by Paul Gilster | Dec 18, 2008 | Exotic Physics |
The best news about recent dark energy findings is that they offer new ways to study the phenomenon. It’s only been ten years since dark energy — thought to be the origin of the universe’s accelerating expansion — emerged from the study of supernovae. Simply put, these exploding stars weren’t slowing as they moved away from us, but were actually speeding up. It was a controversial result, to say the least, and one which remains one of science’s primary riddles. But Chandra X-ray Observatory observations may be providing additional clues.
The team on this work is led by Alexey Vikhlinin (Harvard-Smithsonian Center for Astrophysics), its effort focused on galactic clusters. A model of the cosmos that incorporates dark energy is the only thing that explains why these clusters have grown so slowly during the last five billion years, in what Vikhlinin calls “arrested development of the universe.” Dark energy seems to be working against the gravitational forces that allow clusters to draw in new matter.
Image: The galaxy cluster Abell 85, located about 740 million light years from Earth. The purple emission is multi-million degree gas detected in X-rays by NASA’s Chandra X-ray Observatory and the other colors show galaxies in an optical image from the Sloan Digital Sky Survey. This galaxy cluster is one of 86 observed by Chandra to trace how dark energy has stifled the growth of these massive structures over the last 7 billion years. Galaxy clusters are the largest collapsed objects in the Universe and are ideal for studying the properties of dark energy, the mysterious form of repulsive gravity that is driving the accelerated expansion of the Universe. Credit: NASA/CXC/SAO/A.Vikhlinin et al.
Better still, the results dovetail nicely with the supernovae work, a vindication of the idea that the universe is dominated by dark energy. The fascination of this work is in just how big the questions are. Dark energy could be the cosmological constant described by Einstein, an energy he initially believed could work against the force of gravity to keep the universe from collapsing. These findings keep that prospect alive. The power of dark energy is denoted by the number w, the equation of state, and Vikhlinin’s team derives a value for w not all that dissimilar from that of the supposed cosmological constant. But huge issues remain.
Ponder this: Just as we know little about dark energy, we know little about dark matter, whose presence seems necessary to explain how galaxies form around us in the cosmos. Any studies of galactic clusters have to factor in dark matter, but doing so puts us on tentative ground without a firmer basis for understanding what dark matter actually is. Dennis Overbye picks up on all the uncertainties in a recent New York Times story, in which he notes that the combined supernova and galactic cluster measurements, folded in with observations of the cosmic microwave background, have tightened up our estimate of w:
That looks like an improvement of a factor of 2 on the uncertainty charts that dark energy specialists show to one another in meetings and papers, but critics caution that combining disparate types of measurements can result in an artificially small error that masks underlying uncertainties. Astronomers, for example, still do not have a good theory to explain how their standard candles, supernovas, explode, and theories of cluster growth depend on assumptions about the nature of the dark matter in the universe and the nature of the original fluctuations that give birth to them.
This tantalizing work on galactic clusters, then, implies more than it proves, but the implications are interesting indeed. Thus William Forman (Smithsonian Astrophysical Laboratory), a co-author of the study that will appear in February:
“Putting all of this data together gives us the strongest evidence yet that dark energy is the cosmological constant, or in other words, that ‘nothing weighs something.’ A lot more testing is needed, but so far Einstein’s theory is looking as good as ever.”
Dark energy remains a personal fascination for two reasons. An effect that seems to oppose gravity is of obvious interest in that it promises new insights into physics. A sufficiently advanced civilization might one day harness such a force for propulsion. In the broader sense, dark energy points out how much we have yet to learn, a necessary reminder that a certain humility is a virtue in the study of cosmic effects so recently uncovered. Again we turn to Einstein, no stranger to re-thinking big ideas, who once said “Whoever undertakes to set himself up as a judge of Truth and Knowledge is shipwrecked by the laughter of the gods.”
by Paul Gilster | Dec 17, 2008 | Exoplanetary Science |
The nice thing about our conventional idea of a habitable zone is that liquid water can exist on the surface. The less helpful part of that definition is that water is more readily available much further out in a planetary system, where it usually shows up as ice. Think in terms of the ‘ice line,’ or the ‘snow line.’ Beyond it is the area around the still-forming star where temperatures are low enough to allow hydrogen compounds to condense into ice grains.
