This educational paper [0] titled "Expanding Confusion" (2003) is a classic on the general topic and well worth the read.
Holding the two concepts of an accelerated (!) (in terms of objects [1]) expanding universe and the fixed finite speed of light simultaneously in one's Euclidean head can be dizzying, so be prepare to draw and enjoy the hard earned manual labor of counterintuitive conclusions.
Took me a minute to wrap my head around it, that explanation isn't worded that clearly, but then I got it.
That happens because time is a factor in how light from different parts of the object will reach the observer. Light from its far side takes longer and in that time the object continues to move. You can see behind the object, because its rear end moves out of the way of the light coming from itself during the travel time of that light.
How does the rear end move out of the way? Wouldn't it be blocking the light? It's not like the object could move out of the way faster than the speed the light it traveling at (ftl)
That's amazing. I've watched lots of videos about length contraction and I don't think any of them ever mentioned this (the shape of an object moving a near light speed won't change to an observer, it will just appear as if it had rotated instead of being "squeezed" as every video about this seems to imply)!
You rather showed the opposite. While interesting, this video only explains length contraction, not the Terrell–Penrose effect. In this video, the passing spaceship would appear to be rotated to the observer, not just contracted, as, to quote Penrose via Wikipedia, the light from the trailing part reaches the observer from behind the [spaceship], which it can do since the [spaceship] is continuously moving out of its way"
This nascent series on YouTube , Hypercubist Math, sets out to make four dimensions intuitive to our three-dimensions-accustomed brains. Baseline is just basic calculus, which the inaugural video provides in context:
I actually think the opposite is true. The way I've heard it phrased and explained that makes the most sense to me is "everything moves through spacetime at the same rate" - it's basically the clock speed of the universe. It's just that if you move faster in a space dimension that your relative movement in the time dimension slows down.
It only seems weird to us because our senses and minds evolved in an environment where things we can perceive never differ by relativistic speeds.
While I do like that intuitive explanation, it's lacking in describing all other aspects of the universe.
Like, how the energy required for an object with mass to approximate the speed of light in spacial dimensions goes to infinity, even though it's already traveling at that speed through spacetime.
Sure, one simple sentence is not going to explain the universe. But, at least from the simple relativity side of things, essentially everything falls out of (that is, it's a consequence of) that simple sentence. I.e. starting from that you can derive other consequences. E.g. "how the energy required for an object with mass to approximate the speed of light in spacial dimensions goes to infinity" is actually a direct consequence of that statement: every amount of energy you push into an object with mass causes it to accelerate, but due to the essential "clock speed of the universe", that acceleration is less and less as you approach the speed of light, and thus it takes an infinite amount of energy to reach the speed of light. Another way to think of it is that if it took anything less than an infinite amount of energy to reach the speed of light, then the speed of light couldn't be the universal speed limit, because you could add more energy that would accelerate it further.
On the other hand, my understanding is that quantum mechanics is another beast entirely, and one of the biggest problems in physics, and to developing a "theory of everything", is to unify quantum mechanics with general relativity.
Although nothing will explain everything, still it's fine with the first point: increasing the rotation vector of momentum in spacetime increases mass. The rest follows, since you know that the more mass, the more energy required to accelerate still more.
But if you are interested, a significant amount of the basics of quantum mechanics follow directly from Fourier transforms -- which unfortunately are harder to self-study than spacetime rotations.
I came up with a variant of this that extends the motion vector into “matter” dimensions. Then the logical consequence is that the more matter you have, the less of the unit vector is available for movement in time == time dilation due to mass.
Similarly, only vectors with zero length in the matter direction can have unit length in the space/time direction. This is the “only massless particles move at the speed of light” rule.
FWIW there are other answers in that Stack Exchange that, in my opinion, give a better description of the situation, and in my opinion the primary objection in that particular answer is the definition of "move". Fair enough, but I think it's still a helpful description for laypeople who are not fully versed in the math.
Perhaps nit picky, and I know you were joking, but I think this is the wrong way to think about it. It's not that the other behavior is undefined, it's you essentially have all of these functions that use "C" in their definitions, and then you have "#define C ..." in a header file somewhere.
Nah, its real simple. SR just comes about because you want to keep chemistry working the same on a rocket doing 99% of the speed of light as it is at rest.
Working out all the implications becomes very complex.
But then you probably wouldn't have life to observe it if the simple rules didn't have complex emergent behaviors.
A limit to the speed of causality makes physics so much simpler. Without it you'd need to factor in the interaction of every particle with every other particle in the universe.
Don't you need to do that anyway, because of gravity?
Wouldn't the causality speed limit just cause those gravity interactions to arrive with a time delay rather than being instantaneous?
Which means that to simulate the universe you essentially have to keep a history of how gravity is propagating, which requires keeping more information than if interactions were instantaneous?
In a sense perhaps this applies to light too, because since it has a finite velocity now you have to keep track of how all the photons individually propagate through spacetime, whereas if light traveled instantly this would not be necessary?
EDIT: The advantage I see in a speed limit is that you should be able to compute what happens in a point of spacetime based only on the information that is around that point (which still might have come from any or all other particles in the universe, mind you). For me, this emphasizes how important locality must be and it basically converts the popular "spooky action at a distance" claims into nonsense to me.
I guess that's why I'm a fan of the Many Worlds Interpretation.
