Light Passing Through Space

As the universe expands, the total amount of dark energy also increases. Therefore, dark energy is uniformly distributed throughout the universe, and its density remains constant over time without being diluted by the expansion of the universe.

Space begins where light has passed. Light, varying in energy, splits into multiple colors before recombining as it traverses space. As the point where light originated moves, the light itself shifts, turning red or blue. Redshift refers to the phenomenon where the wavelength of light emitted by celestial bodies receding from the observer lengthens, shifting toward the red end of the visible spectrum. When the light from a star is decomposed, dark lines appear where specific elements absorb light. The positions of these lines are already known experimentally; in receding celestial bodies, these lines are shifted toward the red end. The wavelength changes. While locally intense gravitational fields, like those around black holes, can cause light to lose energy and shift to longer wavelengths as it escapes, leading to redshift, cosmologically, nearly all external galaxies are moving away from the observer. As the universe itself expands, the wavelength of light passing through that expanding space increases. A larger redshift value indicates that the celestial object is receding from the observer at a faster rate.




Space expands, increasing the distance between galaxies. As distances increase, new space forms between them. The farther away a galaxy is, the more space accumulates between it and its neighbors, causing each segment of space to expand simultaneously. Consequently, the final combined recession velocity increases proportionally with distance. The reason galaxies recede faster the farther away they are is due to geometric acceleration—the cumulative effect of expanding space accumulating between galaxies. Redshift reveals both 'distance' and 'recession velocity' simultaneously. By measuring the redshift of light coming from a galaxy, the recession velocity—how fast that galaxy is moving away from the observer—is calculated. Since this is a physical phenomenon caused by the stretching of light's wavelength, it is a fixed value measured independently of a supernova's brightness. 




Light propagates spatially at a constant speed while oscillating. The wave velocity (v) is the product of the frequency (f) and the wavelength (λ). Since the speed of light is constant, the frequency decreases as the wavelength increases. When the light source emitting light moves farther away, the interval between successive light crests reaching the observer increases. When stationary, if a light source emits waves 10 times per second, the observer also receives them 10 times per second. However, if the light source emits light while simultaneously moving backward, the next wave must travel a greater distance to reach the observer. Consequently, the wavelength of light reaching the observer increases, resulting in a lower frequency. Redshift occurs not because the speed of light changes, but because the wavelength changes. Conversely, when a celestial object approaches, the wavelength shortens, causing a phenomenon called blueshift where the light appears more blue.



The reason the wavelength of light, the shape of the wave, lengthens is due to the distance between the light source and the observer. As light is sent backward, the linear motion of celestial bodies moving away from the observer continuously delays the starting point of the light wave, causing the wavelength to lengthen. If the distance between galaxies increases while light travels from the source to the observer, the space itself through which the light passes expands. The elongation of light's wavelength signifies that the space traversed by that light has itself become longer. Therefore, redshift is a direct indicator of changes in spatial distance. The wavelength changes because the distance is increasing. When the universe itself expands, even if galaxies remain stationary, the space between them stretches, causing the wavelength to increase as the light travels.



The redshift phenomenon indicates that the actual distance between the light source and the observer has increased. Light serves as the standard for measuring distance. The reason redshift is evidence that the distance between the two points is increasing is because the speed of light is constant. If the wavelength of light doubles, the speed of light must remain constant, meaning this light has traveled through twice the original spatial distance. Therefore, the distance between galaxies increases accordingly. The very phenomenon of light's wavelength changing signifies that the spatial distance between the two points has changed.



In cosmic expansion, the phrase 'space expands' does not mean new matter is created, but rather that the gaps (distances) between existing matter grow larger. The total amount of ordinary matter in the entire universe does not increase. Vast spaces not bound by gravity, like those between galaxies, move further apart. Consequently, the overall matter density of the universe decreases over time. As space expands, the total amount of dark energy—the energy inherent to space itself—appears to increase proportionally with the volume of space. The vacuum is not completely empty. Invisible but massive dark matter is spread throughout the vacuum. Quantum mechanically, the vacuum is a state where energy fluctuates, a dynamic space where particles and antiparticles are constantly created and annihilated. The universe is becoming more empty, and that empty space is filled with dark energy and quantum fluctuations.



