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> There have been a longstanding problem that the history of the universe and cosmological distance scale haven't made sense. https://medium.com/starts-with-a-bang/the-hubble-tension-sti...

> When I was getting my PhD in condensed matter physics I was going to the department colloquium all the time and seeing astrophysics talks about how some people thought the hubble constant was 40 km/s/Mpc and others thought it was 80 km/s/Mpc. With timescape cosmology maybe they were both right.

You're (mis)remembering a different (old) problem and confusing it with a new one. The problem in the 1970s and 1980s was: what is the local expansion rate of the universe? Where "local" mean "within a few hundred megaparsecs". There were two main groups working on the problem: one group tended to find values of around 50 km/s/Mpc and other values of around 100. Gradually they began to converge (in the early 1990s, the low-H0 group getting values of around 60, the high-H0 group values of around 80), until a consensus emerged that it was in the low 70s, which is where we are now.

The "Hubble tension" is a disagreement between what we measure locally (i.e., a value in the low 70s) and what theory (e.g., LCDM) says we should measure locally, if you extrapolate the best-fitting cosmological models -- based on cosmological observations of the CMB, etc. -- down to now (a value in the upper 60s). This has only become a problem very recently, because the error bars on the local measurement and the cosmological predictions are now small enough to suggest (maybe/probably) meaningful disagreement.

> Another longstanding problem in astronomy is that since the 1970s it's been clear we have no idea of how supermassive black holes could have formed in the time we think the universe has existed. With the JWST there are a flood of results that show the first 500 million years of the universe probably lasted a lot more than 500 million years https://iopscience.iop.org/article/10.3847/2041-8213/ac9b22

That's not a "longstanding" problem, it's a problem from the last 25 years or so. In order for there to be a problem, you have to have what you think are reliable estimates for the age of the universe and evidence for large supermassive black holes very early in the universe. This is something that has emerged only relatively recently.

(Your link, by the way, is to a paper that has nothing to do with black holes.)

Slightly tangential but how does the ad hoc nature of things like cosmic inflation seemingly not bother more people? Quite the opposite, it's a rather lauded discovery. This is more cosmology than astronomy, but at least reasonably related. That topic alone destroyed my interest in an academic pursuit of astronomy.

'Here's an idea that makes no logical sense, has no physical argument whatsoever (let alone evidence) in support of its existence, and just generally seems completely absurd - but if we ignore all of that, it solves a lot of other pesky problems with reality, as observed, practically falsifying other lauded theories.'

Just add more epicycles?

Well, it's not like people pulled it out of thin air. Both inflation and the lambda-CDM models are solutions to the GR equations, so in that sense it's perfectly justifiable to see if general relativity can explain the data. I don't think it's fair to say that it "makes no logical sense".

I assume that OP was talking about the cosmic inflation theory that claims there was a rapid expansion immediately after the big bang. I don't see how that's a solution to the GR equation, could you maybe explain/give a link?

There is an addition of a inflation field. Observations show that such an inflation happened, and there's several ideas of inflation fields that are compatible with the standard model and match observations. The equations that make the observed expansion happen in the presence of such a field the friedmann equations, which are GR solutions.

The field isn't just something that magically expands things, it just effects energy density. The rapid expansion is then a consequence of GR.

Nobody’s happy with dark energy, it’s just the only framework we have that fits the data. All the other ideas might be brilliant and inspired but are measurably worse at describing the real world.

Even the name “dark energy” is a tacit acknowledgment that—along with dark matter—we have no clue what the underlying physics actually is.

How is timescapes measurably worse? It's not a new theory, so perhaps it's been tested, and if it's failed why is it back in the news? (Sometimes failures get back in the news. It's a fair question.)

It starts with the assumption that our observations of cosmic expansion are false and due to time dilation from a clumpy universe, then works out how clumpy the universe must be to account for our observations.

For timescapes to "work" time must run 38% faster in cosmic voids. This works out to a density difference of about 100,000 times what we observe with many existing methods.

> It starts with the assumption that our observations of cosmic expansion are false

It starts with the assumption that our measurements (observations!) of attenuation and red-shift of standard candles are correct. Your characterization is wildly wrong, unless you meant to use a different word than 'observations'.