Since they have a speed limit you have to keep track of all gravity waves associated with all particles of the universe throughout all time and space.
So all particles still interact with all other particles, all the time, it's just that they do it with a time delay.
If there wasn't a speed limit it would be much simpler because all gravity interactions would be instantaneous and you wouldn't have to keep track of gravity waves.
If light acted instantaneously you would have to calculate the effect of it's rays everywhere all at once, which I think is quite expensive given how vast space is.
However, since the speed of light is miniscule compared to the size of the universe you can ignore all but the most local interactions, and just schedule a computation sometime in the future when you know that the light vector will interact with something.
While instant calculations would perhaps make for a simpler system conceptually, the speed limit and locality principle ensures that less processing power is needed (at the cost of a lot of memory).
As someone already mentioned, you would have to account for all the interactions with light -> everywhere at once.
With a effective speed limit to space-time, you can "localize" the computation to the spaces where light has reached. And who knows, maybe light can't travel forever, it might just disappear after crossing some distance we still haven't measured (how we'd do that, who knows).
Giving yet another evidence to the "grand simulation" theory. "The universe" is just a group of simulated worlds connected by interacting photon particles (light).
Right, that's what I meant when I said that the advantage of a speed limit is locality. It allows you to compute the next state of a point in spacetime based only on the points around it.
But my point was that this also makes the universe more complex than an alternative fictional universe where information can be accessed instantaneously across any distance (which still allows for distributed computation, if synchronization or lazy computation is possible).
Possible alternative, but what would be a factor of locality in such an universe? And how would the universe store the infinite "light matter" in it's "memory", since light particle beam being instant means that it has no limits to where it can reach, and will grow depending on distance traveled (which is infinite)?
Some processes that are outside of scope we can sense seems like a too cheap explanation.
We actually don't know and can't really know what simulates the universe.
But we can deduce from various cues that it is being "simulated".
The Double slit experiment is one, experience of deja-vu another, dreams that partly manifest in reality after some time, the apparent speed limit of light, out of body experiences, the fact we are the only local top intelligent lifeform in this part of galaxy, etc...
All signs of processes and memory "bugging out". Except the last one, that one seems to be by design.
You're conflating a lot of things that ought not be conflated
Where are mesoscale "bugs" of cells or the like?
Or is it only on the level of quantum effects and errors of the brain, wherein you're overlaying quantum jargon onto another iconprehensibly complex function?
What’s unique about gravity with what you’re talking about? The Coloumb force also applies between every pair of electrically charged particles, right? And with the same inverse-square function of distance?
Tangential question: is the speed of causality coincidentally the same as the speed of light? Or are they the same because of some underlying principal that inherently links them?
Correct, the speed of light is actually the speed of causality, we just so named it after light because that was what we first discovered as going at c, but many other things in the universe also due because it's the same underlying principle. That is why gravitational waves also travel at c, ie if you removed the sun instantaneously from the solar system, the Earth will continue to orbit for 8 minutes, as that is how long light (and the gravitational force) takes to get from the sun to the Earth.
Not an expert, but can't resist chiming in anyways... One thing to think about it what exactly is causality? There'll be tons of different definitions, but they'll all have one thing in common, events that cause "later" events, and/or events that depend on "earlier" events.
And in a physics sense what is an event? An interaction between two things, right? And since there doesn't exist any force that can interact instantaneously across distance, the speed limit of causality is equal to the speed of our fastest forces.
If we discovered some scifi-esque Tachyon particle that traveled at 2C, we could no longer say the speed of light is the speed of causality.
I'm not an expert either, but I like to think that the speed of causality is the speed at which a piece of information (e.g. a particle, a gravity wave, etc) is traveling through space.
So the speed of light / gravity is essentially the maximum speed of causality, because nothing can travel faster than that.
EDIT: Or you can think in terms of how information propagates through spacetime. In this point of view, the speed of causality is always the speed of light, for everything, including particles with mass.
Yeah that's what I was trying to work towards. Basically that causality is an abstraction, or at least a "higher level" idea. And if we look at the components of it, we can see that interaction between two things is a central part of it. And an interaction between two things in our universe has a maximum bound of the speed of light (and gravity and so on). The speed of causality is just the speed of the fastest thing.
It's the reverse. Light and gravity travel at c because they (at least light) are mediated by particles / systems that have no mass.
Mass is what slows the speed of causality for certain particles. For example, while the photons I emit may travel at C, the massive particles that make up 'me' cannot.
They are the same, because there’s no such thing as the “speed of light”.
Theoretically as I understand it, everything moves at exactly the same speed through space-time, whether it is light, the Earth, etc.
At non-relativistic speeds, this means moving along the time axis at approximately one second per second, with the rest of the movement in space. At relativistic speeds, higher proportions of the “speed” of an object are along the time axis.
As the energy requirements for moving massive objects through space at relativistic speeds are huge, we can only really observe this phenomenon with light, which has no mass, and therefore does not need huge amounts of energy to move through space.
As a result, we call 186k miles per second the “speed of light” when actually it is just the maximum speed anything can travel through space, and due to light being massless, it happens to be the speed that light travels through space too.
This is my understanding as well. All massless particles must travel at the speed of light (they cannot be slowed down) and moreover, they must travel at the speed of light in all reference frames.
Whereas massive particles can remain at rest. There's no such thing as an unmoving photon.
