Atomic clocks on Earth could reveal secrets about dark matter across the universe

Scientists are using atomic clocks to investigate some of the universe’s greatest mysteries, including the nature of dark matter, in a laboratory. In the process, they say they’re bringing cosmology and astrophysics “down to Earth.”

The project, which is a collaboration between the University of Sussex and the National Physical Laboratory (NPL) in the U.K., uses the ticks of these incredibly precise clocks to hunt for hitherto unknown ultra-light particles.

These particles could be connected to dark matter, the mysterious substance that makes up an estimated 85% of all matter in the universe but remains effectively invisible to us because it does not interact with light or, more precisely, electromagnetic radiation. Scientists believe most galaxies are enveloped by a cloud of dark matter, but its presence can only be inferred by the effect it has on gravity.

“Our universe, as we know it, is governed by laws of physics, so gravity is governed by general relativity and particle physics by the Standard Model of particle physics,” Xavier Calmet, project leader and a professor of physics at the University of Sussex, told Space.com. “We call deviations from these laws  ‘breakdown in physics’ — basically, that is a synonym for new physics beyond our current understanding of the universe.”

Related: We still don’t know what dark matter is, but here’s what it’s not

This new physics could be used to explain the nature of dark matter, something that doesn’t fit within the Standard Model.

“One of the biggest mysteries is the nature of dark matter. We know that it is out there, we see its impact in our universe, but we don’t have a valid explanation within the Standard Model of particle physics,” Calmet continued. “There must be new physics, but we do not know how to describe these new particles and how they couple to regular matter.”

According to established laws of physics, clocks should tick at a constant rate, but physics beyond the Standard Model’s scope would result in tiny charges in atomic energy levels. This should affect the rate at which clocks tick, but the variation would be so small it could only be spotted with an incredibly precise clock — and that’s where atomic clocks come in.

“Atomic clocks bring cosmology and astrophysics down to Earth, enabling searches for ultra-light particles that could explain dark matter in a laboratory,” Calmet said.

Atomic clocks measure time using atoms with two potential energy states. When atoms absorb energy, they go to a higher energy state. Then, they eventually release this energy and drop back down to their lower ground state.

In atomic clocks, groups of atoms are prepared by placing them in a higher energy state using microwave energy, and the characteristic and consistent rates at which they vibrate between states — their resonance frequencies — are used to precisely measure time.

So, for example, all atoms of cesium resonate at the same frequency, meaning the standard measure of a second can be defined as 9,192,631,770 cycles of cesium. Because this cycling per second occurs with far less variation than, say, the swinging of a pendulum, this makes atomic clocks incredibly precise.

“It has been recently realized that dark matter could be made of ultra-light particles that interact extremely weakly with regular matter,” Calmet explained. “If that is the case, dark matter would essentially behave as a classical wave that interacts with electrons and protons. This dark matter wave would give some small kicks to these particles.”

Calmet added that these ultra-light dark matter particle kicks to the building blocks of the atom would lead to a time variation in fundamental constants of the universe, such as the fine-structure constant or “alpha” — a measure of how strong particles couple via the electromagnetic force — and the mass of the proton.

“Because atomic clocks are amazingly precise devices, they would be able to detect these kicks and thus discover ultra-light dark matter,” he continued. “By comparing two clocks, one sensitive to changes in alpha and the other one less sensitive to changes in alpha, we can obtain a limit on the time variation of this fundamental constant and thus set constraints on ultra-light particles.”

Calmet thinks the technique could potentially also be used to investigate another problematic aspect of the universe for physicists: Dark energy, the unknown force that is driving the accelerating expansion of space.

While Calmet acknowledges that dark energy is more likely explained by the cosmological constant, a form of energy that acts almost in opposition to gravity to stretch the fabric of space and push apart galaxies, there is a small chance it could be connected to an ultra-light particle. In this vein, future clocks could also be sensitive to that particle and its associated wave.

“While the clocks have not discovered new physics at this stage, we were able to develop a new theoretical framework to probe generic new physics with clocks and were able to derive the first model-independent limits on physics beyond the standard model within this approach,” Calmet concluded. “We are creating a new field at the interface of atomic, molecular, and optical physics and traditional particle physics.

“These are exciting results!”

These results are set to be published in a future edition of the New Journal of Physics.

02

Unknown ultra-light particles linked to dark matter could be found using atomic clocks

Scientists are using atomic clocks to investigate some of the universe’s greatest mysteries, including the nature of dark matter, in a laboratory. In the process, they say they’re bringing cosmology and astrophysics “down to Earth.”

