If you’re interested in learning more about the rich human history and ingenuity underpinning the Hubble “constant”, please do yourself a favor and scroll through The Cosmic Distance Ladder by Terence Tao of UCLA:
https://terrytao.wordpress.com/wp-content/uploads/2010/10/co...
The slides are delightfully visual and comprehensive yet terse, walking you up the rungs of the cosmic ladder from the Earth through the moon, sun, and beyond. I can almost guarantee you’ll learn something new and fascinating.
I really like all the caveats and the time taken to explain things in the first part of that document, but later it starts to rush and gloss over important details and caveats. On page 151 of that link, when it starts talking about using parallax to measure the distance to nearby stars, it says "However, if one takes measurements six months apart, one gets a distance separation of 2AU." This is obviously incorrect because the whole solar system is orbiting around the galactic core, which itself is moving with respect to the CMB rest frame. I did a quick calc based on the 552.2 km/s galactic velocity value from Milky Way wiki [1] and found that it moves an additional 0.97AU in 6 months. I am assuming that this has been accounted for by scientists, and is being simplified to make it more digestible for the reader, but it hides a rather large dependency for every higher rung on the cosmic distance ladder. A cosmic velocity ladder that seems to be based off of Doppler CMB measurements [2]. If we are indeed using measurements many months apart and under or overestimating our velocity through the universe, even a little bit, every higher rung of the ladder would be affected wouldn't it?
In the process of writing this, I thought "Surely we have launched a satellite pair that can take parallax measurements at similar times in different places!" They could range off of each other with Time of Flight, be positioned much further apart than a few AU, and take parallax star measurements at more or less the same time without atmospheric distortion, but it doesn't seem like we have. Both Hipparcos and Gaia were satellites that were deployed to measure parallax, but not as a pair. My reading suggests they used multi-epoch astrometric observations (speed ladder dependent) to generate their parallax measurements and it seems our current parallax and star catalogues are based on the measurements taken by these two satellites. New Horizons got the most distant parallax measurements by comparing simultaneous* earth observations, but it was limited to Proxima Centauri and Wolf 359, far from a full star catalogue.
I would love if someone more knowledgeable can steer me towards a paper or technique that has been used to mitigate the cosmic distance ladder's dependency on this cosmic speed ladder. Regardless of how certain we think we are of our velocity through the universe, it seems to me that sidestepping that dependency through simultaneous* observations would be worthwhile considering how dependency laden the cosmic distance ladder already is.
* Insert relativity caveat here for the use of "simultaneous". What I mean in this context is more simultaneous than waiting months between measurements.
What if the universe doesn't expand at all? What if we're completely wrong and redshift is caused by something else entirely, like some yet-undiscovered phenomenon that occurs to spacetime or electromagnetic waves? How can we be so sure it's space that's expanding, not time?
The more I read about this, the more it feels like phlogiston theory[1]. Works great for describing observations at first, but as more observations are made, some contradict the theory, so exceptions are made for these cases (phlogiston must have negative mass sometimes/there must be extra matter or energy for galaxies to spin as fast as they do), and then finally someone discovers something (oxygen/???) that explains all observations much simpler and requires no weird exceptions.
Not possible. Redshift is not the only observation we have. The totality of all the observations we have cannot be explained in any other way than an expanding universe.
> How can we be so sure it's space that's expanding, not time?
Our best current model does not say "it's space that's expanding, not time". It says that in one particular frame (the comoving frame), the overall spacetime geometry can be described using a "time" that always corresponds to the time of comoving observers and a "space" whose scale factor increases with that time.
> The more I read about this, the more it feels like phlogiston theory
This is an extremely unjustified comparison. Phlogiston theory never accounted well for actual observations.
> as more observations are made, some contradict the theory
None of the observations being discussed contradict the general model of an expanding universe. They only pose problems for the indirect methods we use to convert our direct observations into model parameters.
I agree with you on the overall point, but this statement.
> Not possible.
In all honesty, Cosmology rests on the principal that physics is the same in all directions, over all translations, and over time translation. While this is a good assumption (good luck testing alternatives!!). There are a variety of papers exploring the topic of how much these assumptions would need to be violated to mirror observations.
A good example being
what if the electron was more massive in the past?
All Redshift would then be explained away ;)
P.S.
There are very good reasons to believe that the electron was not more massive in the past.
It’s not just phlogiston, it’s the lifecycle of all scientific theories that they’re used for as long as they make accurate predictions, then we start seeing things they mis-predict, then they’re revised or replaced. You seem to think the expanding universe theory can still be saved by some data artifact or parameter tweaking, but that’s been hunted for years and we’re still at “we just can’t make it match everything we’re seeing”. Historically, that’s what precedes significant revision or replacement.
