Chapter 37:

Hubble News on Dark Energy

The good news from NASA's Hubble Space Telescope is that Einstein was right--maybe.

A strange form of energy called "dark energy" is looking a little more like the repulsive force that Einstein theorized in an attempt to balance the universe against its own gravity. Even if Einstein turns out to be wrong, the universe's dark energy probably won't destroy the universe any sooner than about 30 billion years from now, say Hubble researchers.

"Right now we're about twice as confident than before that Einstein's cosmological constant is real, or at least dark energy does not appear to be changing fast enough (if at all) to cause an end to the universe anytime soon," says Adam Riess of the Space Telescope Science Institute, Baltimore.

Riess used Hubble to find nature's own "weapons of mass destruction" -- very distant supernovae that exploded when the universe was less than half its current age. The apparent brightness of a certain type of supernova gives cosmologists a way to measure the expansion rate of the universe at different times in the past.

Riess and his team joined efforts with the Great Observatories Origins Deep Survey (GOODS) program, the largest deep galaxy survey attempted by Hubble to date, to turn the Space Telescope into a supernova search engine on an unprecedented scale. In the process, they discovered 42 new supernovae in the GOODS area, including 6 of the 7 most distant known.

Cosmologists understand almost nothing about dark energy even though it appears to comprise about 70 percent of the universe. They are desperately seeking to uncover its two most fundamental properties: its strength and its permanence.

In a paper to be published in the Astrophysical Journal, Riess and his collaborators have made the first meaningful measurement of the second property, its permanence.

Currently, there are two leading interpretations for the dark energy, as well as many more exotic possibilities. It could be an energy percolating from empty space as Einstein's theorized "cosmological constant," an interpretation which predicts that dark energy is unchanging and of a prescribed strength.

An alternative possibility is that dark energy is associated with a changing energy field dubbed "quintessence."

This field would be causing the current acceleration -- a milder version of the inflationary episode from which the early universe emerged.

When astronomers first realized the universe was accelerating, the conventional wisdom was that it would expand forever. However, until we better understand the nature of dark energy--its properties--other scenarios for the fate of the universe are possible.

If the repulsion from dark energy is or becomes stronger than Einstein's prediction, the universe may be torn apart by a future "Big Rip," during which the universe expands so violently that first the galaxies, then the stars, then planets, and finally atoms come unglued in a catastrophic end of time. Currently this idea is very speculative, but being pursued by theorists.

At the other extreme, a variable dark energy might fade away and then flip in force such that it pulls the universe together rather then pushing it apart.

This would lead to a "big crunch" where the universe ultimately implodes. "This looks like the least likely scenario at present," says Riess.

Understanding dark energy and determining the universe's ultimate fate will require further observations. Hubble and future space telescopes capable of looking more than halfway across the universe will be needed to achieve the necessary precision. The determination of the properties of dark energy has become the key goal of astronomy and physics today.

Big Telescopes and the First Stars

UK astronomers Elizabeth Stanway, Andrew Bunker and Richard McMahon at the Institute of Astronomy, University of Cambridge, England, have used three of the most powerful telescopes in existence to identify some of the farthest galaxies yet seen. But at the same time, they have encountered a cosmic conundrum: it looks as if there were fewer galaxies forming stars at this early stage in the history of the Universe than in the more recent past. Their results, which will be published in the Monthly Notices of the Royal Astronomical Society, show for the first time, that astronomers may be probing back to the era when the first stars and galaxies were forming.

Stanway, Bunker and McMahon used the unique power of the Hubble Space Telescope and analysed publicly-available images taken in the direction of the southern hemisphere constellation of Fornax (the Oven) with the new Advanced Camera for Surveys as part of the 'Great Observatory Origins Deep Survey' (GOODS) project. They identified half a dozen objects likely to be galaxies 95 per cent of the way across the observable Universe. The redshifts of these galaxies are about 6 and they are so far away that radiation from them has taken about 13 billion years to reach us. They existed when the Universe was less than a billion years old and seven billion years before the Earth and Sun formed. Intervening gas clouds absorbed visible light from them long before it reached Earth but their infrared light can be detected - and it is their infrared 'colours' which lead the researchers to believe that they lie at such immense distances.

They also used infrared images taken with one of the 8-metre telescopes forming the Very Large Telescope (VLT) at the European Southern Observatory (ESO) in Chile to study these galaxies. "The ESO pictures allowed us to distinguish very distant galaxies at the edge of the observable Universe from objects nearby," said graduate student Elizabeth Stanway, who has identified the galaxies as part of her research for a doctorate in astrophysics at Cambridge.

