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In the dark about dark matter.

Astronomers' favorite candidate for the universe's invisible matter is running out of places to hide.

Maybe we should be looking for something else.

The universe is filled with dark matter. To some, this might seem a bold claim--after all, we can't actually see it. But while we have not detected it directly, dark matter's influence is all around us, on galactic and cosmological scales. Over the last several decades, the evidence for this mysterious, gravitationally powerful stuff has become a rich tapestry of many independent observational threads. We know how much dark matter there is in the entire universe, the role it plays in the formation of the galaxies we observe near and far, and that it must be something new.

The question of the nature of dark matter is a thrilling focus right now for physics and astrophysics. An enormous amount of work has gone into experiments that could directly detect dark matter or might somehow produce a dark matter particle. The greatest focus has been on weakly interacting massive particles (WIMPs), beefy hypothetical entities that interact with matter only weakly. In the gloom of the cosmic unknown, WIMPs have been the super-bright lamppost under which we've been looking the most (S&T: Jan. 2013, p. 26).

So far, we haven't found them. Experiments are reaching levels of sensitivity now that rule out more and more types of WIMPs (see page 30).

This is where the search gets exciting. It's also where optimism starts to be tested. How much should we be hedging our bets? How anxious should we be? There are so many possible lampposts out there--if we haven't found dark matter in the most obvious places, will we ever be able to find it at all? Does it even exist? There is a building appreciation that the universe may hold something beyond the WIMP, and that astronomical observations may offer exciting insights that can lead us to it.

An Astrophysical Necessity?

We don't need dark matter to understand the solar system. The planets orbit the Sun in a way that we can describe using Einstein's general theory of relativity. Their movement is perfectly explained by the mass of objects that we can see.

When we move up to the scales of galaxies, though, we notice something unexpected. Galaxies rotate much too quickly (see page 36), as do the huge swarms of galaxies bound together by gravity in clusters.

The whole idea of motion in a gravitating environment is that there is balance. The motions of stars within a galaxy, or galaxies within a cluster, are set by the amount of material that keeps them bound together in a continuous dance. And these stars and galaxies are zipping around at a dizzying speed compared to the amount of 1 mass that we can see.

This puzzle leads to two possible solutions. One is that general relativity doesn't quite describe how gravity works on larger astronomical scales. This is certainly possible! A huge advantage in general relativity's corner, though, is that it is astoundingly successful in describing the universe as a whole. Everything from the hot Big Bang to the way the universe grows and its structure evolves has an elegant, crisp description in the framework of general relativity.

That is, as long as we introduce two special ingredients to the recipe of the universe: an anti-gravitational "force" we call dark energy (S&T: Feb. 2009, p. 22) and dark matter.

Observations actually reveal a lot about dark matter. First, it mainly (but possibly not only) must interact with itself and normal matter through gravity. If it didn't, these two types of matter would be mixed and distributed in profoundly different ways than we observe--and we do observe dark matter's distribution, thanks to its lensing effect on background light (S&T; Sept. 2016, p. 34).

Second, dark matter is neutral: It doesn't have a positive or negative electric charge. If it did, the particles would either repel one another, preventing matter from clumping and creating a dramatically different cosmic structure, or they would build a "dark sector" of atoms, molecules, and so forth that would leave their mark in the cosmic microwave background, a mark we don't see.

Third, it's "cold"--that is, it moves slowly enough that it clumps together easily. We have already been able to (gravitationally) detect how some of this clumping happens, because these clumps are the clouds of dark matter, called dark matter halos, in which galaxies form and live. Thus the name cold dark matter (CDM).

Finally, there's about five times more dark matter than normal matter. Dark matter dominates the universe! It's the fundamental scaffolding that galaxies and clusters are built on.

Particle Physics Weighs In

Let's take a look at another view of these discoveries. Although our particle physics models that explain normal matter are very successful, there are some hints that we're missing something. For example, the so-called Standard Model of particle physics only includes three of the four fundamental forces: the strong, weak, and electromagnetic forces. Scientists haven't yet found a way to comfortably fit gravity into this framework.

