The universe is expanding faster than expected
quantamagazine.org
quantamagazine.org
As a side benefit, it can be soothing and nap inducing, but in the best ways. It's taken a couple attempts to get through some of the episodes, but in the end I find the explanations pretty amazing.
[1] https://www.youtube.com/user/howfarawayisit [2] https://www.youtube.com/playlist?list=PLpH1IDQEoE8QWWTnWG5cK... [3] https://www.youtube.com/watch?v=zKbZeUvPnWI&list=PLpH1IDQEoE... [4] https://www.youtube.com/watch?v=KE4SpkTOY1A&list=PLpH1IDQEoE...
(obviously by then we'll probably uploaded ourselves to computers and turned into totally different beings from what we are now, but the problem of trusting historic data still exists)
Our proof-of-work difficulty is trivial compared to what’s possible a few thousand years from now. So for people a million years from now to trust our records, they’d also have to trust people a thousand years from now not to simply have forged our part of the blockchain, which might be trivial for them given their hash power.
This is one of the key graphics: https://i0.wp.com/particlebites.com/wp-content/uploads/2020/...
You need ten times the mass of Mt. Everest in e=mc^2 energy (i.e. 100% efficiency) per 1 kg of mass to get to Andromeda at 1g acceleration/deceleration. [0]
For this reason I sincerely hope that conservation of mass/energy is not necessarily true. Nothing says it needs to be true other than that it empirically seems to be the case, and there is no known process that violates it, other than the Big Bang itself. If it can be violated, we have hope for the long long long term future.
Scary to think about this stuff.
[0] https://en.wikipedia.org/wiki/Intergalactic_travel
[1] https://en.wikipedia.org/wiki/Timeline_of_the_far_future
I mean, it's bleak perspective, but surely if we all write to our congressperson we can act now to prevent the heat death of the universe, right?
(1) https://www.preposterousuniverse.com/blog/2010/02/22/energy-...
Btw, Andromeda will collide with the Milky Way in about 4 billion years, so at least we won't need ten times the mass of Mt. Everest to get there...
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.)
So given ungodly eons of time (googleplex ^ googleplex * graham's number, etc. of years) something with a very, very, very low probability becomes inevitable.
Obviously the engineering side is also as impressive, being able to look at objects light years away and using the paltry several hundred million miles our Earth orbit takes us around the sun as a means of information discovery.
I'm quite apprehensive that the great filter lies ahead - that technology accelerates too rapidly compared to our wisdom and we end up nearly destroying ourselves. We're getting the ability to program life itself and to likely to democratize the ability to harness the forces inside the atom. Neither of which we're ready for as a species.
I've wondered about this. How can something which is having a cold death (zero kelvin) also have maximum entropy? But I am not at all an expert.
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.)
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.
I once ran a Traveller RPG exploration campaign where one of the systems they visited looked really odd on sensors. Just fuzzy clouds and clumps and ring formations of diffuse metallic debris. It turned out it was all paper clips.
The surprisingly addictive game is below.
This explanation is also the reason why finding basic life (say, bacteria) in the Solar System would be a cause for worry - if life evolved independently twice in the same star system, it would imply abiogenesis isn't that unlikely - thus strongly suggesting the great filter is still ahead of us.
Perhaps it's a combination of factors? Dolphins are social animals that appear to be capable of complex communications among themselves, but don't have hands to manipulate their environment the way we can.
Apes have hands, but don't have complex language.
Earth may only be special in being in a sweet spot between Mars (too small to hold an atmosphere or protect from cosmic radiation, hence no life) and Gliese 832c (with its low-orbit velocity of something like 15km/s, hence much less practical to put stuff in space).
That raises another issue of the planet having a magnetic core...
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!
(the story is - for once, literally - in the URL)
In addition, most humans learn to accept that there’s a good chance nothing they do will have an eternal effect on reality without needing any strange frameworks.
https://en.m.wikipedia.org/wiki/Ultimate_fate_of_the_univers...
absolute zero == zero kelvin == 0K
And that pale blue dot means nothing in a cosmic scale. Stop worrying and enjoy the incredible fortune of being alive.
The entire space is the infinitesimally thin surface which is the balloon surface, yet it is still expanding. Dots on that surface that used to be X distance apart are now > X distance apart in the future. That is all you would know and be able to measure.
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.
> 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.
Light intensity, gravity, and electromagnetism decay at 1/r^2, which is the same rate at which the area of the surface of an expanding 3D bubble grows. So each unit of light / gravity / electromagnetism could be seen as taking up a specific "patch" of expanding area.