Of course, we’re living proof of the fact that planets in the inner system can be covered with oceans. It’s therefore plausible to think in terms of delivery mechanisms, with icy comets bombarding planets in the inner system to produce oceans like those on Earth. But we’re learning to extend our reach beyond conventional habitable zone notions to places much further out, an idea recently given credence by divers hands.
Consider the work of Scott Gaudi (Ohio State), Eric Gaidos (University of Hawaii) and Sara Seager (MIT), familiar names to long-time Centauri Dreams readers. Recognizing the wealth of water resources in outer solar systems, the trio look to cold super-Earths, planets whose water did not have to be delivered by external means. An internal heat source might keep a liquid water ocean viable under the ice, assuming a massive world in the right place, even if that planet were five times farther out than the Earth.
“It turns out that if super-Earths are young enough, massive enough, or have a thick atmosphere, they could have liquid water under the ice or even on the surface,” Gaudi said. “And we will almost certainly be able to detect these habitable planets if they exist.”
By ‘massive enough,’ Gaidos is talking about a super-Earth ten times as massive as the Earth. The scientist reported these results at the American Geophysical Union meeting in San Francisco on Monday. The issue of detection seems clear enough — we’re making such strides in finding exoplanets that tracking down new super-Earths is more or less a given, especially since some are saying that a third of all solar systems probably contain them.
Right now, gravitational microlensing seems to be the best method for detecting planets at 5 AU or more. The planetary signature is found in changes to the magnification caused when a star passes in front of a more distant one as seen from Earth. A planet around the nearer star creates a secondary boost in the lens-like magnification, allowing not just detections at some distance from the star, but also detections around stars much farther away than would be feasible using radial velocity or transit methods. Even so, recent direct imaging successes remind us that the next generation of telescopes may also deliver many a super-Earth.
Proving the astrobiological case for these super-Earths is a tricky matter indeed. It may well take a dedicated lander on the surface of Europa, for example, to tell us about possible life there by drilling into the ice. How do we resolve the question of life on a distant super-Earth? The issue will remain open for years to come, but in short order we’re going to be finding so many of these interesting worlds that we’ll have plenty to speculate about when it comes to life’s formation around other stars.
by Paul Gilster | Dec 16, 2008 | Astrobiology and SETI |
By Larry Klaes
Tau Zero journalist Larry Klaes gives us a look at the immense contribution of physicist Giuseppe Cocconi to SETI. It’s sobering to realize how new a study SETI really is. Frank Drake’s Project Ozma began less than fifty years ago, while estimates of the number of extraterrestrial civilizations are just now scaling back dramatically from the numbers Drake himself and Carl Sagan once used (Claudio Maccone’s recent work on the Drake Equation arrives at an estimate of 250 such civilizations in the Milky Way — more on this soon). If it weren’t for the efforts of Cocconi and Philip Morrison, the theorizing behind the Drake Equation and the development of SETI itself might have been slowed for years, as Larry points out so ably below.
On November 9, the world said farewell to physicist Giuseppe Cocconi, who passed away at the age of 94. Although his life’s work was in particle physics and cosmic ray science, Cocconi will always be best known for co-authoring the paper with a fellow Cornell scientist that sparked the modern Search for Extraterrestrial Intelligence, or SETI.
Born in Como, Italy in 1914, Cocconi was invited to join the faculty at Cornell University in 1947 by none other than the late Cornell physicist Hans Bethe, who won the Nobel Prize for physics in 1967. Cocconi had a fruitful career as a full professor with the university, where his research determined the galactic and extragalactic origin of cosmic rays. Cocconi left Cornell in 1963 to work at CERN, the French acronym for the European Organization for Nuclear Research. Four years later, he became the Director of Research at CERN, the institution that would later build and operate the Large Hadron Collider, the biggest particle accelerator on Earth to date.
