Question about Cosmological expansion?

[MishaFreeman]

I've long wondered about an assumption that we have today and I've never found a direct answer to my question.

Presently we can observe that there is a direct proportionality between an object distance and the factor by which its light is redshifted. We deduce that this observation implies that the universe is expanding.

But is it really a scientific assumption to be made? Couldn't we equally extrapolate other plausible alternatives to explain an electromagnetic redshift without recurring to cosmological expansion?

I've addressed this issue a few other astronomers and cosmology enthusiasts that I know, and the conversation always ends with the reference to the CMBR, but the way I see it, the CMBR is an entirely different phenomenon, all the CMBR really implies is that there is an almost uniform radiation being emitted from all directions, hence we infer that some time in the past the universe was a concentrated mass. Or in other words, the CMBR tell us that there was likely a big bang, but it doesn't tell us anything about cosmological expansion. Isn't this correct?

 

[Mordred]

welcome to the forum.

The redshift isn't an assumption, rather its a confirmed result of expansion. If you doubt expansion is occurring ask yourself the following question, why can we see farther than the radius that the speed of light could travel? The only answer to that is expansion. Scientists have also used various forms of parallax (triangulation techniques) to confirm distances that have been measured by redshift.
So in truth we don't rely solely on the redshift measurements, rather each measurement made is confirmed through more than one technique. This allowed us to confirm the rate of change in the redshift formulas which is important as it also allows us to develop an apparent magnitude to luminosity relation. Which is another relation constantly under scrutiny. A while back several forum members assisted me in writing this article. In the section cosmological distance scale I briefly describe some of the other methods used in distance measures and the luminosity relation.

EXPANSION AND REDSHIFT
1) What is outside the universe?
2) What is causing the expansion of the universe?
3) Is expansion, faster than light in parts of the Universe, and How does this not violate the faster than light speed limit?
4) What do we mean when an object leaves our universe?
5) What do we mean when we say homogeneous and isotropic?
6) Why is the CMB so vital in cosmology?
7) Why is the LambdaCDM so vital to cosmologists?
8) Why are all the galaxies accelerating from us?
9) Is Redshift the same as Doppler shift?
9) How do we measure the distance to galaxies?
10) What is a Cepheid or standard candle

These are some of the common questions I will attempt to address in the following article
First we must define some terms and symbols used.

Planck constant: $h\ =\ 6.62606876(52)\ \times\ 10^{-34}\ J\ s$
Gravitational constant: $G\ =\ 6.673(10)\ \times\ 10^{-11}\ m^{3} kg^{-1} s^{-2}$
Speed of light in a vacuum:$c\ =\ 2.99792458\ \times\ 10^{8}\ m\ s^{-1}$

The parsec (symbol: pc) is a unit of length used in astronomy, equal to about 30.9 trillion kilometers (19.2 trillion miles). In astronomical terms, it is equal to 3.26 light-years, and in scientific terms it is equal to 3.09×10¹³ kilometers
Mpc=1 million Parsecs

Universe: A generalized definition of the universe can be described as everything that is. In Cosmology the universe can be described as everything measurable in our space-time either directly or indirectly. This definition forms the basis of the observable universe. The Hot Big Bang model does not describe prior to 10^-43 seconds. The LambdaCDM or $\Lambda$CDM model is a fine tuned version of the general FLRW (Freidmann Lemaitre Robertson Walker) metrics, where the six observationally based model parameters are chosen for the best fit to our universe.

The Observable universe is 46 Billion light years, or 4.3×10²⁶ meters with an age as of 2013, is 13.772 ± 0.059 billion years.
In the hot big bang model we do not think of the universe as starting from a singularity (infinitely, hot, dense point) instead measurements agree space-time as simply expanding. That expansion is homogeneous and isotropic. If you were to take a telescope and look at the night sky, no matter where you look the universe looks the same or homogeneous meaning no preferred location. As you change directions with the telescope you will find that no matter which direction you look the universe looks the same or isotropic meaning no preferred direction. These terms in cosmology are only accurate at certain scales. Below 100Mpc it is obvious that the universe is inhomogeneous and anisotropic. As such objects as stars and galaxies reside in this scale. This also tells us that there is no center of the universe, as a center is a preferred location. These terms also describe expansion. Expansion will be covered in more detail in the Cosmological Redshift section. Whether or not the universe is finite or infinite is not known. However if it is infinite now so it must be in the beginning.
Common misconceptions arise when one tries to visualize a finite universe such questions include.

"So how do we see farther than 13.772 billion light years?" The answer lies in expansion; as light is traveling towards us, space-time has expanded.
“If the universe is finite what exists outside the Universe?" If you think about this question with the above definition of the universe you will realize that the question is meaningless. One accurate answer in regards to cosmology is nonexistent.
"What makes up the barrier between our universe and outside our universe?" The short answer is there is no barrier. The CMB, (Cosmic Microwave Background) The CMB is thermal radiation filling the Observable universe almost uniformly, This provides strong evidence of the homogeneous and isotropic measurements and distances. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled enough, protons and electrons combined to form neutral atoms. These atoms could no longer absorb the thermal radiation, and so the universe became transparent instead of being an opaque fog. Precise measurements of cosmic background radiation are critical to cosmology, since any proposed model of the universe must explain this radiation. CMB photons were emitted at about 3000 Kelvin and are now 2.73 Kelvin blackbody radiation. Their currently observed energy is 1/1000th of their energy as emitted.

In order to measure an objects motion and distance in cosmology it is important to properly understand redshift, Doppler shift and gravitational redshift. Incorrect usage of any of these can lead to errors in our measurements.

Doppler shift and redshift are the same phenomenon in general relativity. However you will often see Doppler factored into components with different names used, as will be explained below. In all cases of Doppler, the light emitted by one body and received by the other will be red or blueshifted i.e. its wavelength will be stretched. So the color of the light is more towards the red or blue end of the spectrum. As shown by the formula below.

$$\frac{\Delta_f}{f} = \frac{\lambda}{\lambda_o} = \frac{v}{c}=\frac{E_o}{E}=\frac{hc}{\lambda_o} \frac{\lambda}{hc}$$

The Cosmological Redshift is a redshift attributed to the expansion of space. The expansion causes a Recession Velocity for galaxies (on average) that is proportional to DISTANCE.
A key note is expansion is the same throughout the cosmos. However gravity in galaxy clusters is strong enough to prevent expansion. In other words galaxy clusters are gravitationally bound. In regards to expansion it is important to realize that galaxies are not moving from us due to inertia, rather the space between two coordinates are expanding. One way to visualize this is to use a grid where each vertical and horizontal joint is a coordinate. The space between the coordinates increase rather than the coordinates changing. This is important in that no FORCE is acting upon the galaxies to cause expansion. As expansion is homogeneous and isotropic then there is no difference in expansion at one location or another. In the $\Lambda$CDM model expansion is attributed to the cosmological constant described later on. The rate a galaxy is moving from us is referred to as recession velocity. This recession velocity then produces a Doppler (red) shift proportional to distance (please note that this recession velocity must be converted to a relative velocity along the light path before it can be used in the Doppler formula). The further away an object is the greater the amount of redshift. This is given in accordance with Hubble’s Law. In order to quantify the velocity of this galactic movement, Hubble proposed Hubble's Law of Cosmic Expansion, aka Hubble's law, an equation that states:

Hubble’s Law: The greater the distance of measurement the greater the recessive velocity

Velocity = H₀ × distance.

Velocity represents the galaxy's recessive velocity; H₀ is the Hubble constant, or parameter that indicates the rate at which the universe is expanding; and distance is the galaxy's distance from the one with which it's being compared.

The Hubble Constant The Hubble “constant” is a constant only in space, not in time,the subscript ‘0’ indicates the value of the Hubble constant today and the Hubble parameter is thought to be decreasing with time. The current accepted value is 70 kilometers/second per mega parsec, or Mpc. The latter being a unit of distance in intergalactic space described above.
Any measurement of redshift above the Hubble distance defined as H₀ = 4300±400 Mpc will have a recessive velocity of greater than the speed of light. This does not violate GR because a recession velocity is not a relative velocity or an inertial velocity. It is precisely analogous to a separation speed. If, in one frame of reference, one object is moving east at .9c, and another west at .9c, they are separating by 1.8c. This is their recession velocity. Their relative velocity remains less than c. In cosmology, two things change from this simple picture: expansion can cause separation speeds much greater even than 2c; and relative velocity is not unique, but no matter what path it is compared along, it is always less than c, as expected.

z = (Observed wavelength - Rest wavelength)/(Rest wavelength) or more accurately

1+z= λ_observed/λ_emitted or z=(λ_observed-λ_emitted)/λ_emitted

$$1+Z=\frac{\lambda}{\lambda_o}$$ or $$1+Z=\frac{\lambda-\lambda_o}{\lambda_o}$$

λ₀= rest wavelength
Note that positive values of z correspond to increased wavelengths (redshifts).
Strictly speaking, when z < 0, this quantity is called a blueshift, rather than
a redshift. However, the vast majority of galaxies have z > 0. One notable blueshift example is the Andromeda Galaxy, which is gravitationally bound and approaching the Milky Way.
WMAP nine-year results give the redshift of photon decoupling as z=1091.64 ± 0.47 So if the matter that originally emitted the oldest CMBR photons has a present distance of 46 billion light years, then at the time of decoupling when the photons were originally emitted, the distance would have been only about 42 million light-years away.

Cosmological Constant is a homogeneous energy density that causes the expansion of the universe to accelerate. Originally proposed early in the development of general relativity in order to allow a static universe solution it was subsequently abandoned when the universe was found to be expanding. Now the cosmological constant is invoked to explain the observed acceleration of the expansion of the universe. The cosmological constant is the simplest realization of dark energy, which the more generic name is given to the unknown cause of the acceleration of the universe. Indeed what we term as "Dark" energy is an unknown energy that comprises most of the energy density of our cosmos around 73%. However the amount of dark energy per m³ is quite small. Some estimates are around about 6 × 10^-10 joules per cubic meter. However their is a lot of space between large scale clusters, so that small amount per m³ adds up to a significant amount of energy in total. In the De_Sitter FLRW metric (matter removed model)
this is described in the form.

H_o$\propto\sqrt\Lambda$

Another term often used for the cosmological constant is vacuum energy described originally by the false vacuum inflationary Model by A.Guth. The cosmological constant uses the symbol Λ, the Greek letter Lambda.
The dark energy density parameter is given in the form:
$\Omega_\Lambda$ which is approximately 0.685

The Doppler Redshift results from the relative motion of the light emitting object and the observer. If the source of light is moving away from you then the wavelength of the light is stretched out, i.e., the light is shifted towards the red. When the wavelength is compressed from an object moving towards you then it moves towards the blue end of the spectrum. These effects, individually called the blueshift and the redshift are together known as Doppler shifts. The shift in the wavelength is given by a simple formula

(Observed wavelength - Rest wavelength)/(Rest wavelength) = (v/c)

$$f=\frac{c+v_r}{c+v_s}f_o$$

c=velocity of waves in a medium
$$v_r$$ is the velocity measured by the source using the source’s own proper-time clock(positive if moving toward the source
$$v_s$$ is the velocity measured by the receiver using the source’s own proper-time clock(positive if moving away from the receiver)

The above are for velocities where the source is directly away or towards the observer and for low velocities less than relativistic velocities. A relativistic Doppler formula is required when velocity is comparable to the speed of light. There are different variations of the above formula for transverse Doppler shift or other angles. Doppler shift is used to describe redshift due to inertial velocity one example is a car moving away from you the light will be redshifted, as it approaches you the light and sound will be blueshifted. In general relativity and cosmology, there is a fundamental complication in this simple picture - relative velocity cannot be defined uniquely over large distances. However, it does become unique when compared along the path of light. With relative velocity compared along the path of the light, the special relativity Doppler formula describes redshift for all situations in general relativity and cosmology. It is important to realize that gravity and expansion of the universe affect light paths, and how emitter velocity information is carried along a light path; thus gravity and expansion contribute to Doppler redshift

Gravitational Redshift describes Doppler between static emitter and receiver in a gravitational field. Static observers in a gravitational field are accelerating, not inertial, in general relativity. As a result (even though they are static) they have a relative velocity in the sense described under Doppler. Because they are static, so is this relative velocity along a light path. In fact, the relative velocity for Doppler turns out to depend only on the difference in gravitational potential between their positions. Typically, we dispense with discussion of the relative velocity along a light path for static observers, and directly describe the resulting redshift as a function of potential difference. When the potential increases from emitter to receiver, you have redshift; when it decreases you have blue shift. The formula below is the gravitational redshift formula or Einstein shift off the vacuum surrounding an uncharged, non rotating, spherical mass.
$$\frac{\lambda}{\lambda_o}=\frac{1}{\sqrt{(1 - \frac{2GM}{r c^2})}}$$

G=gravitational constant
c=speed of light
M=mass of gravitational body
r= the radial coordinate (measured as the circumference, divided by 2pi, of a sphere centered around the massive body)

The rate of expansion is expressed in the $\Lambda$CDM model in terms of
The scale factor, cosmic scale factor or sometimes the Robertson-Walker scale factor parameter of the Friedmann equations represents the relative expansion of the universe. It relates the proper distance which can change over time, or the comoving distance which is the distance at a given reference in time.

d(t)=a(t)d_o

where d(t) is the proper distance at epoch (t)
d₀ is the distance at the reference time (t_o)
a(t) is the comoving angular scale factor. Which is the distance coordinate for calculating proper distance between objects at the same epoch (time)
r(t) is the comoving radial scale factor. Which is distance coordinates for calculating proper distances between objects at two different epochs (time)

$$Proper distance =\frac{\stackrel{.}{a}(t)}{a}$$

The dot above a indicates change in.

the notation R(t) indicates that the scale factor is a function of time and its value changes with time. R(t)<1 is the past, R(t)=1 is the present and R(t)>1 is the future.

$$H(t)=\frac{\stackrel{.}{a}(t)}{a(t)}$$

Expansion velocity
$$v=\frac{\stackrel{.}{a}(t)}{a}$$

This shows that Hubble's constant is time dependant.
Cosmic Distance ladder, also known as Extragalactic distance scale. Is easily thought of as a series of different measurement methods for specific distance scales. Previous in the article we discussed the various forms of Redshift. These principles are used in conjunction with the following methods described below. Modern equipment now allows use spectrometry. Spectrographs of an element give off a definite spectrum of light or wavelengths. By examining changes in this spectrum and other electromagnetic frequencies with the various forms of shifts caused by relative motion, gravitational effects and expansion. We can now judge an objects luminosity where absolute luminosity is the amount of energy emitted per second.

Luminosity is often measured in flux where flux is

$$f=\frac{L}{4\pi r^2}$$

However cosmologists typically use a scale called magnitudes. The magnitude scale has been developed so that a 5 magnitude change corresponds to a differents of 100 flux.
Rather than cover a large range of those distance scales or rungs on the ladder I will cover a few of the essential steps to cosmological distance scales. The first rung on the ladder is naturally.

Direct measurements: Direct measurements form the fundamental distance scale. Units such as the distance from Earth to the sun that are used to develop a fundamental unit called astronomical unit or AU. During the orbit around the sun we can take a variety of measurements such as Doppler shifts to use as a calibration for the AU unit. This Unit is also derived by a method called Parallax.

Parallax. Parallax is essentially trigonometric measurements of a nearby object in space. When our orbit forms a right angle triangle to us and the object to be measured
With the standardized AU unit we can take two AU to form the short leg. With the Sun at a right angle to us the distance to the object to be measured is the long leg of the triangle.

Moving Cluster Parallax is a technique where the motions of individual stars in a nearby star cluster can be used to find the distance to the cluster.

Stellar parallax is the effect of parallax on distant stars . It is parallax on an interstellar scale, and allows us to set a standard for the parsec.

Standard candles A common misconception of standard candles is that only type 1A supernova are used. Indeed any known fundamental distance measurement or stellar object whose luminosity or brightness is known can be used as a standard candle. By comparing an objects luminosity to the observed brightness we can calculate the distance to an object using the inverse square law. Standard candles include any object of known luminosity, such as Cepheid’s, novae, Type 1A supernova and galaxy clusters.

My thanks to the following Contributors, for their feedback and support.

PAllen
Naty1
Jonathon Scott
marcus

Article by Mordred, PAllen

 

[MishaFreeman]

welcome to the forum.

The redshift isn't an assumption, rather its a confirmed result of expansion. If you doubt expansion is occurring ask yourself the following question, why can we see farther than the radius that the speed of light could travel? The only answer to that is expansion. Scientists have also used various forms of parallax (triangulation techniques) to confirm distances that have been measured by redshift.
So in truth we don't rely solely on the redshift measurements, rather each measurement made is confirmed through more than one technique. This allowed us to confirm the rate of change in the redshift formulas which is important as it also allows us to develop an apparent magnitude to luminosity relation. Which is another relation constantly under scrutiny.

You misinterpreted my question, I didn't say that the redshift is an assumption.

The redshift is what we observe, but the redshift/distance observation is merely that, an observation, my question was why then do we assume that the redshift is an indicative of expansion? Can't there be other phenomenon that somehow naturally redshifts light over distance?

The only thing that the redshift tell us is that the further an object is, the farther its light has traveled, the more redshifted the light will be.

When we assume that the redshift implies that the universe is expanding, we are neglecting other factors that might influence electromagnetic radiation over long distances and naturally redshift it. Isn't it possible that other mechanism, other than dark energy, could stretch the wavelenght of light without recurring to universal expansion?

 

[phinds]

Can't there be other phenomenon that somehow naturally redshifts light over distance?

There have been some proposals. One was "tired light" which has been thoroughly discredited.

 

[MishaFreeman]

There have been some proposals. One was "tired light" which has been thoroughly discredited.

I've read about it before, the case for tired light was that it would imply blurring of long distance objects.

What I'm asking is, in a dynamic universe, there are an unfathomable amount of mechanisms that could cause a natural redshift of electromagnetic radiation, tired light was discredited, dark energy was created out of nothing to comply with predictions and observations, but surely there could be other potential alternatives. Isn't it right?

 

[Mordred]

Not really, there have been numerous attempts to explain cosmological redshift with other causes such as doppler shift, the article above notes that. I was getting the article when your reply was posted. Other attempts do not fit observational data, such attempts include forms of gravitational redshift, tired light, and forms of doppler shift. Even without redshift we can see expansion occurring visually by comparing distances measured at earlier times to measurements now. Though the change in the historical data is miniscule due to the length of time between those measurements they can be determined with reliable precision. As I stated above, scientist never rely on just one methodology. The redshift relation is a critical relation so is constantly under observational scrutiny and confirmation

 

[Mordred]

I've read about it before, the case for tired light was that it would imply blurring of long distance objects.

What I'm asking is, in a dynamic universe, there are an unfathomable amount of mechanisms that could cause a natural redshift of electromagnetic radiation, tired light was discredited, dark energy was created out of nothing to comply with predictions and observations, but surely there could be other potential alternatives. Isn't it right?

dark energy is a convenient popular pop media term for the cosmological constant, which is a vacuum pressure or stress energy tensor. You are correct that other factors can cause shifts, in truth separating one form of shift from another is a difficult task. Numerous attempts were made to define cosmological redshift as just another form of doppler shift. However the method used in one of the more reliable methods by A La Singe and Bunn and Hoggs, required numerous changes in observer and remeasure to work. The method does work but the cosmological redshift formula is far easier and flexible at the cosmological scale.

 

[MishaFreeman]

dark energy is a convenient popular pop media term for the cosmological constant, which is a vacuum pressure or stress energy tensor. You are correct that other factors can cause shifts, in truth separating one form of shift from another is a difficult task. Numerous attempts were made to define cosmological redshift as just another form of doppler shift. However the method used in one of the more reliable methods by A La Singe and Bunn and Hoggs, required numerous changes in observer and remeasure to work. The method does work but the cosmological redshift formula is far easier and flexible at the cosmological scale.

But isn't it the case that all observational data that we collect from the cosmos is in essence electromagnetic radiation, if light, or interactions between light sources is all we can see, then it would stand to reason that any mechanism that would naturally redshift light over distance would be indistinguishable to the same observer (us) from that we're proposing as Einstein's cosmological constant?

 

[Mordred]

lets clear up one thing to start with on dark energy, Redshift allows us to measure expansion. Dark energy is a proposed cause of expansion. If expansion were caused by some other factor we would still measure the same redshift. The problem of what causes expansion plaqued scientists for years since it was discovered. In fact dark energy was proposed as early as the late 1900's and was only recently accepted. Same goes for dark matter. Both the dark energy and dark matter proposals were met with a general denial and attempts to find other causes. MOND for example of dark matter. Torsion proposal from Poplowskii, for dark energy as some examples.

However all the alternate models that attempt to do away with dark energy fail when compared to observational data. You will find this article handy it discuses some of the alternative attempts.


http://arxiv.org/abs/1002.3966

 

[marcus]

...
Presently we can observe that there is a direct proportionality between an object distance and the factor by which its light is redshifted. We deduce that this observation implies that the universe is expanding.
...

It's not quite that simple. The main thing is the 1915 theory of GR, and the SOLUTION to the GR equation discovered in 1922 by a guy named Alex Friedman living in what is now St. Peterburg.

GR predicts dynamic geometry, distances and angles between things can change, if expansion gets started somehow it will tend to continue, but can be slowed down or sped up by various things.
GR has proven amazingly correct every time it has been tested at solar system scale and Earth orbit scale, and where possible also at astronomical scale (with neutron stars and gravitational lensing of light, multiple images caused by light bending).

So GR predicts expansion as one possible solution, and GR has proven so much better than Newton that people tend to trust it. Even before Hubble redshift was observed.

And then Friedman came up with his solution to GR which assumes more or less even distribution of matter to make things simpler to calculate and that has given an amazingly good fit to a huge amount of data.
So all working cosmologists use the Friedman equation (a simplified easier to use version of GR).
And Friedman equation totally predicts expansion or else contraction but that case is obviously not happening. That feature is an essential part of the model, which is basically a formula for the expansion rate H (or alternatively the contraction rate, in a different case).

Now 1922 was before Mr. Hubble observed the redshift pattern and way before people detected the CMB.

So you could say that expansion is a very natural thing which some people were already coming to suspect might be happening. The observation of redshift in the 1930s CONFIRMED this suspicion. And indeed the detection of CMB around 1960s was already EXPECTED. It had been predicted on theoretical grounds using the Friedman equation model which had built-in expansion.

So what you have is a really good theory (gr) of how gravity works---namely by geometry being a dynamic changing responsive field---and it has been checked hundreds of different ways not directly related to cosmological expansion. If you want to throw out GR then you should propose a better theory of gravity.

And our theory of gravity (as dynamic geometry interacting with matter) teaches us to EXPECT expansion as one possibility, contraction would be another but it doesn't fit the data.

and the particular solution to GR that cosmologists find gives the best fit, namely the Friedman model has been confirmed in different ways over and over. There is a list of independent types of observation that work together in support of expansion (which is what people were tending to expect anyway before some of those types of observations were made---like.eg. the abundance of various chemical elements, I don't remember all the details offhand.

GR is in the process of being replaced by a more up-to-date (quantum) theory of geometry, of which there are several rival candidates But the replacement prospects share basic features like expansion distances and wavelengths and existence of black holes and the bending of light etc. Those are well-established features which are part of legacy of GR, not likely to go away even when the theory of how geometry&gravity work is modernized.

 

[MishaFreeman]

It's not quite that simple. The main thing is the 1915 theory of GR, and the SOLUTION to the GR equation discovered in 1922 by a guy named Alex Friedman living in what is now St. Peterburg.

GR predicts dynamic geometry, distances and angles between things can change, if expansion gets started somehow it will tend to continue, but can be slowed down or sped up by various things.
GR has proven amazingly correct every time it has been tested at solar system scale and Earth orbit scale, and where possible also at astronomical scale (with neutron stars and gravitational lensing of light, multiple images caused by light bending).

So GR predicts expansion as one possible solution, and GR has proven so much better than Newton that people tend to trust it. Even before Hubble redshift was observed.

And then Friedman came up with his solution to GR which assumes more or less even distribution of matter to make things simpler to calculate and that has given an amazingly good fit to a huge amount of data.
So all working cosmologists use the Friedman equation (a simplified easier to use version of GR).
And Friedman equation totally predicts expansion or else contraction but that case is obviously not happening. That feature is an essential part of the model, which is basically a formula for the expansion rate H (or alternatively the contraction rate, in a different case).

Now 1922 was before Mr. Hubble observed the redshift pattern and way before people detected the CMB.

So you could say that expansion is a very natural thing which some people were already coming to suspect might be happening. The observation of redshift in the 1930s CONFIRMED this suspicion. And indeed the detection of CMB around 1960s was already EXPECTED. It had been predicted on theoretical grounds using the Friedman equation model which had built-in expansion.

So what you have is a really good theory (gr) of how gravity works---namely by geometry being a dynamic changing responsive field---and it has been checked hundreds of different ways not directly related to cosmological expansion. If you want to throw out GR then you should propose a better theory of gravity.

And our theory of gravity (as dynamic geometry interacting with matter) teaches us to EXPECT expansion as one possibility, contraction would be another but it doesn't fit the data.

and the particular solution to GR that cosmologists find gives the best fit, namely the Friedman model has been confirmed in different ways over and over. There is a list of independent types of observation that work together in support of expansion (which is what people were tending to expect anyway before some of those types of observations were made---like.eg. the abundance of various chemical elements, I don't remember all the details offhand.

GR is in the process of being replaced by a more up-to-date (quantum) theory of geometry, of which there are several rival candidates But the replacement prospects share basic features like expansion distances and wavelengths and existence of black holes and the bending of light etc. Those are well-established features which are part of legacy of GR, not likely to go away even when the theory of how geometry&gravity work is modernized.

So, in other words, cosmological expansion is proven, not directly from redshift itself but rather by the consistent verifications and implications of General Relativity?

But isn't it equally true that General Relativity has as of yet failed to consistently explain phenomenon on large scale structures, such as galactic rotation and cluster filaments?

General Relativity consistent explains all gravitational interactions in a localized scale, but there are observations that as of yet cannot be explained by General Relativity, not without the creation of special variables.

 

[marcus]

But isn't it equally true that General Relativity has as of yet failed to consistently explain phenomenon on large scale structures, such as galactic rotation and cluster filaments?
...

Sure! There are always puzzles and theories are always subject to revision. The Standard Model of particle physics is also enormously successful, but it also leaves some questions unanswered.
GR predicts the existence of DM, and that's good! It's good for it to make yet another prediction. Now we can check various ways to see if DM exists.

You don't throw a successful theory out (which so far has a track record of making predictions which were later confirmed) simply because it makes one more prediction.

Keep an open mind and wait and see whether or not DM is confirmed.

And if it is not, then what? There are proposed variant theories of geometry/gravity which don't involve DM, modifications that fit the galaxy rotation curves by other means but they also involve expansion.

I think 20 years ago there were people seriously working on "tired light" and other ideas for mechanisms which might produce comparable redshift without distances expanding. But each idea they came up with failed. So what you are talking about went out of fashion. Now there are researchers challenging GR and working on variants or replacements, but they have given up on non-expansive explanations of redshift (that was tried a lot and failed). The ideas for alternatives tend to aim at other goals, like getting rid of the need for DM. (even that has not been going so well) or like explaining the slight acceleration in some other way besides a cosmological constant.

 

[Dale]

Please stick to science. Not philosophy or religion.

 

[Chronos]

The only physics that explains redshift is either kinematical, gravitational, or cosmologically based - all of which are strongly confirmed by observational evidence. No alternatives have observational support to date.