Lawrence Krauss on Cosmic Strings

Centauri Dreams recently examined wormholes and their possible survival from the early universe through the mechanism of a negative mass cosmic string. But what exactly is a cosmic string? Here’s Lawrence Krauss on the subject:

“During a phase transition in materials — as when water boils, say, or freezes, the configuration of the material’s constituent particles changes. When water freezes, it forms a crystalline structure. As crystals aligned in various distances grow, they can meet to form random lines, which create the patterns that looks so pretty on a window in the winter. During a phase transition in the early universe, the configuration of matter, radiation, and empty space (which, I remind you, can carry energy) changes, too. Sometimes during these transitions, various regions of the universe relax into different configurations. As these configurations grow, they too can eventually meet — sometimes at a point, and sometimes along a line, marking a boundary between the regions. Energy becomes trapped in this boundary line, and it forms what we call a cosmic string.

“We have no idea whether cosmic strings actually were created in the early universe, but if they were and lasted up to the present time they could produce some fascinating effects. They would be infinitesimally thin — thinner than a proton — yet the mass density they carry would be enormous, up to a million million tons per centimeter. They might form the seeds around which matter collapses to form galaxies, for example. They would also ‘vibrate,’ producing not subspace harmonics but gravitational waves. Indeed, we may well detect the gravitational wave signature of a cosmic string before we ever directly observe the string itself.”

From The Physics of Star Trek (New York: HarperCollins, 1995), pp. 149-150.

Of course, what Landis, Forward and the other authors of the paper “Natural Wormholes as Gravitational Lenses” were talking about was not just a cosmic string, but one possessing negative mass, and it would have to wrap itself around a wormhole in order to stabilize it so it could survive to the present time. Do such structures exist? If so, it is possible that advances in both space-based and ground astronomy will eventually prove the point, but even then, we’ll be left to speculate about where or when such a wormhole might lead.

Closing the Distance to the Perseus Arm

When it comes to making precise observations, nothing can beat the VLBA. Short for Very Long Baseline Array, this system of ten radio antennae is dispersed over the Earth’s surface from Mauna Kea (Hawaii) to St. Croix (Virgin Islands), using 25-meter dishes to create an interferometer 5000 miles wide. The array is controlled from an operations center in Socorro, New Mexico.

All those dishes make for remarkably sharp resolution, the best of any telescope in existence. And they’re needed to make the kind of observations recently performed by a team of astronomers studying the Perseus arm of the Milky Way. The nearest spiral arm to the Sun, the Perseus arm has now been shown to be much closer than previously thought, some 6400 light years as opposed to an earlier estimate of 14,000. The image below shows the location of the Sun and W3OH, a newly formed star in the Perseus arm in the region under study.

Diagram of galactic arms

Image: Mark Reid and his colleagues measured the distance to the Perseus spiral arm and found it to be closer than believed, only 6400 light-years away. Credit Y. Xu et al.

It was the study of stellar motions that created the 14,000 light year estimate, but it had already been challenged by studies of massive young stars whose intrinsic brightness (as compared to apparent brightness as seen from Earth) seemed to argue for a shorter distance. But ponder, as Centauri Dreams often does, just how tricky any sort of measurement of galactic distance must be. We’re situated within the disk, looking not only through a veil of surrounding stars but also faced with impenetrable dust clouds that obscure our view in particular directions. It’s no surprise that we need to re-draw the galactic map on occasion, and this won’t be the last time.

The VLBA work relied on parallax, observing the change in position of a star relative to a far more distant, essentially fixed object like a quasar. The movement of the Earth around the Sun provides the opportunity to gauge this change. In this case, the team worked with radiation from compact radio sources called masers, which amplify radio-wave emissions. The effects are minute, to say the least; Mark Reid (Harvard-Smithsonian Center for Astrophysics), says he spent more than a decade developing the calibration techniques needed to make the new findings.

Get this: the accuracy we’re talking about is 10 micro-arcseconds, which is a factor of 100 better than previous methods. That resolution is equivalent to looking from the Earth at an astronaut standing on the Moon and being able to tell in which hand that person is holding a flashlight!

