Candidates for Dark Matter: MACHOs, WIMPs, "dark photons and the role of neutrinos
MACHOs and WIMPs [1]
Reminder: Have you read the preceding page entitled 'Dark matter, primordial black holes and axions"?
First of all, it was conjectured that dark matter was composed of MACHOS: massive compact halo objects. Found both in spherical halos surrounding each galaxy and near each galaxy’s luminous centre, MACHOs would create the gravitational pull responsible for the otherwise anomalous motions of stars and gas that astronomers observe in the outskirts of galaxies.
However, MACHOs never quite turned up in tentative, indirect searches for their existence. Most notably, astronomers looked for them via microlensing, a variety of gravitational lensing in which a black hole, a brown dwarf or even a planet passes in front of a black hole and temporarily magnifies the star’s light. MACHOs up to around 10 solar masses were ruled out as the primary constituent of dark matter. So attention turned to WIMPs, a single and as yet undiscovered particle.
Dark matter in its simplest form: WIMPs
It is conjectured that WIMPS (weakly interacting massive particles) may contribute the unseen mass, but as with MACHOs, evidence of their existence has yet to be found. The theory was that WIMPS rarely or never interact with other particles of its own kind or ordinary matter. Instead, they congregate into spherical clouds that gravitationally attract normal baryonic matter to form galaxies. Yet despite decades of searches using particle accelerators, underground detectors and space telescopes, nothing corroborating this scenario has been detected, leading some physicists to postulate a more complicated form of dark matter. “Instead of a single type of particle, dark matter might be made of a wider array of dark species. After all, ordinary matter comes in many forms – maybe dark matter is similarly complex”[2].
Reminder: Have you read the preceding page entitled 'Dark matter, primordial black holes and axions"?
First of all, it was conjectured that dark matter was composed of MACHOS: massive compact halo objects. Found both in spherical halos surrounding each galaxy and near each galaxy’s luminous centre, MACHOs would create the gravitational pull responsible for the otherwise anomalous motions of stars and gas that astronomers observe in the outskirts of galaxies.
However, MACHOs never quite turned up in tentative, indirect searches for their existence. Most notably, astronomers looked for them via microlensing, a variety of gravitational lensing in which a black hole, a brown dwarf or even a planet passes in front of a black hole and temporarily magnifies the star’s light. MACHOs up to around 10 solar masses were ruled out as the primary constituent of dark matter. So attention turned to WIMPs, a single and as yet undiscovered particle.
Dark matter in its simplest form: WIMPs
It is conjectured that WIMPS (weakly interacting massive particles) may contribute the unseen mass, but as with MACHOs, evidence of their existence has yet to be found. The theory was that WIMPS rarely or never interact with other particles of its own kind or ordinary matter. Instead, they congregate into spherical clouds that gravitationally attract normal baryonic matter to form galaxies. Yet despite decades of searches using particle accelerators, underground detectors and space telescopes, nothing corroborating this scenario has been detected, leading some physicists to postulate a more complicated form of dark matter. “Instead of a single type of particle, dark matter might be made of a wider array of dark species. After all, ordinary matter comes in many forms – maybe dark matter is similarly complex”[2].
Abell 3827 - beyond WIMPs – with a note on the technique of gravitational lensing
Recent observations and a number of unanswered questions surrounding the WIMP scenario (why, for example, the centres of galaxies are less dense than anticipated) have lately influenced scientists to postulate a more complex dark matter model. Astronomers recently tracked dark matter’s location within four colliding galaxies in the Abell 3827 cluster by using the technique of gravitational lensing.
Recent observations and a number of unanswered questions surrounding the WIMP scenario (why, for example, the centres of galaxies are less dense than anticipated) have lately influenced scientists to postulate a more complex dark matter model. Astronomers recently tracked dark matter’s location within four colliding galaxies in the Abell 3827 cluster by using the technique of gravitational lensing.
Observations made using this technique by the Hubble Space Telescope and the Very Large Telescope in Chile in the case of Abell 3827 and published in May-June 2015 in the Monthly Notices of the Royal Astronomical Society, revealed that the dark matter surrounding at least one of the galaxies significantly lagged behind the ordinary matter there, suggesting that maybe the dark matter particles were interacting with one another and slowing themselves down. Because these interactions did not affect the ordinary matter, it is reasoned that they must have occurred through some force other than gravity that influences only dark matter, for example an exchange of “dark photons”[3].