> The totality of all the observations we have cannot be explained in any other way than an expanding universe.
Surely there are infinite other possible explanations that fit the finite number of data points available to us. Probably what you meant is that the expanding universe theory is the simplest of them all and creates less problems then others.
Isn't it the case that we don't actually know if the universe is expanding, we only know that from our POV things are moving away from us and from each other, based on models and observations that are approximations at best?
In that frame an expanding universe seems to be the simplest and more elegant solution, but it's entirely possible it's not the correct one.
for example: what if, on the antipodes of the universe (assuming something like that exists), things appear to move closer to the relative POV? we'll never know
>> The totality of all the observations we have cannot be explained in any other way than an expanding universe.
If someone has a theory that incorporates "the totality of all observations" then physics is over. Redshift explains most observations, no other concept even comes close, but there are certainly things out there that remain unknown that are not explained by redshift. Dark energy is such a monumental observation that every theory in cosmology must remain caveated.
> Not possible. Redshift is not the only observation we have. The totality of all the observations we have cannot be explained in any other way than an expanding universe.
Well..? What are those other observations that point to expansion?
There is a very old theory called the "Tired Light Hypothesis" which supposes that for some unknown reason light loses energy as it travels over cosmological distances. This would reproduce the observed redshifts, but it has issues predicting pretty much every other cosmological observation.
In particular it doesn't explain observed reductions in surface brightness (expansion has the effect of "defocusing" collimated light). And it doesn't explain observed time dilation effects.
I've always wanted to play a game based on defunct theories. I'm a fan of luminiferous aether myself. What are the impacts on a spacefaring civilization?
Sci-fi already grants alternative physics to enable FTL and other magic. What about hard sci-fi, but wrong-hard sci-fi?
Extra credit: go back to Zeno and all motion is paradoxical, what would you even do in the game?
Time dilation could be that going very fast in the space makes you relatively faster in one direction.
The thing is, atoms also have to travel; so the atoms (and matter in general) have a slightly longer distance to travel, to achieve the same chemical reaction. Which means interactions between atoms is slower, giving illusion of a slower time due to slower inter-atoms reactions.
We can create and observe doppler shift by making things move towards/away from us. Thus it is proven that if something is moving away from us, it will produce a redshift. In the absence of evidence that something else is causing the redshift, the assumption should be that it is a result of things moving away from us.
As an obvious example, doppler shift often needs to be accounted for to communicate with spacecraft.
> In the absence of evidence that something else is causing the redshift, the assumption should be that it is a result of things moving away from us.
But that is not what our best current model of the universe actually says. Our best current model of the universe says that the observed redshift of a distant object tells us by what factor the universe expanded between the time the light was emitted and now (when we see the light). Viewing it as a Doppler shift is an approximation that only works for small redshifts (much less than 1).
X causes Y does not mean that Y implies X. It’s reasonable to suspect X given Y and an absence of other such causal relations, but it’s not necessarily reasonable to spend decades building layers and layers of models that assume X at the most basic level.
1. As other commenter said, X causes Y does not mean that Y implies X. There can be another cause for the Doppler.
And surprisingly,
2. There is at least one known mechanism that cause Doppler WITHOUT moving: when the observer is in a gravity well (ex: earth) and observing a stationary object outside the gravity well (ex: some fixed point in outer space)
>What if the universe doesn't expand at all? What if we're completely wrong and redshift is caused by something else entirely, like some yet-undiscovered phenomenon that occurs to spacetime or electromagnetic waves? How can we be so sure it's space that's expanding, not time?
I suppose that's possible. Does that hypothesis adequately explain our observations?
Is the model we currently have completely "correct"? Almost certainly not. But it appears to be less wrong[0] than earlier models.
If you (or anyone) can show how the above describes our observations better and more completely than our current models, then it's likely "less wrong."
But you offer no evidence or even logical argument to support your hypothesis. As such, it's not much more than idle speculation and essentially equivalent, from a scientific standpoint, as suggesting the universe is a raisin dropped into a sugar syrup solution[1] and absorbing the liquid -- hence the expansion of the universe.
Easiest method is to simply take your idea at face value.
In our first version, imagine all of the stars at rest. Now, we emphatically know this not to be true locally due to all kinds of measurements, but let's go with it. What happens? The moment you let these stars "go," they begin to draw toward one another due to gravity. You would have gravitational collapse. We do not see that.
