If space is infused with ‘dark energy,’ as seems to be the case, we have an explanation for the continuing acceleration of the universe’s expansion. Or to speak more accurately, we have a value we can plug into the universe to make this acceleration happen. Exactly what causes that value remains up for grabs, and indeed frustrates current cosmology, for something close to 70 percent of the total mass-energy of the universe needs to be comprised of dark energy to make all this work. Add on the mystery of ‘dark matter’ and we actually see only some 4 percent of the cosmos.

So there’s a lot out there we know very little about, and I’m interested in mission concepts that seek to probe these areas. The conundrum is fundamental, for as a 2017 study from NASA’s Innovative Advanced Concepts office tells me, “…a straightforward argument from quantum field theory suggests that the dark energy density should be tens of orders of magnitude larger than what is observed.” Thus we have what is known as a cosmological constant problem, for the observed values depart from what we know of gravitational theory and may well be pointing to new physics.

The report in question comes from Nan Yu (Jet Propulsion Laboratory) and collaborators, a Phase I effort titled “Direct probe of dark energy interactions with a Solar System laboratory.” It lays out a concept called the Gravity Probe and Dark Energy Detection Mission (GDEM) that would involve a tetrahedral constellation of four spacecraft, a configuration that allows gravitational force measurements to be taken in four simultaneous directions. These craft would operate about 1 AU from the Sun while looking for traces of a field that could explain the dark energy conundrum.

Now JPL’s Slava Turyshev has published a new paper which is part of a NIAC Phase II study advancing the GDEM concept. I’m drawing on the Turyshev paper as well as the initial NIAC collaboration, which has now proceeded to finalizing its Phase II report. Let’s begin with a quote from the Turyshev paper on where we stand today, one that points to critical technologies for GDEM:

Recent interest in dark matter and dark energy detection has shifted towards the use of experimental search (as opposed to observational), particularly those enabled by atom interferometers (AI), as they offer a complementary approach to conventional methods. Situated in space, these interferometers utilize ultra-cold, falling atoms to measure differential forces along two separate paths, serving both as highly sensitive accelerometers and as potential dark matter detectors.

Thus armed with technology, we face the immense challenge of such a mission. The key assumption is that the cosmological constant problem will be resolved through the detection of light scalar fields that couple to normal matter. Like temperature, a scalar field has no direction but only a magnitude at each point in space. This field, assuming it exists, would have to have a mass low enough that it would be able to couple to the particles of the Standard Model of physics with about the same strength as gravity. Were we to identify such a field, we would move into the realm of so-called ‘fifth forces,’ a force to be added to gravity, the strong nuclear force, the weak nuclear force and electromagnetism.

Can we explain dark energy by attempting to modify General Relativity? Consider that its effects are not detectable with current experiments, meaning that if dark energy is out there, its traces are suppressed on the scale of the Solar System. If they were not, the remarkable success scientists have had at validating GR would not have happened. A workable theory, then, demands a way to make the interaction between a dark energy field and normal matter dependent on where it occurs. The term being used for this is screening.

We’re getting deep into the woods here, and could go further still with an examination of the various screening mechanisms discussed in the Turyshev paper, but the overall implication is that the coupling between matter and dark energy could change in regions where the density of matter is high, accounting for our lack of detection. GDEM is a mission designed to detect that coupling using the Solar System as a laboratory. In Newtonian physics, the gravitational gradient tensor (CGT), which governs how the gravitational force changes in space, would have a zero trace value in a vacuum in regions devoid of mass. That’s a finding consistent with General Relativity.

But if in the presence of a dark energy field, the CGT trace value would be other than zero. The discovery of such a variance from our conventional view of gravity would be revolutionary, and would support theories deviating from General Relativity.

Image: Illustration of the proposed mission concept – a tetrahedral constellation of spacecraft carrying atomic drag-free reference sensors flying in the Solar system through special regions of interest. Differential force measurements are performed among all pairs of spacecraft to detect a non-zero trace value of the local field force gradient tensor. A detection of a non-zero trace, and its modulations through space, signifies the existence of a new force field of dark energy as a scalar field and shines light on the nature of dark energy. Credit: Nan Yu.

