What is Cmb: Definition and 250 Discussions

The cosmic microwave background (CMB, CMBR), in Big Bang cosmology, is electromagnetic radiation which is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all space. It is an important source of data on the early universe because it is the oldest electromagnetic radiation in the universe, dating to the epoch of recombination. With a traditional optical telescope, the space between stars and galaxies (the background) is completely dark. However, a sufficiently sensitive radio telescope shows a faint background noise, or glow, almost isotropic, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum. The accidental discovery of the CMB in 1965 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s, and earned the discoverers the 1978 Nobel Prize in Physics.
CMB is landmark evidence of the Big Bang origin of the universe. When the universe was young, before the formation of stars and planets, it was denser, much hotter, and filled with an opaque fog of hydrogen plasma. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the temperature had dropped enough, protons and electrons combined to form neutral hydrogen atoms. Unlike the plasma, these newly conceived atoms could not scatter the thermal radiation by Thomson scattering, and so the universe became transparent. Cosmologists refer to the time period when neutral atoms first formed as the recombination epoch, and the event shortly afterwards when photons started to travel freely through space is referred to as photon decoupling. The photons that existed at the time of photon decoupling have been propagating ever since, though growing fainter and less energetic, since the expansion of space causes their wavelength to increase over time (and wavelength is inversely proportional to energy according to Planck's relation). This is the source of the alternative term relic radiation. The surface of last scattering refers to the set of points in space at the right distance from us so that we are now receiving photons originally emitted from those points at the time of photon decoupling.

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  1. sjordannc

    I What was expansion speed at the time of the CMB?

    To be clear, I'm looking for the speed at which points in space were moving apart from one another in the universe as it existed 370000 years after the big bang, not the Hubble parameter (expansion rate) derived from our current distance within our current time. Thanks in advance!
  2. GhostLoveScore

    I Why do we still see the CMB today?

    I'm coming upon multiple definitions of what the universe size was at around 378 000 years after the big bang. Some say it was around 42 million ly across, some say it was much larger. Why am I asking this? I was thinking about the cosmic microwave background. If the universe was only 42...
  3. .Scott

    B Images with CMB used to Illuminate Dark Matter

    A collaborative effort is yielding a Dark Matter picture of the universe from microwave (CMB) data collected by the Atacama Cosmology Telescope. Major sections of the sky have already been imaged. Those are shown in the image below as the red/blue speckled regions (from a University of Toronto...
  4. Cerenkov

    B B-Modes in CMB Polarization - any change since BICEP2 in 2015?

    Hello. I was wondering if there have been any further developments on the topic of B-modes in the CMB polarization since this 2015 paper? https://arxiv.org/abs/1502.00612 A Joint Analysis of BICEP2/Keck Array and Planck Data The above paper declares that... 'We find strong evidence for dust...
  5. R

    A What does the TE CMB spectrum reveal?

    Does the polarization spectrum TE measured by the Planck and WMAP satellites show evidence for superhorizon fluctuations at low multipoles and are these evidence for pre-bigbang inflation?
  6. R

    I Estimating Hot & Cold Spots in CMB Sky Maps

    Is it possible to estimate the number of hot and cold spots of average diameter of 1° in a full-sky map of the CMB?
  7. X

    I Cosmic Flatness Deduced from CMB

    The Wilkinson Microwave Anisotropy Probe (WMAP) measured temperature differences across the sky in the cosmic microwave background (CMB). See --->(Wikipedia: https://en.wikipedia.org/wiki/Wilkinson_Microwave_Anisotropy_Probe#Main_result) From these observations researchers concluded that the...
  8. Conn_coord

    I The CMB has a thermal black body spectrum

    https://en.wikipedia.org/wiki/Wien's_displacement_law "Maxima differ according to parameterization ... Using the value 4 to solve the implicit equation yields the peak in the spectral radiance density function expressed in the parameter radiance per proportional bandwidth. (That is, the density...
  9. S Holtom

    B Does the CMB give us absolute space and time in practice?

    Another noob relativity / cosmology question (although at least this time won't turn out to be a coding bug, as no code is involved...) AIUI, according to relativity, there is no privileged reference frame, and any inertial reference frame is as "correct" as any other. But... In practice, in...
  10. SherLOCKed

    I Will the upcoming experiment PICO measure kSZ temperature anisotropy?