My point is subtly different, I think. The point I’m trying to make is that if you’re “at rest” in space, all of your movement is through time. If you are “at rest” in time, all of your movement is through space.
As far as I understand it, having mass is basically a result of moving more slowly through space.
I'm not a physicist, but AFAIU the speed of travel/causality for light is only the maximum speed of causality because photons have a mass of literally zero.
If photons had non-zero mass they could only travel slower than the maximum speed of causality (which would probably be called speed of gravity rather than speed of light, in this alternative universe).
Frankly, it may very well be KISS because the other options were so much more complex. Or they said that if we put speed of light to constant to make it simple, there were so many unforeseen edge cases because of it. The devil is in the details, perhaps?
That should be "all of physics in 6 lines, two flawed overly simplistic arguments and one crackpot theory (and 18 particles and 27 constants buried in the last two items)".
Our senses are evolved to maximise our fitness function within our immediate reality. There's a view that our senses don't reflect truth so much as evolutionary fitness, which involves both compromises and biases.[1]
Our evolutionary environment for the most part has excluded relativistic effects.
Though that raises the interesting question of what sense perceptions of an organism evolving under such circumstances might be.
________________________________
Notes:
1. Donald Hoffman is the principle proponent of this that I'm aware of: <https://www.quantamagazine.org/the-evolutionary-argument-aga...>. I'm not entirely sold on the hard-line version of his argument; it seems to me that there's a general tendency for adherence to truth to be more parsimonious than outright fabulation, in which the nonessential inaccuracies of the sensing system incur additional costs.
Is it reasonable to view the Cosmic Microwave Background Radiation as being the limit of this? The remains of the big bang, maximally scaled up and red shifted as far as things can be today?
I think there's a coherent explanation of the cmb be had there, but it's not the conventional explanation.
Under this alternative, the universe cooled to light transparency some time before the moment depicted by the cmb, and anything "further away" than that hugely magnified scene just happened outside of our light cone. That is to say, it's "elsewhere" (a technical term (https://web.phys.ksu.edu/fascination/Interlude1.pdf)).
Seems to me that in this alternative, cosmic expansion could be explained as gravitational attraction between elsewhere-matter and matter in our light cone.
Imagine there's some argument to be made for why this is not the case, but I don't know it. It would require a bit of explaining re: why that point in history and not some other?
- Is it that the maximal distance is constant and that the cmb is subtly changing in ways we havent noticed (as the point of most-distant-past moves forward in time)
- Or maybe something caused the speed of light to change at that time, pruning the rest of the universe from our view.
Kinda but different scale, the CMB era universe was about 1100 times smaller than that now, so still huge.
There may be a neutrino background behind the CMB, where the universe was even smaller, and the gravitational wave background behind that with even more of a size difference.
Would the universe in those other 2 older events have been 2 orders of magnitude smaller still? Have there been any estimates made for the sizes in each "event"?
Are there even more events further back, or is the next one after gravity the big bang?
What a fascinating subject, thank you for expanding my own little universe!
I'm skim-reading on mobile right now, so here's some more information, but I didn't see anything about how much the universe expanded since 1 second after the big bang, which is the relevant number for the neutrino background:
As in a white hole is the big bang? That has a kind of poetic symmetry to it, with black holes (big crunches?) being the end, and white holes being the beginning of our particular universe.
But our universe has black holes in it. Forgive the layman thinking, but does that mean we're just one of an infinite series of "nested" universes?
The energy in our universe is not unlimited, so perhaps each black hole spawns a new universe, and each has less and less energy in it. Think about, WHY is there a certain amount of energy in the universe? Why not more or less. Maybe it's just universes all the way down.
There were no objects before the CMBR. The universe was so hot that atoms couldn't even form. Once it cooled to the point where hydrogen atoms came into existence, the CMBR became possible. I'm talking at the limits of my knowledge, so allow me to refer you to this video by Fermilab that's pretty good.
I think OP's question related to the observable universe vs what is beyond. We see the CMB (and thus our limit of light) only to a point, but that doesn't mean there's nothing beyond that - otherwise we'd be the literal center of the universe (I recall an old minutephysics video[0] on this).
The CMB is everywhere, but it was emitted by the initial formation of neutral hydrogen (from plasma) in the early universe. When people talk about the CMB being far away they're really talking about the last scattering surface, which is that early plasma as seen 13+ billion years later.
CMB is produced by atoms, right? We see darker/lighter regions in CMB, so we should see a transition somewhere. 300M years is very short period of time, unless everything cooled very very uniformly, which is not the case. Sometimes, somewhere there must be a galaxy past CMB.
> 300M years is very short period of time, unless everything cooled very very uniformly, which is not the case
~300M years is the time between the Big Bang singularity and the CMB, but not really relevant. The entire universe was everywhere as hot as the surface of a star at the time of the CMB, so any evidence of galaxies forming before that is surprising.
The surprisingly high uniformity of the temperature of the CMB — isotropic to roughly one part in 100,000 — is one of the reasons the Big Bang model replaced one of the older competing hypotheses (continuous creation IIRC).
So it is in fact the case that everything cooled very very uniformly and I'm not sure why you think otherwise?
I'm also not clear what you're saying with
> so we should see a transition somewhere
Given the CMB is itself the transition that we see.
> Sometimes, somewhere there must be a galaxy past CMB.
I think here you're mixing up space and time.
