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<\/p>\nHere on Earth, signal degradation is a real problem whenever we transmit information to one another. Signals like sound, light, and gravity spread out through space in three dimensions, becoming weaker and weaker as you travel farther from the source. The medium that the signal travels through alters the signal\u2019s properties as well, as an oncoming train sounds different from the air, with your ear to the ground, or from submerged in a body of water. And if there are interfering signals to contend with \u2014 like sound or light from additional sources \u2014 that \u201cnoise\u201d can also degrade the quality of the signal, at least from the perception of the signal\u2019s recipient.<\/p>\n
Surely these factors, as well as potential other factors, that affect signals as they travel through the expanding Universe, particularly across billions of light-years. But how severe is it? How big of a problem is signal degradation, and is there anything we can do to improve the information we can glean about the original source that generated it? That\u2019s the question of Viraji Ogodapola, who wants to know:<\/p>\n
\u201cWhen light and\/or gravitational waves travel such large distances (billions of light years), don\u2019t they \u2018deteriorate\u2019 in some way? As in, won\u2019t the strength and the quality of the signal fade over time and distance?\u201d<\/p>\n
Signals do change, but the act of traveling through the Universe won\u2019t lead to any sort of deterioration; just an alteration, and one we can usually account for. Here\u2019s the science of what\u2019s going on.<\/p>\n
<\/p>\n
This simplified animation shows how light redshifts and how distances between unbound objects change over time in the expanding Universe. Note that the objects start off closer than the amount of time it takes light to travel between them, the light redshifts due to the expansion of space, and the two galaxies wind up much farther apart than the light-travel path taken by the photon exchanged between them.\n<\/p>\n
Credit<\/a>: Rob Knop<\/p>\n In order to arrive at our eyes (or instruments), a signal has to go through a wide variety of environments. From across the distant Universe, there are many potential effects it can encounter: sources of matter and energy, fields of a variety of types and strengths, environments that change, including by gravitationally growing or shrinking over time, and even the expansion of the Universe. From the moment a signal is emitted to the moment it\u2019s observed, an enormous number of factors can affect it, imprinting themselves upon the original signal, and distorting or degrading it from the initial state it possessed when it was first emitted.<\/p>\n However, it\u2019s also possible that whatever signal was emitted will also decay or deteriorate in some fashion above and beyond the details predicted by the physics and astrophysics effects that we know about. That idea was put forth way back in 1929 by Fritz Zwicky, the same Fritz Zwicky who coined the term supernova and was the first to theorize the existence of dark matter. Known as tired light<\/a>, it was originally an alternative explanation for cosmic redshift: perhaps the wavelength of light isn\u2019t stretching because the Universe is expanding, as illustrated above, but rather because the very act of propagation through space causes it to lose energy very slowly over time.<\/p>\n This animation showcases what happens when a relativistic, charged particle moves faster than light in a medium. The interactions cause the particle to emit a cone of radiation known as Cherenkov radiation, which is dependent on the speed and energy of the incident particle. Detecting the properties of this radiation is an enormously useful and widespread technique in experimental particle physics, and also in astronomy for detecting atmospheric cosmic rays.\n<\/p>\n Credit<\/a>: Public domain image from Vlastni Dilo & H. Seldon<\/p>\n This is analogous to an effect that\u2019s known to happen when fast-moving massive particles travel through a medium: they emit radiation due to interactions between the particle-in-motion and the medium itself. There are many types of radiation that charged particles experience:<\/p>\n and it\u2019s conceivable that photons themselves might experience a similar effect just by traveling through the fabric of space itself. That would, if it occurred, mean that some or even all of what we ascribe to being a cosmological redshift would instead be due to a spontaneous deterioration of the initial signal as it traveled through space.