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Dark Energy’s Elusive Signature

It’s odd to think that there would be a connection between the large-scale structure of the universe and what we hope to achieve with deep space propulsion. But figuring out how things work on the largest scale may offer us valuable clues about what is possible and what is not. If we understand correctly how gravity works at the macro scale, then the evidence for ‘dark energy’ seems persuasive. Something is causing the universe not only to expand but to accelerate its expansion, and that something must operate against the force of gravity, which ought to be slowing the process down.

Which brings us to BOSS, the Baryon Oscillation Spectroscopic Survey, now beginning its operations after taking first light on the night of September 14-15. A part of the Sloan Digital Sky Survey III, BOSS will use the 2.5-meter telescope at Apache Point Observatory in New Mexico to measure the spectra of 1.4 million galaxies and 160,000 quasars by 2014. Out of this we should derive the most accurate data yet about the way the universe is put together at the largest levels. Newly designed spectrographs will be doing the leg work, more efficient at working in the infrared than instruments used by the original SDSS.


Image: One of the “first light” spectra taken by the Baryon Oscillation Spectroscopic Survey (BOSS). The top panel shows the targeted blue quasar, highlighted in the image of the sky, which are thought to be supermassive black holes in distant galaxies. At the bottom is shown the BOSS spectrum of the object which allows astronomers to measure the “redshift”, or distance to this object. BOSS plans to collect millions of such spectra and use their distances to map the geometry of the universe. Credit: D. Hogg, V. Bhardwaj, and N. Ross.

Where dark energy comes into play is that BOSS will be able to study the way protons and neutrons — baryons — interacted with light to create pressure oscillations. These, in turn, would have created variations in density as they traveled through the early universe, finally ceasing when the cosmos was about 400,000 years old as conditions cooled enough to halt their propagation. Nikhil Padmanabhan (Yale University) puts it this way:

“Like sound waves passing through air, the waves push some of the matter closer together as they travel. In the early universe, these waves were moving at half the speed of light, but when the universe was only a few hundred thousand years old, the universe cooled enough to halt the waves, leaving a signature 500 million light-years in length.”

That signature is found in the spacing of the galaxies BOSS will be measuring, whose distribution is described by Daniel Eisenstein (University of Arizona):

“We can see these frozen waves in the distribution of galaxies today. By measuring the length of the baryon oscillations, we can determine how dark energy has affected the expansion history of the universe. That in turn helps us figure out what dark energy could be.”

Galaxies, in other words, are more likely to be separated by about 500 million light years than 400 or 600 million light years, a spacing that scientists are calling a ‘frozen’ sound wave signature. That same structural effect should be seen in the clustering of visible matter in quasars and intergalactic gas as well, and should also have caused clumping in dark matter. What effect dark energy has had on this structure throughout the universe’s expansion should be apparent through the data that BOSS gathers as the project compares these scales at different eras.

Hard to believe that it was only in 1998 that firm evidence for dark energy first surfaced. We’ve been talking about it ever since and with more and more interest. Indeed, along with dark matter, it’s one of the great unanswered questions of physics. Now we can use a method beyond the supernova cosmology that revealed the universe’s acceleration in the first place, one that should complement these data and allow us to get a better handle on the dark energy phenomenon. Look for the first public data release from SDSS-III in December of 2010, yet another case of high-quality data becoming available for broad use on the Internet.

As to propulsion, there is simply no way to know what may come of all this. The continuing study of the universe’s structure may reveal clues to explain its acceleration without offering any helpful insights into exotic ways to push spacecraft. One of the tenets of the Breakthrough Propulsion Physics project all along was that the investigation of matters like these was not only helpful in working out propulsion strategies, but also in sharpening our understanding of physics in exotic realms like these. We gain, then, in deepening knowledge of the physics involved even if dark energy’s seeming ‘anti-gravity’ turns out to be explained by other means. Let’s see what BOSS has to say.


Comments on this entry are closed.

  • Carl October 7, 2009, 8:51

    The original Breakthrough Propulsion Physics page… the one with just the text and the small line diagrams… was my introduction to these ideas. When I told my son I’d read funding was cut for the project, he indicated that was a real step backward.

    Yet this discipline continues as a passion by many scientists; it might not be directly funded, but it continues. I’m happy to see the original project’s objectives are still pursued with intelligence and enthusiasm.

    Dark energy in relation to possible interstellar drives would arise as a possibility in discussion if it looked like a real force. My lay understanding is dark energy is a theoretical, as yet unobserved, solution to apparent cosmic expansion. This signature gap would be a feather in the caps of the theorists. Modern astronomy has come a VERY long way in recent years, and the remarkable stuff is yet to come, I am sure.

  • Paul Titze October 7, 2009, 9:31

    For another thought on “anti-gravity”, Rafelski/Muller in their free online book mention: “if we can understand the coupling of gravitation to matter, we could possibly be able to turn it on and off. Now that is certainly a fascinating thought. Imagine if we could make ourselves weightless”, p159:


    Although it was written in 1985, pretty much all the text remains still relevant today, Johann plans to write a second updated edition to this book. It’s a great read and very relevant to Propulsion Physics.

