Questions tagged [redshift]

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21
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3answers
2k views

Redshifted Photon Energy

A photon emitted from a receding source (Doppler redshift) has less energy when detected at an observer's location. Please explain the energy loss from the perspective of energy conservation.
17
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6answers
1k views

If space is “expanding” in itself - why then is there redshift?

The "kid's" way of understanding the expanding universe is that: "space" is totally "ordinary", and all the galaxies are expanding through it (like an explosion). Of course, that's wrong. The usual ...
37
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1answer
5k views

Can gravitational waves be red-shifted?

Whenever the Doppler effect is mentioned, it's typically in the context of sound waves or electromagnetic radiation. On the cosmological scale, red-shifting is also important because of the enormous ...
7
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2answers
682 views

Why is “gravitational” red-shift neglected in galaxy and galaxy cluster scales?

The red-shift of the light of a star in a galaxy or that of a galaxy in a cluster of galaxies is generally interpreted as how fast the star or the galaxy is moving, i.e. it is interpreted in a purely ...
5
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3answers
476 views

Redshift of merging black holes

How did they found that the gravitational waves where emitted at redshift $z=0.09$? I understand the measurement of redshift for an electromagnetic wave where we have measured in a lab various ...
4
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6answers
1k views

Redshift of distant galaxies: why not a doppler effect?

How can I explain to my 17 year old pupils that the observed redshift of distant galaxies cannot be interpreted as a doppler effect and inescapably leads to the conclusion that space itself is ...
5
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3answers
531 views

Interpretation of cosmological redshift

I was trying to understand why we cannot explain the observed redshift of distant galaxies using special relativity and I came upon this article by Davis and Lineweaver. Unfortunately when I arrive ...
4
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1answer
518 views

What is the cosmological redshift drift effect?

What is the redshift drift effect in cosmology? What are the necessary cosmological conditions for there to be a measurable redshift drift effect?
53
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4answers
11k views

Red shifted to what?

I searched and found a lot of questions and answers about red shift here but none with the answer to mine. (sorry if it is there somewhere and I did not find it.) Everyone is saying the light from ...
10
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4answers
2k views

Does an object's redshift actually decrease with time?

I am trying to determine how an object's redshift (specifically, redshift due just to the expansion of the universe) changes in time. Starting with a definition of the Hubble parameter, $$H \equiv \...
4
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2answers
355 views

A question on the redshift of photons due to cosmic expansion

Given that the universe is expanding over time, in the sense that the (spatial) metric is changing over time, corresponding to the physical distance between objects increasing, naïve intuition leads ...
3
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1answer
169 views

What is the redshifted amplitude of a gravitational wave?

Consider a gravitational plane wave in flat background spacetime, with amplitude $h$ and frequency $f$. For an observer moving with redshift $(1+z)$ relative to the plane wave, what is the observed ...
2
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1answer
756 views

Expanding space and red shift

Assuming space is really expanding - Due to expanding space, distant galaxies are supposed to be moving away from us. When light leaves a distant galaxy, it's wavelength is red shifted to begin with ...
17
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3answers
2k views

Why isn't the De-Broglie wavelength of massive particles redshifted in an expanding universe

It is pretty straight forward how light is redshifted in an expanding universe, yet I still can't understand why the De'Broglie wavelength of a massive particle isn't redshifted in an expanding ...
2
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3answers
234 views

At what cosmological redshift $z$, does the recession speed equal the speed of light? How is it calculated?

At what cosmological redshift $z$, does the recession speed equal the speed of light? What equations are used to calculate this number (since at large redshifts, $z=\dfrac{v}{c}$ won't apply)?
21
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5answers
7k views

Does red shift evidence necessarily imply that the universe started from a singularity?

We are taught that the universe began as a singularity - an infinitely small and infinitely dense point. At the beginning of time there was a 'Big Bang' or, more accurately, 'Inflation'. The main ...
2
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2answers
1k views

Where does the energy go when light is redshifted? [duplicate]

Imagine a galaxy millions of lightyears away and, obeying Hubble's law, moving very quickly away from us. Now imagine the same galaxy emitted a green photon in our direction (a photon with a ...
1
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1answer
226 views

If mass isn't conserved in an expanding universe, why do we assume atomic spectra are constant?