It's reasonable (please permit my use of conventional language rather than 4-vectors) to assume that a galaxy exists on the other side in space of the CMB as we see it now, but that happens at a point in time after the recombination epoch began and space became transparent, and light from that event hasn't reached us yet; when it does, the apparent distance of the CMB will be large enough for the galaxy to appear on this side.
Are you familiar with light cones and the convention of one space axis and one time axis? It might help you visualise it if you draw what's going on.
GLASS-z12 is 33.2Bly away from us. It should be behind some of the CMB produced by BB, isn't?
> Given the CMB is itself the transition that we see.
In BB model, CMB emitted by hot plasma. Where it is, that plasma?
In steady universe model, CMB is light with z=1000, emitted by distant galaxies, in range of 4Tly. It explains high uniformity of temperature. It's like the temperature of a water stream from underground: it's uniform across a climate area because underground temperature averages seasonal temperature shifting.
The latter is what we're talking about when we say the CMB is about 13-point-whatever billion years old.
The difference with the other number is that the universe got bigger in the meantime, and that's where we recon it is now.
> Where it is, that plasma
The plasma itself?
Everywhere. The whole universe, including here.
The bit we see?
An echo made of light emitted at the last moment in time that it stopped being plasma — the light from the plasma that was here is now as far away from us as the plasma that caused the light we can see.
These numbers means that nothing can travel at FTL speed except this galaxy. It travelled 20Bly in 13By at the speed of 1.5 c. Extraordinary claim requires extraordinary evidence. Where is the source of energy for this FTL galaxy? Why this galaxy is not ripped apart into ball of gluon plasma?
Echo requires something to reflect of. Moreover, echo will be an order(s) of magnitude weaker and will have a stamp of the reflective surface on it properties.
In bread analogy, sugar is the source of energy and CO2. In balloon analogy, new air is added to balloon (with lot of turbulence). What is added to our Universe, which causes the inflation? Where we can see it?
In case of Steady Universe model, light just changes it's properties over time, for example, because gravitational waves are stretching photons and photon beams. Gravitational waves are produced by massive objects, which are orbiting each other.
> What is added to our Universe, which causes the inflation?
It's a free parameter in the equations, just like the initial value for the energy in the space or the baryon number.
Or the number of space-like and time-like dimensions.
Or their inherent topology.
Not that it matters, as the point of what I suggested is that it's an analogy for all objects within the space observing the same relationship, and the implications thereof.
> Where we can see it?
In the relationship between distance and redshift. More distant objects move away faster, the further away the faster they move on average, and that relationship best matches "accelerated expansion" than any other model.
Or, more locally, it's (perhaps by coincidence) about the right level to explain the moon's orbit slowly getting bigger.
> In case of Steady Universe model, light just changes it's properties over time, for example, because gravitational waves are stretching photons and photon beams. Gravitational waves are produced by massive objects, which are orbiting each other.
Great!
Unfortunately for you, those gravitational waves can't act anything like the ones predicted by GR which we've actually observed, because those are far too weak (or spacetime too 'stiff', IIRC).
GR has known weaknesses, to be sure, but they're all annoying beyond any observations we've been able to make, and people really are looking as it's considered both important and prestigious to find a way to tie it and quantum physics together properly.
In the meanwhile, the same equations for GR describe the (just about) detectable gravitational influence your body has, and the various demonstrations of gravity influencing the flow of time and path of nearby light.
IIRC, the best atomic clocks are just about at the level where an extra 100kg sitting next to them can change the last digit relative to another otherwise identical clock, but I'm not sure how long you have to sit there.
They're definitely good enough for it to matter which floor of a building you put them on.
> Unfortunately for you, those gravitational waves can't act anything like the ones predicted by GR which we've actually observed, because those are far too weak (or spacetime too 'stiff', IIRC).
Let's play with numbers. Two kinds of gravitational waves are claimed to be observed: 1) HF waves by LIGO/Virgo and 2) LF ones by NANOgrav[1].
I assume, that the meter is defined as c1s/299792458 in steady vacuum*. Same for the second. I assume, that speed of light can go down only, in other words, speed of light cannot be higher than c.
Gravitational wave background strain amplitude calculated to be ~ 2.4E-15 y-1. For simplification, I assume average slowdown (stretching) of light to be 1E-15 per year.
LF gravitational waves are quite powerful, with strain amplitude 2.4E-15 y-1, but their low frequency does almost no impact to the wave length of light. In 1 billion of years, wave length will be enlarged by up to 1,0000024.
HF gravitational waves are much weaker, say 1E-21, but their high frequency, say 20kHz, may increase wave length up to 1.88, which is much closer to expected Red Shift of 7.
a pair of objects orbiting 24 thousand times per second.
This happens when black holes or neutron stars merge and that's it; this means you don't have enough of them to do what you're claiming, not even if I trusted what looks suspiciously like you blindly asserting without evidence how much they should alter wavelengths.
The effect of gravitational waves is barely anything even on the LIGO detector, and they need to use a squeezed quantum state to even notice because it's much smaller than the wavelength of the light even over the length of the entire beam-line.
Also, gravitational waves don't redshift the photons, they change the length of the path the photons take.
-
And as LIGO, NANOGrav etc., are relying on a prediction of the exact same GR equations that also lead to the big bang etc., you trying to shoehorn that in is roughly analogous to a Young-Earth Creationist talking about carbon dating.
