It all started with knowing the distance from the earth to the sun. Nobody had a clue until Richer and Cassini got within 10% in 1672. Then we nailed it down in 1769 with James Cook's voyage to Tahiti, the primary purpose of which was to observe the transit of Venus from the other side of the world.

From there if you know basic geometry, you can observe the nearby stars shift a bit when the earth goes around the sun (parallax), but that only works to about 10k light years.

Then, we discovered a couple unbelievably convenient astrophysics hacks: Cepheid variables (Henrietta Swan Leavitt, 1908) and Type 1A supernovae (Subrahmanyan Chandrasekhar, 1935, the namesake of the Chandra X-Ray Observatory). These allowed us to move out a couple more rungs on the ladder.

From there, the relationship between redshift and distance becomes significant and that takes us to the edge.

https://www.uwa.edu.au/science/-/media/Faculties/Science/Doc...

In the 3rd century BC, Aristarchus calculated that the Sun was between 18 and 20 times farther away from the Earth than the Moon, and proposed the Heliocentric model as a result. The true value is instead approximately 400 times. But it's incredible given that he didn't have lenses, the value of Pi, and that the Geocentric model was considered correct until 1800 years after his death.

https://en.wikipedia.org/wiki/Aristarchus_of_Samos#Distance_...

Nice video about the cosmic distance ladder by Terence Tao: https://www.youtube.com/watch?v=7ne0GArfeMs

He didn’t say nobody had a clue about heliocentrism.

Among total error of 3.1%, Cepheid reddening is 1.4% and the second largest source of uncertainty. SN Ia statistics is the largest with 1.9%. Rarely discussed in popular treatment is anchor distance, the third largest source with 1.3%. It is uncertainty of bottom lungs of the ladder, eg the distance to Large Magellanic Cloud before Cepheid and SN Ia are involved.

I was recently looking for a book which was basically your comment, but more in depth and covered the last couple thousand years. I wanted a to read about the history of astronomy - yknow, what was the state of the art in, say, 1350 or whatever. If you know of anything, I’d be super interested!

I think the pre-quantum mechanics era for physics and astronomy is super interesting. People figured out so much with such primitive tools, and it's all very accessible and easy to understand.

• From variations in the cosmic microwave background (CMB) which are the result of certain conditions in the early universe it is possible to figure out what the expansion rate should be now.

• From looking at very distant galaxies and noting how far away they are and how fast they are receding from us the expansion rate can be calculated.

Theory says that these should give the same expansion rate. When the rates were first found using those two methods they gave different results, but the error bars on both were large enough to overlap. People expected then that further refinement of both methods to decrease the error bars would converge to some common value.

That did not happen. Refinement of the CMB measurements got to 67 +/- 0.5, and refinement of the galaxy distance/speed method got to 73 +/- 1. Those do not overlap.

This non-overlap between the possible ranges given by the two methods is called the Hubble tension, and it is one of the most irksome problems in cosmology.

Possible explanations include:

• Some sort of error in how we measure the variations in the CMB.

• Some sort of error in the distant galaxy distance or speed measurements, which until the James Webb telescope were almost entirely Hubble telescope measurements.

• We're missing something in our understanding of the physics.

These new results add a bunch of data from the James Webb telescope, which observes in different wavelengths than Hubble. These results fit with the Hubble measurements.

They do not resolve the Hubble tension. What they do is remove most doubt that the distant galaxy results involve some sort of Hubble measurement error. I believe cosmologists are pretty confident of the CMB measurements, and so this will be interpreted as telling us that the Hubble tension is not just a problem with our measurements. There is either physics that we got wrong or physics we need to discover.

Redshifts can be way higher than 1 that in your scaling is a magic value.

https://en.wikipedia.org/wiki/List_of_the_most_distant_astro...

And in any case physics still needs to explain what happened to that blackbody radiation.

The amount of change millennials and the generations before and after us have seen is boggling.

I was out in country Victoria, Australia a few months back and the internet was TERRIBLE. I'm talking, JPG's loading line by line terrible. And this was on 'alleged' 4G.

I felt pretty nostalgic for all of 2-3 minutes before I started pulling my hair out. I feel nostalgic about it again now.

Wouldn't that smear the spectrum so that it no longer so matches the black body radiation of a single object of a set temperature?

As for smearing, IIRC black body curve has the central limit property

Out of curiosity: Can we actually differentiate between the expansion of space itself and the drifting of objects within it in the same direction?