Next iteration: we throw the stars, and the galaxies, and the galactic clusters away from one another. No expansion required. Here we have two options. In the first, we did not throw with enough speed, they expand out ... slow to a halt ... and the gravitational collapse again. Again, unobserved. Option two, you have thrown at escape velocity and what you would see is an asymptotically decreasing speed, never quite hitting zero, since gravity works "forever away." Also unobserved.
What you're suggesting is basically the Steady State concept, a kind of static universe. This is a very old idea. So old it was given a kind of courtesy term in general relativity, which would eventually be set to zero.
Here is a rule for any armchair astrophysicists: whatever you think of, that was most likely an idea at one point and was eventually ruled out.
[A sign on two posts, in the grass in front of a building with windows and double doors, a window on each door, and bars facing outwards. There is a cement walk leading to the doors. On the sign is the text:]
Department of Astrophysics
Motto:
Yes, everybody has already had the idea, "Maybe there's no dark matter—Gravity just works differently on large scales!" It sounds good but doesn't really fit the data.
And if the universe was much denser, doesn't that imply that all that matter affected its surroundings gravitationally? And as we know, time runs slower near large masses. And when something falls into a black hole, according to our very own theories, it would also red-shift because of the black hole's gravitational pull without anything having to expand.
For example the entire atomic composition of the stars in the observable universe depends exquisitely on the expansion parameters at the big bang. The ratios can be traced back through the expansion to the quark-gluon soup stage. Changing the expansion rates changes the delicate balance between the particles that form at that stage, and when the various particle fractions "freeze out" during expansion when the temperature cools (btw we're talking about seconds from the big bang here :) which can be subsequently observed in stars all over the universe by spectroscopy. It's pretty beautiful.
There are so many intricate dependencies between these pathways that it's pretty unrealistic to postulate anything else than a big bang + cooldown process IMHO.
This is the equivalent of finding your keys an inch off from where you remember setting them down and concluding that someone broke into your home, stole your keys, took your car for a joyride, and broke back in to place them there.
Expanding universe and Big Bang Theory go hand-in-hand. There are multiple independent observations besides the red shift that make it nearly certain there had to be a BBT event to explain what we see. The universe is too hot, chaotic and clumpy for there not to have been a massive explosion to kick it all into motion. Since there is good confidence BBT happened, transitioning from that event to a steady-state non-expanding universe would require some sort of mechanism to slow then freeze the expansion. Not aware of any support for that model.
The name “big bang” was a pejorative epithet coined by Fred Hoyle, who believed in a steady-state universe that was expanding (as Hubble had argued convincingly) but had some hypothetical mechanism for creating galaxies, such that the universe could have an unbounded past and future.
I remember reading, a long, long time ago, a paper where the authors suggested if the universe was slightly hyperbolic, it would also cause a redshift effect. I can't seem to find it (and as far as I remember it was purely theoretical), but at the time I thought it was an neat idea.
Not that I have the background to know what else they might not have accounted for to reach this conclusion.
While I don't necessarily think at lot of alternative ideas proposed are correct, I always love seeing alternative concepts being considered. Very cool to see ideas that could solve standing issues even if they themselves could have issues.
My guess is that scientists are considering this, but until now no better theory has been presented.
Part of this is the distinction between what is happening and why the model says is happening. Does any physicist believe they have the perfect model? Or is it that they use the model that best fits the observations and are open to any other model, as long as it is either simpler or produces fewer contradictions than the current model (and is just as testable).
I think too often we hear reports of "science says X is what happens" when the reality is more like "science says that the current model based on X happening is what best describes current data and observations".
The _conclusion_ that the Universe is expanding is based on the long accepted premise that the Universe is _flat_. And this premise can not be proven or disproved unless we travel great distances to actually _observe_ if the Universe does, in fact, look the same from any point you look.
The Copernican principle is, indeed, attractive to the modern mind because its neutrality. But it's not neutral. It's just as loaded as any other principle, no matter how crazy it may sound today, philosophical, religious, or merely personal.
The observation of the Hubble constant requires us to measure distance to an object in space. This is very hard to do at the extreme distances required (https://en.wikipedia.org/wiki/Parallax). In the end, the variation in the Hubble constant might be only due to our limited accuracy in measurement.
The universe is a 4 dimensional sphere, so everything could be moving away without increasing 3 dimensional space. Eventually in trillion or quadrillions of years everything would start to blue shift as things move towards us on the other side of the sphere.
So how do I travel backwards in the 4th dimension? Or conversely, where's the source of the force that propels us through the 4th dimension?
