By observing what happens in the vacuum around negatively charged electrons—thought to be swarming with clouds of as-yet-unseen particles—researchers can create models of particle behavior, DeMille said. Alternative physics theories offer answers where the Standard Model falls short. But other theories hint that there are as-yet-undiscovered heavy particles.
Those hypothetical heavyweight particles would deform electrons to a degree that researchers should be able to observe, the authors of the new study said.
These Dark Electrons - Kindle edition by Michael D. Britton. Download it once and read it on your Kindle device, PC, phones or tablets. Use features like. A balloon-borne experiment in Antarctica detected a high number of energetic electrons from space that may be the signature of dark matter.
To test those predictions, new experiments peered at electrons at a resolution 10 times greater than previous efforts, completed in ; both investigations were conducted by the research project Advanced Cold Molecule Electron Electric Dipole Moment Search ACME. For the new study, ACME researchers directed a beam of cold thorium-oxide molecules at a rate of 1 million per pulse, 50 times per second, into a relatively small chamber in a basement at Harvard University.
The scientists zapped the molecules with lasers and studied the light reflected back by the molecules; bends in the light would point to an electric dipole moment. But there were no twists in the reflected light, and this result casts a dark shadow over the physics theories that predicted heavy particles around electrons, the researchers said. Their angular positions are unaffected.
The effect is not detectable for any one structure since the true shape is not known, but can be measured by averaging over many structures assuming Earth is not at a special location in the Universe. The effect was predicted quantitatively by Nick Kaiser in , and first decisively measured in by the 2dF Galaxy Redshift Survey. In astronomical spectroscopy , the Lyman-alpha forest is the sum of the absorption lines arising from the Lyman-alpha transition of neutral hydrogen in the spectra of distant galaxies and quasars.
Lyman-alpha forest observations can also constrain cosmological models.
To build tomorrow's quantum computers, some researchers are turning to dark excitons, which are bound pairs of an electron and the absence of an electron called a hole. The development of spintronics depends on materials that guarantee control over the flow of magnetically polarized currents. Sign in to get notified via email when new comments are made. Retrieved 6 August To test those predictions, new experiments peered at electrons at a resolution 10 times greater than previous efforts, completed in ; both investigations were conducted by the research project Advanced Cold Molecule Electron Electric Dipole Moment Search ACME. If we do, we find the angular momentum and charge of the electron are too large for a black hole of the electron's mass:
Dark matter can refer to any substance that interacts predominantly via gravity with visible matter e. Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard baryonic matter, such as protons or neutrons.
Baryons protons and neutrons make up ordinary stars and planets. However, baryonic matter also encompasses less common black holes , neutron stars , faint old white dwarfs and brown dwarfs , collectively known as massive compact halo objects MACHOs , which can be hard to detect. Candidates for non-baryonic dark matter are hypothetical particles such as axions , sterile neutrinos , weakly interacting massive particles WIMPs , gravitationally-interacting massive particles GIMPs , or supersymmetric particles. The three neutrino types already observed are indeed abundant, and dark, and matter, but because their individual masses—however uncertain they may be—are almost certainly tiny, they can only supply a small fraction of dark matter, due to limits derived from large-scale structure and high- redshift galaxies.
Unlike baryonic matter, nonbaryonic matter did not contribute to the formation of the elements in the early universe Big Bang nucleosynthesis  and so its presence is revealed only via its gravitational effects, or weak lensing. In addition, if the particles of which it is composed are supersymmetric, they can undergo annihilation interactions with themselves, possibly resulting in observable by-products such as gamma rays and neutrinos indirect detection. If dark matter is as common as observations suggest, an obvious question is whether it can form objects equivalent to planets , stars , or black holes.
The answer has historically been that it cannot,   because of two factors:. This question has been debated heavily during recent years. In — the idea of dense dark matter or dark matter being black holes, including primordial black holes , made a comeback  following results of gravitation wave detection. These were again ruled out in December ,  but research and theories based on these still continue as at , including approaches to dark matter cooling,   and the question is by no means settled.
Dark matter can be divided into cold , warm , and hot categories. Primordial density fluctuations smaller than this length get washed out as particles spread from overdense to underdense regions, while larger fluctuations are unaffected; therefore this length sets a minimum scale for later structure formation.
The categories are set with respect to the size of a protogalaxy an object that later evolves into a dwarf galaxy: Mixtures of the above are also possible: Cold dark matter leads to a bottom-up formation of structure while hot dark matter would result in a top-down formation scenario; [ clarification needed ] the latter is excluded by high-redshift galaxy observations.
These categories also correspond to fluctuation spectrum effects and the interval following the Big Bang at which each type became non-relativistic. Candidate particles can be grouped into three categories on the basis of their effect on the fluctuation spectrum Bond et al. If the dark matter is composed of abundant light particles which remain relativistic until shortly before recombination, then it may be termed "hot". The best candidate for hot dark matter is a neutrino Such particles are termed "warm dark matter", because they have lower thermal velocities than massive neutrinos Any particles which became nonrelativistic very early, and so were able to diffuse a negligible distance, are termed "cold" dark matter CDM.
