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. C

    How we are still able to detect CMB photons?

    This might be a silly question. But recently I got to know about CMB(cosmic microwave background) and this question is nagging me. During CMB , the universe was pretty small compared to now, so our current point of existence in space would have been very near the CMB, 13.7 bill yrs ago. So the...
  2. J

    Cosmic acoustics -- why no intermediate waves on CMB map

    I fully understand the representation of the set of waves that are either at full compression or full rarefaction at recombination, thus, yielding a CMB map. But at this time are there no waves that are intermediate, e.g. 50% of the way to full compression or full rarefaction. Why don't these...
  3. Joe Ciancimino

    Q: Measured changes in the CMB radiation?

    The cosmic microwave background was a single event that happened literally everywhere in pretty much a single instance. That moment was in the past and is unchanging. What does change is the wavelength of the radiation as it travels through the expanding universe causing the wavelength to grow...
  4. P

    CMB at 50: Princeton early universe panel discussion

    An interesting discussion at this conference here: http://physics.princeton.edu/cmb50/videos/20150612_session6_2.mp4 David Spergel said the next project should be to look for something called f and L or is is FNL? Can someone explain what this is and what its significance is? What did you guys...
  5. Mysteryciel

    Can the CMB Reference Frame be Changed in the Newtonian Friedmann Equation?

    I know that in physics we can change referance frame.The Newtonian Friedmann equation is ##H^2-{8πGp /3}={-k/a^2}## I know that this equation derived from CMB referance frame.Now can we change this referance frame ? (And If you can give me an article about this issue I will be very happy,I...
  6. U

    CMB Temperature - Will hydrogen be ionized?

    Homework Statement Energy density of radiation ##\rho_r = 8 \times 10^{-14} J m^{-3}## and energy density of matter ##\rho_m = 2.63 \times 10^{-10} J m^{-3}##. Baryon density parameter is ##\Omega_b = 0.04##. Temperature of CMB today is ##2.73 K##. Ionization energy of Hydrogen is ##13.6 eV##...
  7. G

    CMB photons, why are they still around?

    10 billion years ago, photons emitted at the surface of last scattering were being absorbed by whatever stuff was around. Those atoms would absorb those photons and release other photons, but these would not be primordial photons from the CMB anymore but photons emitted at a later time by the...
  8. C

    CMB , Spherical Harmonics and Rotational Invariance

    In Dodelson's "Modern Cosmology" on p.241 he states that the ##a_{lm}##-s -- for a given ##l##-- corresponding to a spherical harmonic expansion of the photon-temperature fluctuations, are drawn from the same probability distribution regardless of the value of ##m##. Dodelson does not explain...
  9. H

    Earth's velocity wrt CMB gives the centre of the universe?

    Alan Guth mentioned in his lectures that to get the accurate linear relationship of Hubble's law, we need to subtract the velocity of the Earth with respect to the cosmic microwave background (CMB) when calculating the velocity of galaxies. Question 1: If space is expanding uniformly...
  10. ScienceIsFun

    Are we center of the universe?

    Hello everyone, As you can see, I am new on this forum. In my first thread, I decided to ask one very interesting question, maybe one of the most shocking questions which exist at this moment. We all know the data which have been provided of WMAP and Planck satellites, and what this data shows...
  11. CassiopeiaA

    Can We See CMB Again If Universe Expands at a Constant Rate?

    If particle horizon is the maximum distance we can see presently in the universe, how come we are able to see CMB? CMB is radiation from surface of last scattering happened at t~380k years. Suppose universe is expanding at a constant rate ( i.e. no acceleration), will we be able to see CMB...
  12. O

    CMB: How Was Matter Distributed Before Photon Decoupling?

    According to the hot Big Bang model, about 380000 years after that violent explosion, the universe was transparent to radiation but before that moment photons were trapped in the soup of ionized plasma. And my question is how does the matter distribute in space before the photons decouple from...
  13. S

    How can you show the CMB shift parameter?