However, the strong nuclear force decays much faster than 1/r^2. In fact, at some distance it goes negative and then decays back to zero. This could be (wild speculation) a force that propagates in many more dimensions. And it's not the only force that does this, the weak nuclear force also decays much more rapidly than 1/r^2 with distance.
I'm not a physicist and I would love nothing more than to hear what holes could be punched into this hypothesis.
I say that partly pejoratively, with a Sabine Hossenfelder sort of jocular but judgmental tone.
I’m personally excited to see how astrophysicists will take dark matter out of its darkness and cast it into intuitive lights, like some have with the rising dough analogy for cosmic expansion.
I'm not a physicist either but believe that this is a well-known idea.
Is it new space?
If I have a long ass string, is the string gradually getting longer, or will it break apart?
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.
How much is the space inside me growing on a daily basis?
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.
in these analogies, the balloon surface is made of rubber, or the stretchy thing is made of whatever stretchy things are made of
is space something tangible? what is it made of? what is its physical property? how can it stretch?
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.
Cosmology is a science in the broadest sense of being a field of human knowledge, but it isn't a science the way that, for example, physics is. It would better be described as a phenomenology[1]. I'm sure many will disagree with this factually more accurate description, because of the emotional role their ideation of science plays in their lives, but I believe it has greater intellectual utility and that a phenomenology can even be of greater value than an experimental science. This framing helps us understand that we should spend less time on trying to come up with dubious "natural experiments"[2] and more on the collection and publication of data in useful formats. And most of all, to be absolutely clear what the assumptions of the model are, even the most trusted ones, because they may well prove incorrect. But maybe this is just an issue in the popular press?
[1] "A description or history of phenomena." (not the definition from what we now call philosophy)
[2] Which aren't experiments at all because selection isn't control.
"All models are wrong - some models are useful"
The farther you venture from the verified useful section of a model (by which I mean - the farther you are from model predications that have been validated with observational evidence), the less you should trust it - ALL MODELS ARE WRONG!
And for most sciences - this isn't a huge deal - we can do lots of observational work easily right now. For cosmology... well - our observational data on the history of the universe it just astoundingly, mind-bogglingly, miniscule in comparison to the events we're interested in.
We have a sampling of data from 99.997% of the timeline — but the sample itself is extremely sparse.
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.
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.
No we have. See above: While redshift goes all the way up to infinity, in terms of cosmological time, there's still only 0.4 billion years between z=11 and z=∞, or about 3% of the age of the universe.
> if our models are correct
What is this whole discussion about?
You're arguing a stance that assumes the model is correct in a discussion about the risks of assuming the model is correct.
But trust me - our model is not correct.
In terms of time, yes. I was thinking in terms of observable spacetime volume. Even though there's only about 0.4 billion years before z = 11, the scale factor increased by a huge factor during that time (a factor of 100 from CMBR emission at z = 1100 to z = 11). That's a lot of spacetime volume from which we have no useful observations.
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.
How so?
> [cosmology] isn't a science the way that, for example, physics is.
Why not?
> I'm sure many will disagree with this factually more accurate description, because of the emotional role their ideation of science plays in their lives
Perhaps i will once i understand what you're going on about.
Since this is a site dedicated to intellectual curiosity, why don't you please present me with both?
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.
If I weren't a YEC, I would focus more on issues with radiocarbon dating and the assumption and not the observation that carbon isotope ratios are constant over the history of the Earth and ignore cosmology entirely. In fact I'm not clear on what YEC has to do with cosmology at all, it strikes me as being outside the scope of the arguments I've seen on the subject.
That would come as news to most theoretical physicists, who rarely set foot in a laboratory.
> we can ... produce quarks
How do you know we can produce quarks?
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?
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.
OK, again this would come as a surprise to most astronomers. Are you aware that we have sent spacecraft to other planets, and that humans have walked on the moon?
This is such a weird statement to make.
Let's not forget how we got where we are:
There are two forces active on macroscopic scales, electromagnetism and gravity. At the turn of the previous century, there were some inconsistencies in the way how electromagnetism fit into the rest of physics, which were resolved by the theory of Special Relativity. That shifted the problem to gravity, leading to the theory of General Relativity.
Friedmann then calculated what solutions General Relativity permitted under the assumption of spatial symmetry, the Friedmann models, which form the foundation of the cosmological standard model.
Most of the things I described so far happened before we even had confirmation that other galaxies existed (Friedmann published his 2nd paper in 1924, while Hubble resolved the 'Greate Debate' in 1923 by discovering Cepheids in the 'Andromeda Nebula', nowadays 'Andromeda Galaxy'). That shifted the focus on trying to figure out the model parameters that made Friedmann's model fit reality, and we've been at it ever since.
