A Dark Flow in the Cosmos

Seeing things that are otherwise invisible means looking for their effect on the things we can see. Examples abound: The presence of dark matter was originally inferred from the shape of galaxies, and the fact that the mass of what we could see couldn’t explain how these cities of stars held together. Dark energy turned up through minute examination of supernovae, shaping the idea that the acceleration of the universe is an ongoing phenomenon. And now we have another unusual effect suggesting the presence of matter beyond the observable universe.

The work grows out of the study of some 700 galactic clusters whose X-rays, emitted by hot gases, cause measurable effects on photons from the cosmic microwave background. This is the Sunyaev-Zel’dovich effect, in which high energy electrons impart some of their energy to the CMB. A variant of the SZ effect helps us study galactic clusters in ways that now suggest the presence of inflation in the early universe. Thus Alexander Kashlinsky (NASA GSFC), who lays out the finding in this news release:

“The clusters show a small but measurable velocity that is independent of the universe’s expansion and does not change as distances increase,” says Kashlinsky. “We never expected to find anything like this… The distribution of matter in the observed universe cannot account for this motion.”

Kashlinsky describes the motion as a ‘dark flow,’ pointing out that it is constant out to at least a billion light years, and suggesting that it extends across the visible universe. The extraordinary period of inflation in the early universe, a provocative theory found in Big Bang models of the cosmos, would indicate that what we see in the sky is but a portion of a much larger picture. We may thus be looking at galactic clusters that are being affected by matter that has been pushed beyond the observable universe.

Image: Hot gas in moving galaxy clusters (white spots) shifts the temperature of cosmic microwaves. Hundreds of distant clusters seem to be moving toward one patch of sky (purple ellipse). Credit: NASA/WMAP/A. Kashlinsky et al.

So what is this matter? The tremendously challenging paper Kashlinsky and team are about to publish describes them as “…pre-in?ationary remnants located well outside the present-day horizon.” That, of course, offers at least the possibility of examining some features of the cosmos before inflation actually occurred. The reference is Kashlinsky et al., “A measurement of large-scale peculiar velocities of clusters of galaxies: results and cosmological implications,” accepted for publication in Astrophysical Journal Letters and currently available online.

Interstellar Flight in Context: A Bet Already Won?

The staggering difficulty posed by interstellar flight pushes us to imagine alternatives to today’s technologies. Using conventional rocketry we’re forced to amass so much propellant that the craft we want to send seem impossible to build, even if we could afford the vast fuel bill. A jacked up rocket engine is, of course, nothing but an old technology pushed to its extreme imaginative limits. And you could sense the constraints in that vision at the recent Joint Propulsion Conference in Hartford (CT), discussed not only in these pages but also here by Ray Villard.

I mention Villard’s comments because while I focused on Robert Frisbee’s antimatter rocket concepts in my Centauri Dreams post, Ray tackles the much broader question of how we place technologies within the context of scientific progress. The news director for the Hubble Space Telescope, Villard is well versed in the rewards and challenges of spaceflight, but he’s nonplussed with some of the reaction to the Hartford conference, much of it focused on the apparent impossibility of interstellar flight. Are these prognostications of doom accurate or do they indicate, as he opines, a simple ‘failure of imagination’?

For extrapolating from today into tomorrow often leads to dead ends. If rocket science doesn’t work, one possibility is to change the paradigm, leaving the fuel behind in the Solar System, or harvesting it along the route. The first alternative evokes beamed propulsion concepts including laser- and particle beam-pushed sails. The latter reminds us of Robert Bussard’s interstellar ramjet, a concept that now seems impractical (such devices seem to generate enough drag to make them function better for braking than acceleration), but one which has never been entirely abandoned. And then, as Villard notes, there are star drives:

The idealized “star drive” uses fundamental properties of matter and space-time to create propulsive forces anywhere in space without the need for carrying fuel. The idea of somehow tapping energy from the vacuum of space may be a little less crazy now. That’s because astronomers have discovered that the universe is dominated by “dark energy” which is stretching the fabric of space at an ever-faster rate. This has rattled our confidence in knowing the true underpinnings of modern physics.

