Cosmic Filaments

A brief introduction to Cosmos

From homeless people living on the street to great minds throughout centuries looked up at the sky during night time trying to grasp all the mysteries that it holds. It is true to a degree that the secrets of the vast space are unfolding as great minds peek under its thick curtain but if we talk about a far greater chunk of space, the cosmos, one can safely say that we have just taken the first few steps into the realm of perplexing wonders.

The cosmos is made up of matter and energy where dark matter and dark energy make up about 95% of it. The remaining five percent is made out of atoms – the kind of matter that makes up you and me. It is the stars and galaxies and gas that we see. However, astronomers have long known that the matter that they’ve seen is less than half of the atomic matter that exists. Several hypotheses have been advanced as to where that matter could be found and a few recent papers suggest that an idea long held by astronomers is probably correct.

Galaxies and Cosmic void within the cosmos

On the very largest scales, the matter of the universe is uniform, however on somewhat smaller scales (say tens or hundreds of millions of light-years), the ordinary matter has a fascinating structure. Cosmic matter looks much like the foam created by shampoo on our hair. In the foam, the bubbles are liquid membranes surrounding empty space. And, from a distance, the foam looks uniform and white. In the universe, the matter is similar. Galaxies are clustered in sheets that surround vast cosmic voids. The origin of this structure is ancient. Shortly after the Big Bang, the hot plasma that made up the cosmos at the time was dense enough to support sound waves. The universe rang like a bell.

Familiar sound waves are caused by density variations in the air, with some locations being slightly denser and others less so. Similar waves in the early universe set up slight density variations in the matter of the universe. And, over the eons, gravity amplified the effect. Where there was more density, gravity pulled matter towards that location. Where there was less, the matter was drawn away. The result is the structure we see today.

The Cosmic filaments and their origin

However, not all of the matter of the universe has assembled into galaxies. If the idea of sound waves permeating the universe after the Big Bang is true, then there are also long cosmic filaments of hydrogen and helium gas connecting clusters of galaxies and being part of the membranes in the observed structure of the universe.

Physics and simulations have long predicted that the large-scale structure of the universe looks like it was spun by some colossal spider. Dark matter seems to have been distributed into filaments spanning lightyears, and that attracted regular matter like dust and gas to gather along those threads, too. Galaxies and clusters tend to clump in the nodes where these filaments meet.

However, these filaments are very tenuous. The density of the gas in filaments between clusters of galaxies is very low – about ten atoms per cubic meter. This corresponds to a vacuum far better than we can achieve on Earth.

Furthermore, these filaments are both large and the gas in them has a variety of temperatures. For instance, earlier studies using telescopes capable of seeing ultraviolet have seen evidence of filaments with a “low” temperature of under 100,000.

Recent break through regarding the observation of the cosmic filaments

Now, astronomers have made the first direct observations of light from this web, by staring at a patch of sky with a powerful deep-field telescope to detect faint dwarf galaxies. Hints of these cosmic webs have been indirectly observed before, through gravitational lensing or thanks to the bright light from quasars illuminating the gas. But now, astronomers have managed to detect light from the hydrogen gas itself.

To do so, the team used the Multi-Unit Spectroscopic Explorer (MUSE) instrument onboard the Very Large Telescope (VLT) in Chile. They pointed this instrument at one patch of sky and stared for 140 hours. This long exposure time allowed the diffuse light of the hydrogen emissions to come into focus, revealing 22 filament structures in the very early universe – between 0.8 and 2.2 billion years after the Big bang.

It was previously thought that any glow from these filaments would have been caused by radiation from the cosmic microwave background heating the gas in the filaments. But when the team ran follow-up simulations, they found that the light was likely coming directly from billions of dwarf galaxies.

Interestingly, while this region had already been studied as part of Hubble’s ultra-deep field survey, the new observations were able to see 40 percent more galaxies that Hubble hadn’t been able to see. And even then, many more galaxies were too faint for MUSE to pick out individually.

Recent discoveries that tell us the Cosmic filaments are spinning

In a new startling piece of research, scientists have discovered that these filaments in the cosmic web which connect galaxies are in fact spinning. For many, it is a big surprise indeed. Previously it was thought that galaxies were the largest structure to inherent spin but now scientists from the Leibniz Institute for Astrophysics Potsdam, have discovered that there are objects which span hundreds of millions of lights that are also rotating.

To identify the spinning motion in the filaments, the scientist used the same technique they use when identifying the spinning motion of stars or galaxies. They examined the relative change in the red/blue shift of the spinning object from one side to the other. In order to apply this on a larger scale, they first had to identify a filament structure. They then divided the galaxies from the survey into regions that would fall on either side of the structure and measured the redshift difference between the two parts.

What they discovered was that the redshift difference between the two groups was much larger than the average velocities of the galaxies. This was clearly a sign that one side of the filament was moving away from Earth. But the alignment needs to be just right to get a strong rotational signal. Hence with this along with other complex mechanisms scientists got to know that the cosmic filaments were indeed rotating. One can even think of it as water spiraling down a tube as its motion of rotation is quite similar to that.

-Iklas Kader Zihad