Cosmic Positioning System Proposed for Outer Solar System | Phys.org

A recent concept, dubbed the Cosmic Positioning System (CPS), aims to dramatically improve our understanding of the universe’s expansion rate – a persistent discrepancy known as the “Hubble Tension.” The project, recently detailed in a NASA Institute for Advanced Concepts (NIAC) Phase I report and a paper published on arXiv, proposes a network of radio antennas spread throughout the outer solar system to make incredibly precise distance measurements.

How a ‘Cosmic Positioning System’ Works

Currently, the rate at which the universe expands, known as the Hubble constant, is measured using two primary methods. Measurements based on the cosmic microwave background radiation – the afterglow of the Big Bang – yield a value of approximately 67.4 kilometers per second per megaparsec. But, measurements using “standard candles” like Cepheid variable stars and supernovae suggest a faster expansion rate, around 73 km/s/Mpc. This difference, the Hubble Tension, has puzzled cosmologists for years.

The CPS proposes a fundamentally different approach: geometric distance measurement. Instead of relying on the brightness or characteristics of objects, CPS would directly detect the curvature of electromagnetic wavefronts – essentially, how the shape of a radio wave changes as it travels vast distances. This is analogous to how GPS works on Earth, but scaled up to cosmological proportions. The system would rely on detecting repeating Fast Radio Bursts (FRBs), intense pulses of radio waves from distant sources. By precisely timing the arrival of these bursts at multiple antennas spread across the solar system, the distance to the FRB source can be triangulated.

The core of the system involves Highly Long Baseline Interferometry (VLBI), a technique that combines data from multiple telescopes to create a virtual telescope with an effective aperture equal to the distance between the telescopes. In this case, the “telescope” would span the outer solar system, with baselines – the distance between antennas – reaching tens of astronomical units (AU). One AU is the distance between the Earth and the Sun. According to the arXiv paper, the nominal design calls for five spacecraft, each equipped with an 8-meter deployable antenna and highly sensitive receivers operating in the 3-6 GHz range.

The Outer Solar System as a Cosmic Baseline

Spreading the antennas across the outer solar system is crucial for achieving the necessary precision. The greater the distance between the antennas, the higher the resolution and accuracy of the distance measurements. The Solar System, as defined by the gravitationally bound system of the Sun and its orbiting bodies, extends far beyond the familiar planets. The Kuiper Cliff, a region marking a sharp drop in the density of objects, lies between 50 and 70 AU from the Sun. The heliopause, where the Sun’s influence wanes and interstellar space begins, has been detected at around 120 AU. The CPS would position its antennas within this vast expanse, leveraging the solar system’s scale as a natural advantage.

Impact on Cosmology and Beyond

If successful, CPS could provide a completely independent measurement of the Hubble constant, potentially resolving the Hubble Tension. Sub-percent constraints on the Hubble constant are predicted with even a handful of FRB detections, with 10-100 sources potentially visible. This would have profound implications for our understanding of the universe’s composition, evolution and ultimate fate. Currently, the discrepancy between different Hubble constant measurements forces theorists to consider possibilities like evolving dark energy – the mysterious force driving the accelerated expansion of the universe – or new physics beyond the standard cosmological model.

Beyond cosmology, the CPS technology could also have applications in fundamental physics, such as testing the equivalence principle and searching for variations in fundamental constants. The precise timing and positioning capabilities could also be valuable for deep-space navigation and communication.

Challenges and Uncertainties

The CPS faces significant technical challenges. Precise wavefront timing is paramount, requiring extremely stable atomic clocks onboard each spacecraft – similar to those used in GPS satellites, but with even greater accuracy. Interstellar refractive delays, caused by the bending of radio waves as they pass through the interstellar medium, must be carefully accounted for. Maintaining accurate positional knowledge of the spacecraft and calibrating the system with ground-based ranging are also critical. The Universe Today article highlights the need for a “truly massive infrastructure” to achieve the required sensitivity.

The reliance on repeating FRBs also introduces uncertainty. Although hundreds of FRBs have been detected, their origins and emission mechanisms are still poorly understood. The number of FRBs bright enough and frequent enough for CPS to detect is also uncertain. The study authors acknowledge these challenges but believe they are manageable with near-term technologies.

What Comes Next: From Concept to Reality

The current NIAC Phase I report represents an initial feasibility study. The next step would be a Phase II study, which would involve more detailed design work and technology development. A full-scale CPS mission would require significant investment and international collaboration. The project team is also exploring the possibility of using existing or planned space missions to test key technologies and validate the concept. Further research into FRB sources and their characteristics will also be crucial. The success of CPS hinges on continued technological advancements and a deeper understanding of the cosmos itself.