David Levy Posted November 20, 2015 Posted November 20, 2015 (edited) The Cosmic Microwave Background of our Universe is as follow: https://en.wikipedia.org/wiki/Cosmic_microwave_background "The CMB has a thermal black body spectrum at a temperature of 2.72548±0.00057 K." Therefore, does it mean that the Universe is a black body? Edited November 20, 2015 by David Levy
Strange Posted November 20, 2015 Posted November 20, 2015 Because the main sources of radiation (stars and gas in galaxies) are not much like black body spectra. More here: http://astronomy.nmsu.edu/nicole/teaching/ASTR505/lectures/lecture26/slide01.html
swansont Posted November 20, 2015 Posted November 20, 2015 It was a blackbody when it became transparent. After that, you lose the coupling that would allow the things in it to remain at a uniform temperature.
David Levy Posted November 20, 2015 Author Posted November 20, 2015 (edited) Because the main sources of radiation (stars and gas in galaxies) are not much like black body spectra. More here: http://astronomy.nmsu.edu/nicole/teaching/ASTR505/lectures/lecture26/slide01.html 1. It is stated: "Galaxy spectra are typically characterized by a strong continuum component, caused by the combination of a range of blackbody emitters spanning a range in temperature." So, why the spectra is not black body, while it is caused by rang of blackbody emitters? 2. Temp amplitude. CMB temp is 2.72548K. It is stronger (if I recall correctly) by at least 1000 times than the temp. amplitude of those main sources of radiation (stars and gas in galaxies). Therefore, the amplitude of this radiation is just a fraction of the CMB temp. So why this radiation could have any real effect on the CMB? 3. In any case, if the Universe is not a black body, then how could it be that its Cosmic Microwave Background is black body? Edited November 20, 2015 by David Levy
swansont Posted November 20, 2015 Posted November 20, 2015 1. It is stated: "Galaxy spectra are typically characterized by a strong continuum component, caused by the combination of a range of blackbody emitters spanning a range in temperature." So, why the spectra is not black body, while it is caused by rang of blackbody emitters? Individually, they are blackbodies. The combination does not yield a blackbody spectrum, because such spectra are temperature-dependent. 2. Temp amplitude. CMB temp is 2.72548K. It is stronger (if I recall correctly) by at least 1000 times than the temp. amplitude of those main sources of radiation (stars and gas in galaxies). Therefore, the amplitude of this radiation is just a fraction of the CMB temp. So why this radiation could have any real effect on the CMB? The CMB is basically isotropic. It's everywhere. Sources from remote sources are bound to be less intense. They have no effect on the CMB because the radiation from the galaxies does not interact with the CMB, and vice-versa. 3. In any case, if the Universe is not a black body, then how could it be that its Cosmic Microwave Background is black body? Already answered: the universe was a blackbody when the radiation was emitted. It isn't anymore.
Strange Posted November 20, 2015 Posted November 20, 2015 (edited) 1. It is stated: "Galaxy spectra are typically characterized by a strong continuum component, caused by the combination of a range of blackbody emitters spanning a range in temperature." So, why the spectra is not black body, while it is caused by rang of blackbody emitters? I guess you don't know what a black body spectrum is. Black-body radiation is the type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body (an opaque and non-reflective body) held at constant, uniform temperature. https://en.wikipedia.org/wiki/Black-body_radiation Note the "constant, uniform temperature". Multiple bodies at different termperatures is not the same thing. The back body spectrum varies with temperature. Also, the next sentence from the page you quoted is: "This produces a fairly flat overall spectrum." In other words, not a black body. It then talks about strong emission and absorption peaks; i.e. not a black body spectrum. 2. Temp amplitude. CMB temp is 2.72548K. It is stronger (if I recall correctly) by at least 1000 times than the temp. amplitude of those main sources of radiation (stars and gas in galaxies). Therefore, the amplitude of this radiation is just a fraction of the CMB temp. So why this radiation could have any real effect on the CMB? I don't understand any of that. What does "stronger" mean? What is "this radiation"? 3. In any case, if the Universe is not a black body, then how could it be that its Cosmic Microwave Background is black body? Because the universe WAS a black body. (See post 2.) The CMB is the light from that time (cooled by expansion.) Edited November 20, 2015 by Strange
David Levy Posted November 21, 2015 Author Posted November 21, 2015 I guess you don't know what a black body spectrum is. https://en.wikipedia.org/wiki/Black-body_radiation Well, the main issue is a simple logic. It is stated that the cosmic microwave background radiation of the universe is "an emission of uniform, black body thermal energy coming from all parts of the sky". Based on this simple info it is quite easy to assume that the Universe should also be a black body. However, if I understand it correctly, the science doesn't agree with this simple logic. Based on the observational cosmology (Stars, galaxies...) the science can't find an explanation for the main source of this black body thermal radiation. Therefore, it is estimated that the BBT must be the main source for that black body radiation: "The cosmic microwave background (CMB) is the thermal radiation left over from the time of recombination in Big Bang cosmology." It is also stated that: "The CMB essentially confirms the Big Bang theory." So, my questions are as follow: How do we know for sure that the BBT can generate a black body radiation? How could it be that even after 13.7 Billion years the left over from this radiation is still a black body thermal radiation?
