Ancient Galaxies Packed with Stars

Just how different were things in the early universe? One answer comes from a study of galaxies whose light has taken eleven billion years to reach us. In this early era — the universe would have been less than three billion years old — researchers have found galaxies so unusually compact that they compress a galaxy’s worth of stars into a space only five thousand light years across. Such objects would be able to fit into the central hub of the Milky Way. What’s more, these ultra-dense galaxies may account for as much as half the number of all galaxies of their mass that existed at this time.

“In the Hubble Deep Field, astronomers found that star-forming galaxies are small,” said Marijn Franx of Leiden University, The Netherlands. “However, these galaxies were also very low in mass. They weigh much less than our Milky Way. Our study, which surveyed a much larger area than in the Hubble Deep Field, surprisingly shows that galaxies with the same weight as our Milky Way were also very small in the past. All galaxies look really different in early times, even massive ones that formed their stars early.”

View within an ultradense galaxy

I sometimes try to imagine the scene from within a globular cluster, a sky awash with tightly packed stars. A compact galaxy must have offered any denizens of eleven billion years ago quite a view as well, as in the artist’s concept above, which shows the view from a hypothetical planet in a distant ultradense galaxy, its sky packed with thousands of stars. We would see 200 times more stars here than we see in Earth’s night-time sky. Image credit: NASA, ESA, G. Bacon (STScI), and P. van Dokkum (Yale University).

Today’s far more slowly rotating galaxies have correspondingly slower moving stars. But these early, compact galaxies show stars moving around their cores at 500 kilometers a second. The speculation now turns to just how such galaxies wound up becoming enlarged over the ensuing aeons. Galactic collisions are an obvious possibility, but there is much to learn about other processes that may have been at work, and the role of dark matter in all this may be consequential. The paper on this work goes out of its way to note that the main mechanism for moving from these early galaxies to the size and density of a more mature galaxy like the Milky Way may not have been identified.

Another wild card: The observed densities may be overestimated because of uncertainties in the analysis, about which the authors list several possibilities. Hubble’s Wide Field Camera 3, set for fall of 2008 installation via Servicing Mission 4, may be able to tighten things up. The paper is van Dokkum et al., “Confirmation of the Remarkable Compactness of Massive Quiescent Galaxies at Z ~ 1.3: Early-Type Galaxies Did Not form in a Simple Monolithic Collapse,” Astrophysical Journal Letters 677, pp. L5-L8 (April 10, 2008). Abstract available.

The ‘Great Filter’ Tackles Fermi

Suppose for a moment that life really is rare in the universe. That when we are able to investigate the nearby stars in detail, we not only discover no civilizations but few living things of any kind. If all the elements for producing life are there, is there some kind of filter that prevents it from proceeding into advanced and intelligent stages that use artifacts, write poetry and build von Neumann probes to explore the stars? Nick Bostrom discusses the question in an article in Technology Review, with implications for our understanding of the past and future of civilization.

Choke Points in the Past

Maybe intelligent beings bring about their own downfall, a premise that takes in more than the collapse of a single society. Alaric’s Goths took Rome in 410, hastening the decline of a once great empire, but the devastated period that followed saw Europe gradually re-build into the Renaissance. And as Bostrom notes, while a thousand years may seem like a long time to an individual, it’s not terribly significant in the overall scheme of a civilization, which theoretically might last millions of years. No, a true filter must be something larger, a potential civilization-killer.

A galaxy like our own

Bostrom’s idea of a ‘Great Filter’ comes from Robin Hanson (George Mason University), and consists of the kind of transition that a civilization has to endure to emerge as a space-faring culture. The key question: Is the filter ahead of us or behind? If behind, wonderful — we have already passed the test and can look with some confidence to the future. Recent work, for example, indicates that human beings were reduced to a band of as little as 2000 individuals some 70,000 years ago, near extinction. Yet somehow migrations out of Africa began 60,000 years ago, and all the tools of civilization would emerge in their wake.

Image: The galaxy NGC 6744, a barred spiral about thirty million light years from Earth. Is it possible that such vast congregations of stars may be utterly devoid of life? Credit: Southern African Large Telescope (SALT).

