Questions tagged [cosmic-microwave-background]

The cosmic microwave background (CMB) is the electromagnetic radiation in the microwave band which can be observed throughout the whole universe, not connected to any astronomical object. Its spectrum follows a very precise black-body radiation with a temperature of about 2.7 K.

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How to extract the "matter fluctuation amplitude" from the CMB power spectrum?

How do you convert the value listed in Planck 2018 results. VI. Cosmological parameters, $A_s = 2.101\times10^{-9}$ to the value of the matter fluctuation amplitude $\sigma_8=0.8111$? I tried ...
Finerichmen's user avatar
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Why is the temperature of electron-proton recombination obtained from Saha equation so small?

I read a blog about the temperature of the Universe at recombination phase. The ionization potential of a hydrogen atom is $13.6\, eV$, throw it into the thermal energy equation $E=\frac{3}{2}kT$, ...
Halcyon Mo's user avatar
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Voids blueshifting CMB photons (in certain cases)?

I have some questions about this interesting work (https://iopscience.iop.org/article/10.1086/517603/pdf) where the authors analysed the effects of voids in the CMB radiation (particularly through the ...
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CMB anisotropies due to moving clusters/voids?

Apparently, there is an effect where a moving overdensity/underdensity (or an object, e.g. a galaxy, moving in an overdensity/underdensity) that is going through an evolution of its gravitational ...
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Why is the horizon problem a problem?

I am curious about physics and astrophysics in particular, although I have no academic basis. Usually, I find it easy to understand the concepts and explanations, but I have particular difficulty in ...
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Do we have to consider decoupled particles in the computation of $g_{*}(T)$?

I am studying the thermal history of the universe and I encountered the definition of effective degrees of freedom $g_{*}(T)$ defined as $$g_{*}(T)=\sum_{Bosons}g_{B}(\frac{T_{B}}{T})^{4}+\frac{7}{8}\...
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Can we measure temperature in an isothermal Universe?

It is envisaged that ,in the future ,Universe can end in a big freeze, where there will be no energy gradient. It is also theorised that Universe was isothermal(with some irregularities I guess) in ...
SacrificialEquation's user avatar
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Why does my perpetual CMBR rocket ship not work? Why does the CMBR have infinite energy?

Take a spaceship and accelerate it to 99.947% the speed of light, effectively blueshifting the Cosmic Microwave Background (CMB) from 1.9mm to 500nm, turning the CMB to a harvestable source of energy. ...
Travis R's user avatar
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Do the colder and warmer poles of the CMB's Axis of Evil corresponds to the terrestrial poles?

Does the terrestrial North or South Pole correspond to the colder and warmer poles in the CMB, as differentiated by the 'axis of evil'? If so, which to which?
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*Re(?)*combination era?

Spurred by reading a recent obituary of Penzias, who with Wilson discovered the cosmic microwave background, I was led to read their letter in a 1965 issue Astrophysical Journal Letters, which was ...
Lee Mosher's user avatar
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How fast do the neutrinos in the neutrino background radiation move through the universe?

The expansion of space drains energy from particles. That's why the CMBR contains less energy nowadays than in the early stages of the universe. What does this mean for the neutrinos present in the ...
Il Guercio's user avatar
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Standing barionic waves in CMB require that the geometry - i.e. the spatial perimeter condition - does not change for a sufficiently long time

Standing barionic waves in CMB require that the geometry - i.e. the spatial perimeter condition - does not change for a sufficiently long time. How long should the standing wave of a byryon ...
Richard Dvorsky's user avatar
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Why are there only two perturbation modes in the early Universe?

Cosmology textbooks and lecture notes often mention that there are two possible modes in the early Universe - adiabatic and isothermal, with the former affecting both the matter field and the ...
cosmosis's user avatar
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Common clock reference of Big Bang

Relativity tells us that there is no preferred reference frame, yet current cosmology does operate on the hypothesis that all points in the observable universe originate from the same big bang ...
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Will cosmic microwave background become invisible in the future?

If my understanding of CMB and Hubble's Law is correct, then CMB photons emitted from more than ~14.4 Glyr during Recombination Epoch would not reach us. The reason is this would correspond to Hubble'...
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Why do we say that temperature of Universe is around 2.7 K?

I am little confused, so please correct me if I am wrong. Temperature of Universe is calculated using cosmic microwave background and it is around 2.7 K. The cosmic microwave background is the relic ...
SacrificialEquation's user avatar
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What is the change in vision and measurement between curved space and flat space, especially for measuring cosmic background radiation?

