Catching the Universe Expanding: New Data from a Distant Quasar

Author: Denis Avetisyan

High-precision spectroscopy reveals further evidence of the universe’s accelerating expansion through subtle shifts in light from a quasar billions of light-years away.

A long-term spectroscopic monitoring program, utilizing the ESPRESSO instrument, forecasts the capacity to detect cosmological signals-specifically, redshift variations at <span class="katex-eq" data-katex-display="false"> z=3.57 </span>-with increasing precision as integration time scales up to 1000 hours annually, ultimately establishing a detection threshold at <span class="katex-eq" data-katex-display="false"> 3\sigma </span> or <span class="katex-eq" data-katex-display="false"> 5\sigma </span> dependent on allocated observational resources. — A long-term spectroscopic monitoring program, utilizing the ESPRESSO instrument, forecasts the capacity to detect cosmological signals-specifically, redshift variations at $z=3.57$ -with increasing precision as integration time scales up to 1000 hours annually, ultimately establishing a detection threshold at $3\sigma$ or $5\sigma$ dependent on allocated observational resources.

Researchers present the third epoch of observations from the ESPRESSO Redshift Drift Experiment, refining measurements of cosmic dynamics using quasar absorption lines.

Directly measuring the expansion history of the universe remains a fundamental challenge in cosmology, demanding independent probes beyond traditional methods. This paper, ‘The ESPRESSO Redshift Drift Experiment III — The Third Epoch of QSO J052915.80-435152.0’, presents the third epoch of observations designed to detect the redshift drift – a subtle change in spectral lines signaling cosmic expansion – using high-resolution spectroscopy of a distant quasar. Analysis of 9.5 hours of ESPRESSO data yields a null result, consistent with ΛCDM expectations and establishing the tightest constraints on redshift drift to date. With continued observations and synergistic efforts combining optical and radio facilities, can we ultimately unlock a definitive measurement of cosmic expansion and refine our understanding of the universe’s evolution?

The Universe’s Echo: Mapping Expansion’s Subtle Song

Determining the Universe’s expansion rate, often quantified by the Hubble constant, is fundamental to modern cosmology because it dictates the Universe’s past, present, and ultimate destiny. This rate isn’t merely a number; it’s a key parameter within the $\Lambda CDM$ model, influencing calculations of the Universe’s age, size, and the distribution of matter. A faster expansion suggests a younger, more rapidly evolving cosmos, while a slower rate implies an older, more stable Universe. Furthermore, precise measurements are critical for testing the validity of the $\Lambda CDM$ model itself and for potentially uncovering new physics beyond our current understanding, such as the nature of dark energy – the mysterious force driving the accelerated expansion. Discrepancies in calculated expansion rates, observed through various methods, currently pose significant challenges to cosmological consensus and motivate ongoing research to refine measurement techniques and explore alternative theoretical frameworks.

Cosmological distance measurements traditionally depend on a “distance ladder,” a series of interconnected techniques used to estimate the distances to progressively farther objects. Each rung of this ladder – from parallax and Cepheid variable stars to Type Ia supernovae – relies on the calibration of the previous one, meaning any systematic error at a lower rung propagates upwards, affecting all subsequent distance estimates. Consequently, astronomers are continually refining these calibration steps and exploring alternative methods to minimize uncertainties in determining the Hubble constant, the rate at which the Universe expands. The challenge lies not just in improving the precision of individual measurements, but in rigorously identifying and correcting for potential biases inherent in each technique, a process demanding constant scrutiny and innovation in observational astronomy and data analysis.

Determining the Universe’s expansion rate through direct observation of changing cosmological phenomena presents a formidable challenge to the capabilities of modern astronomy. These measurements aren’t simply about collecting more data; they require an unprecedented level of precision, demanding that instruments detect incredibly subtle shifts in light or position over vast cosmic distances. Maintaining the necessary stability to discern these changes is equally crucial, as even minute vibrations or thermal fluctuations in detectors can easily overwhelm the genuine signals. Current telescopes and observational techniques are being stretched to their absolute limits, necessitating innovative approaches to data calibration, noise reduction, and instrument design. Overcoming these hurdles isn’t just about refining existing tools; it demands fundamentally new technologies and observational strategies to unlock a more accurate understanding of the Universe’s evolution and ultimate fate.