Having drawn up a list of objects that could be remote galaxies, the astronomers then turned to one of two Keck telescopes, which are the largest in the world and are at the top of the 14000ft mountain of Mauna Kea in Hawaii. Working with California astronomers Professor Richard Ellis (Caltech) and Dr Patrick McCarthy (Carnegie Observatories) they took a spectrum of one of them. They saw the signature of hydrogen gas glowing as it is illuminated by hot, newly-born stars, and measured the redshift to be 5.78. "This galaxy is in the process of giving birth to stars - each year it converts a mass of gas more than 30 times that of our Sun into new stars", according to research astronomer Dr. Andrew Bunker. These additional results have recently been submitted to the Monthly Notices of the Royal Astronomical Society.

"Using the Keck, was very important as it showed that this population of objects discovered by the Hubble Space Telescope really is incredibly distant", said Andrew Bunker, who was part of the team which did the observing in Hawaii. "The galaxy we have proved to be very distant is only 1000 light years across. This is very small compared to our own galaxy, the Milky Way, which is 100 times larger" added Elizabeth Stanway.

But the Cambridge team have also found a cosmic puzzle: on the basis of their sample, they can calculate how may galaxies there are involved in the rapid formation of stars in the very distant universe (redshift 6). They have compared the answer with previous work looking at nearer galaxies, with redshifts around 4. It seems that there are fewer of these galaxies early in the history of the Universe, compared to more recent times.

Theoretical predictions for the star formation history of the universe are highly uncertain, which is why this observational work is essential. "It could be that we are seeing some of the first galaxies to be born", said Richard McMahon, "The light from these first stars to ignite could have ended the Dark Age of the Universe as the galaxies 'turn on', and might have caused the gas between the galaxies to be blasted by starlight - the 'reionization' which has recently been detected in the cosmic microwave background by the WMAP satellite". The results of the Cambridge group combined with the recent results from WMAP satellite complement each other and show that the Dark Age ended sometime between 200 and 1000 million years after the Big Bang with the formation of the first stars.

This team of astronomers are currently building a new instrument in Cambridge called 'DAZLE', which will probe even earlier in the history of the Universe and shed new light on the 'Dark Ages'.

http://www.ast.cam.ac.uk/~bunker/internal/CambridgeGOODS/

NASA Release of the WMAP Cosmic-Background Map

NASA Press Release February 11, 2003

NASA today released the best "baby picture" of the Universe ever taken; the image contains such stunning detail that it may be one of the most important scientific results of recent years.

Scientists using NASA's Wilkinson Microwave Anisotropy Probe (WMAP), during a sweeping 12-month observation of the entire sky, captured the new cosmic portrait, capturing the afterglow of the big bang, called the cosmic microwave background.

"We've captured the infant universe in sharp focus, and from this portrait we can now describe the universe with unprecedented accuracy," said Dr. Charles L. Bennett of the Goddard Space Flight Center (GSFC), Greenbelt Md., and the WMAP Principal Investigator. "The data are solid, a real gold mine," he said.

One of the biggest surprises revealed in the data is the first generation of stars to shine in the universe first ignited only 200 million years after the big bang, much earlier than many scientists had expected.

In addition, the new portrait precisely pegs the age of the universe at 13.7 billion years old, with a remarkably small one percent margin of error.

The WMAP team found that the big bang and Inflation theories continue to ring true. The contents of the universe include 4 percent atoms (ordinary matter), 23 percent of an unknown type of dark matter, and 73 percent of a mysterious dark energy. The new measurements even shed light on the nature of the dark energy, which acts as a sort of an anti-gravity.

"These numbers represent a milestone in how we view our universe," said Dr. Anne Kinney, NASA director for astronomy and physics. "This is a true turning point for cosmology."

The light we see today, as the cosmic microwave background, has traveled over 13 billion years to reach us. Within this light are infinitesimal patterns that mark the seeds of what later grew into clusters of galaxies and the vast structure we see all around us.

Patterns in the big bang afterglow were frozen in place only 380,000 years after the big bang, a number nailed down by this latest observation. These patterns are tiny temperature differences within this extraordinarily evenly dispersed microwave light bathing the universe, which now averages a frigid 2.73 degrees above absolute zero temperature. WMAP resolves slight temperature fluctuations, which vary by only millionths of a degree.