What are these mysterious dark photons? Some models suggest that dark photons can continually transform into ordinary photons and back again via the laws of quantum mechanics. Dark photons may also have non-zero mass, meaning that they can potentially decay into lighter particles, there being a small chance that they can produce pairs of electrons and their antimatter counterparts, or similarly a matter-antimatter pair of muons during the transformation process. So studies are ongoing at the LHC and other particle accelerators for collisions that produce an electron-positron or a muon-antimuon pair[4].
How does an exchange of dark photons work?
Just as ordinary particles with electrical charge (protons) can emit photons (the particles of light that are the carriers of the electromagnetic force), thereby transferring momentum causing the protons to separate, perhaps particles with dark charge can emit “dark photons” – not particles of light but rather particles that interact with dark charge in the same way that photons interact with electrical charge[5]. So when the dark matter from one galaxy collides with the dark matter in another, it is hypothesised that positively and negatively charged dark matter particles should be able to meet and annihilate into dark photons. The particles could repel each other by an exchange of dark photons. The dark matter would seem to be interacting with itself. This “dark electromagnetism” could allow for multiple types of dark particles, some heavy and some light, that attract one another into arrangements akin to atoms and then form disk structures in galaxies that overlap with the spiral arm disks of baryonic matter[6].
Perhaps the simple and complex theories both have something going for them. Dark matter might be a mix of two types of “cold” or slow moving particles, some non-self-interacting (WIMPs) and some self-interacting. Electromagnetism allows ordinary matter to lose energy and settle into flattened disks. Because dark matter is primarily distributed around most galaxies and does not collapse into a disk, it would seem that it cannot lose energy via dark photon emission at the same rate as ordinary matter. This would result in both a spherical cloud of WIMPs around galaxies and a flattened disk of self-interacting particles[7].
Other techniques for detecting dark matter – underground detectors
Apart from searching for collisions between galaxy clusters such as Abell 3827, scientists can continue their search for dark matter in some of the ways they searched for it when it was solely WIMP dependent. One of these is by means of sensitive underground detectors. A consequence of the partially interacting dark matter model, with its concentrated disk of matter in roughly the same plane as the visible matter of the Milky Way, is that this form of dark matter passing through our detectors would be denser than that predicted in WIMP models, which could result in a greater probability of detection[8].
In fact, results from experiments at the Dark Matter (DAMA) detector in Italy’s subterranean dark matter detector in the subterranean Gran Sasso National laboratory (2000), and the Coherent Germanium Neutron Technology (CoGeNT) detector in the Soudan underground Mine in northern Minnesota (2010) suggest that physicists may have already glimpsed particles of dark matter, even though it does not fit the conventional picture that it should[9]. The data point to particles weighing about 7 GeV, or seven times as much as a proton, whereas physicists had expected them to weigh 10 times more. Theoretically, these WIMPS should weigh about as much as the particles that convey the weak nuclear force, the W and Z bosons, which weigh 80 and 91 GeV. However, others such as Dan Hooper from the Fermi National Accelerator Laboratory in Batavia, Illinois express confidence that 7 GeV fits within the bounds of acceptability[10].
The role of neutrinos
Ongoing research at the Batavia Fermilab involves attempts to make beams of dark matter particles, by generating intense beams of neutrinos that shoot at distant detectors. Neutrinos are very light subatomic particles that interact essentially exclusively through the weak nuclear force. If dark matter interacts with ordinary matter via particles like dark photons, it is possible that dark matter is being made in the same beams and can possibly be detected in Fermilab’s MiniBooNE, MINOS or NOvA detectors. [11]
Although they are very small and virtually massless, because their individual numbers are so huge, it is now the consensus that neutrinos exert a profound influence in the cosmos generally. When the early universe was in the process of formation, dark matter, under the influence of gravity collapsed into clumps first and neutrinos at a later point in time, because they are so extremely light. Dark matter is thought to be shaped by neutrinos, and has subtly distorted the CMBR. So by tracing these distortions, physicists may be able to chart dark matter’s structure, and will thus be able to measure the combined masses of the three neutrino types throughout the universe.
Sudeep Das and Tristan Smith have opined that it may thus be possible to measure the mass of the lightest and most elusive of subatomic particles by in effect measuring the entire universe [12]. Before the apparent discovery of interacting dark matter (April 2015), it was reasoned that, based on their arrival time from the supernova 1987A, neutrinos were unlikely to account for dark matter, since they travel at considerably greater speeds because they are so light [13]. Perhaps the techniques presently being utilised at the Batavia Fermilab may shed new light on this question.