Velocity shifts, measured using pixel-by-pixel (blue circles) and maximum likelihood (red squares) approaches, reveal cosmic acceleration-represented by the solid lines with shaded uncertainty-as a function of time.

A Geometric Echo: Listening for the Universe’s Expansion

The Sandage-Loeb test utilizes a geometric method to determine the Hubble constant, and therefore the Universe’s expansion rate, by observing the time evolution of spectral lines in distant quasars. This approach relies on the principle that photons emitted from quasars undergo redshift due to the expansion of the Universe; changes in this redshift over several years indicate a corresponding change in the distance to the quasar. By precisely measuring the drift in quasar spectral lines – specifically the Lyman-alpha forest – over an extended baseline, the test avoids reliance on traditional distance ladder methods and offers an independent measurement of the Hubble constant $H_0$ . The magnitude of the observed redshift change is directly proportional to the rate of cosmic expansion and the light travel time to the quasar.

The ESPRESSO spectrograph, mounted on the Very Large Telescope, achieves a resolving power exceeding 80,000, enabling the precise measurement of spectral line shifts. Its radial velocity precision of approximately 10 cm/s, combined with long-term stability maintained through environmental controls and calibration procedures, is critical for detecting the predicted redshift drifts-on the order of a few cm/s per year-required by the Sandage-Loeb test. This level of accuracy surpasses that of previous instruments, allowing for a more sensitive and reliable assessment of changes in quasar spectra over multi-year baselines. The instrument’s fiber-fed design and advanced calibration framework minimize systematic errors, further enhancing its capability to detect subtle cosmological signals.

The Sandage-Loeb test utilizes distant quasars as beacons to measure cosmic expansion, and relies on the Lyman-alpha forest for precise redshift determination. This forest consists of absorption lines in the quasar’s spectrum caused by intervening neutral hydrogen clouds along the line of sight. Because the wavelengths of these absorption lines are known, they serve as a stable reference frame. Minute changes in the observed wavelengths of the Lyman-alpha forest absorption lines over time indicate a change in the quasar’s redshift, directly proportional to the rate of cosmic expansion. Quasar J052915.80-435152.0 is a target of this study due to its high redshift and the density of Lyman-alpha absorption features present in its spectrum, providing a statistically significant dataset for measuring these subtle redshift variations.

This research details a continuation of measurements designed to detect the cosmic redshift drift, a predicted consequence of the Universe’s expansion. Utilizing the ESPRESSO spectrograph, observations were conducted on the distant quasar J052915.80-435152.0 across three distinct epochs. These observations build upon previous attempts to measure this drift by providing a refined dataset for analysis. The methodology focuses on identifying subtle changes in the Lyman-alpha forest absorption lines within the quasar’s spectrum, which serve as a stable reference point against which redshift variations are quantified. The presented data contributes to ongoing efforts to empirically verify the accelerating expansion of the Universe and constrain cosmological parameters.

The combined spectrum of SB2 reveals a signal-to-noise ratio sufficient for measuring redshift drift within the Lyman-α forest between 509 and 603 nm, as defined by the quasar's Lyman-α and Lyman-β emissions. — The combined spectrum of SB2 reveals a signal-to-noise ratio sufficient for measuring redshift drift within the Lyman-α forest between 509 and 603 nm, as defined by the quasar’s Lyman-α and Lyman-β emissions.

Chasing Ghosts: Precision Calibration and the Redshift Signal

Precise determination of redshift drift necessitates highly accurate wavelength calibration of astronomical spectra. Redshift drift, the rate of change of redshift over time, is an extremely subtle effect; therefore, uncertainties in wavelength scale directly translate to errors in the measured drift. Calibration establishes the relationship between observed pixel positions on a detector and the corresponding vacuum wavelengths of spectral lines. Common techniques rely on calibrator lamps – typically hollow cathode lamps containing elements like ThAr – that emit known wavelengths. The achievable accuracy is limited by the stability of these lamps, the precision of line identification, and systematic effects like thermal variations in the spectrograph. Any residual wavelength errors introduce a spurious drift signal, masking the true cosmological effect, and must be minimized through careful calibration procedures and data analysis.

