Extreme jets take new shape

February 17, 2010 | 2:26 pm

Recent observations of blazar jets require researchers to look deeper into whether current theories about jet formation and motion require refinement. This simulation, courtesy of Jonathan McKinney (KIPAC), shows a black hole pulling in nearby matter (yellow) and spraying energy back out into the universe in a jet (blue and red) that is held together by magnetic field lines (green).

Jets of particles streaming from black holes in far-away galaxies operate differently than previously thought, according to a study published today in Nature. The new study reveals that most of the jet’s light—gamma rays, the universe’s most energetic form of light—is created much farther from the black hole than expected and suggests a more complex shape for the jet.

The research was led by scientists at the Kavli Institute for Particle Astrophysics and Cosmology, jointly located at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University, with participation from scientists from around the world. The study included data from more than 20 telescopes including the Fermi Gamma-ray Space Telescope and KANATA telescope.

High above the flat Milky Way galaxy, bright galaxies called blazars dominate the gamma-ray sky, discrete spots on the dark backdrop of the universe. As nearby matter falls into the black hole at the center of a blazar, “feeding” the black hole, it sprays some of this energy back out into the universe as a jet of particles.

“As the universe’s biggest accelerators, blazar jets are important to understand,” said KIPAC Research Fellow Masaaki Hayashida, who serves as corresponding author on the paper with KIPAC Astrophysicist Greg Madejski. “But how they are produced and how they are structured is not well understood. We’re still looking to understand the basics.”

Researchers had previously theorized that such jets are held together by strong magnetic field tendrils, while the jet’s light is created by particles revolving around these wisp-thin magnetic field “lines.”

Yet, until now, the details have been relatively poorly understood. The recent study upsets the prevailing understanding of the jet’s structure, revealing new insight into these mysterious yet mighty beasts.

“This work is a significant step toward understanding the physics of these jets,” said KIPAC Director Roger Blandford. “It’s this type of observation that is going to make it possible for us to figure out their anatomy.”

Locating the Gamma Rays

Over a full year of observations, the researchers focused on one particular blazar jet, located in the constellation Virgo, monitoring it in many different wavelengths of light: gamma-ray, X-ray, optical, infrared and radio. Blazars continuously flicker, and researchers expected continual changes in all types of light. Midway through the year, however, researchers observed a spectacular change in the jet’s optical and gamma-ray emission: a 20-day-long flare in gamma rays was accompanied by a dramatic change in the jet’s optical light.

Although most optical light is unpolarized—consisting of light rays with an equal mix of all polarizations or directionality—the extreme bending of energetic particles around a magnetic field line can polarize light. During the 20-day gamma-ray flare, optical light streaming from the jet changed its polarization. This temporal connection between changes in the gamma-ray light and changes in the optical light suggests that both types of light are created in the same geographical region of the jet; during those 20 days, something in the local environment altered to cause both the optical and gamma-ray light to vary.

“We have a fairly good idea of where in the jet optical light is created; now that we know the gamma rays and optical light are created in the same place, we can for the first time determine where the gamma rays come from,” said Hayashida.

This knowledge has far-reaching implications about how energy escapes a black hole. The great majority of energy released in a jet escapes in the form of gamma rays, and researchers previously thought that all of this energy must be released near the black hole, close to where the matter flowing into the black hole gives up its energy in the first place. Yet the new results suggest that—like optical light—the gamma rays are emitted relatively far from the black hole. This, Hayashida and Madejski said, in turn suggests that the magnetic field lines must somehow help the energy travel far from the black hole before it is released in the form of gamma rays.

“What we found was very different from what we were expecting,” said Madejski. “The data suggest that gamma rays are produced not one or two light days from the black hole [as was expected] but closer to one light year. That’s surprising.”

Rethinking Jet Structure

In addition to revealing where in the jet light is produced, the gradual change of the optical light’s polarization also reveals something unexpected about the overall shape of the jet: the jet appears to curve as it travels away from the black hole.

“At one point during a gamma-ray flare, the polarization rotated about 180 degrees as the intensity of the light changed,” said Hayashida. “This suggests that the whole jet curves.”

This new understanding of the inner workings and construction of a blazar jet requires a new working model of the jet’s structure, one in which the jet curves dramatically and the most energetic light originates far from the black hole. This, Madejski said, is where theorists come in. “Our study poses a very important challenge to theorists: how would you construct a jet that could potentially be carrying energy so far from the black hole? And how could we then detect that? Taking the magnetic field lines into account is not simple. Related calculations are difficult to do analytically, and must be solved with extremely complex numerical schemes.”

Theorist Jonathan McKinney, a Stanford University Einstein Fellow and expert on the formation of magnetized jets, agrees that the results pose as many questions as they answer. “There’s been a long-time controversy about these jets—about exactly where the gamma-ray emission is coming from. This work constrains the types of jet models that are possible,” said McKinney, who is unassociated with the recent study. “From a theoretician’s point of view, I’m excited because it means we need to rethink our models.”

