The Universe Is Expanding Faster Than It Should Be

The usual prediction I see is that the rate of discovery will continue and in another 2 lifetimes everything we think we know now will seem just as silly and naive as what people believed 2 lifetimes before us.

I don't think that's the case. It's a question of whether we're on a (near) infinite exponential curve of discovery, or on the rapid upward stretch of an S curve. I'm in the S curve camp, and I suspect we're at least half way up it on most topics.

Just look at relativity, quantum mechanics and cosmology. Most of the great discoveries in these fields were actually quite a while ago. Relativity and Quantum mechanics are over 100 years old and most of the advances were made in the early decades. Progress has not stopped but massively slowed down.

In cosmology it's been slower progress, but we've charted time back to milliseconds after T=0 of the big bang, detected thousands of exoplanets, analysed the gravitational waves from black hole collisions and mapped the cosmic microwave background. There's more to learn about dark matter and dark energy for sure, some serious mysteries there, but how much difference will those answers actually make? The Higgs boson discovery was cool, but really just a cherry on top of the standard model rather than a revolutionary discovery that upended science. I'm not sure there are any of those still to come.

The closing chapter on the big questions in science still has to be written, but I think the chances it will force us to completely rewrite, rather than simply revise and update the previous chapters, seems slight IMHO. I'm expecting the GUT to simply update or reframe relativity and quantum mechanics to make them compatible, not throw them out completely.

It truly is astonishing to be alive at this nexus in human development, whatever our individual take on it might be.

I feel like this is more a modern misconception than reality. Play a game of Civilization and you'll start to appreciate just how deep human technology really is. In Roman times, they probably said similar things thinking about how advanced they were compared to the Macedonians a couple hundred years back.

Yea we lost some progress or at least the rate of change greatly slowed from 400 - 1600, but other than that blip I think the progress has actually been fairly linear from 4000 BC to 2021 CE. It just seems like it's not because of the cognitive bias to take for granted all the things that seem so basic compared to computers or nuclear reactors, but in reality, those things actually were very difficult to develop with the knowledge and production techniques available at the time.

This is not even close to being true

    Most humans ever born, are still alive today. Our population growth over the past centuries dwarf populations of the prior centuries.

Or maybe they found us but they just don't care because on cosmic scales we aren't as special as we appear to be from our own limited perspective.

It's humbling because it puts all that hubris away, which most people tend to have an excessive amount of - and begin to focus on living and achieving simple happiness instead of some illusion of grandeur.

(BTW, if you want to know the difference between constructive and nonconstructive mathematics, it's that constructive mathematics doesn't let you say "well, it's definitely 0 or 1" because it leaves open the possibility that there's no proof of either termination or nontermination in the current theory... which is basically the philosophical distinction under discussion).

If my recall is accurate then there is a bit of a question if there exists a "final number".

So here's the problem: maintaining a constant velocity requires no outside force, but acceleration does. So, we're seeing everything accelerate away from us.... but why? Where does that force come from?

The CMB would also need explanation and I'm thinking the curves of galaxy speeds wouldn't match unless you tweak the initial speeds way too much, but I'm not sure how to formulate that right.

It's a bit like noticing that objects can't float. They can either fly away or fall but they can't just sit there. It's not exactly the same thing, since we're talking about the spacetime itself rather than gravity as separate from the objects, but the analogy mostly works.

The other is the speed at which light travels (through a medium).

If we were inside a universe-sized black hole, would we be able to tell? Can we experimentally distinguish a black hole universe from a not-black-hole universe?

Fascinating point!

I share the opinion that they're names for holes, not entities; but I'm not qualified to defend that opinion.

Light exiting from stars deep in a galaxy (which is in turn deep in a galaxy cluster) is subject to gravitational redshift. The greater the mass of the galaxy and cluster, the greater the redshift.

If we add transparent, non-radiating mass to a galaxy and its cluster, the mean redshift of starlight from the galaxy will redshift, and we'll see the result on various absorption and emission lines in the spectrum.

Note, we do not need dark matter for this; increasing the central mass of a galaxy cluster will do the trick too. We can do this by adding more stars, or by adding black holes, or by making a central black hole more massive. All of this will cause an increase in gravitational redshift.

However, adding more central mass to galaxies will be reflected in the trajectories of clouds of gas and collections of stars within the galaxies; likewise, adding more concentrated mass to a galaxy cluster will alter the trajectory of galaxies within it. In general, these objects will have faster orbits -- fast enough that a non-gravitational redshift will be apparent in the arrangement of spectral lines. (We would also need to figure out how to keep any central masses from being extremely bright anywhere in the electromagnetic spectrum, and how to keep them from eclipsing parts of their home galaxies and clusters that are behind them from our perspective.)

