There have been plenty of attempts to resolve the “Hubble Tension” in cosmology. This feature describes how one of the most important variables in cosmology, the expansion of the universe, takes on different values depending on how you measure it. A new NASA Institute for Advanced Concepts (NIAC) Phase I report on the Cosmic Positioning System (CPS) details another potential solution to it - this one involving a network of five far-flung satellites spread throughout the solar system.
To understand why CPS matters, it's best to understand how we currently measure the Hubble constant - the rate of expansion of the universe. Measuring it using the microwave background radiation of the universe gives a value of 67.4km/s/megaparsec - and that number has been built into the standard cosmological model for decades. However, when measuring using other methods, such as Cepheid variable stars and supernovae, astronomers get a number closer to 73 km/s/Mpc. This discrepancy in measured values is known as the Hubble Tension, and it causes lots of issues when it comes to estimating the distance (and therefore age) of cosmological objects.
Modern measurements, such as the Dark Energy Spectroscopic Instrument (DESI) and the James Webb Space Telescope (JWST) have done little to resolve the tension, and have even caused theorists to start looking at whether dark energy could evolve over time. A more sensitive experiment could, in theory, put that speculation to rest. But becoming more sensitive to these minute differences requires some truly massive infrastructure.
Fraser discusses how we know Dark Energy existsCPS is a mission designed to provide that infrastructure. It will consist of five satellites, spaced evenly throughout the solar system, each with a baseline between them of between 20 and 100 AU - that’s 20-100 times the distance from the Earth to the Sun. Using a technique similar to the triangulation used by GPS satellites to monitor position, these satellites can measure the distance to far-off objects directly by keeping a close eye on how long it takes certain information, like a photon, to travel between them. With a long enough distance, and good enough time-keeping, there should be enough signal to differentiate where precisely a signal is coming from.
To make the system work, though, requires some beefy engineering. Each satellite will require an 8 to 9 meter wide antenna, which must be deployable since nothing that large can fit in current generations of rocket fairings. While radio antennas don’t have to have the precision machining of optical or infrared ones, they do still require cooling - down to a frigid 20K by the reports estimates. Being so far away from the Sun would, in many cases, help with that, but it might still require an active cooling system.
But perhaps the most important feature of the system is a clock. The CPS project team suggests using one equivalent to NASA’s Deep Space Atomic Clock, which has some flight heritage from the STP-2 mission from 2019-2021. But, to fit into the CPS package it must be miniaturized, and scaled down from a power perspective as power is at a premium where solar panels only collect a tiny amount of sunlight.
NASA’s atomic clocks would be critical for the CPS mission. Credit - NASA Jet Propulsion Laboratory YouTube ChannelRadioisotope thermal generators would likely be needed to supplement that power requirement, and would enable the use of high-speed analog-to-digital converters to capture the full bandwidth of signals before returning them back to Earth. But those signals wouldn’t just be used to try to resolve the Hubble Tension.
Several other secondary objectives are possible with this same technical set-up. Researchers could analyze the “clumpiness” of dark matter by tracking the “wobbling” of Fast Radio Bursts. They could also detect micro-hertz frequency gravitational waves, including some from supermassive black hole binaries - still an ongoing field of research. But perhaps most interestingly, small tweaks in the gravitational pull of the spacecraft themselves could inform us about the gravity of the Kuiper belt - and the hypothetical “Planet 9” if it’s really out there.
Don’t get your hopes up for such a mission yet. NIAC is for bleeding-edge technology assessments, and most technologies that come out of it are a non-starter for the agency even after successful completion of the program. CPS hasn’t received any additional funding as of yet, and it remains to be seen if it ever will. At least the idea for such a system is out there, and this report shows that it is operationally feasible, assuming a few tweaks are implemented. Maybe someone with a real interest in space exploration will pick up the tab for further development - until then CPS will remain a paper analysis only.
Learn More:
M. McQuinn et al - NIAC project report: Solar system-scale VLBI to dramatically improve cosmological distance measurements
UT - Is the Hubble Tension Starting to Go Away?
UT - If The Supernova Standard Candle Is Wrong, It Could Solve The Hubble Tension
UT - An Interesting Solution to the Hubble Tension: The Universe is Slowly Spinning