Milky Way survived ancient heat wave because of dark matter
July 2, 2009 |16:27 | Cosmology By : Team X
A new theory by scientists says that our Milky Way galaxy survived intense heat generated by the “ignition” of the Universe about half-a-billion years after the Big Bang, because it was already immersed in a large clump of dark matter that trapped gases inside it.Tiny galaxies, inside small clumps of dark matter, were blasted away by the heat that reached approximate temperatures of between 20,000 and 100,000 degrees centigrade, according to the scientists, including experts at Japan’s University of Tsukuba.
The researchers said that the early Milky Way, which had begun forming stars, held on to the raw gaseous material from which further stars would be made.
This material would otherwise have been evaporated by the high temperatures generated by the “ignition”.
Using computer simulations carried out by the international Virgo Consortium (which is led by Durham), the scientists examined why galaxies like the Milky Way have so few companion galaxies or satellites.
Astronomers have found a few dozen small satellites around the Milky Way, but the simulations revealed that hundreds of thousands of small clumps of dark matter should be orbiting our galaxy.
Clearly imagination must be the source from which astronomy•cal theories are developed about the cosmos that surround us. But according to Sagan, skepticism is the very tool to keep things in check, helping to filter out the fantasy from reality.
"I try to challenge my own perceptions, the way I look at things. And one symbol of that was to make these paintings rotate," said the artist Thomas Woodruff at the recent opening of his show at P.P.O.W. gallery in West Chelsea, an event that coincided with further stock market catastrophes. What better antidote for economic turbulence than to be surrounded by a heraldic, hip and healing creative cosmology, moving in a constant cycle of repetition and renewal.
The new work suggests that time existed before the Big Bang, when a more ancient universe collapsed to give birth to the one we live in today.
Astronomers have used ESO’s Very Large Telescope to measure the distribution and motions of thousands of galaxies in the distant Universe. This opens fascinating perspectives to better understand what drives the acceleration of the cosmic expansion and sheds new light on the mysterious dark energy that is thought to permeate the Universe. "Explaining why the expansion of the Universe is currently accelerating is certainly the most fascinating question in modern cosmology," says Luigi Guzzo, lead author of a paper in this week's issue of Nature, in which the new results are presented. "We have been able to show that large surveys that measure the positions and velocities of distant galaxies provide us with a new powerful way to solve this mystery." Ten years ago, astronomers made the stunning discovery that the Universe is expanding at a faster pace today than it did in the past. "This implies that one of two very different possibilities must hold true," explains Enzo Branchini, member of the team. "Either the Universe is filled with a mysterious dark energy which produces a repulsive force that fights the gravitational brake from all the matter present in the Universe, or, our current theory of gravitation is not correct and needs to be modified, for example by adding extra dimensions to space."Current observations of the expansion rate of the Universe cannot distinguish between these two options, but the international team of 51 scientists from 24 institutions found a way that could help in tackling this problem. The technique is based on a well-known phenomenon, namely the fact that the apparent motion of distant galaxies results from two effects: the global expansion of the Universe that pushes the galaxies away from each other and the gravitational attraction of matter present in the galaxies' neighbourhood that pulls them together, creating the cosmic web of large-scale structures."By measuring the apparent velocities of large samples of galaxies over the last thirty years, astronomers have been able to reconstruct a three-dimensional map of the distribution of galaxies over large volumes of the Universe. This map revealed large-scale structures such as clusters of galaxies and filamentary superclusters ", says Olivier Le Fèvre, member of the team. "But the measured velocities also contain information about the local motions of galaxies; these introduce small but significant distortions in the reconstructed maps of the Universe. We have shown that measuring this distortion at different epochs of the Universe's history is a way to test the nature of dark energy."Guzzo and his collaborators have been able to measure this effect by using the VIMOS spectrograph on Melipal, one of the four 8.2-m telescopes that is part of ESO's VLT. As part of the VIMOS-VLT Deep Survey (VVDS), of which Le Fèvre is the Principal Investigator, spectra of several thousands of galaxies in a 4-square-degree field (or 20 times the size of the full Moon) at epochs corresponding to about half the current age of the Universe (about 7 billion years ago) were obtained and analysed. "This is the largest field ever covered homogeneously by means of spectroscopy to this depth," says Le Fèvre. "We have now collected more than 13,000 spectra in this field and the total volume sampled by the survey is more than 25 million cubic light-years." The astronomers compared their result with that of the 2dFGRS survey that probed the local Universe, i.e. measures the distortion at the present time. Within current uncertainties, the measurement of this effect provides an independent indication of the need for an unknown extra energy ingredient in the 'cosmic soup', supporting the simplest form of dark energy, the so-called cosmological constant, introduced originally by Albert Einstein. The large uncertainties do not yet exclude the other scenarios, though. "We have also shown that by extending our measurements over volumes about ten times larger than the VVDS, this technique should be able to tell us whether cosmic acceleration originates from a dark energy component of exotic origin or requires a modification of the laws of gravity," said Guzzo. "VIMOS on the VLT would certainly be a wonderful tool to perform this future survey and help us answer this fundamental question. This strongly encourages our team to proceed with even more ambitious surveys of the distant Universe," says Le Fèvre. The VLT VIsible Multi-Object Spectrograph (VIMOS) can observe spectra of about 1,000 galaxies in one single exposure. This cosmology science machine is installed at the 8.2-m MELIPAL telescope, the third unit telescope of the Very Large Telescope (VLT) at the ESO Paranal Observatory.
