Universe Today

Why is the universe expanding at an accelerating rate, spreading its contents over ever greater dimensions of space? An original solution to this puzzle, certainly the most fascinating question in modern cosmology, was put forward by four theoretical physicists, Edward W. Kolb of the U.S. Department of Energy’s Fermi National Accelerator Laboratory, Chicago (USA): Sabino Matarrese of the University of Padova; Alessio Notari from the University of Montreal (Canada); and Antonio Riotto of INFN (Istituto Nazionale di Fisica Nucleare) of Padova (Italy). Their study was submitted yesterday to the journal Physical Review Letters.

Over the last hundred years, the expansion of the universe has been a subject of passionate discussion, engaging the most brilliant minds of the century. Like his contemporaries, Albert Einstein initially thought that the universe was static: that it neither expanded nor shrank. When his own Theory of General Relativity clearly showed that the universe should expand or contract, Einstein chose to introduce a new ingredient into his theory. His “cosmological constant” represented a mass density of empty space that drove the universe to expand at an ever-increasing rate.

When in 1929 Edwin Hubble proved that the universe is in fact expanding, Einstein repudiated his cosmological constant, calling it “the greatest blunder of my life.” Then, almost a century later, physicists resurrected the cosmological constant in a variant called dark energy. In 1998, observations of very distant supernovae demonstrated that the universe is expanding at an accelerating rate. This accelerating expansion seemed to be explicable only by the presence of a new component of the universe, a “dark energy,” representing some 70 percent of the total mass of the universe. Of the rest, about 25 percent appears to be in the form of another mysterious component, dark matter; while only about 5 percent comprises ordinary matter, those quarks, protons, neutrons and electrons that we and the galaxies are made of.

“The hypothesis of dark energy is extremely fascinating,” explains Padova’s Antonio Riotto, “but on the other hand it represents a serious problem. No theoretical model, not even the most modern, such as supersymmetry or string theory, is able to explain the presence of this mysterious dark energy in the amount that our observations require. If dark energy were the size that theories predict, the universe would have expanded with such a fantastic velocity that it would have prevented the existence of everything we know in our cosmos.”

The requisite amount of dark energy is so difficult to reconcile with the known laws of nature that physicists have proposed all manner of exotic explanations, including new forces, new dimensions of spacetime, and new ultralight elementary particles. However, the new report proposes no new ingredient for the universe, only a realization that the present acceleration of the universe is a consequence of the standard cosmological model for the early universe: inflation.

“Our solution to the paradox posed by the accelerating universe,” Riotto says, “relies on the so-called inflationary theory, born in 1981. According to this theory, within a tiny fraction of a second after the Big Bang, the universe experienced an incredibly rapid expansion. This explains why our universe seems to be very homogeneous. Recently, the Boomerang and WMAP experiments, which measured the small fluctuations in the background radiation originating with the Big Bang, confirmed inflationary theory.

It is widely believed that during the inflationary expansion early in the history of the universe, very tiny ripples in spacetime were generated, as predicted by Einstein’s theory of General Relativity. These ripples were stretched by the expansion of the universe and extend today far beyond our cosmic horizon, that is over a region much bigger than the observable universe, a distance of about 15 billion light years. In their current paper, the authors propose that it is the evolution of these cosmic ripples that increases the observed expansion of the universe and accounts for its acceleration.

“We realized that you simply need to add this new key ingredient, the ripples of spacetime generated during the epoch of inflation, to Einstein’s General Relativity to explain why the universe is accelerating today,” Riotto says. “It seems that the solution to the puzzle of acceleration involves the universe beyond our cosmic horizon. No mysterious dark energy is required.”

Fermilab’s Kolb called the authors’ proposal the most conservative explanation for the accelerating universe. “It requires only a proper accounting of the physical effects of the ripples beyond our cosmic horizon,” he said.

Data from upcoming experiments will allow cosmologists to test the proposal. “Whether Einstein was right when he first introduced the cosmological constant, or whether he was right when he later refuted the idea will soon be tested by a new round of precision cosmological observations,” Kolb said. “New data will soon allow us to distinguish between our explanation for the accelerated expansion of the universe and the dark energy solution.”

INFN (Istituto Nazionale di Fisica Nucleare), Italy’s national nuclear physics institute, supports, coordinates and carries out scientific research in subnuclear, nuclear and astroparticle physics and is involved in developing relevant technologies.

Fermilab, in Batavia, Illinois, USA, is operated by Universities Research Association, Inc. for the Department of Energy’s Office of Science, which funds advanced research in particle physics and cosmology.

Original Source: Istituto Nazionale di Fisica Nucleare

Astrophysicists in recent years have found evidence for a force they call dark energy in observations from the farthest reaches of the universe, billions of light years away.