As theorists consider how the new observations fit models of how jets work, Hayashida, Madejski and other members of the research team will continue to gather more data. “There’s a clear need to conduct such observations across all types of light to understand this better,” said Madejski. “It takes a massive amount of coordination to accomplish this type of study, which included more than 250 scientists and data from about 20 telescopes. But it’s worth it.”

With this and future multi-wavelength studies, theorists will have new insight with which to craft models of how the universe’s biggest accelerators work.

The gamma-ray observations used in this study were made by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope, an astrophysics and particle physics partnership developed by NASA in collaboration with the U.S. Department of Energy Office of Science, along with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the United States. LAT collaboration members were key participants in the development of this research. SLAC National Accelerator Laboratory managed construction of the LAT and now plays the central role in science operations, data processing and making scientific data available to collaborators for analysis.

The optical polarization data that played a crucial role in this study was taken by the KANATA collaboration, using the KANATA telescope located in Higashihiroshima, Japan. The KANATA telescope is operated by Hiroshima University.

The GASP-WEBT observatories participating in this work are Abastumani, Calar Alto, Campo Imperatore, Crimean, Kitt Peak (MDM), L’Ampolla, Lowell (Perkins-PRISM), Lulin, Roque de los Muchachos (KVA and Liverpool), San Pedro Ma´rtir, St Petersburg for the optical–NIR bands, and Mauna Kea (SMA),Medicina, Metsahovi, Noto and UMRAO for the millimeter radio band.

The campaign also included data from NASA satellites Swift and the ROSSI X-ray Timing Explorer, and the Japanese satellite Suzaku.

Kelen Tuttle

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Fermi telescope closes in on mystery of cosmic ray acceleration

January 7, 2010 | 2:09 pm

Supernova remnant W44 as imaged by the Fermi telescope's Large Area Telescope and enhanced with a restoration technique. The green contours indicate the remnant seen with infrared light. (Image courtesy of NASA/DOE/LAT collaboration.)

In all directions of the sky, cosmic rays rocket through space with incredible speed. These “rays”—which mostly consist of protons—are some of the most energetic particles in the universe. For nearly 100 years, they have also been some of the most enigmatic. Now, a new result from the Fermi Gamma-ray Space Telescope’s Large Area Telescope collaboration offers insight into how, exactly, the universe accelerates these particles to such high energies. The high-energy cosmic rays appear to be coming from supernova remnants, the dying remains of exploded stars; the new result reveals the spatial distribution of this emission in one particular supernova remnant.

The acceleration of cosmic rays is a long-standing cosmic mystery. Cosmic rays were first recognized in 1912, but it wasn’t until 1949 that Italian physicist Enrico Fermi first proposed the mechanism behind their acceleration. In the following decades, Fermi’s ideas were developed further by researchers, including Roger Blandford, director of the Kavli Institute for Particle Astrophysics and Cosmology, jointly located at SLAC National Accelerator Laboratory and Stanford University.

The most likely source, researchers determined, is supernova remnants, which result from tremendous stellar explosions. As a star explodes, its material plows into the gas between the stars, compressing it and forming shock waves.  Those shocks are the most likely sites of very efficient acceleration of charged particles called cosmic rays. “But still observations had yet to pinpoint where the particle acceleration really occurs,” said KIPAC Panofsky Fellow Yasunobu Uchiyama.

In a paper published today in Science, the Large Area Telescope collaboration, led by KIPAC researchers Takaaki Tanaka, Uchiyama, and Hiroyasu Tajima, released the first image of a supernova remnant in the giga-electronvolt energy range (about 200 million times the energy of visible light). By revealing the spatial distribution of cosmic rays in the remnant, this result is a significant step toward definitively determining how cosmic rays are accelerated in supernova remnants.

“With Fermi, we finally have succeeded in getting information about spatial distribution from a supernova remnant in this energy band,” said Tanaka. “This band is quite important for the study of particle acceleration in supernova remnants including determining the origin of cosmic rays.”

To reveal this spatial distribution, the LAT observed supernova remnant W44 not in cosmic rays, but in gamma rays. Very soon after cosmic rays are accelerated to high energies in the supernova remnant, they interact with the diffuse gas that pervades the space between stars. Most of these cosmic rays are high-energy protons that collide with hydrogen atoms in this interstellar medium, producing particles called neutral pions that immediately decay into gamma rays. The researchers deduced that the gamma rays detected by the LAT were very likely created in this process based on the observed gamma-ray spectrum.

“This paper proves Fermi capable of determining the origin of gamma rays,” said Tanaka. As the LAT gathers more data, he continued, the certainty will increase.

“In this paper we cannot declare for certain that we’ve finally seen the signature of these protons,” said Uchiyama. “There is another possibility we need to rule out.  But if we can prove this connection, it will be a huge breakthrough. Researchers have been chasing this for nearly 100 years, ever since cosmic rays were first understood.”