A team of physicists and astronomers from the University of Sussex and Imperial College London have uncovered hints that there may be cosmic-strings lines of pure mass energy stretching across the entire universe. Cosmic strings are predicted by high energy physics theories, including superstring theory. This is based on the idea that particles are not just little points, but tiny vibrating bits of string Cosmic strings are predicted to have extraordinary amounts of mass - perhaps as much as the mass of the Sun - packed into each metre of a tube whose width is less a billion billionth of the size of an atom. Lead researcher Dr Mark Hindmarsh, Reader in Physics at the University of Sussex, said: “This is an exciting result for physicists. Cosmic strings are relics of the very early Universe and signposts that would help construct a theory of all forces and particles.” His team took data from NASA's Wilkinson Microwave Anisotropy Probe (WMAP), which is a satellite currently mapping the intensity of cosmic microwaves from all directions, and carefully compared the predictions of what should be seen with and without strings. Dr Hindmarsh said: “We cannot yet see these strings directly. They are many billion light years away. We can only look for indirect evidence of their existence through precision measurements of the cosmic microwave background, of cosmic rays, gravitational radiation, and looking for double images of distant quasars.” The four-person team are members of COSMOS, the UK's world-leading cosmology supercomputing consortium fronted by Stephen Hawking. Using a Silicon Graphics supercomputer they made predictions of how the strings would affect the Cosmic Microwave Background, relic radio waves from the Big Bang which fill the universe. It turned out that the best explanation for the pattern of this radiation was a theory which included strings. Dr Hindmarsh said that better data is required before the existence of cosmic strings can be confirmed. He hopes this will be produced by the European Space Agency's Planck Satellite mission (due for launch this year).
From various independent observations, cosmologists have established that ordinary matter made of protons and neutrons accounts for only 4% of the total energy content of the universe. The remaining 96% is made of puzzling ingredients Dark Matter and Dark Energy. Researchers and at the Laboratory Universe and Theories from the Observatory of Paris and the Belgian Fonds De La Recherché Scientifique have recently suggested the Abnormally Weighting Energy(AWE) Hypothesis to describe the dark side of the Universe as a revolutionary aspect of gravitational Physics. In the past decade, cosmology has entered an era of high precision, and in the future it may become a unique laboratory to test theories of fundamental physics, from gravitation laws to microphysics. Amongst the many questions raised by this science in turmoil, one of the most important is indisputably the one of the energy content of the Universe. Knowing what the Universe is precisely made of, and in which proportions, allows not only to determine its age but also to reconstruct the history, to predict its past and future. In fact in the attempt to solve this question cosmologists have made two of the most promising discoveries in the history of modern physics: the existence of dark matter and dark energy. While dark matter is unavoidable to explain at the same time the angular fluctuations of the cosmic microwave background and the formation and the properties of galaxies, dark energy has been originally invoked to account for the observed recent acceleration of the cosmic expansion. The so-called concordance model of cosmology assumes that this dark energy is in fact the cosmological constant once introduced by Einstein himself as an attempt to incorporate Mach’s principle within general relativity. However, the usual interpretation of the cosmological constant in terms of quantum vacuum fluctuations is in disagreement with observed value by a few dozens orders of magnitude! Furthermore, as the vacuum energy is assumed constant everywhere at all times, it is hard to explain how it became dominant only a few billion years ago. This would mean that we live in a very particular, and even privileged, epoch of cosmic history… Is this an extraordinary coincidence? Yet this anthropic consideration is quite deceiving for scientists. To overcome these difficulties, the authors, Jean-Michel Alimi and André Füzfa, have proposed the AWE Hypothesis (« Abnormally Weighting Energy ») in which the dark sector of cosmic matter violates the equivalence principle on cosmological scales. This principle, as well introduced by Einstein, assumes that all kinds of energies produce and undergo the same form of gravity. This principle is extremely well tested (to a part out of a thousand billion) in laboratories, i.e. at local scales, in contrast what would happen if violation of the equivalence principle would be scale-dependant. In other words, what would happen if the equivalence principle was rigorously verified at local scales, where dark matter and dark energy are present in tiny amount, but is violated on cosmological scales where dark matter and dark energy are dominant? The authors have precisely shown that this could naturally happen if some particles, those of dark matter for instance, do not couple to gravitation in the same way as ordinary matter. These particles would therefore see gravitational fields with a gravitational strength different from ordinary matter. The authors have answered these questions by showing how at a given scale the gravitational strength becomes dependent on dark matter concentration… If the amount of dark