<\/p>\n But there would be consequences of light getting tired that would lead to observable effects: effects that are distinct from what would happen in the case of cosmic redshift<\/a>. One effect would be an energy-dependence on the stretching of light\u2019s wavelengths: shorter wavelengths would lose energy at a different rate than longer wavelengths, leading to a wavelength dependence of the observed light after journeys of hundreds of millions or billions of light years: analogous to the dispersion we see when we pass white light through a prism. Another effect would be the progressive \u201cblurring\u201d of distant sources: where the longer light traveled through the Universe, the less \u201cfocused\u201d distant objects appeared. Yet, when we look for both of these effects, we don\u2019t see them. Distant objects appear just as crisp as nearby ones, and light of all wavelengths redshifts by identical factors. Tired light, as a deterioration effect, is ruled out<\/a>.<\/p>\n The main galaxies of Stephan\u2019s Quintet, as revealed by JWST on July 12, 2022. The galaxy on the left is only about ~15% as distant as the other galaxies of the quintet, while the background galaxies are many scores of times farther away still. And yet, they\u2019re all equally sharp to JWST\u2019s eyes, demonstrating several factors. Sure, we learn that the Universe is full of stars and galaxies practically everywhere we look, but we also learn that light does not get \u201ctired\u201d in the sense of Zwicky\u2019s tired light scenario; the lack of greater blurring with distance rules that out.<\/p>\n Credit<\/a>: NASA, ESA, CSA, and STScI<\/p>\n However, the signal that one does observe from a distant object does indeed exhibit a number of important effects. Whether it \u201cdegrades\u201d the original signal or causes the original signal to \u201cdeteriorate\u201d is a matter of perspective; it depends on what you\u2019re looking for. If you expected the arriving signal to be pristine \u2014 identical in strength and properties to the emitted signal \u2014 then of course it does degrade substantially. The farther away you are, even if there were no other imprints or alterations, the signal would arrive in a much weaker fashion than how strong it was when it was generated.<\/p>\n That\u2019s for a simple reason: a signal has a certain amount of strength, or total energy, inherent to it. That signal spreads out as it propagates away from a source.<\/p>\n If it\u2019s a source of particles, those particles spread out in 3D space, with a spherical \u201cpulse\u201d of particles spreading out like the surface of a sphere, with particle densities decreasing as 1\/r\u00b2, where r is the distance from the initial source.<\/p>\n If it\u2019s a source of light, like a supernova, a star, or an explosive event, that light also spreads out in 3D space, again like a sphere, propagating away from the source, and with the flux density decreasing as 1\/r\u00b2.<\/p>\n And if it\u2019s a source of gravitational waves, it\u2019s the same thing: the energy spreads out like a sphere. However, because gravitational waves aren\u2019t detected by their energy, but rather by a property known as the strain amplitude<\/a>, the detectable signal strength doesn\u2019t fall off like 1\/r\u00b2, but rather simply as 1\/r.<\/p>\n This effect is illustrated, for light, in the diagram below.<\/p>\n The way that light spreads out as a function of distance means that the farther away from a power source you are, the energy that you intercept drops off as one over the distance squared. This also illustrates, if you view a certain specific angular area (illustrated by the squares) from the perspective of the original source, how larger objects at greater distances will appear to take up the same angular size in the sky. Each time you double your distance between a source and observer, the brightness you observe gets quartered. In general, photons (and all light) propagate spherically outward away from the emitting source.\n<\/p>\n Credit<\/a>: Borb\/Wikimedia Commons<\/p>\n But that\u2019s not really a signal degrading or deteriorating over time and space, it\u2019s just getting fainter due to the fact that it spreads out as it travels through our three-dimensional Universe. If our Universe had a different number of dimensions, it would spread out differently, which actually provides meaningful constraints on the existence of extra dimensions<\/a>! The signal gets weaker because of our distance from it, which might make it difficult to detect above the noise floor of the Universe, but that usually just requires longer observation times \u2014 what we call longer integration times<\/a> \u2014 to make the \u201csignal\u201d stand out from the noise, assuming it\u2019s a continuously emitted signal.