The project, which is a collaboration between the University of Sussex and the National Physical Laboratory (NPL) in the U.K., uses the ticks of these incredibly precise clocks to hunt for hitherto unknown ultra-light particles.

These particles could be connected to dark matter, the mysterious substance that makes up an estimated 85% of all matter in the universe but remains effectively invisible to us because it does not interact with light or, more precisely, electromagnetic radiation. Scientists believe most galaxies are enveloped by a cloud of dark matter, but its presence can only be inferred by the effect it has on gravity.

Related: How does an atomic clock work?

“Our universe, as we know it, is governed by laws of physics, so gravity is governed by general relativity and particle physics by the Standard Model of particle physics,” Xavier Calmet, project leader and a professor of physics at the University of Sussex, told Space.com. “We call deviations from these laws  ‘breakdown in physics’ — basically, that is a synonym for new physics beyond our current understanding of the universe.”

This new physics could be used to explain the nature of dark matter, something that doesn’t fit within the Standard Model.

“One of the biggest mysteries is the nature of dark matter. We know that it is out there, we see its impact in our universe, but we don’t have a valid explanation within the Standard Model of particle physics,” Calmet continued. “There must be new physics, but we do not know how to describe these new particles and how they couple to regular matter.”

According to established laws of physics, clocks should tick at a constant rate, but physics beyond the Standard Model’s scope would result in tiny charges in atomic energy levels. This should affect the rate at which clocks tick, but the variation would be so small it could only be spotted with an incredibly precise clock — and that’s where atomic clocks come in.

“Atomic clocks bring cosmology and astrophysics down to Earth, enabling searches for ultra-light particles that could explain dark matter in a laboratory,” Calmet said.

Atomic clocks measure time using atoms with two potential energy states. When atoms absorb energy, they go to a higher energy state. Then, they eventually release this energy and drop back down to their lower ground state.

In atomic clocks, groups of atoms are prepared by placing them in a higher energy state using microwave energy, and the characteristic and consistent rates at which they vibrate between states — their resonance frequencies — are used to precisely measure time.

So, for example, all atoms of cesium resonate at the same frequency, meaning the standard measure of a second can be defined as 9,192,631,770 cycles of cesium. Because this cycling per second occurs with far less variation than, say, the swinging of a pendulum, this makes atomic clocks incredibly precise.

“It has been recently realized that dark matter could be made of ultra-light particles that interact extremely weakly with regular matter,” Calmet explained. “If that is the case, dark matter would essentially behave as a classical wave that interacts with electrons and protons. This dark matter wave would give some small kicks to these particles.”

Calmet added that these ultra-light dark matter particle kicks to the building blocks of the atom would lead to a time variation in fundamental constants of the universe, such as the fine-structure constant or “alpha” — a measure of how strong particles couple via the electromagnetic force — and the mass of the proton.

“Because atomic clocks are amazingly precise devices, they would be able to detect these kicks and thus discover ultra-light dark matter,” he continued. “By comparing two clocks, one sensitive to changes in alpha and the other one less sensitive to changes in alpha, we can obtain a limit on the time variation of this fundamental constant and thus set constraints on ultra-light particles.”

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Calmet thinks the technique could potentially also be used to investigate another problematic aspect of the universe for physicists: Dark energy, the unknown force that is driving the accelerating expansion of space.

While Calmet acknowledges that dark energy is more likely explained by the cosmological constant, a form of energy that acts almost in opposition to gravity to stretch the fabric of space and push apart galaxies, there is a small chance it could be connected to an ultra-light particle. In this vein, future clocks could also be sensitive to that particle and its associated wave.

“While the clocks have not discovered new physics at this stage, we were able to develop a new theoretical framework to probe generic new physics with clocks and were able to derive the first model-independent limits on physics beyond the standard model within this approach,” Calmet concluded. “We are creating a new field at the interface of atomic, molecular, and optical physics and traditional particle physics.

“These are exciting results!”

The results are set to be published in a future edition of the New Journal of Physics.

03

It’s going to take more than early dark energy to resolve the Hubble tension

Our best understanding of the universe is rooted in a cosmological model known as LCDM. The CDM stands for cold dark matter, where most of the matter in the universe isn’t stars and planets, but a strange form of matter that is dark and nearly invisible. The L, or lambda, represents dark energy. It is the symbol used in the equations of general relativity to describe the Hubble parameter, or the rate of cosmic expansion. Although the LCDM model matches our observations incredibly well, it isn’t perfect. And the more data we gather on the early universe, the less perfect it seems to be.

