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Vocapedia > Space > Cosmos > Cosmic web, Dark energy, Dark matter

 

 

 

 

What is the universe made of?        The Economist        20 August 2015

 

The Earth, the sun, the stars,

and everything we can see,

only comprise five percent of the universe.

 

But what about the other 95 percent?

 

Scientists are puzzling over dark matter and dark energy,

the mysterious components that make up the rest.

 

YouTube

https://www.youtube.com/watch?v=5iGgA1ocBkE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The Shadow Universe Revealed        NYT        July 15th, 2014

 

 

 

 

The Shadow Universe Revealed        NYT        July 15th, 2014

 

Recently astronomers have used a cosmic web imager

to visualize simulations of dark matter,

showing how the large scale structure of the universe grows

and the nests in which galaxies are hatched.

 

By Dennis Overbye, Jonathan Corum and Jason Drakeford        NYT        July 15th, 2014

http://www.nytimes.com/video/science/100000003001297/the-shadow-universe-revealed.html

 

Related

http://www.nytimes.com/2014/07/16/science/space/stalking-the-shadow-universe.html

 

 

 

 

 

 

 

 

 

 

 

 

 

 

dark matter

https://www.npr.org/sections/thetwo-way/2018/02/28/
588833582/did-dark-matter-make-the-early-universe-chill-out

 

https://www.youtube.com/watch?
v=5iGgA1ocBkE - The Economist, Aug. 20, 2015

 

http://www.npr.org/blogs/thetwo-way/2015/04/14/
399562522/scientists-release-largest-map-yet-of-dark-matter-in-the-cosmos

http://www.youtube.com/watch?
v=UmW5TAULdAs&list=LLqnbDFdCpuN8CMEg0VuEBqA

 

http://www.theguardian.com/science/2013/oct/30/
dark-matter-lux-detector-particle

http://www.guardian.co.uk/science/2013/apr/03/
dark-matter-space-station-physics

 

http://www.guardian.co.uk/science/2009/dec/17/
dark-matter-detected

 

http://www.nytimes.com/2008/11/25/
science/25dark.html

http://www.guardian.co.uk/science/2008/sep/07/
particlephysics.darkmatter

http://www.guardian.co.uk/science/2008/apr/24/
physics.sciencenews

 

 

 

 

dark energy

https://www.nytimes.com/topic/subject/dark-energy-astronomy 

 

https://www.nytimes.com/2018/02/19/
science/nasa-dark-energy-wfirst.html

 

https://www.youtube.com/watch?
v=5iGgA1ocBkE - The Economist, Aug. 20, 2015

http://www.nytimes.com/2011/10/05/
science/space/05nobel.html

http://www.nytimes.com/2011/01/04/
science/space/04telescope.html 

http://www.guardian.co.uk/science/blog/audio/2009/oct/05/
science-weekly-podcast-astronomy

http://www.theguardian.com/science/2003/dec/19/
universe.spaceexploration

 

 

 

 

Big Rip

https://www.youtube.com/watch?v=5iGgA1ocBkE - The Economist, Aug. 20, 2015

 

 

 

 

Big Crunch

https://www.youtube.com/watch?v=5iGgA1ocBkE - The Economist, Aug. 20, 2015

 

 

 

 

 

 

 

 

 

Studies of Universe’s Expansion

Win Physics Nobel

 

October 4, 2011

The New York Times

By DENNIS OVERBYE

 

Three astronomers won the Nobel prize on Tuesday for discovering that the universe is apparently being blown apart by a mysterious force that cosmologists now call dark energy. They are Saul Perlmutter of the Lawrence Berkeley National Laboratory in Berkeley, Calif., Brian P. Schmidt of the Australian National University in Weston Creek, Australia, and Adam G. Riess of the Space Telescope Science Institute and Johns Hopkins University in Baltimore.

