I also like to consider the experience of time in relation to total life lived.

e.g. A month to a five year-old is the equivalent of a year to a 60 year-old.

So as we experience more time, we feel that time passes more quickly. Although I sure hope there is a plateau to it as we get older.

The most obvious interpretation of general relativity is in terms of B-theory of time: Spacetime is some 'pre-existing', 'eternal' thing over mich matter is distributed. This also fixes the geometry (things like lengths and angles) via Einstein's equations, which more or less state that energy-momentum ∝ Ricci curvature.

In our universe, that distribution comes in layers, ie there's a spatial slicing where the matter distribution appears homogeneous. In that sense, there is a priviledged slicing, which has the unfortunate side effect of making people forget the lessons of special relativity.

Now, the average density of matter changes from layer to layer, and, if our universe were described by the 'closed' Friedmann model, so would the (finite!) volume of the slice. That change is not arbitrary: The layers can be labelled by cosmological time, and with its increase, the average proper distance between galaxies increases as well. That's called the metric expansion of space, because in our idealized model, the metric (a thing defining distances and angles) within a given slice is just a scaled version of the one in a different slice.

Because all of the other theoretical models that attempt to explain the observed redshifts fail to match observations.

> Maybe... just maybe... that's because it isn't actually expanding, but there's some other, unknown effects at play?

Nope. Already been tried. Doesn't work.

> It's about time we think outside the box.

Cosmologists have been trying out of the box ideas for decades. All of them failed. That's why we have the models we have now: they're the only ones that survived that process.

> I get it, both make the calculations work out, but they're basically "we have no clue what this actually is, here's our best guess" kind of variable.

For dark matter, you are correct: we don't know what it is, and the term is basically just another way of saying "whatever we need to add to our model to make it match observations works basically like ordinary matter does in the equations".

But that is not true of expansion itself. All of the alternative models that have been proposed to eliminate the need for dark matter still have an expanding universe. The expansion itself is a much more solid conclusion than dark matter is.

Another way of looking at it, to add to my previous post: dark matter and MOND are thinking outside the box. They cover both possibilities for how to expand on our current theories: either (a) there's more "stuff" out there than our current theories of particle physics know about; or (b) there's more aspects to gravity than our current theory of gravity knows about. Consider whatever "unknown effects" you like: they will end up coming down to one of those two possibilities.

Thinking "inside the box" would be something like: it's far more likely that either the data is wrong or we haven't calculated the predictions of our current theories correctly than that our current theories, which have tons of experimental confirmation, are wrong. Historically, most of the time in science, when there's been a discrepancy between theories and data, that is how the discrepancy has ended up being resolved: either we've figured out something was wrong with the data, or we've figured out that something was wrong with how we calculated the predictions of our current theories.

The reason why cosmologists are driven to consider models like dark matter and MOND is that they have checked and double checked and triple checked both the data and our predictions from our current theories, and the mismatch hasn't gone away. So they are driven to consider "out of the box" ideas, and, as above, dark matter and MOND cover the possibilities.

Where do you get the idea that it "makes the calculations work out"? You got it backwards. The calculations showed that the universe must expand. Georges Lemaitre found that out. People were skeptical at first and said the calculations don't apply, but when the cosmic microwave background was discovered in the sixties, all but the most stubborn hardliners were convinced that the universe must expand and that there must have been a big bang, i.e. a singularity in the finite past.

The microwave background is basically a picture of the universe when it was a baby. There were no galaxies, only hot gas that just cooled down enough to turn from plasma to neutral gas which later clumped into galaxy clusters, galaxies and so forth. Because the speed of light is finite, we can literally see how the universe looked liked billions of years ago. And it looked hot, because it was compressed, and it looked young, because structures hadn't had the time to form yet. The gas composition also shows only light elements, because heavy elements need to be forged in supernovae. Young stars have lots of heavy elements, far away, old stars only consist of hydrogen and a bit of helium.

The tiny, tiny irregularities that we see in the cosmic microwave background match beautifully what we know about thermodynamics, statistical physics, quantum mechanics, general relativity and electrodynamics to a very high degree. It's marvelous, really.

