Cosmology and fundamental physics with the ELT-ANDES spectrograph

Abstract

State-of-the-art 19th century spectroscopy led to the discovery of quantum mechanics, and 20th century spectroscopy led to the confirmation of quantum electrodynamics. State-of-the-art 21st century astrophysical spectrographs, especially ANDES at ESO's ELT, have another opportunity to play a key role in the search for, and characterization of, the new physics which is known to be out there, waiting to be discovered. We rely on detailed simulations and forecast techniques to discuss four important examples of this point: big bang nucleosynthesis, the evolution of the cosmic microwave background temperature, tests of the universality of physical laws, and a real-time model-independent mapping of the expansion history of the universe (also known as the redshift drift). The last two are among the flagship science drivers for the ELT. We also highlight what is required for the ESO community to be able to play a meaningful role in 2030s fundamental cosmology and show that, even if ANDES only provides null results, such 'minimum guaranteed science' will be in the form of constraints on key cosmological paradigms: these are independent from, and can be competitive with, those obtained from traditional cosmological probes.

Keywords: ANDES; Cosmology; Fundamental physics; High-resolution spectroscopy.

Fig. 1

The left panel shows the current state-of-the-art quantities that are relevant to BBN ($1\sigma$ and $2\sigma$ confidence regions are shown by the dark and light shades, respectively): (1) the observational determination of D/H (red contours); (2) the baryon density inferred from the CMB temperature fluctuations (grey contours); and (3) the theory conversion between D/H and the baryon density (blue contours). The uncertainties of these three quantities are well-matched with the current state-of-the-art. Assuming that the precision of these measures in the future will reduce by a comparable amount each, the middle panel illustrates the expected constraints from the ELT-ANDES baseline concept (a factor of $\sim 2$ improvement of D/H), while the right panel illustrates the benefit of including a U band spectrograph in the ANDES design (a factor of $\sim 5$ improvement of the D/H measurement precision)

Fig. 2

Left: An example of CO absorption band observed at $z=2.69$ towards the quasar J1237+0647 with VLT/UVES ($R=50000$, S/N$\sim 40$ ${\text{pixel}}^{-1}$), and as expected with ANDES with $R=100000$, S/N$\sim 100$ ${\text{pixel}}^{-1}$. The numbered tick lines indicate the different rotational levels that compose the band. Right: Simultaneous constraints on CMB temperature, density and kinetic temperature using profile fitting of CO lines coupled with excitation model for CO rotational levels. The blue and red colours represent the constraints obtained using existing UVES spectrum and a forecast for ANDES, respectively

Fig. 3

Comparison of simulated quasar spectra for ANDES versus ESPRESSO. A 1-hour observation of an $r=17.1$ mag quasar with ANDES will provide $\mathrm{S}/\mathrm{N}\approx 100$ per 3.0 km ${\text{s}}^{-1}$ resolution element, $\approx$3.3 times larger than ESPRESSO. To illustrate the effect of a varying $\alpha$, the simulated ANDES absorption lines (orange) have been shifted by $\mathrm{\Delta }\alpha /\alpha =1\times {10}^{-3}$ to produce the light green lines; this is more than three orders of magnitude larger than the 0.3 ppm precision on $\mathrm{\Delta }\alpha /\alpha$ expected from ANDES. Left panels: An idealised, single velocity component in 3 of the $\sim$5–15 transitions typically observed in quasar absorption systems which can constrain $\mathrm{\Delta }\alpha /\alpha$. This would produce a factor of $\sim$5 improvement in statistical uncertainty compared with ESPRESSO. Right panels: A more realistic, multi-component absorber where the higher S/N in ANDES significantly improves the ability to determine the velocity structure as well

Fig. 4

Model-independent constraints on the cosmographic series for $\mathrm{\Delta }\alpha /\alpha$, cf. Martins et al. [53]. The coefficient $a_1$ is predominantly constrained by local experiments (e.g., atomic clock tests), while $a_2$ can only be constrained by astrophysical observations. Solid black contours show current constraints, and red dotted ones show the impact of ANDES measurements of 15 absorbers, under the assumptions outlined in the text, together with the assumption of a fiducial model without variations

Fig. 5

Visualization of the adopted concepts for precision, accuracy, and stability. The right panel shows a sequence of wavelength measurements (small dots) for two spectral lines (blue and orange) taken over a certain period of time. Shown is the deviation of the inferred value from the truth (horizontal black line). To aid the eye, measurements are grouped into bins (thick dots) and the left panel shows histograms for the points falling into the first bin. Precision is the scatter of formally identical measurements (due to stochastic effects), while accuracy describes the offset (or bias) of this distribution from the true value. Over time, this offset will not be stable but be subject to drifts, which characterizes the stability. The ANDES design goals for these properties are indicated in red

Fig. 6

Redshift-wavelength relation for the species of interest to the science cases discussed in the present work: metals for $\alpha$, ${\text{H}}_2$ for $\mu$, DI, HI and HeI* for BBN, CO, ${\text{H}}_2$ and CN for the CMB temperature, and HI for the redshift drift. The foreseen ANDES bands are also displayed. Bearing in mind that most (bright) quasars are at $z\le 3$, this shows that the U band is essential for ANDES to be a competitive fundamental physics probe in the 2030s