    I know for sure PICO will be measuring polarization anisotropies with high fidelity. In addition, the PICO science paper shows that it will make full-sky Compton-y maps but the plots are mostly limited to l=1000. Will PICO be able to measure kSZ temperature anisotropy at l=3000?
  11. L

    Next Generation CMB satellites?

    Hi,this one as my new guy post! Any news on the next generation satellite?NASA?
  12. C

    B Does the CMB itself tell us that it started at 3000K?

    Hi. Could the original temperature of the cosmic microwave background be deduced without a theory of a cooling universe? I.e. if we didn't know that the universe started hotter than 3000K and cooled to below 3000K, would we be able to look at the CMB and say, "Yep, that was definitely...
  13. A

    A Significance of CMB anomalies?

    Significance of https://arxiv.org/abs/2110.00120 ,also https://arxiv.org/abs/2109.11092 on CMB anomaly and holographic models? Support?Confirmation?Interpretation? Not my field,but striking!
  14. J

    I The CMB drag on an object moving through space

    An object moving through a fluid, such as the air, experiences a pressure drag caused by the difference in fluid pressure between the front and back surfaces of the object. Similarly, an object moving through a thermalized photon gas, such as the CMB, also experiences a drag. The cause in this...
  15. N

    B Confused About the Source of CMB Photons: Two Possibilities

    I am still so totally confused about the ultimate source of the photons of the CMB. I am getting really confused by online sources, who are either not very clear, or seem to contradict each other. I feel like I have narrowed it down to two sources; 1. The early universe was full of high...
  16. J

    I Does a photon travel freely through the CMB photon gas?

    A photon often travels billions of years (Gyr) through the CMB photon gas (410 photons per cubic centimeter) to reach us. Does it travel freely? Let’s share our thoughts about this. For discussion purpose, let’s assume the photon has a wavelength of 500 nm, close to the peak of the solar...
  17. S

    I Does the CMB reference frame violate the Cosmological Principle?

    Studies of the Cosmic Microwave Background shows that the Earth is moving roughly 380 km/s with respect to it towards the constellation Leo I think. Yet (I think) the Cosmological Principle and the Michelson-Morely experiments suggest there is no preferred reference frame in the universe --...
  18. J

    I Is the supply of the observable CMB radiation limited?

    About 0.38 Myr after the Big Bang, the universe cooled to about 3,000 K and went through the recombination era. Electrons and protons combined to form neutral hydrogen atoms and the photons became free to travel through space. The freed photons can be considered as photon sources of a small...
  19. D.S.Beyer

    B Exploring Proper Time of CMB: What Does it Mean?

    Did some searches through these forums but didn't find this exact question. I'm sure it's already been asked, but I just missed it, my apologies. Please link. I’ll try and ask this question in 3 different ways, and maybe the idea behind it will become apparent. I know that semantics can really...
  20. Arman777

    A What are the parameters that are the same in CMB data for multiple models?

    In cosmology, CMB tells a lot about which cosmological model can be acceptable or not. For instance we know that, whatever the cosmological model we use ##\theta_*## parameter will be always the same. Is there any other parameters that is listed in this picture is **model-independent** ?(i.e...
  21. A

    I What happened to the range of frequencies after recombination?

    So as the hot plasma of a universe expanded it grew colder and at "recombination" it reached a state where it isn't opaque to em radiation any more unlike dense plasma which is. So we say this is the moment the CMB started. But this got me thinking, after recombination the matter was still some...
  22. P

    I How do we observe the CMB at all?