Now an international team of researchers has used data from powerful computer models, supported by observations from the Hubble Space Telescope, to find evidence of dark energy right in our own cosmic neighborhood.

The data paint a picture of the universe as a virtual sea of dark energy, with billions of galaxies as islands emerging from the sea, said Fabio Governato, a University of Washington research associate professor of astronomy and a researcher with Italy’s National Institute for Astrophysics.

In 1929 astronomer Edwin Hubble demonstrated that galaxies are moving away from each other, which supported the theory that the universe has been expanding since the big bang. In 1999 cosmologists reported evidence that an unusual force, called dark energy, was actually causing the expansion of the universe to accelerate.

However, the expansion is slower than it would be otherwise because of the tug of gravity among galaxies. As the battle between the attraction of gravity and the repellent force of dark energy plays out, cosmologists are left to ponder whether the expansion will continue forever or if the universe will collapse in a “big crunch.”

In 1997, Governato designed a computer model to simulate evolution of the universe from the big bang until the present. His research group found the model could not duplicate the smooth expansion that had been observed among galaxies around the Milky Way, the galaxy in which Earth resides. In fact, the model produced deviations from a purely radial expansion that were three to seven times higher than astronomers had actually observed, Governato said.

“The observed motion was small, and we could not duplicate it without the presence of dark energy,” he said. “When we added the dark energy, we got a perfect match.”

Governato is one of three authors of a paper describing the work, scheduled for publication in the Monthly Notices of the Royal Astronomical Society, an astronomy journal in the United Kingdom. Co-authors are Andrea Maccio of the University of Zurich in Switzerland and Cathy Horellou of Chalmers University of Technology in Sweden. The work was supported by grants from the National Science Foundation and Vetenskapsr?det, the Swedish Research Council.

The authors, part of an international research collaboration called the N-Body Shop that originated at the UW, ran simulations of universe expansion on powerful supercomputers in Italy and Alaska. Their findings provide supporting evidence for a sea of dark energy surrounding galaxies.

“We studied the properties of galaxies close to the Milky Way instead of looking billions of light years away,” Governato said. “It’s like traveling from Seattle to Portland, Ore., rather than from Seattle to New York, to measure the Earth’s curvature.”

Original Source: University of Washington News Release

Image credit: CMU
Current Mars expeditions raise the tantalizing possibility that there may be life somewhere on the red planet. But just how will future missions find it? A system being developed by Carnegie Mellon scientists could provide the answer.

At the 36th Lunar and Planetary Science Conference in Houston this week (March 14-18), Carnegie Mellon scientist Alan Waggoner is presenting results of the life detection system’s recent performance in Chile’s Atacama Desert, where it found growing lichens and bacterial colonies. This marks the first time a rover-based automated technology has been used to identify life in this harsh region, which serves as a test bed for technology that could be deployed in future Mars missions.

“Our life detection system worked very well, and something like it ultimately may enable robots to look for life on Mars,” says Waggoner, a member of the “Life in the Atacama” project team and director of the Molecular Biosensor and Imaging Center at Carnegie Mellon’s Mellon College of Science.

The “Life in the Atacama” 2004 field season?from August to mid-October?was the second phase of a three-year program whose goal is to understand how life can be detected by a rover that is being controlled by a remote science team. The project is part of NASA’s Astrobiology Science and Technology Program for Exploring Planets, or ASTEP, which concentrates on pushing the limits of technology in harsh environments.

David Wettergreen, associate research professor in Carnegie Mellon’s Robotics Institute, leads rover development and field investigation. Nathalie Cabrol, a planetary scientist at NASA Ames Research Center and the SETI Institute, leads the science investigation.

Life is barely detectable over most areas of the Atacama, but the rover’s instruments were able to detect lichens and bacterial colonies in two areas: a coastal region with a more humid climate and an interior, very arid region less hospitable to life.

“We saw very clear signals from chlorophyll, DNA and protein. And we were able to visually identify biological materials from a standard image captured by the rover,” says Waggoner.

“Taken together, these four pieces of evidence are strong indicators of life. Now, our findings are being confirmed in the lab. Samples collected in the Atacama were examined, and scientists found that they contained life. The lichens and bacteria in the samples are growing and awaiting analysis.”

Waggoner and his colleagues have designed a life detection system equipped to detect fluorescence signals from sparse life forms, including those that are mere millimeters in size. Their fluorescence imager, which is located underneath the rover, detects signals from chlorophyll-based life, such as cyanobacteria in lichens, and fluorescent signals from a set of dyes designed to light up only when they bind to nucleic acid, protein, lipid or carbohydrate?all molecules of life.

“We don’t know of other remote methods capable both of detecting low levels of micro-organisms and visualizing high levels incorporated as biofilms or colonies,” says Gregory Fisher, project imaging scientist.

