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 gentle round itself and might detect its impact on the sunshine left over from the primordial plasma of the recent massive bang. We’ve measured these alerts with beautiful precision. We’ve each motive to imagine darkish matter is all over the place. But we nonetheless don’t know what it’s.
We’ve been attempting to detect darkish matter in experiments for many years now, to no avail. Perhaps our first detection is simply across the nook. However the lengthy wait has prompted some darkish matter hunters to marvel whether or not we’re trying within the unsuitable place or within the unsuitable manner. Many experimental efforts have centered on a comparatively small variety of doable identities for darkish matter—those who appear prone to concurrently clear up 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 similar 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 number of good theories and loads of experimental choices for testing them. However the straightforward roads have principally been traveled, and darkish matter has proved extra mysterious than we ever imagined. It’s fully doable that darkish matter behaves in a manner 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 way of any mechanism besides gravity, it is going to be nearly unattainable to detect it in a laboratory. In that case, we will nonetheless hope to find out about darkish matter by mapping its presence all through the universe. However there’s a likelihood that darkish matter will show so elusive we might by no means perceive its true nature.
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On a heat summer time 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 fruits 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 prospects 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 that may represent darkish matter. By now they’ve eradicated most of the easiest, best prospects. However, 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 hanging modifications to the legal guidelines of gravity as we at present perceive them, that are based mostly on Einstein’s normal principle of relativity. Updating the principle in a manner that avoids the necessity for darkish matter—both by adjusting the equations of normal relativity whereas holding the identical underlying framework or by introducing some new paradigm that replaces normal relativity altogether—appears exceptionally tough.
The modifications must mimic the results of darkish matter in astrophysical programs starting from big clusters of galaxies to the Milky Method’s smallest satellite tv for pc galaxies. In different phrases, they would wish to use throughout an unlimited 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 clarify why, if darkish matter is only a modification to gravity—which is universally related to all matter—not all galaxies and clusters seem to comprise darkish matter. Furthermore, probably the most refined makes an attempt to formulate self-consistent theories of modified gravity to clarify away darkish matter find yourself invoking a sort of darkish matter anyway, to match the ripples we observe within the cosmic microwave background, leftover gentle from the massive 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 gentle is a straightforward thought. In reality, we have already got an instance of such darkish matter within the type of neutrinos—almost massless particles which might be ubiquitous however hardly ever work together with different matter. It’s simply that we already know neutrinos can’t account for many of the darkish matter within the universe. At most, they will make up about 1 % of it.
So what concerning the different 99 %? Might 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 Commonplace Mannequin of particle physics? Might darkish matter really feel new forces that the recognized particles don’t (in the identical manner that darkish matter doesn’t seem to really feel electromagnetic forces), or may or not it’s linked to new basic ideas of nature? Might darkish matter clear up excellent puzzles lurking throughout the well-measured physics of the Commonplace Mannequin, or may 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 most well-liked proposals for the id of darkish matter are the weakly interacting huge 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 Commonplace Mannequin. A proton’s mass is slightly 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 items of vitality utilizing Einstein’s E = mc2.) The basic model of a WIMP is a brand new particle that interacts straight with the W and Z bosons recognized to hold the weak nuclear pressure (therefore the “W” in WIMP). Such particles seem naturally in fashions of supersymmetry, the place each recognized particle additionally has a heavier counterpart referred to as a superpartner. A decade and a half in the past our subject hoped that the Giant Hadron Collider close to Geneva would discover superpartners, however we had no such luck. If supersymmetry exists, the superpartners should 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 recognized particles collided. And an analogous response occurred in reverse—when WIMPs collided, they created common particles. If the massive bang hadn’t produced WIMPs initially, the recognized particles would have made them. And collisions of WIMPs that transmuted their vitality into recognized 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 illustration. This mechanism is easy and interesting.
WIMPs attraction to many experimentalists as a result of they have to work together considerably with the recognized particles—that’s how they arrive at the correct amount of darkish matter. There are three basic methods to seek for WIMPs: collider experiments, the place we hope to reprofessionalduce the situations of the early universe by colliding Commonplace 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 strategy particularly exams precisely the identical harmful processes that might have set the abundance of WIMPs within the universe. Due to this fact, if these reactions behave in the identical manner as we speak as they did within the early universe, we have now a definitive prediction for the way typically they happen. For the primary two approaches, the predictions usually are not so clear-reduce. In collider searches, our skill to detect WIMPs depends upon how heavy they’re: extra huge WIMPs might require extra vitality to supply than the collider has out there. And in direct detection, we don’t know the way typically WIMPs will stumble upon common particles.