But it is possible that the Standard Model's greatest challenge is dark matter. If dark matter is indeed a particle, then it should fit into a greater, more encompassing particle physics framework, within which our current Standard Model has its place.

So when astronomers proposed cold dark matter, it excited particle physicists, because it enabled them to investigate what the characteristics of a CDM particle might be. For example, the very early universe was much more dense and hot than the universe today. There should be a time early on when dark matter and normal matter constantly interacted in non-gravitational ways, colliding all the time. Based on what the matter's temperature--and, thus, its density--was at that point, we can write down how likely such an interaction is. This is usually called a cross section and describes how easily a dark matter particle and a normal particle might feel each other's effect.

It turns out that the rough value of this cross section is close to the value we see in other parts of particle physics, in what are called weak interactions, which are responsible for how some particles decay. This led to proposing the WIMP: a family of particles apparently connected with the weak nuclear force and (the calculations tell us) with a mass of up to 10,000 times or more the mass of a proton.

Hunting WIMPs

How might one detect a WIMP? The basic idea is that the chance for a collision between a WIMP and a normal atom is not zero--just extremely low. Trying to catch one of these rare events, researchers set up experiments with carefully isolated and monitored materials and hope a WIMP hits an atom inside. For example, the upcoming LUX-ZEPLIN dark matter experiment--a merger of the recently completed Large Underground Xenon (LUX) and Zoned Proportional Scintillation in Liquid Noble Gases (ZEPLIN) experiments --will use a large chamber filled with liquid xenon. In this experiment, a WIMP collision with one of the xenon atoms would produce a small flash of light and drifting electrons.

The two parameters physicists use to describe the sensitivity of this type of experiment are the cross section and the WIMP mass, which is related to how "cold" the particle is: The lower the mass, the zippier it might be.

Although we haven't detected a WIMP yet, we can now put limits on these two properties, because we know what each experiment would be sensitive to (see graph, facing page). There are many combinations of cross section and mass that we've ruled out. We've even excluded the predicted cross section values that first inspired this work. As sensitivity has pushed more and more into uncharted spaces, the excitement (and trepidation) has mounted. WIMPs are running out of places to hide.

There's a tantalizing line near the bottom of this chart: the neutrino floor. At the sensitivity level marked by the neutrino floor, experiments designed to detect dark matter will instead start detecting lots and lots of neutrinos. Neutrinos are nearly massless particles involved in many processes, including fusion and the creation of neutron stars. Many of those detected on Earth come from the Sun or cosmic rays hitting our planet's atmosphere. Hitting the neutrino floor will be a new window into how these particles work, but if WIMPs live beneath that sensitivity level, the sea of neutrinos we expect to detect will complicate the continuing search for dark matter.

With the stakes so high, the WIMP pursuit has to continue. The sense of unease is real, though. If dark matter isn't WIMPs, then what is it?

Fickle Photons

There are actually several options out there beyond the WIMP lamppost. Many arise as solutions to particle physics puzzles and happen, serendipitously, to also be good dark matter candidates. Two of these candidates stand out.

The first one could solve a persistent hole in particle physics called the strong charge-parity problem, which has been around since 1964. The Standard Model predicts that whatever might happen to a particle should also happen to its antimatter counterpart if you mirror-flip the spatial setup. There are some types of particles and situations for which this symmetry doesn't seem to apply.

One proposed solution is a hypothetical particle dubbed the axion. Axions have tiny masses, no charge, and largely no way of interacting with normal matter. They are also predicted to have a very special property: They convert to photons if they find themselves in extremely strong magnetic fields. The photon's energy will correspond to the mass of the axion particle, following the familiar relationship between energy and mass: E=[mc.sup.2]. There is a cross section for this to happen, too--in other words, a level of probability that tells us about the nature of the axion itself.

We can use this transmogrification to our advantage. To find axion dark matter, we need to seek out places where there are strong magnetic fields--or create them ourselves. Researchers with the Axion Dark Matter Experiment (ADMX), for example, have built a tall cavity within a powerful, superconducting magnet to try to magnetically force any galactic axions that might be passing through the cavity to convert to photons. It's the ultimate parlor trick, since light would essentially appear out of thin air! In this case, though, the light ADMX is sensitive to would be at microwave frequencies. To detect the photons, scientists stick "tuning rods" into the cavity and carefully change the distance between them. If axions exist at masses corresponding to microwave energies, then there should be a separation that makes the cavity resonate at the created photons' frequency. It's kind of like tuning the dial on a radio, searching for a signal.