They were the leaders of two competing teams of astronomers who were trying to use the exploding stars known as Type 1a supernovae as cosmic lighthouses to measure the expansion of the universe. They were hoping to measure how fast the universe, which has been expanding since its fiery birth in the Big Bang 14 billion years ago, was slowing down, and thus to find out if its ultimate fate was to fall back together in what is called a Big Crunch or not. Instead, they reported in 1998, it was inexplicably speeding up, a conclusion that nobody would have accepted if not for the fact that both groups wound up with the same answer.

At the time, “We were a little scared,” Dr. Schmidt said. Subsequent cosmological measurements have confirmed that roughly 70 percent of the universe by mass or energy consists of this antigravitational dark energy.

The most likely explanation for this bizarre behavior is a fudge factor Albert Einstein introduced into his equations in 1917 to stabilize he universe against collapse and then abandoned as his greatest blunder. “Every test we have made has come out perfectly in line with Einstein’s original cosmological constant in 1917,” Dr. Schmidt said.

Quantum theory predicts that empty space should exert a repulsive force, like dark energy, but one that is 10 to the 120th power times stronger than what the astronomers have measured, leaving some physicists mumbling about multiple universes.

Lawrence M. Krauss, a cosmologist at Arizona State University said, “The discovery that the universe is dominated by the energy of empty space has changed everything in cosmology. Nothing could, literally, not be more exciting, because now we know nothing is almost everything!”

In the years since then the three astronomers have shared a number of awards.

Dr. Perlmutter, who led the Supernova Cosmology Project out of Berkeley, will get half of the prize of 10 million Swedish kronor ($1.4 million). The other half will go to Dr. Schmidt, leader of the rival High-Z Supernova Search Team, and Dr. Riess, who was the lead author of the 1998 paper in The Astronomical Journal, in which the dark energy result was first published. They will get their prizes in Stockholm on Dec. 10.

 

 

This article has been revised

to reflect the following correction:

Correction: October 4, 2011

An earlier version of this article

incorrectly stated the publication

in which Adam G. Riess's 1998 paper

on dark energy appeared.

It was The Astronomical Journal, not Science.

The article also stated incorrectly

the amount of the prize.

It is 10 million Swedish kronor ($1.4 million).

    Studies of Universe’s Expansion Win Physics Nobel, NYT, 4.10.2011,
    http://www.nytimes.com/2011/10/05/science/space/05nobel.html

 

 

 

 

 

A Whisper, Perhaps,

From the Universe’s Dark Side

 

November 25, 2008
The New York Times
By DENNIS OVERBYE

 

Is this the dark side speaking?

A concatenation of puzzling results from an alphabet soup of satellites and experiments has led a growing number of astronomers and physicists to suspect that they are getting signals from a shadow universe of dark matter that makes up a quarter of creation but has eluded direct detection until now.

Maybe.

“Nobody really knows what’s going on,” said Gordon Kane, a theorist at the University of Michigan. Physicists caution that there could still be a relatively simple astronomical explanation for the recent observations.

But the nature of this dark matter is one of the burning issues of science. Identifying it would point the way to a deeper understanding of the laws of nature and the Einsteinian dream of a unified theory of physics.

The last few weeks have seen a blizzard of papers trying to explain the observations in terms of things like “minimal dark matter” or “exciting dark matter,” or “hidden valley” theory, and to suggest how to look for them in particle accelerators like the Large Hadron Collider, set to begin operation again outside Geneva next summer.

“It could be deliriously exciting, an incredibly cool story,” said Nima Arkani-Hamed of the Institute for Advanced Study in Princeton, N.J., who has been churning out papers with his colleagues. “Anomalies in the sky tell you what to look for in the collider.”

On Thursday, a team of astrophysicists working on one of the experiments reported in the journal Nature that a cosmic ray detector onboard a balloon flying around the South Pole had recorded an excess number of high-energy electrons and their antimatter opposites, positrons, sailing through local space.

The particles, they conceded, could have been created by a previously undiscovered pulsar, the magnetized spinning remnant of a supernova explosion, blasting nearby space with electric and magnetic fields. But, they say, a better and more enticing explanation for the excess is that the particles are being spit out of the fireballs created by dark matter particles colliding and annihilating one another in space.

