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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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3answers
129 views

How much photon energy has already been destroyed?

We get taught in school that energy can neither be created, nor destroyed. The law of energy conservation is confirmed by many processes. It is an incredibly accurate assumption for every-day life. ...
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Density fluctuations of the present Universe with Large scale structures

Disclaimer The question below is based on a vague knowledge, and hence, statements can be potentially wrong or misleading. An important quantity in Cosmology is the density fluctuation of matter ...
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Order of Magnitude of Cosmic Microwave Background Anisotropies

How does the statement about the anisotropies being of the order of $\frac{\Delta T}{T} \approx 10^-5$ which is often made in the literature (see, e.g. Description of CMB Anisotropies by Scott and ...
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How to justifiy that $\rho_{\text{rad}} \approx \rho_{\text{mat}}$ at recombination time?

In standard cosmology, the recombination time is estimated to be $t_{\text{rec}} \approx 380~000~\mathrm{years}$ after the Big Bang, when matter and electromagnetic radiation becomes decoupled. It's ...
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Why isn't this anomaly in the CMB taken more seriously?

Almost every time I try to find more information about the Axis of Evil, I end up with people trying to ignore the phenomena. The video The (Cosmological) Axis of Evil explains it. However, it's no ...
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Is there cosmic background radiation in de Sitter space?

This is a simple question of understanding. Would a de Sitter universe show cosmic background radiation? (After all, it is often explained said that our universe is similar to a de Sitter universe.) ...
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Where can I download a simulated CMB map?

I tried looking for non-gaussian map but couldn't find one. I would also appreciate if someone can guide me on how to convert a map in fits format to the map array.
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How does Cosmic microwave background radiation provide temperature to the outer space?

We define temperature as average kinetic energy of atoms. We know that outer space has vacuum that is it lacks matter and hence the temperature should be 0 K. But because of CMB radiation, the ...
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How to define the number of causally disconnected patches on the sky?

I'm considering the spatially flat geometry ($k = 0$) of the general FLRW universe: $$\tag{1} ds^2 = dt^2 - a^2(t) (\, dr^2 + r^2 \, d\vartheta^2 + r^2 \sin^2{\! \vartheta} \: d\varphi^2). $$ The ...
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1answer
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Horizon problem: Angular size of the causality patches on the CMB surface

I'm having difficulties in calculating the angular size of the causally connected regions on the cosmic microwave background (CMB), as seen from Earth today. I read in several documents that this ...
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How is the Silk Damping Scale, $k_D$, Calculated?

I'm reading through the lecture notes of Wayne Hu regarding the Damping Scale of the CMB. He give the following steps to calculating the damping scale, $k_D$:$$k_D^{-2}=\int \frac{1}{6(1+R)}\left( \...
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Good references to start researching on dark energy knowing only graduate general relativity

I'd like to research on Dark Energy, but I only know (Schutz-Foster-Carroll) general relativity. Can anyone indicate to me some good books to start to research this topic? I'm interested in how to ...
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The formula for the CMB Damping Tail makes no sense

The formula for the Damping Tail in the CMB BAO analysis generally has the form: $$\mathcal{D}(k)=\int_{0}^{\eta_0}\dot\tau \exp\left\{-\left[\frac {k}{k_D(\eta)} \right]^2\right\}\mathrm d\eta $$I ...
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Can I detect my own rotation by looking at the cosmic background radiation?

This is a simple question. Nevertheless it might be interesting to others. If I move with constant speed across the cosmic background radiation, I see a "dipole": a blue shift of the CBR looking ...
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Why are graphs of the CMB Visibility Function normalized to 1.0?

I've just finished calculating the Visibility Function using electron density data I collected from the RECFAST program. The visibility function is defined as:$$\tag{1}g(\eta)=\dot\tau\space e^{-\tau}...
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1answer
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What are the units of Optical Depth?

