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How dark is the cosmic web?

A visualization of the filaments in the cosmic web. Simulation produced by the EAGLE project.
A visualization of the filaments in the cosmic web. Simulation produced by the EAGLE project. (Image credit: EAGLE Project)

The universe is permeated by a vast, invisible web, its tendrils weaving through space. But despite organizing the matter we see in space, this dark web is invisible. That's because it is made up of dark matter, which exerts a gravitational pull but emits no light. 

That is, the web was invisible until now. For the first time, researchers have illuminated some of the darkest corners of the universe. 

Related: The 11 biggest unanswered questions about dark matter

Weaving the web

A long time ago, the universe was hotter, smaller and denser than it is now. It was also, on average, much more boring. There wasn't much variation in density from place to place. Sure, space was much more cramped overall, but in the young universe, no matter where you went, things were pretty much the same.

But there were tiny, random differences in density. Those nuggets had slightly more gravitational pull than their surrounding neighborhood, and so matter tended to flow into them. Growing bigger in this way, they developed an even stronger gravitational influence, pulling more matter in, causing them to be bigger, and so on and so on for billions of years. Simultaneously, as the nuggets grew, the spaces between them emptied out.

Over the course of cosmic time, the rich got richer and the poor got poorer.

Eventually, the dense patches grew to become the first stars, galaxies and clusters, while the spaces between them became the great cosmic voids.

Now, 13.8 billion years into this massive construction project, the job isn't quite finished. Matter is still streaming out of the voids, joining groups of galaxies that are flowing into dense, rich clusters. What we have today is a vast, complex network of filaments of matter: the cosmic web.

A light in the dark

The vast majority of matter in our universe is dark; it does not interact with light or with any of the "normal" matter that we see as stars and gas clouds and other interesting things. As a result, much of the cosmic web is completely invisible to us. Fortunately, where the dark matter pools, it also drags along some regular matter to join in the fun.

In the densest pockets of our universe, where the gravitational whispers of dark matter have influenced enough regular matter to coalesce, we see light: The regular matter has converted itself into stars.

Like a lighthouse on a distant, black seashore, the stars and galaxies tell us where the hidden dark matter lurks, giving us a ghostly outline of the cosmic web's true structure.

With this biased view, we can easily see the clusters. They pop out like giant cities seen from a red-eye flight. We know for sure there's a tremendous amount of dark matter in those structures, since you need a lot of gravitational oomph to pool together that many galaxies.

And on the opposite end of the spectrum, we can easily spot the voids; they are the places where all the matter isn't. Because there are no galaxies to illuminate these spaces, we know that they are, by and large, truly empty.

But the grandeur of the cosmic web lies in the delicate lines of the filaments themselves. Stretching for millions of light-years, these thin tendrils of galaxies act like great cosmic freeways crossing black voids, connecting bright urban clusters.

Through a dim lens

Those filaments in the cosmic web are the hardest part of the web to study. They have some galaxies but not a lot. And they have all sorts of lengths and orientations; in comparison, the clusters and voids are geometric child's play. So, even though we've known of the existence of filaments, through computer simulations, for decades, we have had a hard time actually, you know, seeing them.

Recently, though, a team of astronomers made a major advance in mapping our cosmic web, publishing their results Jan. 29 to the arXiv database. Here's how they went to business:

First, they took a catalog of so-called luminous red galaxies (LRGs) from the Baryon Oscillation Spectroscopic Survey (BOSS) survey. LRGs are massive beasts of galaxies, and they tend to sit in the centers of dense blobs of dark matter. And if the LRGs sit in the densest regions, then lines connecting them should be made of the more delicate filaments.

But staring at the space between two LRGs isn't going to be productive; there isn't a lot of stuff there. So, the team took thousands of pairs of LRGs, realigned them and stacked them on top of each other to make a composite image.

Using this stacked image, the scientists counted all the galaxies that they could see, adding up their total light contribution. This allowed researchers to measure how much normal matter made up the filaments between the LRGs. Next, the researchers looked at the galaxies behind the filaments, and specifically, at their shapes.

As light from those background galaxies pierced the intervening filaments, the gravity from the dark matter in those filaments gently nudged the light, ever so slightly shifting the images of those galaxies. By measuring the amount of shifting (called "shear" by the scientists), the team was able to estimate the amount of dark matter in the filaments.

That measure lined up with theoretical predictions (another point for the existence of dark matter). The scientists also confirmed that the filaments weren't entirely dark. For every 351 suns' worth of mass in the filaments, there was 1 suns' worth of light output. 

It's a crude map of the filaments, but it's the first, and it definitely shows that while our cosmic web is mostly dark, it's not completely black. 

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of Your Place in the Universe.

Originally published on Live Science.

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Paul Sutter
Paul Sutter

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

  • rod
    Admin said:
    A dark web ties the universe together. Now, we can see it.

    How dark is the cosmic web? : Read more

    "The vast majority of matter in our universe is dark; it does not interact with light or with any of the "normal" matter that we see as stars and gas clouds and other interesting things. As a result, much of the cosmic web is completely invisible to us. Fortunately, where the dark matter pools, it also drags along some regular matter to join in the fun."

