CMB - cosmic microwave background radiation. In preparation for an addendum to The Pearlman Spiral that shows the main differences in interpreting.
CMB - cosmic microwave background radiation In preparation for an addendum to The Pearlman Spiral that shows the main differences in interpreting facts related to CMB with the current standard explanations. Than evaluate if the factual observations of CMB align better with one or the other, or if neutral. listing the known facts: was gas now redshifted light current temp = 2.75 K degrees above absolute zero1 'The temperature is uniform to better than one part in a thousand!'2 so not a localized source current understanding: emitted 300k-400k years after the big bang, 13.7 B years ago. Surface of the last scattering 380k years after the big bang. Due to cosmic expansion dissipated 1000 fold initial size initial temp With 100 times less expansion (273 degrees above absolute zero or 32 degrees Fahrenheit, the temperature at which water freezes to form ice on the Earth's surface) In addition to this cosmic microwave background radiation, the early universe was filled with hot hydrogen gas with a density of about 1000 atoms per cubic centimeter. When the visible universe was only one hundred millionth its present size, its temperature was 273 million degrees above absolute zero and the density of matter was comparable to the density of air at the Earth's surface. At these high temperatures, the hydrogen was completely ionized into free protons and electrons. Since the universe was so very hot through most of its early history, there were no atoms in the early universe, only free electrons and nuclei. (Nuclei are made of neutrons and protons). The cosmic microwave background photons easily scatter off of electrons. Thus, photons wandered through the early universe, just as optical light wanders through a dense fog. This process of multiple scattering produces what is called a “thermal” or “blackbody” spectrum of photons. According to the Big Bang theory, the frequency spectrum of the CMB should have this blackbody form. This was indeed measured with tremendous accuracy by the FIRAS experiment on NASA's COBE satellite. Eventually, the universe cooled sufficiently that protons and electrons could combine to form neutral hydrogen. This occurred roughly 400,000 years after the Big Bang when the universe was about one eleven hundredth its present size. Cosmic microwave background photons interact very weakly with neutral hydrogen, allowing them to travel in a straight lines. The behavior of CMB photons moving through the early universe is analogous to the propagation of optical light through the Earth's atmosphere. Water droplets in a cloud are very effective at scattering light, while optical light moves freely through clear air. Thus, on a cloudy day, we can look through the air out towards the clouds, but can not see through the opaque clouds. Cosmologists studying the cosmic microwave background radiation can look through much of the universe back to when it was opaque: a view back to 380,000 years after the Big Bang. This “wall of light“ is called the surface of last scattering since it was the last time most of the CMB photons directly scattered off of matter. When we make maps of the temperature of the CMB, we are mapping this surface of last scattering.3 1 http://map.gsfc.nasa.gov/universe/bb_tests_cmb.html 2 http://map.gsfc.nasa.gov/universe/bb_tests_cmb.html 3 http://map.gsfc.nasa.gov/universe/bb_tests_cmb.html
When Did the First Cosmic Structures Form? Quasars are the most distant distinct objects that astronomers have been able to directly detect. Because of their intrinsic brightness, the most distant quasars are seen at a time when the universe was one tenth its present age, roughly a billion years after the Big Bang. However, astronomers believe that some objects must have formed earlier than quasars, because the ambient gas in the universe is observed to be ionized at a relatively early time, presumably due to ionizing radiation from a population of early objects. Since ionized gas can interact withcosmic microwave background photons, WMAP observations help to elucidate the nature of the ionized gas and the objects that caused the ionization.4
Where did the photons actually come from? A very good question. We believe that the very early Universe was very hot and dense. At an early enough time it was so hot, ie there was so much energy around, that pairs of particles and anti-particles were continually being created and annihilated again. This annihilation makes pure energy, which means particles of light - photons. As the Universe expanded and the temperature fell the particles and anti-particles (quarks and the like) annihilated each other for the last time, and the energies were low enough that they couldn't be recreated again. For some reason (that still isn't well understood) the early Universe had about one part in a billion more particles than anti-particles. So when all the anti-particles had annihilated all the particles, that left about a billion photons for every particle of matter. And that's the way the Universe is today! So the photons that we observe in the cosmic microwave background were created in the first minute or so of the history of the Universe. Subsequently they cooled along with the expansion of the Universe, and eventually they can be observed today with a temperature of about 2.73 Kelvin.5
Why does the CMB support the Big Bang picture? The basic point is that the spectrum of the CMB is remarkably close to the theoretical spectrum of what is known as a "blackbody", which means an object in "thermal equilibrium". Thermal equilibrium means that the object has had long enough to settle down to its natural state. Your average piece of hot, glowing coal, for example, is not in very good thermal equlibrium, and a "blackbody" spectrum is only a crude approximation for the spectrum of glowing embers. But it turns out that the early Universe was in very good thermal equilibrium (basically because the timescale for settling down was very much shorter than the expansion timescale for the Universe). And hence radiation from those very early times should have a spectrum very close to that of a blackbody. 4 http://map.gsfc.nasa.gov/universe/rel_firstobjs.html 5 //