matter at sub-galactic scales is negligible, so is the amplitude of this effect. This is not the case on cosmological scales where dark matter dominates the energy content of the Universe. The team has shown that over such cosmic distances, ordinary matter has experienced a stronger cosmic expansion, as its own gravitational coupling strength has been adapting to the dark matter domination. This change in the matter gravitational coupling results in an accelerating cosmic expansion until equilibrium is reached such that the gravitational coupling on cosmological scales stabilizes at a value which differs from the one measured in our Solar system. The resulting dark energy mechanism exhibits key features which appear very promising. (i) First, it does not require the existence of negative pressures such as in the case of the cosmological constant or other proposed models like quintessence. (ii) It allows explaining naturally the cosmic coincidence as result of the stabilization mechanism of the gravitational constant during the matter-dominated era. (iii) It fairly accounts for the Hubble diagram of type Ia supernovae by predicting independently the amount of ordinary matter and dark matter as obtained by the detailed analysis of cosmic microwave background anisotropies. This suggests an explanation to the remarkable adequacy of the concordance model while predicting an age of the Universe which is compatible with existing observations. Finally, (iv) in the future this mechanism leads to a decelerated cosmic expansion described by the well-known Einstein-de Sitter cosmological model. Most important is the AWE hypothesis allows reducing dark energy as a new property of gravitation: the anomalous gravity of dark matter.
Using two NASA satellites astronomers have discovered the heftiest known black hole to orbit a star. The new black hole with a mass of 24 to 33 times that of our sun is more massive then scientists expected for a black hole that formed from a dying star. The newly discovered object belongs to the category of "stellar-mass" black holes. Formed in the death throes of massive stars, they are smaller than the monster black holes found in galactic cores. The previous record holder for largest stellar-mass black hole is a 16-solar-mass black hole in the galaxy M33, announced on October 17. "We weren’t expecting to find a stellar-mass black hole this massive," says Andrea Prestwich of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of the discovery paper in the November 1 Astrophysical Journal Letters. "It seems likely that black holes that form from dying stars can be much larger than we had realized." The black hole is located in the nearby dwarf galaxy IC 10, 1.8 million light-years from Earth in the constellation Cassiopeia. Prestwich’s team could measure the black hole’s mass because it has an orbiting companion: a hot, highly evolved star. The star is ejecting gas in the form of a wind. Some of this material spirals toward the black hole, heats up, and gives off powerful X-rays before crossing the point of no return. In November 2006, Prestwich and her colleagues observed the dwarf galaxy with NASA’s Chandra X-ray Observatory. The group discovered that the galaxy’s brightest X-ray source, IC 10 X-1, exhibits sharp changes in X-ray brightness. Such behavior suggests a star periodically passing in front of a companion black hole and blocking the X-rays, creating an eclipse. In late November, NASA’s Swift satellite confirmed the eclipses and revealed details about the star’s orbit. The star in IC 10 X-1 appears to orbit in a plane that lies nearly edge-on to Earth’s line of sight, The Swift observations, as well as observations from the Gemini Telescope in Hawaii, told Prestwich and her group how fast the two stars go around each other. Calculations showed that the companion black hole has a mass of at least 24 Suns. There are still some uncertainties in the black hole’s mass estimate, but as Prestwich notes, "Future optical observations will provide a final check. Any refinements in the IC 10 X-1 measurement are likely to increase the black hole’s mass rather than reduce it." The black hole’s large mass is surprising because massive stars generate powerful winds that blow off a large fraction of the star’s mass before it explodes. Calculations suggest massive stars in our galaxy leave behind black holes no heavier than about 15 to 20 Suns. The IC 10 X-1 black hole has gained mass since its birth by gobbling up gas from its companion star, but the rate is so slow that the black hole would have gained no more than 1 or 2 solar masses. "This black hole was born fat; it didn’t grow fat," says astrophysicist Richard Mushotzky of NASA Goddard Space Flight Center in Greenbelt, Md., who is not a member of the discovery team. The progenitor star probably started its life with 60 or more solar masses. Like its host galaxy, it was probably deficient in elements heavier than hydrogen and helium. In massive, luminous stars with a high fraction of heavy elements, the extra electrons of elements such as carbon and oxygen "feel" the outward pressure of light and are thus more susceptible to being swept away in stellar winds. But with its low fraction of heavy elements, the IC 10 X-1 progenitor shed comparatively little mass before it exploded, so it could leave behind a heavier black hole. "Massive stars in our galaxy today are probably not producing very heavy stellar-mass black holes like this one," says coauthor Roy Kilgard of Wesleyan University in Middletown, Conn. "But there could be millions of heavy stellar-mass black holes lurking out there that were produced early in the Milky Way’s history, before it had a chance to build up heavy elements."