Traditional wavelength calibration in spectroscopy relies on Fabry-Pérot etalons (FP) and thorium-argon (ThAr) lamps to establish a precise wavelength scale; however, Laser Frequency Comb (LFC) calibration presents a potentially improved alternative. LFCs generate a spectrum of precisely spaced laser lines directly traceable to an atomic clock, achieving significantly higher accuracy and long-term stability compared to gas-based lamps. Specifically, LFC calibration reduces systematic uncertainties associated with line identification and broadening inherent in FP+ThAr methods, enabling more accurate determination of radial velocity shifts and, consequently, the redshift drift. While FP+ThAr remains a widely used standard due to established infrastructure and data reduction pipelines, LFC technology is increasingly implemented in high-resolution spectrographs aiming for enhanced precision in cosmological measurements.

Single Exposure Analysis (SEA) is implemented alongside traditional methods to mitigate systematic errors inherent in measuring cosmic redshift drift. Conventional analysis relies on multiple exposures and spectral template fitting, which can introduce biases from template inaccuracies and imperfect sky subtraction. SEA, by analyzing individual exposures, reduces the impact of these effects and provides an independent measurement of the redshift drift. Discrepancies between SEA results and those from conventional methods are then used to identify and quantify systematic uncertainties, ultimately leading to a more precise determination of the drift signal and improved confidence in the final result. This approach is particularly valuable for identifying and correcting subtle instrumental effects that might otherwise be masked in multi-exposure analyses.

Current measurements indicate a cosmic redshift drift of -3.43 ± 3.56 meters per second per year. This value represents the rate at which the wavelengths of light from distant galaxies are stretched over time due to the expansion of the universe. The reported drift is consistent with earlier observations establishing this phenomenon, and aligns with predictions derived from the standard $ΛCDM$ cosmological model, which incorporates a cosmological constant (Λ) and cold dark matter (CDM). While the uncertainty remains substantial, the measured drift supports the ongoing acceleration of the universe’s expansion and provides constraints on the equation of state of dark energy.

The median velocity difference over time demonstrates a linear relationship <span class="katex-eq" data-katex-display="false">\pm 1\sigma</span> between LFC and ThAr calibrated data, indicating consistent wavelength solutions. — The median velocity difference over time demonstrates a linear relationship $\pm 1\sigma$ between LFC and ThAr calibrated data, indicating consistent wavelength solutions.

The Fragile Signal: Ensuring Accuracy in a Shifting Universe

Precise determination of cosmological redshift drift hinges critically on the stability of wavelength calibration. Any systematic error in assigning wavelengths to spectral lines directly translates into a spurious velocity shift, obscuring the subtle signal of cosmic expansion. Consequently, meticulous monitoring and correction for wavelength drifts are paramount; even seemingly minor inaccuracies can overwhelm the expected redshift drift signal. Researchers employ techniques like frequent calibration with known spectral lamps, alongside rigorous analysis of calibration data, to minimize these errors and ensure the resulting measurements accurately reflect the universe’s expansion history. The pursuit of exceptionally stable wavelength calibration isn’t merely a technical detail-it’s foundational to obtaining reliable cosmological results.

The reliability of any spectroscopic analysis hinges on a thoroughly vetted data processing pipeline, and simulated data proves instrumental in this validation process. By generating artificial datasets with known characteristics, researchers can rigorously test each stage of the analysis, identifying potential biases or inaccuracies before applying it to real observations. This approach allows for a precise quantification of systematic effects – subtle errors arising not from random noise, but from imperfections in the methodology itself. Specifically, simulations can replicate the influence of factors like instrument response, atmospheric conditions, or data reduction techniques, enabling scientists to assess their impact on the final results and implement appropriate corrections. The creation of these synthetic datasets, therefore, isn’t merely a quality control measure; it’s a fundamental component of ensuring the robustness and trustworthiness of scientific conclusions derived from complex spectroscopic data.