The observed CMB spectrum is in fact better than the best blackbody spectrum we can make in a laboratory! So it is very hard to imagine that the CMB comes from emission from any normal "stuff" (since if you try to make "stuff" at some temperature, it will tend to either emit or absorb preferentially at particular wavelengths). The only plausible explanation for having this uniform radiation, with such a precise blackbody spectrum, is for it to come from the whole Universe at a time when it was much hotter and denser than it is now. Hence the CMB spectrum is essentially incontrovertible evidence that the Universe experienced a "hot Big Bang" stage (that's not to say that we understand the initial instant, just that we know the Universe used to be very hot and dense and has been expanding ever since). In full, the three cornerstones of the Big Bang model are: (1) the blackbody nature of the CMB spectrum; (2) redshifting of distant galaxies (indicating approximately uniform expansion); and (3) the observed abundances of light elements (in particular helium and heavy hydrogen), indicating that they were "cooked" throughout the Universe at early times. Because of these three basic facts, all of which have strengthened over the decades since they were discovered, and several supporting pieces of evidence found in the last deacade or two, the Big Bang model has become the standard picture for the evolution of our Universe.6 'all the objects within a galaxy, including the dust and gas clouds, the stars, the planets, the life forms, and their legs, are not expanding. ' rmpif we were not in the approximate center the relative positions of distant visible galaxies would move quite a bit but do not ancient star charts debunk that by showing not much change?
My question centers around the surface of last scattering. If I understand correctly, some of the key equations that have been used to determine the physics of the universe depend on two variables, one of which has usually been ignored (the pressure?) because it becomes so small as to be irrelevant. Does this assumption still hold at the time of the surface of last scattering or should the pressure figure back into the equations? Submitted by rassler"AT"cleo.bc.edu The pressure is certainly a very important part of the physics of what's going around 6 http://www.astro.ubc.ca/people/scott/faq_basic.html
redshift 1000 when the Universe becomes neutral and the photons last interact with the matter. So rest assured that when theorists calculate the real thing, they don't forget to include the pressure! In today's Universe pressure on the whole is negligible. The Universe is dominated by regular stuff, which acts like a largely non-interacting fluid on big scales. Such stuff is usually referred to by the term "dust", which here has a technical meaning. So for the recent history of the Universe you can indeed ignore pressure. But earlier in the Universe the radiation content becomes more and more important. And back when the Universe was so hot that all the matter was ionized, the photons were very strongly interacting with all those charged particles. At those early times in the history of the Universe, it would be a hopelessly bad approximation to neglect pressure effects. Indeed the effect of pressure on the generation of CMB anisotropies is very much tied up with the existence and detailed shape of the bumps and wiggles in the "power spectrum" of anisotropies, through which we hope to be able to understand all the physics of the large scale Universe!7
What are the most important physical processes that created the CMB photons? It seems that baryogenesis, nucleosynthesis, and element formation are all important, but I'm not sure where my focus should be. Submitted by rkeyes"AT"rice.edu The answer to this question depends to some extent on what you are really asking. In some sense the CMB photons that we detect were created in the Earth's atmosphere, as they are absorbed and re-emitted along the path of the light. But that's not a very useful answer! Along similar lines, the photons were last scattered at a few hundred thousand years after the Big Bang, and you can think of scattering as absorption of a photon and simultaneous emission of a new one in a random direction. As you go even earlier in the history of the Universe, individual photons interact ever more strongly with matter, and at times before about 1 year photons lose even energy information during scattering processes. So none of the photons we see today contain information about things happening before about the first year in the history of the Universe. What you presumably want to know is where the photons came from in the first place though! The process of nucleosynthesis (formation of the light elements) happened in about the first three minutes. In fact there were already so many photons per baryon (normal matter particle), that the extra photons created from the nuclear energy released at this time are quite negligible. 7 //
At super-high energies particles and anti-particles were being created and annihilated constantly -- appearing out of pure energy (photons) and then annihilating again into pure energy. At higher energies even higher mass particles and anti-particles were appearing and disappearing, and existed in roughly equal numbers to the photons at that time. So early in the history of the Universe there were lots of protons and anti-protons around, for example, earlier than that there also lots of higher mass exotic particles, and so on. The lightest particles we know about are the electrons, and so they annihilated last. Before about 1 second the Universe was full of electrons (e-) and antielectrons (e+) and photons ( ), in about equal amounts. Then as the Universe expanded, the temperature dropped low enough that if you annihilated an e+-e- pair there wasn't enough energy in the average photon to recreate the pair. So eventually the Universe lost most of its e+'s and e-'s, and ended up with mostly 's. Why we have any e-'s left over is a good question (that's baryogenesis), and I talked a little about that before. But this annihilation process is basically what produced the photons in the CMB, with most coming from e+-e- annihilation, some contribution from µ+-µ- and +- - annihilation a little earlier and a tiny bit extra from nucleosynthesis. And of course it's always possible that other unknown physical processes occurring between the first 1 second and the first year could have played some role in generating extra photons as well.