The observations also reveal why ground-based gamma-ray telescopes, which detect even higher energy gamma rays as they zip through the Earth’s atmosphere, have failed to observe gamma rays from this remnant: while many of these protons are produced in the giga-electronvolt energy range, very few are produced in the tera-electronvolt energy range.

“As a result, ground-based tera-electronvolt telescopes miss many important supernova remnants” including SNR W44, Tanaka said. “But these [giga-electronvolt-bright supernova remnants] and the tera-electronvolt-bright supernova remnants are very complementary. With both, we can learn the unknown about the physical processes that produce cosmic rays.”

Kelen Tuttle

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Fermi and the multiwavelength sky

November 12, 2009 | 5:44 am

The study of the cosmos is not a journey that should be traveled alone. The Fermi symposium last week offered plenty of reminders that other telescopes are working with Fermi to study new objects of interest, in as many wavelengths as possible.

After only a few months of data collection, Fermi created a list of bright gamma ray sources it saw in the sky, some of which had not been identified by any other telescope. Other observatories immediately turned their eyes to those objects. In some cases, the objects emitted distinct signals in wavelengths other than gamma rays, such as with some radio pulsars or TeV blazars. This correlating of signals is necessary to officially identify an object, and some point-sources spotted by Fermi have not yet been correlated.

One major contributor to the Fermi/LAT multiwavelength approach is the Swift Gamma Ray Burst Mission, a NASA orbital observatory that includes instruments to collect light in the gamma-ray, X-ray, optical, and ultraviolet wavebands. Before the Fermi telescope had launched, Swift planned a campaign to do targeted monitoring of the brightest gamma ray sources that Fermi identified. Swift has already collected data on all of Fermi’s 23 brightest sources, as well as 25 additional sources.

Now, Swift has made that data available to anyone who wants it. Abe Falcone of the University of Pennsylvania, and a Swift collaboration member, announced in his talk at the meeting that all of Swift’s monitoring data on the Fermi objects are now freely available online. “It’s there for you to just grab and go have fun with,” he said. As nearly all of the top 23 brightest sources observed by Fermi are blazars, Swift is also working closely with the VERITAS collaboration (Very Energetic Radiation Imaging Telescope Array System) to also study the objects in even more energetic wavelengths than Fermi (see our previous post).

Swift’s prime advantage for astronomers is its ability to monitor point-sources of interest over longer periods of time than Fermi. Swift can also monitor those bright sources simultaneously with Fermi, which Falcone explains can be crucial to understanding rapidly changing blazars, which can change their flaring behavior over the course of just minutes.

“With a time variability that short, if you try to look at the same blazar with another telescope that is a month behind Fermi, then you might be sampling a different population of particles all together,” said Falcone, after his talk. He emphasized that by monitoring objects and gamma ray flares in different wavelengths simultaneously, scientists can hope to truly begin to describe and categorize objects like blazars. Up until now, the extreme variability of these objects, and their unpredictable flaring cycle, made it very difficult to draw conclusions about their behavior and make up. Some blazars stay quiet for decades, then suddenly burst into brilliant action for just a few days; while others may burst very frequently every year. “We don’t know why one is one way and one is another,” said Falcone. “Blazars might tell us where cosmic rays from; we could finally understand that age old problem.”

Still some astronomers want even more eyes pointed at the sky. David Paneque of the Kavli Institute for Particle Astrophysics and Cosmology at SLAC National Accelerator Laboratory and Stanford University just wrapped up two observation campaigns for which he recruited twenty different observatories. Paneque selected two blazars to observe, and coordinated the effort so that every two days the telescope observed one blazar for a few hours, and every five days observed the other. Paneque said it was crucial to have all the observatories watching the objects simultaneously, so that the same events could be seen in various wavelengths.

“For me, there is only multiwavelength observation,” said Paneque. “With multiwavelength you can do so much more, and get such a better understanding of what you are looking at.” The campaign lasted four and a half months and included observatories stretching from weak radio waves to the most powerful gamma rays in the TeV range. Paneque had to write computer algorithms to make the data from all the different observatories directly comparable. He presented his work in a poster, but has not yet prepared it for publication.

Paneque isn’t the only one who feels so strongly about multiwavelength surveys. Kent S. Wood of the Naval Research Laboratory stated in his talk that “the age of ASM [all-sky-monitoring] has arrived.” Wood strongly promoted the need not only for targeted multiwavelength astronomical studies, but to do simultaneous all-sky surveys.

Wood dedicated most of his talk to promoting such a complimentary agreement between Fermi and the optical telescope Pan STARRS, which stands for the Panoramic Survey Telescope & Rapid Response System. Located at the University of Hawaii, Pan STARRS will include four optical telescopes on two Hawaiian islands, and is set to be completed in 2012. The first of the four, a prototype telescope, has already been built and began taking images in December 2008. Pan STARRS will be able to survey about three quarters of the night sky, with the dual objective of producing new astronomical data for general study, as well as looking for objects like asteroids that might collide with Earth.