> Or by 24 thousand pairs orbiting 1 time per second,
no, and for the same reason you can't use the output of a quarter million 2.45 Ghz microwave oven magnetrons to produce monochromic teal light (612500 Ghz).
The maths is basically equivalent for EM and gravity waves, except for the constants.
Well, that and the fact it's changing the space-time through which the waves themselves propagate, but the effect is usually small enough to be barely detectable even when you want to.
> Yep, more length to travel - larger wave length. :-/
no, same wavelength, going further on one half of the cycle, then not as far on the other half of the cycle. Same wavelength within the space, it's the space itself which changes.
> I had a discussion about that recently. I have no power to repeat the discussion. You can find it in my comment history.
TBH, that would be a colossal waste of my time. I'm only even bothering to reply to this this now because discussion is supposed to be helpful while I learn things.
> no, and for the same reason you can't use the output of a quarter million 2.45 Ghz microwave oven magnetrons to produce monochromic teal light (612500 Ghz).
Why we need monochromatic light? Gravitational wave background is just noise. A lot of orbiting objects in a galaxy will produce steady noise, due to interference. It's easy to check just by putting a bunch of wave generators with different frequencies in a same pond, and then move. Interference between waves will create noise with higher frequencies than original.
Even small effects are producing significant results over large periods of time. 1 billion years is 31.5E15 seconds.
If we integrate over all frequencies of gravitation noise floor, then we may have a number, which will explain a part of red shift.
More over, gravitational noise is important for Pilot Wave theory, because it may explain the source of energy for the pilot wave.
> no, same wavelength, going further on one half of the cycle, then not as far on the other half of the cycle. Same wavelength within the space, it's the space itself which changes.
It implies FTL speed at the second half of the cycle, which is impossible. If wavelength of light will be enlarged, then it will stay enlarged, because light traveling at c, so c-delta is possible, but c+delta is not.
> TBH, that would be a colossal waste of my time. I'm only even bothering to reply to this this now because discussion is supposed to be helpful while I learn things.
I have the same filling. I only reply because my pleasure to talk with you overcomes the inconvenience of Hacker News.
Maybe we should switch to email, or to a wiki with a proper set of tools for scientific discussion.
You have to be careful with what you mean by "distance" at cosmic scales. Space is expanding with time, and there are several different definitions of "distance" that give very different results at cosmic scales.
The best "distance" measure here is simply redshift. GLASS-z12 is at redshift z=12, as the name suggests. The CMB is at redshift z=1100, so it's father away.
In fact, for very straightforward physical reasons, no light can reach us from beyond the CMB. The universe was opaque before the time of the CMB, because it was ionized and dense. Before the CMB time, photons could not travel very far at all before they hit an electron and were scattered.
Nobody pointed to a source of energy for this "expansion" of "space". Usually, coordinate system doesn't expand with time. An extraordinary claim requires extraordinary evidence.
Yes, CMB emitters are much further away, at a distance of about 4Tly, while BB claimed to be just 14By ago. Your claim, that CMB is produced by BB, requires a lot of stretching.
There's no point in arguing about this here. There's a very well defined, mathematical theory called General Relativity, which explains gravitational phenomena from Mercury's precession all the way to the expansion of the Universe.
If you take the time to learn General Relativity, and to learn how to apply it to cosmology, you will see that there are rigorous mathematical answers to the various questions you're raising.
I want to point out that this isn't esoteric stuff that only a few people understand. General Relativity and cosmology are part of a standard undergraduate physics curriculum. It only takes a few years of study, starting from Physics 101, to get to the point where you can derive the answers to all your questions from scratch.
Doesn't even need that much — their questions so far are at my level, and I keep messing up the much simpler special relatively questions on brilliant.org
"How" this specific expansion happens is an open question — not because nobody has any idea, but because we can't distinguish between three of them and a forth leads directly to the unsolved challenge of combining GR with quantum mechanics.
No, this is not an answer, because it breaks number of laws of physics, such conservation of energy. It looks like an excuse that an answer. It's just heavy stretching of the evidence until it fits the BB model of evolution of Universe.
Static Universe model of evolution doesn't requires such stretching: CMB is just light of distant galaxies. End of story.
>No, this is not an answer, because it breaks number of laws of physics, such conservation of energy.
THE WHOLE POINT of GR is that it explains things that "classical" physics did not, while also explaining everything that classical physics did. Nothing in GR "breaks the laws of physics" because GR largely IS the laws of physics now.
If you want to throw away GR by using a "Static Universe" theory, you have to re-derive a hundred different solutions to problems you bring back into physics by doing so. Einstein literally TRIED to put a static universe into GR because he thought it felt better, and turned out to be dead wrong!
In terms of "what drives the expansion", to us, within the universe, it's just what we see. It could very well be that it's a property of whatever "substrate" or "Stuff/emptyness" that a "Universe" exists in, if "exists" even makes sense in that context. It could be a completely unknowable to us thing. There are very likely phenomena and questions that we cannot ever answer, because we simply have no way of probing them.
All we know is that the way GR says to do the math works out really well for like 99.99% of things, and if you want to come up with a model that doesn't allow space to change "size", you have a shitload of math left to do at a minimum. If you want to understand how we got here, you have 400 years of physics history to read up on. None of this is about the "correctness" of GR either. It just makes the best predictions so far, and in science, all that matters is who makes the best predictions. Want to supersede the GR model? Just predict something correctly that GR cannot, while also predicting everything else correctly.