Theories about the evolution of the universe make specific predictions about the extent of these temperature patterns. Like a detective, the WMAP team compared the unique "fingerprint" of patterns imprinted on this ancient light with fingerprints predicted by various cosmic theories and found a match.

WMAP will continue to observe the cosmic microwave background for an additional three years, and its data will reveal new insights into the theory of Inflation and the nature of the dark energy.

"This is a beginning of a new stage in our study of the early universe," said WMAP team member Prof. David N. Spergel of Princeton University, N.J. "We can use this portrait not only to predict the properties of the nearby universe, but can also use it to understand the first moments of the big bang," he said.

WMAP is named in honor of David Wilkinson of Princeton University, a world-renown cosmologist and WMAP team member who died in September 2002.

Launched on June 30, 2001, WMAP maintains a distant orbit about the second Lagrange Point, or "L2," a million miles from Earth.

WMAP is the result of a partnership between the GSFC and Princeton University. Additional Science Team members are located at Brown University, Providence R.I., the University of British Columbia, Vancouver, BC, the University of Chicago, and the University of California, Los Angeles. WMAP is part of the Explorer program, managed by GSFC.

For more information, including high-quality images, videos and press products, refer to:

http://www.gsfc.nasa.gov/topstory/2003/0206mapresults.html

http://map.gsfc.nasa.gov

WMAP Spacecraft Results Brings Cosmology Into a Precision Era

Comments by JMP about the February 11, 2003, NASA Press Conference

Results released today from the Microwave Anisotropy Probe spacecraft, which was renamed after the late Prof. David Wilkinson, brought cosmology into a new era in that a variety of important parameters are now known to high accuracy. (Wilkinson had been part of the team that first interpreted the cosmic-background radiation and that was beaten out in the observational discovery by Penzias and Wilson.) In particular, the age of the Universe was measured by scientists using WMAP data to be 13.7 billion years with an uncertainly of only 200 million years. Hubble's constant, long sought after, was measured to be 71, in close agreement with the value of 72 announced a couple of years ago by the Key Project on the subject of the Hubble Space Telescope. (The Hubble constant's units are kilometers/second per megaparsec; that is, for every megaparsec of distance, the speed of recession of galaxies increases by 71 kilometers/second.)

The Wilkinson Microwave Anisotropy Probe was launched in late 2001. The results of the first year of data announced today will eventually be supplemented by at least three additional years of data, so the tight constraints announced today will be made even more accurate.

Another parameter announced by WMAP scientists is that the content of ordinary matter in the Universe is only 4.4%. They found that dark matter takes up an additional 27%. The remaining 78.6% is made up of dark energy. Nobody knows what form the dark matter or the dark energy takes. But the evidence is that the dark matter is in the form known as "cold dark matter." The evidence against the presence of much "hot dark matter" is the WMAP's finding that the youngest galaxies formed only 200 million years after the Big Bang. If much hot dark matter had been present, that galaxy formation would have taken longer.

The NASA Press Conference included a long statement about the results by Dr. Charles Bennett of NASA's Goddard Space Flight Center. Additional comments came from Dr. David Spergel of Princeton University and Dr. John Bahcall of the Institute for Advanced Study. Dr. Bahcall said that biggest surprise of the results was that there were no surprising results. WMAP's findings fit in very well with previous determinations of these parameters, though they are to a higher level of accuracy. For example, the previous estimates of the ordinary-matter content of the Universe was 4.2 to 4.6%, and the WMAP result fell in the middle of that range.

Dr. Bahcall summarized that "We are in an implausibly crazy universe, but one whose characteristics we know."

The WMAP findings represent strong endorsement of the Big Bang model of the Universe and for some versions of inflationary cosmology, though some inflationary scenarios are now ruled out. Also, the "quintessence" model that had been a contender for explaining the dark energy now seems less likely than it had, compared with the "cosmological constant" model.

The WMAP results include mainly a high-resolution full-sky map of fluctuations of the temperature of the cosmic background radiation by millionths of a kelvin. The scientists have measured the distribution of fluctuations of different sizes and of different strengths. They have also found polarization in the fluctuations and measured its distribution. The measurements represent a giant step in accuracy over the otherwise similar results from the Cosmic Background Explorer spacecraft a dozen years ago and an expansion to the whole sky from more recent measurements made from telescopes on balloons in Antarctica and other telescopes.

South-Pole Telescope Used for Background Radiation Detailed Images

Berkeley/Carnegie Mellon press release, December 12, 2002

Using a powerful new instrument at the South Pole, a team of cosmologists has produced the most detailed images of the early Universe ever recorded. The research team, which was funded by the National Science Foundation (NSF), has made public their measurements of subtle temperature differences in the Cosmic Microwave Background (CMB) radiation. The CMB is the remnant radiation that escaped from the rapidly cooling Universe about 400,000 years after the Big Bang.