All of which is the reason we keep such a close eye on dark energy research in these pages, for if there is a force of nature that may lead to our re-writing the textbooks, this is it, and its potential uses in propulsion (assuming we advance to the point we can harness it) are mind boggling. We’re at the very edge of speculation here, but at the same time, much that we now take for granted was once equally imponderable.

My guess is that the first interstellar mission, which I assume will be a robotic science probe, will fly with technologies that are as hard for us to imagine as (this is Villard’s phrase) ‘a Roman archer trying to imagine a military laser cannon.’ Tibor Pacher, my erstwhile opponent in the interstellar bet on the Long Bets site, doubtless agrees, but he may think that potential breakthroughs are closer than I do, or he wouldn’t be putting his money behind an interstellar launch as early as 2025. The great unknown here is the pace of computer technology and the possibility of vastly accelerated change.

Because people can vote on Long Bets, I hope you’ll drop by to cast yours. So far the ballots are running 23 to 7 in my favor, but I’m quick to note that neither of us will be collecting any money on New Year’s Eve, 2025. That night I hope to meet with Tibor either in northern Germany or Budapest to open a bottle of Champagne as we toast the victor (Tibor, I lean toward Pol Roger, and I suspect you’ll be buying). The funds that have accumulated interest through the intervening years will go either to the Tau Zero Foundation or SOS-Kinderdorf International, both good causes aimed at enhancing humanity’s future.

In the midst of all this interstellar musing, in came an e-mail from the SETI League’s Paul Shuch, opining that the bet has already been won and, moreover, that Tibor is the victor! Let me quote Paul:

I would argue that the first interstellar missions have already launched, and that (with only a little imagination) they meet the conditions of the bet. They are not spacecraft, but rather streams of photons. Think about it: interstellar microwave transmissions probe other civilizations’ interest in dialog, and pass numerous stars, thus are “flyby probes” in a sense. They are transmitted specifically for the purpose of reaching other solar systems. They have been “launched” (transmitted) several times from Earth, which is clearly within the orbit of Neptune. Some have conveyed scientific information about Earth, which satisfies the condition that they “deliver data for at least one scientific measurement.” They travel at the speed of light, so within the 2,000 year mission duration, will reach stars within 2,000 LY of our own Sun. And they are widely supported by the public, as witness the large number of humans who have submitted messages to the various projects that beam them into space. So, congratulations Tibor, you win!

Remind me not to play poker with Paul, who in the following e-mail exchange added that his argument was a bit of a Kobayashi Maru — a more or less no-win situation, for those of you not familiar with Star Trek lore. The key word here is ‘deliver,’ for now that I read through our Long Bets terms, I see that the operative sentence is ‘As a minimum requirement for the mission the spacecraft shall be capable to deliver data for at least one scientific measurement.’ Now, slapping forehead with palm, I wish Tibor had written (and I had agreed to) ‘return’ instead of ‘deliver’ data! Although I’m not conceding, I do invite Paul to join us for Champagne in 2025, and maybe Ray Villard can join us as well.

Notes & Queries 22 September 2008

Hugh Everett’s ‘many worlds’ interpretation of quantum mechanics spawned not just the idea of a multiverse, but apparently quite a few interpretations on what a multiverse implies. If you’re intrigued by the notion that our cosmos is one of what may be an infinite number of universes, you’ll want to read Dan Falk’s report in Sky & Telescope on the recent multiverse conference held at the Perimeter Institute for Theoretical Physics (Waterloo, ONT). Particularly interesting is the growth of multiverse thinking as string theory has come to the fore, with all the controversy that implies.

And then there’s the notion of ‘eternal inflation,’ which conceives of endless big bangs, each creating a separate cosmos. Laura Mersini-Houghton (University of North Carolina) is concerned about how multiverses spawned by quantum theory, string theory and inflation can be reconciled, as Falk notes:

…it’s not at all clear how these different kinds of multiverses – grounded in quite different physical theories – may be related to one another. Still, the fact that three different lines of reasoning, all rooted in modern physics, seem to be pointing the same way makes some feel there must be a connection. “My gut feeling is that these multiverses have to be related,” said Mersini-Houghton.

