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. Mr Davis 97

    I Intuition for CMB: Exploring Radiation & Expansion

    I am having conceptual issues about what the CMB actually is. I read that it is the remnant radiation at the moment of recombination roughly 380,000 years after the Big Bang. But what about this statement implies that we would be able to observe this radiation today? To put it naively, when I...
  2. bananabandana

    Thomson Scattering In the Early Universe

    Homework Statement Why does Thomson scattering occur in the early universe? Homework Equations $$ e^{-} + \gamma \rightarrow e^{-} + \gamma $$ is a Thomson scattering process if: $$ E_{\gamma} << m_{e}c^{2}$$ (Electrons are essenitally stationary) The Attempt at a Solution [/B] Very confused...
  3. Buzz Bloom

    I What is the Time Ordered Data set for CMB polarization?

    The Wikipedia article https://en.wikipedia.org/wiki/Dark_matter says DASI first detected the CMB polarization, and CBI provided the first E-mode polarization spectrum with compelling evidence that it is out of phase with the T-mode spectrum. A footnote [48] leads to...
  4. JulienB

    I General questions about the CMB

    Hi everybody! I am (like many) very fascinated by the cosmic microwave background, but there are many aspects of it that I do not understand yet. I'm currently in 4th semester of my physics studies, and just starting quantum physics now. Some of those questions might be stupid or (mis)leaded by...
  5. B

    I Observation of photons from CMB by different observers

    If two observers on Earth in different locations around the globe, were both viewing the CMB with their equipment pointing at the same point in the sky, and charting the fluctuations in it, would they correspond or vary from each other greatly. In other words, if you made a graph of the CMB and...
  6. C

    A Angular power spectrum, bias from N weighted events

    My general question is: What is the angular power spectrum C_{l,N,ω} of N weighted (weight ω_i for event i) events from a full sky map with distribution C_l? I'm interested in: Mean of C_{l,N,ω}: <C_{l,N,ω}> Variance of C_{l,N,ω}: Var(C_{l,N,ω}) The question is important, since we observe in...
  7. L

    I Exploring Evidence and Data Pre-CMB in the Early Universe

    I have been looking online and in other resources for some answers to no avail. Thought I would sign up on astrophysics forums to find the answer.What is the evidence and data for the period before the cosmic microwave background (CMB) in the early universe? In other words, is the idea of energy...
  8. G

    I At which time did the CMB become dark?

    At the time the CMB was emitted it was a glowing yellowish-white radiation at 3000 K. From there as space expanded it became redder and redder eventually falling into what for humans is non-visible infrared. At which age of the universe did the CMB background turn from red to infrared? Thanks
  9. substitute materials

    I Is there a drag on Earth caused by its motion relative to the CMB rest frame?

    Motion relative to the CMB rest frame creates a dipole moment in the wavelength of CMB photons in the direction of motion. Observations suggest that the Earth has a motion relative to CMB rest frame of 600 km/s. As indicated in this study, for a spacecraft at a speed we shall dub 'ludicrous...
  10. Chronos

    I Great attractor and the CMB dipole

    The Great Attractor has been a source of controversy since its discovery. Some think it is just a fluke, others feel it to be a clue to a greater mystery. This paper; https://arxiv.org/abs/1702.02483, The Dipole Repeller, offers a deeper look without necessarily resolving the enigma.
  11. G

    A Assumptions behind Maxwell's equations for constant speed

    I need some help in defining what are the assumptions needed to derive a constant speed of light from Maxwell equations. Is it correct to say that this result applies to a sinusoidal wave as an assumption? In my understanding that is (more or less) equivalent to planar waves in vacuum: is it...
  12. K

    I Dark matter and acoustic peaks in the CMB

    Baryon acoustic oscillations acoustic peaks in cosmic microwave background anisotropies provides evidence for cold dark matter but is there any sort of prediction as to the specific properties of this dark matter? predictions as to the mass of this dark matter, total mass, and mass of the...
  13. R

    I Estimate the number of CMB photons in 0.25-litre

    i know that the total energy density of a blackbody radiation is E= 4 segma/C * T^4 so how i can use this to find the number of the CMB photons in 0.25 litre?
  14. K

    I Bullet clusters & CMB accoustic peaks dark matter

    is it possible the dark matter in bullet clusters & CMB accoustic peaks were the result of black holes, originally from baryonic matter, but galaxy rotation curves are MOND. So MOND + dark matter as black holes from baryon, not dark matter particles
  15. Jules Winnfield

    I How is the first multipole calculated from the Plank Study?

    Reading through the Plank 2013 Results we can see that the angular scale is ##0.0104147## or ##0.60^\circ##. However, the Power Spectrum chart clearly shows the first multipole at ##220## ##l##. Using the relation $$\theta = \frac {180^\circ}{l},$$I calculate the first multipole to peak at...
  16. J

    B Why the Milky Way shows band in all-sky energy scans

    I have just read about the Gleamoscope which allows users to dial up visions of the sky in any light that you prefer, for example CMB, Xray, etc. In all of these, the Milky Way shows up as a narrow horizontal band across the middle of theall-sky image. What is it in the analysis that leads to...
  17. Doctor Strange

    I Is BAO a standing or moving wave?