Images of the CMB provide researchers with a snapshot of the Universe in its infancy, and can be used to place strong constraints on its constituents and structure. The new results provide additional evidence to support the currently favored model of the Universe in which 30 percent of all energy is a strange form of dark matter that doesn't interact with light and 65 percent is in an even stranger form of dark energy that appears to be causing the expansion of the Universe to accelerate. Only the remaining five percent of the energy in the Universe takes the form of familiar matter like that which makes up planets and stars.

The researchers developed a sensitive new instrument, the Arcminute Cosmology Bolometer Array Receiver (ACBAR), to produce high-resolution images of the CMB. ACBAR's detailed images reveal the seeds that grew to form the largest structures seen in the Universe today. These results add to the description of the early Universe provided by several previous ground-, balloon- and space-based experiments. Previous to the ACBAR results, the most sensitive, fine angular scale CMB measurements were produced by the NSF-funded Cosmic Background Investigator (CBI) experiment observing from a mountaintop in Chile.

William Holzapfel, of the University of California at Berkeley and ACBAR co-principal investigator, said it is significant that the new ACBAR results agree with those published by the CBI team despite the very different instruments, observing strategies, analysis techniques, and sources of foreground emission for the two experiments. He added that the new data provide a more rigorous test of the consistency of the new ACBAR results with theoretical predictions.

The dark energy inferred from the ACBAR observations may be responsible for the accelerating expansion of the Universe. "It is compelling that we find, in the ancient history of the Universe, evidence for the same dark energy that supernova observations find more recently," said Jeffrey Peterson of Carnegie Mellon University.

The construction of the ACBAR instrument and observations at the South Pole were carried out by a team of researchers from the University of California, Berkeley, Case Western Reserve University, Carnegie Mellon University, the California Institute of Technology, Jet Propulsion Laboratory (JPL), and Cardiff University in the United Kingdom. Principle investigators Holzapfel and John Ruhl at Case Western led the effort, which built and deployed the instrument in only two years.

ACBAR is specifically designed to take advantage of the unique capabilities of the 2.1-meter Viper telescope, built primarily by Jeff Peterson and collaborators at Carnegie Mellon and installed by NSF and its South Pole Station in Antarctica. The receiver is an array of 16 detectors built by Cal Tech and the JPL that create images of the sky in 3-millimeter wavelength bands near the peak in the brightness of the CMB. In order to reach the maximum possible sensitivity, the ACBAR detectors are cooled to two-tenths of a degree above absolute zero, or about -273 degrees Celsius (-459 Fahrenheit). ACBAR has just completed its second season of observations at the South Pole. Researcher Mathew Newcomb kept the telescope observing continuously during the six-month-long austral winter, despite temperatures plunging below -73 degrees Celsius (-100 Fahrenheit).

The construction of ACBAR and Viper was funded as part of the NSF Center for Astrophysical Research in Antarctica. The U.S. Antarctic Program provides continuing support for telescope maintenance, observations, and data analysis. NSF's Amundsen-Scott South Pole Station is ideally suited for astronomy, especially observations of the CMB. The station is located at an altitude of approximately 3,000 meters (10,000 feet), atop the Antarctic ice sheet. Water vapor is the principal cause of atmospheric absorption in broad portions of the electromagnetic spectrum from near infrared to microwave wavelengths. The thin atmosphere above the station is extremely cold and contains almost no water vapor. "Our atmosphere may be essential to life on Earth," said Ruhl, "but we'd love to get rid of it. For our observations, the South Pole is as close as you can get to space while having your feet planted firmly on the ground."

For pictures or more information and drafts of the submitted papers, see:
http://cosmology.berkeley.edu/group/swlh/acbar

Cosmic Background Radiation from the Chilean Imager

National Science Foundation Press Release, May 23, 2002
NSF PR 02-41

Astronomers operating from a remote plateau in the Chilean desert have produced the most detailed images ever made of the oldest light emitted by the universe, providing independent confirmation of controversial theories about the origin of matter and energy.

Pushing the limits of available technology, the Cosmic Background Imager (CBI) funded by the National Science Foundation (NSF) and California Institute of Technology (Caltech) detected minute variations in the cosmic microwave background, the radiation that has traveled to Earth over almost 14 billion years. A map of the fluctuations shows the first tentative seeds of matter and energy that would later evolve into clusters of hundreds of galaxies.