According to Wood, Fermi and Pan STARRS have already created a memorandum to work collaboratively, although he pushed to have more time dedicated to combining the two efforts and having both observatories create simultaneous all sky maps in the visual and gamma ray wavelengths. He also said that scientists at NASA Ames are working on software to bridge the data from the two observatories and make it comparable. Ideally this kind of software would enable Fermi to compare data with any other all-sky observatory.

Many telescopes, observatories, and institutions are working with the Fermi collaboration or utilizing Fermi data, and are making tremendous contributions to turning the Fermi data into knowledge and understanding. Many of those groups were represented at the meeting by talks and by attending representatives.

Calla Cofield

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Antimatter from lightning flashes the Fermi space telescope

November 6, 2009 | 4:58 pm

Artists rendition of a satellite and a terrestrial gamma-ray flash.

Violent and massive events in our universe create brilliant gamma-ray displays that will keep the Fermi Gamma-ray Space Telescope busy for a decade. But recently, Fermi has turned its eyes back to Earth, where it can see evidence of Terrestrial Gamma Flashes, or TGFs, which are believed to originate at the tops of thunderstorm clouds. Fermi announced that it has detected positrons from TGFs, a first result and a major clue about what actually causes them.

Scientists now believe that what they detect as TGFs could be two different phenomena: actual gamma rays, and accelerated particles. Current theory suggest that TGFs originate when electrons caught in lighting storm electric fields become accelerated and collide with air particles, producing gamma rays. The electrons may also eject more electrons from the air particles, causing an avalanche of particles in the electric field, resulting in the second kind of TGF. Fermi has detected a total of 17 TGFs, which are identified as such based on their correlation with lightning flashes.

At the Fermi Symposium, Michael Briggs, a research scientist at the University of Alabama in Huntsville, reported that Fermi has detected TGF gamma rays measuring 511 keV, the exact energy of gamma rays ejected as a result of positron decay. “Fermi’s results on positrons are a first,” wrote Briggs in an email after his talk. “They don’t contradict existing theory, but require an extension.” Eventually, Fermi’s results could confirm the existence of two separate phenomena. Briggs is preparing the results for publication.

Calla Cofield

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Millisecond pulsar timing could find gravitational waves

November 5, 2009 | 7:24 am

Last summer, the Fermi Gamma-ray Space Telescope discovered new pulsars through gamma-ray detection alone and found that millisecond pulsars do emit gamma rays. At the Fermi Symposium in Washington, DC, this week, Scott Ransom of the National Radio Astronomy Observatory announced that Fermi has located new millisecond pulsars based on gamma-ray emissions alone. Fermi may come to greatly assist the radio astronomy community in locating millisecond pulsars, which may eventually assist in the detection of gravitational waves.

“Fermi is giving something back to the radio-astronomy community that I don’t think anyone expected,” said Ransom after his talk at the Fermi Symposium on Monday.

From the Fermi telescope’s frequent scans of the night sky, the collaboration generated a list of noticeably bright sources, which many observatories then investigated in greater detail. Some of these sources (though not the very brightest) turned out to be pulsars, and upon further investigation, NRAO has found three new millisecond pulsars. These discoveries have been made within the last month and have not yet been reported in any academic journals.

Millisecond pulsars are very old pulsars that are thought to increase their rotation speed over the years. While these pulsars are born in the plane of our galaxy, over time they drift out into the darker regions. Astronomers only know of about 80 millisecond pulsars in our galaxy, but they expect that tens of thousands of them could be floating in the darkness all around us.

Because millisecond pulsars tend to drift into isolated areas, they become very difficult to find with radio telescopes. A quick scan of the sky won’t reveal their locations because their radio signals are rather faint, and it would take long exposure times to see them. Ransom says that NRAO can’t dedicate enough time to watching every point in the sky to hope to discover faint pulsars, but Fermi’s frequent scans of the sky eventually make these pulsars visible in gamma rays. With this capability, Ransom says he expects that over time the observatory’s contribution to the search for millisecond pulsars could be significant.

What is perhaps most thrilling about these new discoveries is that the millisecond pulsars can be used to search for gravitational waves. Millisecond pulsars are very accurate time keepers. They pulse in extremely regular, reliable intervals. If gravitational waves pass near them and distort space-time, it could delay the arrival of their signals on  Earth. If astronomers are keeping careful watch on a large number of pulsars simultaneously, they might observe these delays, and find evidence for gravitational waves.

There is currently a project called NANOGrav, or the North American Nanohertz Observatory for Gravitational Waves, which is leading this gravity wave project. Ransom says the three new millisecond pulsars are particularly valuable to NANOGrav because they are actually quite bright, and a lack of bright pulsars has so far been NANOgrav’s biggest hindrance.

Calla Cofield

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Fermi telescope detects gamma rays from starburst galaxies

November 5, 2009 | 2:24 am

M82, the "cigar galaxy" is one of three starburst galaxies now know to radiate gamma rays. Image courtesy of NASA.

