BLOGS

Multiple images of a background image created by gravitational lensing can be seen in the system HS 0810+2554. Credit: Hubble Space Telescope / NASA / ESA

Topics: Astronomy, Astrophysics, Dark Matter, Einstein, General Relativity

Physicists believe most of the matter in the universe is made up of an invisible substance that we only know about by its indirect effects on the stars and galaxies we can see.

We're not crazy! Without this "dark matter," the universe as we see it would make no sense.

But the nature of dark matter is a longstanding puzzle. However, a new study by Alfred Amruth at the University of Hong Kong and colleagues, published in Nature Astronomy, uses light's gravitational bending to bring us a step closer to understanding.

Invisible but omnipresent

We think dark matter exists because we can see its gravity's effects on galaxies' behavior. Specifically, dark matter seems to make up about 85% of the universe's mass, and most of the distant galaxies we can see appear to be surrounded by a halo of the mystery substance.

But it's called dark matter because it doesn't give off light or absorb or reflect it, which makes it incredibly difficult to detect.

So what is this stuff? We think it must be some kind of unknown fundamental particle, but beyond that, we're not sure. All attempts to detect dark matter particles in laboratory experiments have failed, and physicists have debated its nature for decades.

Scientists have proposed two leading hypothetical candidates for dark matter: relatively heavy characters called weakly interacting massive particles (or WIMPs) and extremely lightweight particles called axions. Theoretically, WIMPs behave like discrete particles, while axions behave more like waves due to quantum interference.

It has been difficult to distinguish between these two possibilities—but now light bent around distant galaxies has offered a clue.

New look at 'Einstein rings' around distant galaxies just got us closer to solving the dark matter debate, Rossana Ruggeri, Phys.org.

Rotation curve of the typical spiral galaxy M 33 (yellow and blue points with error bars) and the predicted one from the distribution of the visible matter (white line). The discrepancy between the two curves is accounted for by adding a dark matter halo surrounding the galaxy. Credit: Wikipedia

Topics: Astronomy, Astrophysics, Cosmology, Dark Matter

Although dark matter is central to the standard cosmological model, it's not without issues. There continue to be nagging mysteries about the stuff, not the least of which is the fact that scientists have found no direct particle evidence of it.

Despite numerous searches, we have yet to detect dark matter particles. Some astronomers favor an alternative, such as modified Newtonian dynamics (MoND) or the modified gravity model. And a new study of galactic rotation seems to support them.

The idea of MoND was inspired by galactic rotation. Most of the visible matter in a galaxy is clustered in the middle, so you'd expect that stars closer to the center would have faster orbital speeds than stars farther away, similar to the planets of our solar system. We observe that stars in a galaxy all rotate at about the same speed. The rotation curve is essentially flat rather than dropping off. The dark matter solution is that a halo of invisible matter surrounds galaxies, but in 1983 Mordehai Milgrom argued that our gravitational model must be wrong.

At interstellar distances, the gravitational attraction between stars is essentially Newtonian. So rather than modifying general relativity, Milgrom proposed modifying Newton's universal law of gravity. He argued that rather than the force of attraction as a pure inverse square relation, gravity has a small remnant pull regardless of distance. This remnant is only about ten trillionths of a G, but it's enough to explain galactic rotation curves.

New measurements of galaxy rotation lean toward modified gravity as an explanation for dark matter, Brian Koberlein, Universe Today/Phys.org.

Illustration of the FASER experiment. Image Credit: FASER/CERN.

Topics: CERN, Dark Matter, High Energy Physics, Neutrinos, Particle Physics

Neutrinos are ubiquitous and notorious. Billions are passing through you at this moment. Occasionally described as a “ghost of a particle,” neutrinos are nearly massless, thereby making them extremely difficult to detect experimentally (“Neutrino,” meaning “little neutral one” in Italian, was first used by Enrico Fermi in the early 1930s). Neutrinos were first confirmed in 1956 (thanks to a nearby nuclear reactor), and they’ve since been detected from different sources, including the Sun and cosmic rays, but not yet in a particle collider. Their elusiveness has been the source of much intrigue (and, of course, research funding) within the particle physics community since.

What else makes them so curious? Neutrinos come in three flavors — electron neutrino, muon neutrino, and tau neutrino — and may switch between them through the process of oscillation. Neutrino oscillations have been experimentally confirmed only in the past decade at the Super-K Detector in Japan (physicists Takaaki Kajita and Arthur B. McDonald shared the 2015 Nobel Prize in Physics for it). This discovery signified an important direction in the search for physics beyond the Standard Model because the longstanding theory does not explain neutrino oscillations and describes them as completely massless particles. Something isn’t quite adding up.

Enter: FASER. Initially proposed in 2018, the ForwArd Search ExpeRiment (FASER) is CERN’s newest experiment poised to detect neutrinos, potentially up to 1300 electron neutrinos, 20,000 muon neutrinos, and 20 tau neutrinos. Constructed in an unused service tunnel located about 500 meters from an Atlas experiment interaction point, FASER and its corresponding sub-detector, FASERν, have been designed to probe interactions of high-energy neutrinos (predicted to be between 600 GeV and 1 TeV).

FASER Poised to Further Our Understanding of Neutrinos, Dark Matter, Hannah Pell, Physics Central Buzz Blog

Clocking dark matter: optical clocks join the hunt for dark matter. (Courtesy: N Hanacek/NIST)

Topics: Dark Matter, Modern Physics, Quantum Mechanics

An optical clock has been used to set new constraints on a proposed theory of dark matter. Researchers including Jun Ye at JILA at the University of Colorado, Boulder, and Andrei Derevianko at the University of Nevada, Reno, explored how the coupling between regular matter and “ultralight” dark matter particles could be detected using the clock in conjunction with an ultra-stable optical cavity. With future upgrades to the performance of optical clocks, their approach could become an important tool in the search for dark matter.

Although it appears to account for about 85% of the matter in the universe, physicists know very little about dark matter. Most theoretical and experimental work so far has been focussed on hypothetical dark-matter particles, including WIMPS and axions, which have relatively large masses.  Alternatively, some physicists have proposed the existence of “ultralight” dark matter particles with extremely small masses that span many orders of magnitude (10⁻¹⁶–10⁻²¹ eV/c²).

According to the laws of quantum mechanics, the very smallest of these particles would have huge wavelengths, comparable to the sizes of entire dwarf galaxies – meaning they would behave like classical fields on scales we can easily measure.

Optical clock sets new constraints on dark matter, Sam Jarman, Physics World