In a curved space, the light bends as it travels and acts like it is going through a lens. In a (positively) curved universe, a small object appears larger. If we know the actual size of an object, ...
Saadeh Dayoub's user avatar
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Tightly coupled limit of the Boltzmann equation

When i have studied anisotropic of CMB, I couldn’t understand some technical things if you could help me. I appreciate it. What i couldn’t understand is: When we study tightly coupling limit of the ...
Roden's user avatar
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Are all CMBR entangled with each other even today? [duplicate]

Just curious: since at the very beginning they are all so close together so I suspect all of them are entangled together, it is possible?
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How is the light from the Big Bang still traveling?

How can light from the Big Bang still be traveling at this point in time? How can it be that bright? If I stand at a point with a flashlight, how far can it be seen?
Eric Byrd's user avatar
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Value of $H$ at the emission of the CMB

According to the lambda CDM model, $H_0$ is around 67.7. If this value was calculated from a value for $H$ at the emission of the CMB, what was it, or do we have constraints on its value at that time?
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How large was the visible universe at the time of the CMB?

Given that our best models so far suggest that the universe has been expanding a lot ever since the time the CMB was emitted, I'm wondering: How large was the (visible) universe back at the time it ...
Christoph Lipka's user avatar
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Why only Lyman alpha transition suffer Redshift in the epoch of Recombination in Cosmology?

I was reading Baumann's book on cosmology (topic: Effective three level system). There it is said that in the epoch of recombination to form Hydrogen molecule two type of transition happens. $$ 2S \to ...
Kazi Abu Rousan's user avatar
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How big was the surface of the cosmic background radiation?

The cosmic microwave background radiation is the furthest and oldest visible light in the universe. But the universe has expanded considerably since that light was emitted. At the time that that light ...
John Berryman's user avatar
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Does inflation theory assume a finite universe?

Inflation theory has it that the early universe was causally connected, and could “mix”, hence explaining relative homogeneity of the CMB. The universe then rapidly expanded and became causally ...
Captain Chicky's user avatar
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Convert radiation density into temp in degrees kelvin

in the 1948 paper by Alpher he gives a present radiation density in gm per cm cubed. how is that converted to a temp of 5 degree kelvin
bruce 's user avatar
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How to intuitively understand the Boltzmann equations that come up in Cosmology?

This may be a silly question, but I have been reading through Dodelson's Modern Cosmology textbook and n Chapter 4 we derive the Boltzmann equation for different constituents of matter and radiation. ...
user3461947's user avatar
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What would the cosmic microwave background radiation look like in a hyperspherical universe?

I was thinking about what things would look like in a hyperspherical universe. How would galaxies look as they got closer to the other side of that hypersphere? It seems like if a galaxy were on the ...
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Does General Relativity, without Cosmic Inflation, predict a perfect blackbody for the CMB radiation?

My understanding is that the universe did not have enough time to thermalize before the epoch of recombination, so many patches of the sky were not in causal contact with each other, which means they ...
The Shepard's user avatar
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Constraints on the CMB temperature redshift dependence at matter-radiation equality?

As pointed out in the 2011 paper Constraints on the CMB temperature redshift dependence... here the relation between redshift and the CMB temperature, $T_{CMB}(z)=T_0(1+z)^{1-\beta}$ (with $\beta=0$ ...
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Why is the cosmic background 4K? Where are the other photons? [closed]

The cosmic background is 4K. Where are the other photons? A 4K photon produced at the big bang is detected by our detector. Time stops for the photon and hence we are seeing the big bang. Shouldn't we ...
Ajit Haridas's user avatar
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1 answer
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Transfer function from initial Curvature Perturbations to CMB Temperature Anisotropies has no direction dependence?

In Chapter 7 of Baumann's Cosmology book when discussing how inhomogeneities in the primordial plamsa leads to correlations in CMB temperature anisotropies, he makes a simplification that I'm having ...
delon's user avatar
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Textbooks/papers to learn about the physics of the CMB

I am a PhD student working on observational cosmology. My research is based on the experimental side of it (receiver design, etc.) but I would like to get deep into the CMB and CMB statistics. I have ...
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How does one calculate the Sachs-Wolfe effect?