If we instead redistribute our increase in galactic mass into a gaseous halo-like structure that extends well beyond the outer stars, we get the increased gravitational redshift without requiring stars and gas to travel faster to avoid falling inwards. The halo still has to be electromagnetically quiet, and non-eclipsing, or we would see it. (This all applies to clusters, too).

Stepping away from the "dark matter puzzle" of the spectral lines of molecular and atomic gas clouds in galaxies and clusters of them, we can then look at another aspect of your question.

Galaxy clusters, from their point of view, are just floating passively in space.

The expansion of the universe follows a model wherein if we treat the astrophysical content of galaxies and clusters as sensitive accelerometers that in aggregate show off the peculiar accelerations experienced by these large scale objects, there is no evidence that these accelerometers should report a non-zero magnitude or point in any particular direction. That is, galaxies and galaxy clusters are themselves in free fall, and we can see this because of lines of evidence including expansion-direction-and-magnitude squashings of https://en.wikipedia.org/wiki/Light_echo lines turn out to be null, likewise there are no radio-loud edge shocks for galaxies that appear to be accelerating away from us, there are no shape distortions of the most highly "accelerated" distant galaxies, and so forth. There is simply a spectral redshift.

So, for an isolated galaxy the spectrum it emits depends on a combination of its starshine, any noise from close to its central black hole(s), the noise from any thick gas clouds undergoing gravitational collapse, and the absorption and reemission of diffuse gas clouds heated by these types of emission and absorption-and-reemission sources and the distribution of dark matter within and around the galaxy. The more dark matter, the more redshifted the galaxy's spectral lines will be.

Preserving the Copernican Principle, immersing a set of such isolated galaxies into an expanding cosmos leads to the inhabitants of each galaxy observing a cosmological redshift on the gravitatonally-redshifted spectrum emitted by other sufficiently distant galaxies. The gravitational-redshift depends on the mass of the galaxies including their dark matter. The cosmological redshift only depends on a light-travel distance from the galaxy, no matter how light or how massive that galaxy (or its parent cluster) is.

Next, if we have an arrangement b-->F-->U, where b is a background galaxy, F is a foreground galaxy, and U is an observer, we will that the light from b is cosmologically redshifted before it interacts with the molecular and atomic gas clouds in F, causing a spreading of absorption and reemission lines rather than a thickening. For the very obvious lines from neutral atomic hydrogen, we see this as the https://en.wikipedia.org/wiki/Lyman-alpha_forest which is an important tool in astrophysics and in checking the soundness of our theory of gravitation.

We would get a Lya-forest structure even in the absence of dark matter in b or F or anywhere else; only the fine strucutre would differ. Indeed, we could substitute a diffuse static cloud of atomic hydrogen for F and still see a "forest" structure minus small contributions from the starshine in F and the orbital motions of the gas in F around some central mass. The deep infrared noise from F in that case, however, would not be gravitationally redshifted.

Finally the answer to your question: the cosmological redshift does not in general depend on the "late" distribution of dark matter. (Late as in at least millions of years after the hot big bang). But the gravitational redshift on light emitted by stars and galaxies does depend on dark matter and in general the starshine is reddened by it. The reddening is because of the additional mass of the dark-matter-containing structures, and also because the distribution of the dark matter into halos allows for slower orbits of stars and gas clouds, thus suppressing their non-gravitational non-cosmological redshifts. (In an edge-on spiral galaxy without dark matter we would see a greater difference in the blueshift of one extreme tip of the thin disk and the redshift of the opposite extreme tip of the thin disk, compared to the same spiral galaxy with dark matter; the light-and-radio from the central bulge would be less redshifted in the absence of dark matter).

So, no, the cosmological redshift does not depend on dark matter. The gravitational redshift on light from galaxies, however, does. So does the non-gravitational, non-cosmological redshift on gas clouds whirling around the edges of rotating (e.g. spiral) galaxies, or rising and sinking on radial orbits within elliptical galaxies.

"Tidal effects" are a non-starter, and do nothing to explain galaxy rotation curves. Nor do they explain the excess motions of individual galaxies in galaxy groups and clusters, or the excess pressure of hot, X-ray emitting gas in groups and clusters -- all of which requires either some kind of dark matter, or some kind of alternate theory of gravity, or both.

