OC403: Radiation in the Universe
Ben Franklin Centre for Theoretical Research
PO Box 27, Subiaco, WA 6008, Australia.
This is Number 3 in a suite of web articles about the Oort Cloud, the volume of space immediately outside our Solar System.
Electromagnetic radiation from bodies in the Universe
The light and heat we get from the Sun is just one section of what we call the electromagnetic spectrum. This spectrum is laid out according to the wavelength of the particular radiation. A shorter wavelength means higher energy, while longer wavelengths have less energy.
Fig. OC403-F1. The electromagnetic spectrum. From [C1].
In the diagram, wavelength is shown in the top scale, in metres. Visible light is quite a small section of the spectrum, with wavelengths around 10-6 metres, that is, around a millionth of a metre. On the shorter-wavelength side is first Ultraviolet, then Soft X-Rays, Hard X-rays, and Gamma Rays. These include radiation of increasing energy, and hence the potential for damage to the body.
Moving to the left, to longer wavelengths, we come first to Infrared, then Microwaves, and finally Radio Waves. We can't see Infrared, but we can sense Near Infrared, the section closest to Visible, as heat -- say when we hold a bare arm out in sunlight. The longer wavelengths carry less energy, and so are less of a hazard to life,
The diagram also shows sample objects of similar size to the different wavelengths. Often there is a link between the dimensions of apparatus and the wavelengths of radiation they handle, for example, radio-telescopes may be in the form of great dishes as much as a hundred metres across, while binoculars have lenses only a few centimetres across.
Quite early in history, it was noticed that hotter objects glow with brighter, whiter, colours than cooler ones. When blacksmiths heat up an iron bar in a fire, it first glows with a dull red. As it gets hotter, the bar glows more brightly, with whiter colours. In physics, the "hotter" colours mean shorter-wavelength radiation, carrying more energy.
It turns out that everything in the Universe continually emits radiation, at a wavelength which depends on its temperature. Hot objects emit at short wavelengths, cold ones at longer (lower energy) wavelengths. At the same time, everything is receiving radiation from near and far objects.
At the "room temperatures" in which we usually live, objects emit radiation in the infrared band. But even very cold objects (like those in interstellar space which are distant from any stars) emit radiation, in this case in the longer-wave band we call microwaves.
At the start of the 1900s, a famous German physicist called Max Planck worked out how the radiation from a hot body varied with its temperature. This radiation was not at a single wavelength, but followed a curve with a pronounced peak (the wavelength at which most energy was emitted).
Figure OC403-F2. "Black-body" radiation. From [C4].
Figure F2 above shows how the radiation from stars at different temperatures varies. The biggest peak (solid line) is for a star with a surface temperature of 6000 K (degrees Kelvin), like our Sun. At this peak temperature, the most active wavelength is 483 nm (nanometres, billionths of a metre). The human eye can distinguish light of wavelengths from 380 nm (violet) to 740 nm (red).
But notice that the Sun also radiates in the ultraviolet (left of the colour bars) down to about 100 nm, and in the infrared (right) up to about 2500 nm. The area under any part of the curve represents the amount of energy involved, so you can see that only about half of what the Sun emits is visible light.
With stars cooler than the Sun, the amount of energy they emit drops off quite rapidly with surface temperature, and their peak wavelength also increases. So a star with a 5000 K surface (upper dashed line) has a wavelength peak at 580 nm and an output only half that of the Sun, while a cooler 3000 K star peaks at 966 nm and puts out only about one percent of the Sun's energy.
In the jargon, these curves are called "Black-Body Radiation curves", where "black body" is shorthand for an object which completely absorbs radiation of any wavelength. The curves are of similar shape for bodies at any temperature, even that on Earth, or on quite distant asteroids. Where the radiation is mostly of infrared or longer wavelength, it may be called simply thermal radiation.
The formula which Max Planck developed is called the Planck Radiation Law. It relates the intensity of radiation emitted (by unit surface area into a fixed direction from a body) as a function of wavelength for a given temperature. All objects in the Universe emit thermal radiation, at a wavelength which depends on their temperature.