There are many candidates for CDM including supersymmetric particles. Another approximate dividing line is that warm dark matter became non-relativistic when the universe was approximately 1 year old and 1 millionth of its present size and in the radiation-dominated era photons and neutrinos , with a photon temperature 2. Standard physical cosmology gives the particle horizon size as 2 ct speed of light multiplied by time in the radiation-dominated era, thus 2 light-years. A region of this size would expand to 2 million light-years today absent structure formation. The actual FSL is approximately 5 times the above length, since it continues to grow slowly as particle velocities decrease inversely with the scale factor after they become non-relativistic.
In this example the FSL would correspond to 10 million light-years, or 3 mega parsecs , today, around the size containing an average large galaxy. Conversely, much lighter particles, such as neutrinos with masses of only a few eV, have FSLs much larger than a protogalaxy, thus qualifying them as hot. Cold dark matter offers the simplest explanation for most cosmological observations. It is dark matter composed of constituents with an FSL much smaller than a protogalaxy. This is the focus for dark matter research, as hot dark matter does not seem capable of supporting galaxy or galaxy cluster formation, and most particle candidates slowed early.
The constituents of cold dark matter are unknown. Studies of Big Bang nucleosynthesis and gravitational lensing convinced most cosmologists       that MACHOs   cannot make up more than a small fraction of dark matter. Warm dark matter comprises particles with an FSL comparable to the size of a protogalaxy.
Predictions based on warm dark matter are similar to those for cold dark matter on large scales, but with less small-scale density perturbations. This reduces the predicted abundance of dwarf galaxies and may lead to lower density of dark matter in the central parts of large galaxies. Some researchers consider this a better fit to observations. No known particles can be categorized as warm dark matter. A postulated candidate is the sterile neutrino: Some modified gravity theories, such as scalar—tensor—vector gravity , require "warm" dark matter to make their equations work.
Hot dark matter consists of particles whose FSL is much larger than the size of a protogalaxy. The neutrino qualifies as such particle. They were discovered independently, long before the hunt for dark matter: Neutrinos interact with normal matter only via gravity and the weak force , making them difficult to detect the weak force only works over a small distance, thus a neutrino triggers a weak force event only if it hits a nucleus head-on. The three known flavours of neutrinos are the electron , muon , and tau. Their masses are slightly different. Neutrinos oscillate among the flavours as they move.
It is hard to determine an exact upper bound on the collective average mass of the three neutrinos or for any of the three individually. CMB data and other methods indicate that their average mass probably does not exceed 0.
Thus, observed neutrinos cannot explain dark matter. Because galaxy-size density fluctuations get washed out by free-streaming, hot dark matter implies that the first objects that can form are huge supercluster -size pancakes, which then fragment into galaxies. Deep-field observations show instead that galaxies formed first, followed by clusters and superclusters as galaxies clump together.
If dark matter is made up of sub-atomic particles, then millions, possibly billions, of such particles must pass through every square centimeter of the Earth each second. Another candidate is heavy hidden sector particles that only interact with ordinary matter via gravity. These experiments can be divided into two classes: Direct detection experiments aim to observe low-energy recoils typically a few keVs of nuclei induced by interactions with particles of dark matter, which in theory are passing through the Earth.
After such a recoil the nucleus will emit energy as, e. To do this effectively, it is crucial to maintain a low background, and so such experiments operate deep underground to reduce the interference from cosmic rays. These experiments mostly use either cryogenic or noble liquid detector technologies.
Noble liquid detectors detect scintillation produced by a particle collision in liquid xenon or argon. Cryogenic detector experiments include: Both of these techniques focus strongly on their ability to distinguish background particles which predominantly scatter off electrons from dark matter particles that scatter off nuclei. Currently there has been no well-established claim of dark matter detection from a direct detection experiment, leading instead to strong upper limits on the mass and interaction cross section with nucleons of such dark matter particles.
This results from the expectation that as the Earth orbits the Sun, the velocity of the detector relative to the dark matter halo will vary by a small amount. A special case of direct detection experiments covers those with directional sensitivity. This is a search strategy based on the motion of the Solar System around the Galactic Center. WIMPs coming from the direction in which the Sun travels approximately towards Cygnus may then be separated from background, which should be isotropic.
Indirect detection experiments search for the products of the self-annihilation or decay of dark matter particles in outer space. For example, in regions of high dark matter density e. These processes could be detected indirectly through an excess of gamma rays, antiprotons or positrons emanating from high density regions in our galaxy or others.
A few of the dark matter particles passing through the Sun or Earth may scatter off atoms and lose energy. This could produce a distinctive signal in the form of high-energy neutrinos.