While on a sabbatical leave at CERN in 1959, Cocconi began a discussion with a fellow Cornell physicist named Philip Morrison, who had arrived at the university just one year before him from the Manhattan Project that created the first atomic bombs. Morrison recalled to sociologist and author David Swift in his 1990 book SETI Pioneers: Scientists Talk About Their Search for Extraterrestrial Intelligence (University of Arizona Press, Tucson) that
“…one spring day in 1959, Giuseppe Cocconi came into my office. We were thinking about gamma rays of natural origin when we realized that we knew how to make them, too. We were making lots of them downstairs at the Cornell synchrotron. So Cocconi asked whether they could be used for communicating between the stars. It was plain that they would work, but they weren’t very easy to use. My reply was enthusiastic but cautious. Shouldn’t we look through the whole electromagnetic spectrum to find the best wavelength for any such communication? That was the germ of the idea.”
Cocconi himself explained to Swift that he felt “…a narrow burst of gamma radiation could be a signal that can travel far and straight in galactic space and be peculiar enough to be recognized. And that was the triggering.”
Cocconi and Morrison brainstormed on the most efficient method for communicating among stars in the galaxy. They determined that radio waves would be usable by just about any technological civilization with even a general understanding of astronomy and physics.
Image: Physicist Philip Morrison, Cocconi’s collaborator on their pioneering SETI work.
This was not the first time that radio had been considered for sending messages between worlds. In 1924, radio operators eagerly listened for any transmissions originating from the planet Mars, which was closer than usual to Earth that year. They thought and hoped that any intelligent Martians might also note the orbital closeness of their two worlds and signal their presence to the inhabitants of the third world from the Sun. Needless to say, no definite radio messages were received from the Red Planet, but the idea of using radio as a means of communicating across space was about to gain a higher measure of acceptability.
Once they had established the method, Cocconi and Morrison began to think about the possible frequencies an ETI might transmit on out of the millions of radio channels to choose from. They eventually settled upon the emission frequency of neutral hydrogen, 1420 MegaHertz, the most abundant element in the Universe. The frequency was also chosen for the fact it was in a relatively quiet region of the natural radio spectrum. If an alien civilization wanted its artificial signals to be noticed among all the natural celestial noises, this would be a logical frequency to transmit on. This proposal would later be described as a cosmic ‘water hole’, where intelligent beings might meet in a common area in the radio spectrum to send their interstellar messages, just as animals of different species on this planet often congregate together at a literal water hole to drink.
Shortly after discussing his interstellar communication idea with Morrison, Cocconi realized that a 250-foot wide radio telescope constructed at Jodrell Bank in England just two years earlier would be able to send a detectable signal to the Solar System’s nearest stellar neighbor, Alpha Centauri. Conversely, such a giant dish antenna should also be able to pick up strong electromagnetic transmissions from any technological beings living around the nearest suns. Cocconi wrote to the astronomer in charge of Jodrell Bank, Sir Bernard Lovell, to ask if the steerable instrument that would later bear the English astronomer’s name could be used to search for any intelligent beings in the Milky Way galaxy. Sir Bernard did not share Cocconi’s enthusiasm about the potential for the radio telescope to open up a whole new vista in our understanding of the cosmos and turned Cocconi down.
Undeterred, Cocconi and Morrison put their research findings and ideas into a two-page paper titled “Searching for Interstellar Communications,” which was published in the September 19, 1959 edition of the periodical Nature (available online).
The subject of SETI gained new credibility in the science community with the appearance of an article dealing with this concept written about and supported by two professional physicists and published in such a prestigious journal as Nature. Many of the ideas Cocconi and Morrison proposed for detecting alien transmissions became standards in the SETI field in the decades that followed their paper. For example, to make certain that the recipient of a radio signal from an ETI would know it was artificial in origin, the authors proposed that “…one signal might contain, for example, a sequence of small prime numbers of pulses, or simple arithmetical sums.” Fellow Cornellian Carl Sagan utilized this very proposal in 1985 in his only science fiction novel, Contact, which had a vast galactic society get humanity’s attention by transmitting the first 100 prime numbers (digits which are only divisible by themselves and one) in its opening radio message to Earth.