Other scientists look at strongly magnetic astronomical objects. Some have suggested that a transition to axion-like particles (and back again) could explain why more gamma rays reach us from black-hole-powered beacons called blazars than we think should survive the trip (S&T: July 2012, p. 17). Still others have looked for axion-signature photons emerging from the stellar embers called white dwarfs, which often have enormous magnetic fields--thus far with no success.

Another highly magnetic place is the Sun. Researchers at CERN (which runs the Large Hadron Collider), for example, have built an experiment with a powerful magnet that tracks our star like a telescope does its target, in an attempt to make axions streaming from the Sun convert to X-rays.

More fun arises when we ask whether the distribution of axion dark matter in the universe would be different from what might be expected for WIMPs. Axions are pretty "cold" in the sense of CDM. Overall, when we create giant computer simulations of the universe with CDM (S&T: May 2017, p. 34), the picture for WIMPs versus that for axions should be pretty much the same.

Except in some details. Axions are similar to a particle family we call bosons. Photons are bosons. A feature of bosons is that they don't mind overlapping in space. Many of them can crowd into the same point, and they don't get in one another's way in a conventional sense. This is very different from what happens with bosons' cousins, the fermions, which include electrons, protons, and even WIMPs. There can only be one fermion at any given point of space, like bowling balls. We call this rule the Pauli exclusion principle.

This remarkable property of being able to pile up might leave a detectable signature. Within a galaxy such as our very own Milky Way, streams of stars are constantly swirling around, as stars disperse from their star-formation cradles over time (S&T: Apr. 2017, p. 22). If a dense bit of dark matter were to punch through these streams, we might be able to see the effect on the stars' motions. WIMPs will predominantly form cloud-like clumps, whereas axions might collect into super-high-density arcs, called caustics, and these two structures would plow through streams in a different way.

Caustics are a controversial idea, and recent particle physics work suggests that they might never form. On the astronomy side, we're starting to amass the observational data to test the proposal. Europe's Gaia satellite is measuring the positions and motions of more than a billion stars in the Milky Way and might uncover holes in stellar streams. In the mid-2020s, NASA's infrared WFIRST mission will also survey large areas of the sky, and combined with ground-based measurements of how quickly stars are moving along their line of sight, we may be able to make another level of breakthrough.

Getting Warmer

The second non-WIMP candidate we will discuss is generically called warm dark matter, or WDM. The "warm" basically means that its particles have a somewhat greater general speed than CDM, and so some of the smaller clumps that might form in CDM don't in WDM.

While most of our observations of the universe fit neatly into expectations from CDM, there are small but persistent puzzles. Some of these have to do with the existence of the smaller clumps within our Milky Way's galactic family or even at the earliest times of the universe. Astronomers have been having trouble finding as many small satellite galaxies around the Milky Way as CDM predicted should exist, although recent computer simulations show that the predictions probably were just overzealous because they didn't include normal matter (S&T: May 2017, p. 34).

Another cold versus warm signature might be revealed by counting how many small clouds of clumpy hydrogen gas there are in the vast spaces between galaxies. It's the same idea as with satellite galaxies. If dark matter is too warm to clump gravitationally, then the gas--which would ride along with the dark matter as it coalesced into clouds--will be spread out, too.

These hydrogen clouds would normally be impossible to detect. So astronomers use cosmic spotlights. These are bright objects called quasars, brilliant galactic centers powered by accreting black holes, and the light they emit has properties that we have characterized very well. As quasar light traveling to us intercepts these hydrogen clouds, the clouds absorb some of the light. The absorption happens at particular wavelengths that correspond to the electron energy levels in hydrogen atoms.