The measurements also provide independent evidence for the long-debated theory of inflation, which states that the universe underwent a violent expansion in its first micro-moments. After about 300,000 years it cooled enough to allow the seeds of matter to form and became "transparent," allowing light to pass through. CBI observed remnants of that early radiation. The data are also helping scientists learn more about the repulsive force called "dark energy" that appears to defy gravity and force the universe to accelerate at an ever-increasing pace.

"This is basic research at its finest and most exciting," said NSF Director Rita Colwell. "Each new image of the early universe refines our model of how it all began. Just as the universe grows and spreads, humankind's knowledge of our own origins continues to expand, thanks to the technical expertise and patient persistence of scientists such as these."

"We have seen, for the first time, the seeds that gave rise to clusters of galaxies, thus putting theories of galaxy formation on a firm observational footing," said team leader Anthony Readhead of Caltech. "These unique high-resolution observations provide a new set of critical tests of cosmology, and provide new and independent evidence that the universe is flat and is dominated by dark matter and dark energy."

Readhead, with Caltech colleagues Steve Padin and Timothy Pearson and others from Canada, Chile and the United States, generated the finest measurements to date of the cosmic microwave background. Cosmic microwave background (CMB) is a record of the first photons that escaped from the rapidly cooling, coalescing universe about 300,000 years after the cosmic explosion known as the Big Bang that is commonly believed to have given birth to the universe.

Data from the CBI on temperature distributions in the CMB support a modification of the Big Bang theory; that modification is called inflation theory. Inflation states that the hot plasma of the initial universe underwent an extreme and rapid expansion in its first 10 -32 second. The variations in temperature measured by the CBI are as small as 10 millionths of a degree.

By plotting the peaks of temperature distribution, the scientists showed that the precise CBI data are entirely consistent with inflation and confirm earlier findings by other scientists. In April 2000, an international team of cosmologists led by Caltech's Andrew Lange announced the first compelling evidence that the universe is flat-that is, its geometry is such that parallel lines will neither converge or diverge. Lange's team observed at a different frequency from CBI, using a high- altitude balloon flown over Antarctica.

Since then, two other teams -- using independent methods - have revealed their analyses of the very faint variations in temperature among the comic microwaves. The four instruments have conducted precise measurements of parameters that cosmologists have long used to describe the early universe. Each set of data has offered new clues to the form of the embryonic plasma and has drawn scientists closer to definitive answers. NSF has supported the work of all four teams and their instruments, some of them for more than 15 years.

Five papers on the CBI data were submitted to the Astrophysical Journal for publication.

The CBI consists of 13 interferometers mounted on a 6-meter diameter platform, operating at frequencies from 26 GHz to 36 GHz. Located in the driest desert in the world -- the Atacama -- CBI takes advantage of the low humidity at an altitude of 5,080 meters (16,700 feet). NSF has supported the CBI research since 1995. The National Council of Science and Technology of Chile provided the CBI site.

For more information and images, see:
http://www-dev.nsf.gov/od/lpa/news/advance/pr0241_images.htm and
http://www.astro.caltech.edu/~tjp/CBI/

Cosmic Background Radiation Structure from the Very Small Array

Jodrell Bank Press Release, May 23, 2002

A new picture of the early universe

Physicists from the Universities of Cambridge and Manchester and the Instituto de Astrofisica de Canarias in Tenerife have released the first results of new high-precision observations of the relic radiation from the Big Bang, often called the cosmic microwave background or CMB. These observations have been made with a novel radio telescope called the Very Small Array (VSA) situated on the Mount Teide in Tenerife. The images show the beginnings of the formation of structure in the early Universe. From the properties of the image, scientists can obtain vital information on just what happened in the early universe and distinguish between competing cosmological theories.

Radiation from the Big Bang fireball has been traveling across the universe, cooling as space expands. Today, we see the faint relic radiation in all directions on the sky at a temperature of just 3 degrees centigrade above absolute zero, giving a picture of the universe when it was less than one 50,000th of its present age. Because galaxies must have formed out of the primeval fireball, astrophysicists have predicted that they will have left imprints in the radiation. Across the sky, there should be tiny variations in the temperature of the relic radiation. However, these ripples are very weak---only one 10,000th of a degree C.