For both, we control almost nothing, we observe. To that end we build tools to observe. We want to observe to reject hypotheses or to get new insight on how nature behaves. In particle physics these are detectors, like the Super Kamiokande [1], which is just sitting there waiting for neutrinos to arrive from space. In the case of cosmology these are telescopes, radio telescopes and the like, waiting for photons, gravitational wave chirps to arrive from space.
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...
So Astronomy, as a "hard science", is around 100 years behind...what's the big deal with that?
So if we accept that everything is model, then what you are really advocating is using verbose descriptions in terms of a fairly established model rather than succinct descriptions in a more speculative model. Phrasing it like this makes it clear that there is a balance to be struck between pros and cons. Maybe we are erring on the speculative side, but you will have to make that case.
Edit: response from cygx is completely missing the point about control vs selection.
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.
Not really, no, because current cosmological models are not derived solely by looking at cosmological phenomena and coming up with phenomenological equations that describe them. (That is not to say that past cosmological models didn't do that; only that our current ones, roughly since the early to mid-20th century, don't.) They are derived by starting from laws of physics that already work in other domains, and seeing what those laws say about the universe as a whole.
> to be absolutely clear what the assumptions of the model are, even the most trusted ones
Cosmologists are clear about that.
> maybe this is just an issue in the popular press?
I think it is, since what I see in actual textbooks [1] and peer-reviewed papers is not at all like what you are describing.
[1] A good reasonably current textbook is Liddle's Introduction to Modern Cosmology.
We still have the qm to deal with. Just hope our model is even understandable. But at least that model is refutable.
As for correctness, I suspect we might never see 100% ever though. That is partly we are just have a century of major re-think about the fossilization of good science. And there is just happen our maths and mind can understand quite a bit of the universe.
There is always something to be found is good. And there is no guarantees we will know it all. That is good too. As it keeps the most important of science and philosophy alive:
Curiosity.
I can understand momentum from an explosion, but why would "space itself" expand because of an explosion?
I think the answer to this is “no”. If you could properly ascribe a “cause” to the universe’s expansion (and could prove it), you would definitely be first in line for a Nobel prize, at the very least.
And no, “dark energy” isn’t an answer; it’s a placeholder term for that which we don’t understand about the universe’s expansion.
Is the theory missing in some more fundamental way, like the lack of unification of gravity and quantum mechanics?
Take a rubber band- one of those big wide ones- and draw two dots 1cm apart. Then stretch the rubber band. The dots aren't moving, but the distance between them is growing. That's the big bang.
During the big bang, the distance between things increased at speeds faster than the speed of light. They didn't move at all (no FTL movement), just new space existed between everything. Still happening now, just at a slower pace (but a pace that seems to be increasing?).
Why? Answer that definitively and you'll earn yourself a Nobel prize or two.
Tesla challenged the notion of "curved spacetime" as illogical. People should have listened to him.
Edwin Hubble, who discovered redshift disparity, in his later life questioned expansion as the cause. People should have listened to him.
Many "alternative" physicists over the years and still today deny expansion, dark, matter, dark energy, black holes, and other imaginary phenomena that were invented to patch mis-matches between observations and mathematical predictions.
But its like spitting in the wind against the force of inertia as everyone gets taught these fantasies in school as though they are fact.
Do yourself a favor and look into some alternative cosmology theories. I've posted a list elsewhere in this thread.
Tim Minchin has a great line about this in one of his songs[1]
'Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved'
God is not a hypothesis, but more of an theory in the analytical rather than scientific sense. That is, God is a conceptual framework by through which other information is interpreted, not a source of testable predictions.
The reason it violates Occam's razor is because it is the strongest possible axiom you can include in a system. Why does anything happen? Because God. That's not a very interesting theory in that it can predict anything without offering any reasons or insights.
Not to mention, we have no direct observational evidence of such an entity.
The expansion of the universe, which is known since Hubble observations.Although interesting, it can be explained assuming the big bang (a big assumption of course, but with plenty of experimental evidence).
The second thing,, and the truly mind-blowing one, it is that not only the universe is expanding, but that the expansion is accelerating!! The usual date for this discovery is 1998. This is like throwing a baseball upwards and instead of it keeping its speed or de-accelarating (because of gravity and friction) the ball instead starts to move faster and faster. You would suspect the existence of an unknown energy that is powering this acceleration. That is what is called dark energy , which for all intents and purposes it is a giant black box with a question mark right now.
Astronomers Get Their Wish, and a Cosmic Crisis Gets Worse
There seems to have been a reframing of the "crisis" at some point, with "discrepancy in measurements" becoming "expanding faster than expected" which latter seems to imply the Reiss camp has the expansion rate correct compared to the earlier mystery as to which might be true and why they might be different but both sides on an equal footing otherwise.
Reader (me) misunderstanding, actual change in scientific perception or PR work by somebody?
There are various ways of accounting for it.
Also, these numbers always have error bars. They spend a lot of time coming up with sources of error and characterizing them in order to understand the data fully.
research "tired light". There are many competing theories.
Maybe read "Seeing Red" by Halton Arp, or anything by Tom Van Flandern. Or the book "Pushing Gravity". Or the electric universe guys, or subquantum kinetics, or modern mechanics, or plasma cosmology, or weber dynamics, or infinite universe, pretty much anything but the standard model actually.
be careful though, you might find yourself exposed to ideas that establishment status quo "science" tries hard to ignore.
Edit: changed mass decay to increase in mass. I misremembered the theory.
My reasoning was p = mv. m drops (through various processes in a star emitting energi==mass) and since p is constant, v increases.
Probably totally naive, but it is just a thought which has stuck around with me.
Also, how would this jibe with the parallax-based observations mentioned in the article?
If you're referring to the paper published by Wetterich in 2013 [1], it's because it isn't plausible at all: his theory requires that atoms are shrinking, and that is easily falsified by local experiments.
It would indeed be incredibly exciting if it would lead to a less silly explanation of the redshift than "universe is expanding".
It won't. There is no question about whether the universe is expanding. The issue is over how fast it is expanding; we have two different theoretical models that are giving different answers, and we don't understand (yet) how to reconcile them.
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.
Actually, turns out the math was right with a cosmological constant: that's how the accelerating expansion of our universe is explained. Einstein actually blundered twice: first by putting in the constant for the wrong reason (because he wanted a static universe), but then taking it out again when it turned out that reason wasn't the case (when the expansion of the universe was discovered).
If you just look at how to derive the Einstein Field Equation of General Relativity from first principles, the constant should be there; it isn't an add-on to General Relativity at all, it's part of it. It's just that there's no way to know from those first principles what its value is. That we had to figure out from observations.
Thanks for your point about the constant being actually required. I do not understand the math, but is this similar on how integrals always have a constant as a free parameter that need to be determined by other means?
I assume that Einstein originally set the constant to exactly balance the expansion, but later set it to zero. In bot cases you are picking arbitrar values but the actual value need to determined by empirical observeations.
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.
I wouldn't go that far: Rather, adding it doesn't violate any of the heuristics used to come up with the field equations or action. So to avoid bias, one should keep it around. However, in the absence of observational evidence to constrain its value, it's also justified to start any investigation with its value assumed 0...
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).
Why?
> we have two different theoretical models that are giving different answers, and we don't understand (yet) how to reconcile them.
Maybe... just maybe... that's because it isn't actually expanding, but there's some other, unknown effects at play?
It's about time we think outside the box. To me, as a regular programmer kind of person, the very idea that the universe is expanding makes about as much sense as dark matter. 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.
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.
This must be an incredible map.
I wonder what tech stack these astronomers use.
Hey, we need all governments' commitment to reversing it !
Guess what, other models do not require expansion.
Electric Universe (Thornhill, Birkeland, Scott, et al)
Plasma Cosmology (Alfven, Lerner, et al)
Infinite Universe Theory (Borchardt)
Recycling Universe Cosmology (Mitchell)
Subquantum Kinetics (La Violette)
Modern Mechanics (Bryant)
Push Gravity: (various, book: Pushing Gravity)
The Static Universe (Ratcliffe)
Steady State Universe (Hoyle, older)
That's a good start at least. ;-)
All of the above have one or more books written about them.
In other words, there is no smallest or largest.
There was never any beginning nor will there ever be an end. (If there was a beginning, what caused it?)
There is no outer boundary, limit, or end to the universe. If there were, beyond it would have to be "nothing". How can "nothing" exist, and what would be the border between "something" and "nothing"?
There is no "time", only matter in motion.
Matter is always divisible into something smaller, and composeable into something larger.
Matter in motion at scale n is perceived as a "force" at scales > n.
anyway, fun to think about!
A bit different than what I've always thought about as my personal infinite universe theory, but good food for thought and he makes the math work. Definitely worth the read!
Why would human intuition about what makes sense have any impact on physical reality?
3600 times faster than that ...
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.
I think the experience of time has something to do with focus. When you are young, or on lsd, your brain doesn't filter out all the signals and so time "slows down" as you suddenly need to process more. I know this is a well researched idea, just forget the actual terms for it.
When a year is 10% of your total lifetime, it seems a lot longer than when it's 2% of your lifetime.