By the time we detect the quasar light on Earth, it has picked up a series of wavelength-specific "blackouts," small light-absorption signatures from each of the hydrogen clouds along that enormously long line of sight. Furthermore, each cloud's lines are redshifted by the universe's expansion according to how far away the cloud is from us, revealing not only when in cosmic history the light encountered the cloud but also how many clouds it hit. Once we've parsed the quasar's spectrum, we can count up the clouds and tackle the problem of how warm or cold dark matter may be.

One classic WDM candidate is the sterile neutrino. In our now-familiar Standard Model of particle physics, there are three types of regular, or "active," neutrinos. The Standard Model also predicts that neutrinos should have zero mass, and each neutrino type should stay the same forever. However, physicists have shown that neutrinos do have a (small) mass, and it's possible for each of these so-called "flavors" of neutrino to transform into one another.

These particle conundrums open up new doors. One introduces a type of neutrino that only interacts with matter through gravity--unlike the active neutrinos, which also interact via the weak force. This is why it's called the sterile neutrino. These particles have one revealing feature, though: Given a large enough span of time, a sterile neutrino would occasionally convert to a regular, active neutrino plus a photon. The particle masses that could fill the role of dark matter would produce X-ray photons when they decay.

So if dark matter is made of sterile neutrinos, then in places where there is a lot of dark matter, we might see lots of X-ray photons. If we return to our friendly galaxy clusters, which helped start this quest many decades ago, and observe them in X-ray wavelengths, we might detect a ghostly signal in the cluster's spectra that matches none of the known atomic lines and that could come instead from the dark matter halo that each cluster lives in.

Back in 2014, two teams of astronomers suggested that they might have found this sterile neutrino signal, as a 3.5-kiloelectron-volt bump in X-ray emission from 73 galaxy clusters and the Andromeda Galaxy (S&T: Oct. 2014, p. 16). Astronomers are still arguing about how to interpret these X-rays. They may need to wait a while for the data they need to fully explore sterile neutrinos, though: Although both the Chandra and XMM-Newton space telescopes (see page 57) detected the emission, the Japanese X-ray telescope Hitomi would have given us the best tool to date to investigate this hypothesis, and it tragically broke up only a month after launch (S&T: July 2016, p. 11). NASA and the Japanese Space Agency are exploring a replacement for Hitomi.

Still in the Dark

What a journey. In picking up clues from dark matter's gravitational effects in the universe, scientists are making tantalizing connections with long-standing puzzles in particle physics. At the same time, by studying the predictions from particle physics, astronomers' view of the universe is challenged, informed, and enriched. The world of dark matter has benefitted so much from the classic ideas of CDM and WIMPs. They have given us a bright and clear spotlight, under which we have learned a lot about the universe and how it operates.

And yet, what we have covered in these pages only scratches the surface of what dark matter could be. The question of the nature of dark matter remains a profound one in both physics and astrophysics. Over time, and facing many challenges, we are beginning to turn the lights up on the whole cosmic room.

We may reach a breakthrough at any moment. It's even possible that we might confirm more than one of these theories! There is nothing to say that dark matter must consist of only one type of particle; it's entirely possible that it may be made up of more than one of the candidates we've explored here. That would be a very interesting universe, indeed.

After all, we've been surprised before.

The Standard Model

The Standard Model of particle physics explains matter's fundamental building blocks and how they interact. All matter that we know of is made of elementary particles, which come in two types: quarks and leptons. There are six types of quarks and six types of leptons, split into three pairs each. The lightest and most stable of these pairs (the leftmost in this diagram) make up all stable matter in the universe, with heavier particles decaying to become lighter ones.

The Standard Model also includes four force-carrier particles and the Higgs, which are all a type of particle called bosons. The exchange of the force particles results in three of the four fundamental forces: electromagnetism (photons), the strong force (gluons), and the weak force (Z and W bosons). Gravity is not part of the Standard Model.

Fundamental particles acquire mass by interacting with the Higgs field. However, protons and neutrons, which are each composed of three quarks, mostly take their masses from the energy involved with the strong force holding their constituent quarks together. This means that the Higgs is only responsible for about 1 % of the mass of "everyday stuff."--Camille M. Carlisle


* A particle's cross section is the probability that it will interact with another particle.