Image: Dr. Vanna Cocconi (R) and Dr. Giuseppe Cocconi checking a horizontal row of recording counters which detect cosmic ray showers. Credit: LIFE Magazine, September 1948.
Just months after the publication of “Searching for Interstellar Communications,” a scientist named Frank Drake, who would later become yet another member of the Cornell faculty and participate in a number of extraterrestrial communications projects with Sagan, conducted the first modern SETI project, which he called Ozma. In early April of 1960, Drake used a large radio telescope that belonged to the National Radio Astronomy Observatory (NRAO) at Green Bank in West Virginia to listen for any potential ETI signals transmitting on the 1420 MegaHertz frequency from two nearby Sunlike star systems, Tau Ceti and Epsilon Eridani. Though no artificial signals of alien origin were detected in this relatively short experiment, Drake’s project followed several of the suggestions from Cocconi and Morrison’s Nature paper and further helped to establish SETI as a mainstream science.
In November of 1961, Cocconi and Morrison found themselves traveling to Green Bank as members of a small but highly select group of scientists gathered to discuss the issues surrounding the idea of communicating with extraterrestrial intelligences. Known as the Green Bank Conference, the meeting members included Drake and Sagan. This conference contained the first public discussion of what would later be called the Drake Equation, which the SETI pioneer established in order to identify and calculate the major factors involved in the probabilities of detecting an alien civilization. Although Drake deduced just 10,000 technological societies in our Milky Way galaxy using his equation, Sagan later used the same formula to suggest that perhaps one million ETI might be spread throughout our vast stellar island.
Image: Frank Drake, creator of the Drake Equation, who carried out the first modern SETI observations at Green Bank, West Virginia.
While many aspects of SETI are still in their early stages, our understanding of the Universe has certainly expanded since Cocconi and Morrison released their landmark paper almost half a century ago. There are now several projects such as the Allen Telescope Array
(ATA) in California devoted almost solely to SETI. Scientists have also conducted searches beyond the radio realm, looking for transmissions from laser beams, the infrared signatures of the giant astroengineering structures called Dyson Shells, and even alien probes in our Solar System.
In light of this progress, the final words of Cocconi and Morrison from their Nature article still best describe the search for others in the cosmos:
“The reader may seek to consign these speculations wholly to the domain of science fiction. We submit, rather, that the foregoing line of argument demonstrates that the presence of interstellar signals is entirely consistent with all we now know, and that if signals are present the means of detecting them is now at hand. Few will deny the profound importance, practical and philosophical, which the detection of interstellar communications would have. We therefore feel that a discriminating search for signals deserves a considerable effort. The probability of success is difficult to estimate; but if we never search the chance of success is zero.”
by Paul Gilster | Dec 15, 2008 | Exoplanetary Science |
The prospect of habitable planets around red giant stars fires the imagination, enough so that quite a few readers forwarded me the link to a recent paper looking at this question. I’m reluctant to speak for others, but I suppose a major reason we’re so interested (and I, too, had flagged the paper as soon as it popped up on the arXiv server) is that it changes our view of habitable worlds once again. Not long ago it was only the G-class, Sun-like star that seemed a likely abode of life. Then we started looking hard at M-dwarfs. Do we now extend the search to massive red giants, the descendants of stars once like our own?
Image credit: NASA, ESA and A. Feild (STScI).
Werner von Bloh (Potsdam Institute for Climate Impact Research) and team show that the possibility is real. We’ve long known that life on a planet in Earth’s orbit would not survive the swelling of the Sun, even if it did not actually engulf the planet. But life on Earth would actually die out long before that event, if for no other reason than that carbon dioxide would be removed from the atmosphere as the planetary core cooled, slowing the volcanic activity needed to replenish it. The weathering of silicates on Earth’s continents creates the carbon dioxide sink within which the CO2 gradually accumulates.