“We cannot disprove that the signal could come from an astrophysical object. We also cannot eliminate a dark matter annihilation explanation based upon current data,” said John P. Wefel of Louisiana State University, the leader of the team, adding, “Whichever way it goes, for us it is exciting.”

The results came on the heels of a report earlier this fall from Pamela, a satellite built by Italian, German, Russian and Swedish scientists to study cosmic rays. Pamela scientists reported in talks and a paper posted on the Internet that the satellite had recorded an excess of high-energy positrons. This, they said, “may constitute the first indirect evidence of dark matter particle annihilations,” or a nearby pulsar.

Antimatter is rare in the universe, and so looking for it is a good way of hunting for exotic phenomena like dark matter.

Another indication that something funny is happening on the dark side of the universe is evident in maps of the cosmic background radiation left over from the Big Bang. Those maps, produced most recently this year by the Wilkinson Microwave Anisotropy Probe satellite, show a haze of what seem to be charged particles hovering around the Milky Way galaxy, according to an analysis by Douglas Finkbeiner of the Harvard-Smithsonian Center for Astrophysics.

Adding to the mix and mystery, the European Space Agency’s Integral satellite detected gamma rays emanating from the center of the Milky Way, suggesting the presence of positrons there, but with much lower energies than Pamela and Dr. Wefel’s experiments have seen.

What all this adds up to, or indeed whether it all adds up to anything at all, depends on which observations you trust and your theoretical presumptions about particle physics and the nature of dark matter. Moreover, efforts to calculate the background level of high-energy particles in the galaxy are beset with messy uncertainties. “The dark matter signal is easy to calculate,” Dr. Kane said. “The background is much harder.”

Dark matter has teased and obsessed astronomers since the 1930s, when the Caltech astronomer Fritz Zwicky deduced that some invisible “missing mass” was required to supply the gravitational glue to hold clusters of galaxies together. The idea became respectable in the 1970s when Vera C. Rubin of the Carnegie Institution of Washington and her collaborators found from studying the motions of stars that most galaxies seemed to be surrounded by halos of dark matter.

The stakes for dark matter go beyond cosmology. The most favored candidates for its identity come from a theory called supersymmetry, which unifies three of the four known forces of nature mathematically and posits the existence of a realm of as-yet-undiscovered particles. They would be so-called wimps — weakly interacting massive particles — which feel gravity and little else, and could drift through the Earth like wind through a screen door. Such particles left over from the Big Bang could form a shadow universe clumping together into dark clouds that then attract ordinary matter.

The discovery of a supersymmetric particle would also be a boost for string theory, the controversial “theory of everything,” and would explicate the nature of a quarter of the universe. But until now, the dark matter particles have mostly eluded direct detection in the laboratory, the exception being a controversial underground experiment called Dama/Libra, for Dark Matter/Large Sodium Iodide Bulk for Rare Processes, under the Italian Alps, where scientists claimed in April to have seen a seasonal effect of a “dark matter wind” as the Earth goes around its orbit.

The sky could be a different story. Dark matter particles floating in the halos around galaxies would occasionally collide and annihilate one another in tiny fireballs of radiation and lighter particles, theorists say.

Dr. Wefel and his colleagues have been chasing sparks in the sky since 2000, when they flew an instrument known as ATIC, for Advanced Thin Ionization Calorimeter, around Antarctica on a balloon at an altitude of 23 miles, looking for high-energy particles known as cosmic rays raining from space.

In all they have made three flights, requiring them to spend the winter at the National Science Foundation’s McMurdo Station, which Dr. Wefel described as very pleasant. “It’s not bad until a storm moves in. You put your hand out till you can’t see it. Then you go out and start shoveling snow,” he explained.

The Nature paper includes data from the first two balloon flights. It shows a bump, over theoretical calculations of cosmic ray intensities, at energies of 500 billion to 800 billion electron volts, a measure of both energy and mass in physics. One way to explain that energy bump would be by the disintegration or annihilation of a very massive dark particle. A proton by comparison is about one billion electron volts.

