🗂️ Earth Systems, Geology & Climate · View mindmap

Hubble Flow

Hubble Flow describes the recession of galaxies from one another due to the expansion of space itself, rather than motion through space. This phenomenon is governed by Hubble’s Law, which states that the recession velocity ($v$) of a galaxy is proportional to its distance ($d$) from the observer: $v=H_{0}d$, where $H_0$ is the Hubble Constant.

Key Characteristics

• Metric Expansion: The increase in proper distance between comoving observers is due to the stretching of the metric of space, not kinematic motion.
• Isotropy and Homogeneity: On large scales, the flow appears uniform in all directions, supporting the Cosmological Principle.
• Redshift: Light from receding sources is stretched, resulting in cosmological redshift ($z$).

The Age-Size Paradox

A common misconception arises from the apparent contradiction between the age of the universe (~13.8 billion years) and the radius of the Observable Universe (~46.5 billion light-years). If light travels at $c$, the maximum distance should be ~13.8 billion light-years.

• Resolution: Space has expanded during the time light has been traveling. The proper distance to the source of the oldest light is now much greater than the light-travel distance.
• Integration: See Observable Universe Expansion: Age vs. Current Size Explained for a detailed breakdown of how the current diameter (~92 billion light-years) reconciles with the universe’s age.

Related Concepts

• big-bang
• Dark Energy (drives accelerated expansion)
• cosmic-microwave-background
• Comoving Coordinates

References

• Observable Universe Expansion: Age vs. Current Size Explained (Fermilab, Dr. Don Lincoln)