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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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Is there a Dark Matter Paradox?

According to this preprint, The Importance of Being Symmetric: Flat Rotation Curves from Exact Axisymmetric Static Vacuum Spacetimes, spiral galaxies possess flat rotation curves without assuming Dark ...
timm's user avatar
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What do we mean when we say the CMB has a temperature and how do we measure it?

I have read this: An object without any internal degrees of freedom, like a single photon, can't really have a temperature. But an ensemble of photons can have a temperature. If you put an ensemble ...
Árpád Szendrei's user avatar
1 vote
3 answers
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Does the fact that we are able to see CMBR implies that universe expanded faster than light?

Supposedly, the universe underwent rapid expansion immediately after the big bang, surpassing the speed of light. If we can detect remnants from that era, does this suggest they moved faster than ...
Mr. Spock's user avatar
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2 answers
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Is cosmic background radiation the same photons bouncing around?

Or are "new" photons from perhaps later stages of the expansion encountered?
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Is this tuning fork forever?

Imagine a tuning fork struck in the Boötes Void. At first, the phonons convert to friction \ heat and radiate off into blackbody photons. This happens for a long time, and then at some point, we reach ...
Travis R's user avatar
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GZK Limit in the Far Future and interaction with Vacuum energy?

The GZK limit stops high energy protons from propagating long distances because of the interaction with the CMB. But as the universe ages the CMB will become more red shifted. Will this mean the the ...
EraserDriver's user avatar
1 vote
1 answer
110 views

Could the increasing anisotropy of the universe lead to an additional blueshift?

I'm contemplating the possible sources of a wavelength-shift within our universe: The CMB had a lot more energy when it was produced (around 3000 K). Due to the expansion of the universe, it has been ...
MartyMcFly's user avatar
1 vote
1 answer
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Are there any observations from the beginning of the universe until the CMB?

The question that concerns my mind is that in the time period before the cosmic microwave background, did humans have any observations or not? I mean, are all the materials mentioned in physics about ...
QQQ's user avatar
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Question about the "axis of evil"in cosmology and the Doppler effect due to the solar system's motion

The cosmic microwave background (CMB) can be described by its anisotropies in a direction $\hat{n}$ in the celestial sphere $$ \delta T(\hat{n})=\frac{ T(\hat{n})-\bar{T}}{\bar{T}} $$ where $\bar{T}$ ...
P. C. Spaniel's user avatar
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Why do Baryonic acoustic oscillations imply peaks in the probability distribution of the position of galaxies?

I have been trying to understand baryonic acoustic oscillations (BAO) and there are some parts of the process I don't understand. We start with a hot plasma of baryons and photons. There's also dark ...
P. C. Spaniel's user avatar
11 votes
2 answers
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Redshift of the CMB

Has the $z\sim 1100$ redshift of the CMB been actually measured by comparing the fingerprint (absorption spectrum) of the CMB with the theoretical radiation pattern of a $2.725\,\mathrm{K}$ blackbody, ...
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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
3 votes
2 answers
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Does neglecting dark matter solve the Hubble tension?

If the total mass of the universe is smaller than estimated by neglecting the gravitational pull of dark matter, the estimated expansion rate should be greater. Does this consideration in the CMB ...
Manuel'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
2 votes
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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 ...
vengaq's user avatar
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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 ...
Alaor's user avatar
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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}\...
Filippo's user avatar
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2 answers
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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
20 votes
6 answers
4k views

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?
Svenn's user avatar
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2 votes
2 answers
388 views

*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
2 votes
1 answer
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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
2 votes
1 answer
144 views

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 ...
Freedom's user avatar
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2 votes
3 answers
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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'...
FritzS's user avatar
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10 votes
6 answers
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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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0 answers
26 views

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?
user6760's user avatar
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1 answer
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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
2 votes
2 answers
145 views

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?
user141183's user avatar
2 votes
0 answers
44 views

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
1 vote
1 answer
44 views

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
1 vote
0 answers
58 views

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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0 answers
61 views

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
0 votes
1 answer
100 views

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
2 votes
0 answers
60 views

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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0 answers
54 views

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 ...
David Elm's user avatar
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1 vote
1 answer
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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
0 votes
0 answers
161 views

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 $\beta=0$ ...
Mr Anderson's user avatar
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-3 votes
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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
0 votes
1 answer
113 views

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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0 answers
66 views

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 ...
1 vote
0 answers
53 views

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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0 answers
30 views

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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0 votes
1 answer
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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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0 votes
1 answer
47 views

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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