Most of the matter in our universe is invisible. We will measure the gravitational pull of this “dark matter” on the orbits of stars and galaxies. We will see the way in which it bends mild round itself and may detect its impact on the sunshine left over from the primordial plasma of the new huge bang. We have now measured these indicators with beautiful precision. We have now each purpose to consider darkish matter is in every single place. But we nonetheless don’t know what it’s.
We have now been making an attempt to detect darkish matter in experiments for many years now, to no avail. Possibly our first detection is simply across the nook. However the lengthy wait has prompted some darkish matter hunters to wonder if we’re wanting within the improper place or within the improper approach. Many experimental efforts have centered on a comparatively small variety of doable identities for darkish matter—people who appear more likely to concurrently remedy different issues in physics. Nonetheless, there’s no assure that these different puzzles and the darkish matter quandary are associated. More and more, physicists acknowledge that we might must seek for a wider vary of doable explanations. The scope of the issue is each intimidating and exhilarating.
On the identical time, we’re beginning to grapple with the sobering concept that we might by no means nail down the character of darkish matter in any respect. Within the early days of darkish matter looking, this notion appeared absurd. We had a lot of good theories and loads of experimental choices for testing them. However the straightforward roads have largely been traveled, and darkish matter has proved extra mysterious than we ever imagined. It’s fully doable that darkish matter behaves in a approach that present experiments aren’t well-suited to detect—and even that it ignores common matter utterly. If it doesn’t work together with customary atoms by means of any mechanism besides gravity, it is going to be virtually unimaginable to detect it in a laboratory. In that case, we are able to nonetheless hope to find out about darkish matter by mapping its presence all through the universe. However there’s a probability that darkish matter will show so elusive we might by no means perceive its true nature.
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On a heat summer season night in August 2022 we huddled with a number of different physicists round a desk on the College of Washington. We have been there to debate the end result of the “Snowmass Process,” a year-lengthy research that the U.S. particle physics neighborhood undertakes each decade or so to agree on priorities for future analysis. We have been tasked with summing up the progress and potential of darkish matter searches. The job of speaking simply what number of potentialities there are for explaining darkish matter, and the numerous concepts that exist to discover them, felt daunting.
We’re at a particular second within the quest for darkish matter. For the reason that Nineties 1000’s of investigators have searched exhaustively for particles which may represent darkish matter. By now they’ve eradicated most of the easiest, best potentialities. Nonetheless, most physicists are satisfied darkish matter is on the market and represents some distinct type of matter.
A universe with out darkish matter would require placing modifications to the legal guidelines of gravity as we presently perceive them, that are based mostly on Einstein’s normal principle of relativity. Updating the principle in a approach that avoids the necessity for darkish matter—both by adjusting the equations of normal relativity whereas preserving the identical underlying framework or by introducing some new paradigm that replaces normal relativity altogether—appears exceptionally tough.
The adjustments must mimic the consequences of darkish matter in astrophysical methods starting from large clusters of galaxies to the Milky Method’s smallest satellite tv for pc galaxies. In different phrases, they would wish to use throughout an infinite vary of scales in distance and time, with out contradicting the host of different exact measurements we’ve gathered about how gravity works. The modifications would additionally want to elucidate why, if darkish matter is only a modification to gravity—which is universally related to all matter—not all galaxies and clusters seem to include darkish matter. Furthermore, probably the most refined makes an attempt to formulate self-consistent theories of modified gravity to elucidate away darkish matter find yourself invoking a kind of darkish matter anyway, to match the ripples we observe within the cosmic microwave background, leftover mild from the large bang.
The scope of the darkish matter drawback is each intimidating and exhilarating.
In distinction, positing a brand new kind of matter that merely doesn’t work together with mild is an easy thought. Actually, we have already got an instance of such darkish matter within the type of neutrinos—almost massless particles which might be ubiquitous however not often work together with different matter. It’s simply that we already know neutrinos can’t account for a lot of the darkish matter within the universe. At most, they’ll make up about 1 p.c of it.
