A rendering of how filaments of dark matter might encase galaxies (Credit: AMNH)

The universe is inhabited by billions upon trillions of galaxies, each generally with millions of light-years of space between them. For the most part, said regions are lacking in anything substantial, be it rogue stars, planets or the elusive dark matter. At least, that’s what we thought. Over the last few years, we’ve found that intergalactic space is home to tremendously huge tendrils of interstellar gas (hydrogen and helium, mostly), which form bridges between other galaxies (kind of like the tidal tails of material left behind after galaxies collide). 

In a new study recently published in The Astrophysical Journal Letters, a team of astronomers from the University of Massachusetts at Amherst have announced that these tendrils of material pose a new cosmic light quandary. 

Some Background: 

Lurking within some of the oldest and most distant galaxies in the known universe, astronomers have found huge, highly energetic objects called quasars. It’s believed that each quasar is powered by a central black hole in a galaxy so active, more materials accumulate at such a rapid rate that the black hole can’t possibly consume all of it, thus it builds into an accretion disk. Friction causes these rotating disks to become extremely hot and incredibly luminous, making them celestial beacons of light. They are so luminous, in fact, that any one of them could outshine our galaxy thousands of times over.

Along with galaxies, astronomers use these quasars as celestial “light meters,” Only, new research shows that the combined light from all known galaxy and quasar sources could not possibly account for the measured concentration of intergalactic hydrogen fuel. In fact, the discrepancy is incredibly vast, a stunning difference of more than 400 percent.

“It’s as if you’re in a big, brightly-lit room, but you look around and see only a few 40-watt lightbulbs.” “Where is all that light coming from? It’s missing from our census.”

[Reference: Carnegie Science]

“Computer simulations show a “dimly lit” universe (left) and a “brightly lit” universe (right) that has five times more of the energetic photons that destroy neutral hydrogen atoms. Hubble Space Telescope observations of hydrogen absorption match the picture on the right, but using only the known astronomical sources of ultraviolet light produces the much thicker structures on the left, and a severe mismatch with the observations.” (Caption Source) (Credit: Ben Oppenheimer and Juna Kollmeier)

The light I’m referring to is comprised of highly energetic ultraviolet photons, which can convert electrically neutral hydrogen atoms into electrically charged ions. These characteristics make quasars and galaxies prime sources to probe, the former generate ionizing photons as a byproduct of star consumption. The problem is that observational evidence shows that the ionizing photons created through such processes should never venture beyond a galaxy’s borders to have any influence on intergalactic hydrogen supplies. They are generally absorbed by interstellar gas before they get that far. Once there, they collide with gas, transforming into electrically charged ions that are incorporated into the tendrils

According to Juna Kollmeier, the lead author of the study,“Either our accounting of the light from galaxies and quasars is very far off, or there’s some other major source of ionizing photons that we’ve never recognized.” “We are calling this missing light the photon underproduction crisis. But it’s the astronomers who are in crisis—somehow or other, the universe is getting along just fine.”

[Reference: Carnegie Science]

Potentially Dark Ties:

A chart showing where our mass is (via phys.washington.edu)

As to the apparent source of the discrepancy, Katz and the team even point to dark matter — one of the most elusive mysteries in modern cosmology — as a contender. Astronomers have long known that something was amiss in their calculations. The combined mass of all known objects — from stars, to galaxies, black holes and even interstellar material, extending all the way down to humans and even the most basic building blocks of all matter: subatomic particles — just doesn’t match up to the expected amount… not by a long shot.

We’ve looked for this missing matter in a number of places,  but so far, we’ve not come up with anything substantial, partially because dark matter does not interact with normal matter in any other way. However, we can see its effects, especially when we look at the way galaxies are held together and how they spin. Normal matter alone can not account for this, but ho does dark matter tie into this? 

Well, under this hypothesis, dark matter could decay in such a manner that it might account for their observations.

“The great thing about a 400% discrepancy is that you know something is really wrong,” remarked another co-author of the paper — David Weinberg of The Ohio State University — “We still don’t know for sure what it is, but at least one thing we thought we knew about the present day universe isn’t true.”

 Mechanism of a Mystery:

And it gets even stranger, believe it or not. Apparently, the discrepancy is only apparent in our local universe — the parts that have been charted and studied at length. When we start looking much farther out into the endless void — which, to reiterate, is like looking back into a time when the universe was a much different place — all of the numbers seem to add up. As to why things seem to go haywire locally, the team is confounded by.

One thing is for sure,“You know it’s a crisis when you start seriously talking about decaying dark matter!” Katz remarked.

“The simulations fit the data beautifully in the early universe, and they fit the local data beautifully if we’re allowed to assume that this extra light is really there,” yet another co-author, Ben Oppenheimer from the University of Colorado, remarked. “It’s possible the simulations do not reflect reality, which by itself would be a surprise, because intergalactic hydrogen is the component of the Universe that we think we understand the best.”

[Reference: Carnegie Science]

Similarly, the lead author stresses:

“The imbalance could have to do with our gaseous light meters rather than the light itself.” “In fact, it’s possible that our simulations are predicting the wrong hydrogen distribution relative to reality—i.e. our “light meter” is broken,” she told Motherboard.com, “But we think this is very unlikely. The hydrogen is one of the most well-understood aspects of our calculations. It is used as a tool for ‘precision cosmology’—one of the primary techniques for anchoring the very properties of the universe itself—for example the properties of the dark matter. In order to resolve this crisis, we would need the local hydrogen distribution to be much smoother than our standard predictions.”

[Reference: Motherboard]


Simulations show that there are huge tendrils of dark matter that form a huge web. Learn all about that here. Or learn about a hypothesis that says dark matter might merely be unique atoms here.


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