Riess Et AlEdit
Riess et al. refers to a research program led by astronomer Adam Riess and a collaboration of colleagues that has become a benchmark in measuring the present rate of cosmic expansion, the Hubble constant. Through a careful construction of the cosmic distance ladder, the team ties together geometric distances in the Milky Way and nearby galaxies with standard candles that reach farther into the observable universe. The result is a local, bottom-up determination of how fast the universe is expanding today, expressed in the conventional units of velocity per distance, typically shown as kilometers per second per megaparsec (km s−1 Mpc−1). The approach places particular emphasis on robust calibrations of {{Cepheid variables}} and {{Type Ia supernovae}} as distance indicators, and it relies on a chain of measurements that aims to minimize model dependence and maximize cross-checks against independent anchors such as geometric parallax, the {{Large Magellanic Cloud}}, and the nearby megamaser distance to {{NGC 4258}}.
Riess et al. have repeatedly sharpened the measurement by refining the anchors and the calibration steps, and by extending the distance ladder to cosmological scales where Type Ia supernovae can be used as bright, reach-extending beacons. The method also hinges on precise instrumentation and careful treatment of astrophysical effects such as interstellar extinction, metallicity, crowding in dense stellar fields, and the geometric zero point of parallax. The outcome is a value for the Hubble constant that serves as a direct, empirical reference point for the current expansion rate and, by extension, for the inferred age and evolution of the universe since the Big Bang. For readers following the topic, see Hubble constant and Cepheid variables for the core concepts, and Type Ia supernovae for the distant rung of the ladder.
Background
The Hubble constant has two broad routes of inference. One comes from the local, distance-based ladder that Riess et al. helped pioneer, which anchors the expansion rate in the present era by connecting geometric distances to standard candles. The other route comes from the early universe, where measurements of the Cosmic microwave background anisotropies—most notably from the Planck (spacecraft) mission—constrain the expansion rate indirectly through the standard cosmological model, commonly referred to as the Lambda-CDM model in many summaries. The tension between these two approaches—local measurements tending to yield a higher H0 and CMB-based inferences tending to yield a lower H0—has become a central topic in contemporary cosmology. See discussions of the Hubble tension for a full treatment of the discrepancy and its implications.
In constructing their ladder, Riess et al. rely on several interconnected pieces of astronomy. The first rung uses a geometric distance scale, including parallax measurements of nearby {{Cepheid variables}} as calibrators. The second rung uses these calibrated Cepheids to calibrate the luminosity of Type Ia supernovae in galaxies where both indicators can be observed. The final rung extends to distant supernovae in the Hubble flow, where peculiar motions become negligible and the expansion rate imprints itself on recession velocities. The cross-checks include alternate anchors such as the {{Large Magellanic Cloud}} and the nearby megamaser distance to {{NGC 4258}}. See parallax for the geometric basis and Cepheid variables for the role of these stars in distance measurement, as well as Type Ia supernovae for the bright, standard candles used at greater distances.
Methodology and findings
Riess et al. emphasize a meticulous handling of systematic uncertainties. The analysis typically hinges on:
- Calibrating the {{Cepheid period-luminosity relation}} with multiple independent distance anchors, including parallax measurements from space-based instruments and cross-checks against the {{Large Magellanic Cloud}} distance. See parallax and Large Magellanic Cloud for context.
- Correcting for the effects of interstellar dust and metallicity, which can bias the inferred luminosities of Cepheids.
- Extending the distance scale with {{Type Ia supernovae}} as standardized brightness indicators, allowing a direct link from the local calibrator regime to the distant Hubble flow. See Type Ia supernovae and distance ladder.
- Verifying the zero points with nearby geometric distances, including the megamaser distance to {{NGC 4258}} and other geometric anchors. See Megamaser and NGC 4258.
The resulting value for the Hubble constant from their analyses has consistently fallen in the neighborhood of roughly 73–74 km s−1 Mpc−1, depending on the precise data choices and calibration steps. This figure stands in tension with the value inferred from the early-universe analysis of the Cosmic microwave background under the Lambda-CDM model assumptions, which sits nearer to the mid-60s in the same units. The degree of disagreement—often described as a few standard deviations, sometimes framed more strongly in recent literature—has made Riess et al. a focal point in debates about cosmology and the reliability of distance indicators. See Hubble tension for a consolidated discussion of this discrepancy.
Controversies and debates
The central controversy centers on whether the discrepancy signals new physics beyond the standard cosmological model or reflects unrecognized systematic errors in one or both measurement regimes. Proponents of the local, Riess et al.-driven approach argue that:
- The distance ladder is grounded in multiple, independent rungs, and the cross-consistency among these rungs provides a robust check against large-scale biases.
- New early-universe physics, such as previously unknown relativistic particles or a different energy budget in the early cosmos, could reconcile the two routes without overturning the core success of the ΛCDM framework.
- The tension should be treated as a discovery signal that motivates further, independent verification and refinement of distance indicators, rather than as a reason to abandon the local measurements.
On the other side, critics of the tension stress that:
- Systematic effects in Cepheid calibrations, including metallicity and crowding in dense star-forming regions, may be underestimated or incompletely modeled.
- Parallax zero-point uncertainties and photometric calibration across different instruments can subtly shift the ladder’s anchor.
- The significance of the tension depends on the choice of data subsets and modeling assumptions; different analyses using the same datasets can yield modestly different results.
- In their view, a rigorous, model-agnostic approach should await safer convergence across a broad suite of independent measurements before drawing conclusions about new physics.
From a broader perspective, the scientific community treats the Hubble tension as a healthy sign of robustness in the scientific method: a challenge to test assumptions, improve measurements, and consider new hypotheses only after exhausting conventional explanations. In this spirit, Riess et al. and their critics engage in a dialog that improves both the precision of the local expansion rate and the understanding of whatever new physics might be required to bring the measurements into concordance. See Hubble tension for a survey of the competing positions and proposed resolutions, including references to alternative distance indicators like the Tip of the red-giant branch method and other cross-checks.
Some observers also emphasize the practical implications of resolving the tension. If the local H0 value is indeed higher, the implied age and growth history of cosmic structures, the timing of reionization, and the calibration of distances to distant galaxies feeding into a broad array of observational programs become tightly constrained. Conversely, a resolution that shifts the Planck-based inference would bolster the symmetry of the standard model, reducing the need to invoke exotic new physics. See Planck (spacecraft) and Cosmic microwave background for the competing constraints that shape this debate.
Implications and reception
The Riess et al. contributions have reinforced the importance of independent, locally anchored measurements in cosmology. The work has helped spur a wave of related efforts, including refinements to the distance ladder and the exploration of alternative anchors and methods, such as the TRGB technique. These developments affect how researchers think about the expansion history of the universe, the nature of dark energy, and the possible extensions to the standard model of cosmology. See Distance ladder and Early dark energy for discussions of alternative approaches and their implications.
In the broader scientific ecosystem, the Riess et al. program is often cited as a model of methodological discipline: transparent accounting of systematics, cross-validation with multiple anchors, and continual re-evaluation in light of new data and improved instrumentation. The resulting discourse exemplifies a pragmatic approach to big questions in physics—reliable measurements first, bold theoretical interpretations second, and a willingness to adjust or revise prevailing models when the data warrant it. See Cepheid variables for the foundational role of stellar standard candles and Type Ia supernovae for how distant measurements extend the reach of the ladder.