From Science Times: “Optical Clocks Narrow Down the Search for Dark Matter”

Science Times

From Science Times

Oct 25, 2020
Mark B.

1
Ye group and Baxley/ Joint Institute for Laboratory Astrophysics via Wikimedia Commons) JILA’s experimental atomic clock based on strontium atoms held in a lattice of laser light is the world’s most precise and stable atomic clock. The image is a composite of many photos taken with long exposure times and other techniques to make the lasers more visible.

Researchers have used the accuracy of optical clocks to close in on the mysterious components of Dark Matter*, as well as the coupling between parts – particles and fields – postulated by the standard model of physics.

Standard Model of Particle Physics via http://www.plus.maths.org

The existence of dark matter remains to be proven. Its presence is indirectly observed through its effects on visible objects such as galaxies and stars. One of the effects supposedly caused by dark matter is an oscillation of fundamental physics constants.

On the other hand, optical clocks are extremely precise and accurate timekeeping equipment. They are so accurate in fact, that scientists estimate 20 billion years – longer than the known age of the Universe – before it leads or lags by a second.

A team of researchers, led by Jun Ye from the University of Colorado and the National Institute of Science and Technology (NIST), worked on a new attempt to detect dark matter, submitted in the online repository arXiv [Precision Metrology Meets Cosmology: Improved Constraints on Ultralight Dark Matter from Atom-Cavity Frequency Comparisons]. Through this precision of optical clocks, researchers propose that if the optical clocks still won’t detect the dark matter oscillations, it would suggest that the interaction of dark matter with observable particles in the standard model is lower than the constraints available.


What is Dark Matter and Dark Energy?

Determining Values for Fundamental Constants

Previous works aimed at detecting dark matter have involved large-scale studies, such as those conducted at the CERN Large Hadron Collider (LHC) (CH). Most recently, members of the ATLAS collaboration at the LHC inquired into dark matter using the Higgs boson – using the elementary particles, transforming it into particles that are “invisible.”

Other related efforts include detecting interactions with weakly interacting massive particles (WIMPs), particles whose masses are close to that of a silver atom around 100 gigaelectronvolts (GeV).

Ye’s team, however, used a state-of-the-art strontium optical lattice clock, a hydrogen maser, and its own cryogenic crystalline silicon cavity to try and capture possible interactions between dark matter and particles at the lower end of the mass spectrum, in the range below eV. In comparison, the mass of an electron at rest is close to 500,000 times larger than the limit used in the study.

Setting New Constraints For Future Studies

The optical clock allows researchers to observe variations in alpha (α), known as the fine structure constant and is used to characterize the strength of interactions between photons and charged particles. Researchers compared the frequency of the strontium atoms in the optical clock to those in the silicon cavity, which allows electromagnetic waves to bounce inside its chambers. This phenomenon creates a standing wave whose characteristic frequency can be controlled based on its cavity length. The frequency of the optical clock and the cavity is defined in terms of α and me, or the mass of an electron. Furthermore, data from these two pieces of equipment were also compared to the frequency of a hydrogen maser, a frequency standard using a hydrogen atom as reference.

Researchers were not able to observe the oscillations in fundamental constants caused by dark matter interactions. This result, however, establishes a new set of constraints – narrowing down the possible values for these interactions. Dark matter particles having masses from 4.5 × 10-16 down to 1 × 10-19 electronvolts, the strength of dark matter interactions – in terms of α – is theorized to be up by a factor of five. On the other hand, interactions in terms of me could have constraints by as much as 100 times, for masses 2 × 10-19 and 2 × 10-21 eV.

*Dark Matter Background
Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

Fritz Zwicky from http:// palomarskies.blogspot.com.

Coma cluster via NASA/ESA Hubble.

In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science).


Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL).


Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu.

The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova.

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