Even as the density of the universe decreases, the permittivity of vacuum remain unchanged, and thus the speed of light is maintained at a constant value. Within materials like water or air, atoms and molecules interact with light, slowing its speed. When the density of matter decreases, the speed of light increases. However, the permittivity of vacuum, which determine the speed of light, are not values determined by how many atoms exist within space. They are intrinsic physical properties of spacetime itself. The permittivity of vacuum is treated as a physical constant independent of the density of ordinary matter present in the universe. Even as the universe expands and galaxies move farther apart, no evidence has been found that the properties of the vacuum filling the space between them change. If the expansion of the universe altered the permittivity and thus the speed of light, observers would see stars from the distant past behaving under completely different physical laws than we observe today. Yet observations show that physical constants have been maintained with extreme precision from the early universe to the present. Cosmic expansion lowers the density of matter within space but does not alter the properties of the vacuum (its permittivity). Therefore, the speed of light remains unchanged, and only the phenomenon of redshift occurs, where the wavelength of light lengthens as it loses energy.



Despite the universe's density having decreased tremendously over the past 13.8 billion years, no change in permittivity has yet been detected. This suggests two possibilities. First, the vacuum may be a constant fixed in the geometric structure of spacetime, independent of matter density. Second, the critical point may lie far below the current cosmic density. The universe must expand trillions of times more than it is now to reach the critical point where the permittivity changes. The density of dark energy, which accelerates the current cosmic expansion, is extremely low but remains constant. If dark energy is the key factor determining the properties of the vacuum, then the point where the universe continues to expand until the density of ordinary matter becomes much lower than the density of dark energy could be a physical candidate point where the properties of the vacuum might change.



According to modern cosmology, during the early inflationary period of the universe, the energy state of the vacuum underwent a rapid change that determined the laws of physics. If the future universe's density drops below a critical point, altering the vacuum's ground state (False Vacuum Decay), there exists a theoretical possibility that fundamental physical constants, including the permittivity, could change abruptly in a step-like manner (Varying Fundamental Constants). Near the Planck Density, the extreme point where physical laws break down, the permittivity loses meaning. Physically, the permittivity of the vacuum is related to how virtual particles created and annihilated within the vacuum interact with light (vacuum polarization). If the density of the universe becomes extremely low, or conversely, if energy becomes extremely high, a phase transition could occur where the state of the vacuum changes. From a quantum mechanical perspective, changes in cosmic density could potentially cause a discontinuous change in the permittivity.



General phase transitions, such as water turning into ice, occur due to thermal fluctuations caused by temperature changes. However, quantum phase transitions (QPT) occur at absolute zero temperature by controlling non-thermal parameters such as pressure, magnetic field, or chemical composition. The quantum critical point (QCP) is the precise point in parameter space where a quantum phase transition occurs. At this critical point, the correlation length diverges to infinity (Scale Invariance), and new physical behaviors emerge that differ from classical phase transitions. Near the quantum critical point, quantum fluctuations—where particles and antiparticles are constantly created and annihilated—come to dominate the macroscopic properties of the system.



If the universe's density decreases to trigger a vacuum phase transition, the permittivity could change abruptly and the speed of light could also shift. However, under the current standard cosmological model, this critical point has not yet been precisely calculated. Observational evidence confirms that no such change has appeared even as the universe reached its present density.

---

When the density of the universe becomes extremely low, or conversely, when energy becomes extremely high, a phase transition may occur where the state of the vacuum changes.
https://x.com/i/status/2009197308848820257





A team using NASA’s Hubble Space Telescope has uncovered a new type of astronomical object — a starless, gas-rich, dark-matter cloud considered a “relic” or remnant of early galaxy formation. Nicknamed “Cloud-9,” this is the first confirmed detection of such an object in the universe — a finding that furthers the understanding of galaxy formation, the early universe, and the nature of dark matter itself.

https://science.nasa.gov/missions/hubble/nasas-hubble-examines-cloud-9-first-of-new-type-of-object/

NASA's Hubble Examines Cloud-9, First of New Type of Object - NASA Science

We illustrate that the differences between the high- z  and low- z  SNe in the WLR and CLR, and in HR after the standardization, are fully comparable to those between the correspondingly young and old SNe at intermediate redshift, indicating that the observed dimming of SNe with redshift may well be an artefact of overcorrection in the luminosity standardization. When this systematic bias with redshift is properly taken into account, there is little evidence left for an accelerating universe, in discordance with other probes, urging the follow-up investigations with larger samples at different redshift bins.
https://ui.adsabs.harvard.edu/abs/2022MNRAS.517.2697L/abstract

Evidence for strong progenitor age dependence of type Ia supernova luminosity standardization process