> and due to time dilation from a clumpy universe, then works out how clumpy the universe must be to account for our observations.

Yes, it does work backwards. That's insufficient by itself to say that timescapes is incorrect. It's concerning though because they work out what densities must be in order to explain away all of the attenuation/red-shift disparities. However the reasoning time dilation is not wrong -- in fact, it's blindingly obviously correct if GR is correct, it's just that it might also be wildly insufficient -- it might just be in the noise.

> For timescapes to "work" time must run 38% faster in cosmic voids. This works out to a density difference of about 100,000 times what we observe with many existing methods.

Thanks. This is responsive to my question, but it's not yet dispositive. We could find that:

  - some of our observations are incorrect
  - some of our interpretations of those
    observations are incorrect
  - timescapes explains some but not all
    of the apparent acceleration of the
    expansion of the universe

Also, density differences need to take into account density differences at the time that a ray of starlight we observe today entered each void along the way to us, not current apparent density disparities. It has to be that 8 billion years ago the difference in density between clusters and voids was much starker, though the voids would have been smaller, and perhaps the difference was not enough orders of magnitude.

There's a ton of theoretical work out there that is about building a foundation for future work. This is one of those works. But people tend to overlook the part where the work is just a piece, and many other things most first fall into place and over hype it. This is course isn't the fault of the authors.

What do you mean by "bother"? The universe appears to be expanding, so I assume you don't deny that evidence, correct?

I was doing an astrophsyics Phd 15 years ago, and one of the many reasons I abandoned it is exactly this. To get published, I would have had to start all my papers introducing and assuming the ΛCDM model, even though it just didn't seem right to me (too many "dark" components, too many assumptions, inflation).

To be fair, people a lot smarter than me think it's good, or good enough.

On the topic of early black hole growth I saw this released a couple of months ago, an early black hole apparently growing at 40x the Eddington limit 1.5 billion years after the big bang.

https://chandra.si.edu/press/24_releases/press_110424.html

> A super-Eddington-accreting black hole ~1.5 Gyr after the Big Bang observed with JWST

https://www.nature.com/articles/s41550-024-02402-9

Correct me if I'm wrong, but the term Eddington limit is a bit misleading as it does not describe some physical rate that cannot be exceeded. Lots of super Eddington objects are known.

It's the point where light pressure can blow off the outer layers of a star

https://en.wikipedia.org/wiki/Eddington_luminosity

Objects that pulse like

https://en.wikipedia.org/wiki/Eta_Carinae

can evade it and there are other ways too.

When it comes to super-massive black holes there is the question of how quickly stuff can even get close enough to the black hole to get into the accretion disk.

500M years is a long time for the kind of large star that becomes a black hole (blows up in 10M years or so), but if one black hole is going to merge with another black hole and that is going to merge with another black hole and so on there is no Eddington limit (no EM radiation!) but rather the even slower process of shedding angular momentum via gravitational radiation. (One highlight of grad school was the colloquium talk where we got to hear the signal from two black holes colliding almost 20 years before it was detected for real)

I hope JWST sees

https://en.wikipedia.org/wiki/Stellar_population#Population_...

Note those Pop 3 stars have a higher Eddington limit because they've got hardly any "metal" in them which means light interacts with them differently, although astronomers have the strange (to me) conventional that anything heavier than Helium is a metal which includes, say, oxygen. (As a cond-mat PhD I think a metal is something that has free electrons, which could be one of those elements towards the left side of the periodic table or could be a doped semiconductor or polymer like polyaniline)

Thank you. I did not know that the first cosmic generation of stars had a higher Eddington limit. Why does light interact differently with hydrogen and helium than it does with the other elements? Does it have anything to do with having only a single layer of electrons in the atom?

" He thought that we didn't know what the hell was going on with accretion disks but that a few powerful people in the field created the impression that we did and that there was no dissent because it was so difficult to get established in the field." ... seems to be an epidemic of this stuff in academia tbh.

Most, and I mean scientists and not, don't seem to understand that ALL modelling is curve fitting. Applying maths equations to real data is always thus.