During its first year of operation the VSA has observed three patches of sky, each some 8 x 8 degrees across. It can see detail down to one third of a degree, well matched to the typical size of interesting temperature variations. The VSA has 14 aerials, each somewhat akin to a satellite TV dish but only 15 cm across. The signals from each aerial are combined, forming an interferometric array - a technique pioneered by Cambridge physicists. The array is able to filter out unwanted terrestrial and atmospheric radiation allowing the the extremely faint CMB sky signal common to all the aerials to be detected. This approach allows high precision observations to be made at modest cost - the capital cost of the VSA was 2.6 million GBP. The performance of the VSA also results from using advanced receivers built at Manchester University and from the outstanding atmospheric conditions at the 2.4 km high Teide Observatory on Tenerife. The VSA can therefore measure specific, individual structures in the relic radiation with great precision.

A small number of other experiments have made similar observations. The different experiments work in different ways and face different challenges and sources of error; a key advantage of this diversity is that if their results agree, one can be confident that they are correct. One special strength of the VSA is that it is an interferometer array; another is that it is able to robustly remove the contaminating radiation from radiogalaxies and quasars that lie between us and the CMB relic radiation. The VSA results provide amazing confirmation of the current picture of the Universe.

The VSA observations of the CMB released today reveal the following properties of our Universe:

1) The curvature of space is close to zero -- we live in a spatially 'flat' universe.

2) The material in the universe is dominated by dark matter.

3) There is direct evidence for 'vacuum dark energy' which is currently not well understood, but is causing the universe to accelerate.

4) There are multiple peaks in the CMB power spectrum. This is direct evidence that all the structure in the universe today is due to microscopic quantum-mechanical fluctuations, inflated to astronomical size in the very early universe.

Images and Web Sites
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Images and captions, and links to the scientific papers, are available from the Cavendish Laboratory website at

http://www.mrao.cam.ac.uk/telescopes/vsa

See also:

Press release in both English and Spanish at the Instituto de Astrofisica de Canarias website:

http://www.iac.es/gabinete/noticias/2002/m05d23.htm

Press release and images at the University of Manchester JBO website:

The Jodrell Bank website:

http://www.jb.man.ac.uk/news/vsa/

The PPARC website:

http://www.pparc.ac.uk/Cnt/CM.asp

The VSA is a collaborative project between the Astrophysics Group at Cambridge University's Cavendish Laboratory, Manchester University's Jodrell Bank Observatory, and the Instituto de Astrofi'sica de Canarias (IAC) in Tenerife. The project is funded by the UK Particle Physics and Astronomy Research Council and the IAC.

Neutrinos Make Up Less Than 20% of the Dark Matter

Royal Astronomical Society Press Notice, April 3, 2002

Neutrinos, the lightest of the known elementary particles, weigh a billionth (one part in a thousand million) of a hydrogen atom at most, and can account for no more than one-fifth of the dark matter in the Universe, according to findings by astronomers in Cambridge, who used data from the Anglo-Australian telescope 2dF Galaxy Redshift Survey (2dFGRS). The results were presented by Dr Ofer Lahav of Cambridge University.

The findings come from detailed study of the 2dF (two-degree field) Galaxy Redshift Survey, compiled using the Anglo-Australian telescope in New South Wales, Australia. The telescope has created the world's largest three-dimensional catalogue of galaxies so far, currently consisting of 220,000 galaxies. A team of 30 researchers is analyzing the survey to answer fundamental questions about the Universe.

Neutrinos come in three different varieties, and were long thought to have no mass at all, but observations of neutrinos emitted from the Sun and created by cosmic rays in the Earth's atmosphere have in the last few years revealed that this cannot be the case. A determination of the masses of neutrinos would provide clues about the physics of processes occurring under conditions beyond the reach of current particle physics experiments.

It has long been known that there is more to our Universe than we can see in the starry sky. Indeed, astronomers now know that the visible parts of the Universe, such as stars and galaxies, only constitute a small fraction of its total mass. Neutrinos do not interact with light, and are therefore a candidate for the mysterious invisible dark matter in the Universe. The mass of the neutrinos affects the growth of clumps that evolve into the large structures we observe in the Universe at the present epoch. Since neutrinos are very light, they move at nearly the speed of light over vast regions, smoothing out the clumpiness of the matter.

To study this effect of the tiny neutrinos on the universe, Dr Oystein Elgaroy and Dr Ofer Lahav (both from the Institute of Astronomy, University of Cambridge, UK) together with other 2dFGRS team members, compared the distribution of galaxies mapped out by the 2dFGRS with theoretical calculations of how the matter would be distributed in model universes with different values for the neutrino mass. From this confrontation of theory with observation, they were able to conclude that the neutrinos must have a mass smaller than a billionth of a hydrogen atom. They also concluded that the neutrinos make up less han 20% of the dark matter in the Universe, and that the rest therefore has to be in some as yet unknown form.