    So, as I understand it, the photons from the microwave background were the result of photon decoupling. Now, if I remember correctly, because of the accelerated expansion of the universe, the diameter of the universe is not about 28 Gyr, but about 90 Gyr, the consequence of which is that there...
  23. A

    I CMB creation, distance measurement question

    How do we know it;s redshift for certain? because unlike with stars the CMB doesn't have spectral lines nor other "similar" objects to compare to as it is everywhere and the same. From what I understand first came the theory and model of the Big bang, then Hubble saw that distant galaxies are...
  24. A

    Exploring the Relationship between CMB Radiation and Blackbody Temperature

    Here is something that struck a note to me, they give the CMB radiation in it's frequency which is in Ghz as the name "microwave" implies and then they also give a temperature in Kelvin. But how can light aka EM radiation have a temperature? I thought only matter with mass can have a temperature...
  25. astroYEEET

    Interaction of an astronaut with the CMB flux

    From the density and the mass we can find the volume using d=m/v <=> v=0.06 m^3. Since we consider the astronaut a sphere we find his radius using V(sphere)=4/3*π*R^3 =>R=0.242m. Now we can calculate the surface area with the formula A=4πR^2=0.735m^2. The energy absorbed will be i suppose equal...
  26. W

    I What is power suppression in CMB experiments?

    I have heard that results form CMB experiments show a suppression of power at large angular scales. Can someone explain what "suppression of power" means in laymen terms in this context?
  27. phyzguy

    I New measurement of the Hubble constant is consistent with the CMB value

    This paper just came out with a new measurement of the Hubble constant based on the technique of gamma ray attenuation. The result is consistent with the lower (CMB-based) value. Interestingly, they also do a joint analysis of several non-CMB techniques (BAO+BBN+SN+γ-ray attenuation), and find...
  28. W

    B Early Predictions of Hot Big Bang CMB: Gamov & Alpher

    As I understand it the fact the COBE measured the CMB to be a perfect black body implies strong evidence for the hot big bang. However, I am wondering what was the earliest prediction that this must be so from big bang cosmology? Does it go all the way back to Gamov and Alpher? is it older ...
  29. Cerenkov

    B Can we observe anything beyond the CMB 'wall'?

    Hello. http://www.jb.man.ac.uk/distance/frontiers/cmb/node10.htm The above article refers to the CMB "wall" in sections 2.4.2 and It's my current understanding that this wall prevents observation of the very early universe; i.e, earlier than 380,000 years after the Big Bang. I also...
  30. Q

    I I computing the CMB Damping Tail

    The formula for the Damping Tail in the CMB BAO analysis generally has the form: $$\mathcal{D} (k)=\int_{0}^{\eta_0}\dot\tau e^{-\left[\frac {k}{k_D(\eta)} \right]^2}d\eta $$I can’t make sense of this. ##\dot\tau## is the Thompson (Differential) Opacity; the product of electron density, cross...
  31. V

    A CMB as thermal radiation of cosmic dust?

    Recently, the origin of the cosmic microwave background as a relic radiation of the Big Bang was questioned and an idea of the CMB as thermal radiation of cosmic dust was revived and revisited. Under this theory, the temperature of the cosmic dust is predicted to be 2.776 K which differs from...
  32. F

    A Hot dark matter constrained by the CMB

    Hello, I often read that hot dark matter is constrained by structure formation issues. But I'm now wondering why it is not constrained by the CMB data itself because such hot dark matter should belong to radiation (with density evolving as 1/a^4 rather than 1/a^3) so for instance when we...
  33. J

    I CMB results cause the contradiction of big bang theory

    - The wavelength of light doesn't change at all in the galaxies because the space of the galaxies doesn't expand. - The wavelength of light should be different when it passes through the galaxy or not. However, this is contradictory to the observed result. So, the big bang theory is wrong.
  34. S

    CMB Photon Energy: Average for T=2.73K

    Homework Statement What is the average energy of the CMB photons, in electronvolts, for ##T=2.73K##? Homework EquationsThe Attempt at a Solution I used the grand canonical ensemble for photons and after several calculations I get $$<E>=\frac{8\pi V}{c^3}\int_0^\infty \frac{h\nu^3}{e^{\beta h...
  35. F

    I Trying to reproduce curves with angle of CMB anisotropies

    I am looking for a way to get, by a simple numerical computation, the 3 curves on the following figure: For this, I don't know what considering as abcissa (comoving distance ?, i.e ##D_{comoving} = R(t)r## with ##R(t)## scale factor and ##r## the coordinate which appears into FLRW...
  36. M

    I Hawking points discovered in CMB

    This paper seems to claim, that there is found powerful observational evidence for some anomalies in the CMB that seems to suggest a conformal cyclic cosmology and so called Hawking points. As I understand it, it also claims, that these points, were present in the very early universe and that...
  37. Auto-Didact