“Our fluorescent imager is the first imaging system to work in the daylight while in the shade of the rover. The rover uses solar energy to operate so it needs to travel during daylight hours. Many times, the images we capture may only reveal a faint signal. Any sunlight that leaks in to the camera of a conventional fluorescence imager would obscure the signal,” Waggoner says.

“To avoid this problem, we designed our system to excite dyes with high intensity flashes of light. The camera only opens during those flashes, so we are able to capture a strong fluorescence signal during daytime exploration,” says Shmuel Weinstein, project manager.

During the mission, a remote science team located in Pittsburgh instructed the rover’s operations. A ground team at the site collected samples studied by the rover to bring back for further examination in the lab. On a typical day in the field, the rover followed a path designated the previous day by the remote operations science team. The rover stopped occasionally to perform detailed surface inspection, effectively creating a “macroscopic quilt” of geologic and biological data in selected 10 by 10 centimeter panels. After the rover departed a region, the ground team collected samples examined by the rover.

“Based on the rover findings in the field and our tests in the laboratory, there is not one example of the rover giving a false positive. Every sample we tested had bacteria in it,” says Edwin Minkley, director of the Center for Biotechnology and Environmental Processes in the Department of Biological Sciences.

Minkley is conducting analyses to determine the genetic characteristics of the recovered bacteria to identify the different microbial species present in the samples. He also is testing the bacteria’s sensitivity to ultraviolet (UV) radiation. One hypothesis is that the bacteria may have greater UV resistance because they are exposed to extreme UV radiation in the desert environment. According to Minkley, this characterization also may explain why such a high proportion of the bacteria from the most arid site are pigmented?red, yellow or pink?as they grow in the laboratory.

The first phase of the project began in 2003 when a solar-powered robot named Hyperion, also developed at Carnegie Mellon, was taken to the Atacama as a research test bed. Scientists conducted experiments with Hyperion to determine the optimum design, software and instrumentation for a robot that would be used in more extensive experiments conducted in 2004 and in 2005. Zo?, the rover used in the 2004 field season, is the result of that work. In the final year of the project, plans call for Zo?, equipped with a full array of instruments, to operate autonomously as it travels 50 kilometers over a two-month period.

The science team, led by Cabrol, is made up of geologists and biologists who study both Earth and Mars at institutions including NASA’s Ames Research Center and Johnson Space Center, SETI Institute, Jet Propulsion Laboratory, the University of Tennessee, Carnegie Mellon, Universidad Catolica del Norte (Chile), the University of Arizona, UCLA, the British Antarctic Survey, and the International Research School of Planetary Sciences (Pescara, Italy).

The Life in the Atacama project is funded with a three-year, $3 million grant from NASA to Carnegie Mellon’s Robotics Institute. William “Red” Whittaker is the principal investigator. Waggoner is principal investigator for the companion project in life-detection instruments, which garnered a separate $900,000 grant from NASA.

Original Source: CMU News Release

On the basis of stellar spectra totalling more than 200 hours of effective exposure time with the 8.2-m VLT Kueyen telescope at Paranal (Chile), a team of astronomers [1] has made a surprising discovery about the stars in the giant southern globular cluster Omega Centauri.

It has been known for some time that, contrary to other clusters of this type, this stellar cluster harbours two different populations of stars that still burn hydrogen in their centres. One population, accounting for one quarter of its stars, is bluer than the other.

Using the FLAMES multi-object spectrograph that is particularly well suited to this kind of work, the astronomers found that the bluer stars contain more heavy elements than those of the redder population. This was exactly opposite to the expectation and they are led to the conclusion that the bluer stars have an overabundance of the light element helium of more than 50%. They are in fact the most helium rich stars ever found. But why is this so?

The team suggests that this puzzle may be explained in the following way. First, a great burst of star formation took place during which all the stars of the red population were produced. As other normal stars, these stars transformed their hydrogen into helium by nuclear burning. Some of them, with masses of 10-12 times the mass of the Sun, soon thereafter exploded as supernovae, thereby enriching the interstellar medium in the globular cluster with helium. Next, the blue population stars formed from this helium-rich medium.

This unexpected discovery provides important new insights into the way stars may form in larger stellar systems.

Two Populations
Globular clusters are large stellar clusters some of which contain hundreds of thousands of stars. It is generally believed that all stars belonging to the same globular cluster were born together, from the same interstellar cloud and at the same time. Strangely, however, this seems not to be the case for the large southern globular cluster Omega Centauri.

Omega Centauri is the galactic globular cluster with the most complex stellar population. Its large mass may represent an intermediate type of object, between globular clusters and larger stellar systems such as galaxies. In this sense, Omega Centauri is a very useful “laboratory” for better understanding the history of star formation.