I'll leave this comment here since I'm about to get rate limited: I read/heard a great idea recently, what if gravity is an emergent quantity like heat? Maybe dense fermions just radiate gravitons just like a hot mass radiates photons?
I suspect, in a few decades, when the smoke clears and the very latest sub-infrared, stadium-sized space telescope finds fully formed galaxies several billion years "before" the alleged moment of creation, then the astronomical community will finally start to question the logic of prevailing cosmological theory, from the ground up.
The big bang was first postulated by an agent of the vatican, and scientists raised in any religious context tend to generate experiments that confirm their beliefs.
Naive question: why should the expansion rate need to be uniform or constant everywhere?
I'm likely misinterpreting the article, but it seems to frame things in a way that first assumes expansion should be constant and it's a question of what the right constant value is between the measured/theoretical discrepancies.
(*yeesh, editing all those spelling errors from typing on my phone)
The controversy is that we get 2 different numbers depending on which method (cosmic microwave background vs cosmic distance ladder) we use to calculate the present rate of expansion. These numbers used to have their error bars overlapping, so we assumed they would eventually converge to the true value. But as we get more data the opposite is happening: the numbers are diverging and their error bars are shrinking such that they no longer overlap.
This tells us that either our model of the universe is wrong (therefore the cosmic microwave background method is giving us an incorrect answer) or that something is wrong with how we're calculating the distances along the cosmic distance ladder. The latter was originally the assumption that should be proven true with more and better data from newer telescopes. This is now turning out not to be the case: our cosmic distance ladder calculations seem to have been very good, so it now seems more likely that our model of the universe is wrong.
> our cosmic distance ladder calculations seem to have been very good
Not according to at least one research group described in the article: the Freedman group, which is only getting the higher answer using Cepheids, but gets a lower answer, one consistent with the CMBR calculations, by two other methods. Which raises the possibility that it's the Cepheid part of the cosmic distance ladder that's the problem.
I remember reading that the local group, Laniakea Supercluster and the great attractor [1] are new developments that helped us refine our understanding of H0 but didn't fundamentally remove the controversy.
It's exciting to see how the question drives many new discoveries.
I'll try to paraphrase what it meant: measuring H0 comes down to measuring the relative velocity of galaxies around us. The great attractor was a relatively recent discovery that the "closer" galaxies, the ones we can use in the distance ladder, all have a common component in their velocities which we've recently begun to understand better.
> so it now seems more likely that our model of the universe is wrong.
Whenever a scientist says that it's not possible that the model is wrong, then I just roll my eyes. Of course models can be wrong - and isn't that exciting? Good on them for making sure that there are no errors in the measurements - that's incredibly valuable and absolutely necessary - but I'm really excited to see creative models being thought up that are drastically different. My personal hell is the universe being consistent and boring.
> so it now seems more likely that our model of the universe is wrong.
Anyone with an ounce of sense should have concluded that LCDM was wrong long ago. Hopefully this will finally cause physicists to actually try something different.
Yes, cosmological principle is probably the most fundamental assumption in astronomy.
Most people don't realize that science—and even everything in life—has to start from some axioms/assumptions, just like math. I first realized this fact when I was reading the Relativity book written by Einstein himself, who challenges the assumptions in classical physics.
As time goes, some of the assumptions could be proved to be unnecessary or even wrong. There must be still some assumptions left, though—because without them, we can't talk about science, or anything, really.
Though it is worth noting if this were the case you would expect to see boundaries: if the laws of physics change due to spatial position, the discontinuity should produce an effect of some sort where matter and light transitions between regions.
Seems there are 2 ideas at odds. One is that the universe is infinite, in which case this is all localized and has no bearing on the universe outside of our small observable region. The other is that we are seeing enough of a bounded universe where the observations we make are of a significant enough chunk to make theories about it.
Indeed. Some researchers have proposed quintessence, a time-varying form of dark energy [0].
> A group of researchers argued in 2021 that observations of the Hubble tension may imply that only quintessence models with a nonzero coupling constant are viable.
> what should the expansion rate need the be uniform or constant everywhere?
It doesn't.
"The simplest explanation for dark energy is that it is an intrinsic, fundamental energy of space" [1]. That's the cosmological constant.
Dark energy is a thing because we don't assume that to be the case. Irrespective of your dark energy model, however, there will be a predicted global average.
There has been some interesting recent work that may get rid of the need for dark energy.
Briefly, recent large scale maps of the universe suggest that the universe might not be as uniform as we thought it was. In particular we appear to be in a large region (something like a couple billion light years across) of low density.