The role of particle accelerators
The Large Hadron Collider (LHC) at CERN, the world’s highest-energy accelerator, is another candidate. It was recently (April 2015) ramped up to maximum capacity, giving it a pronounced advantage when searching for heavier versions of dark matter, as well as for dark matter particles whose interactions become increasingly frequent as their energy rises. Because we know dark matter can only interact very weakly with ordinary matter,we cannot expect to observe it directly in the detector, which is made of ordinary matter. Instead, scientists search for dark matter by looking for collisions in which energy is missing. But so far so signs of this have shown up inside the detector, an indication that its interactions with ordinary matter must be very infrequent. Now that it has been revamped to maximum to maximum energy, the LHC may perhaps have the capacity to reveal what its earlier runs could not [14].
The Fermi Gamma-ray space telescope
Another technique utilised in the search for WIMPs has been NASA’s Fermi Gamma-ray space telescope, which recorded high-energy gamma-ray light emanating from the centre of the Milky Way that fitted in well with dark matter predictions on the basis that WIMP annihilations would produce normal matter particles that would in turn create gamma-ray photons[15]. In the dense cloud of dark matter in the Milky Way’s core, dark matter will occasionally collide, creating an electron and a positron in the process. The telescope has in fact already detected such a phenomenon. Gamma rays, denoted by the Greek letter γ, refer to electromagnetic radiation of extremely high frequency and therefore high energy per photon. They are classically produced by the decay from high energy states of atomic nuclei (gamma decay), but are also created by other processes. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium. Villard's radiation was named "gamma rays" by Ernest Rutherford in 1903[16].
If the light particles detected by the Fermi Gamma-ray Space telescope were truly caused by dark matter, it would be the first indirect detection of the particles making up this shadowy substance. Dark matter should be densest at a galaxy’s core (instance the Milky Way), hence that is the best place to look for this light. The new study has found an excess of gamma-ray photons extending at least 5,000 light-years from the Milky Way’s centre. However, most scientists are reserving judgment until the signal is seen by other instruments or in other places. The apparent discovery since of interacting dark matter may also influence the picture.
How does the Fermi gamma-ray telescope work?
Earth’s atmosphere blocks gamma rays, which have energies millions of times that of visible light, so one way astronomers measure them is by getting above the atmosphere. The Fermi Gamma-ray Space telescope is the most powerful gamma-ray observatory ever launched. It contains two main instruments: a burst monitor that keeps watch on the entire sky for evidence of transient gamma-ray bursts and the Large Area Telescope (LAT), which is the most sensitive and highest-resolution gamma-ray detector ever launched.
The LAT is radically different from any optical telescope: it has no mirrors, no lenses and no focal plane. Instead it operates more like a particle physics experiment. Each incoming gamma ray recoils off an atomic nucleus in the telescope and transforms to a positron and electron. These particles are then tracked through onboard detectors and a calorimeter, which measure energy. Further data analysis on the ground filters out background noise and reveals the direction an energy of the original gamma ray. Fermi has a field of view covering a fifth of the sky, which allows it to observe the breadth of the sky every three hours.
The Fermi telescope has also revealed massive structures that tower tens of thousands of light years over the galactic centre. These lobes have been named the Fermi bubbles. The processes that crated these structures are as yet unknown. One explanation is that the bubbles may be inflated by a jet of energy from our galaxy’s central black hole. Alternatively they may be the accumulated wind emanating from a swarm of supernovae[17].
What are these mysterious dark photons? Some models suggest that dark photons can continually transform into ordinary photons and back again via the laws of quantum mechanics. Dark photons may also have non-zero mass, meaning that they can potentially decay into lighter particles, there being a small chance that they can produce pairs of electrons and their antimatter counterparts, or similarly a matter-antimatter pair of muons during the transformation process. So studies are ongoing at the LHC and other particle accelerators for collisions that produce an electron-positron or a muon-antimuon pair[4].
How does an exchange of dark photons work?
Just as ordinary particles with electrical charge (protons) can emit photons (the particles of light that are the carriers of the electromagnetic force), thereby transferring momentum causing the protons to separate, perhaps particles with dark charge can emit “dark photons” – not particles of light but rather particles that interact with dark charge in the same way that photons interact with electrical charge[5]. So when the dark matter from one galaxy collides with the dark matter in another, it is hypothesised that positively and negatively charged dark matter particles should be able to meet and annihilate into dark photons. The particles could repel each other by an exchange of dark photons. The dark matter would seem to be interacting with itself. This “dark electromagnetism” could allow for multiple types of dark particles, some heavy and some light, that attract one another into arrangements akin to atoms and then form disk structures in galaxies that overlap with the spiral arm disks of baryonic matter[6].
