Soumendra Nath Thakur
11-11-2024
Abstract:
In the vast cosmos, distances are commonly expressed in light-years, reflecting the immense spans light can traverse over time. This paper examines the foundational concepts surrounding cosmic distances, including the speed of light, the influence of cosmic expansion, and the effects of redshift on electromagnetic waves. As galaxies and clusters recede from one another due to cosmic expansion, the light from distant sources experiences a continuous lengthening of wavelength and decrease in frequency—a process known as cosmic redshift. This shift causes visible light to transition across the electromagnetic spectrum, ultimately approaching radio waves and losing its mobility as electromagnetic radiation. The maximum observable distance of light is explored, factoring in cumulative redshift and the gradual loss of photon energy. This study offers a cohesive view of how cosmic redshift impacts light’s behavior over intergalactic distances and sets limits on our ability to observe distant objects.
Keywords:
Cosmic distance, Light-year, Redshift, Electromagnetic spectrum, Cosmic expansion, Photon energy, Intergalactic recession, Observational limits
The scale of cosmic distances is generally expressed in light-years or astronomical units (AU). A light-year (LY) represents the distance light travels in a vacuum over one year, approximately 9.46 trillion kilometers, or 300,000 kilometers per second (km/s).
The universe is estimated to have originated approximately 13.8 billion years ago. Over this period, light could theoretically traverse up to 46.1 billion light-years, equivalent to about 4.40 x 10^26 meters or 4.4 x 10^23 kilometers, considering cosmic expansion.
Light is an electromagnetic wave. Visible light occupies the range between infrared (IR) and ultraviolet (UV) on the electromagnetic spectrum, with frequencies between 4 × 10^14 and 8 × 10^14 Hz and wavelengths from 380 to 700 nanometers. Due to the expansion of the universe, galaxies and galactic clusters are receding from each other on an intergalactic scale. This recession increases the proper, or "light-traveled," distances between light-emitting objects and the locations where the light is eventually received.
As galaxies recede, the travel distance for light from these sources extends beyond their original emission distance. This increased separation induces a phenomenon known as cosmic redshift, where the light’s frequency decreases, and its wavelength lengthens. Consequently, visible light from these distant galaxies shifts through the electromagnetic spectrum—from visible wavelengths to infrared, microwaves, and ultimately radio waves—before it fades entirely as electromagnetic radiation and loses its inherent speed.
Photons, the fundamental particles of light, act as carriers of electromagnetic waves. Cosmic redshift leads to an increase in photon wavelength and a decrease in frequency as light travels over vast distances.
The type of signal in the electromagnetic spectrum is defined by the wavelength and frequency of these waves. Visible light falls within the frequency range of 4 × 10^14 to 8 × 10^14 Hz and wavelengths of 380 to 700 nanometers. The full electromagnetic spectrum includes:
Radio waves, with wavelengths from 10 cm to 10 km
Microwaves, with wavelengths from 1 mm to 1 m
Infrared, with wavelengths from 0.7 to 300 micrometers (µm)
Visible light, with wavelengths from about 400 nm (violet) to 700 nm (red)
Ultraviolet, with wavelengths from 3 to 400 nm
X-rays, representing high-energy emissions from hot gases containing atoms
Gamma rays, having the highest energies and shortest wavelengths
When visible light from distant sources shifts in wavelength due to cosmic expansion, it gradually moves from visible light to infrared, then to microwaves, and eventually to radio waves, eventually ceasing to behave as electromagnetic radiation with inherent speed.
Earlier, it was noted that light could theoretically travel a maximum of 46.1 billion light-years, or 4.4 x 10^23 kilometers, within 13.8 billion years. This estimate depends on the continuous recession of both the source and the observer. Due to the increasing separation, the observed distance of light would exceed the emission distance by a slight margin, ultimately causing light to lag behind the observational point, making direct observation impossible.
Furthermore, when light wavelengths elongate beyond radio waves, light loses its mobility as electromagnetic radiation. The maximum observational distance for light can be estimated by calculating the cumulative effect of redshift and energy reduction over the light’s journey, compared to the minimum energy required to sustain its form as a radio wave before losing its inherent mobility.
This summary provides an overview of light’s speed, associated distances, redshift, visibility, and mobility across cosmic scales.
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