Is the CMB simply high energy photons that originated at the origin of the universe that have been red-shifted all the way to the other end of the electromagnetic spectrum because of the expansion of the universe? Or is it something more complicated than that? Submitted by bchaikin"AT"aol.com 11/98 You've got it! Let me qualify that by saying that the CMB photons were made long after the origin of the Universe (whatever that was). Here by "long after" I mean maybe seconds! The CMB derives from a time when the Universe was so hot that a whole bunch of particles were being created and annihilated rapidly, and so were in equilibrium with the photons. There were essentially equal numbers of all the particles you've ever heard of, including photons. As the Universe expanded and cooled various particles annihilated (there wasn't enough energy around to recreate the particle-antiparticle pairs once they had annihilated to photons), increasing the number of photons relative to matter particles. At this point the CMB photons were really CGRB photons, since they were high energy gamma-rays. The last such event (electron positron annihilation) was about
a minute after the Big Bang. So the photons weren't really made in that first instant, but a lot later! These photons stretched in wavelength along with the expanding Universe, as you rightly say. But they were still exchanging energy with the matter, and in fact there were processes which could generate new photons right up until about a year after the Big Bang. At that point the photons were low energy X-ray photons. After that, there were still slow processes that could affect some of the photons (changing their direction or mildly changing their energy), and they only really stopped interacting with the matter when the Universe became neutral around 300,000 years after the Big Bang. At that point the photons were in the near-infrared part of the spectrum. Since then they've interacted almost not at all, and have traveled through the expanding Universe being redshifted into the microwave region.
To me, the cosmic background radiation (alone) merely suggests some past or current universal substance or phenomenon which is both highly isotropic and highly opaque. I am very curious as to the strict limits current CBR evidence places on "Hat-stand" cosmological speculation. For example, does the CBR, by itself, effectively rule out flat space? A periodic universe? A trillion year old universe? A vast universe? Submitted by hazelf"AT"ix.netcom.com 2/99 You are right in essence, that the CMB by itself suggests "some past or current universal substance or phenomenon which is both highly isotropic and highly opaque". But of course it can't be taken on its own, since there are many other well-established facts that we now know about the Universe. For example, we have known for almost 75 years that the universe is expanding. Together with the CMB, this implies that there was an earlier, hotter, denser phase in the history of the Universe, and that the CMB is the relic of this phase. I know of no reasonable alternative. As for whether the CMB rules out various "hat-stand" ideas, it depends how crazy they are! Certainly there is an ongoing investigation into the curvature of the Universe as a whole, and within some bounds we are currently unsure about how curved it is. The best solution currently is flat (but expanding) space, with some amount of vacuum energy density (also called a "cosmological constant") contributing. But it could have negative curvature (an "open" universe), and the closed geometry isn't entirely ruled out either (although currently not favoured). The space-time structure of the Universe could also be periodic in some way, but only if the scale of the periodicity is very large, otherwise you mess up the CMB in a big way. The question of how old the Universe might be is an interesting one. There are lower bounds on the age of the Universe by finding the oldest things in it (e.g. globular clusters). But it's less clear how you might get an upper bound. The best-fitting versions
of big bang models expand from a time around 13 billion years ago, and they don't work too well older than say 15 billion years (assuming nothing particularly funny happens). But I'm sure you can't currently rule out a universe that did basically nothing for a trillion years and then decided to expand rapidly, or one that might have had a previously contracting (or even expanding) phase, or indeed many phases before that. To some extent we then get into metaphysics, since it's not obvious that you could ever test such a hypothesis. Still, the simplest solution is that our current phase is the only one there's been, and that it can't be a trillion years old. My view is that the CMB, together with the wealth of other information we have about the Universe as a whole, paints a pretty coherent picture. And this picture - that the Universe began around 15 billion years ago and has been expanding and cooling ever since - sounds crazy enough! The reason it is believed is that it is a very simple idea, and that it works astonishingly well.