A central difficulty is the fact that increasingly, our various measures of the Hubble parameter aren’t lining up. For example, if we use fluctuations in the cosmic microwave background to calculate the parameter, we get a value of about 68 km/s per megaparsec. If we look at distant supernova to measure it, we get a value of around 73 km/s per megaparsec. In the past, the uncertainty of these values was large enough that they overlapped, but we’ve now measured them with such precision that they truly disagree. This is known as the Hubble tension problem, and it’s one of the deepest mysteries of cosmology at the moment.

Much of the effort to solve this mystery has focused on better understanding the nature of dark energy. In Einstein’s early model, cosmic expansion is an inherent part of the structure of space and time, a cosmological constant that expands the universe at a steady rate. But perhaps dark energy is an exotic scalar field, one that would allow a variable expansion rate or even an expansion that varies slightly depending on which direction you look. Maybe the rate was greater in the period of early galaxies, then slowed down, hence the different observations. We know so little about dark energy that there are lots of theoretical possibilities.

Perhaps tweaking dark energy will solve Hubble Tension, but Sunny Vagnozzi doesn’t think so. In a recent article uploaded to the arXiv preprint server (and later published in the journal Universe), he outlines seven reasons to suspect dark energy won’t be enough to solve the problem. It’s an alphabetical list of data that shows just how deep this cosmological mystery is.

Ages of distant objectsThe idea behind this one is simple. If you know the age of a star or galaxy a billion light-years away, then you know the universe must have been at least that old a billion years ago. If this age disagrees with LCDM, then LCDM must be wrong. For example, there are a few stars that appear to be older than the universe, which Big Bang skeptics often point to as disproving the Big Bang. This doesn’t work because the age of these stars is uncertain enough to be younger than the universe. But you can expand upon the idea as a cosmological test. Determine the age of thousands of stars at various distances, then use statistics to gauge a minimum cosmological age at different epochs, and from that calculate a minimum Hubble parameter.

Several studies have looked at this, drawing upon a range of sky surveys. Determining the age of stars and globular clusters is particularly difficult, so the resulting data is a bit fuzzy. While it’s possible to fit the data to the range of Hubble parameters we have from direct measures, the age-distance data suggests the universe is a bit older than the LCDM allows. In other words, IF the age data is truly accurate, there is a discrepancy between cosmic age and stellar ages. That’s a big IF, and this is far from conclusive, but it’s worth exploring further.

Baryon acoustic oscillationRegular matter is made of baryons and leptons. The protons and neutrons in an atom are baryons, and the electrons are leptons. So Baryonic matter is the usual type of matter we see every day, as opposed to dark matter. Baryon acoustic oscillation (BAO) refers to the fluctuations of matter density in the early universe. Back when the universe was in a hot dense state, these fluctuations rippled through the cosmos like sound waves. As the universe expanded, the more dense regions formed the seeds for galaxies and galactic clusters. The scale of those clusters is driven by cosmic expansion. So by looking at BAO across the universe, we can study the evolution of dark energy over time.

What’s nice about BAO is that it connects the distribution of galaxies we see today to the inflationary state of the universe during the period of the cosmic microwave background (CMB). It’s a way to compare the value of the early Hubble parameter with the more recent value. This is because early inflation put a limit on how far acoustic waves could propagate. The higher the rate of expansion back then, the smaller the acoustic range. It’s known as the acoustic horizon, and it depends not only on the expansion rate but also on the density of matter at the time. When we compare BAO and CMB observations, they do agree, but only for a level of matter on the edge of observed limits. In other words, if we get a better measure of the density of matter in the universe, we could have a CMB/BAO tension just as we currently have a Hubble tension.

Cosmic chronometersBoth the supernovae and cosmic microwave background measures of the Hubble parameter depend on a scaffold of interlocking models. The supernova measure depends on the cosmic distance ladder, where we use various observational models to determine ever greater distances. The CMB measure depends on the LCDM model, which has some uncertainty in its parameters such as matter density. Cosmic chronometers are observational measures of the Hubble parameter that aren’t model dependent.

One of these measures uses astrophysical masers. Under certain conditions, hot matter in the accretion disk of a black hole can emit microwave laser light. Since this light has a very specific wavelength, any shift in that wavelength is due to the relative motion or cosmic expansion, so we can measure the expansion rate directly from the overall redshift of the maser, and we can measure the distance from the scale of the accretion disk. Neither of these require cosmological model assumptions.

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