So what in regards to the different 99 p.c? May darkish matter be the tip of an iceberg of discovery, the primary revelation of a number of new particles that aren’t a part of the Customary Mannequin of particle physics? May darkish matter really feel new forces that the identified particles don’t (in the identical approach that darkish matter doesn’t seem to really feel electromagnetic forces), or might or not it’s linked to new elementary rules of nature? May darkish matter remedy excellent puzzles lurking inside the well-measured physics of the Customary Mannequin, or might it reveal the earliest moments of the universe’s historical past? Proper now the solutions to all these questions are a definitive “maybe”—however the potential energy of such a discovery drives us onward.
Two of the preferred proposals for the id of darkish matter are the weakly interacting large particle (WIMP) and the axion of quantum chromodynamics (QCD). These concepts have formed how theorists take into consideration darkish matter and impressed most of the experiments looking for it.
WIMPs are hypothetical secure particles with plenty similar to these of particles within the Customary Mannequin. A proton’s mass is just below 1 GeV/c², and most WIMP searches have centered on the mass vary between 10 and 1,000 GeV/c². (Particle physicists discover it handy to measure plenty in models of vitality utilizing Einstein’s E = mc2.) The traditional model of a WIMP is a brand new particle that interacts instantly with the W and Z bosons identified to hold the weak nuclear pressure (therefore the “W” in WIMP). Such particles seem naturally in fashions of supersymmetry, the place each identified particle additionally has a heavier counterpart referred to as a superpartner. A decade and a half in the past our subject hoped that the Massive Hadron Collider close to Geneva would discover superpartners, however we had no such luck. If supersymmetry exists, the superpartners have to be heavier than we’d initially anticipated. Moreover, though many variations of supersymmetry predict WIMP darkish matter, the converse isn’t true; WIMPs are viable darkish matter candidates even in a universe with out supersymmetry.
Tim M. P. Tait and Jen Christiansen
One of many causes many physicists love the WIMP thought is that these particles naturally would have generated the identical quantity of darkish matter within the universe that we observe. Because the pondering goes, when the cosmos was a lot smaller, denser and warmer than it’s now, even weak interactions have been sufficient to supply WIMPs when identified particles collided. And an analogous response occurred in reverse—when WIMPs collided, they created common particles. If the large bang hadn’t produced WIMPs initially, the identified particles would have made them. And collisions of WIMPs that transmuted their vitality into identified particles would have destroyed most WIMPs, leaving solely a residual abundance. A WIMP with a mass round that of the Higgs boson would produce the correct quantity of darkish matter, as an example. This mechanism is straightforward and interesting.
WIMPs enchantment to many experimentalists as a result of they have to work together considerably with the identified particles—that’s how they arrive at the correct amount of darkish matter. There are three traditional methods to seek for WIMPs: collider experiments, the place we hope to reprofessionalduce the circumstances of the early universe by colliding Customary Mannequin particles collectively to generate darkish matter; direct-detection experiments, which use extraordinarily delicate detectors to search for seen particles “jumping” when they’re struck by a darkish matter particle; and oblique detection, the place we glance out into house to seek for acquainted particles being produced when darkish matter particles collide and annihilate each other. The third method specifically exams precisely the identical harmful processes that may have set the abundance of WIMPs within the universe. Due to this fact, if these reactions behave in the identical approach immediately as they did within the early universe, we have now a definitive prediction for a way usually they happen. For the primary two approaches, the predictions aren’t so clear-lower. In collider searches, our means to detect WIMPs relies on how heavy they’re: extra large WIMPs might require extra vitality to supply than the collider has out there. And in direct detection, we don’t know the way usually WIMPs will stumble upon common particles.
Astrophysical observations—oblique detection—have revealed a number of indicators that could be hints of darkish matter annihilation, however there are additionally extra mundane explanations for what we see. For instance, the Galactic Middle GeV Extra is a glow of gamma-ray mild from the center of the Milky Method; it has the proper fee and the proper vitality to be a WIMP-annihilation sign. It was found in 2009, so why haven’t we declared victory? Sadly, we all know that sure spinning neutron stars can produce gamma rays at comparable energies, and it’s fairly doable that the surplus is the primary signal of a brand new inhabitants of such stars. We hope this query will probably be resolved within the coming years: discovering a counterpart sign in a direct-detection or collider experiment would help the darkish matter interpretation, whereas discovering radiation from the neutron stars at different wavelengths would rule it out.
