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A New Angle on Dark Energy

The best news about recent dark energy findings is that they offer new ways to study the phenomenon. It’s only been ten years since dark energy — thought to be the origin of the universe’s accelerating expansion — emerged from the study of supernovae. Simply put, these exploding stars weren’t slowing as they moved away from us, but were actually speeding up. It was a controversial result, to say the least, and one which remains one of science’s primary riddles. But Chandra X-ray Observatory observations may be providing additional clues.

The team on this work is led by Alexey Vikhlinin (Harvard-Smithsonian Center for Astrophysics), its effort focused on galactic clusters. A model of the cosmos that incorporates dark energy is the only thing that explains why these clusters have grown so slowly during the last five billion years, in what Vikhlinin calls “arrested development of the universe.” Dark energy seems to be working against the gravitational forces that allow clusters to draw in new matter.


Image: The galaxy cluster Abell 85, located about 740 million light years from Earth. The purple emission is multi-million degree gas detected in X-rays by NASA’s Chandra X-ray Observatory and the other colors show galaxies in an optical image from the Sloan Digital Sky Survey. This galaxy cluster is one of 86 observed by Chandra to trace how dark energy has stifled the growth of these massive structures over the last 7 billion years. Galaxy clusters are the largest collapsed objects in the Universe and are ideal for studying the properties of dark energy, the mysterious form of repulsive gravity that is driving the accelerated expansion of the Universe. Credit: NASA/CXC/SAO/A.Vikhlinin et al.

Better still, the results dovetail nicely with the supernovae work, a vindication of the idea that the universe is dominated by dark energy. The fascination of this work is in just how big the questions are. Dark energy could be the cosmological constant described by Einstein, an energy he initially believed could work against the force of gravity to keep the universe from collapsing. These findings keep that prospect alive. The power of dark energy is denoted by the number w, the equation of state, and Vikhlinin’s team derives a value for w not all that dissimilar from that of the supposed cosmological constant. But huge issues remain.

Ponder this: Just as we know little about dark energy, we know little about dark matter, whose presence seems necessary to explain how galaxies form around us in the cosmos. Any studies of galactic clusters have to factor in dark matter, but doing so puts us on tentative ground without a firmer basis for understanding what dark matter actually is. Dennis Overbye picks up on all the uncertainties in a recent New York Times story, in which he notes that the combined supernova and galactic cluster measurements, folded in with observations of the cosmic microwave background, have tightened up our estimate of w:

That looks like an improvement of a factor of 2 on the uncertainty charts that dark energy specialists show to one another in meetings and papers, but critics caution that combining disparate types of measurements can result in an artificially small error that masks underlying uncertainties. Astronomers, for example, still do not have a good theory to explain how their standard candles, supernovas, explode, and theories of cluster growth depend on assumptions about the nature of the dark matter in the universe and the nature of the original fluctuations that give birth to them.

This tantalizing work on galactic clusters, then, implies more than it proves, but the implications are interesting indeed. Thus William Forman (Smithsonian Astrophysical Laboratory), a co-author of the study that will appear in February:

“Putting all of this data together gives us the strongest evidence yet that dark energy is the cosmological constant, or in other words, that ‘nothing weighs something.’ A lot more testing is needed, but so far Einstein’s theory is looking as good as ever.”

Dark energy remains a personal fascination for two reasons. An effect that seems to oppose gravity is of obvious interest in that it promises new insights into physics. A sufficiently advanced civilization might one day harness such a force for propulsion. In the broader sense, dark energy points out how much we have yet to learn, a necessary reminder that a certain humility is a virtue in the study of cosmic effects so recently uncovered. Again we turn to Einstein, no stranger to re-thinking big ideas, who once said “Whoever undertakes to set himself up as a judge of Truth and Knowledge is shipwrecked by the laughter of the gods.”

Comments on this entry are closed.

  • James M. Essig December 18, 2008, 23:03

    Hi Paul;

    A recent report on the Science News website suggest that the strength of the dark energy field might be constant.

    I wonder however if it is possible that the non-gravitational aspects of space time that influence the rate of universe expansion have multiple components. What appears to be a relatively constant dark energy or cosmological constant value may be a coincidence of two or more causal agents. The apparent increase in the rate of expansion may be due to the fact that two or more variables effecting the non-gravitational kinematics components of space time expansion are roughly currently in balance such that their net effects appear as a constant dark energy strength value.

    As time progresses over cosmic time frames, perhaps one or more of any components of a composite non-gravitational space time expansion mechanism will dominate perhaps leading to either: a big rip, a constant rate of expansion at distant future cosmic epochs, a decrease in the rate of cosmic expansion but nevertheless in an open universe, a marginally open universe whose expansion rate goes to zero at infinite time durations, a closed universe that will eventually collapse, or perhaps a closed universe the will collapse and back on itself and be reborn in an finite number of or an infinite number of rebirths and collapses, in other words, a cyclical big bang universe.

