Chapter 1: The Nature of the Observable Universe

When the term "universe" is discussed by astronomers, it predominantly refers to the observable universe. The term "observable" does not pertain to our technological ability to detect signals from far-off galaxies or whether any celestial bodies exist beyond our perception. Instead, it signifies a boundary defined by the speed of light. No information can traverse faster than light, establishing a maximum distance from the universe's inception beyond which we cannot receive signals; there simply hasn't been sufficient time for light to reach us. This boundary is known as the particle horizon, drawing a parallel to the event horizon found around black holes.

The universe is generally considered isotropic, meaning that every point can be viewed as the center of its own observable universe. In 2016, the Hubble Space Telescope identified a galaxy called GN-z11, located in the Ursa Major constellation. GN-z11 ranks among the most distant galaxies discovered, sitting at the periphery of our observable universe. Due to the isotropic characteristic of space, if you resided in this relatively small galaxy, Earth would be positioned at the edge of your observable universe.

The light emitted by GN-z11 is estimated to be around 13.4 billion years old, dating back to just 400 million years post-Big Bang. The early observation of this galaxy posed challenges to certain theoretical frameworks regarding galaxy formation. One might assume that this age indicates GN-z11 is 13.4 billion light-years distant from Earth. However, the actual distance measures approximately 32 billion light-years. How is this discrepancy possible? Given that the universe is roughly 13.8 billion years old, how could light have traveled such a vast distance?

The distinction between the 'light-travel' distance and the true distance arises from the expansion of spacetime. Picture two points on a deflated balloon, each representing a galaxy. As the balloon inflates, the points drift apart—illustrating how the very fabric of space, akin to the rubber of the balloon, expands. Edwin Hubble noted that galaxies are receding from one another, with more distant galaxies moving away at greater speeds than those that are closer. Hubble likened this to raisins in a loaf of bread, a more fitting analogy than the balloon, as space expands in three dimensions rather than two.

If one raisin is positioned twice as far from a point as another, it will recede at double the speed.

The expansion of space also affects the wavelength of light. If we envision a line moving across the balloon between the two points, it elongates as the balloon expands. This phenomenon mirrors what occurs with light; as it travels, its wavelength increases. This lengthening of the wavelength, alongside a decrease in light energy, is termed redshift. By measuring the redshift of light from distant galaxies, astronomers can ascertain cosmological distances.

Consequently, during the time it takes for light to journey from GN-z11 to Earth, the space between these two points has expanded significantly. In fact, the distance from Earth to the edge of the observable universe is approximately 46 billion light-years. As time advances, the observable universe will continue to grow, yet paradoxically, we will be able to observe fewer celestial objects. The universe is expanding faster than the speed of light, meaning that galaxies currently on the fringe of our observation will become increasingly redshifted until they vanish from view, no longer transmitting signals.

Calculations indicate that the farthest distance from Earth capable of sending signals that will eventually reach us, accounting for infinite time and expansion, is 16 billion light-years. This distance is referred to as the 'reachable universe'; in essence, we could still access galaxies closer than 16 billion light-years if we departed from Earth today. The 'future visibility limit', beyond which objects will never be observable at any future time, stands at 62 billion light-years. This limit equals the current visibility threshold (46 billion light-years) combined with the reachable limit (16 billion light-years).

To ponder: what is the actual distance that light from GN-z11 has traveled? If light's speed is constant and the duration it has traveled is also fixed, how far has it traversed while space continues to expand?

Beyond the Observable Universe [4K] - Explore the boundaries and mysteries of the universe that lie beyond what we can see.

Chapter 2: Implications of Cosmic Expansion

Our Observable Universe | How the Universe Works - Understand the mechanics of our universe and the observable limits we experience.