If the CMB was created by the Big Bang and the proceeding few moments of the early universe, then shouldn't it have had plenty of time to whizz off into outer space, way faster than the non-light-speed matter (e.g. us) have been moving out from the Big Bang site, in which case we shouldn't be able to observe it, I'd have thought? Submitted by vince.bowdren"AT"jobstream.co.uk 3/99 This is similar to other question which I have received many times. The answer is very simple - your mental picture is incorrect! Since the Big Bang model, the expanding Universe, the speed of light etc., are all far from everyday experience, there are many ways in which people can get the wrong image in their heads. The first thing to get straight is that there's nothing outside the Universe! By definition the Universe is everything there is: we live inside it; and it isn't expanding into anything. The next thing you have to get clear is that the Big Bang happened everywhere at once, and shortly afterwards all of the CMB photons were created and suffered their last interactions with matter. So those photons are indeed shooting off into space in all directions at the speed of light. The CMB photons we see today are coming to us from way across the Universe (about 13 billion light years away, if for example the Universe is 13 billion years old). That's true no matter what direction in the sky we look. It might help to think what happened to the photons that were made right here, all that
time ago. Those particular CMB photons have been whizzing off at the speed of light in all directions, and are now being detected by distant observers (say 13 billion light years away) as part of their Cosmic Microwave Background.
Has the redshift of a practically ideal blackbody like the CMB actually been measured or calculated from a model? If measured what absorption line was used as a reference? Submitted by LABELE"AT"aol.com 9/99 It sounds like you have already answered that question! Since there is no reference line, you can't measure the redshift. In fact there's no way to tell the difference between a blackbody at lower temperature and a blackbody which has been redshifted - the spectrum retains exactly the same shape as the Universe expands. If you think about it, if that were not the case, then the CMB spectrum would change shape as the Universe expanded, and hence it would be pretty unlikely that we'd observe it to be such a precise blackbody shape today. To put it another way, you can think of the CMB as being roughly 3 Kelvin blackbody radiation existing today, or as 30 Kelvin blackbody radiation which has been redshifted by a factor of 10, or as 300 Kelvin radiation which has been redshifted by a factor of 100, etc. They are all equivalent. As far as the theoretical picture is concerned, the CMB photons last interacted with matter when the temperature of the Universe was about 3000 Kelvin, and the photons have redshifted about a factor of 1000 since then. So you can think of them as being a view from that redshift. But the photons themselves were produced much earlier, in a much hotter phase. And so it's probably best to picture them as the glow from period in the early Universe when the temperature was billions of Kelvin, and since the Universe has expanded by factors billions since then, we observe the CMB today at 3K. Although there are general arguments (involving synthesis of the light elements in the early Universe for example) for predicting that the CMB should have roughly the order of magnitude of temperature that's observed (and these arguments were discussed as early as the 1940s), I've no doubt other arguments would be put forward if it had turned out to be a quite different temperature. There's no fundamental reason known which can explain why the current temperature is 3K rather than, say, 5K or 0.2K. ..In the standard cosmological picture the CMB photons last scattered with matter in the Universe at about half a million years after the Big Bang. At that time the radiation was about 3000 Kelvin. The photons have been redshifted by a factor of about 1000 since then. By applying Wien's law you can then see that the CMB photons, with their wavelengths stretched by 1000, will appear as a blackbody with a temperature about 1000 times smaller, or about 3 Kelvin. It's the fact that the Universe is expanding (with
freely-travelling photons having their wavelengths expanded in the process) which argues that the Universe must have been hotter in the past.
