Figure 2: The two approaches for generating a laser photometric ratio star (LPRS), and their respective expected impacts on measurements of dark energy cosmological parameters, are shown in this figure. The diagrams at top left and top right show atomic levels (not to scale) for neutral sodium atoms (Na I) within the Earth’s upper atmosphere, starting from their ground state (3 S1/2); and showing the atomic states reached via excitations from light from one, or from two, ground-based lasers. In both of the atomic level diagrams, the allowed and the laser-excited atomic states are shown as solid black lines; the “forbidden” 343 nm electric quadrupole de-excitation in the upper left diagram is shown as a gray solid downward-pointing arrow; while “ghost” levels, that are entirely inaccessible from any of the laser-excited states, are shown as dash-dotted lines and in shadowed gray text. (The “5” in the 3 D5/2 state is red to distinguish that atomic state from the [slightly higher-energy] 3 D3/2 state; and the “detuning parameter” δ is approximately 3.9 GHz.) The dotted black horizontal line in the upper right diagram represents the off-resonant energy corresponding to the frequency of the first (i.e., the yellow-orange) laser in the dual-laser approach. As is shown in these upper two diagrams, both of these LPRS approaches result in “fully-mandated cascades” from the 820 nm (or 819 nm) de-excitation to the 589 nm (or 590 nm) de-excitation, resulting in a mandated 1:1 ratio between those produced photons. Using calibrations provided by these approaches, the lower two plots show the expected constraints on the cosmological dark energy equation of state parameters w0 andwa, obtained using simulated catalogs of type Ia supernovae corresponding to the expected first three years of observation at the Vera C. Rubin Observatory. In either approach for generating an LPRS, the LPRS results in large expected improvements in the observational constraints on the dark energy cosmological parametersw0 and wa, with the greater of the two expected improvements being from the dual-laser LPRS approach [13].
measured at wavelengths of 589 nm, vs. astronomical magnitudes measured at wavelengths of 820 nm, to be performed at up to 100-fold better precision than the present approximately 1% uncertainties on such measured SNe Ia magnitude ratios [13]. Such an LPRS would, thus, allow for unprecedented precision on future measurements of dark energy.
Figure 2 shows two different approaches that can be used to create such an LPRS (located at an observatory, for example the Rubin Observatory in Chile). In the single-laser approach, a powerful near-UV laser tuned near 343 nm to excite the 3 D3/2 sodium state would be aimed at the sky above an observatory, and would create a cascade of 819/820 nm photons produced in a 1:1 ratio with 589/590 nm photons in the upper atmosphere. And, in the alternative, dual-laser approach, two co-aligned lasers, one near 589 nm and a second near 820 nm, would work together to excite the 3 D5/2
sodium state, which would then de-excite in a cascade of
820 nm photons produced in a 1:1 ratio with 589 nm photons in the upper atmosphere. Although the single-laser approach is slightly simpler in concept; the dual-laser approach would be both simpler to construct in practice, and also would provide a far brighter LPRS that would result in greater improvement on measurements of dark energy than would the single-laser approach. The dual-laser approach requires two lasers instead of one; however, the relatively larger cross-section of the resonances involved in the dual-laser approach means that one can produce a dual-laser LPRS that is over 1000× brighter than that from the single-laser approach, while using lasers that each require only about 5% of the optical output power of the laser that would be required if using the single-laser approach [13].
An LPRS would precisely calibrate results from a ground-based observatory, and thus would not directly calibrate space observatories such as JWST; however, by using an LPRS at its ground-based observatory to precisely calibrate a set of stable white dwarf stars, one could then use that set of white dwarf stars to (indirectly, but still precisely) calibrate JWST and other space telescopes — as well as other, separate, ground-based observatories [12b,c].
In the dual-laser LPRS approach, the repeated pulses from the two lasers would be timed such that the STIRAP (STImulated Raman Adiabatic Passage) [14] technique for the excitation of the upper-atmospheric sodium atoms would be implemented. STIRAP is a multi-laser technique that has been commonly used within physical chemistry laboratories around the world since the early 1990s [15], however the STIRAP technique has not yet been utilized in the open atmosphere. An implementation of a two-laser LPRS may thus mark the first utilization and observation of “STIRAP in the sky” — in addition to usage of an LPRS for calibration in cosmology and for the understanding of dark energy, as well as for atmospheric physics and chemistry.
Dark energy is, at present, consistent with being the “cosmological constant” from Einstein’s equations of general relativity; and it has been reasonably consistent with being a cosmological constant ever since its initial discovery [1,2,5]. However, amongst other problems [16], this value of a cosmological constant is both unexplained, and unexpected, by the effective quantum field theory that is the Standard Model of particle physics [17]. Also, its relation, if any, to the vastly larger cosmological constant-like expansion that appears to have occurred within the first 10–33 s after the Big Bang, known as “cosmological inflation,” remains unexplained [18]. So, is dark energy a cosmological constant, or not…? It may take some human-generated light — precise artificial stars — in order to determine the true nature of our Universe’s dark side.
PEER REVIEW
The peer review history for this article is available at https://publons.com/publon/10.1002/ntls.2022xyz.
This Highlight has been internally reviewed and edited by ab andcd .
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