    Dealing with CMB, people sometimes refer to the shift parameter; R = \sqrt{\frac{\Omega^0_m}{\Omega^0_k}}sinh(\sqrt{\Omega^0_k}\int^{z_{dec}}_{0}{\frac{dz}{E(z)}}) I know that it is related to the position of the first acoustic peak, however, the amount is around 1.7. What does this mean?! does...
  14. Chronos

    An explanation for hemispherical anisotropy in the CMB?

    This paper, http://arxiv.org/abs/1503.03859, suggests a possible explanation for hemispherical anisotropy in the CMB based on non-commutative space time.
  15. B

    Why can we still detect the CMB ?

    According to Wayne Hu here, http://background.uchicago.edu/~whu/intermediate/intermediate.html, the universe started hot and dense and then expanded and cooled. In the hot, dense conditions of the early universe, photons were tightly glued to matter. When the universe was about 300,000 years...
  16. ChrisVer

    How Does the CMB Power Spectrum Relate to Angular Scales?

    http://pdg.lbl.gov/2013/reviews/rpp2013-rev-cosmic-microwave-background.pdf Here one reads in sec. 26.2.4 that: However it states that a single Y_{lm} corresponds to angular variations of \theta \sim \pi /l. I am not getting these statements. Also I find it difficult to understand, since...
  17. ChrisVer

    CMB dipole anisotropies- we are moving towards cmb?

    Don't kill me, but I have a problem understanding and answering a question of what it means to be moving towards/away from CMB rest frame. The CMB is a sphere around earth, and corresponds to the last scattering surface of the Universe. If we are moving with respect to it, doesn't that mean we...
  18. ChrisVer

    Understand CMB Anisotropies & Interpret Figures

    Can someone help me understand what information we extract from such kind of figure? In fact the C_l^{TT} parameter gives us information on the amplitude of the temperature fluctuations \Delta T/T... However I don't understand why there is such a peak formation (1 very large, 2 smaller and 2...
  19. H

    Anti-matter annihilation and the CMBR

    Recently I watched a lecture on Anti-matter and the Standard Model... ...At one point Dr. Quinn, makes the statement that the CMB is the resulting energy left over from the annihilation of the matter and anti-matter which arose during cosmic inflation (leaving behind only one part in 30 million...
  20. ChrisVer

    CMB dipole anisotropies & Hubble's Law

    It is known that the dipole anisotropies of the CMB radiation, can give the relative velocity of the Earth with respect to the CMB "rest" frame... In first order, this is given by \beta \approx 1.2 \times 10^{-3} or u_{CMB} \approx 360 km/s . I have one question here: Why is the velocity given...
  21. D

    Baryon acoustic oscillations and sound horizon

    I'm having some trouble understand this correctly, so I was hoping someone could enlighten me a bit :) Okay, so in the early Universe most of the hydrogen and helium was formed, and then kept in equilibrium, and ionized via photons. So we have a plasma of baryonic matter, including dark matter...
  22. T

    Why does inflation generate gravity waves?

    I've been trying to understand the whole BICEP story. Here's what I think I understand so far: Inflation produces primordial gravitational waves. During recombination, photons decouple from matter and the CMB is formed. Propagating gravity waves stretch space in one direction and squeeze it in...
  23. A

    CMB Prediction or Post-diction?

    One of my friends majoring in physics with me was telling me that the CMB radiation was not "predicted," by the big bang cosmology, but was in fact a "post-diction," and that the range of values for temp. obtained by Penzias were in fact way off, 5K to 50K and that other "cosmologies," can more...
  24. D

    True map of universe from computer simulation with CMB initial state

    Do you think we will ever be able to create a simplified computer simulation of the universe using the cosmic microwave background as the initial state that would generate the true locations of galaxies or at least galaxy clusters, and then be able to find our own galaxy or galaxy cluster within...
  25. marcus

    CMB cold spot explanation proposed (looks good)

    Joe Silk is one of the co-authors. One expects large voids in the distribution of galaxies because of the cobwebby way dark and ordinary matter have condensed. A large void comparatively close by (within a few billion LY of us) could cool ancient background light passing thru on its way to us...
  26. Z

    Size of Universe at CMB last Scattering

    I hope that this will be a simple question, but I had difficulty finding an answer by searching. If the Cosmic Microwave Background radiation was radiated over 13 billion years ago, and we are able to see it now, then does this mean that the radius of the universe at that time must have been...
  27. D

    Is the CMB a Distorted Image of the Early Universe?