The constellation of satellites envisioned for the GDEM experiment would fly in close formation, separated by several thousand kilometers in heliocentric orbits. They would use high-precision laser ranging systems and atom-wave interferometry, measuring tiny changes in motion and gravitational forces, to search for spatial changes in the gravitational gradient, theoretically isolating any new force field signature. The projected use of atom interferometers here is vital, as noted in the Turyshev paper:

GDEM relies on AI to enable drag-free operations for spacecraft within a tetrahedron formation. We assume that AI can effectively measure and compensate non-gravitational forces, such as solar radiation pressure, outgassing, gas leaks, and dynamic disturbances caused by gimbal operations, as these spacecraft navigate their heliocentric orbits. We assume that AI can compensate for local non-gravitational disturbances at an extremely precise level…

From the NIAC Phase 1 report, I find this:

The trace measurement is insensitive to the much stronger gravity field which satisfies the inverse square law and thus is traceless. Atomic test masses and atom interferometer measurement techniques are used as precise drag-free inertial references while laser ranging interferometers are employed to connect among atom interferometer pairs in spacecraft for the differential gradient force measurements.

In other words, the technology should be able to detect the dark energy signature while nulling out local gravitational influences. The Turyshev paper develops the mathematics of such a constellation of satellites, noting that elliptical orbits offer a sampling of signals at various heliocentric distances, which improves the likelihood of detection. Turyshev’s team developed simulation software that models the tetrahedral spacecraft configuration while calculating the trace value of the CGT. This modeling along with the accompanying analysis of spacecraft dynamics demonstrates that the needed gravitational observations are within range of near-term technology.

Turyshev sums up the current state of the GDEM concept this way:

…the Gravity Probe and Dark Energy Detection Mission (GDEM) mission is undeniably ambitious, yet our analysis underscores its feasibility within the scope of present and emerging technologies. In fact, the key technologies required for GDEM, including precision laser ranging systems, atom-wave interferometers, and Sagnac interferometers, either already exist or are in active development, promising a high degree of technical readiness and reliability. A significant scientific driver for the GDEM lies in the potential to unveil non-Einsteinian gravitational physics within our solar system—a discovery that would compel a reassessment of prevailing gravitational paradigms. If realized, this mission would not only shed light on the nature of dark energy but also provide critical data for testing modern relativistic gravity theories.

So we have a mission concept that can detect dark energy within our Solar System by measuring deviations found within the classic Newtonian gravitational field. And GDEM is hardly alone as scientists work to establish the nature of dark energy. This is an area that has fostered astronomical surveys as well as mission concepts, including the Nancy Grace Roman Space Telescope, the European Space Agency’s Euclid, the Vera Rubin Observatory and the DESI collaboration (Dark Energy Spectroscopic Instrument). GDEM extends and complements these efforts as a direct probe of dark energy which could further our understanding after the completion of these surveys.

There is plenty of work here to bring a GDEM mission to fruition. As Turyshev notes in the paper: “Currently, there is no single model, including the cosmological constant, that is consistent with all astrophysical observations and known fundamental physics laws.” So we need continuing work on these dark energy scalar field models. From the standpoint of hardware, the paper cites challenges in laser linking for spacecraft attitude control in formation, maturation of high-sensitivity atom interferometers and laser ranging with one part per 1014 absolute accuracy. Identifying such technology gaps in light of GDEM requirements is the purpose of the Phase II study.

As I read this, the surveys currently planned should help us hone in on dark energy’s effects on the largest scales, but its fundamental nature will need investigation through missions like GDEM, which would open up the next phase of dark energy research. The beauty of the GDEM concept is that it does not depend upon a single model, and the NIAC Phase I report notes that it can be used to test any modified gravity models that could be detected in the Solar System. As to size and cost, this looks like a Large Strategic Science Mission, what NASA used to refer to as a Flagship mission, about what we might expect from an effort to answer a question this fundamental to physics.

The paper is Turyshev et al., “Searching for new physics in the solar system
with tetrahedral spacecraft formations,” Phys. Rev. D 109 (25 April 2024), 084059 (abstract / preprint).