<\/p>\n However, what arrives still won\u2019t be identical to what was emitted, because of all of the effects of \u201cstuff in the way\u201d that have measurable impacts on the (eventually) observed signal. Perhaps the best way to illustrate this is to imagine what happens to that signal, from the moment of its emission through every step along its journey until it arrives at the observer\u2019s eyes or in the observer\u2019s instruments, in a sequential fashion. Even though, again, it\u2019s not necessarily a signal degrading or deteriorating, it is a signal \u201closing its original quality\u201d because of all of the interfering effects that it experiences along the way.<\/p>\n As electromagnetic waves propagate away from a source that\u2019s surrounded by a strong magnetic field, the polarization direction will be affected due to the magnetic field\u2019s effect on the vacuum of empty space: vacuum birefringence. By measuring the wavelength-dependent effects of polarization around neutron stars with the right properties, we can confirm the predictions of virtual particles in the quantum vacuum.\n<\/p>\n Credit<\/a>: N. J. Shaviv\/Sciencebits<\/p>\n We can begin with the emitted signal itself. The first thing it will encounter, immediately upon being emitted, is the environment around the emitting source. This includes, in order as you propagate away from it:<\/p>\n the electric and magnetic fields surrounding the emitting source, including the fields generated by the source itself,<\/p>\n the medium of particles that surround the source, including the imprints of their temperature and ionization state,<\/p>\n and the strength of the gravitational potential, which causes the outgoing signal the wavelength of light, gravitational waves, or the kinetic energy of massive particles to lose energy, either redshifting (if massless) or slowing (if massive) as they climb out of it.<\/p>\n That\u2019s three particular and separate imprints on the emitted signal before it ever begins its journey through intergalactic space. This can have effects on things like the signal\u2019s polarization, absorption lines that show up in the spectrum of the signal, by stretching the wavelength of the signal through gravitational redshift, and even, if the medium is hot enough, to boost the energy of the signal through the Sunyaev-Zel\u2019dovich effect<\/a>: causing it to appear colder in the expected (emitted) wavelengths, but hotter at shorter wavelengths.<\/p>\n An event like AT2018cow, now known as either FBOTs or Cow-like events, is thought to be the result of a breakout shock from a cocooned supernova. With five such events now discovered, the hunt is on to uncover precisely what causes them, as well as what makes them so unique. In order to understand the light we\u2019re observing from this class of objects, we have to accurately model the environment around it, so that we understand which components of the observed signals are from the explosion and which ones are imprinted from the surrounding material.<\/p>\n Credit<\/a>: Shanghai Astronomical Observatory, China<\/p>\n Then, as that signal travels through intergalactic space, it\u2019s going to do a number of things. First, it\u2019s absolutely going to redshift. Once you leave a gravitationally bound system, whether it\u2019s a galaxy, a group of galaxies, a cluster of galaxies, or a bound cosmic filament or any other bound portion of the cosmic web, you\u2019ll find yourself not just in the abyss of intergalactic space, but in a region where space itself is expanding. As it expands, the wavelength of any light or gravitational waves traveling through it will lengthen: a cumulative effect that keeps piling up as it propagates from the source to the observer. We know that, particularly at large distances, nearly all of this observed redshift is cosmological, with gravitational redshifts and the redshifts due to peculiar velocities (the relative initial motions of the source and the observer) making up the rest.<\/p>\n But there are plenty of other entities in space, with the most common ones being galaxies, protogalaxies, dark molecular clouds of gas, and the ionized warm-hot intergalactic medium<\/a>. These signatures typically absorb or emit light in a wavelength-dependent fashion, and so those absorption or emission features can then be imprinted onto the traveling light, but not onto gravitational waves, as these are electromagnetic interactions, and gravitational waves don\u2019t possess those in any way at all. When there\u2019s an intervening cloud of neutral matter in the way, for example, the original signal will be partially absorbed at a specific set of wavelengths by that matter.<\/p>\n Distant sources of light\u200a\u2014\u200afrom galaxies, quasars, and even the cosmic microwave background\u200a\u2014\u200amust pass through clouds of normal matter. The absorption features we see enable us to measure many features about the intervening gas clouds, including the abundances of the light elements inside and the degree of ionization.