"It is fascinating that we can use enormous structures like galaxies to learn about the properties of the lightest of all the particles in the Universe," says Oystein Elgaroy.

"The dark matter problem has bothered astronomers for over 70 years. If indeed neutrinos have mass, the composition of matter and energy in the universe is even more complicated than the astronomers have so far imagined," says Ofer Lahav.

Recently a group of particle and nuclear physicists announced that they had observed a new type of nuclear decay process involving neutrinos. Their result is still being debated by scientists around the world but, as it stands, it implies that the three neutrinos have very nearly the same mass, and that its value is roughly a few parts in ten billion of the mass of a hydrogen atom.

"Our result from the galaxy survey does not rule out a neutrino mass as deduced from the particle physics experiment," says Oystein Elgaroy. The redshift surveys of millions of galaxies that will be completed in the next few years will set even tighter limits on the mass of the neutrino".

Ofer Lahav adds: "The latest cosmological data suggest that the universe is a mysteriously dark place. It is probably made of four entities, three of them rather exotic: ordinary matter, neutrinos, another form of dark matter which is 'cold' and energy (so-called 'dark energy' or vacuum energy) represented by the cosmological constant, suggested originally by Einstein".

First Image and Spectrum of a Dark Matter Object

Astronomers have observed a Dark Matter object directly for the first time. Images and spectra of a MACHO microlens - a nearby dwarf star that gravitationally focuses light from a star in another galaxy - were taken by the NASA/ESA Hubble Space Telescope and the European Southern Observatory's Very Large Telescope. The result is a strong confirmation of the theory that a large fraction of Dark Matter exists as small, faint stars in galaxies such as our Milky Way.

The Riddle of Dark Matter
The nature of Dark Matter is one of the fundamental puzzles in astrophysics today. Observations of clusters of galaxies and the large scale structure of individual galaxies tell us that no more than a quarter of the total amount of matter in the Universe consists of normal atoms and molecules that make up the familiar world around us. Of this normal matter, no more than a quarter emits the radiation we see from stars and hot gas. So, a large fraction of the matter in our Universe is dark and of unknown composition.

For the past ten years, active search projects have been underway for possible candidate objects for Dark Matter. One of many possibilities is that the Dark Matter consists of weakly interacting, massive sub- atomic sized particles known as WIMPs. Alternatively Dark Matter may consist of massive compact objects (MACHOs), such as dead or dying stars (neutron stars and cool dwarf stars), black holes of various sizes or planet-sized collections of rocks and ice.

The MACHOs
In 1986, Bohdan Paczynski from Princeton University realised that if some of the Dark Matter were in the form of MACHOs, its presence could be detected by the gravitational influence MACHOs have on light from distant stars. If a MACHO object in the Milky Way passes in front of a background star in a nearby galaxy, such as the Large Magellanic Cloud, then the gravitational field of the MACHO will bend the light from the distant star and focus it into our telescopes. The MACHO is acting as a gravitational lens, increasing the brightness of the background star for the short time it takes for the MACHO to pass by. Depending on the mass of the MACHO and its distance from Earth, this period of brightening can last days, weeks or months. The form and duration of the brightening caused by the MACHO - the microlensing light curve - can be predicted by theory and searched for as a clear signal of the presence of MACHO Dark Matter. MACHOs are described as `microlenses' since they are much smaller than other known cases of gravitational lensing, such as those observed around clusters of galaxies.

The MACHO Project
Astronomers from the Lawrence Livermore National Laboratory, the Center for Particle Astrophysics in the United States and the Australian National University joined forces to form the MACHO Project in 1991. This team used a dedicated telescope at the Mount Stromlo Observatory in Australia to monitor the brightness of more than 10 million stars in the Large Magellanic Cloud over a period of eight years. The team discovered their first gravitational lensing event in 1993 and have now published approximately twenty instances of microlenses in the direction of the Magellanic Clouds. These results demonstrate that there is a population of MACHO objects in and around the Milky Way galaxy that could comprise as much as one half of the Milky Way total (baryonic/normal-matter) Dark Matter content.

