Dark Energy From the Ground Up: Make Way for BigBOSS

August 7, 2009
Ever since the big bang and the epoch of inflation, the universe has been expanding. Now it’s expanding at an accelerating rate, because of a mysterious something called dark energy.

(PhysOrg.com) -- Several ways have been proposed to examine dark energy, in hopes of finding out just what it is. One of them, "supernovae" for short, certainly works: it's how dark energy was discovered in the first place. Other independent techniques, such as weak gravitational lensing and baryon acoustic oscillation, also promise great power but are as yet unproven.

These three techniques all have a share of the proposed Joint Dark Energy Mission (JDEM), a satellite design managed by NASA with the participation of the U.S. Department of Energy. DOE's JDEM Project Office is headquartered at Lawrence Berkeley National Laboratory and led by Michael Levi of the Physics Division.

During deliberations on JDEM's reference design in the fall and winter of 2008-2009, some members of the JDEM Science Coordination Group (SCG), which included Saul Perlmutter and David Schlegel of Berkeley Lab's Physics Division, questioned whether a satellite was really the best platform for all three of the proposed methods.

"Of the three main things JDEM is supposed to do, the NASA design focuses on baryon acoustic oscillation," says Schlegel. "It's good science, but I wondered whether it could be done just as well, or better, from the ground."

Space is the place - sometimes

The goal of all experiments that seek to determine the nature of dark energy is a detailed expansion history of the . For supernova studies, which depend on measuring the redshift and brightness of distant Type Ia supernovae, there's no question that space is the place.

The Mayall 4-meter telescope at Kitt Peak National Observatory can be adjusted to observe a 3-degree field of view with a high-precision spectrograph. A new secondary mirror (top) and a corrector and field flattener will send the light to a flat focal plane (above primary mirror) where the light from each target object is carried to spectrographs by optical fibers.

Beginning in the 1980s, methods for finding Type Ia supernovae "on demand" were developed by the international Supernova Cosmology Project (SCP), based at Berkeley Lab and headed by Perlmutter, and adopted in 1994 by a rival team, the High-Z Supernova Search Team. In the fall of 1997 the SCP concluded that the universe is expanding at an accelerating rate, propelled by a mysterious something soon to be called dark energy. The unexpected acceleration was soon confirmed by the High-Z Team.

Most of the early studies were done from the ground but included a handful of supernova measurements made with the Hubble Space Telescope. To measure expansion rate with enough precision to choose among competing models of dark energy, however, exquisite spectrometry of thousands of distant Type Ia supernovae will be needed. This can only be done by flying a big telescope and an adequate spectrograph in space. That's why the SCP inaugurated a DOE satellite proposal in 1999 called the /Acceleration Probe, SNAP, which eventually inspired JDEM.

Early on, SNAP included the capacity to measure weak gravitational lensing, which looks at subtle measures of the distortion of space by both ordinary and dark matter to reveal how the distribution of matter in the universe has changed over time. Weak lensing will also greatly benefit from a space-borne telescope.

Baryon acoustic oscillation (BAO) is distinct from both these methods. "Baryon" is cosmology-speak for ordinary matter, and "acoustic oscillation" is a fancy name for the way galaxies tend to bunch up at roughly 500 million light-year intervals. These density oscillations have their origin in the pressure waves (like sound waves, thus acoustic) that moved through the liquid-like plasma of the early, hot universe.

By the time the universe was 300,000 to 400,000 years old, it had expanded and cooled enough for atoms to form, releasing light to go on its way unimpeded - the era of decoupling. But the density oscillations left their mark as minute temperature differences in the cosmic microwave background (CMB). The denser regions, where matter was clumped more tightly, were the seeds of today's galaxies and groups of galaxies.

The cosmic microwave background provides the starting point for a natural ruler to measure how much, and how smoothly, the universe has expanded since the era of decoupling. The ruler is extended forward in time by measuring variations in the density of galaxies - especially old, bright, red galaxies and quasars - across billions of light-years. The expansion history of the universe emerges when the markings of the ruler, as seen in more recent cosmic structures, are calibrated against the scale frozen in when the universe was less than 400,000 years old.

