# Tag Info

## Hot answers tagged redshift

80

A complementary answer to Chris's, the middle row is the spectrum at rest. A blue shift does not mean that the object ends up blue. It just means that the entire spectrum is shifted up in frequency. Note that this is a schematic diagram and not actual data. When a star emits light, the color of its light as observed on earth depends on its motion ...

71

There are certain physical processes that always produce a light of the same wavelength. For instance, hydrogen changing from the $n=2$ to the ground state always emits a photon with an energy of $10.2~\rm eV$, corresponding to light with a wavelength of $122~\rm nm$. There are many processes like this, which form "spectral lines" that should be the same ...

52

Redshifts doesn't actually mean the light is red, or was ever red. That's what is confusing you. "Red" and "blue" in this context are shorthand ways to say "towards longer wavelengths/lower energies" (red) and "towards shorter wavelengths/higher energies" (blue), because in the visible light spectrum, red is at the low energy end of what we can see, and ...

35

The instantaneous change occurs when you consider the Doppler shift in only one dimension. In three dimensions you can consider the correction when the velocity vector and the separation vector are not parallel. Usually such corrections go like $\cos\theta$, where $\theta$ is the angle between the two vectors, but more complicated things are possible. ...

31

Specific wavelengths in a spectrum, whether later shifted or not, are identified by the absorption bands of common elements near the emitters. For example, the spectral absorption lines of hydrogen are well known, and there is pretty much always some hydrogen near where the light was emitted. The absorption lines are shifted, but the various lines relative ...

28

The de Broglie wavelength of a massive particle is redshifted in an expanding universe. The de Broglie wavelength is given by: $$\lambda = \frac{h}{p}$$ so a red shift of the de Broglie wavelength simply means that the momentum is decreasing, which for a massive particle means that its velocity relative to us is decreasing. And that is exactly what we ...