However, it appears that the more information astronomers acquire about the stars in this cluster, the less they seem to understand the origin of these stars. But now, new intriguing results from the ESO Very Large Telescope (VLT) may show a possible way of resolving the present, apparently contradictory results.

Last year, an international team of astronomers [1], using data from the Hubble Space Telescope (HST), showed that Omega Centauri, unlike all other globular clusters, possesses two distinct populations of stars burning hydrogen in their centre. Even more puzzling was the discovery that the bluer population was more rare than the redder one: they accounted for only a quarter of the total number of stars still burning hydrogen in their central core. This is exactly the opposite of what the astronomers had expected, based on the observations of more evolved stars in this cluster.

Over Two Weeks of Total Exposure Time!
The same team of astronomers then went on to observe some of the stars from the two populations in this cluster by means of the FLAMES instrument on the Very Large Telescope at Paranal. They used the MEDUSA mode, allowing to obtain no less than 130 spectra simultaneously.

Twelve one-hour spectra were obtained for 17 stars of the blue population and the same number stars from the red one. These stars have magnitudes between 20 and 21, i.e., they are between 500,000 and 1 million times fainter than what can be seen with the unaided eye.

The individual spectra of stars from each population were then co-added. This produced a “mean” spectrum of a blue-population star and another of a red-population. Each of these spectra represents a total of no less than 204 hours of exposure time and accordingly provides information in unrivalled detail about these stars, especially in terms of their chemical composition.

The scientific outcome matches the technical achievement!
From a careful study of the combined spectra, the astronomers were able to establish that – contrary to all prior expectations – the bluer stars are more “metal-rich” (by a factor two) than the redder ones. “The latter were found to have an abundance of elements more massive than helium corresponding to about 1/40 the solar abundance [2] “, explains Raffaele Gratton of INAF-Osservatorio Astronomico di Padova in Italy. “This is indeed very puzzling as current models of stars predict that the more metal-rich a star is, the redder it ought to be”.

Giampaolo Piotto (University of Padova, Italy), leader of the team, thinks that there is a solution to this celestial puzzle: “The only way we can explain this discrepancy is by assuming that the two populations of stars have a different abundance of helium. We find that while the red stars have a normal helium abundance, the bluer stars must be enriched in helium by more than 50% with respect to the other population!”

These stars are thus the most helium-rich stars ever found, and not by just a few percent! It took some 8 billion years for the Milky Way Galaxy to increase its helium abundance from the primordial 24% value (created by the Big Bang) to the present solar 28% value, and yet in a globular cluster that formed only 1 or 2 billion years after the Big Bang, stars were produced with 39% of helium!

Contamination from supernovae
The obvious question is now: “Where does all this helium come from?”

Luigi Bedin (ESO), another member of the team, suggests that the solution might be connected to supernovae: “The scenario we presently favour is one in which the high helium content originates from material ejected during the supernovae explosions of massive stars. It is possible that the total mass of Omega Centauri was just right to allow the material expelled by high-mass supernovae to escape, while the matter from explosions of stars with about 10-12 times the mass of the Sun was retained.”

According to this scenario, Omega Centauri must therefore have seen two generations of stars. The first generation, with primordial helium abundance, produced the redder stars. A few tens of million years later, the most massive stars of this first generation exploded as supernovae. The helium-enriched matter that was expelled during the explosions of stars with 10-12 times the mass of the Sun “polluted” the globular cluster. Then a second population of stars, the bluer ones, formed from this helium-rich gas.

The scientists acknowledge that certain problems still remain and that the last word may not yet have been said about this unusual globular cluster. But the new results constitute an important step towards the solution of the biggest mystery of all: why is Omega Centauri the only one among the galactic globular cluster that was able to produce super helium-rich stars?

More information
The research presented here appeared in the March 10 issue of the Astrophysical Journal, Vol. 621, p. 777 (“Metallicities on the Double Main Sequence of Omega Centauri Imply Large Helium Enhancement” by G. Piotto et al.) and is available for download as astro-ph/0412016.

Notes
[1]: The team is composed of Giampaolo Piotto, Giovanni Carraro, Sandro Villanova, and Yazan Momany (University of Padova, Italy), Luigi R. Bedin (ESO, Garching), Raffaele Gratton and Sara Lucatello (INAF- Osservatorio Astronomico di Padova, Italy), Santi Cassisi (INAF- Osservatorio Astronomico di Teramo, Italy), Alejandra Recio-Blanco (Observatoire de Nice, France), Ivan R. King (University of Washington, USA), and Jay Anderson (Rice University, USA).

[2]: Helium is the second most abundant chemical element in the Universe, after hydrogen. The Sun contains about 70% hydrogen and 28% helium. The rest, about 2%, is made of all elements more heavier than helium. They are commonly referred to by astronomers as “metals”.

Original Source: ESO News Release