    If you dig deep enough into the cosmology model, all the dark stuff can give rise to dark stars too, forming about 200 million years after the Big Bang event. So does the model used in this report, also require dark stars to be real?
  • Stephen J. Bauer
    I find this representation of the cosmic web to be very telling in the furthering of the hypothesis that the universe is somewhat evolved into a working brain. It might be purported as the simplest evidence for promoting a sense of consciousness in the makeup of the universe. This pictorial representation is similar to what has been seen as the structure of neural activity in the brains of organic beings. Furthermore it lends credence to a universe built from a foundation in 'dark energy' and 'dark matter'.

    Considering the 'Big Bang' inflation theory, try thinking of the proposed one dimensional singularity as the pre-existing fabric of space-time without any real matter, rather than a singular point as modeled after a gravitational singularity . Considering the 'Big Bang' theory from a pre-existing fabric of space-time without any real matter, as a proposed one dimensional determinant, its inception starts with the unfolding perspective of this dimensional determinant for space-time fabric towards existence. The sequence is somewhat understood from an expansion from our one dimensional space-time into a two dimensional space-time fabric, and then into a three dimensional space-time fabric, and so on. The expectation is that ordinary matter creation took place within a pre-existing dark energy medium of space-time. Indeed, the existence of matter would be an intrusion upon this pre-existing universal medium of space-time which maintains a zero sum difference that is the balance of our cosmological continuum. Take away all matter and you would still have a vessel in which the matter once existed. I would only be logical for the vessel to be one of dark energy; dark energy unaffected by this promulgation of matter.

    With this understanding, any perturbation in this medium would engender a warping of this pre-ordered dark energy template of space-time fabric in the evolutionary perspective of its dimensional unfolding. Wherein the creation of matter as a whole induces a complementary displacement, or warping, in the dark energy medium of the space-time fabric, its promulgation is interdependent on its insistence and persistence. For within this warping, there is yet another pertubation in the whole matter created; a dual relationship of newly created positive density matter in an envelopment of negative density matter. The complementary displacement insulates the newly created positive density matter in an envelopment of negative density matter. This envelope of negative density matter, known as dark matter, then infiltrates the spaces in matter, providing it with the ability to interact, bond, and evolve. Indeed it would require much more dark matter to fill the spaces among ordinary matter down to its smallest constituent parts.

    So if dark matter is what engenders a force of gravity for ordinary matter to bond, then the accretion and accumulation of ordinary matter is just the resultant consequence of this force. And if the black holes are nothing but dark matter, then it would also follow that dark matter can be accumulated, separate of ordinary matter. It would therefore also follow that the gravitational force is more representative of negative density mass than positive density mass.

    Upon this hypothesis then, one can expect that there is a require transition to separate ordinary matter from its complementary dark matter. It starts first with the disintegration of matter, as a whole, as it interacts with the event horizon of the black hole. As the positive density mass is 'squeezed' upon its own gravitational acceleration toward the black hole, liken to the spaghettification effect, its matter changes to allow for its disintegration via transmutation and the massive release of photons due to alpha decay and beta decay. This is the effect wherein positive density mass is collected within the event horizon, into a plasma, increasing its photon density. This 'squeezing' effect is like extracting out the dark matter from the whole matter, allowing for the ordinary matter to be reduced to its smallest constituent components. The dark matter is then absorbed into the black hole, and the remnants of ordinary matter are discarded and radiated out at high velocity back into the cosmos; to start, once again, to reintegrated into the universe via bonding and evolving.

    If you're interested in exploring this concept more, please review the alternative theories presented in the book, 'The Evolutioning of Creation: Volume 2', or even the ramifications of these concepts in the sci-fi fantasy adventure, 'Shadow-Forge Revelations'. The theoretical presentation brings forth a variety of alternative perspectives on the aspects of existence that form our reality. #shadowforgerevelations
  • rod
    FYI. Cosmology has many problems today, here is an example. How a Dispute over a Single Number Became a Cosmological Crisis The report wraps up with: “There is a small correction somewhere needed to bring the numbers into agreement,” Suntzeff says. “That is new physics, and that is what excites cosmologists—a kink in the wall of the Standard Model, something new to work on.” Everybody knows what they have to do next. Observers will await data from Gaia, a European Space Agency observatory that promises, in the next couple of years, unprecedented precision in the measurement of distances to more than a billion stars in our galaxy. If those measurements do not match the values that astronomers have been using as the first rung in the distance ladder, then maybe the problem will have been systematic errors after all. Theorists, meanwhile, will continue to churn out alternative interpretations of the universe. So far, though, they have not found one that withstands community scrutiny. And there, barring any breakthrough, the tension—problem, crisis—will have to reside for now: in a quasi-unscientific universe harboring a predicted Hubble constant of 67 that belies the observation of 74. The standard cosmological model remains one of the great scientific triumphs of the age. In half a century cosmology has matured from speculation to (near) certainty. It might not be as complete as cosmologists believed it to be even a year ago, yet it remains a textbook example of how science works at its best: it raises questions, it provides answers and it hints at mystery."

    H0 67 to 74 using the cosmology calculators shows the Hubble time is 14.252E+9 years for the age of the universe at 67. H0 74, Hubble time is 12.905E+9 years old, using the flat universe model. Using the open universe model, H0 67, Hubble time is 11.869E+9 years old universe. H0 74, Hubble time is 10.746E+9 years old.
    Clearly the Big Bang model has problems.