28

It does have an effect, but whether or not you have to worry about it depends on how sensitive your measurements are. The rotational period of the Earth is about $24\operatorname{hours}$. Depending on your latitude, that could contribute to your velocity as little as $0\operatorname{m}\operatorname{s}^{-1}$ (at the poles) or as much as $464\operatorname{m}\... 22 Yes, this is possible using nonlinear optics. This kind of frequency shift can be done using acousto-optic modulators and electro-optic modulators, and it is normally done in a transmission geometry. The basic idea is that you have a block of material whose refractive index depends on the acoustic pressure or on the local electric field, and then you make ... 21 A red or blue shift is created when the light source is moving relative to the detector. In your thought experiment, you emit light and you receive it, so there is no red or blue shift. For your idea to work, light must be emitted not by you, but by the universe equally in all directions. Such emission is known as Cosmic Microwave Background. By measuring ... 18 The object cannot occupy your same place as he passes you, so let us assume that the trajectory is a straight line that passes next to you. As the object approaches, the component of the velocity in your direction diminishes, to the point of being zero when the object is next to you. Thus the doppler effect will change continuously, from blue to zero to red ... 12 The present cosmic microwave background was emitted when the initial plasma formed during the Big Bang had cooled down enough to convert into gas - mostly hydrogen and helium, which then was transparent to radiation. The reason it was at this time is that hot gas does not glow or absorb much at all: think, for example, of the flame from a candle: there is a ... 9 You need to solve this problem $$H(a) = \frac{\dot{a}}{a} \tag{1}$$ where$a = 1 / (1 + z)$is the scale factor,$H$is the Hubble parameter and follows the Friedmann equation $$\left(\frac{H(z)}{H_0}\right)^2 = \Omega_{m,0}(1 + z)^{3} + \Omega_{\Lambda,0} + \Omega_{\gamma,0}(1 + z)^{4} = E(z) \tag{2}$$ and I have assumed that the curvature is flat. ... 9 Redshift is said to have happened when the wavelength of electromagnetic radiation increases. Blueshift, a sister phenomenon of redshift, is said to have happened when the wavelength of electromagnetic radiation decreases. In the visible light spectrum, the red color has a longer wavelength, around$700nm$, compared to other colors; Violet, for example, has ... 9 Here is simple explanation: Suppose a light source and a observer are in an expanding space. Now think of two subsequent crests of wave emitted by the lightsource. The second crest is emitted slightly later than the first one, hence the space has expanded slightly in the meantime. Consequently, the second crest has to travel further to reach the observer, ... 9 The exact number depends on the cosmological model and its parameters. In special relativistic models (e.g. the Milne model), the redshift at the speed of light is of course infinite. However, in all viable cosmological models, recession velocities exceed the speed of light for objects with redshifts greater than$z\sim 1.5$. The general relativistic ... 8 When talking of cosmology one needs a model, and the standard model for cosmology at present is the Big Bang Model. This developed fitting observations and taking into account General Relativity ( which includes gravitational redshift.) . In this model the universe started from a small spacetime singularity which is not explorable with the physics we know ... 8 The redshift of a source actually changes in a more complicated way: when the source entered our cosmological horizon (i.e. at the moment its light reached Earth for the first time), its redshift was$\infty$, because it was located at the edge of our observable universe. Over time, this redshift then decreases to a minimum value, but eventually the ... 8 It's pretty easy to explain if we take a classical view of an electromagnetic wave. As an EM wave from a distant star propagates towards us, the space it propagates through is expanding. Since the space is expanding, the peaks & troughs of the EM wave are getting farther apart from each other. That corresponds to an increase in wavelength and a decrease ... 8 The other answers (as of this posting) stick mainly to "light"... but the same concepts exist in other forms of "waves" Red/Blue Shifts and associated "Doppler effects" I want to add associated ideas that you can, literally, hear: Sirens and Train Horns. The basic idea of "shifts" is that the waves that you see, hear, experience change based on how they ... 8 First of all, let's get a better picture of why microwave doors keep waves inside the oven in the first place. Using a generous amount of hand-waving: imagine that an electromagnetic wave is incident on a circular hole in the microwave oven door. Furthermore imagine that at some moment in time, the electric field is pointed towards the right side of the ... 8 As Jon Custer said in a comment, the light returning from a receding mirror will be redshifted. This is easiest to see if you consider the problem in the rest frame of the mirror. A symmetry argument shows that light returning from a stationary mirror can't be Doppler shifted. If the experimenter emits two light wavefronts a time$δt$apart, the worldlines ... 7 What are the observational/experimental facts: 1)Atoms have definite spectra, with a fixed pattern, a fingerprint of the atom 2) The further away ( measured by luminocity) galaxies all around ours the more shifted the fingerpring pattern towards the red part of the spectrum. 3) This happens uniformly all around. The model that fits these facts is ... 7 Your proposal is almost exactly the Michelson-Morley experiment, done in 1887. It compared the speed of light in perpendicular directions, in an attempt to detect the relative motion of matter through the stationary luminiferous aether ("aether wind"). The result was negative. 6 FYI Interestingly I found this article, http://arxiv.org/abs/0707.0380v1 where the authors address the exact issue of this question, and indeed whether the whole pedagogical idea of "space expanding," is crap. For example, section 2.6.2, is a question identical to the OP here. 2.6.2 Is everything expanding? An extension of the argument against ... 6 A Galaxy cluster could have$10^{14}$solar masses within a radius of 5 Mpc. In this case$GM/Rc^2 \sim 10^{-6}$, equivalent to a velocity shift of less than 1 km/s. Our own Milky Way has a mass of around$10^{12}\$ solar masses within 100 kpc. This gives a gravitational redshift of about 100 m/s. These are completely negligible compared to cosmological ...

6

There is no such thing as "greenshift" First off, the terms "blueshift" and "redshift" correspond to light getting shorter or longer wavelengths, respectively. We use these terms because blue light has a shorter wavelength than red light, and we use them irrespective of the actual wavelength of the light. That is, if an ultraviolet photon with a wavelength ...

6

Redshift is a kinematical effect, not a dynamical one. Therefore, the explanation of redshift is the same in classical mechanics and in quantum mechanics. For example, the derivation of the red-shift formula in non-relativistic classical mechanics is valid in non-relativistic quantum mechanics. Similarly, the derivation in (special) relativistic classical ...

6

No theories that have held up or are currently seen as in any way a competitive explanation of the observations. Yes, redshift as we've measured it, and theory, indicate an expanding universe. The idea that it's something else has a long history, and not a single success. The latest one (maybe there's more recent alternative theories, but let's say) is ...

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