> If you want to throw away GR by using a "Static Universe" theory, you have to re-derive a hundred different solutions to problems you bring back into physics by doing so.
GR will be a special case in a new theory, which will explain laws of Universe better, which may join together GR and QM. If a formula does a good job, then it will be used anyway. We are not throwing away Newton physics just because GR does a better job in some cases.
> In terms of "what drives the expansion", to us, within the universe, it's just what we see.
Are you talking about a "light sail" effect? Yes, EM radiation creates pressure on dust particles, which pushes them away, but gravitation doesn't let it go. The same effect happens at size of galaxy. I'm not sure about superclusters, but it looks like we are falling into Great Attractor then into Shapley Attractor with all that dust.
So yes, this is possible, but EM radiation must be stronger than gravitation.
> It just makes the best predictions so far, and in science, all that matters is who makes the best predictions.
Predictions are very important, because they allow to prove or falsify a theory, but this is a game for theoretical physicists only. There is only one reality, which can be describer in many ways. Many different formulas can fit the same data. Many different techniques can be used to achieve the same result.
Moreover, every formula works in a range, then it doesn't work. Pi is an irrational number, which cannot be reproduced correctly in reality, thus every formula or path, which contains the irrational number, can be reproduced by physical reality with limited precision only. Multiply the error by many iterations, and new physics will emerge in the same place.
The only way to prove a theory, as I see it, is to make physical demonstrations at human scale, an analog, and then study it.
Hydrodynamic quantum analogs allows us to see pilot wave at work, so no mysteries in double slit experiment anymore: it just self-interference of the pilot wave. The same can be done for space effects.
It's easier to make computer model, to make predictions, but to make a correct model, we need to understand physics first. Egg and chicken. In case of a physical demonstration, nature performs all these calculations for free, automatically. Even when they are partially correct, they are still helpful.
> this is not an answer, because it breaks number of laws of physics, such conservation of energy
GR doesn't conserve energy. What follows is a bit beyond my level so I may be misremembering, but IIRC Noether's theorem is that conservation laws are always identical to some symmetries, and the symmetry for energy (time?) just isn't true in GR.
(I don't think it's even true in SR because space and time are observer dependent, but at least in SR you can get a different conserved quantity because all observers agree on a space-time interval; but as I implied in a different comment where I mentioned brilliant, this is my hobby not my profession).
> CMB is just light of distant galaxies. End of story.
The CMB is a perfect blackbody. Galaxies are far from a blackbody. Your explanation fails if one knows even a tiny amount about astronomy.
Before you criticize Big Bang cosmology, you should learn the theory. That means studying General Relativity, learning to derive the Friedmann Equations, learning about the (utterly overwhelming) observational evidence for the theory, etc. Then you'll be in a position to ask intelligent questions about the theory.
I promise you that if you learn the theory, you'll understand that the questions you're asking either don't make sense or have obvious answers. For example, conservation of energy does not hold in General Relativity. You keep saying that expansion is an ad hoc assumption that breaks physical laws. However, if you solve the Einstein Field Equations, you'll see that the universe must be either expanding or contracting. This fact bothered Einstein so much that he tried to modify General Relativity to get rid of it, something he regretted when observational evidence firmly established that the universe was indeed expanding. This was all the way back in the 1920s, and the evidence is so overwhelming now, a full century later, that it's impossible to deny.
> The CMB is a perfect blackbody. Galaxies are far from a blackbody.
CMB is not emitted by a single galaxy or even group of galaxies. It's light of trillions of supeclusters, like our Visible Universe, averaged. I expect that almost any local unevenness should be polished out when averaged over such large area and distance. We are not seeing stream of photons from individual emitters, we see random photons from extremely huge range of emitters at extremely huge range from us.
If clump together all radiation from all our Visible Universe into single stream of photons, then we will see something very similar.
> For example, conservation of energy does not hold in General Relativity.
If you average a bunch of different types of galaxies, you do not get a blackbody.
Do you know what does give you a blackbody? An optically thick medium with a uniform temperature, which is what the CMB "last scattering surface" is.
I just have one question for you: do you think that physicists are all a bunch of dunces? You're doing extremely simple questions. Do you think that physicists haven't worked out the basics of the theory? Again, instead of raising extremely simple objections, your time would be better spent understanding the theory first.
>> For example, conservation of energy does not hold in General Relativity.
> Then something is wrong.
Energy conservation only holds locally, when space is nearly flat. The true conservation law in General Relativity is more complicated (energy-momentum conservation).
> If you average a bunch of different types of galaxies, you do not get a blackbody.
Black body averages emission of trillions of trillions of atoms. Why it will not work for emission of trillions of trillions of galaxies? Can you prove that?
> Energy conservation only holds locally, when space is nearly flat.
> Black body averages emission of trillions of trillions of atoms. Why it will not work for emission of trillions of trillions of galaxies? Can you prove that?
No, that's not what a blackbody is. A blackbody is an optically thick medium in thermal equilibrium. Galaxies are not blackbodies (not even close), and when you average a bunch of non-blackbody spectra, you don't get a blackbody. You'll get a spectrum with all sorts of atomic and molecular features. There is actually something called the "Cosmic Infrared Background," which is caused by distant galaxies, but it's not a blackbody and it has much larger amplitude variations than the CMB (because galaxies are distributed in a clumpy way).
> Space is flat in all directions.