Dark energy is needed to make the solution to Einstein's field equations for the whole universe match observations. However that solution was derived based on a universe with matter distributed uniformly. At the time it was first derived that appeared to be the case--we thought the Milky Way was the whole universe.
When we learned that the Milky Way was just a small galaxy in a vastly larger universe than had thought we were in and that there were bazillions of other galaxies, those galaxies appeared to be distributed uniformly enough the the solution to the field equations still worked.
Later we found that there is some large scale structure in the distribution of galaxies, like superclusters, but those seemed uniform enough throughout the universe that things still worked.
If that couple of billion light year low density region turns out to exist (large scale mapping of the universe is hard enough that it may just be observational error) the universe may not actually be uniform enough to for the field equations based on uniform matter distribution to actually work.
Some researchers worked out the solutions to the field equations for a universe that has such large low density bubbles big enough to invalidate the uniform universe solution, and found that such a universe would have an expansion force without the need to invoke any kind of dark energy.
There was a recent PBS Space Time episode that covered this: "Can The Crisis in Cosmology be SOLVED With Cosmic Voids" [1]. The above is my summary of what I remember from that. See the episode for a better explanation and references to the research.
It's not constant (the early universe inflated quite quickly), and it doesn't need to be uniform, but it sure does appear to be. We measure it via redshift, pulsar timing arrays, and the temperature fluctuations of the CMB, and it looks pretty much the same in all directions.
Spacetime is apparently extremely rigid as it supports the transmission of gravitational waves originating billions of light-years away, as detected by the LIGO experiments. This suggests smooth and gradual uniform expansion, at least spatially. Temporal variation (speeding up and slowing down uniformly at all points) might be possible but seems hard to explain.
The issue here is that it's not constant depending on the type of star we use to measure it. It's not a discrepancy in location in space. Or at least that's how I read the article.
That's the first thing that occurred to me too. It could also not be constant even at the same place, i.e. could it not be speeding up and slowing down as the universe expands?
Certainly when I look at convection currents in the ocean or the atmosphere, I see plenty of variation. Shoot, the earth's atmosphere constantly produces moving blobs of relatively high and low pressure.
"researchers started using Cepheids to calibrate the distances to bright supernovas, enabling more accurate measurements of H0."
It seems like if there were some error in the luminosity measurement for cepheids, it would propagate to the measurements with supernovas...
I would expect that stacking measurement techniques (as is common with cosmology, where distances are vast and certainty is rare) would also stack error, like summing the variance in gaussians...
The other solution is to increase the accuracy of parallax. This is what the Gaia project is doing. It can measure distance to stars in galactic center to 20%. It will measure distance to 2 billion stars and be super accurate within 300 ly.
New Horizons has taken some star pictures from the Kuiper Belt and you can easily spot the parallax of some nearby stars just by eyeball. I'm not sure that it has a good enough camera for any kind of precision measurement, but it was really cool to see that.
I think the problem with such a mission right now is the high probability we could launch a faster mission in the very near future - i.e. with NASA looking at spaceborne nuclear propulsion again, we could send much more capable telescopes out faster - which is not just an "I want it now" benefit: time in space is time you run potentially having components wear out or break. So getting them onto their missions ASAP is a huge de-risking element.
These uncertainties in Cepheid luminosities are accounted for in Type Ia distance measurements. Particularly with Gaia we can now calibrate the luminosities of Cepheids in our galaxy using parallax observations.
(Knowing this field I'm sure there are some astronomers who argue that there are still some systematic uncertainties that are not fully being accounted for, but from what I understand it's pretty hard to account for it with the Gaia results at this point.)
"But according to Freedman, the galaxies’ supernovas seemed to be intrinsically brighter than the ones in farther galaxies. This is another puzzle cosmologists have yet to understand, and it also affects the H0 value. "
The opening sentence of this article is 100% wrong. Hubble was a good scientist and correctly made no assumptions regarding his observations that objects that are further away by parallax are more red shifted.
The assumption that these observations indicated an expanding universe was delivered to us by LeMaitre; if you believe in an expanding universe with a finite age, then give credit where it is due...
One of the frustrating aspects of cosmology is how difficult it is to actually apply the scientific method to it. You can't make a couple stars in a lab and see how they behave, the same way you can for particle physics. Fundamentally, most of cosmology comes down to observation, not true experimentation, where the experimenter is directly acting and comparing that to a control group. There are some experiments that can be done, but there are just some fundamental limitations. This is also the case in the so-called "soft sciences" like economics and psychology. But it's even true in some corners of the "hard sciences" like evolutionary biology.
Readit News
The slides are delightfully visual and comprehensive yet terse, walking you up the rungs of the cosmic ladder from the Earth through the moon, sun, and beyond. I can almost guarantee you’ll learn something new and fascinating.
Deleted Comment
In the process of writing this, I thought "Surely we have launched a satellite pair that can take parallax measurements at similar times in different places!" They could range off of each other with Time of Flight, be positioned much further apart than a few AU, and take parallax star measurements at more or less the same time without atmospheric distortion, but it doesn't seem like we have. Both Hipparcos and Gaia were satellites that were deployed to measure parallax, but not as a pair. My reading suggests they used multi-epoch astrometric observations (speed ladder dependent) to generate their parallax measurements and it seems our current parallax and star catalogues are based on the measurements taken by these two satellites. New Horizons got the most distant parallax measurements by comparing simultaneous* earth observations, but it was limited to Proxima Centauri and Wolf 359, far from a full star catalogue.
I would love if someone more knowledgeable can steer me towards a paper or technique that has been used to mitigate the cosmic distance ladder's dependency on this cosmic speed ladder. Regardless of how certain we think we are of our velocity through the universe, it seems to me that sidestepping that dependency through simultaneous* observations would be worthwhile considering how dependency laden the cosmic distance ladder already is.
[1] https://en.wikipedia.org/wiki/Milky_Way
[2] https://arxiv.org/pdf/astro-ph/9312056
* Insert relativity caveat here for the use of "simultaneous". What I mean in this context is more simultaneous than waiting months between measurements.
The more I read about this, the more it feels like phlogiston theory[1]. Works great for describing observations at first, but as more observations are made, some contradict the theory, so exceptions are made for these cases (phlogiston must have negative mass sometimes/there must be extra matter or energy for galaxies to spin as fast as they do), and then finally someone discovers something (oxygen/???) that explains all observations much simpler and requires no weird exceptions.
[1] https://en.wikipedia.org/wiki/Phlogiston_theory
Not possible. Redshift is not the only observation we have. The totality of all the observations we have cannot be explained in any other way than an expanding universe.
> How can we be so sure it's space that's expanding, not time?
Our best current model does not say "it's space that's expanding, not time". It says that in one particular frame (the comoving frame), the overall spacetime geometry can be described using a "time" that always corresponds to the time of comoving observers and a "space" whose scale factor increases with that time.
> The more I read about this, the more it feels like phlogiston theory
This is an extremely unjustified comparison. Phlogiston theory never accounted well for actual observations.
> as more observations are made, some contradict the theory
None of the observations being discussed contradict the general model of an expanding universe. They only pose problems for the indirect methods we use to convert our direct observations into model parameters.
> Not possible.
In all honesty, Cosmology rests on the principal that physics is the same in all directions, over all translations, and over time translation. While this is a good assumption (good luck testing alternatives!!). There are a variety of papers exploring the topic of how much these assumptions would need to be violated to mirror observations.
A good example being
what if the electron was more massive in the past?
All Redshift would then be explained away ;)
P.S.
There are very good reasons to believe that the electron was not more massive in the past.
> Redshift is not the only observation we have.
What else is there?
Surely there are infinite other possible explanations that fit the finite number of data points available to us. Probably what you meant is that the expanding universe theory is the simplest of them all and creates less problems then others.
Isn't it the case that we don't actually know if the universe is expanding, we only know that from our POV things are moving away from us and from each other, based on models and observations that are approximations at best?
In that frame an expanding universe seems to be the simplest and more elegant solution, but it's entirely possible it's not the correct one.
for example: what if, on the antipodes of the universe (assuming something like that exists), things appear to move closer to the relative POV? we'll never know
If someone has a theory that incorporates "the totality of all observations" then physics is over. Redshift explains most observations, no other concept even comes close, but there are certainly things out there that remain unknown that are not explained by redshift. Dark energy is such a monumental observation that every theory in cosmology must remain caveated.
Well..? What are those other observations that point to expansion?
In particular it doesn't explain observed reductions in surface brightness (expansion has the effect of "defocusing" collimated light). And it doesn't explain observed time dilation effects.
Sci-fi already grants alternative physics to enable FTL and other magic. What about hard sci-fi, but wrong-hard sci-fi?
Extra credit: go back to Zeno and all motion is paradoxical, what would you even do in the game?
Not to mention contradicting the laws of conservation of energy and momentum.
I have an interesting addition to it:
Time dilation could be that going very fast in the space makes you relatively faster in one direction.
The thing is, atoms also have to travel; so the atoms (and matter in general) have a slightly longer distance to travel, to achieve the same chemical reaction. Which means interactions between atoms is slower, giving illusion of a slower time due to slower inter-atoms reactions.