* LEONIDAS MOUSTAKAS manages the astrophysics section at the Jet Propulsion Laboratory in Pasadena, California. He explores what astronomical observations can teach us about dark matter and is also a deputy project scientist with NASA's upcoming WFIRST mission.

Caption: SEEING THE UNSEEN This composite image reveals the distribution of matter in the galaxy cluster Abell 1689. Using the observed positions of 135 lensed images (smears) of 42 background galaxies, astronomers calculated the locations and amount of matter concentrations. The matter map, tinted blue, is overlaid here on an image from the Hubble Space Telescope. If the cluster's gravity came only from the visible galaxies, the lensing distortions would be much weaker.


Caption: TIGHT SQUEEZE More than a dozen experiments around the world have failed to detect WIMPs, ruling out an increasingly large range of particle characteristics (red). The white region between the red and orange curves marks what's left for scientists to explore. It still includes a fair amount of "generic WIMP" territory, a favored region (purple). The bumps in the neutrino background are from different kinds of neutrinos. Liquid xenon experiments are pushing the red curve down, whereas those using lighter elements are pushing it toward the lower left.

Caption: CATCHING DARK MATTER Left: The idea behind experiments such as LUX-ZEPLIN (LZ) is to catch WIMP dark matter particles interacting with normal matter. In LZ's case, the normal matter is about 10 metric tons of liquid xenon. When a WIMP collides with a xenon atom, the atom emits light and causes a burst of electrons in the tank. Sensors at the top and bottom detect the initial light flash. An electric field pushes the electrons to the top of the chamber, where they generate a second flash of light (red). Right: A team member installs photomultiplier tubes in the bottom array of the LUX experiment, LZ's precursor.

Caption: FORCING PHOTONS TO CHANGE The Axion Dark Matter Experiment (ADMX) hides a microwave cavity inside a large superconducting magnet (the cavity is about as wide as the inner circle on the top of the setup in this photo). The magnetic field should convert any axions of a certain mass that are passing through the cavity into microwave photons. Researchers slowly change the position of rods inside the cavity, trying to make the cavity resonate. The resonant frequency would correspond to the photons' frequency and therefore to the axions' mass.

Caption: SOLAR AXION SEARCH The CERN Axion Solar Telescope (CAST) points a cryogenic, dipole-magnet "telescope" at the Sun, in an attempt to convert solar axions into X-rays.

Caption: Cosmic Slice 3D MAP Astronomers created this 3D slice of the cosmic web by mapping the distribution of hydrogen gas, which left its imprint in the spectra of distant background galaxies. (This project used star-forming galaxies, not quasars as discussed in the text.) Brighter colors represent higher density. of cosmic structure.

Caption: COLD VS. WARM DARK MATTER The average speed of dark matter particles affects how easily the particles clump together--and, therefore, how easily small lumps of material can form. Cold dark matter (which is slower, left) clumps more easily than warm dark matter (faster, right), as apparent in these simulations of a 5-million-light-year-wide box

Caption: HINT OF DARK MATTER? An unexpected "bump" in the X-ray emission from the Perseus Cluster (background image, shown in X-rays) and more than 70 other galaxy clusters might be created when sterile neutrinos transform into active ones. The slight bump is circled in the spectrum. The cluster's X-ray glow in the background image spans roughly 500,000 light-years.

Caption: CANDIDATE MASSES The playground of potential dark matter candidates spans roughly 90 orders of magnitude in particle mass. (The above is thus not an exact scale.) All matter can be described as both a particle and a wave, and dark matter must fit within a galaxy, so the bottom limit corresponds to a particle wavelength that is larger than a galaxy. The upper limit of about 1 solar mass comes from gravitational lensing studies, which have essentially ruled out primordial black holes down to this size. There are many candidates in addition to the three most popular ones shown here, but we've excluded them for simplicity's sake. For context, the proton's mass is about 1 GeV.
Baryonic matter    4.9%
Dark matter       25.9%
Dark energy       69.2%

Note: Table made from pie chart.
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Author:Moustakas, Leonidas
Publication:Sky & Telescope
Date:Aug 1, 2017
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