Of the many ways to write a mass in general relativity (Komar, adm and the like), it seems that none of them are conserved in an expanding universe (or more generally for nonstationary metrics). ...
8
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2answers
645 views

What wavelength was the CMB originally?

What wavelength was the light that became the CMB after factoring out redshift? Is it safe to assume that what we're seeing in the CMB was emitted at a distance of approx: ...
3
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1answer
193 views

How large or small can frequency in the EM spectrum get?

The largest frequency range is gamma rays, but does the EM spectrum 'stop' somewhere? Like is there a limit to how large a frequency can get? Or how small frequency can get? Is it one of those things ...
2
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1answer
2k views

Redshift of mass-dominant universe

What was the value of redshift $z$ when matter started to dominate the universe? Is there any way to calculate it without knowing the time?
0
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1answer
52 views

Why is void making CMBR lose energy as inflation speeds up?

The expansion of space is speeding up, so many voids are getting bigger. CMBR lose energy when moving through this void and then gain back the same energy as it leaves the void except in the case when ...
0
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3answers
119 views

How do astronomers know what wavelength the body will emit when it is at rest? [duplicate]

In the book of The First Three Minutes by Weinberg, at page 21, he talks about how do astronomers measure the speed of a luminous body along the line of sight by Doppler affect, i.e. the fractional ...
5
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3answers
146 views

Cosmological Inflation: If Photon expands, why not other matter?

I keep repeatedly reading in many Stack Exchange and Quora questions that space of universe expands but particles (matter) don't, see e.g. this Phys.SE post. The reason given is that particles are ...
5
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3answers
527 views

Does observed red shift indicate expanding universe? [duplicate]

The observed red shift of objects is assumed to indicate that those objects are moving away from Earth but is it possible that there could be another explanation for the observed red shift that does ...
3
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1answer
53 views

For an observer NOT in comoving coordinates, won't the Hubble factor be anisotropic?

Since the Earth isn't exactly in comoving coordinates, I would think redshift surveys have to adjust for this? I couldn't, upon searching, immediately find which direction our CMB dipole shift was in,...
1
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1answer
242 views

Effect of redshift on energy conservation [duplicate]

Light coming from galaxies that are going away from us is redshifted. Since the energy of a photon is purely dependent on its frequency one may conclude that the energy of these photons decreases. The ...
1
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2answers
545 views

We say Light is Red-Shifted or Blue-shifted from faraway stars and galaxies [closed]

We say Light is Red-Shifted or Blue-shifted from faraway stars and galaxies. Can we find out the distance at which it changed its frequency. So in another solar system, it might seem to be Green ...
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2answers
100 views

Redshifting light in an expanding universe

It's evident and well known that light traveling across an expanding FLRW universe is redshifted via an equation: $$\frac{\lambda_{arriving}}{\lambda_{emitted}}=\frac{a_{now}}{a_{then}}$$ Where $a$ is ...
1
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1answer
73 views

CMB - Excess Energy? [duplicate]

Today in physics we were looking at how the energy of a photon is the product of Planck's constant and the frequency of the photon, therefore the lower the frequency, the lower the energy of the ...
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1answer
275 views

Acceleration of Universe expansion

I have been recently researching the topic of the expansion of the Universe. Specifically, the 1998 discovery by Saul Perlmutter that claims the Universe is expanding at an accelerated pace. From my ...
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1answer
46 views

How do you know that objects are accelerating from each other in space?

I know that Hubble redshift comes from the relation between the distances of objects and the Hubble parameter: $$H_0 = \frac{\dot{R}}{R}.$$ But how we differentiate between redshifts effects from the ...
0
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2answers
80 views

On Distant Lanterns and Relative Speed [closed]

It is said, that by measuring distant sources, we can gather information about the past state of the universe. The further we measure, the faster things appear to recede, which brings me to my ...