Nor should we forget the interesting philosophical questions such thinking suggests. What happens if every possible outcome happens with 100 percent probability? A many-worlds quantum theory leads to that result, but how does quantum theory live with the disappearance of probability itself? Hilary Greaves (Oxford) went at that one in this small session (about twenty participants) that examined not only the concept of a multiverse but the possibility that it doesn’t exist. Thus the Perimeter Institute’s own John Moffat, a specialist in general relativity, who says the multiverse “…is not the kind of science we’ve been doing since Galileo.” A good multiverse has plenty of room for skeptics.


If things closer to home carry more appeal, last month’s conference The Great Planet Debate: Science as Process got into familiar and controversial terrain in its discussion of how to define a planet. We now have eight planets as per the International Astronomical Union, but feeling at the conference seems to have been widespread in favor of revising that definition. The trick, of course, is in just how to do that. Possible definitions are all over the map, and I send you to this Planetary Science Institute news release to get the overview. Personally, I prefer the wider perspective that Larry Lebofsky (PSI) has to offer:

“We all have a conceptual image of a planet. Therefore, we need a term that encompasses all objects that orbit the Sun or other stars. The debate is a great teaching moment. Whether dwarf planets are grouped together with the classical planets is not as important as the process by which scientists arrived at their conclusions. Scientists look at the same information in different ways; there may be more than one ‘answer.’ Facts change. What we know now may not be what we know in two or three years. Learning to think critically and understanding how scientists organize facts to develop theories are lessons that will serve students for a lifetime.”

I’m assuming that what Lebofsky means is not so much that facts themselves change, but that our data continue to bring us new information about those facts. In any case, a call to think critically about the data influx is a worthwhile reminder that the way we do science carries implications for thinking and learning in any discipline.


This brief squib from the BBC relates European plans for a potential mission known as Marco Polo, now in feasibility studies. It’s an asteroid lander with the possibility of sample return, involving a small near-Earth asteroid of less than a kilometer in size. Mission launch would be approximately 2017. As I’ve often opined in these pages, an asteroid mission is a needed first step as we begin to develop the understanding — and the tools — we may one day need for possible asteroid deflection. The more we learn about the objects that could someday collide with Earth, the better prepared we’ll be should the need ever arise.

Exoplanets on the Fringe

Most Centauri Dreams readers will be familiar with the concept of interferometry by now. The idea is to combine light from multiple telescopes, allowing the combined array to act like a single telescope with a diameter equivalent to the distance between the telescopes. Thus we have the European Southern Observatory’s VLTI (Very Large Telescope Interferometer), which uses two telescope elements some 200 meters apart. The VLTI has now put a new instrument called PRIMA into operation, with useful exoplanetary implications.

PRIMA (Phase Referenced Imaging and Microarcsecond Astrometry) is designed to pick out the tiny motions a star makes as it is influenced by unseen planetary companions. We’ve long studied such wobbles in stars through radial velocity methods — these analyze the light from the star, determining through Doppler shifts in the star’s spectrum how a companion object may be influencing it. But PRIMA will find the wobbles through actual imaging, using incredibly precise astrometric measurements. The images below, although not based on PRIMA, illustrate the basic operation of interferometry.

Image: The difference between two stars of different diameter is illustrated. While the image of the smaller star displays strong interference effects (i.e., a well visible fringe pattern), those of the larger star are much less prominent. The “visibility” of the fringes is therefore a direct measure of the size; the stronger they appear (the “larger the contrast”), the smaller is the star. Credit: ESO.

This ESO news release talks about measuring angular differences of about ten micro-arcseconds using the PRIMA instrument, good enough for gas giant detections. We’ll do much better than that once we have future space-based observatories tuned for astrometry, but for now this is state of the art stuff. The first starlight fed into PRIMA reached it on September 2. In interferometer lingo, this is not ‘first light’ but ‘first fringes,’ since it refers to the first time the light from different instruments was combined to produce the pattern of bright and dark lines known as interferometric fringes. The differences between the signals allow astronomers using these instruments to measure angles to exquisite precision.

