# Tag Info

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I think your confusion has to do with terms and semantics, rather than physics: The cosmological redshift has nothing to do with the velocity of the emitter and the observer with respect to each other. That's why it's not a Doppler shift. The cosmological redshift is caused by the expansion of space. It is a direct measure of the relative size of the ...

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At a distance of $d = 87\,\mathrm{Mpc}$, with a Hubble constant of roughly $H_0 = 70\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$ cosmological expansion should make the host galaxy UGC 11723 recede at $v=H_0 \,d\simeq6100 \,\mathrm{km}\,\mathrm{s}^{-1}$. However, galaxies also move through space, at typical velocities from several $100\,\mathrm{km}\,\... 6 The reason we think that the cosmological redshift is caused by the metric expansion of space is 1) that there is a well-known, physical mechanism that can cause this effect, and 2) that this mechanism is a prediction of a well-established and thoroughly tested theory, namely the theory of general relativity. The physical mechanism in question can be ... 6 I'm assuming you're talking about physical distances (as opposed to any of the other distance measures in cosmology). The comoving distance to a galaxy at redshift$z$is $$d_C(z) = \frac{c}{H_0}\int_0^z \frac{dz}{\sqrt{ \Omega_r(1+z)^4 + \Omega_m(1+z)^3 + \Omega_k(1+z)^2 + \Omega_\Lambda }},$$ ... 5 Yes, radio spectra have been used extensively to find the distances and locations of HI regions and molecular clouds inside the Milky Way. Observations of the 21 cm hydrogen line and/or several carbon monoxide lines (in particular,$\text{CO}(1\to0)$) enable us to make radial velocity measurements of clouds within the galaxy. From there, some geometry (see ... 4 Yes, there is a transverse relativistic Doppler shift. You can think of it as being caused by time dilation. https://en.m.wikipedia.org/wiki/Relativistic_Doppler_effect There can be a redshift or a blueshift depending on when, where and who does the measurement. e.g. a receiver with a source going around it in a circular orbit. The receiver sees a lower ... 4 No. Or at least such an effect has never been observed, neither in the locality of the Earth or in light detected from distant sources. If a photon has an interaction with a quantum field (such as an electromagnetic field) this causes a scattering. Scattering would cause a blurring and dimming of distant sources. This is not observed. Such an effect, if ... 3 Redshift is not related to the time it takes to travel from the source to the viewer, not to the distance from the source to the viewer. But light is red-shifted by travelling out of a gravitational well. One way to see this is to consider the equivalence principle: A person at the front of an accelerating spacecraft would see light shone from the back to ... 3 You're confusing error and uncertainty. In school labs or other situations where the true value of a quantity is known, the difference between a measured value and that true value is sometimes referred to as the "error" in the measurement. A lower error is taken as an indication that the results are successful. When making observations, the true value of ... 3 Is the amount of energy that dark energy applies to push objects apart equal to the amount of energy lost because light from distant galaxies is red shifted? No, dark energy existed prior to the expansion and the shift towards blue or red is based on the direction of movement of the emitter relative to the observer, so the amount isn't equal. Sources: ... 3 Difference between 3D real space and 3D redshift space? Why is 3D real space ever even considered? Is it some kind of highly idealistic cosmology-dependent only scenario, ignoring peculiar velocities? Simple explanation: 3D real space is the actual distance, position, velocity, (even spin), etc. of an object; this can be used to measure Earth's ... 3 The non-relativistic formula can be derived analogously to the derivation of the Doppler effect for sound. Namely, draw waves as consecutive parallel lines moving at velocity$c$whose distance is$\lambda$(if the source is not moving) and let the source move away at$v$. You will find that the distance has increased and became the observed wavelength, ... 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 ... 2 Since an object moving perpendicular to a given "line of sight" has a constantly changing range , there is a Doppler shift, blue when approaching and red when leaving. The shift drops to zero at the point of crossing the line of sight because at that instant the radial speed is zero, as you suggested. So, the general magnitude is calculated using ... 2 A big picture reason is: because it's not really clear if it's scientifically sound to include multiple sources of data obtained from different methods. Astronomical measurements are very difficult, with huge numbers of confounding factors involved. Distance measurements are notoriously difficult because we can't actually measure interstellar distances ... 2 From Friedmann Equation, distance as a function of redshift is: $$d(z)=\frac{c}{H_0}\int_0^z \frac{dx}{\sqrt{\Omega_{R_0}(1+x)^4+\Omega_{M_0}(1+x)^3+\Omega_{K_0}(1+x)^2+\Omega_{\Lambda_0}}}$$ The Hubble-Lemaître Law: $$v=H_0 \cdot d$$ We want$\boxed{v=c}$The distance that fulfils this condition is known as Hubble Distance, (or Hubble Radius, or Hubble ... 2 I don't really understand all the equations you wrote out and I'm not sure you can perform the simplification the way you suggest. In particular, I don't agree with Equations (3) and (4). Indeed, using Bayes theorem in the bottom term of Equation (1) leads to$$\int_{z_{min}}^{z_{max}} \int_{z_i^-}^{z_i^+} dz dz_p n(z) p_{ph} (z | z_p)\frac{p_{ph}(z_p)}{... 1 While @planetmaker's comment is true if the lines come from the same source, you can have lines emerging from different physical processes which still appear to come from the same location. An example is absorption (or more rarely emission) lines from galactic winds, which are typically blueshifted with respect to the "systemic" redshift, i.e. the &... 1 Your intuition is correct - a moving source emitting wavefronts periodically will be closer to the previously emitted wave in the direction of motion, and farther from the previously emitted wave in the opposite direction - see the simulation here. You are also correct that the size of the effect depends on the speed of the observer relative to the speed of ... 1 The term "Hubble flow" refers to the homologous expansion of space and the resulting recession of all galaxies from each other (if they're not close enough to be gravitationally bound). This effect causes the "cosmological redshift", i.e. the redshift that light from distant galaxies attain as it travels through space. In addition to this ... 1 Not quite. Gravitational redshift is proportional to$M/R$, where$M$is the mass interior to a radius$R$. However density$\rho$is proportional to$M/R^3$. So gravitational redshift does not depend directly on density. If you are considering radiation emitted from the surface of an object then the redshift is proportional to either$\rho R^2$or$\rho^{1/...