Perhaps the simple and complex theories both have something going for them. Dark matter might be a mix of two types of “cold” or slow moving particles, some non-self-interacting (WIMPs) and some self-interacting. Electromagnetism allows ordinary matter to lose energy and settle into flattened disks. Because dark matter is primarily distributed around most galaxies and does not collapse into a disk, it would seem that it cannot lose energy via dark photon emission at the same rate as ordinary matter. This would result in both a spherical cloud of WIMPs around galaxies and a flattened disk of self-interacting particles[7].
Other techniques for detecting dark matter – underground detectors
Apart from searching for collisions between galaxy clusters such as Abell 3827, scientists can continue their search for dark matter in some of the ways they searched for it when it was solely WIMP dependent. One of these is by means of sensitive underground detectors. A consequence of the partially interacting dark matter model, with its concentrated disk of matter in roughly the same plane as the visible matter of the Milky Way, is that this form of dark matter passing through our detectors would be denser than that predicted in WIMP models, which could result in a greater probability of detection[8].
In fact, results from experiments at the Dark Matter (DAMA) detector in Italy’s subterranean dark matter detector in the subterranean Gran Sasso National laboratory (2000), and the Coherent Germanium Neutron Technology (CoGeNT) detector in the Soudan underground Mine in northern Minnesota (2010) suggest that physicists may have already glimpsed particles of dark matter, even though it does not fit the conventional picture that it should[9]. The data point to particles weighing about 7 GeV, or seven times as much as a proton, whereas physicists had expected them to weigh 10 times more. Theoretically, these WIMPS should weigh about as much as the particles that convey the weak nuclear force, the W and Z bosons, which weigh 80 and 91 GeV. However, others such as Dan Hooper from the Fermi National Accelerator Laboratory in Batavia, Illinois express confidence that 7 GeV fits within the bounds of acceptability[10].
The role of neutrinos
Ongoing research at the Batavia Fermilab involves attempts to make beams of dark matter particles, by generating intense beams of neutrinos that shoot at distant detectors. Neutrinos are very light subatomic particles that interact essentially exclusively through the weak nuclear force. If dark matter interacts with ordinary matter via particles like dark photons, it is possible that dark matter is being made in the same beams and can possibly be detected in Fermilab’s MiniBooNE, MINOS or NOvA detectors. [11]
Although they are very small and virtually massless, because their individual numbers are so huge, it is now the consensus that neutrinos exert a profound influence in the cosmos generally. When the early universe was in the process of formation, dark matter, under the influence of gravity collapsed into clumps first and neutrinos at a later point in time, because they are so extremely light. Dark matter is thought to be shaped by neutrinos, and has subtly distorted the CMBR. So by tracing these distortions, physicists may be able to chart dark matter’s structure, and will thus be able to measure the combined masses of the three neutrino types throughout the universe.
Sudeep Das and Tristan Smith have opined that it may thus be possible to measure the mass of the lightest and most elusive of subatomic particles by in effect measuring the entire universe [12]. Before the apparent discovery of interacting dark matter (April 2015), it was reasoned that, based on their arrival time from the supernova 1987A, neutrinos were unlikely to account for dark matter, since they travel at considerably greater speeds because they are so light [13]. Perhaps the techniques presently being utilised at the Batavia Fermilab may shed new light on this question.
The role of particle accelerators
The Large Hadron Collider (LHC) at CERN, the world’s highest-energy accelerator, is another candidate. It was recently (April 2015) ramped up to maximum capacity, giving it a pronounced advantage when searching for heavier versions of dark matter, as well as for dark matter particles whose interactions become increasingly frequent as their energy rises. Because we know dark matter can only interact very weakly with ordinary matter,we cannot expect to observe it directly in the detector, which is made of ordinary matter. Instead, scientists search for dark matter by looking for collisions in which energy is missing. But so far so signs of this have shown up inside the detector, an indication that its interactions with ordinary matter must be very infrequent. Now that it has been revamped to maximum to maximum energy, the LHC may perhaps have the capacity to reveal what its earlier runs could not [14].