Systemic: Working with Extrasolar Data

Centauri Dreams readers will want to know about Systemic, a research collaboration led by astronomer Gregory Laughlin (University of California, Santa Cruz). Scheduled for launch in 2006, the Systemic Collaboration is a simulation that will study a catalog of 100,000 stars, some of which are surrounded by planetary systems created by the team. The idea is to observe these stars by running their radial velocity information through the Systemic Console, a java applet that has just been released in beta form for early use and debugging.

Radial velocity measurements have been a key tool in the hunt for extrasolar planets, using slight perturbations in a star’s motion as evidence for distant planetary systems. A radial velocity measurement, according to Laughlin, is …”the component of the velocity of the star along the line of sight from the Earth to the star.” We can get measurements of motion along this line of sight down to 1 meter per second, telling us how stars are moving in relation to the Sun, and also allowing us to detect the slight wobble in motion that may be introduced by orbiting planets.

To check out the method first-hand, try the first of three Systemic tutorials, this one based on data from HD 4208, a Sun-like star some 110 light years from Earth. The tutorial explains how to use the Systemic Console to examine this star’s radial velocity and manipulate the data to explore possible planetary configurations. The exciting thing about this work is that everyone from professional researchers to educators and the interested public can have a hand in refining our tools for extrasolar planet detection.

Be aware, too, that the Systemic site contains an ongoing weblog laced not only with gorgeous photography but Laughlin’s insightful posts, the most recent of which discusses, in addition to the Lagrangian points associated with Jupiter and the Sun, the question of whether a stable orbit exists on the opposite side of the Sun from the Earth and in the Sun’s habitable zone — a twin of Earth, in other words. The intriguing result is that such an orbit is nonlinearly stable. Laughlin describes the scenario this way:

As one planet tries to pass the other one up, it receives a forward gravitational pull. This forward pull gives the planet energy, which causes it to move to a larger-radius orbit, which causes its orbital period to increase, which causes it to begin to lag behind. Likewise, the planet which is about to be passed up receives a backward gravitational pull. This backward pull drains energy from the orbit, causes the semi-major axis to decrease, and causes the period to get shorter. The two planets are thus able to toss a bit of their joint orbital energy back and forth like a hot potato, and orbit in a perfectly stable variety of a 1:1 orbital resonance, known as a horseshoe configuration. The horseshoe orbit is an example of the negative heat capacity of self-gravitating systems, which is one of the most important concepts in astrophysics: If you try to drain heat away from a self gravitating object, it gets hotter.

Have we observed any planetary systems with planets in this configuration? It seems unlikely, but the idea can’t be ruled out. Laughlin again: “It is dynamically possible that 51 Peg b (or any of the other extrasolar planets that do not transit within the predicted window) is actually two planets participating in a stable 1:1 orbital resonance…”

Laughlin is a major player in the extrasolar planet game and the Systemic Collaboration is a form of distributed science at its best. Those interested in rolling their sleeves up and getting into the hard data — in the process making a contribution toward fine-tuning our exoplanet hunting techniques — should bookmark the Systemic site not only for work with the console, but for provocative posts on a wide variety of astronomical subjects.

The Art of the Wormhole

Last week Centauri Dreams discussed the possible signature of a wormhole in astronomical data, as worked out in a 1994 paper titled “Natural Wormholes as Gravitational Lenses.” A wormhole moving between Earth and another star would show an odd but identifiable form of lensing — two spikes of light with a dip in the middle. But what would a wormhole look like if you could actually see it? Space artist Jon Lomberg had some thoughts on that and shared them in the following e-mail.

The wormhole entry was fascinating. I had the opportunity to try to visualize how a wormhole would look during the production of the film CONTACT. For the novel on which the film was based, Carl Sagan had asked Kip Thorne [Feynman Professor of Theoretical Physics at CalTech, and author of Black Holes and Time Warps: Einstein’s Outrageous Legacy] for guidance to keep the wormhole as scientifically plausible as possible. During the film’s production, I consulted with Kip to determine the appearance of a wormhole. Kip and his student Scott Hughes tried to calculate the paths of photons traveling through the wormhole, seen from outside the wormhole.