Note that the amount of Planck Radiation emitted is proportional to the surface area of the emitting body. We will see the relevance of this later, such as when we look at the surface areas of small bodies in space.
Figure OC403-F3. Black-body radiation curves from various objects. From [C6].
Figure F3 shows black-body radiation curves for various objects, including the Sun, a red-hot object, and the Earth's surface, at temperatures of 6000 K, 1800 K, and 300K respectively (subtract 273 to convert to Centigrade, so 300K is 27 deg. C). The last curve is for much cooler objects at 2.7 K, only 2.7 degrees above Absolute Zero. It is labelled "Cosmic background radiation". We will shortly show that this "CMBR" comes from objects far out in the Oort Cloud.
The Planck Radiation Law applies to ordinary (fusion) stars, and to planets, moons, and smaller bodies in space. It does not apply to Vortex Stars, which produce light by a different mechanism, which we'll look at later.
About Max Planck
Max Planck is a major figure in the history of science. He was a German theoretical physicist who originated quantum theory, which won him the Nobel Prize in Physics in 1918. Planck made many contributions to theoretical physics, but his fame as a physicist rests primarily on his role as an originator of quantum theory -- which revolutionized human understanding of atomic and subatomic processes, just as Albert Einstein's theory of relativity revolutionized the understanding of space and time.
Figure OC403-F4. Max Planck. From [C5].
Max Planck was born in 1858, and lived to the age of 89. His work on black-body radiation was published in 1900, when he was already 42. "Planck's Law" describes the electromagnetic radiation emitted by a black body in thermal equilibrium at a definite temperature.
Planck had a full, if sometimes tragic, personal life. He had four children with his first marriage, all of whom died during his lifetime. There is more detail on his life and background at P3: Living In The Universe [C4].
CMBR -- Radiation from the Oort Cloud
The Universe is awash with radiation from all the objects it contains. Living close, as we do, to our star the Sun, we might assume that visible light would be a major part of this radiation, but this is not true. Out in deep space (approximated on Earth by looking into space at midnight on a moonless, cloud-free day), the major component is CMBR, Cosmic Microwave Background Radiation.
Figure OC403-F5. The Universe Radiation Wheel. From [C7]. Derived from data in [C8].
From Figure F5 above, it can be seen that visible light makes up less than 3% of the energy of radiation sluicing through the Universe. CMBR makes up over 70%, with Infrared next at about 27%. Higher-energy radiation, such as Gamma Rays and X-Rays, is very small, much less than 1%.
Notice that this division of radiation into slices is based on the energy content of the radiation division, not on the number of photons -- because CMBR photons are of very low energy, if presented by photon numbers the Universe Radiation Wheel would be almost entirely CMBR. For more detail on this representation of radiation in the Universe, see UG101: Recycling the Universe: Neutron Stars, Black Holes, and the Science of Stuff [C7].
We can now go on to look at the discovery of CMBR and its influence on current scientific thought.
The discovery of CMBR
Cosmic Microwave Background Radiation, CMBR, was discovered accidentally in 1965 by Arno Penzias and Robert Wilson at the Bell Telephone Laboratories in Murray Hill, New Jersey. For this, they won a Nobel Prize.
Penzias and Wilson were investigating microwave radio emissions from the Milky Way and other natural sources, using a large horn-shaped antenna. When the two scientists tuned their equipment to the microwave portion of the spectrum, they discovered an annoying background static that wouldn't go away.
Figure OC403-F6. The horn antenna on which Penzias and Wilson discovered CMBR in 1965. From [C9].
No matter where they pointed the antenna, or when, the microwave static was the same. They spent months running down every possible cause for the static, including pigeon droppings inside the antenna, but they couldn't find a source or a solution.
Eventually they realized that this radiation came from the sky, in fact from everywhere in space. It is primarily in the microwave portion of the electromagnetic spectrum, and is invisible to the naked eye. It fills the universe and can be detected everywhere we look. In fact, if we could see microwaves, the entire sky would glow with a brightness that was astonishingly uniform in every direction.
