Darkmatter

[Time02112]

Researchers Question Dark Matter Quantity
October 3, 2000 08:10 CDT

An article published in the latest issue of the Astrophysical Journal lends strong support for a controversial theory that rejects the cold dark matter hypothesis central to what most scientists believe about the composition of the Universe.

In the October issue, University of Maryland astronomer Stacy McGaugh details cosmic microwave background predictions that he made last year and which subsequently proved correct. The cosmic microwave background is the faint radiation that scientists believe to be a remnant of the energy released in the Big Bang. Measurements of cosmic microwave background matching McGaugh's 1999 prediction were reported in the journal Nature in March of this year by scientists conducting an experiment known as Boomerang.

The accuracy of his predictions, writes McGaugh, points to a universe that consists entirely of "ordinary" matter. This contradicts the widely held paradigm that 90 percent of the universe is made up of unseen matter, termed cold dark matter. Cold dark matter is widely thought to consist of a new kind of particle rather than the protons, neutrons, and other known particles that constitute ordinary matter.

"What I predicted correctly in an article in the October 1999 Astrophysical Journal is the amplitude of the second peak relative to the first peak in the power spectrum of the cosmic microwave background," McGaugh said.

In March, when the Boomerang results were announced, many cosmologists publicly rejoiced that the position of the first peak in the power spectrum of the microwave background indicated the universe was "flat," a key prediction of inflation, one of the central tenets of modern cosmology. However, cosmologists were puzzled by the small amplitude of the second peak relative to the first because it didn't fit what they expected to see based on another key tenet, the theory of cold dark matter.

"On the other hand, the relative amplitudes were precisely what I had expected should cold dark matter not exist," said McGaugh.

According to McGaugh, the basis for his correct predictions lies in a little known alternative theory to dark matter called MOND, for modified Newtonian dynamics. "Until 1994, I was like most astronomers and didn't think much of, or about, MOND," he said. "But that year a problem cropped up in my data for the rotation curves of low surface brightness galaxies. The data made no sense in the conventional dark matter context. I pounded my head against the wall for many months trying to make sense of it when by chance I attended a talk by Moti Milgrom, the Israeli physicist who conceived of MOND.

"Without knowing who I was or what problem I was struggling with, he derived a series of predictions for how low surface brightness galaxies ought to behave in MOND. Everything that was so confusing in the dark matter context was actually a prediction of MOND."

"It was a classic example of the kind of hypothesis testing that forms the basis of science. In this case MOND's predictions came true, cold dark matter's did not," McGaugh said.

McGaugh's newest article on modified Newtonian dynamics is attracting the attention and interest of many astronomers and physicists. But while acknowledging the accuracy and potential significance of his work, McGaugh says that even friends and colleagues on campus remain skeptical of MOND.

Cole Miller, an assistant professor of astronomy at Maryland who has had numerous friendly debates with McGaugh about MOND points out that there are possibilities that fit within the context of the theory of cold dark matter that could explain why the second cosmic microwave background peak is lower than cosmologists expected. "Though Stacy's idea is very interesting, it's not really possible to point to the Boomerang findings or to any current observation and conclusively say which theory <MOND or cold dark matter> is correct."

Miller said that for largely philosophical reasons most scientists are not likely to embrace Stacy's position at this point. "MOND introduces a new fundamental constant in a way that seems ad hoc, and which is aesthetically displeasing to most cosmologists. On the other hand, cold dark matter postulates new and so far unobserved particles, so it's good to keep an open mind about both possibilities," he said.

McGaugh said he agrees that it's only natural to be skeptical of an idea as radical as MOND the first time you hear it. "But to be fair, I think we need to apply the same degree of skepticism to dark matter."

Source: U of Maryland News Release

Cosmiverse Staff Writer http://www.cosmiverse.com/space100301.html

 

[dob]

I founded interesting that this post from Time02112 (dating from october 4, 2000), was touching one of the last subject we touched here in the previous replies. About Dark-Matter.