Image: If the distance between the two telescopes is increased when a particular star is observed, the fringes become less and less prominent. At a certain distance, the fringe pattern disappears completely. This distance is directly related to the angular size of the star. Credit: ESO.

Dark Matter’s Galactic Implications

Segue 1 is one of the tiny satellite galaxies orbiting the Milky Way whose dark matter component has caused great astronomical interest. As we saw in this post a couple of weeks ago, these ultra-faint objects have been turning up in Sloan Digital Sky Survey data, surprising astronomers by their mass, which indicates they’re dominated by dark matter.

Consider them top-heavy with the stuff: Segue 1 turns out to be a billion times fainter than the Milky Way, yet a study by members of the same team shows that it is a thousand times more massive than would be expected by its visible stars. The new regime of faint galaxies offers intriguing observational clues to galaxy formation while putting dark matter’s properties on display. Thus Marla Geha (Yale University):

“These dwarf galaxies tell us a great deal about galaxy formation. For example, different theories about how galaxies form predict different numbers of dwarf galaxies versus large galaxies. So just comparing numbers is significant.”

Supercomputer simulations are also put to work to study how dark matter interacts with galaxies. A new paper shows that while most early clumps of dark matter eventually merged to form a halo around the Milky Way, the largest would have been torn apart to form a disk of dark matter within the galaxy itself. If that’s the case, the dark matter disk would be less dense than the halo. Because the dark matter halo does not rotate around galactic center like the Sun, dark matter should be flowing toward us at considerable speed. The disk, on the other hand, rotates along with the stars and thus produces little of this dark matter ‘wind.’

Image: A composite image of the dark matter disk (red contours) and the Atlas image mosaic of the Milky Way obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. Credit: J. Read & O. Agertz.

This has interesting implications for detection, according to Laura Baudis (University of Zurich), who is one of the lead investigators for the XENON direct detection experiment that is looking for dark matter at the Gran Sasso Underground Laboratory in Italy. Baudis is quoted in this Royal Astronomical Society news release:

“Current detectors cannot distinguish these slow moving particles from other background ‘noise.’ But the XENON100 detector that we are turning on right now is much more sensitive. For many popular dark matter particle candidates, it will be able to see something if it’s there.”

Well, we’ll see. Dark matter’s presence in and around our galaxy seems increasingly clear but nailing down what it consists of has been an elusive challenge, to say the least. Doing so would be hugely important because cold dark matter (CDM) is part of the overall model being continuously refined by such studies. That model, called ?CDM or Lambda-CDM, includes a cosmological constant ? that makes up 72 percent of the energy density of the universe, yet another area of immense scientific interest. And the development of a dark matter disk is, the authors of the new study believe, inevitable under this model. From the paper:

In this paper, we study how the Milky Way disc affects the accretion of satellite galaxies in a ?CDM cosmology, and how these satellites in turn affect the Milky Way disc. The Milky Way disc is the dominant mass component of the Milky Way interior to the solar circle. It is important because dynamical friction against the disc causes satellites to be preferentially dragged into the disc plane… As satellites are torn apart by tidal forces, they deposit both their stars and their dark matter into a thick disc. The latter point is the key new idea presented in this work: a dark matter disc must form in a ?CDM cosmology and we set out to quantify its mass and kinematic properties.

While we are seeing the dark matter puzzle examined through simulation and observation, we are a long way from fully integrating its effects into theories of galaxy formation. The work is knotty, highly theoretical and carries the almost surreal excitement of making sense out of something we cannot see. The simulation paper is Read et al., “Thin, Thick and Dark discs in ?CDM,” Monthly Notices of the Royal Astronomical Society 389 (2008), pp. 1041-1057 (abstract). The paper on Segue 1 is Geha et al., “The Least Luminous Galaxy: Spectroscopy of the Milky Way Satellite Segue 1,” accepted by the Astrophysical Journal and available online.