by Liz Kruesi
06 August 2016
from Sci-Hub Website

Spanish version

Have black holes hurled

half the universe into the wilderness?
Billions of years ago,

half the universe's visible matter disappeared.

The monstrous culprit

might have been unmasked - and with it,

the future of our galaxy.
 

We've all been there.

You had it in your sight just a moment ago, then you turn your back for a second and it's nowhere to be seen. It's bad enough with car keys or the remote control. Just imagine if you'd lost half the universe.

Yet that's exactly where cosmologists find themselves. The problem lies not with dark matter or dark energy, those mysterious substances that measurements tell us make up most of the cosmic mix. This is about normal, visible matter.

Our best estimates say it makes up only about 5 per cent of all the stuff out there. Yet we're hard pressed to find even half that.

The other half went missing about 13 billion years ago and nobody's seen it since. Tracking this missing matter down isn't just a question of pride. Working out where it went - and how - could shed light on the fate awaiting everything else in the universe.

Now suspicion has fallen on an unexpected culprit:

black holes.

But these are black holes set not to suck - but to blow.
 

We've long known that our universe plays host to vast quantities of invisible stuff. Since the 1930s, astronomers have wrestled with the fact that galaxies are spinning much faster than their apparent densities should allow.

Calculating the amount of mass needed to keep the stars from flying off into the darkness revealed that only a few per cent of each galaxy's total mass is visible to us. The rest is labeled dark matter, a fundamentally different type of substance that interacts with the ordinary universe only through gravity.

Measurements of the cosmic microwave background, rays of light that have been travelling the universe since 380,000 years after the big bang, tell us that dark matter once outweighed normal matter by more than 5 to 1.

As there is no process we know of by which normal matter would change to dark matter, or vice versa, much of modern cosmology assumes that ratio stays the same throughout cosmic history.

 

Where the other half lives

We've been hard-pressed to find that same ratio today.

Astronomers measure how much of both kinds of matter the universe has by mapping how background light is bent by its combined gravitational pull. But when they tot up the mass of all the stuff we can see - from stars to gas to black holes to planets - they get a surprising result.

The visible components make up closer to one- tenth than one-fifth of the total mass.

The same is true when they look out to greater distances, which is equivalent to looking further and further back in time. The results suggest a staggering conclusion: sometime in its first billion years, half of the normal universe settled down, forming stars and galaxies.

The other half went missing.

"The word missing is kind of a colloquial way of putting it," says Princeton University astrophysicist Neta Bahcall.

No one really disputes that the matter must be out there somewhere.

But where is it? It's the largest scavenger hunt in history, and the list of hiding places is as vast as the universe itself.

Fortunately, there are clues to help narrow the search - starting with the fact that this missing matter is invisible. To the naked eye, much of the universe is. Our senses are attuned to light in a narrow part of the spectrum, emitted by objects in the temperature range relevant for human activity - anything from a few degrees below zero up to a few thousand °C, the temperature of the sun's surface.

Even with a powerful telescope designed to detect this light, you miss out on much that the night sky has to offer.

Most of the universe is unimaginably cold: at around -270 °C it is less than 3 degrees above absolute zero. The slowly vibrating molecules here emit light at much lower frequencies - in the microwave range.

Over decades, we have slowly refined our capability to pick up this light, but scanning these frequencies has not turned up the missing mass.

When we turn to the other extreme, where matter is so hot that it emits ultraviolet waves, our technology starts to fall behind. And as for the X-rays emitted by matter heated to millions of degrees, our capabilities are even poorer.

All of that leads us to think that the missing matter probably emits X-ray light, burning brightly enough to pass unseen at around a million °C.

That's roughly the temperature of the sun's corona, its fiery outermost layers.

 

Cosmic clean-up

Black holes at the heart of galaxies could act as cannons, firing visible matter into the voids, where we cannot see it.

 

At which point you might cry foul: we can see the corona.

That's true, but it is much more difficult to see than the sun's cooler surface, and it is the density of matter in the corona - as well as our proximity to the sun - that gives our detectors a fighting chance.

This provides another clue to the identity of the missing matter:

it must be wispy and dispersed.

But where might such sparse-but-sizzling material be lurking?

Over the past few years, astronomers have begun to speculate that it could lie surprisingly close to home, in galaxies like our own Milky Way. With today's telescopes, we can account for nearly all the Milky Way's stars, as well as the gas and dust that float between them.

Assuming the usual dark matter ratio holds true, these components should tally up to roughly 16 per cent of the galaxy's mass. But even here we come up short. Half of the normal matter that's supposed to be visible isn't.

It may seem counter-intuitive to propose invisible clouds of roiling hot gas in the apparent calm of the Milky Way. But galaxies are not peaceful places. The relentless pull of their gravity acts on diffuse streams of gas surrounding them, drawing the matter in to fuel star formation and galaxy growth.

As those streams of protons and neutrons, collectively known as baryons, fall in, they gain energy, raising their temperature and speed. At some point, the free-falling gas hits other material accreted thousands of years earlier.

This collision is so violent that it stops the newly congregating material in its tracks, heating it up to 2 million °C and propelling some of it away from the galaxy's central regions.

 

"Some bits of matter

are cooling in the bath.

Others are going down the drain"
 

Think of the galaxy like a bathtub, says Michael Anderson of the Max Planck Institute for Astrophysics in Garching, Germany, with the inflowing streams of baryons playing the role of water.

"Some of them are cooling in the bathtub and forming stars. They're also going out the drain."

And there, a competition of forces ensues.

The galaxy's gravity pulls the hot baryons back towards its centre while their high-energy jiggle pushes them away.

The consequence is they surround the galaxy in a diffuse halo of hot gas.