We’re looking, then, at the end of photosynthesis. At an age of roughly 6.5 billion years, long before the Sun enters its red giant phase, the Earth becomes uninhabitable. But as a planet like ours gradually dies, dormant ‘super-Earths’ — planets more massive than the Earth but with similar chemical and mineralogical composition — may be heading toward an entirely different fate. A super-Earth well beyond 1 AU in a Sun-like star’s system could be thawed into life during the star’s red giant expansion. Frozen earlier in the system’s history and well beyond what von Bloh defines as the ‘photosynthesis-sustaining habitable zone’ (pHZ), the planet would have sustained no weathering and consequent loss of CO2.
Warm such a planet up by the swelling of the central star and weathering begins, with a CO2 equilibrium being established. A super-Earth dominated by oceans would hold onto its CO2 the longest because of the reduced amount of land surface. The numbers here make it clear that land surface is crucial in the length of time a planet can sustain photosynthesis. The paper discusses how long a planet might take to transit the pHZ:
For planets of a distinct size, the most important factor is the relative continental area. Habitability was found most likely for water worlds, i.e., planets with a relatively small continental area. For planets at a distinct distance from the central star, we identi?ed maximum durations of the transit of the pHZ. A comparison of planets with different masses revealed that the maximum duration of the transit increases with planetary mass. Therefore, the upper limit for the duration of the transit for any kind of Earth-type planet is found for most massive super-Earth planets, i.e., 10 M? , rather than 1 M? planets, which are rendered uninhabitable after 6.5 Gyr…
The best case scenario is an ocean-dominated world not far beyond the present orbit of Mars, a ten Earth-mass world that could maintain itself in the star’s habitable zone for a whopping 3.7 billion years. Searching for habitable planets, then, we may want to add red giants to the mix, another surprising development that points to the widening range of astrobiological investigations. And maybe we should start thinking in terms of two life cycles around stars like our own, the first as the star burns through the main sequence, the second a perhaps lengthy efflorescence of life on a previously frozen world.
The paper is von Bloh et al., “Habitability of Super-Earth Planets around Other Suns: Models including Red Giant Branch Evolution,” in press at Astrobiology and available online.
by Paul Gilster | Dec 13, 2008 | Outer Solar System |
Europa is interesting enough without throwing in a new theory about energy sources. But Robert Tyler (University of Washington) has been studying the possibilities in Europan tides, using computer simulations that offer a different way of getting energy out of this icy world. We’ve speculated that Europa experiences enough tidal flex from Jupiter to create possible energy sources for life. What Tyler is saying is that the moon may experience not just internal pressures but large waves pushing through the submerged ocean. These waves, of course, could be a way of distributing heat and dissipating tidal energies.
This being the case, the assumption that energy may come from flexing at the core, as well as pressures on the oceanic ice sheets, has to be supplanted by a different view:
“If my work is correct then the heat source for Europa’s ocean is the ocean itself rather than what’s above or below it,” Tyler says. “And we must form a new vision of the ocean habitat that involves strong ocean flow rather than the previously assumed sluggish flows.”
Causing the waves is obliquity, the axial tilt of the moon in relation to its orbital plane, which results in a tidal force not previously considered in Europan terms. Earth’s axial tilt is 23 degrees. Europa’s hasn’t been measured, but Tyler believes that even at minimum values, it should be sufficient to produce significant heating. If obliquity does cause waves in an ocean we’ve long assumed as calm, then we have another way of explaining how Europa’s ocean manages to stay liquid. After all, the surface of this world is extremely cold — minus 160 degrees Celsius — while any sources of heat produced by radioactive decay seem meager.
Obliquity could be highly significant in the overall energy picture. From the paper:
“…the minimum kinetic energy of the flow associated with this resonance (7.3 X 1018 J) is two thousand times larger than that of the flow excited by the dominant tidal forces, and dissipation of this energy seems large enough to be a primary ocean heat source.
Tyler’s work gives us another take on the energy possibilities on Europa, and by extension on other moons suspected to have oceans, such as Ganymede and Callisto. And that has to play well with astrobiologists speculating on life’s development under distant ice. The paper is Tyler, “Strong ocean tidal flow and heating on moons of the outer planets,” Nature 456 (11 December 2008), pp. 770-772 (abstract).