Some people are born geniuses and one of them is Astronomer Robert Quimby who has been successful in finding the most luminous supernova ever. Quimby discovered the current record holder, supernova 2006gy, last year as part of his Texas Supernova Search project. Now he announces that a supernova he discovered earlier in the project is actually twice as luminous. Using follow-up studies to pinpoint its distance, supernova 2005ap peaked at more than 100 billion times the brightness of the Sun. This supernova is a Type II, Quimby said, because it contains hydrogen. Most Type II supernovae are thought to result when the cores of massive stars, those seven to eight times or more heavy than the Sun, collapse under their own weight and trigger an explosion. This particular Type II is 300 times brighter than average, Quimby said, and lies in a dwarf galaxy in the constellation Coma Berenices, well behind the famous Coma cluster of galaxies.“It’s clearly not the same as 2006gy,” Quimby’s colleague and supernova expert J. Craig Wheeler of The University of Texas at Austin said. “It’s a puzzle.” Quimby completed his Ph.D. under Wheeler’s supervision at Texas in May, and has just begun a post-doctoral appointment at Caltech. His Texas Supernova Search uses the 18-inch ROTSE-IIIb robotic telescope on McDonald Observatory’s Mount Fowlkes, a tiny neighbor to the giant 10-meter-class Hobby-Eberly Telescope (HET). Quimby studied 2005ap with HET just a few days after its discovery. The results were intriguing, Quimby said. The supernova’s spectrum hinted at the presence of a highly shifted absorption line of oxygen III (an oxygen atom that has lost two of its electrons). Quimby knew that if the feature was oxygen III, then 2005ap was “possibly very far away and thus very luminous.” Follow-up observations with the Keck Telescope in Hawaii by Quimby’s colleague Greg Aldering of Lawrence Berkeley National Lab not only confirmed Quimby’s HET detection of oxygen III, but added another, equally shifted element to the spectrum: magnesium. Together, the studies confirmed 2005ap’s distance of 4.7 billion light-years. (In astronomical terms, this equates to a redshift of z = 0.2832.) It was this distance measurement, combined with measurements of the supernova’s apparent brightness that allowed the calculation of its intrinsic brightness, or “luminosity,” and uncovered 2005ap as the most powerful supernova yet. “Before 2006gy, I thought this should not be plausible,” Quimby said. “There I was finding my first supernovae — I was just happy to get anything. It turned out to be the most luminous supernova ever found.” How is that Quimby has found the brightest supernova yet, twice in a row? “I’ve worked too damn hard for this to be luck,” he said. Quimby explained, “I’m searching a huge volume of space, comparable to all previous nearby supernova surveys combined.” Also, Quimby will find supernovae that other studies ignore: he doesn’t filter out non-Type Ia supernovae, which is what many studies do that are searching for supernovae for cosmology studies, and he does search dwarf galaxies as well as galaxies with active black holes at their centers, which other studies avoid. Others also avoid supernovae near the cores of galaxies. In fact, 2006gy was found in the core of a galaxy, and that galaxy has a weakly active central black hole, Wheeler said. “There’s no question that [his results] have gotten everybody’s attention,” Wheeler said. The University of Michigan-run ROTSE collaboration, whose main mission is the search for gamma-ray bursts, has decided to expand the supernova search to its entire network. Its robotic telescopes in Australia, Turkey, and Namibia will soon join the unit at McDonald Observatory in this search. The Sloan Digital Sky Survey Supernova Search, for which the HET provides confirming spectra, is also reconsidering its search filters in response to these discoveries, Wheeler said.