    Cheers, Paul.

  • Erik Anderson October 7, 2009, 12:45

    Very interesting, Paul. And while the 2010 SDSS-III data release may be the first seriously comprehensive survey of photometric galactic redshifts to be published, it won’t be the last. Pan-STARRS is already engaged in a similar observing program using a 1.8m aperture telescope with better sky coverage (from Maui, HI) — it will finish its 3-year observing program in 2012. If the LSST ever comes online (presently scheduled in 2016), it will be doing the same with an 8.4m telescope in Chile.

    Believing as I do that nature of ‘dark energy’ is purely an optical phenomenon (exaggeration of redshifts — not an actual physical force), what I hope the resultant mappings of galactic density can show us is a history of cosmic expansion that deviates more and more with LCDM model predictions at higher and higher values of z. Only time will tell!

  • andy October 7, 2009, 17:54

    I don’t get where the propulsion idea comes into this: as far as I can tell, there is absolutely no evidence at all the dark energy would be useful for spacecraft propulsion in any way, nor is there any evidence that it can be manipulated. Just because something is not fully explained does not give us a license to invent magic properties. At present any such “dark energy drives” are more science fiction than science, and can be filed along with all the {scientific buzzword of the minute} drives that science fiction authors invent to get round the horrifyingly vast distances between the stars. I wonder if the 70 Ophiuchi refuelling station sells the right brand of handwavium to power it?

  • Paul Titze October 8, 2009, 9:23

    And for yet another recent “repulsive gravitational field” theory within the framework of general relatively checkout the just released article on:


    Cheers, Paul.

  • James M. Essig October 10, 2009, 21:02

    Hi Folks;

    This is a favorite topic of mine.

    Dark energy is a fascinating subject. Dark energy could be any of the following: Einstein’s self confessed biggest blunder of the cosmological constant, a real energy field that exists as a kind of anti-gravity, some aspect of the zero point virtual mattergy particle energy fields and the like.

    It is interesting to consider ways in which we might harness hidden or latent energy associated with space time such as the zero point energy fields.

    I have often wondered if there was not some sort of possible chemical reaction that would somehow release huge quantities of energy drawn from the zero point latent energy fields which by some calculations, should have an energy density of 120 orders of magnitude greater than that of the energy density of the observable universe. Since energy density of the observable universe is about 5 x 10 EXP – 30 grams per cubic centimeter, one cubic centimeter might have 5 x 10 EXP 90 grams of mattergy available in the form of the ZPF energy. This works out to be about 5 x 10 EXP 84 metric tons equivalent or about 10 EXP 34 times the entire real mass energy content of the observable universe, in every cubic centimeter of space.

    Now the chemical reactions I am contemplating are not the same as those ordinary chemical reactions that result from electrochemical potential energy derived from the electrical fields of the charged particles that make up atoms wherein such electric fields are themselves an expression of virtual photon exchange or zero point field photon exchange in the ordinary sense of Quantum Field Theory and quantum-electro-dynamics or QED, but are rather of a different type that would act to depolarized the net vacuum energy fields which are essentially viewed as a near complete cancellation of negative and positive ZPF energy thus resulting in the appearance in some models of a relatively very miniscule net ZPF vacuum energy density.

    If such ZPF depolarization can occur by means of special artificial chemical reactions that are exceeding rare in nature, then the amount of energy required for ultra-high gamma factor space travel, and even perhaps for wormhole travel and warp drive might yet be generatable.

    Since the strong nuclear force is 100 times stronger than the electromagnetic force, a nuclear reaction analogue of such ZPF depolarization effects might result in even greater energy releases perhaps in the form of ionizing radiation such as reified ZPF gamma rays, and reified ZPF fusion or fission fragments.

    My thinking is that any such associated experimentation that would be accomplished with the strong interaction should be done with extreme caution since the energy density and interaction levels of the strong force are about 100 million times greater than that associated with ordinary chemical reactions. Because of the much greater energy density and intensity of the strong nuclear reactions relative to the ordinary chemical reactions, the possibility of jarring a local portion of our universe in which the nuclear reactions would occur might cause a breach in any energy well based minimum in which our universe may rest. This could lead to a catastrophic phase change that would race throughout our universe at a velocity of at least C potentially destroying all structures, heavenly bodies, laws of physics, particles and fields, and space time in its path as we know it.

    If such ZFP reification technologies are possible, even though they might pose some risk to our universe or at least the local portions thereof, any such technology might be safely controlled, and the problem of manned travel to any portion of our observable universe to say the least might be completely cracked. It might be the case that any universe destabilizing effects would be very hard to achieve even by such technology and so the effects of such ZPF nuclear reactions might remain too muted to cause any serious threat unless some bazaar and unethical program was instituted to weaponize the effect, which in my opinion would be very unwise.

    Studying the Dark Energy Fields and way to harness them might lead to the above conjectured technologies or perhaps even something more exotic-a process that can only help our endeavor to travel to the stars in the Milky Way and beyond.