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Comment by taylodl | Hacker News Reader

I don't see where this explanation is strange, it's straightforward GR (General Relativity), as much as one can call GR "straightforward." It's a simpler explanation than inventing Dark Energy, for which we have no direct observational evidence.

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> It's a simpler explanation than inventing Dark Energy

Intuitively I never liked the Dark Energy theory. Doesn't seem valid to just make up an answer supported by no direct evidence for something you can't explain. It sounds a lot like religion in fact.

But I never took physics after high school, so I always assumed I just didn't understand and had faith that these wise men knew what they were talking about.

I don't think it's accurate to compare it a theocratic phenomenon. Ask any physicist, they will say that science requires checking our theories against the evidence and changing our theories as new evidence comes to light. It's not like they issue scriptures to all grad students about what you must believe.

In practice, science requires balancing multiple criteria. Does a model have the right level of simplicity and complexity? Can I convince my peers that it's correct? Should I trust the results of a crank over the results of someone who has a long history of excellent work, and how much onus is there on my me and my research group to reproduce every single published result?

Keep in mind, no one expects to be responsible for a revolution, so you get an effect similar to "poll herding" -- it is desirable to have a consensus, and as consensus emerges around a theory for how to describe some new observations, you get people going along with it even if they maintain personal reservations. In fact, it has been clear to many physicists that dark energy is just an ad hoc explanation, and there has been serious contention about what better model could exist, and there have been several competing models. But every model had its flaws. It's possible that the timescape model will also have flaws that cause it to be re-evaluated.

I recommend a book called "What is this thing called science?" by Chalmers [1]. When I was an undergrad it gave me a more nuanced understanding of the Philosophy of Science.

[1] https://en.wikipedia.org/wiki/What_Is_This_Thing_Called_Scie...

> to just make up an answer supported by no direct evidence for something you can't explain. It sounds a lot like religion in fact.

Well, it sounds like the Aether, really.

Dark energy isn't really a theory, it's more of a placeholder for the observation that the universe's expansion is accelerating. There are a variety of ways you can potentially explain this observation. The simplest theory is the standard lambda-CDM model for dark energy where it is taken to be a cosmological constant. This model has worked well for about 20 years or so, but it is starting to come in tension with newer high precision observations.

That seems reasonable; I guess I'd rather hear "for reasons we don't understand the universe appears to be expanding and the expansion is accelerating by a constant factor we're calling 'lambda'." But I guess "dark energy" is fewer words, if that's what it means.

That’s what it means. The field is pretty transparent about it. It’s often poorly conveyed in pop science, but that’s how it goes.

As I remember it (from 40+ years ago, and well before dark energy was discussed), lambda naturally arises as a constant of integration in GR rather than being "invented" as in the usual story. Assuming this to be true, I'd suggest that any model should either allow for a non-zero lambda, or give a reason for it to be zero. I.e. even if an alternative for dark energy is found, we still have to deal with lambda.

Yes, given the equations of GR, there's no particular reason to assume that the cosmological constant should be zero. (Though one of the major problems in theoretical physics is that back-of-the-envelope estimates as to what it "should" be give numbers that are ~120 orders of magnitude larger than its observed value. So if dark energy is a cosmological constant, it's not really understood why it is non-zero, but extremely small.)

Isn't that why they came up with "dark matter" i.e. invisible, undetectable mass.

No, dark matter is quite different. There are a number of different lines of evidence for dark matter. One of these is sort of similar to dark energy, in that it is needed to explain the slower-than-expected expansion of the universe at early times. But the most compelling line of evidence for dark matter (IMO) is baryon acoustic oscillations in the CMB.

I thought that lambda was directly in the differential equation form?

    Rμν − (1/2)*gμν*R + Λ*gμν  = (8*π*G / c^4) * Tμν

Lambda-CDM is a model, yes, but DE sure is a theory, or hypothesis if you prefer. Look at https://en.wikipedia.org/wiki/Dark_energy#Nature and reference [23] and note that there's a proposed mechanism for the acceleration of the expansion of the universe.

[23] http://philsci-archive.pitt.edu/398/

Idk. My son once used standard knowledge of the quantum froth / particle / anti-particle pair production in vacuum to estimate the light pressure due to that production in great voids and came up with an answer that's off from standard cosmology by just a factor of two (i.e., this explained half the acceleration of the expansion of the universe). It's pretty easy to take assumptions like this and extrapolate. It's not clear that dark energy is obviously wrong, but it's weird because the universe presumably needs energy in order to accelerate its expansion, and... where's that coming from? The standard answer is: from the light traversing the universe (especially the great voids) which loses energy as the universe accelerates its expansion. But what is the mechanism by which that happens? I've yet to see such a mechanism proposed, and that is what makes Dark Energy weird.

I would not compare DE to religion though. It's DM that's a bit of a bridge too far for me, though even there it's hard to know yet.

Also note that timescapes is only about DE not DM. I don't think you'll find an explanation for the galaxy rotation curves anomalies in GR.

I've always known that Dark energy/matter is bunk the minute I read about it. It uses <something> variables. More simply put they make up numbers so the math works. Sounds like something a child would do to try to fool their parents. I'll finish the homework later mom see I made IOU's in the answer fields. I've never understood how this theory was not laughed out of the field rather than strongly supported.

Then again I'm not a physicist.

I was wondering the same. It's just an application of GR in a way I thought people would have been doing already, right?

The equations are really simple for a homogeneous universe

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

It's only recently that people have been able to handle the more complex case of an inhomogeneous universe. Scientists are often like the drunk who is looking for his keys under the lightpost.

> the more complex case of an homogeneous universe

s/homogeneous/inhomogeneous/ ?

Good call. I remember pretty clearly fighting with the spell checker on that one and I think it managed to change it when I wasn't looking. It's fixed!

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