Chapter 1: Understanding Hubble's Law

The Hubble Law enables astronomers to estimate the distances to far-off galaxies by factoring in their speed of recession and the Hubble constant. This constant illustrates how the recession speed of galaxies correlates with their distance from us. Various techniques exist to ascertain the Hubble constant's value; however, they yield results that are close yet not identical. Recently, researchers sought to gauge the Hubble constant through an analysis of cosmic microwave background radiation, but this effort has led to an impasse. What caused this situation? Let's explore further.

Section 1.1: The Expanding Universe

It's widely accepted that the universe is not static; rather, it is perpetually expanding, and this expansion is accelerating. The precise cause of this acceleration remains elusive, but it is attributed to the enigmatic force known as "dark energy." This phenomenon results in galaxies drifting apart, with those farther away receding at a greater speed. Edwin Hubble was the first to articulate this relationship in detail during the early 20th century.

Section 1.2: Hubble's Law Explained

Hubble's Law can be expressed as follows: there exists a linear correlation between the distance to galaxies and their recession velocities. In simpler terms, the farther an astronomical object is from an observer, the faster it appears to move away.

Subsection 1.2.1: Measuring the Hubble Constant

To accurately determine the Hubble constant, two critical parameters must be established: the recession velocity of a galaxy and the distance separating us from it. The redshift of an object provides insights into its recession speed; as an object moves away rapidly, the electromagnetic waves it emits stretch, resulting in an increase in wavelength, making the object appear "redder." The extent of this reddening is directly proportional to its velocity.

For relatively nearby stars, distance is typically gauged using the parallax method. However, for objects situated far beyond our galaxy, this technique is ineffective. In such cases, scientists often turn to red giants in distant galaxies, which exhibit similar absolute stellar magnitudes, allowing for accurate distance measurements.

Cepheid variable stars also serve as a key tool for distance measurement. Their pulsation periods correlate with their brightness, enabling scientists to ascertain their absolute luminosity, which in turn helps determine their distance.

Historically, scientists have established the Hubble constant to be approximately 74 ± 1.4 km/s/Mpc, meaning that an object situated one megaparsec away recedes at a velocity of 74 km/s.

Chapter 2: The Hubble Constant Dilemma

Recently, a new approach involving the study of cosmic microwave background radiation yielded a Hubble constant value of 67.4 ± 0.5 km/s/Mpc, which starkly contrasts with previous findings.

The "Planck" space telescope's estimation leverages observations of cosmic microwave background characteristics and predicts their evolution based on the Lambda-CDM standard cosmological model. This comprehensive model elucidates the accelerated expansion of the universe, the distribution of elemental matter, and the large-scale structure of the cosmos.

Both measurement methods present their findings with considerable accuracy, leading to two primary hypotheses regarding the discrepancy in results. Either scientists may have overlooked certain parameters while measuring galaxy distances, or, albeit less likely, there could be flaws inherent in the Lambda-CDM model itself.

Numerous modifications to the model have been proposed, but each comes with its own limitations. While some adjustments alleviate tension, they compromise the model's predictive capabilities regarding other data. Consequently, researchers are diligently gathering more data to address this quandary.

The first video, "Why is the Hubble Constant NOT Constant?" provides insights into the ongoing debate surrounding the Hubble constant and its implications in cosmology.

The second video, "The Hubble tension: what is wrong with our cosmological model?" delves deeper into the inconsistencies in the current cosmological framework and what they could mean for our understanding of the universe.

If you wish to see more articles about space appear in your feed, make sure to clap! Don’t forget to subscribe to our channel and submit your questions, which I will address in future articles. If you appreciate my work, consider supporting me by becoming a Medium member for just $5 a month, helping us create even better content.