    If nature has fundamentally designed a cyclical universe, perhaps each cycle varies with time, has the same temporal duration, and/or increases with time as some models involving the conservation of thermodynamic information from one bounce to the following bounce would suggest.

    Just as the four known forces may have some sort of relation in unified field theory schemes, thus effectively giving unified fields such a super-gravity sub-components (for lack of a better word), perhaps dark energy has multiple components which may branch off from each other and become seperated in a manner similar or analogous to the symmetry breaking events that lead to the multi-stepwise seperation of the unified field into the four seperate forces we know of currently.

    Other mechanisms may dwarf the effects of dark energy whether or not the strength of dark energy varies with time or whether or not dark energy has sub-components. For instance, phase changes leading to futher bifurcation of any or all of the 4 known forces might occur leading to one or more episodes of inflation, or the production of exotic particles and forces that have never existed before, but which nonetheless, could result in a volumetrically large to huge increase in the real mattergy content of our universe, at which point, the attractive force of gravity may once again dominate the rate of universe expansion.



  • ljk December 19, 2008, 0:48

    Title: Dark Energy and the Return of the Phoenix Universe

    Authors: Jean-Luc Lehners and Paul J. Steinhardt

    Categories: hep-th astro-ph gr-qc

    Comments: 5 pages, 3 figures

    In cyclic universe models based on a single scalar field (e.g., the radion determining the distance between branes in M-theory), virtually the entire universe makes it through the ekpyrotic smoothing and flattening phase, bounces, and enters a new epoch of expansion and cooling.

    This stable evolution cannot occur, however, if scale-invariant curvature perturbations are produced by the entropic mechanism because it requires two scalar fields (e.g., the radion and the Calabi-Yau dilaton) evolving along an unstable classical trajectory.

    In fact, we show here that an overwhelming fraction of the universe fails to make it through the ekpyrotic phase; nevertheless, a sufficient volume survives and cycling continues forever provided the dark energy phase of the cycle lasts long enough, of order a trillion years.

    Two consequences are a new role for dark energy and a global structure of the universe radically different from that of eternal inflation.

    http://arxiv.org/abs/0812.3388 , 162kb

  • Administrator December 19, 2008, 9:23

    Apropos of your comment, Jim, note this additional bit from the Dennis Overbye story I linked to:

    Paul Steinhardt of Princeton University pointed out that the uncertainties in the cluster results are larger if you consider the possibility that w could vary with cosmic time, “which is what you should do if you are trying to test the idea that the dark energy is not a cosmological constant,” he said in an e-mail message.

  • James M. Essig December 19, 2008, 15:55

    Hi Paul and ljk;


    The article at the link you provided sounds like a fascinating read. Thanks for providing the link. I tend to like the cyclical big bang model because of the way it represents a pheonix like rise from the ashes. When our universe’s star forming era ceases, perhaps the universe will enter its collapse phase only to be reborn with simmilar or perhaps differeing laws of physics, numbers and types of fundamental particles and constants, degrees of curvature etc. I will read that paper this evening after I finish my holiday shopping.


    Thanks for the bit from the Dennis Overbye story.

    If the universe may be of a cyclical form with perhaps the potential for an infinite number of rebirths, the question as to how many previous cycles it passed through becomes of interest. Perhaps, in a multiverse theory, other universes within the multiverse are simply past and future versions of our big bang’s cycles. However, one might then consider whether or not there are additional multiverses, perhaps in no way related to previous or forward cycles within our universe.

    I might be going out on a limb here, but one wonders from an ontological perspective or from a metaphysical perspective as to whether or not there might be innumerably many such completely causaly, thermodynamically, and perhaps even ontologically and existentially uncoupled cosmoses.

    The scientific and mathematical possibilities stagger the mind. The philosophical possibilities are practically beyond comprehension.



  • James M. Essig December 19, 2008, 20:19

    Hi ljk;

    I finally did have a chance to read the entire paper at http://arxiv.org/abs/0812.3388 .

    As I lead the caption under figure 3 of the subject paper,

    “FIG. 3: The global structure of the two- field cyclic universe:
    large smooth and flat regions are interspersed with small regions
    that have collapsed and have stopped cycling. The tiny
    regions of lighter shade will turn into the entire habitable regions
    during the next cycle.”

    I noted that although the Figure 3 is an abstraction portraying the kinematics of spatial-temporal topological aspects related to the cyclical universe theory in light of the discussion of the two proposed scalar fields, it occurred to me how much the picture appeared to resemble a crystal in terms of its energy states such as diagrams of the abstractions known as Fermi Surfaces within the field of solid state physics, or perhaps to the fluctuating magnetic domain regions within magnetic materials.

    It occurred to me that just as the relavant abstract discription presented in Figure 3 implies reified topological and morphological aspects of the scalar fields and the various regions within the universe, perhaps there is; by argument of symmetry by philosophy or metaphysics, that each of these cells or “regions” are perhaps by no means the fundamental macroscopic entities encompasing our universe just as the magnetic domains of a crystal or Fermi surfaces are by no means the fundamental aspects in the ultimate material that comprises crystalline matter and magnetic domains within magnetic materials.