Strange Posted November 21, 2015 Posted November 21, 2015 Well, the main issue is a simple logic. I don't think so. It is stated that the cosmic microwave background radiation of the universe is "an emission of uniform, black body thermal energy coming from all parts of the sky". Based on this simple info it is quite easy to assume that the Universe should also be a black body. Only if you ignore all the other sources of radiation. How do we know for sure that the BBT can generate a black body radiation? Because it was all at the same temperature. (It is obviously more complex than this.) How could it be that even after 13.7 Billion years the left over from this radiation is still a black body thermal radiation? What would cause it to change? There are some good sources of information on line if you want to learn, for example: http://www.astro.ucla.edu/~wright/cosmolog.htm http://ocw.mit.edu/courses/physics/8-286-the-early-universe-fall-2013/video-lectures/
MigL Posted November 21, 2015 Posted November 21, 2015 To be fair, 300 mil yrs after the Big Bang, the universe was extremely close to a black body ( but not quite ) and the CMB remains so to this day. This is because at any time after the inflationary period, there were areas of the universe that were not in causal contact with each other, and isotropy could no longer be maintained. This is also one of the reasons I often argue for a finite universe, at least pre-inflation. The whole purpose of inflation is to impose causal contact between all areas of the universe, at one point in the past, and so guarantee isotropy to the present day. The only way this could happen with an infinite, pre-inflation universe, is to have discontinuities at the `domain` boundaries. This would make for very interesting ( but unlikely ) complications at these discontinuities.
swansont Posted November 21, 2015 Posted November 21, 2015 Well, the main issue is a simple logic.[/size] [/size] It is stated that t[/size]he cosmic microwave background radiation of the universe is "an emission of uniform, [/size]black body[/size] thermal energy coming from all parts of the sky".[/size] Based on this simple info it is quite easy to assume that the Universe should also be a black body.[/size] It's easy to assume that if you ignore physics. "Easy" isn't the right metric, though. However, if I understand it correctly, the science doesn't agree with this simple logic.[/size] No, because the logic is flawed. Based on the [/size]observational cosmology (Stars, galaxies...) the science can't find an explanation for the main source of this black body thermal radiation.[/size] Are you kidding? Your link explains how observational cosmology explains the CMB, and you quote it. Therefore, it is estimated that the BBT must be the main source for that black body radiation: [/size]"The cosmic microwave background (CMB) is the [/size]thermal radiation[/size] left over from the time of [/size]recombination[/size] in [/size]Big Bang cosmology[/size]."[/size] It is also stated that: "[/size]The CMB essentially confirms the Big Bang theory.[/size]"[/size] [/size] So, my questions are as follow:[/size] How do we know for sure that the BBT can generate a black body radiation?[/size] How could it be that even after 13.7 Billion years the left over from this radiation is still a black body thermal radiation?[/size] The BBT doesn't generate blackbody radiation. It's a theory, not a blackbody. The BBT predicts a blackbody spectrum, though, because objects emit radiation according to their temperature. It still has a blackbody spectrum because it hasn't interacted with anything.