So I just pulled it from the archive...

(T-02112: I'm shure that you remember it!)

 

[Time02112]

Seeing the dark side 21 Apr 01

ASTRONOMERS in the US have mapped a cluster of invisible galaxies for the first time. They spotted the cluster by analysing the effect its gravity had on the light from more distant galaxies. The discovery opens a new window on the Universe, because a large part of its mass may be "dark matter".

Last year, a team of European astronomers caught a glimpse of a dark galaxy as it distorted light from the more distant bright ones they were imaging. But astronomers have never been able to confirm such sightings or work out what the galaxies were like. "It was a true mass detection but difficult to confirm," admits Peter Schneider of the University of Bonn, a member of the team.

Now Tony Tyson, David Wittman and colleagues at Lucent Technologies' Bell Labs in New Jersey have made a similar discovery, which they later confirmed by picking up very faint light from the cluster.

They looked at 31,000 distant galaxies within a square patch of sky half a degree across using the Blanco 4-metre telescope at the Cerro Tololo Inter-American Observatory in Chile. They then entered data on the apparent shapes of the galaxies into a computer and combined them to produce an average shape.

They reasoned that because most galaxies are elliptical, the average of many galaxies with different orientations should be circular. In fact the average was an ellipse, indicating that dark matter in front of the galaxies was distorting the images. Analysing what sort of bodies would produce such distortion, the team was able to construct a three-dimensional map of the positions of 26 dark galaxies in a cluster.

Because the Universe is expanding, light from distant objects gets stretched, shifting it to redder wavelengths. By comparing the "red shifts"-which are proportional to distance-of the background galaxies to the amount the dark cluster distorts their light, Wittman says he was able to estimate the distance to the dark cluster. At the moment his claim is controversial. "I don't think you can get the red shift using <such> data," says Schneider.

But if the team is right, astronomers may soon be able to produce 3D maps of truly dark galaxies that cannot be seen any other way. Tyson believes the only way to test current theories about dark matter is to study such galaxies. "Astronomers need no longer be biased towards what glows in the dark," he says.

Eugenie Samuel


From New Scientist magazine, vol 170 issue 2287, 21/04/2001, page 11

Further reading:

More at:] http://xxx.lanl.gov/abs/astro-ph?0104094

 

[Time02112]

Black whole 13 Jan 01

Someone has cleverly weighed the Universe at last (16 December 2000, p 26). Last time I tried, I couldn't get it to stay on the weighbridge. It just kept floating away. I take it that what you really meant was that someone has determined its mass.

Recently, I estimated the mass. I was out by a factor of 2. Sorry about that. Using the latest determination, if we were to squeeze all of that mass into a black hole, the Schwarzschild radius of the hole (the distance from its centre to its effective edge) would be about 15.6 billion light years. The latest thinking puts the age of the Universe at about 15 billion years. Not long ago, therefore-and possibly still-the Universe was inside a black hole.

The matter inside a black hole collapses to a singularity. The Universe is supposed to have been expanding ever since the big bang. Will someone please explain this flat contradiction?

Due to an editing error, we did indeed say "weight" on page 29, when of course we meant "mass". The person responsible has been taken out and shot-Ed

P. Warlow


From New Scientist magazine, vol 169 issue 2273, 13/01/2001, page 55

 

[Time02112]

Universe in the balance 16 Dec 00

At last we know just how much the cosmos weighs. The answer shows that theories of the Universe's origin are spot on, says cosmologist Jeff Peterson. Trouble is, we still haven't a clue what most of the stuff is made from

HOW do you weigh the Universe? Astronomers have been asking this question for decades, and using every trick they can think of to get at the answer. Frustratingly, the results never added up. Different techniques gave different answers.

Now a new cosmic weight-scale has been pressed into service to try to resolve the conundrum. It's the faint afterglow of the big-bang fireball in which the Universe was born. This glow can still be seen in every part of the sky. Map its structure, the idea goes, and you can work out the cosmic mass.