Grounded cosmology

But does one need a telescope in space to measure baryon acoustic oscillations? David Schlegel didn't think so. In 2006 he and his colleague Nikhil Padmanabhan, both members of the Sloan Digital Sky Survey (SDSS), completed the largest three-dimensional map of the universe ever made until then, in which they first detected the 500-million-light-year scale of baryon oscillations. Now Schlegel leads the SDSS's Baryon Oscillation Spectroscopic Survey, BOSS, whose goal is to map one and a half million galaxies and quasars and measure the varying densities of hydrogen gas in the universe. It will be the first survey with a chance at using BAO to measure the universe's expansion history.

As a member of JDEM's Science Coordination Group, however, Schlegel was taken aback by NASA's emphasis on baryon acoustic oscillation. "I was surprised that JDEM, a $600-million mission, was going down what seemed a risky scientific pathway," he says.

"Last winter, I was scheduled to give a talk at an SCG meeting the next day in Washington with no idea what I was going to say," Schlegel recalls. "On my way down from New Haven on the train I just decided to work out the numbers to see if what JDEM wanted to do with BAO could be done from the ground. Remarkably, no one had done that. Instead of asking what kind of instrument we needed to do the science, the approach had been, 'here's the instrument we're giving you, what can you do with it?'"

Schlegel's back-of-the-envelope BAO calculations looked "encouraging," as he put it, and his presentation to the SCG raised a few eyebrows. But he realized existing programs were no threat to JDEM's "figure of merit" for BAO - a more or less abstract number based on the 2006 DOE-NASA-National Science Foundation Dark Energy Task Force's calculation of how useful a given experimental result would be for measuring dark energy. JDEM's figure of merit is 313. The figure of merit for BOSS, the biggest ground-based BAO search underway so far, is 107.

Nevertheless, Schlegel couldn't shake the idea, and in February of 2009, once NASA had finalized their design ideas, he started thinking about it more seriously.

"To match what JDEM proposes to do, we would need a bigger telescope than the SDSS telescope in New Mexico we're using for BOSS. Optimum would be a 4-meter telescope that could accommodate a spectrograph with a wide field of view, covering three degrees of the sky," Schlegel says. (For comparison, the full moon is half a degree in diameter.) "There are only 14 4-meter telescopes in the world, seven of them U.S.-operated. And whether any of them had three-degree field-of-view imaging capability, I wasn't sure."

Robot actuators individually position each optical fiber precisely where it needs to be to collect the light from a specific astronomical object programmed by computer. The light is analyzed by blue, visible, and infrared spectrographs. For videos of the prototype actuators, see "Additional Information" below.

Two of the candidate telescopes are operated by the National Science Foundation's National Optical Astronomical Observatory (NOAO), which oversees the Kitt Peak National Observatory in Arizona with its 4-meter Mayall Telescope, and the Cerro Tololo Inter-American Observatory in Chile, which also has a 4-meter telescope. Schlegel's inquiries indicated that the NOAO astronomers would indeed be interested in exploring the possibility of BAO studies.

Says Schlegel, "So I asked Michael Sholl, the optical designer in the JDEM project office here, whether the 4-meter Mayall could be adapted for a spectrograph with a three-degree field of view. He said, 'I'll look into it and get back to you.'"

A string of luck

Schlegel fully expected Sholl to tell him it couldn't be done. And when Sholl knocked on his office door and said, "I'm really sorry, I can't get to three degrees." Schlegel thought that was the end of it - until Sholl added, "The best I can do is 2.94."

Says Schlegel, "I dropped everything. We were in business." It turned out that the telescope design which would allow a three-degree (oh all right, a 2.94-degree) spectrograph was common to only three of the world's 4-meter telescopes. NOAO's Kitt Peak and Cerro Tololo had two of them.

The spectroscopic instrument that would fit these telescopes had already been developed at Berkeley Lab using Laboratory Directed Research and Development funds, but wasn't completed in time to be installed on BOSS. BOSS's spectrograph uses optical fibers fitted into holes in metal "plug plate" masks, drilled in the precise position of galaxies mapped from previous photos. To obtain redshift and other spectral information, each fiber conducts the light of a single galaxy to a sensitive CCD. Each plate is limited to 1,000 fibers. BOSS will use some 1,500 virtually hand-made plates to gather the light of 1.5 million quasars and galaxies.

The new spectrograph does away with plug plates altogether. Target galaxy positions are stored in a computer, which directs the positioning of an array of thousands of optical fibers for each exposure. A single aluminum block machined to the curvature of the modified telescope's 1 meter focal plane is divided into 5,000 cells, each perforated by a cylindrical hole.