I've read several papers that describe how to calculate the Optical Depth to recombination. They use some version of this formula:$$\tau(\eta)=\int_\eta^{\eta_*} n_e\sigma_Ta\space d\eta$$I'm trying ...
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About the applicability of the FRW metric in the present epoch vs the time of last scattering

The form of the metric of the Universe, the FRW metric, is obtained from the principles of spatial homogeneity and isotropy which are supported by observation. If we probe the present Universe, it ...
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1answer
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Why Cosmic Microwave Background is still present? [duplicate]

This is a novice question. It is theorized that Cosmic Microwave Background (CMB) is remnant of radiation from very early time of universe creation. As these were photons that have been traveling, ...
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Has the speed of light ever been measured in vacuum?

According to: https://en.wikipedia.org/wiki/Cosmic_microwave_background the CMB (Cosmic Microwave Background) "is faint cosmic background radiation filling all space" Also, https://en.wikipedia.org/...
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What is the relation between $X_e$ and $n_e$?

I'm trying to compute the Damping Tail of the CMB. A critical parameter to this calculation is the mean free path of the photons in the plasma:$$\lambda_C=\dot\tau^{-1}$$Where$$\dot\tau=n_e\sigma_Ta$$...
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Where can I find the latest, cleanest data set of Planck CMB data?

In order to better understand the spherical harmonic math, I would like to build a temperature power spectrum chart from the temperature data. I want to be able to generate the $D_l$ data in the ...
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Pair production form high energy cosmic microwave background photons and related calculations

If assume that fluctuating fermion-antifermions pairs in space are products of photon-photon fusion reactions. Thus their total electric charge and total color are null. And the pairs spins are ...
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Is the “cosmological constant tension” the prime reason that we believe the expansion of the universe is accelerating?

Background: For a few years now there is significant difference in the calculation of the cosmological constant between standard candles methods and CMB methods. CMB measurement comes from a very ...
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thermal equilibrium in deep space

Imagine an object- say, a sooty asteroid- in deep space, far enough from any star to receive only negligible radiation from it. We make it sooty so as to make it a good absorber of radiant energy and ...
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Does space expansion affect the CMB photon frequency?

I have read this question: Effect of expansion of space on CMB where Ted Bunn says: For definiteness, let's consider a wave packet of electromagnetic radiation with some fairly well-defined ...
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BH evaporation vs CMB

This is not a duplicate, I am not asking about how hawking radiation works or how CMB works or whether BHs can really evaporate, or not. My quesion is about how CMB (considering space expansion's ...
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What's the advantage of Conformal Time?

I'm trying to follow the analysis of CMB Acoustics and several charts are done using conformal time instead of chronological time. Conformal time corresponds to the quotient of the particle horizon ...
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Why does Penrose expect visible Hawking Points in CMB?

I've only just started reading into Conformal Cyclic Cosmology. My understanding of the hypothesis is that, with some conformal transformation of space, the very end of the universe at $t=\infty$ ...
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1answer
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Could decaying dark energy solve the Hubble tension? [closed]

The redshift-based distance ladder values of the Hubble constant are higher than that inferred by the cosmic microwave background. This discrepancy in the last few years has become highly ...
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What exactly is the problem that Inflation solves?

There's copious documents about how Inflation solves the problem that General Relativity predicts a lumpy CMB. That influation 'smooths' out the curvature fluctuations and, so, predicts a CMB at ...
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Does CMB rule out that the universe is infinite?

If the universe were infinite, the energy of the big bang would have been long dissipated, and very little or nothing would hit us. Does the fact that CMB still comes roughly the same from every ...
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Where does the 379,000 year recombination prediction of the Big Bang theory come from?

The Big Bang theory postulates that recombination happened 379,000 years after the Big Bang. However, I have never seen a single statement as to how this number came about. I don't imagine they ...
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On universe homogeneity. Why “shouldn't be” so? [closed]

I start by apologising if I have took sparse sentences to literally. If I didn't understand something that's is why I am asking. In more or less divulgative literature, here, and also in the few ...
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Is the universe be expanding at different rates depending on its temperature and density?

