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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Does the CMB have a digital ‘structure’, if so how large individually and how many total pieces are there? [closed]
I would swear Neil Turok made a statement in one of his talks that the CMB is digital in structure. If I understood correctly, is there an individual size and total number to the ‘bits’ that make up ...
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Does an eternal, or a long, past explain the Horizon problem?
I recently learned about the Horizon Problem (which is basically, how can the CMB be so homogenous when there are causally disconnected regions?).
As far as I understand, one approach to explain this ...
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Why didn't inflation happen right after creation and it had to wait $10^{-36}$ seconds to occur?
What caused this minimum delay in the universe faster than light expansion?
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Spatial Curvature of Universe at recombination vs now
From my understanding, we use the CMB data to measure the spatial curvature of the universe today. Why is it the value for today if the CMB data reflects the universe at recombination (380K years ...
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Multipole moment in the power spectra of the CMB
Why is the multipole moment l in the power spectra of the CMB corresponds to the radius (instead of diameter) of the sound wave horizon of the baryon-photon fluid?
It seems to me that it should be the ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 $\bar{T}$ ...
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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 ...
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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 ...
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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 ...
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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 $E=\frac{3}{2}kT$, ...
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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 ...
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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. ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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, ...
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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 ...
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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?
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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 ...
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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 ...
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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 ...
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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 ...
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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
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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. ...
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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 ...
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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 ...
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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 ...
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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 ...