I start from $$\Delta T/T_{0} = - \frac{1}{3c^2}\Phi$$ where $\Phi$ is the gravitational potential which can be written in its Fourier expression as $$\Phi_k = \frac{-3}{2}\left(\frac{H_0}{k}\right)^2 ...
ArK's user avatar
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In theory, shouldn't there be multiple BAO peaks and even troughs?

Shouldn't acoustic oscillations created by primordial matter anisotropies create multiple peaks and troughs when those oscillations are frozen by decoupling ? It's not unthinkable to imagine that the ...
ticster's user avatar
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Numerical Approximation of the CMB Anisotropy Monopole $\Theta_0$ and the Scalar Metric Perturbation $\psi$

TL;DR: How can $\Theta_0+\psi$ be modelled, what are viable Ansätze for this, including numerical factors? $\Theta_0$ being the temperature fluctuation monopole and $\psi$ being the scalar metric ...
kalle's user avatar
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How fast would you have to go in space to be warmed to a comfortable temperature?

How fast would you have to go in space, not near a star, such that the Cosmic Microwave Background is blueshifted enough to warm you to a comfortable temperature (70 F or 21.111 C)? Expressing it in ...
Jonathan's user avatar
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Energy lost by CMB since recombination from energy density and scale factor

To calculate the energy lost by CMB since recombination from energy density and scale factor, one should compute today's energy density of CMB * a^-1 or today's energy density of CMB * a^1 with a ...
Manuel's user avatar
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Finding initial conditions from the temperature autocorrelation function

So I was reading Mukhanov and in section 9.4 titled Correlation functions and multipoles, he talks about obtaining the auto-correlation function \begin{equation} C(\theta) = \bigg\langle \frac{\delta ...
QFTheorist's user avatar
3 votes
3 answers
643 views

What's so "quantum" about quantum fluctuations in the CMB?

I've heard that fluctuations in the CMB provide support for inflationary theory, as they are thought to be amplified quantum fluctuations of the inflaton field. My question is, what is so "...
Munthe's user avatar
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How would we create a device to detect communication using the x-ray and gamma bandwidth? Like a radio. But x-ray gamma spectrum [closed]

I realize the dipole would have to be small enough (in the nucleus of an atom range) and we don't have any mechanism that is small enough to demodulate the frequencies at this rate. But is there some ...
ZiiZii's user avatar
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How/Where to find the theoretically expected cross-correlation between CMB and Large-Scale Structure in real sapce?

I have cross correlated the CMB map (WMAP 7 Year map) and the large-scale structure tracer- quasar (NBC-KDE Quasar catalog from SDSS-6 avilable here) in real space by following the this paper by ...
Abhishek Sachan's user avatar
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Question on cosmological parameters and slow-roll potential

Given a slow-roll potential $V (\phi)$, how do the cosmological parameters (see e.g. the Dodelson book on Cosmology, section 8.7) relate to this potential? I'm a bit confused how parameters such as ...
Geigercounter's user avatar
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1 answer
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If Inflation never happened before the CMB, and the present expansion were projected backwards, how old would the Universe be since the Big Bang?

The Cosmic Microwave Background Radiation is the oldest thing we can directly observe. To explain the near uniformity of temperature of the CMBR and the flatness of space, Cosmic Inflation was ...
rajnz00's user avatar
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2 votes
1 answer
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What do WMAP CMB Temperature map values represent exactly?

I understand that Cosmic Microwave Background Radiation Map provided by WMAP survey in HEALPix pixelization format is nothing but an array of temperatures associated with the cmb radiation coming from ...
Abhishek Sachan's user avatar
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How can we use CMB to measure the value of the cosmological constant?

Another mystery facing cosmologists is the accelerating expansion of the universe. In 1929, astronomer Edwin Hubble showed that the universe was expanding, but for this expansion to be justified, ...
Saadeh Dayoub's user avatar
-1 votes
1 answer
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Size of the observable universe and CMBR [duplicate]

Why can we detect the Microwave Background radiation (the earliest light that we can see) and not the light from stars beyond the observable universe? Should not the microwave radiation also be ...
Christopher Cruickshank's user avatar
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Number of spots in CMB map

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? (A full-sky spherical surface has 4pi sr or 41253 sq°). And does this number ...
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What conclusions can be drawn from the CMB TE polarization spectrum?

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?
Rene Kail's user avatar
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3 answers
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Why does the CMB have a spectrum like a black-body radiation?

Equilibrium distributions of particles (Maxwell, Boltzmann, Saha) are achieved by the particle collisions. On the other hand, photons do not interact with each other. From the introductory course in ...
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