Globally, spacetime is not flat (i.e., it is not Minkowski). Spacelike surfaces of constant coordinate time are flat, but the whole manifold is not flat. If this is all a bunch of gobbledygook to you, then you need to learn the basics of General Relativity.
(I suspect also GR, but not for any reason you give — the maths presumes no singularities from what I've been told, and yet they happen anyway with easy initial conditions).
For the broader point, if there were galaxies trillion of light years away whose light had time to reach us, they'd be trillions of years old by now, and therefore we'd expect a lot more galaxies near us to be that age too.
We don't see any evidence of nearby galaxies that old; denying the conclusion means falsifying the hypothesis.
Also, they'd have to go on forever to not look clumpy, and then we would still need a source of red-shift to stop them being as bright as the surface of a star in all directions.
I know that. I'm heretic. Moreover, I'm too stupid to understand all these great theories. I need simple explanations.
> For the broader point, if there were galaxies trillion of light years away whose light had time to reach us, they'd be trillions of years old by now, and therefore we'd expect a lot more galaxies near us to be that age too.
Of course, not. Space is mostly empty. If elementary particles are generated constantly from pure energy (which doesn't violate laws of conservation) just of pure luck at cosmic scale, then light from distant neighbors slowly pushed this newborn dust into the center of a gigantic void, where it started to concentrate. In such case, we will have huge gap of void between our region of space and our neighbors.
> Also, they'd have to go on forever to not look clumpy, and then we would still need a source of red-shift to stop them being as bright as the surface of a star in all directions.
Surface area of a distant object reduces at r^2, while brightness of the distant object diminishes at r^3. Moreover, the probability of hitting something grows with d^1, so total brightness diminishes with (d^3*d)/d^2 = d^2. The number of objects in the sky increases with area = d^2. So, d^2/d^2 = const. I see no infinity. At average, the brightness of sky must be very similar in all directions. The larger the distance - the closer to average brightness must be.
CMB must be almost ideal.
> If elementary particles are generated constantly from pure energy (which doesn't violate laws of conservation) just of pure luck at cosmic scale, then light from distant neighbors slowly pushed this newborn dust into the center of a gigantic void, where it started to concentrate. In such case, we will have huge gap of void between our region of space and our neighbors.
Requires simultaneous behaviour from all directions at great distances while also not having that behaviour here, and also having us being really close to the physical center of this phenomenon rather than off to one side — even a fraction of a percent would be easily noticeable given the CMB is so close to the same in all directions; we see a red/blue-shift dipole from us moving at 370-ish km/s relative to it's comoving rest frame, so that's the scale of fractional away-from-perfect-centre you'd have to explain.
> Surface area of a distant object reduces at r^2, while brightness of the distant object diminishes at r^3.
If space was flat, which is your presumption, those would both be 1/r^2.
> Moreover, the probability of hitting something grows with d^1
You should be able to tell that's wrong by it being an unbounded function, when probability stops at 1.
> Nobody pointed to a source of energy for this "expansion" of "space".
Several have been made, the suggestions have issues.
> Usually, coordinate system doesn't expand with time.
Define "usually". Do you have experience of other universes?
> An extraordinary claim requires extraordinary evidence.
Indeed, but this comment box is too small to do the evidence justice.
Edit: that's unhelpful in retrospect, so I suggest the Youtube channel "PBS Space Time". The videos build on each other, so start at the beginning and work through the back catalogue.
> Yes, CMB emitters are much further away, at a distance of about 4Tly,
> Define "usually". Do you have experience of other universes?
I have experience with coordinate systems. I can bend or expand space-time on my computer all day long, to simulate reality, but I cannot do that in the real world at all.
> Indeed, but this comment box is too small to do the evidence justice.
Looking for the paper or a blog post! However, I suspect that you will just stretch evidence until it will match your model.
> I have no idea where you got this belief from.
Just by looking in the window, I see that some object are close, other are far away, then even further away, and so on, up to 4Tly. Nothing extraordinary. No Big Bangs, no FTL speeds, no hidden sources of energy of epic size, just ordinary physics.
Reminds me a bit of my dad; he did radar simulation for military IFF and one of his work anecdotes was about increasing the number of decimal(!) digits of pi the software used.
He stopped boasting about that when I pointed out the extra digits were less relevant than the curvature of spacetime caused by Earth itself.
> Just by looking in the window, I see that some object are close, other are far away, then even further away, and so on, up to 4Tly
I had dreams like that once. Woke up to find I was suffering from testicular torsion.
If you seriously believe you can see 4e12 light years through your window, that's probably hallucinogens of some kind (not necessarily intentional).
I 100% sure that I cannot bend or stretch imaginary coordinate system outside of my imagination. Can you point to real physical process which causes stretching or bending of the mathematical abstraction?
> If you seriously believe you can see 4e12 light years through your window, that's probably hallucinogens of some kind (not necessarily intentional).
I cannot see objects smaller than a star or galaxy with naked eye. However, we can see light stretched to the microwave range.
> Can you point to real physical process which causes stretching or bending of the mathematical abstraction?
"General relativity" as we keep telling you.
> I cannot see objects smaller than a star or galaxy with naked eye.
Only a factor of about a trillion in the size of those two things.
> However, we can see light stretched to the microwave range.
With your eyes? No. And certainly not through your window, whose own thermal emissions relative to the CMB makes your previous claim roughly as unphysical as saying you can look through the sun's photosphere to see Jupiter during an occultation.
