The ESPRESSO Redshift Drift Experiment III -- The Third Epoch of QSO J052915.80-435152.0

Abstract

The Sandage-Loeb test probes cosmic expansion directly by measuring the redshift drift in quasar absorption features in a model-independent way. In this series of papers, we have launched an observational campaign to assess whether current instrumentation is capable of measuring this effect and what systematic effects might interfere with a detection. We report the observations and analysis of the third epoch of ESPRESSO observations of the bright quasar J052915.80-435152.0 (SB2, z=3.962), extending the temporal baseline to $\sim2$ years, and providing the tightest constraints on the redshift drift in the series so far. We acquired 9.5 hours of ESPRESSO observations, complementing the 12 hours presented in the first paper of the series, with one year of separation from the second epoch. The complete dataset was analysed and compared to spline-based Lyman-$α$ forest models calibrated on simulations, to measure the presence of any velocity drift among the spectra. The measurement was carried out with two independent methods. Both approaches give a consistent null result, $\dot{v} = -3.5 \pm 3.6 ~{\rm m s^{-1} yr^{-1}}$ (or $\dot{z} = (-5.3\pm5.6)\times 10^{-8}~{\rm yr^{-1}}$ in redshift space), in agreement with $Λ$CDM expectations, systematic effects remain subdominant at the present level of noise. By extrapolating the results from the observed sightline to the complete QUBRICS Golden Sample, we show that ESPRESSO alone could detect the signal on century timescales, while a joint ESPRESSO+ANDES programme would reach first detection before 2080. A future analysis of the other quasars of the QUBRICS Golden Sample is required to improve this estimate. We show that the program would greatly benefit from a complementary effort with radio facilities targeting low-z HI 21 cm absorption lines. Such synergy could reduce the experiments' timeline by up to $\sim10$ years.

Figures (9)

• Figure 1: Combined spectrum of SB2. Top panel: Flux density in arbitrary units is shown in black, with the purple line denoting the flux density error. Bottom panel: S/N per $1~{\rm km\,s^{-1}}$ pixel. The green shaded area highlights the Lyman-$\alpha$ forest considered in the redshift drift measurement, bound by the Lyman-$\alpha$ and Lyman-$\beta$ emissions of the quasar, namely between 509 - 603 nm.
• Figure 2: Velocity shifts of every single epoch with respect to the combined mean model, as a function of time, obtained via the Pixel-by-pixel (Sect. \ref{['sect:pix2pix']}, blue circles) and the Maximum Likelihood approaches (Sect. \ref{['sect:Likelihood']}, red squares). For both approaches, the cosmic acceleration is computed as a linear fit of the data points and shown as a solid line. Shaded areas report the uncertainty in the fit.
• Figure 3: Top panel: Velocity shift between transmission model and single exposure flux, subdivided in the three observational epochs: first (green), second (red) and third (black). The horizontal dashed lines report the average velocity shift for each epoch, weighted by single exposure velocity shift uncertainty, and their uncertainty as shaded areas. Middle panel: median S/N per $1~{\rm km\,s^{-1}}$ pixel at continuum in the forest for each exposure. Bottom panel: velocity shift uncertainty for each exposure. The dotted grey line reports the expected precision computed as a simple ${\rm S/N}^{-1}$ scaling.
• Figure 4: Extrapolation of the present results based on three epochs to a long-term ESPRESSO redshift drift experiment targeting SB2 with a continuous monitoring programme of 10, 100, and 1000 hours of integration per year. Solid green lines define the forecasted measurement precision of such a programme, depending on the allocated time, as a function of the experiment baseline. Horizontal black dashed line defines the magnitude of the cosmological signal in a $\Lambda$CDM scenario at $z=3.57$, i.e. the mean redshift of SB2's forest. Vertical arrows denote the timestamp at which a 3$\sigma$ (solid) and 5$\sigma$ (dotted) detection is achieved for each time allocation, with the same colour coding.
• Figure 5: Extrapolation of the present results based on three epochs to a long-term ANDES redshift drift experiment targeting SB2 with a continuous monitoring programme of 10, 100, and 1000 hours of integration per year, assuming a $\epsilon_A=10\%$ efficiency.