Many experimental searches have been undertaken to look for such emission from dark matter annihilation or decay, examples of which follow. The Energetic Gamma Ray Experiment Telescope observed more gamma rays in than expected from the Milky Way , but scientists concluded that this was most likely due to incorrect estimation of the telescope's sensitivity. The Fermi Gamma-ray Space Telescope is searching for similar gamma rays. At higher energies, ground-based gamma-ray telescopes have set limits on the annihilation of dark matter in dwarf spheroidal galaxies  and in clusters of galaxies.
They could be from dark matter annihilation or from pulsars. No excess antiprotons were observed. In results from the Alpha Magnetic Spectrometer on the International Space Station indicated excess high-energy cosmic rays that could be due to dark matter annihilation.
An alternative approach to the detection of dark matter particles in nature is to produce them in a laboratory. Because a dark matter particle should have negligible interactions with normal visible matter, it may be detected indirectly as large amounts of missing energy and momentum that escape the detectors, provided other non-negligible collision products are detected.
Because dark matter remains to be conclusively identified, many other hypotheses have emerged aiming to explain the observational phenomena that dark matter was conceived to explain. The most common method is to modify general relativity.
General relativity is well-tested on solar system scales, but its validity on galactic or cosmological scales has not been well proven. A suitable modification to general relativity can conceivably eliminate the need for dark matter. The best-known theories of this class are MOND and its relativistic generalization tensor-vector-scalar gravity TeVeS ,  f R gravity  and entropic gravity. A problem with alternative hypotheses is that the observational evidence for dark matter comes from so many independent approaches see the "observational evidence" section above.
Explaining any individual observation is possible but explaining all of them is very difficult. Nonetheless, there have been some scattered successes for alternative hypotheses, such as a test of gravitational lensing in entropic gravity. The prevailing opinion among most astrophysicists is that while modifications to general relativity can conceivably explain part of the observational evidence, there is probably enough data to conclude there must be some form of dark matter. In philosophy of science , dark matter is an example of an auxiliary hypothesis, an ad hoc postulate that is added to a theory in response to observations that falsify it.
It has been argued that the dark matter hypothesis is a conventionalist hypothesis, that is, a hypothesis that adds no empirical content and hence is unfalsifiable in the sense defined by Karl Popper. Mention of dark matter is made in works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties. Such descriptions are often inconsistent with the hypothesized properties of dark matter in physics and cosmology. From Wikipedia, the free encyclopedia. Hypothetical form of matter comprising most of the matter in the universe. Not to be confused with antimatter , dark energy , dark fluid , or dark flow.
For other uses, see Dark matter disambiguation. Discovery of cosmic microwave background radiation. Religious interpretations of the Big Bang theory. Simulated Large Hadron Collider CMS particle detector data depicting a Higgs boson produced by colliding protons decaying into hadron jets and electrons. String theory Loop quantum gravity Loop quantum cosmology Causal dynamical triangulation Causal fermion systems Causal sets Event symmetry Canonical quantum gravity Superfluid vacuum theory.
This section needs additional citations to secondary or tertiary sources such as review articles, monographs, or textbooks.
Please add such references to provide context and establish the relevance of any primary research articles cited. Unsourced or poorly sourced material may be challenged and removed. December Learn how and when to remove this template message. Type Ia supernova and Shape of the universe. What is dark matter? How is it generated? Is it related to supersymmetry? Not to be confused with Missing baryon problem. White, The evolution of large-scale structure in a universe dominated by cold dark matter.
Alternatives to general relativity. Dark matter in fiction. See Baryonic dark matter. Strictly speaking, electrons are leptons not baryons ; but since their number is equal to the protons while their mass is far smaller, electrons give a negligible contribution to the average density of baryonic matter. Baryonic matter excludes other known particles such as photons and neutrinos.
Hypothetical primordial black holes are also generally defined as non-baryonic, since they would have formed from radiation, not matter. The Dallas Morning News.
Annual Review of Astronomy and Astrophysics. Planck Collaboration 22 March Retrieved 21 March An invisible, essentially collisionless component of matter that makes up about 25 percent of the energy density of the universe Retrieved 10 June Evidence, candidates and constraints". Towards a new paradigm for structure formation". Monthly Notices of the Royal Astronomical Society. It is incidentally suggested that when the theory is perfected it may be possible to determine the amount of dark matter from its gravitational effect.
Three Parts Dark Matter. The New York Times. Retrieved December 27, Retrieved 6 August Observational evidence and detection methods". Reports on Progress in Physics. It is basically the same except that dark energy might depend on scale factor in some unknown way rather than necessarily being constant. Retrieved 8 December Physics for the 21st Century. Rik Myslewski, The Register. Wilkinson Microwave Anisotropy Probe. National Aeronautics and Space Administration.
Retrieved 9 January See Wayne Hu The Astrophysical Journal Supplement.