We now know that CMBR is just normal thermal (black-body) radiation from all our Oort Cloud bodies. But due to an unfortunate coincidence at the time of its discovery, until recently it was ascribed to a totally different source -- the Big Bang, purported to be an event at the start of the Universe. This fundamental misconception has held back proper analysis of almost all the vast mass of CMBR data assembled to date.
The unfortunate coincidence was that a scientist in a neighbouring New Jersey university, Robert Dicke, heard about Penzias and Wilson's work before it was published. He put together a paper on his idea of CMBR's origin, and managed to get it published in the same issue of the Astrophysical Journal in which Penzias and Wilson revealed their work. Because of this close coupling, Dicke's article never received the critical attention which it deserved, and the myth of CMBR's origin with the Big Bang was perpetuated.
There is more about CMBR's discovery and its faulty attribution in P3: Living In The Universe [C4]. and in P2: The Oort Soup as the real origin of Cosmic Microwave Background Radiation [C10] .
Let us look in more detail at the CMBR curve, and what it can tell us.
The CMBR curve
In 1989, NASA launched a satellite called COBE (Cosmic Background Explorer) to make measurements of the CMBR. These measurements showed that the spectrum of CMBR received from any point in the sky was almost uniform -- there were only small variations from one direction to the next. (The small variations have their own importance, which will be looked at later).
Figure OCC-F7. CMBR data from the COBE satellite. From [C11].
Figure F7 shows the points on the spectrum as recorded by COBE (black squares), imposed on the theoretical black-body curve for bodies at a temperature of about 2.7 K, a little less than 3 degrees above Absolute Zero. It can be seen that the fit is excellent. One point worth noting is that the CMBR cuts out (on the left) at a wavelength of about 0.5 mm -- we come to an explanation of this later.
It has long been accepted that Oort Cloud material exists at low temperatures of under 3 K, because of its distance from the Sun's warmth. Naturally enough, the temperature will fall, the further from the Sun. Some would say it's almost self-evident that CMBR comes from the Oort Cloud -- after all, if the Oort Cloud contains the distribution of bodies suggested above, their normal thermal radiation would have to be very similar to that observed in CMBR.
We can mark this point with a formal Proposition.
As mentioned above, CMBR does vary a little at different points of the sky, with peak wavelengths varying very slightly from those corresponding to a temperature of 2.7 K. Figure F8 following is a representation of the whole sky in what's called a Mollweide projection.
Figure OC403-F8. CMBR over the whole sky. From [C12].
On this map, points with increasing amounts of redness indicate places of slightly higher "temperature" (CMBR peak shifted to slightly shorter wavelength), while bluer points indicate more "coolness" (longer wavelength). But the total variation between the coldest (blue) and the hottest (red) parts of the sky corresponds to a temperature variation of only 0.0005 K, less than a thousandth of a degree. An extended explanation of CMBR Mollweide maps (including a map of the Earth with this projection) may be found at [C13].
From the present viewpoint, variations in CMBR over different parts of the sky can be explained by slight variations in movement of clusters of Oort Bodies, away from or towards the Sun. The map in Figure F8 above has been adjusted to remove a gross effect called the CMBR Dipole, which will be explained later as an effect of the Solar System moving with its own rapid motion through Oort Space, relative to the rest of the Oort Cloud. Those shifts in apparent temperature are due to the well-known Doppler Effect.
Why does the CMBR curve peak where it does?
The general CMBR curve peaks at wavelength which corresponds, according to Planck's Law, with a temperature of 2.7 K. What does this mean in reality? It is more or less inarguable that the CMBR is derived from bodies which have surfaces at around 2.7 K. Why should they be so cold?
Of course, for matter in our own Solar System, and to some extent the Oort Cloud, average equilibrium temperature largely depends on how much radiation energy is being received from the Sun. Figure F9 shows how the planets are colder, the further they are from the Sun
Figure OC403-F9. Planet temperatures decrease with distance from Sun. From [C14].
Assuming that the Sun radiates uniformly in all directions, if you know how much total energy it radiates, it's easy to calculate how much energy is received at a given distance -- the same amount is spread over a spherical surface, as the sphere gets bigger, there is less energy per unit area. The temperature achieved for an ideal body at that distance is called the Equilibrium Temperature.