But that’s a filter that still gets intelligent life well on its way, and surely with the number of stars in our galaxy, that would imply at least a few civilizations should have made it through besides ourselves, their presence obvious by now. No, to explain the Fermi paradox, we would like to go further back, making the emergence of complex life of any kind problematic. Making it, in fact, so rare that a galaxy devoid of it (other than here on Earth) is an explicable outcome. That kind of filter gives us hope, because we’ve survived it even though no one else has. The galaxy may be empty of life, but it is also a vast frontier awaiting our expansion.

The Shape of Future Menace

But maybe the filter is still ahead of us. If so, we may be able to see its outline in fairly familiar terms, such as nuclear war, asteroid impact, genetically engineeered disease used as weaponry, and so on. Or maybe, and more likely, it’s something we cannot foresee:

The study of existential risks is an extremely important, albeit rather neglected, field of inquiry. But in order for an existential risk to constitute a plausible Great Filter, it must be of a kind that could destroy virtually any sufficiently advanced civilization. For instance, random natural disasters such as asteroid hits and supervolcanic eruptions are poor Great Filter candidates, because even if they destroyed a significant number of civilizations, we would expect some civilizations to get lucky; and some of these civilizations could then go on to colonize the universe. Perhaps the existential risks that are most likely to constitute a Great Filter are those that arise from technological discovery. It is not far-fetched to imagine some possible technology such that, first, virtually all sufficiently advanced civilizations eventually discover it, and second, its discovery leads almost universally to existential disaster.

Better to have the Great Filter behind us. Then, at least, we know that we are here and that the experience was survivable. And the parameters of the filter have implications for our search for life. Bostrom hopes we find no sign of life elsewhere because such a find would imply that life is commonplace, that the Great Filter kicked in after the point in evolution that that life represents. Well and good if the discovered lifeforms were simple — we could still assume the filter operated early in evolutionary history and that we are past it. But if we found complex life, this would eliminate a larger set of early evolutionary transitions as the filter, and would imply that it is ahead rather than behind us.

Explaining the Great Silence

Remember, we are trying to explain why we are not finding signs of intelligence elsewhere, no von Neumann probes, no artifacts from civilizations that should have had plenty of time to expand through the galaxy. In Bostrom’s view, no news from the stars may actually be good news. It could imply that life itself is improbable, that the Great Filter happened well in our past and we somehow survived it, and that therefore we may be able to make the transition to a higher and better civilization. We are the one species lucky enough to make it this far, and while we cannot rule out the possibilities of other Great Filters lying ahead, we can at least hope we have weathered the worst.

All of which seems to put Earth back into the center of the universe again, a bizarre exception to the overwhelming norm. Bostrom thus has no choice but to explain the observation selection effect, a way to make sense out of our good fortune in being the lucky exception to the rule:

Consider two different hypotheses. One says that the evolution of intelligent life is a fairly straightforward process that happens on a significant fraction of all suitable planets. The other hypothesis says that the evolution of intelligent life is extremely complicated and happens perhaps on only one out of a million billion planets. To evaluate their plausibility in light of your evidence, you must ask yourself, “What do these hypotheses predict I should observe?” If you think about it, both hypotheses clearly predict that you should observe that your civilization originated in places where intelligent life evolved. All observers will share that observation, whether the evolution of intelligent life happened on a large or a small fraction of all planets. An observation-selection effect guarantees that whatever planet we call “ours” was a success story.

Into a Barren Universe

Bostrom is director of the Future of Humanity Institute at Oxford, a transhumanist philosopher (this is George Dvorsky’s description) who notes that even if the Great Filter were in our past, this would not absolve us from future danger. But this is a man who would like to see all that interesting technology, from nanotech to life extension, kicked in to provide us with a ‘posthuman’ existence whose outline we cannot presently imagine. He’s actively pulling against finding life anywhere else because he’s convinced that life’s rarity implies most organisms run into a buzzsaw before they can colonize space. We survivors, then, may find no one else to talk to, but we should have a fighting chance to use our technologies in a transformative way.