It isn't as easy as it sounds. The structure in this afterglow-the cosmic microwave background (CMB)-is very subtle. What's more, from the surface of the Earth the faint features of the CMB are obscured by the dirty window of our damp, cloudy atmosphere. To get round this, researchers have set up shop in some of the most arid deserts on the planet: the Atacama plateau in Chile, for instance, and high on the dry, icy plateau at the South Pole, site of the telescope built by my research team. Others have suspended their telescopes from helium-filled balloons and floated them high into the stratosphere, above most of the water vapour that causes the problems.

This year, all these efforts are finally bearing fruit. Thanks to a flurry of results published in the past 18 months or so, we finally know what the Universe weighs. And the answer is great news for theorists. It tallies with their long-held conviction that the Universe began with a dramatic expansion known as inflation. However, there's bad news too. The new results imply that our Universe is dominated by strange forms of matter that we can't see and don't understand.

It was back in 1981 that Alan Guth from the Massachusetts Institute of Technology first proposed that an episode of energy release that he called "inflation" happened in the first minute or so of the Universe's existence. During inflation, the part of the Universe we can see today swelled by a factor of 1060. Then, so the theory goes, the Universe's expansion slowed to a more normal rate.

Why propose something that sounds so strange? Well, it solves lots of thorny puzzles in cosmology. In particular, it explains why the Universe seems to be flat, rather than curved. It's hard to picture a three-dimensional universe being curved, but space in any dimensions can have positive curvature, like a ball, or negative curvature like a saddle. Whether the Universe is flat or curved depends on what it weighs-or more precisely, on its density. If the density is just right, the Universe will be flat. If it's higher than this critical value, the gravitational pull of the matter forces space to have positive curvature. If it's lower than the critical value, space is negatively curved.

And here's the problem cosmologists faced before the inflation idea appeared. If you start off with a perfectly flat universe early on, it stays flat forever. If, on the other hand, space starts off slightly curved, it quickly becomes dramatically more curved. It's almost impossible for a universe to hover close to flatness for any length of time unless it has no curvature at all. Even in 1981, the signs seemed to be that the density of the Universe was at least somewhat close to the critical value. So some process early on must have made the Universe flat.

Inflation fits the bill perfectly. It automatically creates a flat Universe because it stretches out any wrinkles in the curvature-just as blowing up a balloon flattens out its surface. Inflation fills space with material whose density has precisely the critical value. So theorists assumed that inflation must have happened and that the Universe must be at its critical density. In their view, all that was left to do was confirm this by observation.

The trouble is that for decades optical observations have thrown up results that fall short of the critical density. In their efforts to inventory all the matter in the Universe, astronomers have mapped the rotational velocities of galaxies to see how much matter was holding them together. They have also looked at clusters of galaxies, and even measured how light is bent by gravity as it passes massive objects on its way to Earth. Over and again, they measured a density that was close to, but still crucially shy of, the critical value. There seemed to be only 30 per cent of the expected matter out there.

That's where the microwave background comes in. Imprinted upon it are the frozen images of a time when the Universe rang with vibrations. These vibrations are the key to weighing the Universe.

A hundred thousand years after the start of the big bang, conditions were similar to those inside the Sun today. An almost uniform plasma of electrons and hydrogen and helium ions filled the entire Universe, all bathed in a brilliant glow of light-the blaze of the big bang itself. At this early stage, the free electrons played a key role. They scattered the photons so that they careened from free electron to free electron like a relativistic pinball machine, rendering the Universe opaque.

Meanwhile, throughout the Universe matter was gradually gathering around the areas of slightly higher density that were eventually to become the galaxies and clusters that we see in the Universe today. Pulled by gravity, matter fell towards these slightly denser regions. But, bombarded by the scattering photons, it was forced out again. In and out the plasma bounced, never fully collapsing, but never quite pulling out of these gravitational hot spots. The material of the early Universe quivered like a shaken bowl of jelly.

