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 .
ORCID