    Hi, I have been doing a coarse lately and am currently studying refraction. And it got me thinking about refractions effects on the Cosmic Microwave Background. If the cmb is light from the early universe then I think it would be ridiculous to assume that this light has traveled through nothing...
  28. D

    Question about the CMB and WMAP

    A very basic question that is stalking me for more than 20 years. Every popular science cosmology book states: A variation in temperature in the CMB of 1 part in 100.000, and then, from place to place or even from point to point. And then the WMAP picture. Everywhere hot en cold spots...
  29. Chronos

    Dark energy and the CMB cold spot

    Two papers went up today regarding infrared all sky surveys by WISE and 2MASS that affirm the CMB cold spot - http://arxiv.org/abs/1405.1555, A Supervoid Imprinting the Cold Spot in the Cosmic Microwave Background; http://arxiv.org/abs/1405.1566, Detection of a Supervoid Aligned with the Cold...
  30. C

    Why can we see the CMB in every direction we look?

    Hi guys, As you can probably tell this is my first post, so I thought I ask a question that I've had trouble getting my head round. I think I know the reason but I'm not a 100% certain. Why is it that we see the CMB radiation from every direction we look? I would really appreciate if...
  31. WannabeNewton

    Quick question on CMB anisotropy in Earth frame

    Hi guys. Consider the mean occupation number and specific intensity of the CMB photons in the CMB frame as given by the blackbody formulas: ##\eta = \frac{1}{e^{h\nu/k_{B}T_0} - 1}## and ##I_{\nu} =\frac{2h\nu^3}{e^{h\nu/k_{B}T_0} - 1}## with ##T_0## the thermal bath temperature in the CMB...
  32. C

    Describe & explain the significance of the 3K CMB

    Ok so I'm a bit confused as to how to attempt to answer this question? Describing its significance vs explaining its significance? Here is my attempt, please be ruthless when correcting me as I feel like there are some gaps in my knowledge and misconceptions I have: Where I put '/' I...
  33. marcus

    Signs of Quantum Gravity in CMB Ancient Light

    Thanks to GJ for pointing out these three links: http://backreaction.blogspot.ca/2013/10/quantum-gravity-in-cosmic-microwave.html Quantum Gravity in the Cosmic Microwave Background? http://www.nature.com/news/how-to-see-quantum-gravity-in-big-bang-traces-1.13834 How to see quantum...
  34. B

    How do GWs generate B-modes in the CMB polarization pattern?

    I have been searching through some of the literature, as well as this forum - but I have not found a clear explanation on how exactly the B-modes in the CMB polarization pattern are caused by gravitational waves. I have seen Wayne Hu's pages ( see for instance...
  35. Spinnor

    CMB polarization data plots, almost a vector field, then what?

    The data points of the polarization of the CMB are a magnitude and an orientation that varies between 0° and 180°. What kind of mathematical field is that, not quite a vector field? Thanks for any help!
  36. Spinnor

    Fluctuations of the CMB, symmetry between the hot and cold?

    Consider any of the latest maps of the temperature fluctuations of the CMB. Such a map can be considered a 2 dimensional topographical map of the surface of a sphere, high points hotter, low points cooler. Consider the contours that divide such a map into the two regions, fluctuations hotter...
  37. M

    Early CMB Temperature: What Changed?