Then, 300,000 years after the big bang, the slowly falling temperature of the Universe reached 4500 kelvin. Electrons no longer had enough energy to resist being captured by nuclei. Atoms formed, and because photons had no more free electrons to scatter off, the Universe became transparent. But the photons did not disappear, they simply continued in whatever direction their last scattering sent them. Some of these photons happened to scatter in our direction and we can still detect them today. They make up the CMB and they have been travelling unimpeded towards us for almost 12 billion years.

Imprinted on this afterglow should be an image of the compressed and rarefied regions frozen at age 300,000 years, showing up as bright and dim regions on the sky. Measure that pattern, the idea goes, and you learn the density of the Universe.

Here's how it works. Different-sized regions had different periods of oscillation-the smaller the region, the faster it oscillated. For instance the largest patches had not even completed their first "bounce" when the Universe became transparent, and the smallest patches had been through several cycles. It's the regions that were exactly halfway through their first oscillation cycle when the free electrons disappeared that should show up most strongly in the microwave background. "Halfway through a cycle" describes the point at which the material was at its maximum compression, giving the strongest contrast against the sky. Theorists have worked out exactly how big such regions would have been 300,000 years after the big bang. Knowing how the Universe has expanded, they can also work out how big the same regions should appear on the sky today.

Here's where the connection with the Universe's weight comes in. Those regions of compression look bigger to us than they would if the Universe were low-density. That's because matter exerts a gravitational pull on light, curving its trajectory. As the microwave background photons travelled towards us, their paths were bent by the matter in the Universe. The more matter there is in the Universe, the more the light paths are bent and the bigger the regions will appear on the sky. So to weigh the Universe, all you have to do is calculate how big those oscillating regions must have been, see how big they actually look in the microwave background, and work out how much matter is needed to create that distortion in the image (see "Good vibrations").

During the 1990s a series of CMB observations began to show that the sky did indeed contain the signature of those ancient wobbles. But for most of the early microwave telescopes, the images were too smeared-out to resolve the individual bright and dim patches.

Then in 1998, my telescope at the South Pole-called Viper-and the Mobile Anisotropy Telescope in the Atacama Desert each mapped out a few square degrees of sky with much higher resolution. In both sets of results, the half-cycle regions seemed to be present. But the observations covered very little sky and it was hard to tell if the structure they were finding was truly representative.

Then in April and May this year results from two balloon-borne telescopes, Boomerang and MAXIMA, were reported. Launched from the McMurdo Station on Ross Island, Antarctica, the Boomerang telescope spent 10 days riding the polar stratospheric vortex in a long arc about the South Pole. By the time it returned, it had mapped a whopping 400 square degrees-around one per cent of the sky-which is plenty enough to see whether the results are representative. In addition, even though the MAXIMA telescope only had a one-night flight from Palestine, Texas, the team succeeded in mapping 100 square degrees. In the data from each of these two experiments the half-cycle regions stand out strongly (see Graph). Between them, the two projects have enough data to make an accurate determination of the density of the Universe. (Convert this to a weight by considering the volume of the visible Universe and you get 100 trillion trillion trillion trillion tonnes, give or take a few kilograms.) The measured density of the Universe matches the critical value to within about 6 per cent. It looks as though the balloon projects have nailed it: the Universe is flat, and the theorists and their ideas about inflation seem to be right.

So is cosmology now all figured out? Far from it. Our cosmological models are full of gaps. For one thing, there is the discrepancy between the results from optical observers and the microwave background telescopes. It's not necessarily a conflict-they may both be right. The optical observations focus on concentrations of matter such as stars and galaxies. In contrast, the cosmic background reveals the average density not just of matter, but of energy too. Energy exerts a gravitational pull on the paths of CMB photons just as matter does. And the latest idea is that the missing component of the Universe's weight comes from some type of dark energy (New Scientist, 11 April 1998). Still, nobody knows for sure what this energy is, or why it has the value it does.