Dr. Wefel noted, however, that according to most models, a pulsar could generate particles with even more energy, up to trillions of volts, whereas the bump in the ATIC data seems to fall off at around 800 billion electron volts. The ATIC results, he said, dovetail nicely with those from Pamela, which recorded a rising number of positrons relative to electrons, but only up to energies of about 200 billion electron volts.

Reached in China, where he was attending a workshop, Neal Weiner of New York University, who is working with Dr. Arkani-Hamed on dark matter models, said he was plotting ATIC data gleaned from the Web and Pamela data on the same graph to see how they fit, which was apparently very well.

But Piergiorgio Picozza, a professor at the University of Rome and the Pamela spokesman, said in an e-mail message that it was too soon to say the experiments agreed. That will depend on more data now being analyzed to learn whether Pamela continues to see more positrons as the energy rises.

Moreover, as Dr. Kane pointed out, Pamela carries a magnet that allows it to distinguish electrons from positrons — being oppositely charged, they bend in opposite directions going through the magnetic field. But the ATIC instrument did not include a magnet and so cannot be sure that it was seeing any positrons at all: no antimatter, no exotic dark matter, at least at those high energies.

But if he is right, Dr. Wefel said that the ATIC data favored something even more exotic than supersymmetry, namely a particle that is lost in the fifth dimension. String theory predicts that there are at least six dimensions beyond our simple grasp, wrapped up so tightly we cannot see them or park in them. A particle in one of these dimensions would not appear to us directly.

You could think of it as a hamster running around on a wheel in its cage. We cannot see the hamster or the cage, but we can sort of feel the impact of the hamster running; according to Einsteinian relativity, its momentum in the extra dimension would register as mass in our own space-time.

Such particles are called Kaluza-Klein particles, after Theodor Kaluza and Oscar Klein, theorists who suggested such an extra-dimensional framework in the 1920s to unify Einstein’s general theory of relativity and electromagnetism.

Dr. Wefel’s particle would have a mass of around 620 billion electron volts. “That’s the one that seems to fit the best,” he said in an interview. The emergence of a sharp edge in the data, he said, “would be a smoking gun” for such a strange particle.

But Dr. Arkani-Hamed said that Kaluza-Klein particles would not annihilate one another at a fast enough rate to explain the strength of the ATIC signal, nor other anomalies like the microwave haze. He and his colleagues, including Dr. Weiner, Dr. Finkbeiner and Tracy Slatyer, also of Harvard, drawing on work by Matthew Strassler of Rutgers, have tried to connect all the dots with a new brand of dark matter, in which there are not only dark particles but also a “dark force” between them.

That theory was called “a delightful castle in the sky” by Dr. Kane, who said he was glad it kept Dr. Arkani-Hamed and his colleagues busy and diverted them from competing with him. Dr. Kane and his colleagues favor a 200 billion-electron-volt supersymmetric particle known as a wino as the dark matter culprit, in which case the Pamela bump would not extend to higher energies.

Dr. Wefel said he had not kept up with all the theorizing. “I’m just waiting for one of these modelers to say here is the data, here is the model,” he said. “Fit it out. I’m not sure I’ve seen it yet.”

Dr. Picozza said that it was the job of theorists to come up with models and that they were proliferating.

“At the end of the story only one will be accepted from the scientific community, but now it is too early,” he said in an e-mail message.

Sorting all this out will take time, but not forever.

Pamela is expected to come out with new results next year, and the first results from the Fermi Gamma-ray Space Telescope, launched last summer, should also be out soon. Not to mention the Large Hadron Collider, which will eventually smash together protons of seven million electron volts. It is supposed to be running next summer.

“With so many experiments, we will soon know so much more about all of this,” Dr. Weiner said. “In a year or two, we’ll either not be talking about this idea at all, or it will be all we’re talking about.”

    A Whisper, Perhaps, From the Universe’s Dark Side, NYT, 25.11.2008,
    http://www.nytimes.com/2008/11/25/science/25dark.html

 

 

 

 

 

The universe gives up

its deepest secret

It is the invisible material
that makes up most of the cosmos.
Now, scientists have created
the first image of dark matter

 

Published: 08 January 2007
The Independent
By Steve Connor, Science Editor
 

 

One of the greatest mysteries of the universe is about to be unravelled with the first detailed, three-dimensional map of dark matter - the invisible material that makes up most of the cosmos.

Astronomers announced yesterday that they have achieved the apparently impossible task of creating a picture of something that has defied every attempt to detect it since its existence was first postulated in 1933.

Scientists have known for many years that there is more to the universe than can be seen or detected through their telescopes but it is only now that they have been able to capture the first significant 3D-image of this otherwise invisible material.

Unlike the ordinary matter of the planets, stars and galaxies, which can be seen through telescopes or detected by scientific instruments, nobody has seen dark matter or knows what it is made of, though calculations suggest that it is at least six times bigger than the rest of the visible universe combined.

A team of 70 astronomers from Europe, America and Japan used the Hubble space telescope to build up a picture of dark matter in a vast region of space where some of the galaxies date back to half the age of the universe - nearly 7 billion years.

They used a phenomenon known as gravitational lensing, first predicted by Albert Einstein, to investigate an area of the sky nine times the size of a full moon. Gravitational lensing occurs when light from distant galaxies is bent by the gravitational influence of any matter that it passes on its journey through space.

The scientists were able to exploit the technique by collecting the distorted light from half a million faraway galaxies to reconstruct some of the missing mass of the universe which is otherwise invisible to conventional telescopes.

"We have, for the first time, mapped the large-scale distribution of dark matter in the universe," said Richard Massey of the California Institute of Technology in Pasadena, one of the lead scientists in the team. "Dark matter is a mysterious and invisible form of matter, about which we know very little, yet it dominates the mass of the universe."

One of the most important discoveries to emerge from the study is that dark matter appears to form an invisible scaffold or skeleton around which the visible universe has formed.

Although cosmologists have theorised that this would be the case, the findings are dramatic proof that their calculations are correct and that, without dark matter, the known universe that we can see would not be able to exist.

"A filamentary web of dark matter is threaded through the entire universe, and acts as scaffolding within which the ordinary matter - including stars, galaxies and planets - can later be built," Dr Massey said. "The most surprising aspect of our map is how unsurprising it is. Overall, we seem to understand really well what happens during the formation of structure and the evolution of the universe," he said.

The three-dimensional map of dark matter was built up by taking slices through different regions of space much like a medical CT scanner build a 3-D image of the body by taking different X-ray "slices" in two dimensions.

Data from the Hubble telescope was supplemented by measurements from telescopes on the ground, such as the Very Large Telescope of the European Southern Observatory in Chile and the Japanese Subaru telescope in Hawaii.

Details of the dark matter map were released yesterday at the annual meeting of the American Astronomical Society in Seattle and published online by the journal Nature. The map stretches half way back to the beginning of the universe and shows that dark matter has formed into "clumps" as it collapsed under gravity. Other matter then grouped around these clumps to form the visible stars, galaxies and planets.

"The 3-D information is vital to studying the evolution of the structures over cosmic time," said Jason Rhodes of the Jet Propulsion Laboratory in Pasadena.

Astronomers have compared the task of detecting dark matter to the difficulty of photographing a city at night from the air when only street lights are visible.

Scientists said the new images were equivalent to seeing a city, its suburbs and country roads in daylight for the first time. Major arteries and intersections become evident and a variety of neighbourhoods are revealed.

"Now that we have begun to map out where dark matter is, the next challenge is to determine what it is, and specifically its relationship to normal matter," Dr Massey said. "We have answered the first question about where the dark matter it, but the ultimate goal will be to determine what it is."

Various experiments on Earth are under way to try to find out what dark matter is made of. One theory is that it is composed of mysterious sub-atomic particles that are difficult to detect because they do not interact with ordinary matter and so cannot be picked up and identified by conventional scientific instruments. Comparing the maps of visible matter and dark matter have already pointed to anomalies that could prove critical to the understanding of what constitutes dark matter.