(I notice that I pointed out the ignorant nonsense of that video when you posted a link to it 7 months ago -- https://news.ycombinator.com/item?id=27085979#27090683)

> Let me ask you; How long was the time before you were alive, and how long will the time be after you are dead?

This is misleading as it posits that since there's a "before you" and an "after you", "you" are standing within some time continuum.

> There has to be a beginning and end.

Wrap your head around this: if there is a "beginning" and an "end" to our universe, both would define discrete stop points, thus there would be some way to say "before" or "after" these discrete points, and thus the universe sits inside something that has time.

But then either this something of a container either has infinite time (which goes back to square one of things not having beginning and end) or has a beginning and end itself.

But again, if this container has a discrete beginning and end then you can apply the same logic that you applied to our universe in the first place. So there's another container of the container, to which you can recursively apply the same logic (assuming logic itself somehow applies to this container). So if you want all these container things to have beginnings and ends, it implies there's an infinite set of nested containers.

The alternative is that time exists within our universe, but our universe doesn't sit within time. Thus "beginning" and "end" are not discrete points but concepts that only exist as limits (in the mathematical sense of "limit"): by travelling the arrow of time forward or backward you can only move closer and closer to either but never actually reach these because these points don't actually exist.

Such a universe could be freestanding (as in: it's all there is), or it could have a container of sorts, and maybe even neighbour universes. But since time only exists within our universe, there could very well be no concept of time in the container (or other universes), or similar concepts but behaving entirely differently. In any case these (the container, the other universes) would be unbelievably incomprehensible to us.

These concepts of unreachable zero, unreachable infinite, or unreachable value do exist in other areas: absolute zero is basically unreachable, reaching c velocity requires infinite energy†...

† which creates other interesting conundrums such as a pair of photons travel at the same delta speed whether they go away or towards each other, or there's no "velocity = 0" either, or tachyonic vs baryonic realms.

[0] http://www.geom.uiuc.edu/~banchoff/Flatland/

Also, not having a border is not the same as being infinite. Zogg from Betelgeuse started a great series on the topic [1], but disappeared after two episodes. I highly recommend it! But, long story short, it's entirely possible to have a finite universe that, nonetheless, has no edge. Go far enough in one direction and you might end up back where you started.

[1] https://www.youtube.com/watch?v=_k3_B9Eq7eM

Assuming there's a physical edge to our universe, there's still the problem of the "observable universe" -- there's a lot more of the universe that we will simply never be able to see or reach, because it's moving away from us too quickly. That defines another kind of edge, one that's much "closer". Entities beyond our observable universe can't affect us gravitationally; it takes time for the effects of gravity to propagate from one point to another, as "gravitational waves", and the universe is inflating too quickly for those to reach us.

You might say that things further away from us, but within the observable universe, might be affected by things beyond, because they're "closer" to it than we are. I think that's correct, but by the same token, I think the information about those effects would never reach us in finite time.

That's why I was thinking in terms of extra spatial dimensions. You might have a bunch of matter "nearby" but totally inaccessible to you, because we're incapable of moving along the extra axis. But that gets back to the research I linked.

Now imagine there are two such spheres near to each other. Two beings whose physical laws and interactions were entirely confined to their respective surfaces would be unaware of each other and unable to influence each other in any way - gravity for example is only able to travel around the surface of the sphere, not into whatever 'space' lies in between. The surface of each sphere is its own universe.

There's no reason to believe our "common sense" or "intuitive understanding" has any relevance to how the universe is or isn't.

According to an ant's common sense humans for sure have antennas somewhere on their bodies, even if not visible to an ant.

It's alright to critique something to try and improve it, you know?

Lastly, it's not really besides the point, as our models and possibly our observations being wrong is entirely the crux of this "problem".

Absolutely. If one person does their best work and is still frustrated after trying dozens of options, it is fair for them to be scratching their head and saying, "This shouldn't be happening."

The article itself provides exceptionally precise reporting (though IANAA) of mutual work toward a grand challenge in astronomy --- which is still leaving people scratching their heads at discrepancies after trying dozens of model or analysis variations. The word 'should' in the title (I believe it only appears there) to me conveys well-enough (it is only a title) that there are human elements/expectations and legitimate after-the-best-of-care puzzles remaining.

I don't know if it is a good comparison, but if the time for the g-2 result reveal came, and the result was a 20-sigma discrepency, I would expect every experimentalist and every theorist involved to immediately scowl and say, "F***, this should never have happened."

Also perfectly ordinary and ideomatic: To say, This should be compiling without errors.

Great science reporting in the article, hence my rant to defend it.