With the direction in which we are traveling at c called “time”, and the other three which we're not moving through called “space”;

diverging rays having spatial velocity relative to each other equal to tan(alpha) i.e. flat projection;

and at any given point on the 4-sphere's 3D surface everything appear to be diverging in 3D because the emission is spherically symmetric;

and trajectories sufficiently paraxial to us fits the small angle approximation but those diverging widely has much more apparent perspective distortion,

with an error ratio of tan(alpha)/sin(alpha) = sec(alpha) = 1/Lorentz_Factor?

Exactly - the "universe is expanding" explanation has the benefit of having tons of physical evidence for it, whereas your explanation has the drawback of being meaningless not-even-wrong word salad.

Cf the last paragraph of https://en.wikipedia.org/wiki/Hubble_volume

Observations indicate that the expansion of the universe is accelerating, and the Hubble constant is thought to be decreasing. Thus, sources of light outside the Hubble horizon but inside the cosmological event horizon can eventually reach us. A fairly counter-intuitive result is that photons we observe from the first ~5 billion years of the universe come from regions that are, and always have been, receding from us at superluminal speeds.

Hubble ended his book Observational Approach to Cosmology[+] with the statement:..."if the recession factor is dropped, if redshifts are not primarily velocity-shifts, the picure is simple and plausible. There is no evidence of expansion and no restriction of time-scale, no trace of spatial curvature, and no limitation of spatial dimensions. Moreover, there is no problem of internebular material. The observable region is thoroughly homogeneous; it is too small a sample to indicate the nature of the universe at large. The univers might even be an expanding model, provide the rate of expansion, which pure theory does not specify, in inappreciable. For that matter, the universe might even be contracting."

[+] https://ned.ipac.caltech.edu/level5/Sept04/Hubble/paper.pdf

source: https://plasmauniverse.info/people/contributors.html

https://en.wikipedia.org/wiki/General_relativity#Relationshi...

planetary have very important applications!

But I'd argue not nearly as many as "Here are all the formula that govern a falling apple."

I guess if humans have will and are part of the universe, then the universe has it too, but I don't think that's what you meant.

Also, I forget off the top of my head, but there's some oddities depending on your chosen frame of reference when looking @ hawking radiation (skip to part about relativity of the vacuum) that I think could apply to the cosmic event horizon as well

https://youtu.be/isezfMo8kWQ?si=5m_L6JtZ7p7Ls6xH

https://en.wikipedia.org/wiki/Hubble%27s_law#Determining_the... (caption: "Value of the Hubble constant in (km/s)/Mpc, including measurement uncertainty, for recent surveys[54]")

I remembered the age of the universe as as 13.7 billion years, but I wasn't sure why that was.

Well, the initial WMAP results in 2003 supported an age of 13.7 billion years. Later results nudged this upwards to 13.8 billion years. Of course, all the results have error bars.

Not to mention the underlying philosophical assumptions, such that the "rate of time" has been constant across all of... time.

Aka: How do we know a "year" 13 Billion "years" ago bears any resemblance to one now? What would it mean for it not to?

[1] https://en.m.wikipedia.org/wiki/Noether%27s_theorem

It's almost an appeal to common sense. (which, admittedly, is often the best argument we have). IMHO there's often a bit of over-confidence among scientists about the universe being 13 billion years old and what happened during the Big Bang, if it just relies on such a common sense argument.

I know it's unscientific to suggest maybe laws of physics are not time invariant across such scales, because until we have a time machine we can't test this theory, but then flipping the argument (that laws of physics are definitely time invariant) is also technically unscientific -- we're basically assuming this without strong evidence.

This goes back to the old debate on the problem of induction in science, (see David Hume, Karl Popper, etc.) and I think it isn't emphasized enough in modern discussions that, perhaps, there's a small chance that these foundational concepts in physics could be invalid.

To the extent that relates to epistemology that would be more like anti-proof I guess? It does not prove that the laws are time invariant, rather it raises the bar to demonstrate that the laws are not time invariant because that means energy need not be conserved. You can not get one without the other without attacking even deeper fundamentals of modern scientific models. So you must either demonstrate the linked claims, which are pretty foundational themselves, or you must overturn basically everything; both of which demand very robust evidence.

We measure time in vibrations of a cesium isotope IIRC

https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...

It's great to know that say dating using carbon-14 decay is still useful over those time ranges on planet earth (I don't know if that is something we care about given that fossils don't tend to contain much carbon, but coal and oil deposits are around 400 million years old).

I don't want to imply that this is too small of a sample size, but I will imply that nuclear decay, and the movement of galaxies across the universe might be unrelated. Don't know. Not sure how we'd measure that. Supernova observations would tell us about nuclear fusion and it's limits. Does it tell us about nuclear fission? I don't know.

The laws of physics invariance under time is a core to our understanding. It would be very disrupting if we found otherwise.

If a computer chugs along doing { counter++; } at 1 clock cycle per clock cycle for 13.7 billion clock cycles, it will think 13.7 billion clock cycles have passed when counter is 13.7 billion.

On the other hand, if a computer chugs along at one clock cycle per clock cycle for 1 clock cycle, and reads &counter and sees 13.7 billion, it will think 13.7 billion clock cycles have passed.

Either way it's perfectly capable of introspecting it's source code and logically stepping back until the memory location was 0 to see how many clock cycles would have been required to reach it's current state, but that sort of reasoning is completely devoid of meaning without both perfect knowledge of what the true start state was, and a guarantee that no external influences have occurred.

Here in reality, we know neither the our start state nor our isolation level, but the hubris of many is too string to not at least try finding some logical step-back functions and iterating them until they don't know how to go any further, then proudly proclaiming that "The Start". (after all - how could it not be The Start, look, I can iterate the inverse of my step-backward function from then to now and it matches! QED!)

To exist in the state you describe - with neither start state nor isolation level measurable in any tamper-proof way - and to yet still dedicate one's life to observing and pondering the complexity of the resulting cosmos is, to my eye, laudable and beautiful.

Where you see hubris, I see humility. Everyone who attempts to expand the corpus of human understanding of cosmology knows that the endgame is somewhere short of perfection. And yet they are inspired to carry on. It seems to me that matters of state, economics, medicine, technology, and many other fields will benefit from a similar disposition.

Let's say I walk into a room and observe someone writing a tally mark on a chalk board once every second. I count 4x10^17 tally marks. I might assume that 4x10^17 seconds ago that same person entered the room and started tallying. I might even observe for the next 4x10^17 seconds they continue to tally. Heck I might even see a recorder going that when I play back at what I assume to be 1x speed, has chalk scratches at regular intervals for 4x10^17 seconds. I still don't have any actual evidence that they started those 8x10^17 seconds ago.

This talk about the "rate of time" doesn't make any sense to me. A second takes one second, and always has.

Isn't this like asking whether the length of a metre might have changed over time? Or the mass of a kilogramme? It looks to me like a category error.

How do we know that galaxies are accelerating away from us and not moving at constant speed? People often point to the observation that the further away a galaxy is, the faster it appears to be moving away from us, implying acceleration.

However, wouldn't we expect to see the same observation even without any acceleration? Imagine there are some objects in space all moving in random directions and speeds, relative to Earth. After long enough time, all objects will appear to be moving away from Earth, even if they were moving towards it initially. And after long enough time, the objects that move fastest should be farthest away, by the simple definition of speed!

In short, even if galaxies weren't accelerating, we would still see that the further away a galaxy is, the faster it recedes.

There's a more basic question: How do we know the galaxies are moving? It seems (I haven't seen any other response, like... ever) that we have one and only one way to measure the speed of galaxies: the red shift.

It's impossible to triangulate those huge distances and the time scale would also be a barrier, so no way of confirming the red shift calculations with a different method. That means that if the red shift was caused by any other effect, say the light "degrading" after millions of years of travelling the void, all the calculations would be invalid.

I've asked about this many times and the answers are in the line of "we don't know any other reason for the light to red shift" and "the current theoretical frame is consistent", even if there isn't any other measure to be consistent with.

There was a prediction (expansion is related to Big Bang) that the far away galaxies, being younger, would have a different composition. This prediction seems to be failing, but advances in instrumental could give us a more precise answer in the future.

The theory has to explain why the light gets redshifted, but does not get blurred, and the spectral lines do not get broadened. This severely restricts the type of interactions possible. Also the theory has to explain the consistency between redshifts within our own galaxy, to that of far-away galaxies.

The interpretation of redshift as velocity is also the primary reason cosmologists think the universe is expanding.

https://en.wikipedia.org/wiki/G%C3%B6del_metric

There are versions of this which do provide another reason for red shift.

Personally I keep this in mind as a means to free my thinking from a single narrative.

Galaxies are BIG. Andromeda is faint, but the same angular size as the moon. It's 2.5Mly away, but it's also 150kly across. Over a long enough time line you could do triangulation on it. In fact it's moving toward our galaxy, but very, very slowly compared with its diameter at 110kps. But yeah, in theory you could do triangulation on it over a very long period of time.

Astronomy is the paleontology of photons. You should take an astronomy course if you really want to know, but essentially a "ladder" was built of distances, starting with the very near and slowly building outward using various techniques and discoveries of physics as they became available. This is called the cosmic distance ladder. You start with stellar parallax, then after that you go farther with "standard candles" (particular types of variable stars). But then you have to get even further out, where you can no longer see an individual star, and then you rely on specific breeds of supernovae. Only then do you get to redshift, and compounding tons of data from step three seems to verify the redshift estimates. By the time you get to the Hubble constant, it was a huge rift between two communities over what was still a factor of two difference.

It's quite fascinating, but I can't really dump out an entire book into a comment.

More precisely, we see that galaxies started accelerating away from us a few billion years ago; before that they were decelerating (moving away from us but with the "speed" decreasing instead of increasing).

> People often point to the observation that the further away a galaxy is, the faster it appears to be moving away from us, implying acceleration.

That observation tells us that the universe is expanding, but by itself it does not tell us whether the expansion is accelerating or decelerating or neither. So you are correct that that observation alone is not sufficient to show that the expansion is accelerating.

What we look at to see how the expansion rate changes with time is a comparison of three pieces of observed data, galaxy by galaxy: redshift, brightness, and angular size. The relationship between these three quantities is what cosmologists use to construct a model of the expansion history of the universe, which in turn tells us things like what I said above, that the expansion has been accelerating for the last few billion years but before that it was decelerating.

I think the story is that dark energy is indeed created, in the new emptiness resulting from the expansion of space.

I believe it's supposed to be spacetime that expands, not 'space'. But it's beyond me to explain what that even means; as I understand it, spacetime refers to the whole Universe, across all of time. To 'expand' means 'to become larger over time'. But if the thing that's expanding includes time itself, then I'm bewildered.

-Galaxies are not accelerating, space is expanding.

> Imagine there are some objects in space all moving in random directions and speeds, relative to Earth...

-No, In your scenario then end result would be a most static average distance between all objects in the universe. As an infinite number of objects come from infinite distances, there would ALWAYS be objects in the neighborhood.

I think what you're imagining is a bunch of objects in a box, give them random vector and then remove the box. If they maintain course, all will eventually move outside the original box boundaries, and away from each other. (not the way the universe is).

They know space is expanding. The primary mechanism we know this is the speed with with we measure an object (moving away) is redshifted. Objects at the same distance from Earth, but opposite regions of space are moving at the ~SAME measured velocity.

There simply is no existing theory which can account for what we are seeing besides space expanding. I'm not big on thinking we understand it all, but in this particular measurement, there is basically zero doubt. Space is expanding, which has the affect of accelerating all objects in the universe away from you, with an acceleration relative to the distance. The more space between you, the more opportunity to expand.

But then, I'm back to what does "space is expanding" mean? What is doing the expansion?

In your explanation, I think we'd expect to see some very distant, very slow-moving galaxies moving toward us. And there may be some very fast-moving galaxies close to us that just started really far away. Objects would be entering our local universe from outside it, and that simply doesn't happen.

Thank you, but I suppose I'm not really questioning the big bang piece. My question was mostly in regards to the continued acceleration piece. Feel free to disregard the "in random directions" part of my original post.

I'm picturing more of an explosion in empty space. A firework or granade of sorts. Any individual dust/shard of the explosion still sees all other objects moving away from it and the rest of my question stands. But I suppose this would imply a "center" to the explosion, which I've also heard is not the case.

Theres a few other comments offering more clarity to the acceleration piece. Thank you everyone!

No, that's not what the big bang is.

> this would imply a "center" to the explosion, which I've also heard is not the case

That's correct. The big bang does not work like anything ordinary that you are used to imagining. The math is straightforward and unambiguous, but there is no good ordinary language description that corresponds to the math.

Another thing: suppose I point a laser beam to the space and by chance this laser beam never finds any kind of matter in its way, where is this laser going to? To an infinite void? Is it correct to say that stars radiate energy to the infinite then?

Photon will hit something, or will travel until it will be redshifted to obvilion, or will travel until end of the medium (photon is a wave, so it waves something).

That sounds suspiciously like postulating the 'ether'. Surely what a photon 'waves' is the electromagnetic field, which is not a medium, and which fills the whole of spacetime. There is no 'end of the medium'.

Sure, but to argue that this explains what we observe today, you would need to show that as of today it has been “long enough,” which is its own can of worms to open.

You might say “obviously it has been long enough for full sorting, because we observe a fully sorted data set of speed correlated with distance.” But that would be begging the question.

The surprising bit is that the far away stuff seems to be even more red shifted than that, so we're not just expanding, but the rate of expansion seems to be accelerating.

Not in our actual model of the universe, no. The redshift of light is determined by the spacetime geometry and the worldlines of the emitter and receiver. That is a general formula that works in any spacetime.

> Gravitational redshift is a thing

Not for the universe as a whole, no. Gravitational redshift is only meaningful in certain kinds of spacetimes, namely stationary spacetimes (which, roughly speaking, describe objects that either don't change with time at all, or which are periodic, like a rotating planet or star). The spacetime that describes our universe as a whole is not stationary and there is no meaningful concept of gravitational redshift.

> our model of gravity is incomplete

In the sense that we do not have a quantum theory of gravity, yes. But that does not affect anything under discussion here. Our current theory of gravity, GR, works fine for treating the expansion of the universe and whether or not it is accelerating.

Then why are there phantoms in the data that need dark matter and dark energy to make the supposed working model fit them?

The terms "dark matter" and "dark energy" are just names for, respectively, "stress-energy that acts like the matter we can see, but we can't see it", and "stress-energy that acts like a cosmological constant". Neither of those things poses any problem for GR, since both types of stress-energy are allowed for in the theory.

"Dark matter" poses a problem for particle physicists, who have so far been unable to find any fundamental particles that would produce the observed properties. "Dark energy" only poses a problem if for some reason you don't like having a nonzero cosmological constant.

It's clear to me why we haven't made any new discoveries in cosmology in the past two decades. It's this exact attitude of "the model is the truth". All models are wrong. The data can help you improve it, but you have to at least want to improve it.

And trust me, all scientists know that all models are wrong. This isn't some unique insight that is beholden to amateur scientists on the sidelines.

Is that true?

At least for my hobbyist understanding of the progress of cosmology, quite a lot seems to have happened in the past two decades. Confirmation of the Higgs Boson at CERN [0] kept me up all night to watch the press conference; I found it extremely exciting. (Maybe you count this strictly as observational particle physics and not cosmology, but I might appeal for it to be allowed in the context of your critique).

And what of TFA? Isn't what we're reading now a new discovery in cosmology?

What about the rush of exoplanet discoveries?

What about the dramatically different galactic properties now observed in increasingly strange corners of the observable universe (including some which perhaps give insight into some of the properties of "dark matter" or whatever it ends up being)?

0: https://home.web.cern.ch/news/news/physics/new-results-indic...

How do you propose they do it differently?

What evidence do you have that what they are doing now doesn't work, and does the all the evidence of how they work support your hypothesis?

Be detailed, because your comment just has some motivational speaker nonsense but no depth. For example, in the last 20 years cosmology has:

+ Refined its model of stellar formation based on observational data of the number of planets found observationally, and used this to validate and invalidate several model adjustments.

+ Observed galaxies that appear not to have dark matter, and by their existence and behavior validate some theories of dark matter, and validated others, which predict such galactic behavior. (e.g. some theories attempting to update gravity).

+ Run simulations of stellar and glactic formation that predicted structures in the universe that were later observed.

Everywhere they look they are finding things the models don't explain well, and refining the models - that is literally using the data to improve the models.

If you think you can come up with something better, then do it - all you gotta do is make up some mumbo jumbo and write down any old equation. It probably should:

- provide the same results as were observed when the plugging in the experimental parameters of existing experiments.

- explain "wierd stuff" in the data that existing models couldn't.

- predict future observations of the known phenomena with the same or better accuracy as the old model

- predict currently unobserved and unpredicted phenomena

Go ahead and take a stab real quick - I'm sure you can do it. I mean Gallieo did it, so did Newton and Einstein. Next up is willis936.

I didn't even realize how many people held that belief til that article about how the universe isn't as big as we thought

The conclusion that the expansion is accelerating was a quite recent result: 1990s, I believe. It's based on careful measurements of supernova explosions of a type with computable intrinsic brightness in increasingly distant galaxies, and the exact pattern seen in their apparent redshifts vs. apparent brightness. It was a shocking discovery when it came out, with two separate teams announcing the result pretty much neck and neck. There's also independent and compatible evidence for acceleration from the exact pattern of variations in the temperature of the cosmic microwave background seen at different points in the sky.

If the universe were expanding uniformly, we would see galaxies moving away from us. The further away they are, the faster they would move. Distance and velocity would have a linear relationship, with the Hubble Constant as the scaling factor.

But what we actually see is that, if we measure precisely enough, galaxies further away are moving faster than that. The conclusion is that the expansion is accelerating.

No, you have it backwards. Accelerating expansion means, roughly speaking, that we see galaxies further away moving away slower than a "uniform" expansion would predict. Remember that we are seeing galaxies further away as they were a longer period of time ago--so "accelerating expansion" means the universe was expanding slower then, when the light was emitted, than it is now.

Actually, though, we don't observe the distance to a galaxy directly. We infer it from other observations. The actual observed quantities are redshift, brightness, and angular size, and the relationship between those three observed quantities is what tells us the expansion history of the universe.

What if the cosmological constant (from which, if I understand correctly, the Hubble constant is derived) is not constant? Could a changing comological "constant" explain the discrepancy between the various methods of calculating the Hubble constant?

From Wikipedia [0]: The cosmological constant "was revived and reinterpreted as the energy density of space, or vacuum energy, that arises in quantum mechanics. It is closely associated with the concept of dark energy."

Do vacuum energy and dark energy have to be constant?

[0] https://en.wikipedia.org/wiki/Cosmological_constant

Having a dynamic dark energy is a proposed solution to the Hubble tension: https://arxiv.org/abs/2103.01183

That is, besides a whole family of potential other solutions:

Dark energy in extended parameter spaces [289] Early Dark Energy [235] Early Dark Energy [229] Dynamical Dark Energy [309] Phantom Dark Energy [11] Decaying Warm DM [474] Metastable Dark Energy [314] Dynamical Dark Energy [11, 281, 309] Neutrino-DM Interaction [506] PEDE [392, 394] GEDE [397] Interacting dark radiation [517] Elaborated Vacuum Metamorphosis [400–402] Vacuum Metamorphosis [402] Self-Interacting Neutrinos [700, 701] IDE [314, 636, 637, 639, 652, 657, 661–663] IDE [314, 653, 656, 661, 663, 670] IDE [656] Self-interacting sterile neutrinos [711] Critically Emergent Dark Energy [997] Unified Cosmologies [747] Generalized Chaplygin gas model [744] f (T ) gravity [814] Scalar-tensor gravity [856] Galileon gravity [876, 882] ¨Uber-gravity [59] Modified recombination [986] Power Law Inflation [966] Reconstructed PPS [978] Super ΛCDM [1007] f (T ) [818] Coupled Dark Energy [650]

The cosmological constant does in fact have to be constant within the constraints of general relativity. The mathematical machinery of GR only allows two parameters: Newton's constant G, representing the coupling of matter to gravity, and the cosmological constant. Both have to be true constants, numbers with a unit.

However, this is only true of the most basic theory of dark energy, where you directly add a constant to the general relativistic lagrangian. More complicated theories, like, for example, quintessence, involve adding new dynamical fields to the theory. The "effective cosmological constant" associated to such theories, quantifying the effect these fields are having on the expansion of the universe, can dynamically change over time, and some of these theories are proposed to solve the Hubble tension. To be clear, although none of these theories are fringe or pseudoscience, they haven't been accepted as a final explanation either, resolving these issues is still a work in progress, at this point they're all simply interesting hypotheses.

[0] 'theJWST just made the "Crisis in Cosmology" WORSE' https://www.youtube.com/watch?v=hps-HfpL1vc&t=858s

I think the main point of resistance is the incompatibility with observations. All Tired light models have been falsified: https://en.wikipedia.org/wiki/Tired_light#Specific_falsified...

If you're aware of a model that can fit some or all of our observations, please share it!

https://www.youtube.com/playlist?list=PLpH1IDQEoE8QWWTnWG5cK...

The title should be "Hubble Tension almost certainly not caused by measurement error."

Is there an implicit caveat asterisk on all such statements that is like “as far as we can possibly tell from the data we have” and the reality is we really, really don’t know for sure?

- We have multiple observatories on and around the planet - The Earth is moving around the Sun - The Sun is moving around the centre of the galaxy - The galaxy is moving towards the great attractor, etc

The Copernican principle states that humans, on the Earth or in the Solar System, are not privileged observers of the universe, that observations from the Earth are representative of observations from the average position in the universe. This has been tested in various ways: https://en.wikipedia.org/wiki/Copernican_principle#Tests_of_...

A nice PBS spacetime video about the topic: https://www.youtube.com/watch?v=q-6oU3jXAho

You can also ask how do we know that the laws of physics haven't changed over time. We don't. But at some point you have to make a few basic assumptions in order to have any hope of making scientific progress.

https://youtu.be/TGpjGVNVYEg?t=397

It's beyond my depth to explain why he's wrong.

[1]: https://iopscience.iop.org/article/10.3847/2041-8213/ad1ddd

Expansion of the universe: https://en.wikipedia.org/wiki/Expansion_of_the_universe :

> While objects cannot move faster than light, this limitation only applies with respect to local reference frames and does not limit the recession rates of cosmologically distant objects

Given that v is velocity in the opposite direction, and c is the constant reference frame speed of light; do we account for velocity in determining whether light traveling at c towards earth will ever reach us?

  v - c c
  v + c > c if v>0

How far away in light years does a mirror in space need to be in order to see dinosaurs that existed say 100 million years ago?

As far as I know this is not necessary because the speed of light is constant regardless of the velocity of both the source and the observer (this is Einstein's special relativity: https://en.m.wikipedia.org/wiki/Special_relativity)

No, because the speed of light is constant for all observers. From our frame of reference on earth, light from distant receding galaxies is always moving towards us at exactly the speed of light, c. Those galaxies also observe the light moving away from them at exactly c.

That seems contradictory and unintuitive, that two observers moving away from each other both measure light moving c relative to themselves, but that’s reality.

It’s another measurement that changes: if c is always constant, then it must be the passage of time and the distance travelled that we observe differently.

Light is a wave. Photon is a complex thing (Hopfion?), but it's a wave, so it waves something, a medium. Speed of wave propagation in a medium is constant. IMHO, it's intuitive.

Nothing in the universe can travel faster than the speed of light. This does not hold for the universe itself. It can and does expand faster than the speed of light, using specific reference frames (i.e., big enough).

So, space can increase FTL. Particles do not travel faster than light tho, that is nonsense.

From the article:

"The bottom line is that the so-called Hubble Tension between what happens in the nearby Universe compared to the early Universe’s expansion remains a nagging puzzle for cosmologists. There may be something woven into the fabric of space that we don’t yet understand."

Before we know better it can be just that spacetime was expanding at a different rate (we still would need at least one another Planck that operates in roughly same range to confirm this).

Hubble wavelenght range - 0.1 to 0.8 μm Webb wavelenght range - 0.6 to 28.3 μm Planck wavelenght range - 330 to 10000 μm

My understanding is that Planck was observing photons that have happened much more earlier.

That's quite a strong claim. I'm skeptical. Sources?

> Have physicists looked into this?

They shelved it right next to "God Made The Universe" in the "Unfalsifiable Propositions" section, under the title "Grad Students Made The Universe."

Software engineers presenting weird pseudo science as serious physics is one way this manifests.

I could be wrong.

It's also astounding that celestial objects emit enough photons that thousands per second travel within the arc that goes the distance from the star to somewhere inside the radius of your pupil.

And if this wasn't the case, then we'd never even be aware of any of this.

We haven't detected dark matter or dark energy outside of their visible effects on the larger universe. Maybe we can't interact with it with baryonic matter. A dark energy engine made out of dark matter would be invisible.

So... the negative is just a convention to represent work done against the "field", and positive is work done by the field? ie more of a vector than actually being negative energy? I think I get that bit now.

So now I'm wondering if that still applies to an expanding universe vs eg a rocket leaving earth. If things aren't moving further apart by work (ie force x distance) being done against their gravitational fields so much as space time expanding, is there an increase in potential energy? And then if objects are moving apart faster than escape velocity, could that still be seen as increasing potential energy?

I think I'm confusing myself further...