How come radiation "cools off" instead of retaining a constant wavelength? Submitted plotinus"AT"otenet.gr 2/01 You can think of this just like any other form of slow expansion of a gas. Expansion acts to cool a gas, which is for example, the principle on which refrigerators work. Physicists refer to this as "adiabatic expansion". You can think of the CMB as a gas of photons, adiabatically expanding through the Hubble expansion of the Universe. This photon gas loses energy. And since the energy of a photon is inversely proportional to wavelength (Planck's law), then the wavelength increases as the Universe expands. Another way to think about this is just that the wavelengths of the photons stretch along with all the other distances - the interpretation of cosmological redshift is just that when the photons left their source, distances were proportionally smaller, and the wavelengths got stretched (i.e. redshifted) on their way to us. ..The CMB is photons! Photons are particles of electromagnetic radiation, or in other words, light. The ones in the CMB were made in the early Universe as high energy photons, and have been cooling as the Universe expanded - so that we observe them as microwave radiation today. Why don't we see cosmic background radiation well into wavelengths longer than microwave? I understand the concept of "last-scattering surface" and its relevance to why we don't see *shorter* cosmic background wavelengths, but I don't understand why we shouldn't also "see" longer wavelength remnants of an even younger, more redshifted universe.
Submitted by bozone"AT"ripco.net 4/01 Remember that the photons are red-shifting on their way to us. In principle we can't tell the difference between photons emitted very early, which have redshifted a lot, and photons emitted later, which haven't redshifted so much. The early Universe is hotter and hotter. And this is balanced exactly by the redshift factor. The "last-scattering surface" is not the time when the radiation formed, just when it last interacted strongly with matter. At that time they were redshifted by about a factor of 1000, and at a temperature of about 3000 Kelvin. But before that they would have been at 30,000 Kelvin, and redshifted by a factor of 10,000 on their way to us. And before that they would have been at 3000,000 Kelvin, and redshifted by a factor of 100,000. Etc.
I've noticed something that I've never seen explained, and was hoping you could shed some light on; Fact 1: The Universe has an average temperature of 2.73 K. Fact 2: Absolute zero is 0K = -273 deg. C. So, the avg. temp. is exactlyB (-)100 times absolute zero. Is this a coincidence, or is there a real connection between the two? Submitted Andy Smith 1/10 You're not the first person to note this coincidence! See this page for several (crazy!) ideas involving CMB temperature numerology. This is of course a coincidence. The CMB temperature is changing with time for one thing. And the numbers are actually 2.725 and 273.15, so not exactly a factor of 100 anyway.
How is the blackbody spectrum connected with the cosmic microwave background radiation? Submitted by sreerajt90"AT"gmail.com 10/10 The CMB has spectrum is extremely well fit with a blackbody shape. In fact it may be the best blackbody that we have ever measured!
What experimental evidence do we have that the CMB fills the entire universe as required by theory? Submitted by vorleons"AT"hotmail.com 7/07 I don't think this is "required by theory". The ubiquity of the CMB is what drives the theoretical picture (not the other way around). The CMB is observed to be very close to isotropic, i.e. the same in every direction. Since CMB photons travel at the speed of light, then it would take an extremely controived universe to only have CMB photons arriving at us here and now from every direction, without having them everywhere else too! In fact the tiny amplitude CMB anisotropies have a pattern which constrains details of the cosmological model - and part of that is the need for radiation which fills the Universe and which dominated the total energy density at early times. I can't think of any way of avoiding having CMB photons everywhere.
Is there any way to be sure that the CMB fills the entire universe? Isnt it possible that it only exists around certain entities (perhaps it only exists around Earth, for that matter)? Submitted by whitetrash_01"AT"hotmail.com 4/01 Obviously we only have direct information about CMB photons which have arrived at detectors right here on Earth. But they had to come from somewhere at the speed of light! Since they're observed to come from all directions, then everywhere in the Universe seems likely to be filled with these things. The only way to avoid that conclusion would be to have all the photons aimed at us, and that puts us in a very special position indeed! That would contradict the observation of zilllions of galaxies in the Universe, which show that our local part of space isn't much different from any other. There are also some fairly direct ways of showing that the CMB photons exist elsewhere. One way is to find the effect on the CMB of the photons having changed their energies a little as they travel through the hot gas in a cluster of galaxies. This effect has been measured for many distant galaxy clusters, indicating that the CMB photons have to be coming from at least as far away as those objects. The observation of structures (anisotropy) on the CMB sky is also good evidence for a Universe filled with CMB photons. That's because the size distribution of the structures on the CMB sky has turned out to be just like you'd predict in the hot Big Bang model, where the CMB photons are in fact left over from a much hotter early phase of the history of the Universe. Understanding CMB in light of The Pearlman Spiral:
