PodCastAllLangs said," Redshift could also result from a different speed of light (or rather, a different rate of passage of time) at these distances & periods in our past, compared to our perception of it here & now (including against our reference lab spectrum). "
The redshift of light, the movement of light from the narrow visibility achieved in our eyesight to the invisible infrared, can now be seen clearly thanks to the James Webb Space Telescope (JWST), for which it was designed.
Astronomers use
redshifts to measure how the universe is
expanding, and thus to determine the distance to our universe’s most distant (and therefore oldest) objects. What is a redshift? It’s often compared to the high-pitched whine of an ambulance siren coming at you, which drops in pitch as the ambulance moves past you and then away from you. That change in the sound of an ambulance is due to what’s called the
Doppler effect. It’s a good comparison because both sound and light travel in waves, which are affected by their movement through air and space.
Sound can only move so fast through the air; sound travels at about 750 miles (1,200 kilometers) per hour. As an ambulance races forward and blares its siren, the sound waves in front of the ambulance get squished together. Meanwhile, the sound waves behind the ambulance get spread out. This means the frequency of the sound waves is higher ahead of the ambulance (
more sound waves will strike a listener’s ear, over a set amount of time) and lower behind it (
fewer sound waves will strike a listener’s ear, over a set amount of time). Our brains interpret changes in the frequency of sound waves as
changes in pitch.
Like sound, light is also a wave traveling at a fixed speed: 186,000 miles (300,000 km) per second, or some one
billion kilometers per hour. Light, therefore, plays by similar rules as sound.
But, in the case of light, we perceive changes in wave frequency as changes in color, not changes in pitch.
In our expanding universe, a measurement of speed translates to a measurement of distance and time.
Here’s a recent example. Astronomers said in early January 2020 that the most distant
quasar known at this time –
quasar J0313-1806 – has a record-setting redshift of
z = 7.64. In accordance with astronomers’ interpretations of redshift, we’re seeing quasar J0313-1806 – a highly luminous galaxy nucleus in the early universe, thought to be powered by a supermassive black hole – just 670 million years after the Big Bang, or more than 13 billion light-years away.
Or consider an even more distant object, not a very bright quasar, but instead just a regular galaxy in the early universe.
GN-z11 is a high-redshift galaxy found in the direction of the constellation Ursa Major, the Great Bear. GN-z11 is currently the oldest and most distant known galaxy in the observable universe, with a redshift of z = 11.09. That redshift corresponds to a distance of 13.4 billion light-years. So we see this object as it existed 13.4 billion years ago, just 400 million years after the Big Bang.
Astronomers make use of markers in the
spectrum of starlight. This is the study of
spectroscopy. If you shine a flashlight beam through a prism, a rainbow comes out the other side. But if you place a clear container filled with hydrogen gas between the flashlight and the prism, gaps appear in the smooth rainbow of colors, places where the light literally goes missing.
The hydrogen atoms are tuned to absorb very specific frequencies of light. When a beam of light consisting of many colors passes through the gas, those frequencies get removed – absorbed – from the beam. The rainbow becomes littered with what astronomers call
absorption lines. Replace the hydrogen with helium, and you get a completely different pattern of absorption lines. Every atom and molecule has a distinct absorption fingerprint that allows astronomers to tease out the chemical makeup of distant stars and galaxies.
When we pass starlight through a prism (or similar device suitable for telescopes, such as
diffraction gratings), we see a forest of absorption lines from hydrogen, helium, sodium, and so on. However, if that star is hurtling away from us, all those absorption lines undergo a recessional Doppler shift and move toward the red part of the rainbow. This is what we call a
redshift.
For stars heading toward us, the
opposite happens, and the lines are shifted toward the blue end of the spectrum; they are
blueshifted (generally, astronomers only use the term redshift to simplify things, and just put a negative sign in front of it if it’s a blueshift).
By measuring how far away the lines are located from where they’re supposed to be in the spectrum, astronomers can calculate the speed of a star or a galaxy relative to Earth, and even how a galaxy rotates: by measuring a different redshift for one side of the galaxy compared to the other, you can see which side is moving away from you and which side is moving toward you.
With this tool, the motion of the universe is revealed and a host of new questions can be investigated.
