A New Probe of Dark Energy

What we know about dark energy can be pretty much summed up in these words: “We know that it dominates the universe. In fact, it comprises an estimated 73 percent of the universe, while so-called dark matter accounts for 23 percent, and matter of the familiar kind — the stars, galaxies, all known life — comprises only four percent.”

The speaker is David Lambert, a University of Texas at Austin astronomer and the director of UT’s McDonald Observatory. And it has always seemed to Centauri Dreams that these numbers — what we know and see of the universe is no more than four percent of the total — should inspire a certain humility. Yes, we know more than those before us, but just how far we are from comprehending the nature of ‘reality’ seems obvious. There are real reasons to wonder whether the human mind is capable of ever understanding ultimate reality.

Perhaps a quote from Martin Rees about the beginning of the universe is appropriate:

“There are lots of ideas of what might have happened at the very beginning, but we can’t say whether there are other big bangs apart from ours. If there are, we can’t say whether they are before or after or alongside ours, because to make such a statement implies that you can have a single coordinate system covering them all and a single clock that can be coordinated and synchronized between the different universes. So we can’t trace things right back to the beginning, we can’t say whether our universe is the only one, and we can’t even say whether there are only three dimensions of space.”

But now we’re on the edges of string theory, so perhaps it’s time to focus on something less abstract. How about cold cash? Money talks in cosmology, or at least the study of it, and the news that businessman Harold C. Simmons has ponied up a $5 million challenge grant — he’ll match the next $5 million in private support received — to the University of Texas is heartening. Simmons is trying to support people like the aforementioned Dr. Lambert in their quest to run HETDEX — the Hobby-Eberly Telescope Dark Energy Experiment, at McDonald Observatory.

And Lambert, who calls the nature of dark energy ‘…the number-one question in all of science,” believes that the combination of large telescope, plentiful observing time and an instrument that will produce a three-dimensional map of up to one million galaxies may help us solve the riddle. Or, at least, make the kind of incremental progress that is at the heart of tackling the universe on its own terms, doggedly pushing back the frontiers of the knowable without being blinded by prior assumptions. As for Simmons, what an example of the role philanthropy can play in opening up the cosmos!

Cometary Breakup Continues

73P/Schwassmann-Wachmann is proving to be a far more interesting object than first anticipated. The comet is closing toward the Sun and will swing around it on June 7, passing the Earth along the way at a distance of 11.7 million kilometers. The fascination comes from watching its ongoing disintegration, which has broken the comet into more than 30 separate fragments.

Nor is the show over. The larger fragments appear to be continuing their breakup. In the image below, taken by the Hubble Space Telescope, you can see one of the major fragments breaking into smaller chunks, dozens of which trail behind the main piece. The chunks are evidently pushed back along the tail by outgassing from their Sun-facing surfaces, and the smallest of them seem to be dissipating completely over a multi-day period.

Breakup of a Comet

Image: The second image from a three-day observation with Hubble showing the breakup of Comet 73P/Schwassmann-Wachmann 3’s Fragment B. Credit: NASA, ESA, H. Weaver (APL/JHU), M. Mutchler and Z. Levay (STScI).

As far back as 1995, 73P/Schwassmann-Wachmann was seen to be active, brightening a thousand-fold during the period of observation with five separate nuclei subsequently identified. The comet’s orbit takes it around the Sun every 5.4 years, but the next detection in 2000-2001 revealed only three fragments. This time around the show is considerably more interesting. It seems likely that what we are seeing now is an accelerated disintegration of the comet, making it open to conjecture how many of these fragments will be around to view in 2011-12.

Fragments B and C should be visible with binoculars on closest approach to Earth, which will occur on May 11. Neither object should be that remarkable unless another outburst occurs. But if it does, backyard astronomers may have quite a show to look at. The breakup process, observed in numerous earlier comets, is always accompanied by significant brightening. It was, in fact, the initial breakup of Shoemaker-Levy 9 in 1992 that made this famous comet bright enough to be detected eight months later. Shoemaker-Levy had its date with destiny when its fragments plowed into Jupiter in 1994, but 73P/Schwassmann-Wachmann poses no threat whatsoever to Earth.

Via Solar Array to the Outer Planets

New Scientist is covering the work of Rudolph Meyer (UCLA), who envisions a vehicle that sounds for all the world like a cross between a solar sail and an ion engine. And in a way, it is: Imagine a flexible solar panel a solid 3125 square meters in size, and imagine this ‘solar-electric membrane’ weighing no more than 16 grams per square meter, far lighter than today’s technology allows. I’ll be anxious to see the paper when it’s published in Acta Astronautica, but the gist of the design seems to be this: the solar membrane would power an ion engine array which, conventionally enough, draws xenon ions through a powerful electric grid to create thrust.

The membrane, stabilized by additional ion engines at the corners, could reach remarkable speeds. Meyer talks about 666,000 kilometers per hour — that’s one year to Pluto, and an obvious invitation out into the Kuiper Belt. No show stoppers here, but clearly a design heavily dependent on advances in thin film arrays. I always listen to Geoffrey Landis (NASA GRC) about such matters; he is, after all, the man Robert Forward declared to be his successor in interstellar studies. And Landis is quoted as saying of Rudolph’s idea, “…the extremely high-energy ion-propulsion vehicles he proposes may be a practical alternative technology for future missions to the edge of interstellar space.”