    I had this question pop up while watching something: Was early CMBR still CMBR or was the M something else, possibly even visible light? Scouring wiki yielded the following: Since decoupling, the temperature of the background radiation has dropped by a factor of roughly 1,100 due to the...
  38. Chronos

    CMB anisotropies and the ISW effect

    Two papers on arxiv today, http://arxiv.org/abs/1312.0025 and http://arxiv.org/abs/1312.0275, propose to resolve a large part of the CMB anisotropies suggested by WMAP and Planck based on the Integrated Sachs-Wolfe effect. The authorship appears independent, the former group is from India, and...
  39. L

    How has CMB research affected modern technology

    So for my physics class, we are trying to find modern applications for different areas of research. I know the HST had a lot of help in the areas of breast cancer research and eye surgery but what are some applications for modern technology that have stemmed from cosmic microwave background...
  40. F

    CMB polarization: the E and B modes

    Hi all! In Ruth Durrer's book "The Cosmic Microwave Background" there is this picture (Fig. 5.2 page 193) about the E and B mode of the CMB polarization: http://imageshack.us/photo/my-images/11/s7o6.png/ http://imageshack.us/photo/my-images/11/s7o6.png/ They represent the E and B...
  41. P

    What Is the Ratio of Baryonic to Dark Matter in the CMB?

    What is the observed DM ratio in CMB ? Is there observational evidence for the DM ratio in the CMB? Other then the info-graph from Wikipedia which I was explained is based on the assumption of DM being the same total amount back then but the mass of relativistic particles was higher. Would...
  42. marcus

    Meissner sees Penrose-type circles in Planck CMB map

    Meissner et al just posted a paper where they see those circles in the high res. microwave sky of Planck. Who knows if this is real, or what it would mean if it were confirmed? Meissner has a followup paper in preparation with Penrose and others. Either way I think it's pretty interesting...
  43. A

    Could a Black Hole Explain the Big Bang & CMB?

    Hi all, I'd like to apologise in advance for the layman's terminology in the idea below. I'd be delighted to get any feedback/thoughts on whether CMB observations could be explained in this way. Could the universe have started as a giant black hole that reached a critical mass and collapsed...
  44. G

    What is 'in front' of the CMB?

    The furthest away (back in time) we can see is the CMB. This was the surface of last scatter, the surface of a hot gas bubble which due to redshift appears today as CMB. The actual temperature of that surface was around 3000 K. Moving from the CMB towards us, the next thing we can observe are...
  45. M

    Acoustic Wave & CMB: Exploring Anisotropy & 7 Peaks

    Dear all, many people refer to the CMB anisotropy picture by WMAP or Plack and call it acoustic waves. I thought there is the anisotropy of the CMB radiation plotted, what exactly can I imagine to be the acoustic wave of it? Secondly, what exactly is plotted n the famous plot where the...
  46. Chronos

    CMB Hemisphere of Evil: Planck Asymmetry Examined

    This paper critically examines the 'hemisphere of evil' CMB asymmetry reported by Planck - CMB Aberration and Doppler Effects as a Source of Hemispherical Asymmetries, http://arxiv.org/abs/1304.3506. Its worth a look if you find this aspect of Planck vaguely disturbing [as I did].
  47. T

    How should I see the CMB picture

    These days have been full of news of the Planck Telescope. The most popular is the actualization of the CMB picture (the ellipse with dots red and blue). I know the idea of it. Each dot represents the "temperature" in some direction (from the earth) in some sort of (θ;ψ) chart -0≤(θ;ψ)≤2\pi ...
  48. S

    How did the CMB Lorentz frame get chosen?

    The cosmic microwave background is anisotropic: the temperature is distributed as a dipole with the temperature at the poles differing by ±0.00335 K from the mean. This defines a Lorentz frame: we can boost by several hundred km/s to make the dipole vanish, on average. This new frame could...
  49. J

    CMB Sphere Size: Distance & Diameter

    Hi What was the distance between our point in space and the points from which the Cosmic Microwave Background radiation emanated that we detect today at the moment that that radiation was created. Or similarly, what was the diameter of the sphere, the surface of which we detect today as CMB.
  50. C

    The observable universe, the actual uinverse, and CMB

    Hi all. I relish hearing from our great cosmological explainers like Neil DeGrasse Tyson and Brian Greene and watch whenever I find something new on youtube, but one thing that I don't understand and haven't heard anyone specifically address is that, if the part of the universe we are able to...
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