Within the subsequent decade or so future massive gamma-ray telescopes (such because the Cherenkov Telescope Array being in-built Chile and Spain and the Southern Vast-subject Gamma-ray Observatory deliberate for someplace in South America) might check the WIMP mechanism for producing darkish matter as much as the very best plenty the place it’s viable. But even when we don’t observe darkish matter annihilation, there are loopholes to avoid wasting WIMP principle. In some fashions, the annihilation course of that created WIMPs within the early universe switches off at later instances. In these circumstances, nonetheless, WIMPs ought to usually nonetheless present up in collider experiments and direct detection.
If we ask “What could dark matter be?” the chances are almost countless.
Direct-detection experiments have made superb progress in bettering their sensitivity to uncommon occasions. Inside 10 years the subsequent technology of experiments might be so delicate that they’ll begin detecting neutrinos from the solar streaming by means of the detector. Till we attain that time, there are not any different processes that would masquerade as darkish matter, and no seemingly insurmountable technical challenges stand in the way in which. There are nonetheless many easy WIMP fashions that would present up on this vary.
The QCD axion is a really completely different kind of darkish matter candidate, and till not too long ago we haven’t had almost the identical means to check it. Just like the WIMP, it might be a brand new elementary particle, although a lot tinier: axions are far lighter than any identified particle, even neutrinos. If these particles exist—whether or not they make up all of the darkish matter or not—they might reremedy long-standing puzzles in our understanding of the sturdy pressure, which holds atomic nuclei together. Plus, axion theories make distinct predictions: if you realize the mass of the axion, you possibly can estimate how strongly it interacts with the identified particles. Sadly, these interactions rely on the axion mass and will be exceedingly weak for the lighter axions.
Nonetheless, axion interactions might have placing results as a result of to account for darkish matter they must be so plentiful that they’d manifest as a wave reasonably than as particular person particles. In accordance with quantum mechanics, each elementary particle can be a wave and has an related wavelength inversely proportional to its mass. At scales smaller than this wavelength, the classical image of a particle breaks down. Axions are so mild that we might count on to see such quantum results over distances similar to the everyday dimension of an experiment on Earth.
Due to how weakly QCD axions are anticipated to work together with common matter, fewer experiments have seemed for them, they usually have searched in solely a tiny fraction of the doable mass vary. New detection methods and quantum sensor applied sciences, nonetheless, have opened up prospects for looking the QCD axion over many orders of magnitude in mass. The newest model of a long-running experiment referred to as ADMX-G2 is extraordinarily delicate, and upcoming initiatives resembling DMRadio promise to significantly lengthen the search.
Over the subsequent decade dramatic experimental advances will check each the WIMP and the QCD axion over the majority of their pure mass vary for the primary time. The theoretical groundwork has been laid, and the plans for experiments are in place. We might go away it at that—there’s an excellent probability that these methods will give us the answer.
And but … regardless that the WIMP and the axion are stunning concepts, there isn’t any assure that the universe conforms to our aesthetic preferences. And if we ask, “What could dark matter be?” the chances are almost countless.
An total panorama of theories manages to explain all the things that darkish matter must do to elucidate the universe, however every invokes completely different particles and forces to make it occur. Theorists have totally mapped out which concepts have a hope of working and which of them are inconsistent with observations. Most of the viable hypotheses are surprisingly completely different from WIMPs or axions. Some, as an example, embrace large combination objects composed of many tinier constituents—akin to darkish matter atoms composed of various darkish particles.
There’s a restrict to how small darkish matter particles will be. In the event that they have been a lot lighter than axions—about 25 orders of magnitude lighter than the mass of the electron—their wavelengths might be near the scale of star clusters or small galaxies. If this have been the case, the distribution of darkish matter and its gravitational footprint can be observably completely different.