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In Equation (16) of the paper you link to, $z$ is the observed redshift. In the first paragraph of section 2.2 The heart of the method is to use a measured redshift, z, to infer a velocity, v(z)

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Determining the photometric redshift means looking at the light from the galaxy through a limited number of color filters (or bands), and infering the redshift from that data. For instance, the light coming from the galaxy can be measured in the visible light band, the infrared band,... This constitutes the photometry. Then the redshift is determined either ...

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This was too long for a comment, but is not a real answer since I'm not completely sure, but: My guess is that "representing the galaxies" refers to the "type" of galaxy that you intent to observe, where by "type" I mean e.g. Lyman-break galaxies (Steidel et al. 1996), Lyman $\alpha$ emitters (Partridge & Peebles 1967), sub-millimeter galaxies (Blain et ...

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Welcome to StackExchange. Good question. Hubble's Law says that an object's velocity away from an observer is directly proportional to its distance from the observer. In other words, the farther away something is the faster it is moving away from us. The redshift tells how fast a star is receding from us and we can therefore get the distance. Hubble's ...

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After a lot of research, I found the answer to the second question, since the FITS files in sdss Images have more than 1 primary hdus, you have to pass the following fits images like an array *.fits[0] to the swarp program, where the first hdu in the fits file contains the image pixel. solution here For the first question, yes the method to do that is ...

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This is exactly what is done for template fitting. The shifted SED is one of the intermediate data products of template fitting codes such as Le Phare or Phosphoros. For instance, with Phosphoros, you can shift your SEDs to any number of redshifts, then find the shifted SEDs (also called model grids) in the IntermediateProducts folder in the Phosphoros ...

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