    The sound horizon is the distance that a wave of plasma can move from the end of Inflation to Recombination (roughly 300,000 years). In several papers and talks, this is described as a moving wave (see https://www.youtube.com/watch?v=JSqIBRbQmb0 at the 23 minute mark). The velocity of the wave...
  18. Doctor Strange

    A How is the red-shift of CMB obtained?

    The value for the red-shift of the CMB is 1090 according to the latest Plank study. How do they arrive at this number? You can look at how hydrogen lines of supernovae light are shifted, but how do you tell how far light is shifted when looking at the light from a primordial soup? Is there...
  19. UHECRon

    A Photon densities within galtactic disc compared to CMB

    I am interested in what the average photon densities are within galactic environments due to non CMB sources. In extra galactic environments, the CMB certainly dominates, however as you enter more dense and luminous areas (e.g. , the galactic disc) broad spectrum photon densities from local...
  20. bapowell

    Insights A Poor Man's CMB Primer. Part 4: Cosmic Acoustics - Comments

    bapowell submitted a new PF Insights post A Poor Man's CMB Primer. Part 4: Cosmic Acoustics Continue reading the Original PF Insights Post.
  21. Bill McKeeman

    I Gravitational field inside a void

    Sci Am August 2016 discusses a supervoid detected in the direction of the CMB cold spot. The analysis assumes the gravitational potential is less in the center of the void than near its edges (thus near its surrounding galaxies). On the other hand the gravitational field inside a spherical...
  22. E

    I How Similar Are the Images from BOSS and CMB Planck Spacecraft?

    In http://physicsworld.com/cws/article/news/2016/jul/15/dark-energy-study-maps-1-2-million-galaxies-in-the-early-universe we can see picture which was also measured. One paper about this is also: http://arxiv.org/pdf/1607.03155v1.pdf This picture is very similar as CMB picture measured by...
  23. DuckAmuck

    I Causally separated regions in CMB?

    So I have heard it mentioned that there are causally separate regions in the CMB. For instance a point A and point B that we can see here on earth, but are outside of each other's light cones. My question is then, how far apart are these points A and B at minimum to be causally separate in this...
  24. Bill McKeeman

    A Transition from opaque universe to release of CMB

    What is known about how long it took to transition from an opaque universe to the release of the cosmic microwave background?
  25. 1

    B Where did the energy of red shifted CMB go?

    i know it sounds stupid, but i can't seem to find an answer to it where did the energy went? you can't just just destroy energy, but when you red shift a gamma CMB into a microwave CMB, the energy has go to go somewhere...? it's not like one gamma photo spitted into many microwave, cause that...
  26. E

    I Calculation of mean energy value of photons in recombination

    I am interested in the calculation of the mean energy value of CMB (Cosmic Microwave Background) photons from which the recombination is performed. The subject on French Wikipedia says : "Intuitively, one might say that recombination occurs when energy average of photons is of the order of the...
  27. W

    I CMB Education Tool - Adjust Contrasts & Visualize Effects

    Hi . When you look at the CMb with low contrast it should look like a uniform colour. But when you turn up the contrast you see the differences in temperature at 1 part in 100,00. I seem to remember seeing a tool somewhere where there was a slider you could adjust and see this change in a visual...
  28. M

    A Tension between local and CMB measurements of Hubble constant

    "A 2.4% Determination of the Local Value of the Hubble Constant" by Riess et al has led to some excited news stories recently. I don't see it discussed anywhere here. Looking for the essence of the paper, I note three things: The two measurements considered are "the Hubble constant ... measured...
  29. twistor

    A What's the truth behind CCC's concentric circles prediction for the CMB?

    http://arxiv.org/pdf/1510.06537v1.pdf I found this semirecent paper about CCC's concentric circles prediction for the CMB. Is this just another piece of the debate, or is its significance enough to increase the plausibility of Penrose's model? Thank you in advance for your answers, and please...
  30. resurgance2001

    I Cosmic Microwave Background Radiation Anisotropies

    Hi This question may have already been answered elsewhere. If so please accept my apologies in advance. I am confused! The textbok(s) I am reading describe a whole bunch of different causes for there being temperature fluctuations in the CBM, so I am confused about which one(s) of these...
  31. H

    B Why is the CMB relatively uniform?

    This might be a stupid question due to a simplistic under standing of the Big Bang. But why is the CMB uniform across the sky. Why is it not sognificantly higher on one side from the other. Surely if the Big Bang was a point then everything would eminate from that point resulting in one side...
  32. C

    Energy from cosmic microwave background

    Hello everyone I've a question about the cosmic microwave background which my nephew asked me a couple of days ago. How much energy hits the Earth each year from the CMB? I'm assuming here that it is measurable in this way, but apologies if not. Thanks in advance for your help. Chris
  33. G

    Are black holes more redshifted than the CMB?

    I don't know if this question makes any sense, sorry if it doesn't. I have often read that any matter (say an object, for more mundane clarity) falling into a black hole (providing it radiated) would be perceived by an outside observer as never actually crossing the event horizon. It's "image"...
  34. Dr. Strange

    Why does dark matter pool in the CMB plasma?

