How Cosmic Signals Weaken Across the Universe: The Physics Behind Deep-Space Communication Explained
·5 min read
On Earth, signal degradation is a persistent challenge when transmitting information across distances. Signals such as sound, light, and gravitational waves propagate through three-dimensional space, gradually losing intensity as they travel away from their source. The medium through which these signals travel can significantly alter their characteristics. For instance, the sound of an approaching train is perceived differently when heard through air compared to when it's transmitted through ground or water. Additionally, interference from other signals—whether acoustic or optical—can further compromise the quality of the received signal, especially from the perspective of the recipient.
These factors, along with others, are expected to influence signals traveling through the expanding Universe, particularly over billions of light-years. But to what extent does this occur? How significant is the impact of signal degradation, and is there a way to enhance the accuracy of our observations regarding the original source of the signal? This question was posed by Viraji Ogodapola:
"When light and/or gravitational waves travel such large distances (billions of light years), don't they 'deteriorate' in some way? As in, won't the strength and the quality of the signal fade over time and distance?"
While signals do undergo changes during their journey, the process is not one of deterioration but rather alteration, which we can typically account for. Here’s the scientific explanation behind what occurs.
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.
To reach our eyes or instruments, a signal must traverse a vast array of environments. From distant regions of the cosmos, numerous potential effects can influence it: sources of matter and energy, fields of various types and intensities, dynamic environments, including those influenced by gravitational forces that grow or shrink over time, and even the expansion of the Universe itself. From the moment of emission to the moment of observation, countless factors can affect the signal, imprinting upon it and altering its initial state.
However, it is also possible that the signal may decay or deteriorate beyond the known astrophysical effects. This idea was introduced in 1929 by Fritz Zwicky, who coined the term "supernova" and was the first to propose the existence of dark matter. Known as tired light, this hypothesis proposed an alternative explanation for cosmic redshift: perhaps the wavelength of light isn't stretching due to the expansion of the Universe, but instead due to the gradual loss of energy as it propagates through space.
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.
Credit: Public domain image from Vlastni Dilo & H. Seldon
This concept is analogous to an effect observed when fast-moving massive particles pass through a medium: they emit radiation due to interactions between the moving particle and the medium. There are several types of radiation that charged particles experience:
and it is conceivable that photons themselves might experience a similar effect while traveling through the fabric of space. If this were true, it would imply that some or all of what we attribute to cosmological redshift could instead be due to the spontaneous deterioration of the original signal as it traverses space.
However, there would be observable consequences if light were to "tire": effects distinct from those associated with cosmological redshift. One such consequence would be an energy-dependent wavelength stretching: shorter wavelengths would lose energy at a different rate than longer ones, leading to a wavelength-dependent observation after long journeys. This would be akin to the dispersion seen when white light passes through a prism. Another consequence would be the progressive blurring of distant sources: as light traveled farther through the Universe, the appearance of distant objects would become less defined. However, when we look for these effects, we do not observe them. Distant objects appear just as sharp as nearby ones, and light of all wavelengths redshifts by identical factors. Tired light, as a deterioration effect, is ruled out.
The main galaxies of Stephan’s 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’re all equally sharp to JWST’s 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 “tired” in the sense of Zwicky’s tired light scenario; the lack of greater blurring with distance rules that out.
Nevertheless, the signal observed from a distant object does exhibit several important effects. Whether it "degrades" the original signal or causes it to "deteriorate" depends on perspective; it hinges on what one is looking for. If one expects the arriving signal to be pristine—identical in strength and properties to the emitted signal—then yes, it does degrade substantially. The farther away the source, even without any additional imprints or alterations, the weaker the signal becomes.
This occurs because a signal has a certain amount of strength or total energy. As it propagates away from a source, it spreads out in three dimensions.
If it's a source of particles, those particles spread out in 3D space, with a spherical "pulse" of particles spreading like the surface of a sphere, with particle densities decreasing as 1/r², where r is the distance from the initial source.
If it's 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/r².
And if it's a source of gravitational waves, it's the same thing: the energy spreads out like a sphere. However, because gravitational waves aren't detected by their energy, but rather by a property known as the strain amplitude, the detectable signal strength doesn't fall off like 1/r², but rather simply as 1/r.
