Not-So-Clumpy Dark Matter Poses Cosmological Challenge

Astronomers analyzing a new sky survey have found that the distribution of dark matter in the modern universe is smoother than predicted from observations of a far younger universe.

KIDS Dark Matter Map

Analysis of weak gravitational lensing produced this map of dark matter in a region surveyed by the Kilo-Degree Survey.
Kilo—Degree Survey Collaboration / H. Hildebrandt & B. Giblin / ESO

The large-scale distribution of dark matter in the universe is less clumpy than expected. That surprising conclusion is based on a thorough statistical analysis of the shapes of some 15 million remote galaxies.

Galaxy images are ever so slightly distorted by the gravity of intervening matter — a phenomenon known as weak gravitational lensing (see my article in Sky & Telescope’s September issue). But the international Kilo-Degree Survey (KIDS), which studies this so-called cosmic shear, finds an effect that’s about 10% smaller than predicted by the standard cosmological model.

“If the tension […] persists […], modification of the current concordance model will become necessary,” according to the KIDS team in an upcoming issue of Monthly Notices of the Royal Astronomical Society.

Mapping the (Dark) Cosmic Web

Current wisdom says that the newborn universe was pretty homogenous, as evidenced by the cosmic microwave background (CMB). The Big Bang’s afterglow shows only minute temperature variations across the sky, which means that density varied only minutely as well. But starting from these tiny fluctuations, dark matter clumped over time into a huge 3D cobweb of filaments and sheets. Gravity then drew in ordinary matter, which coalesced into superclusters, clusters, and individual galaxies.

Matter in the threads of this intergalactic cosmic web represents about 25% of all matter in the universe, according to Massimo Viola (Leiden Observatory, The Netherlands), who led the new study together with Hendrik Hildebrandt (Argelander Institute for Astronomy, Germany).

The only way to map this mostly dark cobweb is by studying the very subtle gravitational fingerprint it leaves on the shapes of background galaxies. KIDS, carried out with the European Southern Observatory’s 2.6-meter VLT Survey Telescope at Cerro Paranal in Chile, is one of three ongoing, large cosmic shear surveys. So far, it has mapped five areas totaling 450 square degrees (we’d need some 2,200 full moons to cover the same area on the sky).

The other two large cosmic shear surveys — the Dark Energy Survey (DES) at the 4-meter Blanco telescope at the Cerro Tololo Inter-American Observatory, also in Chile, and the Hyper Suprime-Cam (HSC) survey on the Japanese 8.2-meter Subaru telescope at Mauna Kea, Hawai‘i — haven’t published definitive results on this topic so far, though the DES did release analysis of preliminary "science verification" data that's roughly consistent with the KIDS result.

Based on observations by the European Space Agency’s Planck mission, cosmologists had expected dark matter to be fairly clumpy on the largest scales. Planck precisely mapped the CMB’s temperature fluctuations, then astronomers combined that information with what we know about the universe’s expansion history — along with some assumptions about the gravitational properties of dark matter — to yield a prediction of the current dark matter distribution. But the distribution KIDS found is smoother than predicted. The apparent discrepancy poses a challenge for astronomers to explain.

Kilo-Degree Survey, Dark Energy Survey, and Hyper Suprime Cam Survey

The Kilo-Degree Survey, Dark Energy Survey, and Hyper Suprime Cam are all tracing weak gravitational lensing to map the universe's dark matter. The KIDS result is only the first of what's to come. This graphic first appeared in the September 2016 issue of Sky & Telescope. (Click for larger image.)

Smooth Dark Matter Causes Tension

According to KIDS Principal Investigator Koen Kuijken (Leiden Observatory, The Netherlands), earlier cosmic shear observations by smaller and shallower survey programs already hinted at a smoother-than-expected dark matter distribution.

“Maybe the role of dark energy in the expansion history of the universe is different from what is generally assumed,” he says. “Or dark matter has different properties than popular cold dark matter models predict. It may even be the case that gravity behaves differently at the largest cosmic scales.” Kuijken is confident that theorists will soon come up with alternative models to explain the latest observations.

However, weak-lensing pioneer Tony Tyson (University of California, Davis), the chief scientist of the future Large Synoptic Survey Telescope (LSST) scheduled to see first light in 2022, says the disagreement between KIDS and Planck isn't all that worrisome.