The Fermi Gamma-ray space telescope
Another technique utilised in the search for WIMPs has been NASA’s Fermi Gamma-ray space telescope, which recorded high-energy gamma-ray light emanating from the centre of the Milky Way that fitted in well with dark matter predictions on the basis that WIMP annihilations would produce normal matter particles that would in turn create gamma-ray photons[15]. In the dense cloud of dark matter in the Milky Way’s core, dark matter will occasionally collide, creating an electron and a positron in the process. The telescope has in fact already detected such a phenomenon. Gamma rays, denoted by the Greek letter γ, refer to electromagnetic radiation of extremely high frequency and therefore high energy per photon. They are classically produced by the decay from high energy states of atomic nuclei (gamma decay), but are also created by other processes. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium. Villard's radiation was named "gamma rays" by Ernest Rutherford in 1903[16].
If the light particles detected by the Fermi Gamma-ray Space telescope were truly caused by dark matter, it would be the first indirect detection of the particles making up this shadowy substance. Dark matter should be densest at a galaxy’s core (instance the Milky Way), hence that is the best place to look for this light. The new study has found an excess of gamma-ray photons extending at least 5,000 light-years from the Milky Way’s centre. However, most scientists are reserving judgment until the signal is seen by other instruments or in other places. The apparent discovery since of interacting dark matter may also influence the picture.
How does the Fermi gamma-ray telescope work?
Earth’s atmosphere blocks gamma rays, which have energies millions of times that of visible light, so one way astronomers measure them is by getting above the atmosphere. The Fermi Gamma-ray Space telescope is the most powerful gamma-ray observatory ever launched. It contains two main instruments: a burst monitor that keeps watch on the entire sky for evidence of transient gamma-ray bursts and the Large Area Telescope (LAT), which is the most sensitive and highest-resolution gamma-ray detector ever launched.
The LAT is radically different from any optical telescope: it has no mirrors, no lenses and no focal plane. Instead it operates more like a particle physics experiment. Each incoming gamma ray recoils off an atomic nucleus in the telescope and transforms to a positron and electron. These particles are then tracked through onboard detectors and a calorimeter, which measure energy. Further data analysis on the ground filters out background noise and reveals the direction an energy of the original gamma ray. Fermi has a field of view covering a fifth of the sky, which allows it to observe the breadth of the sky every three hours.
The Fermi telescope has also revealed massive structures that tower tens of thousands of light years over the galactic centre. These lobes have been named the Fermi bubbles. The processes that crated these structures are as yet unknown. One explanation is that the bubbles may be inflated by a jet of energy from our galaxy’s central black hole. Alternatively they may be the accumulated wind emanating from a swarm of supernovae[17].
[1] Juan Garcia-Bellido and Sébastien Clesse, "Black Holes from the Beginning of Time", Scientific American, July 2017, 30 at 32.
[2] Dobrescu and Lincoln, "Mystery of the Hidden Cosmos", Scientific American, July 2015, 21at 27.
[3] Clara Moskowitz, “Dark matter drops a clue”, Scientific American, June 2015, 9.
[4] Dobrescu and Lincoln, "Mystery of the Hidden Cosmos", Scientific American, July 2015, 23.
[5] Dobrescu and Lincoln, op.cit., 27.
[6] Ibid, 25.
[7] Ibid 25. The remainder would then be composed of “hot” or fast moving particles, but if dark matter were primarily composed of “hot” particles, they would never have clumped together enough to form the spherical clouds that gave rise to individual galaxies. But some small component of the total dark matter could still be “hot”. About 95% of dark matter is thought to be “cold”: ibid.
[8] Ibid, 25, 26.
[9] Adrian Cho, “Have physicists already glimpsed particles of Dark Matter? Science, Vol 331, 4 March 2011, 1132-3.
[10] Ibid.
[11] Neutrinos, the discovery that they do in fact have mass, and their role in the formation of dark matter, are further considered in the page The path to the standard model (in the material below footnote 24).
[12] “The neutrinos secrets, written on the sky”, Scientific American, Special Collector’s Edition, August 2013, 26.
[13] Ray Jawardhana, “Coming soon: A Supernova near you”, Scientific American, December 2013, 60 at 63.
[14] Dobrescu and Lincoln, op cit, 27.
[15] Clara Moskowitz, “A glimpse at the unseen – mysterious light from the centre of the Milky Way may be our first look at dark particles”, Scientific American, July 2014, 18.
[16] http://en.wikipedia.org/wiki/Gamma_ray
[17] Finkbeiner, Su, Malyshev, ibid, pp 28-33.