> the very idea that the universe is expanding makes about as much sense as dark matter

The universe does not care about what makes sense to us. It just is. And the expansion is an observational fact.

On a side note, I find it a bit presumptuous to assume that a layperson knows better than legions of professional cosmologists. These are very smart people who work full time for the better parts of their lives on these problems while you clearly haven't put in the time to understand the fundamentals. They wouldn't come to the conclusions they come to if they didn't think they had merit. You can be curious about it, you can have questions about it (please do!), you don't have to understand it, but please trust experts on their opinion and show some humility and some respect. I'm sure you wouldn't appreciate a physicist who tried to tell you how to design your programs either. (We're known to write horrible software, just look at ROOT.) Imagine some amateur telling you "for-loops don't make sense to me, why don't you use GOTO". No offense, and sorry to say it like this, but it really bugs me a bit.

The good news (or bad depending on your take) is that we're nowhere close to being an advanced enough civilization to have any real stake in making predications about the future state of the universe other than "it will look basically the same as it does today", which we've conveniently used to position ourselves very accurately on the surface of the earth since the sextant and star charts.

https://en.m.wikipedia.org/wiki/Ultimate_fate_of_the_univers...

absolute zero == zero kelvin == 0K

If you have a model that makes testable predictions, it's science. Cosmology is not only science, but physics, making your original comment contrasting the two kind of nonsensical...

Because your comment I responded to was arguing something different - namely, the number of required assumptions.

Due to confinement, the existence of quarks rests on quite a lot of them.

That doesn’t mean there can’t be a higher spatial dimension that our observable universe is part of; we just haven’t observed it or can’t observe it.

I guess we’re sort of like ants crawling on the surface of an opaque, seemingly-indestructible balloon. We’re pretty clever ants, but we probably aren’t gonna peer inside the balloon anytime soon.

Unless the answer has changed in the last several years, no, the consensus is that it's expanding too fast and has too little mass (and thus gravity) to collapse again.

(Disclaimer: not even an amateur at this, at least not for 40 years.)

It depends on what you mean by "light loses energy over cosmological distances". See below.

> We already know that energy isn't conserved at those scales.

This suggests that what you mean by "light loses energy over cosmological distances" is that the universe is expanding while the light travels, and in an expanding universe, total energy is not conserved. That is true, and that is the redshift due to the universe expanding--it's not something different.

If, however, you are working from the hypothesis that the universe is not expanding, then energy is conserved even over cosmological distances, so I don't understand what you mean by invoking "energy isn't conserved" as an explanation for the redshift.

If you're referring to the "tired light" hypothesis, that's been known to be unviable for decades.

In our universe, however, the space that's created is the same as the space that was just there. It'd be like if the rubber band stretched, but didn't lose any density while it did so.

Insane, I know.

Unfortunately, it doesn't answer your more fundamental question of "if I make 2 marks in space, and then those expand apart, what is in the new space, and how do I know they really are farther apart?"

I have never heard a good description of this on human scales, only at stellar and galactic scales.

[0]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3181613/

[1]: https://en.wikipedia.org/wiki/Turbo_button

Yes, that's what I was trying to say. I didn't mean that including it mathematically in the equations necessarily requires one to adopt a non-zero value for it; you are quite correct that one shouldn't do that unless one has observational evidence to back it up (and cosmologists in fact didn't adopt a non-zero value until observational evidence required it).

If you are not a YEC, then the first thing I'm going to do is ask you why you think that cosmology and physics are different?

If you are a YEC then I don't need to ask you that question because I already know the answer. (It's because you have accepted the truth of the Bible as a foundational assumption. From this, the idea that modern cosmology is not science is a logically valid conclusion.) Instead, the first thing I'm going to do at this point is to ask you why you are trying to conceal it.

Also, if you are a YEC, I would be interested to know if you were surprised that I was able to correctly guess this.

And even though that seems like a large distance, it is a very small redshift compared to the total time the universe has existed. The cosmic microwave radiation background, for comparison, was emitted at a redshift of about 1100. So a redshift of 11 only covers 1 percent of the expansion of the universe since the CMBR was emitted.

> we've seen 97% of the universe's history

No, we haven't. See above.

> Including the microwave background in that pushes the number to 99.997%.