As an example, for a body such as the Earth at 1 AU from the Sun, and treating it as ideal spherical black-body absorber,
the equilibrium temperature is 279 K, or 6 deg C [C14]. Various factors move a planet's temperature away from this equilibrium temperature, for example it will not be a perfect absorber -- and in particular, it may receive energy from its internal processes.
The same formula used to calculate planetary temperatures can be used to calculate how far away an object would be to have an equilibrium temperature of 2.7 K. On a graph of the CMBR spectrum, that is the peak of the curve -- it can be called the CMBR Median (in a distribution, the median is the most popular point). The standard formula gives a value for the CMBR Median at about 14.8 light years.
Figure OC403-F10. The CMBR spectrum. From [C15].
Let us now look at what may be the physical reality behind the CMBR curve, what it implies about the distribution of bodies in Oort Space. There is a caution here. When a great new formula is discovered, such as that for Planck Radiation, there is a tendency to expect Nature, the real world, to conform with the formula. It does not work like that.
In the real world, Nature does as it does; it's Man's task to continually try to get a closer idea of what's going on. In the case of CMBR, a reasonable picture of what's going on is as follows.
In this scenario, solid bodies of every size, from molecules up to sub-stars, are distributed throughout Oort Space. Most will have been subjected to gravitational influences over many millions of years, and will have formed every conceivable sets of planet-moon and sub-solar-system assemblies.
A big proportion of the Oort Bodies will be moon or asteroid size, without their own internal-heating processes. The further such bodies are from the Sun, the lower will be their equilibrium temperature. The most favoured distance, in the case of our own Oort Cloud, will be the CMBR Median distance of about 14.8 light-years.
Why should there be a median at all, why doesn't it get even colder as bodies are more than 14.8 light-years away? The answer seems to be, that as you move further and further from the Sun, eventually you come into the sphere of influence of other stars, which will add their radiation to your equilibrium temperature, moving it up above 2.7 K.
Figure OC403-F11. Travelling through space to the influence of other stars. From [C16].
Since the stars nearest to the Sun are about 4 light-years away, it may seem surprising that the distance to reach another star's influence could average as much as 14.8 light-years. But when you think that even the sixth-nearest star-set is almost 8 light-years away [C17], and these nearest 6 are not evenly spread over the sky, the figure of 14.8 light-years for the CMBR Median seems to be in the right area at least.
There are also other factors which would affect the simple 14.8 light-year figure. As well as stars, a large notional sphere surrounding the Sun would contain other energy-generating bodies, such as Brown Dwarfs (super-Jupiters). Here is what Wikipedia has to say [C17] about such a sphere, of radius 16.3 light years.
"Some 52 stellar systems beyond our own, the Solar System, currently lie within 16.3 light-years of the Sun. These systems contain a total of 63 stars, of which 50 are red dwarfs, by far the most common type of star in the Milky Way. Much more massive stars, such as our own, make up the remaining 13. In addition to these "true" stars, scientists have identified 11 brown dwarfs (objects not quite massive enough to fuse hydrogen), and four white dwarfs (extremely dense collapsed cores that remain after stars such as our Sun have exhausted all fusible hydrogen in their core and have shed slowly their outer layers).
Despite the relative proximity of these 78 objects to Earth, only nine are bright enough to be visible to the naked eye from Earth".
This scenario does explain the left-hand side of the CMBR curve (the "Cold Leg") -- why it drops away rapidly from the peak, and does not fall to zero (in which respect it varies from the theoretical black-body curve, as in Figure F2). The Cold Leg represents bodies well beyond the 14.8 light-year CMBR Median, and the probabilities are such that fewer and fewer "lines of clear sight" will exist beyond the Median.
There is more detail on these topics at P3: Living In The Universe [C4].
Calculations on the Oort Cloud
What has gone before is a mostly qualitative picture of the Oort Cloud. Skilled mathematicians may be able to apply analysis to the data to derive more quantitative results which will clarify the whole scenario.
Energy Balances. A pertinent question, when considering energy balances in the Oort Cloud, is to ask "If all bodies in the Oort Cloud are continuously radiating CMBR, where does this energy come from?".