Strange Posted November 21, 2015 Posted November 21, 2015 Therefore, it is estimated that the BBT must be the main source for that black body radiation It is also important to note that the CMB (with exactly that temperature) was predicted well before it was observed. That is why the observation of the CMB killed off steady state models. 2
David Levy Posted January 8, 2016 Author Posted January 8, 2016 (edited) Due to the expansion, the CMB had been decreased to the current level of 2.73K. http://www.phy.duke.edu/~kolena/cmb.htm "Therefore, the drop in the CMB temperature by a factor of 1100 (= 3000 K/2.73 K) indicates an expansion of the universe by a factor of 1100 from the moment of decoupling until now". Hence, as the universe will increase its size do to the expansion, the CMB should decrease its level. However, no one gives us a confirmation that the expansion is uniform by 100%. Therefore, it is possible that in some direction the expansion of the universe is different from other direction. So it might be that the universe expansion isn't homogeneous If that is correct, then by definition we might get some minor changes in the CMB level based on direction. Hence, could it be that the famous Foregrounds map of the CMB is a direct reflection of none homogeneous expansion of the universe? Edited January 8, 2016 by David Levy
Strange Posted January 8, 2016 Posted January 8, 2016 (edited) However, no one gives us a confirmation that the expansion is uniform by 100%. Yes they have. http://asd.gsfc.nasa.gov/archive/arcade/cmb_spectrum.html Therefore, it is possible that in some direction the expansion of the universe is different from other direction. So it might be that the universe expansion isn't homogeneous Or we might be moving relative to the CMB. (And you mean "isotropic" not "homogeneous".) If that is correct, then by definition we might get some minor changes in the CMB level based on direction. We do. http://map.gsfc.nasa.gov/ http://planck.cf.ac.uk/science/cmb Edited January 8, 2016 by Strange 3
imatfaal Posted January 8, 2016 Posted January 8, 2016 Due to the expansion, the CMB had been decreased to the current level of 2.73K. http://www.phy.duke.edu/~kolena/cmb.htm "Therefore, the drop in the CMB temperature by a factor of 1100 (= 3000 K/2.73 K) indicates an expansion of the universe by a factor of 1100 from the moment of decoupling until now". Hence, as the universe will increase its size do to the expansion, the CMB should decrease its level. However, no one gives us a confirmation that the expansion is uniform by 100%. Therefore, it is possible that in some direction the expansion of the universe is different from other direction. So it might be that the universe expansion isn't homogeneous If that is correct, then by definition we might get some minor changes in the CMB level based on direction. Hence, could it be that the famous Foregrounds map of the CMB is a direct reflection of none homogeneous expansion of the universe? The CMB is homogeneous isotropic (thanks Strange - Mr 7000+ posts) to IIRC at least one part in a thousand. And the foregrounds which are deducted are the results of theory, found experimentally, and removed once sure they are an artifact
MigL Posted January 8, 2016 Posted January 8, 2016 I always get those two mixed up also, ( sheepish grin )
David Levy Posted January 8, 2016 Author Posted January 8, 2016 (edited) isotropic = (of an object or substance) having a physical property that has the same value when measured in different directions. However: http://planck.cf.ac.uk/science/cmb "The Cosmic Microwave Background (or "CMB" for short) is radiation from around 400,000 years after the start of the Universe. Ever since the Big Bang, the Universe has been cooling and expanding. By around 400,000 years through its life it was cool enough (though still around 3000 Celsius). The expansion of the Universe has stretched out the CMB radiation by around 1000 times, which makes it look much cooler. So instead of seeing the afterglow at 3000 degrees, we see it at just 3o above absolute zero, or 3 Kelvin (-270o C)." So, is it correct that as the universe continue with the expansion process, the CMB should be decreased proportionally? For example, when the size of the universe will be twice, the value of the CMB should be half. Based on the following explanation: The rate of expansion is that required to get from zero size(1) to its current size in 13.8 billion years. So to get to twice that size would take another 13.8 billion years(2). Therefore, after another 13.8 billion years from now (- 400,000 years, as our starting point is 400,000 years after the B.B.) the value of the CMB should be: 2.7 / 2 = 1.35 K Is it correct? Edited January 9, 2016 by David Levy
David Levy Posted January 13, 2016 Author Posted January 13, 2016 isotropic = (of an object or substance) having a physical property that has the same value when measured in different directions. However: http://planck.cf.ac.uk/science/cmb "The Cosmic Microwave Background (or "CMB" for short) is radiation from around 400,000 years after the start of the Universe. Ever since the Big Bang, the Universe has been cooling and expanding. By around 400,000 years through its life it was cool enough (though still around 3000 Celsius). The expansion of the Universe has stretched out the CMB radiation by around 1000 times, which makes it look much cooler. So instead of seeing the afterglow at 3000 degrees, we see it at just 3o above absolute zero, or 3 Kelvin (-270o C)." So, is it correct that as the universe continue with the expansion process, the CMB should be decreased proportionally? For example, when the size of the universe will be twice, the value of the CMB should be half. Based on the following explanation: Therefore, after another 13.8 billion years from now (- 400,000 years, as our starting point is 400,000 years after the B.B.) the value of the CMB should be: 2.7 / 2 = 1.35 K Is it correct? No answer? Does it mean that it is correct? So, let me ask again: Do you agree that few billion years ago, the CMB level was higher? Do you agree that in the future, the CMB level should be lower due to the expansion process? Hence, by every passing day - the value of the CMB is decreasing by tiny micro fraction of temp.