    The universe gives up its deepest secret, I, 8.1.2007,
    http://news.independent.co.uk/world/
    science_technology/article2134891.ece

 

 

 

 

 

Steve Connor:

The mystery that has endured

since Big Bang

 

Published: 08 January 2007
The Independent

 

For anyone who has been mesmerised by the sheer number of stars that make up a clear night sky, it seems incredible that what we can see, even with a telescope, is but a small fraction of what is actually out there. In fact, more than 80 per cent of the material of the universe is invisible to even the best instruments.

It is called "dark" matter because, unlike the "bright" matter of the visible stars, galaxies and planets, it is invisible, even though its gravitational presence can be felt. What dark matter is made of, however remains a mystery.

Fritz Zwicky, a Swiss astronomer, was the first to postulate the existence of dark matter in 1933 when he observed clusters of galaxies beyond our own Milky Way. Zwicky said that these distant galaxies were moving too fast to be held together by the gravity of the visible stars they contain.

Confirmation of Zwicky's idea came in the 1970s when astronomers measured the speed at which stars moved inside and on the outer edges of galactic discs. To their surprise, the outer stars were travelling just as fast as the inner stars. Gravitational theory suggested that the outermost stars should be travelling more slowly.

The only reasonable explanation was that each galaxy had up to 10 times more mass than could be seen. This extra material was creating the additional gravity that kept the outer stars from slowing down.

The latest findings from the Hubble space telescope, released at the American Astronomical Society in Seattle yesterday, suggest that dark matter forms an invisible "scaffold" around which the ordinary matter of the stars and galaxies have formed. The map of dark matter has been likened to a three-dimensional X-ray of the skeleton on which the "flesh" of the visible universe is hung.

Knowing the whereabouts of the dark matter is critical to understanding how galaxies formed and how they began to accumulate into clusters over the 13.7 billion years since the Big Bang.

Critically, comparing the distribution maps of bright and dark matter may point to important differences between them. Several experiments on Earth are designed to capture the elusive subatomic particles that may account for the missing mass.

Many scientists now feel that we are on the verge of discovering what it is that has formed such an immense part of the universe.

    Steve Connor: The mystery that has endured since Big Bang, I, 8.1.2007,
    http://comment.independent.co.uk/commentators/article2134892.ece

 

 

 

 

 

9 Billion-Year-Old ‘Dark Energy’

Reported

 

November 17, 2006

The New York Times

By DENNIS OVERBYE

 

A strange thing happened to the universe five billion years ago. As if God had turned on an antigravity machine, the expansion of the cosmos speeded up, and galaxies began moving away from one another at an ever faster pace.

Now a group of astronomers using the Hubble Space Telescope have discovered that billions of years before this mysterious antigravity overcame cosmic gravity and sent the galaxies scooting apart like muscle cars departing a tollbooth, it was already present in space, affecting the evolution of the cosmos.

“We see it doing its thing, starting to fight against ordinary gravity,” Adam Riess of the Space Telescope Science Institute said about the antigravity force, known as dark energy. He is the leader of a team of “dark energy prospectors,” as he calls them, who peered back nine billion years with the Hubble and were able to discern the nascent effects of antigravity. The group reported their observations at a news conference yesterday and in a paper to be published in The Astrophysical Journal.

The results, Dr. Riess and others said, provide clues and place new limits on the nature of dark energy, a mystery that has thrown physics and cosmology into turmoil over the last decade.

“It gives us the ability to look at changes in dark energy,” he said in an interview. “Previously, we knew nothing about that. That’s really exciting.”

The data suggest that, in fact, dark energy has changed little, if at all, over the course of cosmic history. Though hardly conclusive, that finding lends more support to what has become the conventional theory, that the source of cosmic antigravity is the cosmological constant, a sort of fudge factor that Einstein inserted into his cosmological equations in 1917 to represent a cosmic repulsion embedded in space.

Although Einstein later abandoned the cosmological constant, calling it a blunder, it would not go away. It is the one theorized form of dark energy that does not change with time.