Hubble Obtains the First-Ever Image of a MACHO
In order to learn more about each microlensing event, the MACHO team has used Hubble to take high-resolution images of the lensed stars. One of these images showed a faint red object within a small fraction of an arc-second from a blue, main sequence background star in the Large Magellanic Cloud. The image was taken by Hubble 6 years after the original microlensing event, which had lasted approximately 100 days. The brightness of the faint red star and its direction and separation from the star in the Large Magellanic Cloud are completely consistent with the values indicated 6 years earlier from the MACHO light curve data alone. This Hubble observation further reveals that the MACHO is a small faint, dwarf star at a distance of 600 light-years with a mass between 5% and 10% of the mass of the Sun.

VLT Adds Spectral Information
To further confirm its findings, members of the MACHO team sent in a special application for observing time on the FORS instrument on the European Southern Observatory's (ESO) Very Large Telescope (VLT)) to make spectra of the object. ESO responded swiftly and positively to the request. Although it was not possible to separate the spectra of the MACHO and background star, the combined spectrum showed the unmistakable signs of the deep absorption lines of a dwarf M star superimposed on the spectrum of the blue main sequence star in the Large Magellanic Cloud.

The Nature of Dark Matter
The combination of the microlensing light curve from the MACHO project, the high-resolution images from Hubble and the spectroscopy from the VLT has established the first direct detection of a MACHO object, to be published in the international science journal Nature on 6 December. The astronomers now have a complete picture of the MACHO: its mass, distance and velocity. The result greatly strengthens the argument that a large fraction of the `normal' Dark Matter in and around our Galaxy exists in the form of MACHOs and that this Dark Matter is not as dark as previously believed!

Future searches for MACHO-like objects will have the potential to map out this form of Dark Matter and reach a greater understanding of the role that Dark Matter plays in the formation of galaxies. These efforts will further strengthen the drive to reveal the secrets of Dark Matter and take a large step towards closing the books on the mass budget of the Universe.

For more information, please see: http://hubble.esa.int/hubble/news/newsarchive.cfm

MAP is in Place

After its three-month journey in space, NASA's Microwave Anisotropy Probe (MAP) moved into its new home a million miles from Earth and is ready to chart the oldest light in the cosmos.

"We can now begin the process of observing the remnants of the early Universe," said Dr. Charles L. Bennett, MAP Principal Investigator from NASA's Goddard Space Flight Center in Greenbelt, Md. "There is great anticipation within the astronomy community about this mission because of the potential it has to give us key clues to the content, shape, history and the ultimate fate of our Universe."

MAP, launched June 30, 2001, and was placed into a highly elliptical orbit around the Earth. From there, the spacecraft team executed a series of maneuvers using on-board thrusters to bring MAP around the Earth three times and position it for a gravity-assist boost from the Moon. The lunar swing-by occurred a month after launch, on July 30.

Since then, MAP has cruised toward L2, a quasi-stable position one million miles from Earth in the direction opposite the Sun. While previous missions have passed through the L2 neighborhood, MAP is the first mission to use an L2 orbit as its permanent observing station.

All of MAP's spacecraft and instrument systems are performing admirably. "Both the operations team and the science team are ecstatic because of MAP's outstanding performance," added Bennett. "Everything is going extremely well."

MAP will scan the skies over two years, collecting information on the faint cosmic glow in five distinct wavebands of light. The data will be analyzed and made into a full sky map for each waveband. The first sky map results are expected about December 2002.

The space probe will collect the information needed to make a map of the entire sky in the microwave light left over from the Big Bang. The entire universe is bathed in this afterglow light. This is the oldest light in the universe and has been traveling for 14 billion years. The patterns in this light across the sky encode a wealth of details about the nature, composition and destiny of the universe.

The images of the infant universe are viewed by measuring tiny temperature differences within the microwave light, which now averages 2.73 degrees above absolute zero. The extraordinary design of MAP allows it to measure the slight temperature fluctuations to within millionths of a degree. The unprecedented accuracy of MAP has the potential to revolutionize current views of the universe.

MAP was produced in partnership between Princeton University, N.J., and Goddard. Goddard and Princeton University produced the MAP hardware and software. In addition to Goddard and Princeton, science team members are located at the University of Chicago, the University of California, Los Angeles, Brown University, Providence, R.I., and the University of the British of Columbia, Vancouver.

http://map.gsfc.nasa.gov

The latest on inflation

http://physics.stanford.edu/linde.

A direct link to his 1994 Scientific American article on inflationary cosmology is at

http://www.sciam.com/specialissues/0398cosmos/0398linde.html

COBE Slide Set

A slide set showing cosmic-background-radiation images is available from the Astronomical Society of the Pacific.