# Tag Info

21

Until the Universe was 380,000 years old, it was filled with a gas of protons an electrons. There was also radiation, in thermal equilibrium with the matter, and because it was so hot, the protons and electrons couldn't form neutral hydrogen, since every time it "tried", an energetic photon would knock off the electron. This gas was everywhere. And photons ...

18

It is the late time integrated Sachs Wolfe effect. As they travel towards us, apart from the general expansion, photons from the CMB gain energy when they fall into potential wells (where matter is). Of course, they lose it again as they emerge on the other side of the well, but the cosmic expansion means that the well isn't quite as deep by the time that ...

17

The temperature of the cosmic microwave background scales as the inverse of the cosmic scale factor $a$. i.e. When everything was at half the separation it is now, then the CMB was twice the (absolute) temperature. The scale factor in turn is reciprocally related to the redshift by $a/a_0 = (1 +z)^{-1}$, where $z$ is the redshift and $a_0$ is the present-day ...

13

This is a great question. I know of a couple of really big things about inflation people want to be able to nail down by using the cosmic microwave background. The first is measuring what are known as E- and B- modes, which are the curl-free and divergence-free components to the modes of cmb radiation: Essentially, measuring large scale Gaussian B-modes ...

11

The CMB patterns do indeed change over time, although statistically they remain the same, and although it will not be noticeable on human timescales. The CMB we observe now comes from a thin shell with us in the center, and with a radius equal to the distance that the light has traveled from the Universe was 379,000 years old and until now. As time passes, ...

10

Firstly, there is no centre to the universe and the universe seems to continue indefinitely in all directions. It is best to imagine the universe as infinite in size. Now we need to explain why the CMB appears to be a sphere surrounding us. Let's imagine, for a moment that I have filled the entire infinite universe with (magic) mud. This mud glows, and ...

9

First, let me clear up a misunderstanding: Particle horizon The "edge" of the observable Universe is called the particle horizon, and lies roughly 47 Gly (billion lightyears) away. It is always receding, both because the Universe expands and because light from increasingly large distances eventually reach us. In comoving coordinates (the coordinate system ...

8

The simple answer to your question is "yes" - the universe expanded at much greater speeds than $c$ during the inflationary epoch. This period of time was very quick but very dramatic, lasting from about $10^{-36}$ to $10^{-32}$ seconds. The universe expanded, in this very short period, by a factor of $10^{26}$. That's pretty incredible, when you think about ...

8

No - the decreasing energy in the CMB is already well modeled in the Friedmann equations. The term in the density parameter that is proportional to $a^{-4}$ is the contribution of radiation energy density to the evolution of the universe, the term proportional to $a^{-3}$ is matter density (mostly dark, but includes ordinary matter), $a^{-2}$ is the ...

6

The Boomerang Nebula (or Bow Tie Nebula) is a cloud of gas being expelled from a dying low-mass star, at $164~\mathrm{km}~\mathrm{s}^{-1}$. In general, when a gas expands, it cools (see extended explanation below). If the gas were optically thin to the CMB — that is, if it were sufficiently dilute that CMB photons could easily penetrate — it ...

6

The cosmic microwave background is a result of an almost perfect blackbody emitter. That means the spectrum covers a broad wavelength range with a peak that is given by Wien's law: $$\lambda_{peak} = \frac{2.9\times 10^{-3}}{T},$$ where the wavelength is in metres and temperature in Kelvin. The microwave background is formed at the epoch of (re)combination, ...

6

I don't remember reading any paper title specifically about the hot spots you refer to, so I guess that they are not as "strange" as the cold spot. Assuming that the temperature fluctuations are well described by a Gaussian distribution with mean 2.72548 and variance 0.00057 source, the probability of having the cold spot of the observed size and ...

6

The anisotropies in the CMB are caused by four effects; three at the surface of last scattering (SoLS), and one after: Temperature differences Denser regions will be more compressed and thus hotter, on average. Hence, an overdensity will result in a hotter spot, with a fractional fluctuation $\Delta T/T_0$. Gravitational redshift Photons climbing up (or ...

5

We observe the temperature of the CMB as a ~2.7 K blackbody, but that's the redshifted version we observe. The CMB is also know as the "surface of last scattering" at the point of recombination when nuclei and electrons combined to form neutral atoms the universe went from opaque to transparent. This happens at a temperature of ~3000 K. From this we can ...

5

"Spinning dust" is a mechanism proposed to explain a particular feature in the foreground emission of CMB; a bump around $\nu\sim20\,\mathrm{GHz}$. Dust grains acquire charge through photoelectric emission and collisions with electrons and ions (Draine & Lazarian 1998). As zephyr comments, if the dust is a poor conductor, its charges will, in general, ...

5

The furthest we can "see" is the cosmic microwave background at a redshift of about 1100. The proper distance of the CMB-emitting gas that we see today is about 46 billion light years. If you are talking about galaxies, then the first are thought to have formed at redshifts of about 20 (current distance 36 billion light years) and beyond that are the ...

4

Yet we also have a third axiom: There are some parts of the universe we can never observe because they are receding away from us at a superluminary speed. This is simply false, so of course it gets you in trouble if you insist on taking it as axiomatic. The average recession velocity goes luminal at redshift $z\approx 1.4$, while there are ...

4

When we observe the CMBR we are observing the surface of last scattering, however the comoving points that make up the surface of last scattering (which infact will actually have a comparitvely very small, but non-zero thickness rather than being a 2D surface) change with time, which in theory should correspond to a change in the observed pattern (of ...

4

Assuming you mean the Cosmic Microwave Background ... The CMB is entirely in a very narrow range of the microwave part of the spectrum. Galaxy emissions are all over the spectrum. The CMB comes uniformly from all over the sky. It's like looking at painting that is all one color. Galaxy light looks like pinpoints on a dark background. (Nearby galaxies are ...

4

The CMB lets us measure how close to flat the universe is right now. On the other hand, inflation tries to explain how we got from whatever the early universe was to right now. The motivation for the latter being that even extremely small deviations from perfect flatness in the early universe should have resulted in very obvious deviations from flatness ...

4

In principle yes, in practice no. As seen in the temperature power spectrum below, the Planck satellite detects power (i.e. "a signal") even on the smallest probed scales, which is a few arcminutes. Fig. 1 from Planck Collaboration et al. (2016) with my own approximate corresponding angles annotated in red. The smallest scale shown in the figure is $\ell =... 3 The answer to your first question is "Yes, the temperature referred to is the 'normal' temperature, reflecting the average kinetic energy of the gas particles". The answer to your second question is a bit more complex: Cooling function Gas cools by various processes, with an efficiency depending on the temperature, the density, and the composition of the ... 3 Well obviously the CMBR arrives (towards us, not receding) at us at the speed of light as it is a form of electromagnetic radiation and local velocities are not affected by expansion. What I guess you may mean though is the surface of last scattering, which is the surface from which the CMBR that we receive at a given instant was emitted in the early ... 3 B-modes in the raw CMB data can be caused by primordial gravitational waves, as well as by cosmic dust. After subtracting the cosmic dust polarization from the BCEP2 data, the residual signal is too weak with respect to noise to be statistically sufficiently significant to claim a discovery. Or stated in a different way: The observed B-modes can be ... 3 Polarization of an electromagnetic wave refers to the extent to which the electric field (which is always perpendicular to the direction of wave motion) oscillates in one particular direction, rather than randomly in the plane perpendicular to the wave motion. Complete linear polarization means the electric field oscillates to and fro along one particular ... 3 In this context, the$\kappa\$ you are referring to is called the dimensionless matter density field. It is gravitational lensing jargon, and is usually just referred to as the 'convergence' field. What you have written there (that a field can be split up into divergence-less and curl-less components) is generally speaking true for any field which can be ...

3

No. Space expansion doesn't slow down, unlike the expansion of cosmic horizon. That means the cosmic horizon actually swallows galaxies as they travel away from us due to space expansion, reducing their number instead of increasing it. Nevertheless, we still haven't gathered all cosmic radiation from Big Bang that occurred within our current event horizon, ...

3

It is said that due to the CMB radiation our universe has been expanding? No. It's the the other way round. The CMB is the result of the expansion of space. In the initial stages after the birth of the universe, there was hot plasma everywhere and light could not travel much farther. The universe was opaque at that time. But slowly as the Universe cooled ...

3

At the time when the universe was ~380,000 years old it was completely filled with a plasma and light. Light could only travel very short distances (I forget the specifics, but think a couple meters at the most) before being absorbed. So the entire universe was a fog, if you were there and there was an object 10 meters in front of you, you couldn't see it ...

3

You seem to struggle with a very common thing; the understanding that The Big Bang is not something that happened in space, but rather to space. It could be good for you to read some answers to misconceptions about The Big Bang here, or the answer to a question about it on Physics Stack Exchange. The Big Bang caused everything in the universe to move away ...

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