# Tag Info

## Hot answers tagged cosmic-microwave-background

27

Yes, our (i.e. the Sun's) motion in the "global", or comoving, reference frame can be measured accurately from the dipole of the cosmic microwave background. The latest results from the Planck Collaboration et al. (2018) yielded a velocity of $$369.82\pm0.11\,\mathrm{km}\,\mathrm{s}^{-1}$$ in the direction $$\begin{array}{rcl} \ell & = & 264.021º\... 21 The CMB is produced as the ionisation fraction of hydrogen falls from a high value to a very small value. Contrary to what is written in the Quora answer you may have been misled by, this happens at a temperature of about 3000 K. The value given by 13.6 eV/k_B (remember to multiply by the electric charge to put the energy in SI units) is not even "... 14 The idea that belts or spheres of dust might be responsible for (some) microwave emission is not crazy. Indeed we know that dust does emit microwaves and indeed the contribution of such dust has to be removed from the CMB signal before it can be interpreted cosmologically. There is some debate about some of the larger scale anisotropies (particularly the &... 14 Would it be possible to look deep into a certain part of space and time to find some galaxy that contributed to the matter that makes up the Milky Way today? No, that's not possible. If we could do that, it'd mean that the matter traveled from there to here faster than its light got here, and matter can't travel faster through space than light does. ... 10 The rubber sheet only is not meant to be a qualitative model, it gives one concept and one concept only: Mass causes curvature of spacetime. You can't get any more than that from the rubber sheet. If you have that idea in your head already then you are ready to drop the image because: The sheet is 2d but spacetime is 4d The 2d sheet is embedded in 3d ... 9 Yes, the universe is believed to be infinite in size. That's what you get if the curvature is zero or negative, assuming a simple topology. The curvature has to be positive for a finite universe, once again, assuming a simple topology, and no weird stuff like edges. Now it's possible that the universe has a very small positive curvature, so that it's finite,... 7 By request: Beyond the fact that the cosmic microwave background (CMB) is a direct prediction of the big bang model, there is the question of how you would produce it in any other way. It is remarkably close to being isotropic and remarkably close to being a blackbody spectrum - i.e. it is almost a perfect blackbody radiation field. A blackbody radiation ... 7 If I understand you correctly, you want to know the distance from the point from which we observe the CMB, to the edge of the observable Universe. During inflation, the observable Universe expanded from ridiculously small to some ten meters in radius, so that part can be safely ignored compared to the distances now^1. The distance^2 to the "CMB shell"^... 6 It is not very clear what you mean. There are many cosmic sources of microwave radiation that emit so much energy and/or are so close to us that they confuse measurements of the cosmic microwave background. For example "compact sources" such as giant molecular clouds, supernova remnants, H II regions; "diffuse" sources due to dust in the galaxy and in our ... 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 Yes. Gravitational wave observatories like the proposed eLISA laser interferometer may be able to detect gravitational waves that originate from the early moments of the big bang itself. If some part of the big bang energy goes into gravitational waves then those waves will be redshifted by expansion and waves produced in the first <10^{-10} s should be ... 5 The CMB is visible at a distance of 13.8 billion light years in all directions from Earth, leading scientists to determine that this is the true age of the Universe. This is wrong in a few ways. First, we do have good reason to think that the CMB was produced around 13.8 billion years ago, but that doesn't mean it's 13.8 billion light years away. The light ... 5 Because B_\lambda is not just B_\nu, with \nu replaced by c/\lambda. The relationship between the two functions is that$$ B_\lambda\ d\lambda = B_\nu\ d\nu$$since one is defined in terms of flux per unit wavelength, the other as flux per unit frequency. Thus$$B_\lambda = B_\nu\ \left|\frac{d\nu}{d\lambda}\right| = \frac{c}{\lambda^2} B_\nu, \tag{1}...

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I just wrote an answer about this. The rubber sheet is a good model of 2D Newtonian gravity, with a 1/r force law. If you make a rigid surface in the shape of the 3D gravitational potential, like the gravity wells you find in some science museums, and roll small balls on it, it's a pretty good model of orbits in 3D Newtonian gravity, with the correct 1/r2 ...

5

You would have to catch up to the light that carries the information you seek. It's traveled for a few billion years at this point (Earth is ~4.3B). So, you could watch the formation of Earth (Milky Way, whatever), if you could instantly teleport billions of light years away from here. When we watch distant galaxies and starts, what we're seeing is "old" ...

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If it is really infinite then shouldn't such measurements be inconsequential as an infinite curved universe would still appear perfectly flat to a local observer? No, it wouldn't necessarily. It could, but it's not mandatory. Imagine an infinite line that just takes a sharp turn at some point. It would definitely be "curvy" at that point, even though it's ...

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No, the oldest light we can see is as old as the Universe$^\dagger$. This is simply because light started traveling at that time, so after one year you'd se light that had been traveling for one light-year, after 1 million years you'd see light that had been traveling for a million light-years, and so on. There is (most likely) more Universe beyond the ...

4

Yes, I think it is a constraint, but don't think I can tell you how different it could be from 2.73 K to cause a problem. The CMB tells us that the universe was once much hotter ($>3000$ K), so that hydrogen nuclei and electrons had sufficient kinetic energies to remain uncombined. In the big-bang model, the universe expands and cools, then at ...

4

The expansion of the universe is described by a scale factor that we normally call $a(t)$. We take the scale factor to be equal to one right now, so if $a(t)$ is less than one that means the universe has contracted while if $a(t)$ is greater than one the universe has expanded. General relativity allows us to calculate how the scale factor depends on time. ...

3

Above I have plotted few Planck's radiation contribution: The violet is due to CMBR, Green is due to Milky way Galaxy, and Blue is due to contribution from both. One way to resolve out CMBR is by modeling milky way from higher frequency of the graph since CMBR contribution is negligible at higher frequency. At particular direction one can associate ...

3

If a signal of power $P$ is spread isotropically it will have power density $P_s = P/4\pi r^2$ at distance $r$. If there is a noise power density $P_n$ then the signal to noise ratio will be $$\text{SNR} = \frac{P_s}{P_n} = \frac{P}{4\pi r^2 P_n}.$$ Conversely, signals can be detected when the SNR>1, or $$r < \sqrt{\frac{P}{ 4\pi P_n}}.$$ Now we need to ...

3

If an extraterrestrial civilization has a SETI project similar to our own, could they detect signals from Earth? In general, no. Most earthly transmissions are too weak to be found by equipment similar to ours at the distance of even the nearest star. But there are some important exceptions. High-powered radars and the Arecibo broadcast of 1974 (which ...

3

CMB fluctuations The CMB fluctuations are often analyzed through their power spectrum $P(k)$, which is a measure of the extent to which it is "clumpy" on a given scale $\ell$, with corresponding wavenumber $k = 2\pi/\ell$. The origin of this power spectrum is laid in the very early early Universe, just after the Big Bang, and it is of utmost ...

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An appropriate explanation might begin by explaining that the cosmic microwave background (CMB) was formed in the expanding universe about 400,000 years after the big bang. An expanding gas cools, and when the gas in the universe cooled to about 3000 K, a transition tok place whereby the protons and electrons, hitherto free particles, combined to form ...

3

The CMB is basically the cooling of the high energy radiation of the early universe to achieve a nearly uniform dispersion of microwave shifted energy. It came to be via the decoupling, an early universe expansion-related event (~380,000 years after the big bang) that allowed particles to fall out of thermal equilibrium for the first time. This decoupling ...

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If you were in deep space, i,e., a spot somewhere between galaxies, then the night sky would look a lot like the sky seen by astronauts in the ISS, when they look away from both the ecliptic plane and the galactic plane, minus the individual Galactic stars. That is, it is like our deep sky photos in those directions that are dominated by galaxies, but no ...

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There shouldn't be any correlation. The CMB light that we see is from a spherical region in the early universe. Its homogeneity strongly suggests that the interior of the sphere was just as homogeneous, but we can't actually see CMB light from the interior. The galaxies that we can see formed from matter inside the sphere, and quite far from the edge. ...

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We could not have formed "outside" the expanding universe, as there would not be any substance to form our galaxy, and solar system -including our Sun and planet.

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The comoving radius of the sphere of matter that we see as the CMBR can be calculated from measured cosmological parameters, and is around 46 billion light years. The comoving size encompassed by our past light cone back to the beginning of time is not known, but it has to be much larger—at least hundreds of billions of light years across, and potentially ...

2

The peaks in the temperature and polarization spectra determine the angular size of the sound horizon at the time of recombination fairly accurately: $$\theta = \frac{r}{D(z)}$$ The sound horizon, which is represented by r, is the comoving distance a sound wave could travel from the beginning of the universe to recombination and is a standard ruler is any ...

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