery may have been related to the following Russian-Italian observation by Mikhail Sazhin et al.:
- astro-ph/0406516 and astro-ph/0302547
- see also poster2004USA.pdf
The team has observed a pair of galaxies 10 billion light years away and gravitational lensing is supposed to be the origin. The angular separation of the pair is roughly 2 arc-seconds so that the two images of the galaxy (or two galaxies?) almost touch. OK, now the unusual part:
Gravitational lensing, as Andy Neitzke explains me, usually produces an odd number of images. This is a consequence of the Morse theory. Moreover, these images have typically very different intensities if the source of the lensing is a point-like object. The Russian-Italian pair is special because
- no candidate galaxy whose gravity would be the source of the lensing has been seen
- exactly two (even number) of images have been observed
- their intensity is equal, at least in three different intervals of wavelength (within the experimental error) - in fact, a similar pair of radio sources of equal strength may already have been found by Winn et al. in 2000 (the separation is 1.13 arc seconds there)
- the images are not distorted; it's very unusual for gravitational lensing as you can see if you play with the simulation here:
- http://iam.ubc.ca/~newbury/lenses/ ...
- the team of Sazhin has now found 11 pairs of double images in the 16-arc-second square around their original object CSL-1
Moreover, there seems to be one more rumor from Robert Helling, Cambridge, England: the WMAP people may have already obtained the coordinates of CSL-1, and they may have looked in that direction. It is "not ruled out" that they've seen a repetition of the image of the cosmic microwave background, something that you would expect the hypothetical cosmic string to do.
The oldest double image - another evidence
The first gravitational lensing (by an ordinary point-like object, and therefore distorted) was observed in 1979 by the Jodrell Bank telescope, the UK. It's a double quasar Q0957+561A,B. Normally, the oscillations of one image A are mimicked by the other image B roughly 417 days later, because of the different lengths of the two paths. But recently our Harvard astro-colleagues have observed some additional, 100-day-long oscillations in intensity (plus minus 4% or so) which were repeated by both images roughly four times without any time delay. Well, it's the right moment to insert a link to the paper about these strange oscillations:
OK, something is moving in between us and the lensing object - and it must be very close to us because the lag is zero. Moreover, because the images are separated by 6 arc seconds which is a rather large angle, the nearby object(s) causing the oscillations must be pretty large. Because a hypothetical binary star or a similar object would have to be unacceptably heavy to give you the speedy oscillations, the source of the additional variations should be an oscillating cosmic string, they say. A string is able to oscillate very quickly.
However, what seems more comprehensible is that they can measure how this object (string?) is moving, and it is moving by the velocity 0.7c across our line of sight. OK, at any rate, they believe that this string is oscillating, and the smooth period of 100 days of the oscillation is translated to the radius comparable to 1000 astronomical units - and the string should be in our galaxy, just 10,000 light years from the Sun!
Note that this old double-imaged quasar is at a different location than CSL-1, so it would probably be an artifact of a different cosmic string, if I understand it well. It would be interesting to know whether the estimated tensions of these two cases are close.
More tests to be done include the spectral analysis of CSL-1 to determine whether they're really identical images, as well as the attempts to find more examples in the skies. Joe Polchinski explains why the emission of gravitational waves is an interesting signal - a signal that can be seen by LIGO and/or VIRGO and/or LISA, as Damour and Vilenkin pointed out. Only the last paragraph of that article in New Scientist is dedicated to the fact that we still have no idea whether these cosmic strings are generic field-theoretical cosmic strings, or fundamental strings (or D-strings) from string theory.
Finally, let me say that the article looks much more serious and potentially exciting than what I expected. And don't get me wrong: the observation of cosmic strings, even if they're not the known strings from string theory, is a Nobel-prize-scale insight, I think.
Some trivial quantitative statements
The gravitational field of a cosmic string in 3+1 dimensions is simple - the spacetime is flat everywhere except for the locus of the string, but it has a deficit angle, much like a usual cone (multiplied by two more flat dimensions along the stringy worldsheet). The deficit angle is something like "8.pi.G_{Newton}.T" where T is the tension of the string - roughly the energy density per unit length of the string. The gravitational lensing by cosmic strings is special - the cosmic strings create two identical images, and they are visually separated roughly by the deficit angle itself. There are various upper bounds and lower bounds what the tension of the cosmic strings should be - i.e. how large the deficit angle of potential realistic cosmic strings can be if the cosmic strings exist. These upper and lower bounds marginally contradict each other (the uncertainty seems to be large enough so that the picture may still be consistent), but all these bounds are close to the deficit angle slightly below 10^-6. Because the tension has units of "squared mass" and because the deficit angle is roughly the tension in Planck units as we've said, we see that the mass scale associated with the string tension is roughly 10^-3 of the Planck scale - which is nothing else than the GUT scale. This fits the estimate for the tension that you expect from some GUT cosmic strings or the fundamental strings in string theory themselves - in the nearly old-fashioned models where the string scale is close to the GUT scale.
Let me also say some elementary comments about the popularity of cosmic string models that decreased rapidly around 2000 (and may be revived now): the cosmic strings have also been proposed as alternatives against inflation to explain structure formation. And because their equation of state is roughly "pressure equals -1/3 of energy density" in average, you may imagine that they cause the acceleration of the Universe instead of the cosmological constant. Both of these applications now seem highly unlikely with WMAP: WMAP says "pressure is below -0.78 of energy density at 95% confidence level". Also, the inflationary predictions of non-uniformities of the cosmic microwave background (CMB) agree with WMAP while the cosmic-string-dominated models are more or less falsified - the latter would lead to a much smoother profile of the temperature variations.
One more trivial calculation of mine: if they claim that the length of the stringy loop lensing CSL-1 is about 10^14 meters (10^49 Planck lengths) and the tension is 10^-6 squared Planck masses, the total mass is comparable to 10^43 Planck masses which is roughly 10^35 kilograms. So this loop of string would have something like 50,000 solar masses, unless I made an error. That would be a huge chunk of mass.
In this particular case, I decided not to include any links to the articles inspired by this topic on other blogs simply because their quality does not seem sufficient for you to learn something new out of them - their authors have not tried to learn the basic questions about the phenomenon of gravitational lensing.