The breakfast room was buzzing on Wednesday morning at the Fermi Gamma-ray Space Telescope symposium. The collaboration is definitely excited about the newly observed gamma rays coming from “starburst” galaxies–sites of high numbers of star births and subsequent star deaths. These gamma rays have multiple implications for the astronomical community, most notably providing clues about the origin of cosmic rays.

This is the first observation of gamma rays from starburst galaxies, and astronomers are already eager to revisit other known starburst galaxies, this time looking for gamma ray emissions. If these galaxies emit gamma rays, they would certainly contribute to the diffuse gamma ray background, which is a kind of white noise of gamma rays that comes from every direction in the universe. The source of the background is unidentified but it is expected to have many sources, such as blazars. This background must be subtracted from studies of gamma ray point sources, and it interferes with searches for dark matter, so deconstructing its origins could have widespread implications.

The gamma rays are most likely generated by cosmic rays; but rather than finding that cosmic rays shower down on the galaxies from distant sources, the Fermi results suggest that they are produced inside the galaxies themselves. Cosmic rays are believed to be created when particles are accelerated in the wake of supernova explosions. These cosmic rays then collide with other particles in the galaxy, accelerating them and causing them to radiate gamma rays. So it makes sense that sites of frequent massive star deaths would generate cosmic rays. Fermi scientists also believe the cosmic rays then become trapped in the galaxies by magnetic fields.

The discovery will change the Fermi catalog of objects because “diffuse-gamma-ray-emitting starburst galaxies” are now officially their own object. That is, they will be added as a unique item to the list of objects in the sky. Many point sources of gamma rays have gone unidentified in the past, and Fermi scientists are trying their best to identify and characterize as many as possible.

The Fermi All Sky Map, showing the diffuse galactic background from the Milky Way. Courtesy of NASA/DOE/International LAT Team

What is also notable about this discovery is that the starburst galaxies are more like the Milky Way, and much closer to it, than the very exotic and violent galaxies that contain blazars and other energetic objects more easily seen by Fermi. The Milky Way emits a galactic diffuse emission in gamma rays (which is what creates the colorful band image seen in the Fermi all-sky map). According to a press release from NASA, this is the first time astronomers have seen diffuse emission from star-forming regions in galaxies other than our own.

And there’s more.

Three starburst galaxies were found to emit gamma rays, and two of them, M82 and NGC 253, have also been identified as sources of very-high-energy (VHE) gamma rays by HESS and VERITAS, respectively. While Fermi detects gamma rays with energies up to 300 GeV, observatories like VERITAS (Very Energetic Radiation Imaging Telescope Array System) detect gamma rays in the TeV range (where 300 GeV is .3 TeV).

This the first example of a VHE gamma ray source associated with a starburst galaxy. VERITAS collected the TeV rays from M82 over the course of two years.

Finding that these galaxies radiate in the TeV and the GeV range implies that there is a spectrum of cosmic ray energies, and this could help scientists better understand cosmic rays, where they come from and what they do.  “The spectrum is where the real science is,” said Keith Bechtol, a graduate student at Stanford University and SLAC National Accelerator Laboratory, after his talk on Wednesday announcing the discovery of the gamma rays from the three starburst galaxies. “If you want to really understand something, you look at the spectrum.” Bechtol says the next step will be looking and other starburst galaxies for gamma rays.

VERITAS has had a paper accepted in Nature on the subject of these star formation galaxies and cosmic rays.

Calla Cofield

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Gamma-ray burst restricts ways to beat Einstein’s relativity

October 28, 2009 | 1:00 pm

In this illustration, one photon (purple) carries a million times the energy of another (yellow). Some theorists predict travel delays for higher-energy photons, which interact more strongly with the proposed frothy nature of space-time. Yet Fermi data on two photons from a gamma-ray burst fail to show this effect, eliminating some approaches to a new theory of gravity. Click for an animation that shows the delay scientists had expected to observe. Credit: NASA/Sonoma State University/Aurore Simonnet

Sometimes a single photon can tell us a lot. Especially when it comes barreling toward us with extremely high energy from a gamma-ray burst billions of light-years away. If you catch just the right photon in just the right way, it might even tell you something about the fundamental structure of space-time and provide guidance toward unifying gravity and quantum physics.

In a paper first published on the arXiv on August 13, and then published in Nature today, the Fermi Gamma-ray Space Telescope team presents a reconstruction of the gamma-ray burst GRB090510, observed on May 10, 2009. The burst included a 31 GeV photon, one of the highest-energy photons ever observed from a gamma-ray burst. Although no grand sweeping statements can be made, this observation does place some limits on the kinds of theories physicists can develop about the nature of space-time.