> Sometimes, somewhere there must be a galaxy past CMB.
If there is we'd have to wait for the light from it to get to us, by which time the CMB will have receded further and it would then be in front of the CMB.
A transition from plasma to the cold mater in the form of galaxies we see.
> Why not?
As you see, there are big clusters everywhere. It means that some regions were cooler from the start, to form these cluster in so short period of time. It means that regions around them were hotter, thus they should emit light longer.
> If there is we'd have to wait for the light from it to get to us, by which time the CMB will have receded further and it would then be in front of the CMB.
300My is a short period of time. Why they cannot sometimes overlap?
Take this a further step. Assuming we had telescopes big enough and sensors sensitive enough, what does the structure of the deepest parts of space look like? Are there pre-galaxy-formation structures which are smaller than galaxies and yet take up huge swaths of sky? Are there structures from some point in the past that take up so much space on the sky that not very many of them can "fit", and, if so, do the calculations work out so that an equivalent explanation for having not very many of them is that the [region of the] universe [which is observable to us] was just that much smaller back then?
The furthest we can see is the physical limit of universe: we can literaly see thefirst photons after universe became transparent. That is the CMB (cosmic microwave background, and you can easily google a real picture). The problem is, while these are the oldest photons we will ever be able to see, they still are from when universe was cca 400 000 years old, and by that time it was 100 million lightyears wide. That picture tells us that the universe was extremely homogenous (altho not perfectly), and basically no such structures you talk about.
If we would like to see even further, we must give up on photons completely, and probably probe the ultra deep space gravitation waves. Those should give us picture even of completely opaque universe, as it was before then. So far we can only "see/hear" the brightest/loudest events in the universe with our gravitational waves observatories, but the fact we can even do that is astounding nontheless: we built a new sense for humanity, that no other known creature in the universe posses. LISA project should hear more.
Fascinating. A related question: When we look at Andromeda, which has a diameter of 220,000 light years, we are looking at it slightly edge on. Shouldn't the stars on the back edge be in a relatively different place in the sky than the stars on the front edge since the galaxy has moved relative to us over that 220K light years?
I would expect something kind of stretched out and warped like taffy. i.e. the front edge of the galaxy would be stretch out ahead of the back edge of the galaxy since the back edge is running behind time-wise.
You have apparently not figured out (1) by how much? and (2) in which direction? - Andromeda rotates so slowly that after 220K years the far side has made only 1/1000th of a rotation. Please draw on a picture of Andromeda how far the far side moves after 1/1000th of a rotation. That's how small the image warping is.
Never forget that the universe does not have a "preferred scale" and has fractal complexity and emergent behavior from the smallest two particle interactions to the largest black holes circling each other.
Our 10 pound hunk of fat is not able to comprehend that.
One thing that always bothers me is time dilation when it comes to observing distant objects like this. If it takes 4 light years for the light to travel one way (and the one way speed of light still hasn't been measured!), that is for the observer at the origin, the photons we observe, for them it is much less than 4 years, is it even in years? So if I look at alpha centauri with a telescope, it isn't really aloha centauri 4 years ago that I am observing right? It's much more recent than that?
Otherwise, if a 30yo person travels at the speed of light from alpha centauri to earth, when the person arrives will they be a 31yo(~) person who arrived 4 year later, effectively time traveling to the future? And if they return right away, will folks at alpha centauri meet a 32yo(~) person who came back 8 years later? If so, then maybe superman had the right idea about flying really fast around earth to travel in time, just not to the past.
Perhaps some billionaire will decide to spin around the solar system really fast for a few decades and skip a century or so? Haha!
A photon travelling between Alpha Centauri and Earth does not experience time it effectively arrives instantly. We as observers however will see that it took 4 years to make its journey. The idea of whether it's recent or not is irrelevant, it is all relative (to the photon it's recent, to us the light is 4 years old).
Huh. That would make such objects even harder to detect, since the light is spread out over a larger area, so the amount of light hitting each pixel of the detector is less than if it wasn't magnified.
On the other hand, it means you can see details you might not be able to otherwise.
> Things that are far away look smaller, but things that are REALLY far away look bigger, because when their light was emitted, the universe was small and they were close to us.
Clearly because Munroe hasn't adjusted for screen size inflation. If we extrapolate based on whatever Samsung or Apple are selling these days and the 8210, will we even see meaningful redshift before the Planck constant chimes in?
Speaking of the expansion of the universe, in a very distant future when the expansion speed is so high that most of the galaxies won't be visible from earth, their astronomers will be thinking the whole universe contains only a few galaxies. But wait, what if the universe we observe today also misses some parts that can't be observed anymore?
And based on the size of the magnification of the galaxies throughout time, we can tell whether the universe is expanding at a constant rate or accelerated or decelerated at certain point.
This XKCD strip would be genius if he used actual different historical cell phones - for awhile they were getting smaller, and then they started to get larger again!
Holding the two concepts of an accelerated (!) (in terms of objects [1]) expanding universe and the fixed finite speed of light simultaneously in one's Euclidean head can be dizzying, so be prepare to draw and enjoy the hard earned manual labor of counterintuitive conclusions.
[0]https://arxiv.org/pdf/astro-ph/0310808.pdf
[1]https://bigthink.com/starts-with-a-bang/universe-expansion-n...