The article is by Paul Marks, “Will a flying carpet take us to Pluto?” in New Scientist (29 April 2006), available here but only for subscribers.

Update: Geoffrey Landis was kind enough to forward the complete text of his comments to New Scientist (the magazine quoted only the last sentence). Landis wrote: “Professor Meyer suggests an interesting thought-experiment about what may be possible in the future. The solar array needed for his mission requires reducing the mass of solar arrays by several orders of magnitude from existing technology. Much development work, including our work at NASA Glenn, has been addressed at reducing the weight of space solar arrays by adapting thin-film technologies to high-efficiency photovoltaic technology, and the performence he quotes, although extremely agressive, may be possible in the future. If this can be achieved, the extremely high energy ion-propulsion vehicles he proposes may be a practical alternative technology for future missions to the edge of interstellar space.”

How aggressive is Meyer’s idea? Landis notes in his accompanying message that Meyer is quite optimistic about the weight of his solar-electric membrane — “…he was assuming that they could get the performance of single crystal cells, with the weight of a solar sail.” But that’s the kicker, and if it can be achieved the idea may be practicable.

A Powerful Resource on ‘Hot Jupiters’

That oh so interesting planetary system around 51 Peg continues to fascinate exoplanet hunters. After all, this was the first planetary discovery around a main sequence star, and the formidable dimensions of the radial velocity dataset accrued before and since the discovery may lead to other planets in the same system. For the latest on 51 Peg, check the online proceedings of last August’s colloquium, in which eighty astronomers provide their thoughts on ‘hot Jupiters’ and discuss recent observational facts about this intriguing system.

Centauri Dreams is always delighted to see conference proceedings made available online. The only catch here is that the format is PDF — yes, it’s a standard of sorts, but there has to be a better way…

Ultraviolet Limits on Habitable Worlds

Habitable zones, and our idea of what constitutes them, change over time. We know, for example, that the habitable zone around a given star should migrate outward as main sequence stars become brighter with age. Thus the notion of the ‘continuously habitable zone’ (CHZ) has emerged, the region where a planet remains in habitable conditions for a specified period of time. If you want to look for technological civilizations, that time frame might be 4 billion years, paralleling the experience of life on our own planet. If you’re content to look for microbes, as little as a billion years might suffice, perhaps less.

Among the numerous factors involved in creating the CHZ, ultraviolet radiation is significant. A new paper points out the need to assess UV and the limits it places upon emerging biospheres. Get too much of it and you inhibit photosynthesis, as well as damaging DNA and various proteins. Get too little and you dampen a primary energy source for the synthesis of biochemical compounds. So just as older markers like the presence of liquid water can define a habitable zone, so too can the properties of ultraviolet radiation constrain the formation of life.

In defining the UV habitable zone, Andrea Buccino (Instituto de Astronomía y Física del Espacio, Buenos Aires) and colleagues try to establish both an inner limit (not so close to the star as to damage DNA) and an outer (beyond which there is not sufficient UV for biogenesis to occur). They then apply their criteria to all nearby stars with exoplanets that have been studied in UV by the International Ultraviolet Explorer (IUE). This provides a dataset of 23 stars harboring 32 different planets.

Intriguingly, in most cases the ultraviolet habitable zone is closer to the star than the traditional habitable zone. “In those cases, UV radiation inside the traditional HZ would not be an efficient source for photolysis, and therefore the formation of the macromolecules needed for life would be much more difficult, if not completely impossible,” the authors write. In fact, stars like 51 Peg and HD160691 show no overlap between the UV region and the habitable zone; fully 59 percent of the sample fits in this category.

The authors find seven cases where the traditional habitable zone and the UV zone overlap at least partially, allowing the presence of a habitable planet. But in three of these cases, the presence of a giant planet would make terrestrial-style orbits unstable. Five extrasolar systems have giant planets inside the traditional habitable zone but four of these are at the extremes of the ultraviolet zone. The conclusion:

Applying all these criteria to those stellar systems whose central star has been observed by IUE, we obtained that an Earth-like planet orbiting the stars HD216437, HD114752, HD89744, ? Boo and Rho CrB could be habitable for at least 3 Gyr. A moon orbiting ? And c would be also suitable for life. While, in the 59% of the sample (51 Peg, 16CygB, HD160691, HD19994, 70 Vir, 14 Her, 55 Cnc, 47 UMa, ? Eri and HD3651), the traditional HZ would not be habitable following the UV criteria exposed in this work.

And this about F stars: the two studied in the sample, HD114762 and ? Boo, would demand layers of atmospheric protection far higher than that of the early Earth to protect against life-threatening UV.

Centauri Dreams‘ take: How planets cope with incoming ultraviolet is also an area that needs more research. The authors point out possible attenuating effects from planetary atmospheres, oceans, orbital factors and more, all of which have bearing on the result. It is clear that our traditional notions of what makes for habitability are in a state of revision as we learn more about our own ecosphere. The paper, slated for publication in Icarus, is Buccino, Lemarchand and Mauas, “Ultraviolet Radiation Constraints around the Circumstellar Habitable Zones,” available here.