What in regards to the different finish of the mass scale? The smallest clumps of darkish matter we are able to instantly observe are tens of hundreds of thousands of instances the mass of the solar. Particular person darkish matter particles must be smaller than that, however how a lot smaller? If darkish matter have been fabricated from dense, darkish objects—usually referred to as large compact halo objects (MACHOs), as a tongue-in-cheek distinction to WIMPs—then their gravity might deflect mild and disrupt orbits as they barreled by means of the galaxy in methods we might see. MACHOs might take the type of tiny black holes, born within the first moments after the large bang. These black holes wouldn’t kind from stars—as a result of darkish matter predates stars—and might be a lot lighter than the solar. The one approach these black holes might account for all the darkish matter can be if that they had about the identical mass because the asteroids in our photo voltaic system, between round 100 billion and 100,000 trillion metric tons. That may give them particular person plenty one one-thousandth of the mass of the moon, making them 75 orders of magnitude heavier than the smallest doable darkish matter particles. (For comparability, the ratio between the radius of our observin a position universe and the radius of a proton is just about 41 orders of magnitude.) That’s numerous floor to cowl.
And within the huge area between these two extremes, we have now a plethora of choices. The method that would produce WIMPs within the early universe would additionally work for a lot of different particles. If darkish matter have been lighter than a proton and born by means of this mechanism, it might be simply certainly one of many new particles inhabiting a “dark sector” of physics. These different particles would usually be unstable, so there can be only a few of them out in house. But they might present up in particle accelerators, particularly in the event that they have been additionally comparatively mild. Mild darkish matter and darkish sectors might additionally exist with out counting on the WIMP mechanism to supply the correct amount of darkish matter—there are myriad different potentialities for easy methods to generate the noticed abundance of darkish matter.
If the darkish sector is on the market, we want new experimental strategies to seek out it. Basic WIMP detectors, as an example, lose sensitivity as soon as the darkish matter is way lighter than atomic nuclei as a result of they search for a powerful “kick” on nuclei by incoming darkish matter. New know-how can search indicators of electrons (that are 2,000 instances lighter than protons) being kicked instead or use much more artistic methods to detect tiny vitality transfers from darkish matter to plain particles. The latest creation of ultrasensitive quantum sensors might assist.
The one approach we all know to look over such a variety of potentialities is to construct many small experiments, every delicate to various kinds of darkish matter, reasonably than focusing our sources on a number of enormous initiatives. We will additionally use these small experiments to develop new applied sciences and check out novel concepts; if a type of methods proves highly effective or detects one thing that might be an preliminary trace of darkish matter, we might then scale it up.
Oblique-detection searches in house already span an enormous vary of vitality scales. If the darkish matter have been slowly decaying into seen particles, with a typical lifetime so long as a billion instances the present age of the universe, we’d realize it by now for a lot of doable darkish matter plenty. We will check primordial black holes, as an example, with this type of search; that is how we all know that if black holes make up all of the darkish matter, they’ll’t be lighter than about 100 billion metric tons (lighter black holes decay sooner).
And even when we don’t see a sign, we’ll proceed to study extra about darkish matter by mapping its gravity in house. Present and upcoming devices will measure the distribution of stars and distant galaxies with unbelievable precision and depth. Developments in precision cosmology and synthetic intelligence are driving methods to assist us glean as a lot as we are able to from these information. Such observations might present new clues to the elemental nature of darkish matter that can complement what we are able to study within the lab.
After all of the Snowmass discussions, the physics neighborhood opted to embrace a balanced technique. We plan to delve deeply into our favourite theories of darkish matter whereas additionally looking out extensively (at a shallower stage) to discover as many potentialities as we are able to.
If we’re fortunate, certainly one of these experiments will make a transparent detection. As soon as that occurs, it can set off a paradigm shift. The broad and assorted search will collapse to give attention to that sign, and we’ll plan future experiments to higher perceive it. A discovery would additionally immediate theorists to check the larger image of easy methods to join darkish matter with the remainder of the particle zoo we’re conversant in.
However what if none of those experiments finds a sign? Maybe physicists on the subsequent Snowmass Course of, a few decade therefore, must use null outcomes to chart the course for future searches. We will’t deny this end result can be disappointing, however it might nonetheless depend as a serious achievement. Science strikes ahead one step at a time, and the outcomes that train us the place to not search for the subsequent insights are simply as necessary as people who verify a selected thought is appropriate. If we might predict with certainty what darkish matter will grow to be, it might imply that we already know the reply, making our jobs a lot much less thrilling. And though we are able to’t say precisely when and even whether or not we’ll discover darkish matter, we all know that the universe is stuffed with it. We’re optimistic that the subsequent years of our quest will lead us to a deeper understanding of what it’s.
