The first results from the Dark Energy Spectroscopic Instrument (DESI) are a cosmological bombshell, suggesting that the strength of dark energy has not remained constant throughout history. Instead, it appears to be weakening at the moment, and in the past it seems to have existed in an extreme form known as “phantom” dark energy.

The new findings have the potential to change everything we thought we knew about dark energy, a hypothetical entity that is used to explain the accelerating expansion of the universe.

“The subject needed a bit of a shake-up, and we’re now right on the boundary of seeing a whole new paradigm,” says Ofer Lahav, a cosmologist from University College London and a member of the DESI team.

DESI is mounted on the Nicholas U Mayall four-metre telescope at Kitt Peak National Observatory in Arizona, and has the primary goal of shedding light on the “dark universe”.  The term dark universe reflects our ignorance of the nature of about 95% of the mass–energy of the cosmos.

Intrinsic energy density

Today’s favoured Standard Model of cosmology is the lambda–cold dark matter (CDM) model. Lambda refers to a cosmological constant, which was first introduced by Albert Einstein in 1917 to keep the universe in a steady state by counteracting the effect of gravity. We now know that universe is expanding at an accelerating rate, so lambda is used to quantify this acceleration. It can be interpreted as an intrinsic energy density that is driving expansion. Now, DESI’s findings imply that this energy density is erratic and even more mysterious than previously thought.

DESI is creating a humungous 3D map of the universe. Its first full data release comprise 270 terabytes of data and was made public in March. The data include distance and spectral information about 18.7 million objects including 12.1 million galaxies and 1.6 million quasars. The spectral details of about four million nearby stars nearby are also included.

This is the largest 3D map of the universe ever made, bigger even than all the previous spectroscopic surveys combined. DESI scientists are already working with even more data that will be part of a second public release.

DESI can observe patterns in the cosmos called baryonic acoustic oscillations (BAOs). These were created after the Big Bang, when the universe was filled with a hot plasma of atomic nuclei and electrons. Density waves associated with quantum fluctuations in the Big Bang rippled through this plasma, until about 379,000 years after the Big Bang. Then, the temperature dropped sufficiently to allow the atomic nuclei to sweep up all the electrons. This froze the plasma density waves into regions of high mass density (where galaxies formed) and low density (intergalactic space). These density fluctuations are the BAOs; and they can be mapped by doing statistical analyses of the separation between pairs of galaxies and quasars.

The BAOs grow as the universe expands, and therefore they provide a “standard ruler” that allows cosmologists to study the expansion of the universe. DESI has observed galaxies and quasars going back 11 billion years in cosmic history.

“What DESI has measured is that the distance [between pairs of galaxies] is smaller than what is predicted,” says team member Willem Elbers of the UK’s University of Durham. “We’re finding that dark energy is weakening, so the acceleration of the expansion of the universe is decreasing.”

As co-chair of DESI’s Cosmological Parameter Estimation Working Group, it is Elbers’ job to test different models of cosmology against the data. The results point to a bizarre form of “phantom” dark energy that boosted the expansion acceleration in the past, but is not present today.

The puzzle is related to dark energy’s equation of state, which describes the ratio of pressure of the universe to its energy density. In a universe with an accelerating expansion, the equation of state will have value that is less than about –1/3. A value of –1 characterizes the lambda–CDM model.

However, some alternative cosmological models allow the equation of state to be lower than –1. This means that the universe would expand faster than the cosmological constant would have it do. This points to a “phantom” dark energy that grew in strength as the universe expanded, but then petered out.

“It’s seems that dark energy was ‘phantom’ in the past, but it’s no longer phantom today,” says Elbers. “And that’s interesting because the simplest theories about what dark energy could be do not allow for that kind of behaviour.”

Dark energy takes over

The universe began expanding because of the energy of the Big Bang. We already know that for the first few billion years of cosmic history this expansion was slowing because the universe was smaller, meaning that the gravity of all the matter it contains was strong enough to put the brakes on the expansion. As the density decreased as the universe expanded, gravity’s influence waned and dark energy was able to take over. What DESI is telling us is that at the point that dark energy became more influential than matter, it was in its phantom guise.

“This is really weird,” says Lahav; and it gets weirder. The energy density of dark energy reached a peak at a redshift of 0.4, which equates to about 4.5 billion years ago. At that point, dark energy ceased its phantom behaviour and since then the strength of dark energy has been decreasing. The expansion of the universe is still accelerating, but not as rapidly. “Creating a universe that does that, which gets to a peak density and then declines, well, someone’s going to have to work out that model,” says Lahav.

Scalar quantum field

Unlike the unchanging dark-energy density described by the cosmological constant, a alternative concept called quintessence describes dark energy as a scalar quantum field that can have different values at different times and locations.

However, Elbers explains that a single field such as quintessence is incompatible with phantom dark energy. Instead, he says that “there might be multiple fields interacting, which on their own are not phantom but together produce this phantom equation of state,” adding that “the data seem to suggest that it is something more complicated.”

Before cosmology is overturned, however, more data are needed. On its own, the DESI data’s departure from the Standard Model of cosmology has a statistical significance 1.7σ. This is well below 5σ, which is considered a discovery in cosmology. However, when combined with independent observations of the cosmic microwave background and type Ia supernovae the significance jumps 4.2σ.

