Understanding the Journey of Photons in the Universe

The journey of light propagation in the cosmos is a fascinating journey through the vast expanse of space. Photons, those massless particles of light, travel from one end of the universe to the other until they interact with matter, marking their interaction points.

The oldest photons in the universe date back to the time just 400,000 years after the Big Bang. During this period, the universe transformed from a state of heat and plasma into one with neutral atoms, allowing these photons to freely travel through the cosmos. This period marked the formation of the Cosmic Microwave Background (CMB), a remnant radiation that has been traveling for approximately 13.8 billion years.

Despite the immense distance these photons have covered, there is no guarantee that they will continue traveling indefinitely. With no mass for them to decay into, photons will presumably travel for many billions or perhaps trillions of years. However, it is speculative and uncertain whether they will travel forever. The concept of "forever" or "infinite duration" invites philosophical questions and lacks scientific certainty.

The Continuous Journey of CMB Photons

Consider the journey of these CMB photons as they continue to propagate through space. As of today, the number of CMB photons in the observable universe is around 1.5 billion times the total number of atoms. These photons were formed just 380,000 years after the Big Bang, yet 13.8 billion years later, an overwhelming majority—approximately 99.9%—of the universe's photons remain CMB.

While these photons have been traveling for an extensive period, their effects are not as pronounced as one might initially predict due to their significant redshifting. The CMB photons have been redshifted by a factor of over a thousand, resulting in a substantial reduction in their intensity. The temperature of the universe at their formation was 3,000 Kelvin, but today, it has dropped to a mere 2.725 Kelvin.

Intensities and Contributions of Different Types of Radiation

To fully understand the cosmic landscape of light propagation, one must consider the intensity contributions from different types of radiation. The current radiation intensity in the universe, measured in nanowatts per steradian per meter squared, is distributed as follows:

Cosmic Microwave Background (CMB): 960 Infrared (IR): 30 Optical: 2 X-rays and Gamma-rays: nearly negligible Ultraviolet (UV): nearly negligible Radio: nearly negligible Total: 10^14.3

The CMB stands out as the single most precise category, having been formed nearly 13.8 billion years ago. Despite the passage of time, its energy remains dominant. Even today, the CMB's energy is approximately 20 times greater than that of all other sources combined.

While the CMB has been traveling through space for an impressive distance, the vast majority (about 46.5 billion light years) can only be seen within our own local sky. This localized visualization highlights the immense scale of the universe and the limitations of our observational capabilities.

Ultimately, the journey of photons through the cosmos is both extensive and intriguing. Whether they will continue to travel indefinitely or come to an end is not definitively known. However, the journey of CMB photons provides us with valuable insights into the history and evolution of the universe.