    I'm trying to understand the mechanisms of the anisotropies in the CMB. The general idea is that there are fluctuations in some field (e.g. inflation) and the baryonic and dark matter rush into the space compressing the fluid. The photon energy pushes back on just the baryonic matter while the...
  35. sandrogiongo

    Why is there no continuity on the temperature of the CMB?

    On the epoch of last scattering, the universe became transparent and the typical CMB photon was "free" to travel the universe. The corresponding radiation was of a black body temperature of ~3000K. My question is: after the last scattering, the universe was still hot and, I presume, emitting...
  36. Dr. Strange

    How is the first peak of the CMB calculated?

    This is a follow up to an old thread: https://www.physicsforums.com/threads/the-first-acoustic-peak-in-cmb.475412/. In the final draft of the Plank study - http://arxiv.org/pdf/1303.5076v3.pdf - they quote the sound horizon at 1.04147 x 10-2. This equates to a multipole moment of 302...
  37. G

    CMB Depth: 115000 LY & Its Impact on Analysis

    The Wiki article on the CMB says that recombination lasted for around 115,000 years, from 372,000 years after the BB (when all baryonic matter was in the form of ions) until around 487,000 years after the BB when all electrons had bonded to nuclei. So Thomson scattering gradually decreased from...
  38. wolram

    Cold Spot in CMB: Real or Systemic Error?

    Is this a systemic error? Quote from wiki The cold spot is mainly anomalous because it stands out compared to the relatively hot ring around it; it is not unusual if one only considers the size and coldness of the spot itself.[7] More technically, its detection and significance depends on using...
  39. G

    Why Does Increased Baryon Loading Shift Power Spectrum to Higher Multipoles?

    About the power spectrum: if increased the number of baryons the frequency of the oscillation reduced.I want to ask why this fact moves the power spectrum to higher multipoles and no to smaller?smaller frequency doesn't mean longer wavelength and thus smaller multipoles?
  40. bapowell

    Insights A Poor Man's CMB Primer. Part 2: Bumps on a Blackbody - Comments

    bapowell submitted a new PF Insights post A Poor Man's CMB Primer. Part 3: Bumps on a Blackbody Continue reading the Original PF Insights Post.
  41. P

    Time in relativity and the CMB

    As understand relativity there is no "absolute time" however I have its possible to use the CMb as a way round this? Is this right and how does it work?
  42. A

    How to measure CMB anisotropies *In Practice*

    Hi everyone Given the definition of ##C_{l}##, ##C_{l}=\frac{1}{2l+1} \sum a_{l,m} Y_{l,m}##, I was wondering how it is possible to measure the ##C_{l}##s in practice. How does one compute this quantity, having a map of the temperature anisotropies of the CMB?
  43. bapowell

    Insights A Poor Man's CMB Primer. Part 1: The Birth of a Cosmic Background Radiation - Comments

    bapowell submitted a new PF Insights post A Poor Man's CMB Primer. Part 2: The Birth of a Cosmic Background Radiation Continue reading the Original PF Insights Post.
  44. G

    Question about first peak of CMB

    Hello,i would like to ask something about the first peak in power spectrum of cmb giving by WMAP experiment.The y-axis is temprature difference between two points and the horizontal is the angle seperation.the first peak giving as the fundamental frequency of sound wave right? if we suppose that...
  45. bapowell

    Insights A Poor Man's CMB Primer Part 0: Orientation - Comments

    bapowell submitted a new PF Insights post A Poor Man's CMB Primer. Part 1: Orientation Continue reading the Original PF Insights Post.
  46. A

    Gravitational lensing and Dark Energy

    Hi everyone I have been told that gravitational lensing affects CMB power spectra (TT, TE, EE) for high values of l (i.e. the least, low peaks on the right, say l>1000). But how? Isn't the effect of the varying gravitational potential along the line of sight the cause of Integrated Sachs-Wolfe...
  47. B

    Exploring the Age of Photons: My First Post

    My first Post :) I understand photons are are not going to age. But they still have a time in which they are created. Maybe this is known and I am just late. While learning about photons, right away I got he question if we can measure its born date.I mean everything decays right? So we should...
  48. A

    Is the CMB Data Showing that We are the Center of the Universe?

    Two days ago, I have opened theme about fine-tuning on which I got good answers... Now, there is another topic, on which I have question... The reason why I am asking this question here, is because I want to get correct answer... Internet is full of trash and misinformation, and this is a...
  49. G

    I CMB Radiation Q: Can It Be Detected from 42 Million Light Years?

    Hello i have a question about cmb radiation.Cmb was release about 13.7 billion years ago and reach us from a point about 42 million light years.At the past were the age of the universe was smaller and the time that cmb release was smaller too,we were able to detect the cmb?I mean the cmb was...
  50. G

    Question about quantum superposition in CMB photons

    Hi, CMB photons reaching our telescopes have traveled for over 13 billion years without interacting with anything. If I understand correctly, from the event of the photon emission, its wavefunction gradually spreads, encompassing more and more possible states for the photon, coexisting in...
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