"In each hole live a couple of robots," says Schlegel, "actuators that can position the fibers to an accuracy of 15 microns" - 15 millionths of a meter. The robots put the tip of the fiber right where the light from the distant galaxy falls - even, if necessary, outside the hole - and positions the fibers in the focal plane with no dead space, gathering the light of some 4,000 galaxies at a time.

To accommodate the spectrograph, the existing telescopes would need to be modified with a 2-meter secondary mirror. It so happened that a glass blank for just such a mirror, intended for the SNAP , had already been bought and paid for by DOE. DOE offered it to NASA for JDEM, but NASA wasn't interested. It was available.

Schlegel realized that he and his colleagues were looking at the possibility of mounting a three-degree spectrograph on existing telescopes that could gather millions of galaxies with extraordinary spectral resolution - precision that would allow the study not just of density variations of galaxies but in the hydrogen gas that fills the universe, something JDEM could not do, covering a much wider range of redshifts than JDEM, and looking much farther back in time.

Because the new approach had evolved from the existing BOSS, it was tagged BigBOSS.

Into the fray

"In a March 3 phone call to Kitt Peak we decided to go for broke," Schlegel says. "Every 10 years the National Academy of Science's Decadal Survey lays out a roadmap for future astronomy and astrophysics research. White papers describing proposals were due April 1. Most people work for years on these proposals; we did it in four weeks."

The joint DOE-NSF BigBOSS white paper was submitted to the Decadal Survey on time and has since gathered a string of approvals from government committees; the Decadal Survey report is due at year's end. "We made a Hail Mary pass and hit every committee," says Schlegel. "Our case is strong."

BigBOSS proposes to advance in two stages, the first at Kitt Peak covering the northern sky, the second at Cerro Tololo. BigBOSS North would look at the distribution of 30 million galaxies and a million quasars. After this survey is complete the project would move to Chile, where BigBOSS South would add another 20 million galaxies and quasars. Both surveys would measure distortions in hydrogen gas.

The construction of the spectrograph and telescope modifications are estimated to take three years, beginning in 2011, at a cost of $71 million, with operations costing $2.5 million a year for 10 years.

Compared to JDEM's figure of merit, 313, BigBOSS North alone would achieve 240, and North and South together would achieve 338. At three years to build and 10 years to cover the whole sky, assuming five million targets a year, BigBOSS could take longer than JDEM, which might launch in 2016 at the earliest. The cost is less than a sixth JDEM's, however, and the risk of failure is minimal - BigBOSS uses existing facilities and proven technology.

"BOSS will be the first survey to produce a 3-D map of red galaxies and quasars and clouds of hydrogen gas in the universe, and BOSS is the first BAO survey from which it may be possible to measure the expansion history of the universe. BigBOSS's map will be far bigger and more detailed," says Schlegel.

"But BigBOSS offers more. One of the most interesting questions in cosmology is the relationship between dark energy and the early inflationary epoch of rapid expansion. Something was happening then, and we wonder if it's repeating in some way. BigBOSS will have the best sensitivity to the inflationary epoch. In some ways this could be the best argument for BigBOSS of them all."

More information: "BigBOSS: The Ground-Based Stage IV Experiment," by David J. Schlegel, Chris Bebek, Henry Heetderks, Shirley Ho, Michael Lampton, Michael Levi, Nick Mostek, Nikhil Padmanabhan, Saul Perlmutter, Natalie Roe, Michael Sholl, George Smoot, and Martin White of Lawrence Berkeley National Laboratory, and Arjun Dey, Tony Abraham, Buell Jannuzi, Dick Joyce, Ming Liang, Mike Merrill, Knut Olsen, and Samir Salim of the National Optical Astronomy Observatory, is posted at arXiv:0904.0468v3.