This burst and photon have already received a lot of attention over the past few months and will get a lot more in the coming days, probably with a series of arguments and claims about how this kills or doesn’t kill various theories of quantum gravity. Based on what has already happened on the Web, the discussion is likely to become pretty rancorous with a small re-inflammation of the string wars on the way. But behind all the shouting, there is a lot of interesting science going on here and the real potential to increase understanding of astrophysics and provide some guidance for the future of quantum gravity theories.

What quantum gravity says about space-time

To set the scene, we need to know a little about quantum gravity. At this time, there is no complete theory of quantum gravity, but there are various frameworks for trying to understand the problem. String theory and loop quantum gravity are probably the best known of these, but there are many other attempts to build quantum physics and gravity into one theory.

Without going into details of specific frameworks, many quantum gravity frameworks adopt the idea that space-time itself might be quantized, or made up of some kind of fundamental unit or grainy structure, down on the level of billionths of billionths of the size of an atom’s nucleus. If that is indeed the case, there are many reasons to think that light moving through that quantized space-time might “feel” the quanta as it travels. One consequence could be that light of different energies would travel at different speeds.

The biggest problem is that no frameworks of quantum gravity really make concrete predictions of anything much. Instead, much of this research is phenomenology, a realm that sits somewhere in between fundamental theory and experimental observation. Phenomenological theories are designed to match experimental observation, and are often inspired by ideas from fundamental theory, but generally can’t be derived purely from fundamentals.

For example, some quantum gravity frameworks suggest that space-time is quantized. This could show up as the speed of light depending on the energy of the photons. But if it does, then the speed of light should vary with energy in some generic way that can described by an equation. In particular, it seems reasonable that the speed of light should be modified by a small correction that depends on the ratio of the energy of the light to some other energy scale. Quantum gravity frameworks suggest that the appropriate energy scale is called the Planck energy. And because energy scales and distance scales are related in these frameworks, that energy corresponds to a length called the Planck length, which is about the scale on which space starts to look like it is made up of little chunks rather than a continuum. The Planck length is about 10^-35 meters—a proton is about 100 billion billion times larger in diameter.

That equation that relates speed of light to energy can’t yet be deduced strictly from the fundamental quantum gravity ideas but these phenomenological equations should have the right structure and properties. Importantly, the equation can be tested against observation. If scientists could observe light of different energies traveling over long distances, they might be able to notice any differences in speed and see if the equation matches.

If that phenomenological equation does matches observation, it gives theorists a hint that their underlying theories could be on the right track and that the details should predict the kind of universe that equation describes. If not, then theorists had better look elsewhere.

Light from gamma-ray bursts

Physicists realized a decade or so ago that if light of different energies travels at different speeds, then the current generation of gamma-ray space telescopes might be able to observe those effects. What would be needed is to observe light coming from a compact source in a short burst a long way from us. By being compact and from a short burst, physicists could be confident that the light is all starting from the same place at the same time. By being a long way off, the light has enough of a chance to separate out, with the highest energy light being delayed slightly relative to the lower energy light. It might seem strange that it is the highest-energy light that is delayed, but a hand-waving argument for this is that the highest energy photons are more likely to interact with the graininess of space-time and so feel its effects more and are slowed most.

GRB090510 is precisely the kind of burst that physicists were hoping for to make this kind of measurement and test, and the Fermi telescope managed to collect good clean data to analyze.

Of course, nothing in astrophysics is as simple as it might sound, and the situation here is quite messy. For starters, nobody knows precisely what happens in a gamma-ray burst. Without knowing the mechanism of the burst, they can’t pin down precisely where and when the gamma-ray photons come from. That means physicists can’t just compare the arrival times as they depend on energy and calculate a speed. Instead, physicists can place some limits on how much quantum gravity effects could influence the speed working under a set of assumptions about the astrophysics involved. In particular, they can look at their phenomenological equation and see what the measurements say about the energy scale of quantum gravity.

The energy scale of quantum gravity

At what scale should quantum gravity become important? We mentioned earlier that the Planck energy and Planck length might be the reasonable scales. One key reason to choose these scales is that they occur naturally as scales defined by the other various constants of nature such as the speed of light and the strength of gravity. Indeed, one way to look at this is to say that any other choice of energy or length scale would require justification. That is precisely the position particle physicists find themselves in as they try to understand why particles have the masses they have and why various phenomena seem to occur at particular energies. Particle physics doesn’t seem to live in the simplest possible world, so to solve those issues physicists need to invoke ideas like symmetry breaking and other concepts more complicated than what the simplest possible universe would require.

Conceptual arguments and choosing the simplest path forward implies that the appropriate scale to observe effects seems to be this Planck scale, and it is often represented by the Planck mass in the equations. (The Planck energy, length, and mass are all related to each by multiplying or dividing by constants of nature.)

If the dependence of speed on energy has the simplest form possible, then the time delay between high- and low-energy photons depends on the energy difference between them and an appropriate mass scale for quantum gravity. So by measuring the time delay and energy difference, physicists can determine limits on this mass scale and see whether it matches up with the expected Planck mass.