\n<\/p>\n Credit<\/a>: Ed Janssen\/ESO<\/p>\n If there are multiple clouds in the way, you\u2019ll see multiple, independent absorption features, with each unique feature corresponding to the properties of the intervening matter at the specific location \u2014 and hence, at a specific redshift and wavelength \u2014 it\u2019s found at. This shows up in quasar and galactic absorption lines, as illustrated above, in what\u2019s known as the Lyman-\u03b1 forest<\/a>. It\u2019s named as such because quasars are so distant that the sheer number of intervening molecular clouds, and the sheer number of absorption features, means that there are so many of them that instead of a \u201ctree\u201d of an absorption line, what shows up imprinted on the observed spectrum actually looks more like a \u201cforest.\u201d<\/p>\n Of course, there are also regions that have hot, ionized material in the way, such as around active galaxies or in passing through galaxy clusters that have hot, X-ray emitting intracluster mediums. That triggers the thermal Sunyaev-Zel\u2019dovich effect, where the overall spectrum of light gets boosted to higher energies: causing an apparent \u201ccoldness\u201d in the expected wavelengths but a \u201cwarmness\u201d at shorter wavelengths. (There\u2019s also a kinetic Sunyaev-Zel\u2019dovich effect due to the motions of the particles inside those gas cloud.)<\/p>\n And as these signals, whether light or gravitational wave signals, pass through any region of space that has a significant amount of mass in it, the curvature of space causes them to gain energy, or to gravitationally blueshift. As they exit again, and climb out of that gravitational potential again, they gravitationally redshift. If the structure maintains the same mass and mass distribution, i.e., the same gravitational potential, then these two effects will cancel. But if the structure grows or shrinks, the difference in those potentials will imprint themselves onto the signal: an imprint known as the integrated Sachs-Wolfe effect<\/a>.<\/p>\n This X-ray\/infrared composite image shows galaxy cluster CL J1001+0220, the earliest known mature, X-ray emitting galaxy cluster. Although this was the earliest known galaxy cluster of any type in 2016, several younger protoclusters have since been identified. The light from background objects behind this cluster will be boosted to higher energies on account of the hot, ionized medium that the background light must travel through: the thermal Sunyaev-Zel\u2019dovich effect.<\/p>\n Credits<\/a>: X-ray: NASA\/CXC\/Universit\u00e9 Paris\/T.Wang et al; Infrared: ESO\/UltraVISTA; Radio: ESO\/NAOJ\/NRAO\/ALMA<\/p>\n Finally, after all of those effects, the light makes it to the Local Group, the Milky Way, and all the way to us in the Solar System. But once again, there are all sorts of things in the way that affect that signal once more. For gravitational waves, it\u2019s just the gravitational potential, which again creates a gravitational blueshift as the signal \u201cfalls into\u201d it. But for electromagnetic signals, like light, there\u2019s:<\/p>\n the neutral material in gas clouds,<\/p>\n the light-blocking dust at a specific set of wavelengths,<\/p>\n the configuration and temperature of atoms, molecules, and ions in the galaxy\u2019s interstellar medium,<\/p>\n and similar effects along these lines. This can induce all sorts of changes, from the average temperature observed to the polarization of the arriving light. In general, the intervening material will imprint itself onto your light, no matter where it comes from, and it\u2019s up to you \u2014 the observer who receives and analyzes this data \u2014 to disentangle the various effects.<\/p>\n By the time you observe this light, it is no longer identical, in all of these ways, to the original light that was emitted by the source. It is fainter, it is more susceptible to the various sources of noise in our instruments and that are associated with our modern measurement techniques, and it has potentially all of these aforementioned effects imprinted on it. We have to be able to properly account for all of them, wherever it\u2019s relevant, if we wish to extract correct information about the source. When there are limits to how well we can disentangle these effects, there are induced uncertainties: including in the location and properties of every distant object we observe.<\/p>\n When the entire sky is viewed in a variety of wavelengths, certain sources corresponding to distant objects beyond our galaxy are revealed. This first all-sky map from Planck includes not only the cosmic microwave background, but also extragalactic contributions and the foreground contributions from matter within the Milky Way itself. All of these must be understood to tease out the appropriate temperature and polarization signals.\n<\/p>\n![\"\"](\"https:\/\/www.newsbeep.com\/nz\/wp-content\/uploads\/2026\/01\/0_gHn30CAZI4P0NMT_.gif\")
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