As an obvious example, doppler shift often needs to be accounted for to communicate with spacecraft.
But that is not what our best current model of the universe actually says. Our best current model of the universe says that the observed redshift of a distant object tells us by what factor the universe expanded between the time the light was emitted and now (when we see the light). Viewing it as a Doppler shift is an approximation that only works for small redshifts (much less than 1).
1. As other commenter said, X causes Y does not mean that Y implies X. There can be another cause for the Doppler.
And surprisingly, 2. There is at least one known mechanism that cause Doppler WITHOUT moving: when the observer is in a gravity well (ex: earth) and observing a stationary object outside the gravity well (ex: some fixed point in outer space)
I suppose that's possible. Does that hypothesis adequately explain our observations?
Is the model we currently have completely "correct"? Almost certainly not. But it appears to be less wrong[0] than earlier models.
If you (or anyone) can show how the above describes our observations better and more completely than our current models, then it's likely "less wrong."
But you offer no evidence or even logical argument to support your hypothesis. As such, it's not much more than idle speculation and essentially equivalent, from a scientific standpoint, as suggesting the universe is a raisin dropped into a sugar syrup solution[1] and absorbing the liquid -- hence the expansion of the universe.
[0] https://en.wikipedia.org/wiki/The_Relativity_of_Wrong
[1] https://en.wikipedia.org/wiki/Compote
In our first version, imagine all of the stars at rest. Now, we emphatically know this not to be true locally due to all kinds of measurements, but let's go with it. What happens? The moment you let these stars "go," they begin to draw toward one another due to gravity. You would have gravitational collapse. We do not see that.
Next iteration: we throw the stars, and the galaxies, and the galactic clusters away from one another. No expansion required. Here we have two options. In the first, we did not throw with enough speed, they expand out ... slow to a halt ... and the gravitational collapse again. Again, unobserved. Option two, you have thrown at escape velocity and what you would see is an asymptotically decreasing speed, never quite hitting zero, since gravity works "forever away." Also unobserved.
What you're suggesting is basically the Steady State concept, a kind of static universe. This is a very old idea. So old it was given a kind of courtesy term in general relativity, which would eventually be set to zero.
Here is a rule for any armchair astrophysicists: whatever you think of, that was most likely an idea at one point and was eventually ruled out.
The relevant XKCD is Astrophysics - https://xkcd.com/1758/ ( https://www.explainxkcd.com/wiki/index.php/1758:_Astrophysic... )
The transcript from explain:
There are so many intricate dependencies between these pathways that it's pretty unrealistic to postulate anything else than a big bang + cooldown process IMHO.
So, historically, they did not go hand-in-hand.
https://en.wikipedia.org/wiki/Fred_Hoyle#Rejection_of_the_Bi...
Not that I have the background to know what else they might not have accounted for to reach this conclusion.
Part of this is the distinction between what is happening and why the model says is happening. Does any physicist believe they have the perfect model? Or is it that they use the model that best fits the observations and are open to any other model, as long as it is either simpler or produces fewer contradictions than the current model (and is just as testable).
I think too often we hear reports of "science says X is what happens" when the reality is more like "science says that the current model based on X happening is what best describes current data and observations".
Deleted Comment
The Copernican principle is, indeed, attractive to the modern mind because its neutrality. But it's not neutral. It's just as loaded as any other principle, no matter how crazy it may sound today, philosophical, religious, or merely personal.
Deleted Comment
I'll leave this comment here since I'm about to get rate limited: I read/heard a great idea recently, what if gravity is an emergent quantity like heat? Maybe dense fermions just radiate gravitons just like a hot mass radiates photons?
Deleted Comment
Anti-mass is mass where Gravity goes Out instead of In.
The anti-mass and mass accelerated away from eachother at the start and the redshift is it's repulsion away.
The big bang was first postulated by an agent of the vatican, and scientists raised in any religious context tend to generate experiments that confirm their beliefs.
I'm likely misinterpreting the article, but it seems to frame things in a way that first assumes expansion should be constant and it's a question of what the right constant value is between the measured/theoretical discrepancies.
(*yeesh, editing all those spelling errors from typing on my phone)
This tells us that either our model of the universe is wrong (therefore the cosmic microwave background method is giving us an incorrect answer) or that something is wrong with how we're calculating the distances along the cosmic distance ladder. The latter was originally the assumption that should be proven true with more and better data from newer telescopes. This is now turning out not to be the case: our cosmic distance ladder calculations seem to have been very good, so it now seems more likely that our model of the universe is wrong.