As I understand it, when observing different regions of the universe we get a different rate of expansion (the two Hubble constants), specifically the more distant observation seems to return a lower ...
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1answer
67 views

How far was the surface of last scattering at the moment of recombination?

If I understand correctly, then the surface of last scattering (the last particles off of which CMB photons scattered before traveling for 13 billion years and reaching us) is currently at a distance ...
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1answer
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Why can’t you see at the start of the Big Bang?

I’m quite confused with regards to photon emission throughout the creation of the universe. From what I’ve heard, there was no light (of any frequency) in the universe until 300,000 years after the ...
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What wavelength was the CMB originally?

What wavelength was the light that became the CMB after factoring out redshift? Is it safe to assume that what we're seeing in the CMB was emitted at a distance of approx: ...
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1answer
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Future of the cosmic microwave background pattern

Does the comoving size and the observed angular diameter of CMB hot and cold patches change with time in the future?
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What is the temperature of the black night sky, outside the atmosphere?

The sky at night is rather black. If it were completely black, it would correspond to a black body at absolute zero. But the sky is not completely black. Is there a way to assign a temperature value ...
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Tests of inflation theory

So I'm following a course in Early Universe Cosmology right now. For the next lecture we have to solve a few questions about inflation, just as preparation. I read some closely related lecture notes ...
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1answer
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Why is Cosmic Microwave Background Radiation attributed to the Big Bang? [duplicate]

When searching for CMBR all articles start by stating as a fact that it is a remnant of the Big Bang. Why? Why is it well understood and accepted? Why sure that it's not an effect of something else ...
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Scale factor at surface of last scattering

Is there an exact theoretical scale factor associated with the CMB? More specifically, since the CMB is associated with a temperature of around 3000K (as I understand), is there an exact thermodynamic ...
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2answers
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Do we need dark matter and dark energy, if the behaviour of the universe in its initial stages was similar to that of the Sun? [closed]

According to CMBR the universe was a cloud of plasma and was a perfect black body, $380,\!000$ years after big bang. But the Sun in our solar system also is in the state of plasma, thus making it a ...
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Proving that universe is a blackbody

How can you theoretically explain why cosmic microwave background seems to be coming from a perfect blackbody given that the universe is no longer at thermal equilibrium?
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1answer
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Which decoupled first? The dark matter or the CMB photons?

According theories that work best for the structure formation which happened first in the history of the evolution of the Universe, the decoupling of dark matter or the decoupling of photons? How sure ...
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How can CMB anisotropies be due only to inhomogeneities?

I'm studying from these lecture notes (Author: Wayne Hu, title: Lecture Notes on CMB Theory: From Nucleosynthesis to Recombination). At the very end of page 10 and beginning of page 11 he says: ...
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1answer
113 views

The Cosmic Microwave Background Paradox

I was reading an article on Olbers' Paradox (why the universe isn't as bright as the sun) and the more I read on it, the more the same question came to mind... We know the observable universe is ...
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How come we can treat space as a blackbody?

I thought that the condition to be a black body was that an object had to absorb all incident radiation without reflecting any. My understanding is, electrons in atoms absorb the incoming radiation. ...
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How to relate CMB acoustic peak to BAO in galaxy surveys to better than a factor 2?

Question for a friendly cosmologist. Let's define the sound horizon as $s = \int_0^{t_{ls}} c_s (1+z) dt$ where $c_s$ is speed of sound in primordial plasma (roughly $c/\sqrt{3}$ but you can include ...
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1answer
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What will happen with the cosmic microwave background radiation if the Big Rip theory is true?

There are three things I can imagine happening: It appears one light year further away every year, which would be a bit weird, because it would be behind the universal event horizon, but still ...