"The same thing that gives us the atmosphere around the Earth gives us the atmosphere around a galaxy," says Joel Bregman, an astrophysicist at the University of Michigan.

We might not be able to spot this halo directly, but we should be able to see its effects.

As light from distant galactic centers shines across the universe, the clouds of gas and dust it passes through absorb light of specific colors, revealing details about their temperature and composition.

For the last decade, Joel Bregman and his colleagues have been staring at these cosmic searchlights through our own galaxy in an attempt to identify just how much invisible gas the Milky Way contains.

Bregman's observations have shown that our galaxy could be encircled by just such an atmosphere, hosting roughly the same amount of baryonic matter as the galaxy's luminous parts, yet spread across a volume many thousands of times as vast.

So, problem solved? Not quite...

These hot haloes help solve the problem of underweight galaxies, but Bregman calculates that this only accounts for half of the baryonic matter needed to balance the cosmic scales. And explaining the whereabouts of the rest is where things start to get really interesting.

There are some mundane explanations.

Bregman, for example, believes we could find the rest within galaxies if we look even farther out from their centers, in the distant regions where their gravitational pull fades away.

Neta Bahcall thinks we might not even need to do that.

We've ventured out far enough to account for all the missing matter already, she says - the only problem is that the measurements have been carried out by different teams who haven't had the chance to combine their results.

She's working on combining them now, and although she cautions that there will inevitably be some uncertainty in the final result, she believes there's a fair chance we can make things add up. Others are not convinced.

After all, galaxies themselves are a tiny fraction of the universe's volume.

Held together in vast clusters, they lie at the intersections of filaments that thread together to form what is known as the cosmic web. In the gaps between the cosmic web's filaments are voids billions of times emptier than any vacuum we can build on Earth.

Perhaps the missing matter is going to turn up here.

 

Planned X-ray probes

give us our best shot at spotting missing matter

 

Searching for any matter hiding in these voids is fraught with complexity.

And it raises another thorny question:

what in the cosmos could propel the matter out of the clutches of the cosmic web and into voids millions of light years away?

That would require far more energy than most galactic processes could generate.

Shy Genel of Columbia University in New York is one astronomer pointing the finger in a surprising direction:

at black holes.

For the past five years, Genel has been a core member of the Illustris project, one of the biggest and most complex universe simulations ever built.

Run on a global network of supercomputers, Illustris is the closest we can get to recreating the universe in the lab. Part of its power lies in allowing researchers to dictate the strength of any processes they like, even if the detailed physics remains blurry.

So you can, say, adjust the parameters of your black hole behavior as you want, and see what sort of universe emerges.

Genel's initial interest lay in following how galaxies live and die, a problem indirectly related to the mystery of the missing baryons. In their attempts to build a model universe that resembles our own, however, Genel and his collaborators were surprised to find themselves with black holes strong enough to catapult vast quantities of matter into the voids (see "Cosmic clean-up", far above).

We tend to think of black holes as inescapable whirlpools, not cosmic cannons, but their powerful gravitational fields make them among the most efficient energy generators in the universe.

As material from the surrounding galaxy falls towards a black hole, nearly 10% is converted to energy. This is enough to accelerate some of the remaining particles very close to the speed of light, causing them to shoot out from the accretion disc in two jets in opposite directions.

The jets barrel through the galaxy, sharing that energy with the gas they encounter, says Genel.

"In some cases, they go well beyond the host galaxy of the black hole."

Astronomers have seen exactly that effect in galaxy clusters, as enormous bubbles of hot gas expand away from the galactic centre.

The Illustris simulation showed that black holes could be powerful enough to throw baryons out far beyond the outer reaches of a galactic cluster. But there was a problem.

In the first - and so far only - full Illustris run, the effect of black holes was so strong that some galaxy clusters lost nearly 90 per cent of their baryons.

It's an overestimation that the team has been working over the past year to correct, recalibrating their descriptions of black holes to make them more realistic, in line with the observed figure of around 50%.

 

Mostly missing

Only a small fraction of the universe is visible to us today.

If they succeed, it means not only that the missing baryons could be floating through the cosmic voids, but that black holes are pumping out more to join them all the time.

We already know that over the next few billions of years, galaxy clusters will drift farther and farther apart thanks to dark energy and the universe's ever-faster expansion. But if baryons are disappearing down the plughole faster than the faucet can replace them, these clusters might start crumbling apart as well.

Our universe's distant future is starting to look rather bleak.
 

"Their gravitational fields

turn black holes

into cosmic cannons"
 

Of course, a computer simulation can only tell you so much.

"[This field] is being led by the observations, not by simulations," says Bahcall.

So what we'd really like to do is find out if this gas is actually there.

And while our current ability to see X-ray radiation across the universe is severely limited, our eyes are about to be opened. The next planned large-scale X-ray instrument should be able to see some of the diffuse gas at the million-Kelvin temperature range - provided it's where astronomers think it is.

The European Space Agency's Athena mission will be 100 times more sensitive than the X-ray telescopes currently in orbit. The telescope won't launch until 2028 at the earliest. And NASA has its own project in the works.

ARCUS is a telescope designed to use the bright lights of distant galaxies to sample all of the hot X-ray gas within the gravitational boundaries of many galaxies. If it gets the go-ahead, it could be in operation by 2023.

Bregman can hardly wait.

"If we don't see most of the baryons at that point, I'll be thoroughly disappointed," he says. "That is our best shot."

Discovering them would be huge for cosmology.

Our understanding of galactic evolution depends on understanding what happens to baryon clusters, and our reconstructions of the early universe emerge from the way baryons behave.

Like a pair of headlights on a darkened country road, baryonic matter is the only visible sign we have of something much larger going on in the background.

This missing 2.5 per cent of the cosmos could be our most valuable tool for understanding everything else.