And here is where I truly disagree with Bostrom:

…surely it would be the height of naïveté to think that with the transformative technologies already in sight–genetics, nano­technology, and so on–and with thousands of millennia still ahead of us in which to perfect and apply these technologies and others of which we haven’t yet conceived, human nature and the human condition will remain unchanged. Instead, if we survive and prosper, we will presumably develop some kind of posthuman existence.

I see no evidence in history that the basics of human nature are amenable to change, whether or not such change would be a positive or negative thing. Nor can I go along with those who think we will be able to control our own evolution into some kind of higher lifeform, but long-time readers know my doubts that a genuine ‘transhumanism’ is possible to us. That would be another discussion, though, and I leave this one with the thought that if complex life of any kind is rare, we may have survived only to move outwards into an unexpectedly bleak universe.

Surface Oceans Around Distant Stars

Would large amounts of water on the surface provide a glint of light in both the infrared and visible spectrum if we study a distant exoplanet long enough? That’s the premise of an investigation now in progress, one aiming to find Earth-like planets in the habitable zone of a star. Darren Williams (Penn State Erie) and Eric Gaidos (University of Hawaii) have something more in mind than analyzing a planetary atmosphere for signs of water. They want to spot planets with water on the surface.

If the goal sounds chimerical now, bear in mind that various planet-hunting missions like Terrestrial Planet Finder (in its various incarnations) and Darwin are being designed to allow direct observation of planets as small as the Earth. Such observatories, which may be in place within two decades or less, could also examine the visible and infrared light curve of such planets over the course of an entire orbit.

“We are going to look at the planets for a long time,” says Williams. “They reflect one billionth or one ten billionth of their sun. To gain enough light to see a dot requires observation over two weeks with the kinds of telescopes we are imagining. If we stare that long, unless the planet is rotating very slowly, different sides of the planet will come through our field of view. If the planet is a mix of water, we are going to see the mix travel around the planet.”

According to the paper on this work, half of all detected extrasolar planets will have orbital inclinations that make it possible to detect surface oceans. When we looked at this idea back in January, one thought particularly stood out from the team’s paper: “… of all the extremely difficult measurements astronomers hope to make with a TPF-class telescope, time-series photometry and polarimetry that can lead to the identification of specular reflection from surface water might be the easiest.” This Penn State news release catches up with the current thinking of the researchers.

The pale blue dot photographWhat to do while we wait for a mission capable of making such detections? One idea is to do what Voyager did long ago in the famous ‘Pale Blue Dot’ photograph, looking back at our own planet to study its signature at various wavelengths. Voyager’s view was fascinating largely for what it represented — our world from the outer reaches of the Solar System — but Williams has now enlisted the Mars Express and Venus Express missions to occasionally view the Earth and examine its various phases. That data should provide a useful baseline for the kind of studies that may one day find Earth’s twin.

Image: The famous ‘pale blue dot’ photograph taken by Voyager 1. Looking back at our planet from distant viewpoints will tell scientists much about detecting water on distant worlds. Credit: NASA/JPL.

And if we do find a world in the habitable zone with liquid water on the surface? The odds for life will obviously go up, supplemented by whatever spectrographic data we gather. As to intelligence, we all have our viewpoints on its likelihood, but such observations may prove inconclusive. About the only certainty we’ve had in our explorations of our own Solar System is that we have continued to be surprised, so it’s reasonable to assume we’ll run into more than our share of enigmas around other stars.

The paper is Williams and Gaidos, “Detecting the Glint of Starlight on the Oceans of Distant Planets,” upcoming in Icarus and available online. Let me also recommend Greg Bear’s Queen of Angels for a look at just how tantalizing (and confounding) the detection of exoplanetary life can be, in this case by a probe in the Centauri system.

Dark Matter: Flashes Beneath the Earth

Dark matter is interesting in its own right, a mysterious ‘something’ that according to WMAP data must account for 23 percent of the universe (the breakdown now thought to be 72 percent dark energy, 23 percent dark matter, 4.6 percent atoms and less than 1 percent neutrinos). From a propulsion standpoint, dark matter intrigues us because it may represent a reaction mass conceivably useful for future space flight. It’s also Nobel Prize territory for the team that identifies it, which is why so many teams are looking, with one team’s provocative results drawing criticism.