No, it doesn't, because even though we can see the CMBR, we can't see anything useful in between its redshift and the redshift of 11--not because there's nothing there, but because what's there is too distorted and faint to see. (The only reason we can see the CMBR is that it's black body radiation at a temperature we can independently predict from our knowledge of the physics of recombination, so we can tailor extremely sensitive instruments to looking for its precise signature.) So there is a lot of universe that we haven't seen.

You might as well be looking at a single frame of a movie and telling me you know the whole plotline because it happens to have the same pixels on screen for the whole shebang.

We have a sampling of data from 99.997% of the timeline — but the sample itself is extremely sparse.

Not really, no. It's a consequence of the assumptions made when the Einstein Field Equation (EFE) is derived from a Lagrangian using the principle of least action.

The assumptions are that the Lagrangian should be a Lorentz scalar (which is required of any Lagrangian) and that it should include no more than second derivatives of the metric. The Ricci scalar R meets this requirement and is the Lagrangian that was originally used by David Hilbert to derive the EFE (without a cosmological constant). But a simple constant (the cosmological constant) also meets the requirement, and therefore should be included in the Lagrangian; including it leads to the cosmological constant term in the EFE.

Einstein didn't include the constant at all in his original equation, published in 1915.

In (IIRC) 1917, he realized that his original equation did not allow a static solution for the universe as a whole. He also realized that including the cosmological constant term in his equation would be mathematically valid, and that if he picked just the right value for the constant, he could obtain a static solution for the universe. At that time, it was generally believed that the universe was static on large scales.

Then, later, when it was discovered that the universe is expanding, Einstein dropped the cosmological constant term. He later called including that term in 1917 "the greatest blunder of my life", because if he had just gone with his original field equation, without the constant, he could have predicted the expansion of the universe more than a decade before it was discovered.

> In bot cases you are picking arbitrar values

When the expansion of the universe was discovered, yes, it was already recognized that it is valid to include the constant in the Einstein Field Equation, so it couldn't just be un-included. Its value was just assumed to be zero since that was consistent with all observations that were known then.

In Einstein's original 1915 field equation, however, the constant wasn't "set to zero". It wasn't included at all; nobody even realized at that time that it was valid to include it.

Collecting a catalog of phenomenological observations is the defining characteristic of a phenomenology. One may feign hypotheses as one likes to fit that data, but that activity, valuable though it may be, is qualitatively distinct from the proper science of predicting the phenomena before they are observed. Of course phenomenologies can provide the necessary insight for predictive science. Like all human endeavors boundaries can blur.

Sadly much so called science is just computer aided trunk wiggling[1].

[1] https://quotefancy.com/quote/1342664/John-von-Neumann-With-f...

I agree that it's fair to describe cosmologists as a species of theoretical physicist, given the long standing connection between the observation of heavily bodies and the birth of the modern physical sciences. Still the relation is rather remote.

> How do you know we can produce quarks?

I personally don't. I'm relying on hearsay from a buddy with a PhD in particle physics who worked at Los Alamos. I don't think he's a liar so I'll take him at his word about what's possible in particle physics labs.

> Also, where does astronomy fit into your taxonomy? Was Newton doing science or phenomenology when he came up with the inverse square law? Plate tectonics? What about (drum roll, please) biology?

Astronomy is a phenomenology on account of nobody has a lab big enough to create stars in, or run any other astronomical scale experiments. Newton was doing natural philosophy, which in his case had elements of both what we now call science and phenomenology. Plate tectonics would strictly speaking be a phenomenology. Biology is a bloody wet mess that's mostly phenomenology, but there are disciplines in it which are mature science to the point of being engineering, like breeding domesticated plant and animal species.

There may not be a universal law which says everything will be OK, but there is one which says everything will be 0K in the end.

Isn't there some QM law that says that with infinitesimal probability anything can materialize at any point in space? Meaning that after everything has collapsed, you can (will!) still re-materialize somewhere in space. An infinite number of times!

It's also worth noting that the entropy can be very large, even if the temperature is absolute zero. (You just need a system with a lot of different ground-states that all have the same energy.)

That raises another issue of the planet having a magnetic core...