"KIDS galaxies are much more nearby than those in deeper surveys," he says, "so the lensing signal is correspondingly smaller, making systematic errors more prominent." According to Tyson, solving the issue requires a deep and large-area survey of billions of galaxies over half the sky, with controlled systematics. "LSST is designed to address this."

One thought on “Not-So-Clumpy Dark Matter Poses Cosmological Challenge

  1. Yehiel Gotkis

    Here it is an alternative concept
    Vortices are formed all the way around us. Some of them are too large, or too small, or located too far away, or require sophisticated tools to research their activity. However, if different kinds of vortices possess common generics, observing the easily observable ones (for example, liquid vortices), could be of great help in building and testing theoretical models applicable for the hardly accessible and/or observable ones, as for example, for the black holes. Also, vortices activated in a transparent liquid allow to make observations in reasonable time, and under variety of conditions, and, what is especially important, to observe the developments taking place below the funnel bottom.
    The arrangement of the liquid vortex observed confined the vortex developments in the thin surface volume thus making the it to act as a two-dimensional rather than a tri-dimensional one. It allows to consider them as reasonably generic ones and relate them to the black hole ones, which basically take place in the two-dimensional spacetime framework.
    The collection of videos recorded by myself for a liquid vortex located in the middle of a reasonably large pool could be accessed at YouTube under my name YehielGotkis .
    Recently, with sophisticated astronomical tools used to observe the black holes, a bunch of new findings on the newly discovered matters were published. Surprisingly, or maybe not surprisingly, many of similar effects clearly occurred with our liquid vortex. The straightforward observation of the liquid vortex dynamics also allowed to come up with some out-of-box ideas, which, being looking as generic ones, are potentially applicable for the other types of vortices, including the black holes.
    In the notes on these matters published at Linked In some time ago I addressed a number of observable features, which seemingly may occur also for other kinds of vortices, including the cosmic black holes:
    • Emanation of outgoing double-spiral waves, which, being asymmetric, carry momentum and can transfer the associated kinetic energy to the distant material objects. The cause for the double spiral waves generation is associated with the spinning funnel bottom asymmetry, which most of the time was observed to be shaped as a spinning distorted digit 8. The asymmetry appeared to be caused by a couple of string-like features twisted as a double spiral “attached” to the funnel bottom and extending beneath the vortex bottom. This feature, thanks to the water transparency, was clearly observable from aside.
    • Spaghettification – the term, which in Astronomy is associated with vertical stretching and horizontal compression of objects to form long thin shapes (like spaghetti) in a strong inhomogeneous gravitational field. Spaghettification was normally observed when floating foam patches were pulled into the vortex funnel. When a handful of dry shredded leaves was spread over the surface around the liquid vortex a galaxy-like structure normally was quickly developed.
    Scrutinizing the vortex’ pulling-in action produced a shocking grasp.
    1. In the water pool (where the vortex is located) the upper liquid surface could be considered as being consistent with the gravitationally spacetime in cosmic case. The floating in the water pool debris and foam could be thought as material object resemblances.
    2. The liquid vortex sucked (obviously) both the flowing in water and the floating objects. If this or similar course of action takes place for the spinning black holes, then black holes should pull-in not only the material objects but also the spacetime itself whirling around the black hole in the same way as the water is whirling around the liquid vortex.
    3. As in the liquid vortex, an asymmetry in the spinning black hole mass distribution, even a slight one, must inevitably cause generation of propagating away spiral gravitational waves carrying momentum and energy. Meaning that spinning black holes dynamically distort the spacetime spreading energetic gravitational (spiral) waves. Meaning that the Universe must be filled with energy possessing gravitational ripples. Like the ocean surface.
    And here is the most intriguing point – the spacetime vortex model allows to explain the two known paradoxical cosmic observations, namely, the anomaly in the galaxies rotation curves and the Universe accelerated expansion with no necessity to introduce the two famous but challenging to prove hypotheses:
    1. The existence of black matter, introduced to explain the anomalous acceleration of the spinning matter angular velocity in proximity to the galaxy centers. Our vortex model explains it simply as resulting from the additive contribution of the spacetime whirling momentum in the black hole proximity
    2. The existence of black energy – Our vortex model explains the accelerated expansion effects as induced by centrifugal force from the spinning spacetime still keeping on at the galaxy peripheries where the attractive gravitational force are already diminished.
    I believe I am not mistaken with these my speculations.

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