Cosmic dynamics in the era of Extremely Large Telescopes

Abstract

The redshifts of all cosmologically distant sources are expected to experience a small, systematic drift as a function of time due to the evolution of the Universe's expansion rate. A measurement of this effect would represent a direct and entirely model-independent determination of the expansion history of the Universe over a redshift range that is inaccessible to other methods. Here we investigate the impact of the next generation of Extremely Large Telescopes on the feasibility of detecting and characterising the cosmological redshift drift. We consider the Lyman alpha forest in the redshift range 2 < z < 5 and other absorption lines in the spectra of high redshift QSOs as the most suitable targets for a redshift drift experiment. Assuming photon-noise limited observations and using extensive Monte Carlo simulations we determine the accuracy to which the redshift drift can be measured from the Ly alpha forest as a function of signal-to-noise and redshift. Based on this relation and using the brightness and redshift distributions of known QSOs we find that a 42-m telescope is capable of unambiguously detecting the redshift drift over a period of ~20 yr using 4000 h of observing time. Such an experiment would provide independent evidence for the existence of dark energy without assuming spatial flatness, using any other cosmological constraints or making any other astrophysical assumption.Comment: Accepted for publication in MNRAS, 27 pages, 19 figure

Observational probes of cosmic acceleration

“…For our fiducial cosmological model, the predicted change over a ∆t = 10 year observational span is +1.32 cm s −1 , −1.21 cm s −1 , and −3.66 cm s −1 for sources at z = 2, 3, and 4, respectively. Corasaniti et al (2007) estimate that observations of 240 quasars over a span of 30 years using the CODEX spectrograph proposed for the European Extremely Large Telescope could measure H(z) over z = 2−5 with an aggregate precision of ≈ 2% (see Liske andCodex Team 2006 andQuercellini 2007 for similar discussions and Liske et al 2008 for detailed calculations of CODEX performance). The BAO component of the fiducial Stage IV program that we present in §8.1 (which assumes 25% sky coverage and errors that are 1.8 times those of linear theory sample variance) yields errors of 0.6 − 0.7% in H(z) per bin of 0.07 in ln(1 + z) at z = 2 − 3 (see Table 6).…”

The status of varying constants: a review of the physics, searches and implications

“…ELT-HIRES can measure the redshift drift signal deep in the matter era, using the Ly-α forest and various additional metal absorption lines [231]. Apart from the fundamental aspect of being able to watch the expansion of the universe in real time, one should note that when using these observations to constrain specific models their importance is not so much that they are more constraining than other observational probes but that they tend to probe orthogonal directions in the relevant model parameter spaces, thereby breaking limiting degeneracies.…”

Ceres’ sunlight atlas

“…The reasons are manifold and range from the intensive search for extra solar planets of lower and lower mass to astronomical checks of changes in the coupling fundamental constants, to projects aimed at measuring the change in the expansion velocity of the Universe Levshakov et al 2007;Liske et al 2008).…”