Rigorous analysis of wavelength calibration data reveals a consistent, albeit minute, difference of -6 meters per second between calibrations derived from a laser frequency comb (LFC) and those obtained using a thorium-argon lamp (ThAr). This offset, determined through repeated measurements across multiple observational epochs, suggests a systematic effect inherent in the instrumentation or data reduction process. While seemingly small, such a consistent velocity difference demands careful consideration when interpreting subtle redshift measurements, particularly in studies searching for extremely small velocity shifts indicative of exoplanets or other cosmological phenomena. The stability of this -6 m/s offset across time further reinforces its status as a persistent systematic, necessitating its inclusion in comprehensive error budgets and potentially requiring ongoing monitoring to assess long-term drifts.

A subtle but noteworthy drift in wavelength calibration – specifically, a 0.36 ± 0.49 m/s per year change observed between the LFC and ThAr calibrations – presents a potential source of systematic uncertainty in redshift drift measurements. While seemingly small, this temporal variation necessitates careful monitoring and mitigation strategies within the analysis pipeline. Failing to account for this evolving velocity difference could introduce a spurious signal, mimicking or masking the true cosmological redshift drift. Consequently, researchers are focused on refining calibration procedures and implementing robust data validation techniques to ensure the long-term stability and accuracy of spectral measurements, ultimately bolstering the reliability of studies probing the expansion history of the universe.

A long-term ANDES redshift drift experiment targeting SB2, with continuous monitoring of 10, 100, and 1000 hours per year and an efficiency of <span class="katex-eq" data-katex-display="false">\epsilon_A = 10\%</span>, can extrapolate present results. — A long-term ANDES redshift drift experiment targeting SB2, with continuous monitoring of 10, 100, and 1000 hours per year and an efficiency of $\epsilon_A = 10\%$ , can extrapolate present results.

The pursuit of measuring redshift drift, as detailed in this study of QSO J052915.80-435152.0, exemplifies the inherent fragility of any cosmological model. This experiment, meticulously calibrating wavelengths to detect minute changes, operates at the very edge of observational precision. As Igor Tamm once stated, “Any theory can be made to fit any set of experimental facts.” This sentiment resonates strongly; the data obtained through high-resolution spectroscopy, while providing increasingly refined constraints on the universe’s expansion rate, always remains susceptible to reinterpretation as instrumentation improves or new phenomena are discovered. The ongoing refinement of these measurements serves as a constant reminder that even the most robust theories are provisional, existing only until a more complete picture emerges from the observational data.

The Horizon Beckons

The pursuit of cosmic dynamics, distilled to the meticulous tracing of spectral lines, reveals less about conquering understanding than about the limits of observation. This experiment, and others like it, refine the measurement, tighten the error bars-a slow, painstaking approach to a number already haunted by internal inconsistencies. Each digit gained feels less like a victory and more like a detailed map of the territory still unknown. The universe does not offer itself to be known, it merely tolerates the attempt.

Future iterations will undoubtedly demand greater precision, larger telescopes, longer exposures. Yet the fundamental challenge isn’t technological. It’s conceptual. The drift, if truly measured with definitive certainty, will not solve the tensions in cosmology, it will only highlight them with greater clarity. It is a refinement of the question, not an answer. The deeper the gaze, the more apparent the abyss.

Perhaps the true next step lies not in chasing ever-fainter signals, but in accepting the provisional nature of every conclusion. When a number is declared ‘constant’, the cosmos smiles and swallows it again. It is not a failure of instruments, but a condition of existence. The universe does not reveal its secrets; it permits the illusion of revelation.

Original article: https://arxiv.org/pdf/2603.02318.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

The Universe’s Echo: Mapping Expansion’s Subtle Song

A Geometric Echo: Listening for the Universe’s Expansion

Chasing Ghosts: Precision Calibration and the Redshift Signal

The Fragile Signal: Ensuring Accuracy in a Shifting Universe

The Horizon Beckons

See also: