# Tag Info

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 ...

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

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 ...

3

The (late time) ISW is caused by the evolution of cosmic structures as photons of the cosmic microwave background traverse them on their way to our detectors. It may cause a redshift or blueshift with respect to the redshift predicted for a homogeneous expanding universe. A bit more detail: If a photon "falls" into a potential well, its frequency and energy ...

3

The specific radius of $8 h^{−1} \text{Mpc}$ is used because the value of $\sigma_8$ turned out to be close to unity. To quote e.g. from Amendola & Tsujikawa (2010), p. 39: If the cells have a radius of $8 h^{−1} \text{Mpc}$, it turns out that $\sigma_R$ is close to unity. Conventionally the normalization of the power spectrum is therefore given by ...

2

I'm under the impression that the sound horizon is a fixed physical distance, so its measured comoving distance at any redshift should always be the same. Am I right or wrong in this assumption? You are both right and wrong at the same time. Right in the sense that, yes, in comoving coordinates, the distance is the same at all redshifts. But when we use ...

2

Short answer. Great question! Ask yourself what absorbs better, something that is white, or black.Then ask yourself what radiates better. The answer is the same thing. It turns out that absorptivity and emissivity are linked pretty closely. Kirchov's Law of Thermal Radiation can be stated as: For an arbitrary body emitting and absorbing thermal ...

2

I think the answer is that it's primarily a matter of association on the sky, though there are some (weak) additional constraints. The first thing to note is that there are three observations that are associated on the sky: the X-ray emission (which has been known about since the mid 1990s), the newly detected 140 MHz LOFAR emission, and evidence for a ...

1

If time started in our Universe at the Big Bang, then in what time dimension (outside of ours) do collisions between Universes occur? In inflationary cosmologies, time did not start at the Big Bang. There are many models, but in at least some of them, there is an "inflating" universe which extends infinitely in past (and possibly future) time, constantly ...

1

Whichever direction we look in the universe, we are looking back in time. If we observe a galaxy that is $10$ million light years away then we are observing photons that were emitted $10$ million years ago. If we observe a galaxy that is $1$ billion light years away then we are observing photons that were emitted $1$ billion years ago. And the further away ...

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