The Italian and Chinese physicists on the DAMA Project have held out since 2000 for their claim that they are detecting dark matter beneath the Gran Sasso mountain in Italy. The modulation is yearly and could represent the Earth’s motion through a dark matter stream as it orbits the Sun. The larger DAMA/LIBRA experiment now reaffirms the phenomenon, which appears as flashes in the team’s sodium iodide detector. With the rate of flashes highest in June and lowest in December, the findings are provocative.

Such flashes could signal WIMPs (weakly interacting massive particles), which pass through most matter as if it were not there. But are the flashes DAMA/LIBRA is seeing enough to claim a detection of dark matter, or do they merely open up a range of possibilities? Cosmic Variance recently let Juan Collar (University of Chicago), himself a member of a dark matter team, loose on the question. Collar believes the modulation the team is finding is unmistakable, but disputes the team’s conclusions:

…to conclude from something this mundane that the experiment “confirms evidence of Dark Matter particles in the galactic halo with high confidence level” or that there is “an evidence for the presence of dark matter particles in the galactic halo at 8.2 sigma confidence level” is simply delusional. There is evidence for a modulation in the data at 8.2 sigma, stop. Compatible with what would be expected from some dark matter particles in some galactic halo models, full stop. Anything beyond this is wanting to believe, and it smears on the rest of us in the field. Of course, of course… there is no other observed process in nature that peaks in the summer and goes through a low in winter, so this must be dark matter, right? (Occam is turning in his grave, rusty razor still in hand. He is thinking a remake of that opening scene in “Un chien andalou“, with help from this little lady. I am channeling him loud and clear).

Collar is obviously a lively guy with eclectic tastes. I had to look up Un chien andalou, which turns out to be a surrealist film by Luis Buñuel and Salvador Dalí, one whose Wikipedia summation leaves me inclined to avoid it. He may be right about Occam, though. What about the possibility of experimental error in the DAMA/LIBRA experiments? Collar thinks we can’t rule it out, and is critical of the team’s attempts thus far to do so.

But this is not a slash and burn job on a particular experiment. Collar calls the work of DAMA/LIBRA ‘phenomenal’ on many fronts, and finds much to admire in it. But his appraisal is leavened with deep skepticism, and it’s one I wanted to call to your attention because of Collar’s high visibility in this area (he’s quoted in this New York Times article on the difficulty of dark matter detection) and because his lengthy post is a great example of what weblogs can do to spread good science from the source.

Centauri Dreams takes no position at all on DAMA/LIBRA other than to report the ongoing controversy, which all dark matter watchers will follow with interest. As I say, whoever does confirm a dark matter detection is easily in Nobel range. On that score, be aware as well of the Large Underground Xenon detector (LUX), which will look for evidence of dark matter in an abandoned South Dakota goldmine. Here we’re talking about 600 pounds of liquid xenon suspended in a huge water tank, some 4800 feet underground in the Homestake mine near Lead, South Dakota. The signature of dark matter would again be the flashes given off by WIMPs as they hit xenon atoms. Expect this experiment to start coming together later in the year as the mine is prepared to receive its equipment.

Down and Dirty in the Data hosts the 51st Carnival of Space, a lengthy compilation indeed, from which I’ll draw Ian Musgrave’s interesting post on a possible transit at 83 Leonis as the feature of the week. If you want to find out what it’s like to get your hands dirty juggling the data, trying to sift out signal from noise and working with all the imponderables that go into spotting the signature of a transiting world, have a look. Ian finds a noisy 83 Leonis but one that just might show a transit. A self-described ‘mathematically challenged biologist,’ this is a writer whose work is always worth watching. In this case, what he’s doing reflects the broadening participation of amateurs in exoplanet projects, an idea Greg Laughlin has championed, so it’s no surprise to see that Ian has drawn from Laughlin’s expertise in his current work.