The answer is that each body is at its own energy equilibrium. Every body is continually absorbing energy at all applicable wavelengths -- nearer to a star, more of this energy will be at visible-light wavelengths, while further away, most will be CMBR from its neighbours. Each body accumulates energy from these incident photons, until a certain storage level is reached, when it will itself fire off a CMBR photon. There may be a long time between firings, if the body is very isolated. In Vortex Stars and Vortex Radiation, we will look at a simple model of the process involved.
Mass of Matter Radiating. Assuming a distribution model of mass in the Oort Cloud, it should theoretically be possible to calculate how much CMBR is emitted at what wavelengths. There are possibilities here, but there is a complication.
Planck's law gives the amount of radiation from unit area of a radiating body. In practice, this will depend on the body's size and shape, and the extent to which it is aggregated.
For example, assuming other factors to be equal, a spherical Oort body 200 km across will have four times the surface area of one 100 km across, and 8 times the mass. So the radiation produced per unit of mass in the larger body will be only half that of the smaller body. The amount of CMBR produced in the Oort Soup has no simple direct relationship to its mass.
Our observations of smaller bodies in the Solar System show that if their diameter is much below 1000 km, they will usually be non-spherical, sometimes quite irregular in shape. Such irregularities increase their relative surface area, and hence their ability to radiate.
* * * * * * * * * * * * * * * * * * * *
References and Links
[C1]. The Electromagnetic Spectrum. https://www.flickr.com/photos/advancedphotonsource/5940581568 .
[C2]. Black body. https://en.wikipedia.org/wiki/Black_body .
[C3]. Blackbody Radiation. http://astronomy.swin.edu.au/cosmos/B/Blackbody+Radiation .
[C4]. David Noel. P3: Living In The Universe: (What CMBR tells us about Dark Matter, and much more). http://aoi.com.au/Living/ .
[C5]. Max Planck, German Physicist. https://www.sciencesource.com/archive/Max-Planck--German-Physicist-SS2287499.html .
[C6]. The 3K Cosmic Background Radiation. http://hyperphysics.phy-astr.gsu.edu/hbase/bkg3k.html .
[C7]. David Noel. UG101: Recycling the Universe: Neutron Stars, Black Holes, and the Science of Stuff. http://aoi.com.au/Recycling/index.htm .
[C8]. Terence Witt. Our Undiscovered Universe. p.361. Aridian Press, 2007. ISBN 9780978593131.
[C9] Cosmic Microwave Background Discovered 50 Years Ago Today. https://www.amnh.org/explore/news-blogs/news-posts/cosmic-microwave-background-discovered-50-years-ago-today.
[C10] David Noel. P2: The Oort Soup as the real origin of Cosmic Microwave Background Radiation. http://aoi.com.au/OortSoup/index.htm .
[C11] COBE Satellite. http://hyperphysics.phy-astr.gsu.edu/hbase/bkg3k.html .
[C12] Planck CMB SMICA map at high resolution. https://figshare.com/articles/Planck_CMB_SMICA_map_at_high_resolution/795296 .
[C13] Cosmic Microwave Background Anisotropy. http://www.astro.ucla.edu/~wright/CMB-DT.html .
[C14] If you were a human floating towards the sun, at what distance from the sun would you feel an Earth-like temperature?. https://www.quora.com/If-you-were-a-human-floating-towards-the-sun-at-what-distance-from-the-sun-would-you-feel-an-Earth-like-temperature .
[C15] Spectrum of the Cosmic Microwave Background. https://map.gsfc.nasa.gov/media/ContentMedia/990015b.jpg .
[C16] Olber's Paradox: Why is the Sky Dark at Night?. http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/olbers.html .
[C17] List of nearest stars and brown dwarfs. https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs .
Go to the Oort Cloud Home Page
Go to the previous article in the OC suite
Go to the next article in the OC suite
Version 1.0 published November 2019 as Segment B of the book "The Oort Cloud: Almost all the Universe". AOI Press, ISBN 9798614884314.
Version 2.0 placed on web at "AOI.com.au", 2022 Jun 23.