    One might argue by further symmetry and analogy that the scalar fields, phi 1 and phi 2, are by no means the ultimate existential or ontological driving mechamisms nor the ultimate mechanisns that underwrite the existence and evolution of cyclical universe cycles, whether such cycles be finite or infinite in number. The argument by analogy would be akin to the fact that the very atoms that immeadiately causally and thermodynamic underpin the Fermi Surface characteristics of crystaline matter and the domains in magnetic materials are not the ultimate primitive sub-stratum with respect to their constituent physical mattery in themselves although they are one step closer to such than the respective Fermi surfaces and mangetic domains.

    The goal of determining just how many levels of existential or ontological entities exist before we arrive at the ultimate prime matter, or ultimate and irreducable substratum that underwrites all physical existence, entities, and accidental properties of our entire physical cosmos may in the end be a philosophical and perhaps even a mathematical and scientific enterprise that never ends. The study of such underlying mechanisms might be even far more sublime that the consideration that hidden quantum variables might actually exists.



  • spaceman December 24, 2008, 2:48

    Given that this measurement is not capable of allowing us to distinguish between the many dark energy models, I think the importance of it is that it’s results very strongly suggest that there is somekind additional field or energy causing the expansion of space to accelerate. Thus, this result is more than anything else, an epistemological triumph in that it emphatically confirms what cosmologists have been thinking for about the past decade or so. In other words, one can quibble about the supernove data and be skeptical about the overall standard model, but now that two completely different techniques relying on different physics–supernovae data and the X-ray clusters–both come to more or less the same conclusion is epistemologically formidable. Obviously more work, both theoretical and observational, will be needed before we can confidently say what the dark energy consists of.

  • ljk February 16, 2009, 22:49

    Wide Field Imager in Space for Dark Energy and Planets

    Authors: Andrew Gould (IAP, OSU)

    (Submitted on 12 Feb 2009)

    Abstract: A wide-field imager in space could make remarkable progress in two very different frontiers of astronomy: dark energy and extra-solar planets. Embedding such an imager on a much larger and more complicated DE mission would be a poor science-approach under any circumstances and is a prescription for disaster in the present fiscal climate.

    The 2010 Decadal Committee must not lead the lemming stampede that is driving toward a DE mega-mission, but should stand clearly in its path.

    Comments: 4 pages. Decadal White Paper: Gould_wide_field_imager_PSF_CFP.pdf

    Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0902.2211v1 [astro-ph.CO]

    Submission history

    From: Andrew Gould [view email]

    [v1] Thu, 12 Feb 2009 21:00:05 GMT (5kb)


  • ljk April 27, 2009, 10:32

    A primer on problems and prospects of dark energy

    Authors: M. Sami

    (Submitted on 22 Apr 2009)

    Abstract: This review on dark energy is intended for a wider audience, beginners as well as experts. It contains important notes on various aspects of dark energy and its alternatives. The section on Newtonian cosmology followed by heuristic arguments to capture the pressure effects allows us to discuss the basic features of physics of cosmic acceleration without actually resorting to the framework of general theory of relativity.

    The brief discussion on observational aspects of dark energy is followed by a detailed exposition of underlying features of scalar field dynamic relevant to cosmology. The review includes pedagogical presentation of generic features of models of dark energy and its possible alternatives.

    Comments: Invited article; submitted to Curr. Sci

    Subjects: High Energy Physics – Theory (hep-th); High Energy Astrophysical Phenomena (astro-ph.HE); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics – Phenomenology (hep-ph)

    Cite as: arXiv:0904.3445v1 [hep-th]

    Submission history

    From: Mohammad Sami [view email]

    [v1] Wed, 22 Apr 2009 12:19:07 GMT (814kb)


  • ljk September 25, 2009, 12:59

    Does the Universe Accelerate Equally in all Directions?

    Authors: R. Cooke, D. Lynden-Bell (IoA, Cambridge)

    (Submitted on 22 Sep 2009)

    Abstract: We employ the Union compilation of Type Ia supernovae with a maximum likelihood analysis to search for a dark energy dipole. To approach this problem, we present a simple, computationally efficient, and largely model independent method.

    We opted to weight each SN by its observed error estimate, so poorly measured SNe that deviate substantially from the Hubble law do not produce spurious results.

    We find, with very low significance, a dipole in the cosmic acceleration directed roughly towards the cosmic microwave background dipole, but this is almost certainly coincidental.

    Comments: 7 pages, 6 figures, Accepted for publication in MNRAS

    Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

    Cite as: arXiv:0909.3861v1 [astro-ph.CO]

    Submission history

    From: Ryan Cooke [view email]

    [v1] Tue, 22 Sep 2009 08:50:41 GMT (72kb)