The Universe's Expansion: A Cosmic Conundrum

When considering the Universe beyond our galaxy, astronomers have two primary methods to understand its evolution. One approach involves analyzing light from well-understood celestial objects at varying distances, allowing us to infer how the Universe's structure alters as light traverses space. The other method relies on detecting ancient signals from the Universe's formative moments to gather insights on the evolution of spacetime. While both techniques are precise and reliable, they yield conflicting results regarding the rate of cosmic expansion. Luc Bourhis seeks clarification on this issue, posing the question:

> "As you’ve noted in several of your writings, the cosmic distance ladder and the analysis of the Cosmic Microwave Background Radiation (CMBR) provide contradictory values for the Hubble constant. What explanations have cosmologists proposed to reconcile these findings?"

To address this inquiry, we must first understand the underlying conflict and then explore potential resolutions.

Understanding the Cosmic Expansion

The concept of an expanding Universe can be traced back nearly a century to Edwin Hubble's groundbreaking discovery of Cepheid variable stars within spiral nebulae. This revelation confirmed that these nebulae were separate galaxies, enabling distance calculations and ultimately leading to the understanding that the Universe is expanding.

A decade prior, Vesto Slipher had observed that the spectral lines of these nebulae exhibited significant redshifts. This indicated either that the galaxies were moving away from us or that the space between us and them was stretching, consistent with Einstein's predictions of spacetime behavior. As more data emerged, the evidence overwhelmingly supported the notion of an expanding Universe.

The construction of the cosmic distance ladder involves measuring distances from our Solar System to nearby stars and galaxies, each step carrying its own uncertainties. Although potential biases could skew the inferred expansion rate, observational data has ruled out extreme discrepancies. With multiple independent methods forming the distance ladder, it is challenging to attribute the observed discrepancies to a singular erroneous measurement.

Once it became clear that the Universe was expanding, it followed that it was once smaller, hotter, and denser. As light travels from its source to our eyes, it must traverse the expanding Universe, allowing us to measure the extent of its redshift and, consequently, derive the expansion history. Currently, various celestial objects serve as standard candles, collectively indicating an expansion rate of 73 km/s/Mpc with an uncertainty of merely 2–3%.

In contrast, investigations into the CMB yield a significantly different result, suggesting a rate of 67 km/s/Mpc with a mere 1% uncertainty. This discrepancy raises critical questions about our understanding of the Universe's expansion and the forces at play.

Theories Addressing the Discrepancy

Possible explanations for the conflicting results can be categorized into three main scenarios:

1. Errors in the Early Relics Method: The group studying early cosmic signals may harbor fundamental inaccuracies, leading to lower-than-expected values.
2. Errors in the Distance Ladder Method: Conversely, there may be systematic errors in the distance ladder approach, inflating the results.
3. Both Methods are Correct: This scenario posits that new physics could exist, accounting for the differing outcomes.

Given the robustness of both methodologies, belief in their findings is warranted. If both groups are accurate, it necessitates the consideration of new physics to explain the observed phenomena. Several potential avenues are currently under investigation:

1. Temporal Variability of Dark Energy: Dark energy may not remain constant over time, potentially influencing the expansion rate differently across cosmic epochs.
2. Extended Neutrino Coupling: If neutrinos retain interactions with other matter for longer than previously thought, it could lead to an accelerated expansion rate.
3. Miscalculated Cosmic Sound Horizon: Reevaluating the measurements related to the cosmic sound horizon may reveal inconsistencies that could explain the discrepancies.
4. Interactions Between Dark Matter and Neutrinos: Potential interactions between these two components could lead to variations in expansion measurements.
5. Early Existence of Dark Energy: If dark energy were present in the early Universe but diminished before CMB observations, it might clarify the observed conflict.

In conclusion, the ongoing exploration of these theories highlights the complexities of understanding the Universe's expansion. The advancements in observational technology and methodologies promise to provide deeper insights, potentially addressing the current discrepancies. As new data emerges, we may uncover profound truths about the cosmos that challenge our existing paradigms.

Starts With A Bang is now featured on Forbes and republished on Medium with support from our Patreon contributors. Ethan has authored two books, Beyond The Galaxy and Treknology: The Science of Star Trek from Tricorders to Warp Drive.