What the measurement says about the mass scale of quantum gravity

When the Fermi team did the calculations, using the most conservative estimates for how astrophysics plays into this, they determined that the mass scale must be at least 1.2 times the Planck mass, and by using reasonable but less conservative assumptions, they derived lower limits on the mass scale of up to 100 times the Planck mass. One way to interpret this is to say that there is no variation of the speed of light coming from any quantum gravity effects at less than 1.2 times the Planck mass. And given that some quantum gravity frameworks predict that effects should be showing up at that point, perhaps those models are simply wrong, and there is no changing speed of light.

There are, however, quite a few caveats. The limit on the mass scale is only true if the quantum gravity effects show up in the simplest possible phenomenology where the time difference is proportional to the energy difference scaled by the quantum gravity mass. Some models suggest that the time difference might be proportional to the square of the energy difference scaled by the quantum gravity mass. That would be a much smaller time difference and not observable in this kind of experiment.

Additionally, other quantum gravity models could still have quantized space-time but wouldn’t show an energy-dependent speed of light in this form. Instead, speed might depend on the polarization of the light (called birefringence, like the optical property of calcite crystals which create two images when you look through them). There are other options floating around as well.

To be fair to the claim, though, ruling out the simplest dependence of speed on energy at the expected Planck scale is a significant constraint on future theories of quantum gravity.

The future of quantum gravity theories

In one sense, this result doesn’t change a whole lot. None of the quantum gravity frameworks were really predicting concrete results so it is not really an explicit test of quantum gravity, nor does it rule out any particular frameworks. It does, however, provide guidance to theorists about what kinds of theories might be viable as they develop their ideas further, and that is pretty important to the field. Any experimental constraints are good constraints, especially as string theory is now in a position of predicting almost anything and this result limits some of the versions of string-like theory. Meanwhile, loop quantum gravity had a class of ideas that have now been ruled out.

What is particularly clear is that the technique of observing gamma-ray bursts is powerful for placing real observational limits on the kinds of predictions that quantum gravity can get away with. The Fermi Gamma-ray Space Telescope likely to see many more bursts that can even more tightly constrain quantum gravity theories and some theorists have suggested that combining the observations of gamma-ray bursts with neutrino observations, it might be possible to develop tighter constraints or even measurements of how much light speed changes with energy if the speed of light does indeed depend on energy.

Read more

Fermi Telescope Caps First Year With Glimpse of Space-Time — NASA press release, October 28, 2009
Constraining Modified Dispersion Relations with Gamma Ray Bursts — BackReaction blog, June 26, 2009
Prospects for constraining quantum gravity dispersion with near term observations — Preprint by Giovanni Amelino-Camelia and Lee Smolin, June 23, 2009

Update: Some other news coverage that has appeared since this was published

An intergalactic race in space and time — Nature (news section)
Quantum gravity theories wiped out by a gamma ray burst — ars technica (looks like it got the embargo time wrong so it came out early)
Special relativity passes key test — Physics World

NASA’s Fermi Gamma Ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy, along with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

David Harris

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Gamma-ray burst hits highest energy yet

September 11, 2009 | 3:58 pm

This story first appeared in SLAC Today on September 11, 2009.

For the second time in as many years, a Large Area Telescope collaboration meeting was punctuated by a stellar firework. Last week’s meeting, which ran from August 29 through September 4 at SLAC National Accelerator Laboratory, was briefly interrupted on Wednesday when the LAT, the main instrument onboard the Fermi Gamma-ray Space Telescope, recorded a large gamma-ray burst.

Although the Gamma-ray Burst Monitor, the other instrument aboard the Fermi telescope, sees gamma-ray bursts almost daily, the LAT detects far fewer because it views only about a sixth of the sky at any given time and detects only the bursts that emit the highest-energy gamma rays. Including last week’s burst, the LAT total now stands at ten.

When the alert came in last week notifying collaboration members of the possibility of a newly detected burst, the researchers leapt to action, alerting astronomers around the world so that they too could turn their instruments toward it. At the same time, the LAT automatically stopped its regular scan of the sky to continue recording the burst.

“We knew this was likely to be an exciting one—it was immediately clear that it was a very big burst,” said SLAC astrophysicist and LAT collaboration member Jim Chiang.

Using additional data collected that afternoon, researchers determined that the burst included the highest energy gamma-ray so far measured from a gamma-ray burst: 33 GeV.

Roger Blandford, director of the Kavli Institute for Particle Astrophysics and Astrophysics, said that three main scientific messages can be gathered from this type of burst. “First, when you see high-energy gamma rays, it means the source must be rushing toward us with high speed. Second, from these gamma rays we’ve come to believe that most bursts are associated with the birth of a black hole in a supernova explosion.” And third, Blandford said, observations are showing that short- and long-duration bursts, which were previously considered to be different in some fundamental way, from our perspective are looking increasingly similar. But that last understanding is still a work in progress, he said.