Mordred Posted January 13, 2016 Posted January 13, 2016 (edited) Natural consequence of expansion. When you increase a volume with the same number of particles. (Aside from any phase changes) the temperature will lower. This is already well known. pV=nRt. The temperature variations is Inversely proportional to the scale factor Edited January 13, 2016 by Mordred
David Levy Posted January 13, 2016 Author Posted January 13, 2016 (edited) Natural consequence of expansion. When you increase a volume with the same number of particles. (Aside from any phase changes) the temperature will lower. This is already well known. pV=nRt. The temperature variations is Inversely proportional to the scale factor Thanks So, the CMB level is directly effected by the expansion process. However, The this process is not uniform: Where there are concentrations of matter, bound together by electromagnetic forces and/or gravity, then there will be no expansion. There will be no expansion in clusters and where there are concentration of stars Therefore, how can we assume that the CMB is isotropic if it is based on a none uniform process? In other words: If theoretically, in one direction of the universe there are more clusters and more concentrated stars, then the expansion might not be equal to other direction with less clusters and less concentrated stars. Less expansion means - higher level of CMB. Higher expansion means - less level of CMB As a direct outcome - directions with more clusters and more concentrated stars might have in the future higher level of CMB comparing to directions with less clusters and less concentrated stars. Hence, how can we explain this paradox? If the CMB is isotropic then the expansion must be uniform. If the Expansion isn't uniform then the CMB can't be isotropic. Do you agree? Edited January 13, 2016 by David Levy
swansont Posted January 13, 2016 Posted January 13, 2016 If the Expansion isn't uniform then the CMB can't be isotropic. Do you agree? No. You are talking about two separate things when you speak of motion of clusters and the expansion of the universe. Therefore, after another 13.8 billion years from now (- 400,000 years, as our starting point is 400,000 years after the B.B.) the value of the CMB should be: 2.7 / 2 = 1.35 K Is it correct? No, I don't think so. Linear expansion does not give a proportional expansion in volume. If you double the linear distance, you increase the volume by 8x. And expansion isn't linear anyway.
imatfaal Posted January 13, 2016 Posted January 13, 2016 Thanks So, the CMB level is directly effected by the expansion process. However, The this process is not uniform: There will be no expansion in clusters and where there are concentration of stars Therefore, how can we assume that the CMB is isotropic if it is based on a none uniform process? In other words: If theoretically, in one direction of the universe there are more clusters and more concentrated stars, then the expansion might not be equal to other direction with less clusters and less concentrated stars. Less expansion means - higher level of CMB. Higher expansion means - less level of CMB As a direct outcome - directions with more clusters and more concentrated stars might have in the future higher level of CMB comparing to directions with less clusters and less concentrated stars. Hence, how can we explain this paradox? If the CMB is isotropic then the expansion must be uniform. If the Expansion isn't uniform then the CMB can't be isotropic. Do you agree? It seems to boil down to <<If we now state that the universe is neither isotropic nor homogeneous on a large scale will the predictions we have made on the basis of the seemingly correct assumption that the universe IS isotropic and homogeneous still hold?>> Not necessarily no - and we lose our power to predict. Our predictions so far are very very accurate - therefore we choose to keep our assumption that the universe is isotropic and homogeneous on a large scale
David Levy Posted January 13, 2016 Author Posted January 13, 2016 (edited) No, I don't think so. Linear expansion does not give a proportional expansion in volume. If you double the linear distance, you increase the volume by 8x. And expansion isn't linear anyway. Yes, fully agree. However, the main idea was to show that the CMB level is directly based on the expansion process. No. You are talking about two separate things when you speak of motion of clusters and the expansion of the universe. Sorry, I'm not sure that I fully understand your answer. So, let me ask you the following: Do you agree that the CMB level is directly based on the expansion process? If yes, then: Do mean that the CMB is isotropic as the expansion is uniform? Or do you mean that even if the expansion is not uniform the CMB must be Isotropic? Edited January 13, 2016 by David Levy
swansont Posted January 13, 2016 Posted January 13, 2016 Yes, fully agree. However, the main idea was that the CMB level is directly effected by the expansion. The main idea, sure, you gave a numerical example which is grossly wrong, which calls into question your grasp of that idea. Sorry, I'm not sure that I fully understand your answer. So, let me ask you the following: Do you agree that the CMB level is directly based on the expansion? Do mean that the CMB is isotropic as the expansion is uniform? Or do you mean that even if the expansion is not uniform the CMB must be Isotropic? Yes to the first two. But this does not seem to address your misconception about clusters at all.
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now