“Try not to make them look like black holes,” Kip advised. “It shouldn’t look like a vortex, a spinning accretion disk or anything like that.” Turns out that a wormhole doesn’t look like a hole at all. Rather than a hole in space, it would look like a blister in space, with a convex surface bulging out at you. A highly distorted, partially rotated image of the scenery on the other end might be discerned. Because of the large amount of negative energy required to keep the wormhole open, I envisioned some lensing distortions of the space around this side of the wormhole.

If you recall, once Ellie goes through the wormhole, she first emerges around Vega, gets a glimpse of the surroundings, and is almost immediately drawn into another wormhole, emerging over an icy moon orbiting an inhabited planet with a multiple star system in the sky. The following concept illustration [below] shows a wormhole approaching the viewer. A distorted image of the galactic center (the next stop) is seen on the convex shape of the wormhole’s mouth.

Ice planet wormhole

The image below is the scene she would travel into, a grand galactic panorama. A third wormhole is approaching in the distance, with a close view of the galactic center within it. This scene was cut, but the art in this sequence suggests how the wormhole could have appeared with digital graphics.

Panorama of galaxy

But at the conclusion of our work, the director and effects producer decided that the wormhole should not appear the way we had shown it, but should look like a vortex, exactly as Kip had cautioned against. Kip and I both hope that some future production will attempt to show these enigmatic objects in a way that distinuishes them from the very different phenomenon of a black hole.

Images: Copyright Jon Lomberg (

The Wake of a Pulsar

When a neutron star gives off pulses of radiation every time it rotates, it’s called a pulsar. The radiation, which moves along the star’s magnetic field lines, is often compared to a lighthouse beam sweeping across an ocean. Now a pulsar called Geminga has been found to leave a comet-like trail of high-energy electrons as it muscles its way through the nearby interstellar medium at about 120 kilometers per second. Geminga is close in interstellar terms, a mere 500 light years from Earth, and because it is moving across our line of sight, it offers unprecedented material for observation.

The ‘cometary’ tail shows up on data gathered by the Chandra X-ray Observatory; the same team found twin x-ray tails stretching billions of kilometers behind the object in 2003, using data from ESA’s XMM-Newton. As for Geminga itself, this incredibly dense core of an exploded star is about 20 kilometers across but contains the mass of our Sun. Although most pulsars emit radio waves, Geminga is silent at those wavelengths and shows up as a gamma-ray source that was only later identified in the x-ray and optical wavelengths. The gamma rays are evidently produced from the acceleration of electrons and positrons as Geminga spins at a rate of four times per second.

Besides being fascinating in its own right, this fast-moving object gives insights into the interstellar medium through which it passes. Its bowshock compresses the gas and magnetic fields it encounters by a factor of four; its tails are the bright edges of the shockwave it carves out as it moves. The current work suggests that high-energy electrons escaping the pulsar’s magnetosphere are creating the complex cometary and x-ray tails now being observed.

“Astronomers have known that only a fraction of these accelerated particles produce gamma rays, and they have wondered what happens to the remaining ones,” said Dr. Patrizia Caraveo of the Italian National Institute for Astrophysics (INAF), a co-author on the Astronomy & Astrophysics article that reports this work. “Thanks to the combined capabilities of Chandra and XMM-Newton, we now know that such particles can escape. Once they reach the shock front, created by the supersonic motion of the star, the particles lose their energy, radiating X-rays.”

The paper is De Luca, A., Caraveo, P.A., et al., “On the complex X-ray structure tracing the motion of Geminga,” Astronomy & Astrophysics 445 (2006), L9-L13. From the abstract: “Geminga is thus the first neutron star to show a clear X-ray evidence of a large-scale, outer bow-shock as well as a short, inner cometary trail.” An INAF press release (Italian only) is here. ESA reports on the discovery of ‘hot spots’ on Geminga and two other neutron stars in this April news release.

Centauri Dreams note: I love the name Geminga. INAF reports that the discoverer of this pulsar, Giovanni Bignami (Centre d’Etude Spatiale des Rayonnements, Toulouse) named the object for ‘Gemini gamma-ray source’ in 1973, but in his Milanese dialect, ‘ghè minga’ means ‘it is not there.’ The reference is to the fact that Geminga showed up as a gamma-ray only source until 1993, when x-ray and optical measurements were finally made.