Then there's the problem that optical observers can't explain the nature of all the matter they measure. They know that some of it is just ordinary stuff like stars and planets. But they also require at least five times as much exotic "dark" matter as ordinary matter to explain the way that galaxies rotate, and to explain the fast orbits of galaxies within clusters. Could the Universe really have two mysterious ingredients, dark matter and dark energy?

There are also many open issues within inflation theory. Even if current observations point to an inflationary episode in the history of the Universe, they don't tell us how inflation occurred or at what temperature. So far, we don't have a hint as to what sent the big bang booming.

Some cosmologists are so dissatisfied with all these mysterious ingredients they prefer to question the laws of gravity. Stacy McGaugh of the University of Maryland has recently shown that we can understand our Universe without the need for exotic dark matter if we accept that gravity might be slightly higher at low accelerations than Newton or Einstein predict. However, even with a modified theory of gravity, McGaugh needs some kind of dark energy to explain the cosmic observations. It looks as though it will be some time yet before the Universe gives up all its secrets.

Good vibrations
IN THE hot, early Universe, matter tried to collapse into regions of higher density, where the gravitational pull was stronger. But the pressure of photons left over from the big bang pushed the matter outwards again. In and out it bounced, in a series of well-defined oscillations.

When the Universe had cooled enough to become transparent, the photons trapped in the hot plasma were suddenly free to travel through space. Frozen into this microwave background-the faint afterglow of the big bang-is the pattern of oscillations that existed when the Universe reached the critical temperature. Researchers can measure the imprint of these oscillations in the microwave background. Small regions oscillated more quickly than large ones, so different sized regions were at different points in their "bounces" when the imprints were frozen in. Because the cycles tended to die down in amplitude after the initial collapse and rebound, a region which we see at the maximum compression of its first bounce will show up more strongly than one at the same point of its second cycle.

Hidden in images of the microwave background are different frequencies corresponding to oscillations by different-sized regions. Researchers use mathematical techniques to filter out these frequencies. The result of this process is called a power spectrum, a plot displaying the amount of each frequency present. Searching for a particular oscillation amounts to searching for a peak in the power spectrum.

Results published earlier this year from two experiments-Boomerang and MAXIMA-show a strong first peak in the power spectrum (see Graph). This corresponds to regions that had gone through half a cycle when the imprint was frozen into the microwave background. Because they were caught at their maximum compression, and hence their maximum density contrast, they are the easiest to see. By measuring the frequency of this peak, researchers can work out how big the half-cycle regions now look to us on the sky. They look bigger than they would in a low-density universe, because the light has been bent on its way to us by the gravitational pull of intervening material. That shifts the peak to a lower frequency. Thus the position of the peak indicates how much material there is in the Universe, and hence how much it weighs.

In the future it should be possible to confirm this result, and learn much more, by looking for more peaks in the spectrum. To do this, NASA has built the Microwave Anisotropy Probe. Scheduled for launch in the middle of next year, MAP will produce a detailed image of the entire sky. Hopefully, it will spot the second peak in the spectrum, corresponding to regions that had gone through one entire cycle and then overshot slightly. If so, that will help pin down the nature of the dark matter in the Universe. It may also see the third peak-due to regions that were at maximum compression on their second oscillation. This would improve our measurement of the density of the Universe and fill in details of what the conditions were when the vibrations started.

Meanwhile, the telescopes at less lofty altitudes continue the quest. Another Antarctic balloon launch, carrying the "Top Hat Telescope", is planned for January. Two new interferometer telescopes, DASI at the South Pole and CBI at Atacama, have been collecting data and should weigh in soon with their results. And a new receiver, ACBAR, is being installed on Viper which will give it a resolution three times greater than MAP.

Further down the road, ESA plans to launch the Planck satellite in 2007. Planck will image the entire sky with a sensitivity better than MAP, and with twice the resolution.

Jeff Peterson


From New Scientist magazine, vol 168 issue 2269, 16/12/2000, page 26