Not according to at least one research group described in the article: the Freedman group, which is only getting the higher answer using Cepheids, but gets a lower answer, one consistent with the CMBR calculations, by two other methods. Which raises the possibility that it's the Cepheid part of the cosmic distance ladder that's the problem.
It also sound like progress that we seem to have 2 “scales” to play with to try to develop a consistently measurable distance.
It's exciting to see how the question drives many new discoveries.
[1] https://en.wikipedia.org/wiki/Great_Attractor
I'll try to paraphrase what it meant: measuring H0 comes down to measuring the relative velocity of galaxies around us. The great attractor was a relatively recent discovery that the "closer" galaxies, the ones we can use in the distance ladder, all have a common component in their velocities which we've recently begun to understand better.
Whenever a scientist says that it's not possible that the model is wrong, then I just roll my eyes. Of course models can be wrong - and isn't that exciting? Good on them for making sure that there are no errors in the measurements - that's incredibly valuable and absolutely necessary - but I'm really excited to see creative models being thought up that are drastically different. My personal hell is the universe being consistent and boring.
Anyone with an ounce of sense should have concluded that LCDM was wrong long ago. Hopefully this will finally cause physicists to actually try something different.
See https://en.wikipedia.org/wiki/Cosmological_principle
Basically, if this weren't to hold true, a lot of astronomy would fall over, even physics.
Most people don't realize that science—and even everything in life—has to start from some axioms/assumptions, just like math. I first realized this fact when I was reading the Relativity book written by Einstein himself, who challenges the assumptions in classical physics.
As time goes, some of the assumptions could be proved to be unnecessary or even wrong. There must be still some assumptions left, though—because without them, we can't talk about science, or anything, really.
Deleted Comment
> A group of researchers argued in 2021 that observations of the Hubble tension may imply that only quintessence models with a nonzero coupling constant are viable.
[0] https://en.wikipedia.org/wiki/Quintessence_(physics)
It doesn't.
"The simplest explanation for dark energy is that it is an intrinsic, fundamental energy of space" [1]. That's the cosmological constant.
Dark energy is a thing because we don't assume that to be the case. Irrespective of your dark energy model, however, there will be a predicted global average.
[1] https://en.wikipedia.org/wiki/Dark_energy
Briefly, recent large scale maps of the universe suggest that the universe might not be as uniform as we thought it was. In particular we appear to be in a large region (something like a couple billion light years across) of low density.
Dark energy is needed to make the solution to Einstein's field equations for the whole universe match observations. However that solution was derived based on a universe with matter distributed uniformly. At the time it was first derived that appeared to be the case--we thought the Milky Way was the whole universe.
When we learned that the Milky Way was just a small galaxy in a vastly larger universe than had thought we were in and that there were bazillions of other galaxies, those galaxies appeared to be distributed uniformly enough the the solution to the field equations still worked.
Later we found that there is some large scale structure in the distribution of galaxies, like superclusters, but those seemed uniform enough throughout the universe that things still worked.
If that couple of billion light year low density region turns out to exist (large scale mapping of the universe is hard enough that it may just be observational error) the universe may not actually be uniform enough to for the field equations based on uniform matter distribution to actually work.
Some researchers worked out the solutions to the field equations for a universe that has such large low density bubbles big enough to invalidate the uniform universe solution, and found that such a universe would have an expansion force without the need to invoke any kind of dark energy.
There was a recent PBS Space Time episode that covered this: "Can The Crisis in Cosmology be SOLVED With Cosmic Voids" [1]. The above is my summary of what I remember from that. See the episode for a better explanation and references to the research.
[1] https://www.youtube.com/watch?v=WWqmccgf78w
It seems like if there were some error in the luminosity measurement for cepheids, it would propagate to the measurements with supernovas...
I would expect that stacking measurement techniques (as is common with cosmology, where distances are vast and certainty is rare) would also stack error, like summing the variance in gaussians...
In 50-100 years they'd get a much better angular fix on stars that are too distant for Earth-orbit-sized angular measurements.
https://en.wikipedia.org/wiki/Stellar_parallax
[1] https://en.wikipedia.org/wiki/Solar_gravitational_lens
(Knowing this field I'm sure there are some astronomers who argue that there are still some systematic uncertainties that are not fully being accounted for, but from what I understand it's pretty hard to account for it with the Gaia results at this point.)
The assumption that these observations indicated an expanding universe was delivered to us by LeMaitre; if you believe in an expanding universe with a finite age, then give credit where it is due...
I think people forget that, due to the longer wavelengths to which it's sensitive, Webb actually has a poorer angular resolution than Hubble.