    A Penrose: CCC predicts BICEP2 B-modes due to void magnetic fields, correlated to CMB Hawking Points

    Very recently, Roger Penrose, along with Daniel An and Krzystztof Meissner, have submitted a new paper to PRL. In the new paper they present new empirical findings which support CCC. It doesn't seem to be out yet though, I can't find it anywhere, but I will place the link here when it is out...
  38. BadgerBadger92

    Question about the Big Bang and CMB

    This may seem like a naive or obvious question, which is why I posted it here. How do we know the cosmic microwave background came from the Big Bang? How can we tell how old radiation is?
  39. B

    B Photon mean free path (regarding CMB black body assumption)

    Per Wikipedia (Outer Space) referencing Davies, P. C. W. (1977), "...the mean free path of a photon in intergalactic space is about 10E23 km, or 10 billion light years." Per Lawrence Krauss (1999), it is longer than the size of the visible universe. What is the current thinking about this?
  40. DoobleD

    I Vertical axis of the CMB power spectrum

    Why is the vertical axis in the CMB power spectrum usually chosen as ##l(l+1)C_l/2\pi## instead of simply ##C_l## ? The only answer I found come from this post on stackexchange, but the answer doesn't seem very complete. Anyone knows ?
  41. G

    I How long will the CMB be detectable?

    The CMB has already stretched from some 3000K to 3K, so 99.9% of all the possible stretching until asymptotically approaching absolute zero. Even if our telescope technology continues improving, with the ongoing expansion there may come a time in the future when the CMB temperature will be so...
  42. Buzz Bloom

    I Qs re aspects of the Holmdel Horn Antenna used to find the CMB

    I have been trying to understand some trechnical aspects of the Holmdel Horn Antenna used to first confirm CMB. My references are: (1) https://en.wikipedia.org/wiki/Antenna_gain (2) https://en.wikipedia.org/wiki/Isotropic_radiator (3)...
  43. Sanborn Chase

    I Does the pattern of the CMB change?

    Is the pattern of temperature variation of the cosmic microwave background static, or does it vary over time?
  44. T

    A Planck 2015 CMB temperature individual pixel variance

    Hi, I'm hoping someone can point me in the right direction please. I'm using the Planck 2015 CMB temperature (intensity) SMICA pipeline maps (Nside = 2048) and am trying to determine the temperature variance of each individual pixel. Variance and hit-count were provided with the 2013 CMB maps...
  45. Arman777

    I Observable Universe Tempature and CMB

    It may sound stupid but something bothers me and I want to ask This question come to my mind due to another thread, https://www.physicsforums.com/threads/entropy-and-derivations-is-my-logic-faulty.935533/page-2#post-5910474 In Friedmann Equation we are assuming that universe is homogeneous and...
  46. nomadreid

    I CMB and the reference point for vacuum energy

    From the basic definition of vacuum energy as being tied in with the Uncertainty principle, I would expect this not to include the Cosmic Background Radiation. Right? On the other hand, in figuring out (a) the Casimir effect, one attributes the force to the field between the plates carrying...
  47. bapowell

    Insights A Poor Man's CMB Primer. Part 5: Quantum Seeds - Comments

    bapowell submitted a new PF Insights post A Poor Man's CMB Primer. Part 5: Quantum Seeds Continue reading the Original PF Insights Post.
  48. T

    I Are photons from the CMB being emitted from beyond our cosmological horizon?

    Is there more to our universe than what we can observe? If so, does that mean that photons from the CMB are traveling towards us from beyond our cosmological horizon?
  49. T

    I Why hasn't the light from the CMB already passed us?

    If the cosmic microwave background occurred at the moment that electrons and protons joined together and photons were now free to travel across the universe, then why haven't those photons already passed it. I'm having a hard time understanding why we can see the CMB forever if it was simply a...
  50. X

    I What is the C_l in the Fisher Matrix For An CMB Survey?

    I'm trying to construct a Fisher Forecast for the upcoming S4 CMB survey. I don't understand what the C_l is in this formula. It is H(z) and the Angular Distance? Or is it some covariance matrix and if it is a covariance matrix how do I calculate it considering the experiment hasn't been done...