Source: Lawrence Berkeley National Laboratory (news : web)

Explore further: BOSS: The Baryon Oscillation Spectroscopic Survey

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4 / 5 (4) Aug 07, 2009
We should have more people like Schlegel in science!!
5 / 5 (2) Aug 07, 2009
one of the best articles i have read all week -- cudos to the author
4 / 5 (2) Aug 07, 2009
Que Omatumr in 3, 2, 1...
3 / 5 (2) Aug 07, 2009
Yes! excellent!
1 / 5 (3) Aug 07, 2009
It would sure be a whole lot cheaper to just study and measure the "Dark Matter" in our own solar system! Since it is obviously pervasave just one 12by12 area could be voided of all we know. What is left would be "Dark Matter!"
2.6 / 5 (5) Aug 07, 2009
There are alternative theories about galaxy bunch up.
A group of cosmologists has found that the whole bunch up pattern charted in all of known space resembles the resonance oscillation of a quantum wave function like an atomic orbital. In other words
the empty voids in the map of galaxies correspond to places where a low quantum wave-function probability existed and the dense regions correspond to high density parts of wave-functions
that were theorized to have existed at the big bang, and that were preserved during the inflationary and regular expansion eras.
Aug 07, 2009
This comment has been removed by a moderator.
3.3 / 5 (7) Aug 07, 2009
just study and measure the "Dark Matter" in our own solar system!
Indeed, from above theory follows, dark matter would manifest here like weak deceleration, which correspond the product of Hubble constant and speed of light. Exactly this deceleration was observed as a Pioneer spacecraft analogy.

At large distance scale the situation gets more complicated though, because areas of dense vacuum exhibit parity violation due their negative curvature of space-time and they tend to collect antimatter particles and atom nuclei, so that dark matter isn't completely space-time deformation effect, but it traps a substantial amount of real matter.


It means, at large distances CMB slows down not just photons, but the material particles too - so that dark matter becomes more complex phenomena here and it becomes more dense, then simple product of Hubble constant and speed of light - which we can experience as a "dark energy" from insintric perspective.
3 / 5 (8) Aug 07, 2009
What we can expect is, at even larger distances (i.e. outside of boundary of observable Universe) dark matter would collect even heavier objects like stars and galaxies. We can see, energy dispersion effects just lead to fractal Universe concept due its principal nonlinearity, where path of energy can be dispersed and curved ad infinitum. But we cannot observe it directly, because we can experience Universe by its transversal waves only.


At watter surface the same dispersion, which enables us to observe "dark matter" effect from outside leads to the cluttering of environment both from insintric perspective perspective, so we are experiencing chaotic nature of Universe on both sides of observational scale. At quantum scale it manifest by quantum uncertainty, at cosmological scale we can see it as a chaotic and hot past of Universe (so called Big Bang).

If you cannot understand this model, you can simply put a silly question: what would I see from my 2D universe, if I become a tiny undulating bubble at water surface? We can even find an answer by computer simulation of large particle system surface.
4.3 / 5 (6) Aug 08, 2009
Que Omatumr in 3, 2, 1...

We used to dream of getting Omatumr. Instead we got Alexa.

We also got a pretty good post from Neil Farbstein. See what not spamming can do you for you Neil. You even write better. But you might want to try hitting the enter key TWICE instead of once for a new paragraph now and then.

It would sure be a whole lot cheaper to just study and measure the "Dark Matter" in our own solar system

Cheaper only if Dark Matter is something exotic that exists pretty much everywhere. If it is simply baryonic matter that haven't detected it probably isn't something we are going to find in the Solar System.

Of course the article was about dark ENERGY not matter. Dark Energy is much more problematic than Dark Matter. That there is at least SOME matter that we haven't seen is fairly certain. What kind of matter it is and how much is the question. Dark Energy may be unreal. This sort of experiment should be able to tell us a lot about the universe whether it finds evidence for Dark Energy or not.

Aug 08, 2009
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Aug 08, 2009
This comment has been removed by a moderator.
Aug 08, 2009
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5 / 5 (1) Aug 08, 2009
1. The mass of a particle is mostly determined by its vibrational energy? The force in a stretched spring is proportional to its displacement and acts in the opposite direction to the direction of extension. The potential energy stored in the spring is proportional to the extension squared.

If one thinks of the BB universe like a vibrating string, could the dark energy be the KE/PE of the string?

And if this string were the fabric of our universe it could be alternately stretching and compressing. The same energy which is stretching the universe now could later be the same energy which will bring it all back with a compression?

With a metal spring coil, gravity plays little role. But gravity seems to dominate forces at a long distance. The force in a spring acts against the direction of extension, In a stretching universe gravity acts to put a brake on the extension and so it an attractive force. My suspicion is that when the universe starts to compress, there will be a force which acts against the compression, until it eventually starts to inflate again. That force may still be gravity, but it would need to be a repulsive gravity.