And galaxies aren’t the only things that can be investigated with redshifts. Astronomers have learned to tease out the subtle tug of a distant planet on its parent star, thus revealing the planet to astronomers. If a star in our Milky Way galaxy has a hidden planet – and if astronomers see that the star sometimes exhibits a slight redshift and other times a slight blueshift – the astronomers infer that star is alternating between moving toward and away from us. They refer to this movement as a “wobble” of the star in space.
Something must be pulling on the star, causing it to wobble. By measuring how far the absorption lines shift, an astronomer can determine the mass of the invisible companion and its distance from the star, and come to the conclusion that a planet is in orbit around the star!
As a planet orbits a star, it tugs the star back and forth with tiny movements. Astronomers see the star wobbling as an alternating red and blueshift of its spectrum. Image via ESO.
In addition to finding other worlds, redshifts also led to one of the most important discoveries of the 20th century. In the 1910s, astronomers at Lowell Observatory and elsewhere noticed that the light from nearly every galaxy was redshifted: most galaxies in the universe were racing away from us! A Belgian scientist,
Georges Lemaître, who was also a priest, recognized that the recession velocities of the galaxies could be explained by a startling truth: the universe is expanding! In 1929, American astronomer Edwin Hubble matched up redshifts with distance estimates to the galaxies and uncovered something remarkable: the farther away a galaxy, the faster it’s receding. This relation, the Hubble law, was
renamed in 2018 by the International Astronomical Union to the
Hubble–Lemaître law.
What came to be known as the
cosmological redshift was the first piece of the Big Bang theory, and ultimately a description of the origin of our universe.
The list of the
most distant astronomical objects is always changing as astronomers find higher and higher redshifted objects on the brink of the observable universe. Galaxies, quasars and even gamma-ray bursts travel for eons across the cosmos, delivering their faint red light, and revealing a little more of the secrets of the universe.
Edwin
Hubble and colleagues found a correlation between distance to a galaxy (horizontal axis) and how quickly it’s moving away from Earth (vertical axis). The movement of galaxies in a nearby cluster adds some “noise” to this plot. Image via William C. Keel/ Wikipedia.
Bottom line: A redshift reveals how an object in space (star/planet/galaxy) is moving compared to us. It lets astronomers measure a distance for the most distant (and therefore oldest) objects in our universe.
See:
https://earthsky.org/astronomy-essentials/what-is-a-redshift/
See:
https://www.sciforums.com/threads/speed-of-light-redshift.136797/
According to Einstein's theory space and time are both distorted for objects moving relative to each other. There is another way to look at it that I like better. The Lorentz Aether Theory contended that all things were distorted when they moved. It did not warp space and time. All the distortion was in the material things that moved. Space and time remained constant. Things that moved experienced dimensional and time distortion.
This allowed H. Ziegler to assign a cause for relativity phenomena.
The Lorentz transformations are still used to calculate the distortions even in Einstein's theory. But the Lorentz theory is largely forgotten as is H. Ziegler. Doppler shifting happens because motion tends to shrink and stretch waves whether it is sound or light. The constant speed of light doesn't effect, noris it effected by the Doppler shift.
The statement "the speed of light is constant" refers to the
local speed with which light passes through any given point in spacetime, according to an observer that is also passing through that point. The significance of this caveat for cosmology is explained here:
What does general relativity say about the relative velocities of objects that are far away from one another?
For the present question, the "local" caveat is important because successive wavecrests are spatially separated from each other: successive wavecrests cannot both pass through the same spacetime point. So there is no contradiction between "the speed of light is constant" and cosmological redshift.
Whenever a
given wavecrest passes through a given point in spacetime, its speed is the usual constant 𝑐 according to an observer who is also passing through that point. In contrast, the relative "speed" between two spatially-separated entities (such as successive wavecrests) is not fundamentally limited; it's not even obvious how such a speed should be
defined. This is related to the possibility of a
cosmological horizon.
The speed of light is a constant for any and all frames of reference (FoRs).
We have three kinds of "shifts "
[1] Doppler red or blue shift:
Is the change in frequency of a wave [mainly sound] when the emitter or the receiver is moving, resulting in a change in frequency
[2] Cosmological red or blue shift:
This occurs when there is an expansion of space between the emitter and receiver rather then either the emitter or receiver moving.
Again as happens with Doppler, it results in a change of frequency, and occurs when we view very distant objects in the Universe
[3] Gravitational red shift:
This occurs when light is emitted from a dense gravitational source resulting in a reduction in frequency and subsequently "time dilation. "
Hartmann352