“The LAT is a superb instrument that keeps on giving,” said Blandford. “It’s outperformed our highest expectations in almost all areas. Everyone who’s associated should take pride.”

Kelen Tuttle

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Fermi telescope captures a solar eclipse

July 24, 2009 | 3:31 pm

Voltage data from the solar-charged battery onboard the Fermi Gamma-ray Space Telescope for the days prior to and including the July 22 solar eclipse. (Image: Fermi Gamma-ray Space Telescope Collaboration.)

The Fermi Gamma-ray Space Telescope was launched to study gamma rays, not sunshine.  Yet that’s what it has done, most recently last week, when one of its instruments registered signals from a solar eclipse.

Shawne Workman writes in today’s edition of SLAC Today :

During Wednesday’s total solar eclipse, the moon blacked out the sun from vantage points in India, Asia and the Pacific Ocean for as long as 6 minutes, 39 seconds. Of course, it also shadowed the intervening space, between the moon and Earth—including, by chance, a swath of the Fermi Gamma-ray Space Telescope’s orbit.

The telescope passed through the eclipse at roughly 3:30 Universal Time (3:30 a.m. at zero longitude, or 8:30 p.m. PDT). The main power voltage to the Large Area Telescope took a dip as the sun’s power-charging rays hid behind the moon. The eclipse created a downward spike in the LAT’s regular cycle of increasing voltage as the battery charges in the sun, followed by a drop as the battery discharges during the telescope’s brief night.

It turns out that the sun itself is a source of gamma rays, although very faint ones.  They’re created when high-energy cosmic rays hit the sun’s atmosphere.  So the LAT can watch the sun moving across its field of view in a matter of hours against a background of stars, and is monitoring those emissions around the clock and in high quality for the first time, according to principal investigator Peter Michelson. Read more here from the Feb. 19, 2009 issue of SLAC Today.  Here’s a scientific poster  about that work, as well as coverage of Michelson’s talk in science journalist Ivan Semeniuk’s Embedded Universe blog.

Glennda Chui

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Dark matter may be brighter than expected

July 17, 2009 | 6:24 pm

Simulated all-sky map showing a galactic dark matter halo in dark blue. (Image: Science).

The Fermi Gamma-ray Space Telescope may find dark matter in our galaxy more easily than expected. Theoreticians have demonstrated that small clumps of dark matter in our galaxy and others like it may be more visible than previously thought.

Dark matter particles moving around within self-bound clumps tend to move slower than those bound only to the larger halo that wraps around the entire galaxy. As a result, these smaller halos may generate a higher flux of gamma rays. If the theory proves true, the Fermi Gamma Ray Telescope could detect hundreds of small dark matter halos in the Milky Way alone. But, according to the authors of a paper in this week’s Science, even a negative result will move their work forward.

Researchers are still uncertain exactly what dark matter is, but a predominant theory suggests it is made of supersymmetric particles that act as their own antiparticles. While dark matter does not emit or reflect light, the subatomic particles can annihilate each other – leaving behind a wreckage of subatomic detritus that may subsequently decay into gamma rays and reveal the location of the otherwise invisible matter.

In 2008, theoretical physicist Michael Kuhlen and his collaborators in the Via Lactea Project ran a complex computer simulation of dark matter behavior in a galaxy like the Milky Way. Many researchers use simulations like this one; most of the work lies in finding different ways to analyze what the simulations spit out.

Kuhlen’s first analysis showed that only a handful of small dark matter clusters, or halos, would be visible by the Fermi telescope. But more recently, Kuhlen took another look at the results and applied what is known as the Sommerfeld effect. This takes into account the relative velocities of dark matter particles within the larger structure. The Sommerfeld effect is well known in nuclear physics and commonly observed in experiments. But its relevance for dark matter annihilation was not appreciated until recently. This is the first time it has been applied to a dark matter simulation.

Kuhlen, who is now at The Institute for Advanced Study in Princeton, and collaborators Piero Madau of UC Santa Cruz and Joseph Silk of Oxford used the Sommerfeld effect to show that slower moving dark matter particles, like those in small halos, will have higher rates of annihilation and subsequently high gamma ray flux. Rather than a handful of small dark matter clusters, Fermi could see hundreds.

But Fermi has collected data for more than a year, and it hasn’t seen any dark matter yet.

“That immediately starts putting constraints on testing Sommerfeld-enhanced models,” says Kuhlen.

Because scientists don’t fully understand dark matter, there are multiple models exhibiting Sommerfeld enhancement, and the group considered more than one in their paper. But Kuhlen is also considering the impact of his paper from this converse side: it’s more likely the simulation results will rule out some of these models by testing them against observations.

“Of course it would be tremendously exciting if Fermi did discover hundreds of sub-halos, and provided strong evidence for one of these models,” he says. “But I think the true power is…that a lot of these models will be strongly constrained if Fermi actually does not detect any sub-halos or only detects a few. And that’s maybe not as exciting as actually detecting a signal, but in terms of science and research it’s progress.”

Calla Cofield

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