2. But that is not what appears to have happened in the BB, which started at a micoscopic size then rapidly inflated, then inflated more slowly?

In such a case, perhaps the BB event took place in a part of the universe which was contracting? It could be that the compressions were strong enough for our BB universe to be squeezed out of the outer framework or out of the wider universe? Gravity was repulsive in the outer framework and our BB universe is still being repulsed away from its original location?

Is it possible for our BB framework to be being repelled gravitationally by matter in an outer framework while at the same time, matter within the BB framework gravitationally attracts matter within the framework?

3. Is there a threshold effect happening for BB event(s)? if such things happened all the time, at all scales, would not that undermine the law of conservation of energy? But when it does happen on smaller scales, that is energy popping out of the vacuum, it is said to be only allowed if it is fleeting or evanescent. Is our BB universe fleeting if looked at in an appropriate way? Or has its immense energy passed a threshold value which lets it have permanence? Ie are BB events quantised?
5 / 5 (1) Aug 09, 2009
On the other hand, if the universe were analogous to a vibrating tightened piece of elastic, there could be a position where the universe would lie if it were not in motion. When vibrating it is extended alternately on either side of that rest position. The main force to do this would be attractive. No need for repulsive forces. When vibrating and passing through the rest position the KE is a maximum and the PE is zero. Then immediately after the rest position the elastic starts to expand or inflate. If we has access only to infinitesimal amounts of time increments here, at this particular phase of the motion, and had no knowledge of the prior motion, we could be forced to look for repulsions to be driving the expansion of the elastic? Or be looking for large external masses to be attracting the elastic? Or for 'dark energy' to drive the expansion. Could the dark energy be KE from an earlier phase of the vibration? Unfortunately, this does not seem to fit in with a BB event.
1 / 5 (1) Aug 10, 2009
Enough Is Enough

Beyond Einstein-Hubble And Beyond Darwin

On The Origin Of Origins

Dark Matter-Energy And Higgs Particle?
Energy-Mass Superposition
The Fractal Oneness Of The Universe
All Earth Life Creates and Maintains Genes

A. On Energy, Mass, Gravity, Galaxies Clusters AND Life, A Commonsensible Recapitulation
The universe is the archetype of quantum within classical physics, which is the fractal oneness of the universe.

Astronomically there are two physics. A classical physics behaviour of and between galactic clusters, and a quantum physics behaviour WITHIN the galactic clusters.

The onset of big-bang's inflation, the cataclysmic resolution of the Original Superposition, started gravity, with formation - BY DISPERSION - of galactic clusters that behave as classical Newtonian bodies and continuously reconvert their original pre-inflation masses back to energy, thus fueling the galactic clusters expansion, and with endless quantum-within-classical intertwined evolutions WITHIN the clusters in attempt to delay-resist this reconversion.

B. Updated Life's Manifest May 2009

All Earth life creates and maintains Genes. Genes, genomes, cellular organisms - All create and maintain genes.

For Nature, Earth's biosphere is one of the many ways of temporarily constraining an amount of ENERGY within a galaxy within a galactic cluster, for thus avoiding, as long as possible, spending this particularly constrained amount as part of the fuel that maintains the clusters expansion.

Genes are THE Earth's organisms and ALL other organisms are their temporary take-offs.

For Nature genes are genes are genes. None are more or less important than the others. Genes and their take-offs, all Earth organisms, are temporary energy packages and the more of them there are the more enhanced is the biosphere, Earth's life, Earth's temporary storage of constrained energy. This is the origin, the archetype, of selected modes of survival.

The early genes came into being by solar energy and lived a very long period solely on direct solar energy. Metabolic energy, the indirect exploitation of solar energy, evolved at a much later phase in the evolution of Earth's biosphere.

However, essentially it is indeed so. All Earth life, all organisms, create and maintain the genes. Genes, genomes, cellular organisms - all create and maintain genes.

Dov Henis
(Comments from 22nd century)
not rated yet Aug 10, 2009
Genes are THE Earth's organisms and ALL other organisms are their temporary take-offs.

Another victim of gene fixation.

Haven't you noticed that genes absolutely require ribosomes to be expressed. The ribosome had to came first so how can genes be all that matters? Something came before the ribosome so it cannot be all that matters. The key is reproduction and genes are merely a tool for that.


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