Cosmic strings observed?

I hope that serious astro-people read this newsgroup! The possible
observations of cosmic strings look pretty exciting. See the links in the
following article on my blog:

http://motls.blogspot.com/2004/12/as...ng-theory.html

If someone has something new about these stories, it would be great if she
could let me know.

David Goss has pointed out the following article in the future issue of
New Scientist to me.

* http://newscientist.com/ ...

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

That's of course too cool and one is naturally skeptical. I am doubly
skeptical because New Scientist has not been a terribly serious journal in
my eyes in the last 5 years. Nevertheless I think it's fun to bring your
attention to this article because observing a string in the telescope is
the favorite scenario of many leaders of our field how string theory
should eventually be proved. ;-)

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 paper

* hep-th/0410082

and its citations and references. The paper explains how the cosmic
superstrings can be distinguished from other, more ordinary types of
cosmic strings. (The article in New Scientist only dedicates the last
paragraph to the difference between the fundamental strings and other
types of cosmic strings.)

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:

* astro-ph/0410073

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:

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 http://widefield.lbl.gov/ ... poster2004USA.pdf

When I saw the full article, this rumor was confirmed. The authors keep
the coordinates of the object in secret and they want to get a telescope
time before someone else studies it. Joe Polchinski is skeptical whether
the object is a cosmic string, but Henry Tye is more confident because the
tension seems to be much bigger than the upper bound allowed for pulsars.
However Eanna Flanagan claims that the pulsar bound is wrong.

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)
* the images are not distorted; it's very unusual for gravitational
lensing as you can see if you play with the simulation he
* http://iam.ubc.ca/~newbury/lenses/ ...

Tom Kibble from London believes that these things together indicate a
presence of a cosmic string. In fact, I have not told you the main
interesting observation yet:

* the team of Sazhin has now found 11 pairs of double images in the
16-arc-second square around their original object CSL-1

A galaxy as the source of gravitational lensing is statistically expected
to create 2 images (pairs) in average, while the cosmic strings would give
you something in between 9 and 200; note that 11 belongs to this interval.
;-) The main skeptics' explanation is that the double images are just
random pairs of galaxies etc. that happen to be similar to each other.
Incidentally, I also recommend you an Italian page about CSL-1 and why
they believe it is a double image caused by a cosmic string:

* http://people.na.infn.it/~longo/ ...

The oldest double image - another evidence

The first gravitational lensing 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 are mimicked by the other image 417 days later,
because of the different lengths of the two paths. But recently they
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:

* astro-ph/0406434

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 causing the oscillations must be pretty large. Because a
hypothetical binary star or a similar object would be unacceptably heavy,
the source of the additional variations should be an oscillating cosmic
string, they say.

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.
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