Sean Carroll, a cosmologist at the California Institute of Technology who was not on the team, said: “Had they found the evolution was not constant, that would have been an incredibly earthshaking discovery. They looked where no one had been able to look before.”

The paper by Dr. Riess and his colleagues represents a sort of progress report from the dark side, where astrophysicists have found themselves more and more as they try to understand what is happening to the universe.

This encounter with the invisible began eight years ago, when two competing teams of astronomers were using exploding stars known as Type 1a supernovas as cosmic distance markers to determine the fate of the universe.

Ever since the Big Bang 14 billion years ago, the galaxies and the rest of the universe have been flying apart like a handful of pebbles tossed in the air. Astronomers reasoned that gravity would be slowing the expansion, and the teams were trying to find out by how much and, thus, determine whether all would collapse one day into a “big crunch” or expand forever.

Instead, to their surprise, the two teams, one led by Saul Perlmutter of the University of California, Berkeley, and the other by Brian Schmidt of the Mount Stromlo and Siding Spring Observatories in Australia, found that the universe was speeding up instead of slowing down.

But the ground-based telescopes that the two teams used could track supernovas to distances of just seven billion light-years, corresponding to half the age of the universe, and the effect could have been mimicked by dust or a slight change in the nature of the supernova explosions.

Since then, Dr. Riess, who was a member of Dr. Schmidt’s team, and his colleagues have used the Hubble telescope to prospect for supernovas and dark energy farther out in space or back in time.

The new results are based on observations of 23 supernovas that are more than eight billion years in the past, before dark energy came to dominate the cosmos. The spectra of those distant supernovas, Dr. Riess reported, appear to be identical to those closer and more recent examples. By combining the supernova results with data from other experiments like the NASA Wilkinson Microwave Anisotropy Probe, Dr. Riess and his colleagues could begin to address the evolution of dark energy.

“That’s one of the $64,000 questions,” he said. “Is dark energy changing?”

So far, he said, the results are consistent with the cosmological constant, but other answers are also possible. The possibility that it is the cosmological constant is a mixed blessing. Physicists concede that they do not understand it.

Dr. Carroll of Caltech said, “Dark energy makes us nervous.”

Einstein invented his constant to explain why the universe does not collapse. After he abandoned it, the theory was resuscitated by quantum mechanics, which showed that empty space should be bubbling with staggering amounts of repulsive energy. The possibility that it really exists in the tiny amounts measured by the astronomers has flummoxed physicists and string theorists.

Because it is a property of empty space, the overall force of Einstein’s constant grows in proportion as the universe expands, until it overwhelms everything. Other theories of dark energy like strange force fields called quintessence or modifications to Einstein’s theory of gravity can change in more complicated ways, rising, falling or reversing effects.

Astronomers characterize the versions of dark energy by their so-called equation of state, the ratio of pressure to density, denoted by the letter w. For the cosmological constant, w is minus one.

Dr. Riess and his group used their data to make the first crude measurement of this quantity as it stood nine billion years ago. The answer, he said, was minus one — the magic number — plus or minus about 50 percent. By comparison for more recent times, with many more supernovas observable and thus more data, the value is minus one with an uncertainty of about 10 percent.

“If at one point in history it’s not minus one,” Dr. Riess said, “then we have killed the very best explanation.”

Getting to the precision needed to kill or confirm Einstein’s constant, however, will be very difficult, he conceded. One of the biggest sources of uncertainty is the fact that the Type 1a explosions are not completely uniform, introducing scatter into the observations.

The Hubble is the sole telescope that can pursue supernova explosions deeply enough to chart the early days of dark energy. The recent announcement that the National Aeronautics and Space Administration will send astronauts to maintain and refurbish the Hubble once again, enabling it to keep performing well into the next decade, is a lift for Dr. Riess’s project. A new camera could extend observations to 11 billion or 12 billion years back.

9 Billion-Year-Old ‘Dark Energy’ Reported,
NYT,
17.11.2006,
http://www.nytimes.com/2006/11/17/
science/space/17dark.html

 

 

 

 

 

 

 

 

 

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