Astrophysical observations—oblique detection—have revealed a number of alerts that is perhaps hints of darkish matter annihilation, however there are additionally extra mundane explanations for what we see. For instance, the Galactic Heart GeV Extra is a glow of gamma-ray gentle from the center of the Milky Method; it has the correct price and the correct 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 related 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 can 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) may take a look at 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 save lots of WIMP principle. In some fashions, the annihilation course of that created WIMPs within the early universe switches off at later instances. In these instances, nevertheless, 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 following era of experiments might be so delicate that they are going to begin detecting neutrinos from the solar streaming by way 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 just lately we haven’t had almost the identical skill to check it. Just like the WIMP, it will be a brand new basic particle, although a lot tinier: axions are far lighter than any recognized particle, even neutrinos. If these particles exist—whether or not they make up all of the darkish matter or not—they may reclear up long-standing puzzles in our understanding of the sturdy pressure, which holds atomic nuclei together. Plus, axion theories make distinct predictions: if the mass of the axion, you may estimate how strongly it interacts with the recognized particles. Sadly, these interactions depend upon the axion mass and might be exceedingly weak for the lighter axions.
Nonetheless, axion interactions may have hanging results as a result of to account for darkish matter they must be so plentiful that they might manifest as a wave fairly than as particular person particles. Based on quantum mechanics, each basic 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 gentle that we may anticipate to see such quantum results over distances similar to the everyday measurement of an experiment on Earth.
Due to how weakly QCD axions are anticipated to work together with common matter, fewer experiments have appeared for them, they usually have searched in solely a tiny fraction of the doable mass vary. New detection methods and quantum sensor applied sciences, nevertheless, 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 tasks similar to DMRadio promise to tremendously prolong the search.
Over the following decade dramatic experimental advances will take a look at 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 may depart it at that—there’s a very good likelihood that these methods will give us the answer.
And but … although the WIMP and the axion are lovely 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 every part that darkish matter must do to clarify the universe, however every invokes completely different particles and forces to make it occur. Theorists have completely 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 illustration, embody huge 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 might 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 could be observably completely different.
What concerning the different finish of the mass scale? The smallest clumps of darkish matter we will straight observe are tens of hundreds of thousands of instances the mass of the solar. Particular person darkish matter particles needs to be smaller than that, however how a lot smaller? If darkish matter have been made from dense, darkish objects—typically referred to as huge compact halo objects (MACHOs), as a tongue-in-cheek distinction to WIMPs—then their gravity may deflect gentle and disrupt orbits as they barreled by way of the galaxy in methods we may see. MACHOs may take the type of tiny black holes, born within the first moments after the massive 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 manner these black holes may account for all the darkish matter could be if they’d about the identical mass because the asteroids in our photo voltaic system, between round 100 billion and 100,000 trillion metric tons. That might 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 observready universe and the radius of a proton is just about 41 orders of magnitude.) That’s various 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 way 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 could be only a few of them out in house. But they may present up in particle accelerators, particularly in the event that they have been additionally comparatively gentle. Gentle darkish matter and darkish sectors may additionally exist with out counting on the WIMP mechanism to supply the correct amount of darkish matter—there are myriad different prospects for find out how to generate the noticed abundance of darkish matter.
If the darkish sector is on the market, we’d like new experimental strategies to search out it. Basic WIMP detectors, as an illustration, 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 expertise 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 may assist.
The one manner we all know to look over such a variety of prospects is to construct many small experiments, every delicate to various kinds of darkish matter, fairly than focusing our assets on a number of large tasks. 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 may 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 take a look at primordial black holes, as an illustration, with this type of search; that is how we all know that if black holes make up all of the darkish matter, they will’t be lighter than about 100 billion metric tons (lighter black holes decay quicker).
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 strategies to assist us glean as a lot as we will from these information. Such observations may present new clues to the elemental nature of darkish matter that can complement what we will 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 extensively (at a shallower stage) to discover as many prospects as we will.
If we’re fortunate, certainly one of these experiments will make a transparent detection. As soon as that occurs, it’ll set off a paradigm shift. The broad and assorted search will collapse to deal with that sign, and we’ll plan future experiments to raised perceive it. A discovery would additionally immediate theorists to check the larger image of find out how to join darkish matter with the remainder of the particle zoo we’re accustomed to.
However what if none of those experiments finds a sign? Maybe physicists on the subsequent Snowmass Course of, a few decade therefore, should use null outcomes to chart the path for future searches. We will’t deny this consequence could be disappointing, however it will 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 following insights are simply as essential as those who affirm a specific thought is appropriate. If we may predict with certainty what darkish matter will develop into, it will imply that we already know the reply, making our jobs a lot much less thrilling. And though we will’t say precisely when and even whether or not we’ll discover darkish matter, we all know that the universe is full of it. We’re optimistic that the following years of our quest will lead us to a deeper understanding of what it’s.