This effect is illustrated, for light, in the diagram below.
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.
But that's not really a signal degrading or deteriorating over time and space, it's 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! 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 — what we call longer integration times — to make the "signal" stand out from the noise, assuming it's a continuously emitted signal.
However, what arrives still won't be identical to what was emitted, because of all of the effects of "stuff in the way" 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's eyes or in the observer's instruments, in a sequential fashion. Even though, again, it's not necessarily a signal degrading or deteriorating, it is a signal "losing its original quality" because of all of the interfering effects that it experiences along the way.
As electromagnetic waves propagate away from a source that's surrounded by a strong magnetic field, the polarization direction will be affected due to the magnetic field's 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.
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:
the electric and magnetic fields surrounding the emitting source, including the fields generated by the source itself,
the medium of particles that surround the source, including the imprints of their temperature and ionization state,
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.
That's 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's 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'dovich effect: causing it to appear colder in the expected (emitted) wavelengths, but hotter at shorter wavelengths.
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're 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.
Then, as that signal travels through intergalactic space, it'll do a number of things. First, it'll absolutely redshift. Once you leave a gravitationally bound system, whether it's 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'll 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.
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. 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't possess those in any way at all. When there's 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.
Distant sources of light — from galaxies, quasars, and even the cosmic microwave background — must 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.
If there are multiple clouds in the way, you'll see multiple, independent absorption features, with each unique feature corresponding to the properties of the intervening matter at the specific location — and hence, at a specific redshift and wavelength — it's found at. This shows up in quasar and galactic absorption lines, as illustrated above, in what's known as the Lyman-α forest. It's 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 "tree" of an absorption line, what shows up imprinted on the observed spectrum actually looks more like a "forest."
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'dovich effect, where the overall spectrum of light gets boosted to higher energies: causing an apparent "coldness" in the expected wavelengths but a "warmth" at shorter wavelengths. (There's also a kinetic Sunyaev-Zel'dovich effect due to the motions of the particles inside those gas clouds.)
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.
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'dovich effect.
Credits: X-ray: NASA/CXC/Université Paris/T.Wang et al; Infrared: ESO/UltraVISTA; Radio: ESO/NAOJ/NRAO/ALMA
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's just the gravitational potential, which again creates a gravitational blueshift as the signal "falls into" it. But for electromagnetic signals, like light, there's:
the neutral material in gas clouds,
the light-blocking dust at a specific set of wavelengths,
the configuration and temperature of atoms, molecules, and ions in the galaxy's interstellar medium,
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's up to you—the observer who receives and analyzes this data—to disentangle the various effects.
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's 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.
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.
Credit: ESA, HFI and LFI consortia, 2010; CO map from T. Dame et al., 2001
And yet, it's a testament to how far we've come, scientifically, that we can do precisely this for a wide variety of astronomical objects. We have an old saying in astronomy: that one astronomer's signal is another astronomer's noise. Different sub-fields are often pitted against one another for a variety of reasons, but the truth is that we need astronomers studying all of these various aspects — because they're interested in them and because they want to untangle and solve all of the mysteries within their particular field — in order to improve our ability to extract meaningful, correct information about all aspects of the Universe.
If you want to understand the CMB, the relic radiation from the Big Bang, then you have to understand the foreground emission of the galaxy and the properties of the interstellar medium. If you want to understand the abundances of the light elements from pristine gas clouds, you need to understand the specifics of quasar absorption in the intergalactic medium. If you want to understand the nature of various classes of supernovae, you need to understand the physics of the dust that enshrouds them. These are not "sub-fields at war" with one another; these are complementary sub-fields, and improving the uncertainties in any one of them helps us reveal the deeper truth underlying all of the others.
Sure, you can view a journey through the Universe as the "degradation" or "deterioration" of the emitted signal, because it gets less powerful and less pristine at every step along its cosmic journey. However, the way that it gets less powerful and obtains more imprints throughout its travels teaches us not just about the source, but about everything that lies between it and ourselves. That provides useful information about the Universe that we couldn't get in any other way, and that's how I prefer to view it!
