D1 Observable Universe: Three-Dimensional Spectroscopic Cartographies

Topic

The centerpiece of this model is a three-dimensional map of the universe, constructed from the spectra of millions of galaxies obtained simultaneously by two major spectroscopic surveys.

One of these surveys employs a large-format camera and a spectrograph with hundreds of optical fibers mounted on a 2.5-meter-diameter telescope; the more recent survey, by contrast, uses five thousand robotic fiber positioners—targeting five thousand objects at once on a 4-meter-diameter telescope—to capture spectra across ten distinct bands simultaneously.

Each optical fiber channels light from an object into a spectrograph, which disperses the light and measures the exact wavelengths of its emission and absorption lines; the redshift of each galaxy is derived from these measurements.

The most remarkable feature of the newer survey is its robotic positioning system: thousands of independent actuators position each fiber in mere seconds with precision down to a few microns, enabling the imaging of an entire field of the sky and the seamless repetition of the process. In 2025, this survey yielded the most precise measurement to date of baryon acoustic oscillations across a wide redshift range, uncovering a nascent indication that dark energy might not be perfectly constant.

This same scale of baryon acoustic oscillations also serves as a cosmic standard ruler: measuring its angular size as a function of redshift determines the universe's expansion rate without requiring any assumptions about the physics of dark energy. Thus, from the three-dimensional map of the universe that opens this document to the cosmic standard ruler that concludes it—taking in robotic fiber positioners and the redshift of each galaxy along the way—the observable universe allows millions of individual spectra, captured one by one, to collectively map out the complete picture and the universe's own expansion rate.

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