“Big rip” avoided

Confirmation of a phantom era and a current weakening would be mean that dark energy is far more complex than previously thought – deepening the mystery surrounding the expansion of the universe. Indeed, had dark energy continued on its phantom course, it would have caused a “big rip” in which cosmic expansion is so extreme that space itself is torn apart.

“Even if dark energy is weakening, the universe will probably keep expanding, but not at an accelerated rate,” says Elbers. “Or it could settle down in a quiescent state, or if it continues to weaken in the future we could get a collapse,” into a big crunch. With a form of dark energy that seems to do what it wants as its equation of state changes with time, it’s impossible to say what it will do in the future until cosmologists have more data.

Lahav, however, will wait until 5σ before changing his views on dark energy. “Some of my colleagues have already sold their shares in lambda,” he says. “But I’m not selling them just yet. I’m too cautious.”

The observations are reported in a series of papers on the arXiv server. Links to the papers can be found here.

The post DESI delivers a cosmological bombshell appeared first on Physics World.

Physicists in Germany have found an alternative explanation for an anomaly that had previously been interpreted as potential evidence for a mysterious “dark force”. Originally spotted in ytterbium atoms, the anomaly turns out to have a more mundane cause. However, the investigation, which involved high-precision measurements of shifts in ytterbium’s energy levels and the mass ratios of its isotopes, could help us better understand the structure of heavy atomic nuclei and the physics of neutron stars.

Isotopes are forms of an element that have the same number of protons and electrons, but different numbers of neutrons. These different numbers of neutrons produce shifts in the atom’s electronic energy levels. Measuring these so-called isotope shifts is therefore a way of probing the interactions between electrons and neutrons.

In 2020, a team of physicists at the Massachusetts Institute of Technology (MIT) in the US observed an unexpected deviation in the isotope shift of ytterbium. One possible explanation for this deviation was the existence of a new “dark force” that would interact with both ordinary, visible matter and dark matter via hypothetical new force-carrying particles (bosons).

Although dark matter is thought to make up about 85 percent of the universe’s total matter, and its presence can be inferred from the way light bends as it travels towards us from distant galaxies, it has never been detected directly. Evidence for a new, fifth force (in addition to the known strong, weak, electromagnetic and gravitational forces) that acts between ordinary and dark matter would therefore be very exciting.

A team led by Tanja Mehlstäubler from the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig and Klaus Blaum from the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg has now confirmed that the anomaly is real. However, the PTB-MPIK researchers say it does not stem from a dark force. Instead, it arises from the way the nuclear structure of ytterbium isotopes deforms as more neutrons are added.

Measuring ytterbium isotope shifts and atomic masses

Mehlstäubler, Blaum and colleagues came to this conclusion after measuring shifts in the atomic energy levels of five different ytterbium isotopes: ^{168,170,172,174,176}Yb. They did this by trapping ions of these isotopes in an ion trap at the PTB and then using an ultrastable laser to drive certain electronic transitions. This allowed them to pin down the frequencies of specific transitions (²S_1/2→²D_5/2 and ²S_1/2→²F_7/2) with a precision of 4 ×10⁻⁹, the highest to date.

They also measured the atomic masses of the ytterbium isotopes by trapping individual highly-charged Yb⁴²⁺ ytterbium ions in the cryogenic PENTATRAP Penning trap mass spectrometer at the MPIK. In the strong magnetic field of this trap, team member and study lead author Menno Door explains, the ions are bound to follow a circular orbit. “We measure the rotational frequency of this orbit by amplifying the miniscule inducted current in surrounding electrodes,” he says. “The measured frequencies allowed us to very precisely determine the related mass ratios of the various isotopes with a precision of 4 ×10⁻¹².”

From these data, the researchers were able to extract new parameters that describe how the ytterbium nucleus deforms. To back up their findings, a group at TU Darmstadt led by Achim Schwenk simulated the ytterbium nuclei on large supercomputers, calculating their structure from first principles based on our current understanding of the strong and electromagnetic interactions. “These calculations confirmed that the leading signal we measured was due to the evolving nuclear structure of ytterbium isotopes, not a new fifth force,” says team member Matthias Heinz.

“Our work complements a growing body of research that aims to place constraints on a possible new interaction between electrons and neutrons,” team member Chih-Han Yeh tells Physics World. “In our work, the unprecedented precision of our experiments refined existing constraints.”

The researchers say they would now like to measure other isotopes of ytterbium, including rare isotopes with high or low neutron numbers. “Doing this would allow us to control for uncertain ‘higher-order’ nuclear structure effects and further improve the constraints on possible new physics,” says team member Fiona Kirk.

Door adds that isotope chains of other elements such as calcium, tin and strontium would also be worth investigating. “These studies would allow to further test our understanding of nuclear structure and neutron-rich matter, and with this understanding allow us to probe for possible new physics again,” he says.

The work is detailed in Physical Review Letters.

The post Atomic anomaly explained without recourse to hypothetical ‘dark force’ appeared first on Physics World.