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A Solar System–Sized Experiment: New Proposal for Precision Cosmology and More

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A graphic of a spacecraft with a large dish antenna flying past Jupiter. The Milky Way and numerous stars are in the background.
Artist’s impression of Pioneer 11, one of the first spacecraft to reach the outer solar system, as it passed by Jupiter. [NASA]

Using a network of faraway telescopes in the outskirts of the solar system, astronomers could measure the distance to much farther away galaxies with exquisite precision. A recent study describes how this tactic works and explores what else we could learn with such a bold experiment.

Very, Very Long Baselines

Distance is notoriously a tricky quantity to measure in astrophysical contexts, and astronomers have struggled to size up the universe since Hubble first drew his famous diagram. While they have certainly made progress over the last century, it’s natural to wonder if modern technology could enable an entirely new, more precise way to measure the gaps between galaxies.

A plot showing a source of radio emission surrounded by concentric circles, each meant to represent a wavefront. Below the source are 3 detectors in a horizontal line. The same wavefront is touching, ahead of, and behind the three detectors respectively, illustrating how it strikes them at different times.

A sketch of three detectors and a fast radio burst source. Since the wavefront is slightly curved, the same emission will strike each detector at different times. Using measurements of those differences, astronomers can back out the distance to the source. [Boone and McQuinn 2023]

This thinking led Kyle Boone and Matthew McQuinn (University of Washington) to propose a bold new experiment. Their idea, described in a recent publication in the Astrophysical Journal Letters, is to scatter a fleet of radio telescopes throughout the solar system and instruct them to all observe the same flashing, repeating fast radio burst at the same time. Since each flash is emitted equally in all directions at the same time, the wavefront will be slightly curved when it arrives and will strike each satellite at a very slightly different time. Add up these nanosecond delays between each, and with some geometry you can back out the distance to the source.

Such a mission would require solving numerous intense, but feasibly surmountable, engineering challenges. Chief among these, astronomers would have to know the distances between the telescopes to within just a few centimeters, a demanding requirement considering the millions of miles separating them and the many subtle forces that affect their motion. Also, each satellite would need to nurture an ultra-precise atomic clock in the face of the unforgiving vacuum of space. But, should engineers resolve these hindrances, a constellation of four or more telescopes drifting in the outer solar system could pin down the distance to each observed flash to within 1% uncertainty.

Spanning Distances and Disciplines

A line plot of fractional uncertainty vs. distance. Five curves are shown for five different spacecraft baselines, ranging from 12.5 AU to 200 AU. Generally, the larger the baseline, the smaller the fractional uncertainty, and the closer the source, the smaller the fractional uncertainty.

The uncertainty in a measurement of the distance to a source as a function of the true distance to the source for a number of different satellite configurations. Each color represents a different possible baseline separation, and the thickness of each region marks how the uncertainty changes if the resolution of their separation varies between 0.5 and 2 cm. Note that for a source closer than 100 megaparsecs (approximately 300 million light-years), a 25 AU baseline could measure its distance to better than 1%. [Boone and McQuinn 2023]

This experiment was conceived explicitly with precision cosmology in mind, and as Boone and McQuinn show, would be demonstrably revolutionary in that field. However, should astronomers be audacious enough to build a solar system–sized hammer, there are more than a few outstanding nails the same hardware could bludgeon. Take dark matter, for example: several models suggest that invisible clumps of the stuff should occasionally fly through the solar system at high speed. This experiment would necessarily be sensitive enough to notice the slight gravitational tug of such an encounter, meaning even a non-detection of occasional jostles could help constrain our theories of dark matter’s form. Similarly, the much debated “Planet 9” would be unable to evade such an exquisitely sensitive instrument: over time, even from hundreds of AU away, any large planets lurking in the outer solar system would eventually nudge these radio telescopes out of place.

While this study may never grow into more than a thought experiment, such an exercise is constructive nonetheless and gives the astronomical community a chance to reflect on its current capabilities and muse about its future. That said, a more hopeful interpretation is to take this as a starting point for a grand, exacting, colossal mission that could one day uncover secrets of the universe, and our own backyard, all at once.

Citation

“Solar System-scale Interferometry on Fast Radio Bursts Could Measure Cosmic Distances with Subpercent Precision,” Kyle Boone and Matthew McQuinn 2023 ApJL 947 L23. doi:10.3847/2041-8213/acc947