[pdonis]

> It would indeed be incredibly exciting if it would lead to a less silly explanation of the redshift than "universe is expanding".

It won't. There is no question about whether the universe is expanding. The issue is over how fast it is expanding; we have two different theoretical models that are giving different answers, and we don't understand (yet) how to reconcile them.

[mpoteat]

Not a physicist, but why isn't the simple explanation that light loses energy over cosmological distances not also a valid explanation of redshift? We already know that energy isn't conserved at those scales.

[pdonis]

> why isn't the simple explanation that light loses energy over cosmological distances not also a valid explanation of redshift?

It depends on what you mean by "light loses energy over cosmological distances". See below.

> We already know that energy isn't conserved at those scales.

This suggests that what you mean by "light loses energy over cosmological distances" is that the universe is expanding while the light travels, and in an expanding universe, total energy is not conserved. That is true, and that is the redshift due to the universe expanding--it's not something different.

If, however, you are working from the hypothesis that the universe is not expanding, then energy is conserved even over cosmological distances, so I don't understand what you mean by invoking "energy isn't conserved" as an explanation for the redshift.

If you're referring to the "tired light" hypothesis, that's been known to be unviable for decades.

[gpderetta]

Energy is not conserved at those scales because of general relativity, which also provides the mechanism for an expannding universe. Einstein himself didn't like this result of GE and tried to hack around it with the cosmological constant. Turns out the math was right.

[pdonis]

> Turns out the math was right.

Actually, turns out the math was right with a cosmological constant: that's how the accelerating expansion of our universe is explained. Einstein actually blundered twice: first by putting in the constant for the wrong reason (because he wanted a static universe), but then taking it out again when it turned out that reason wasn't the case (when the expansion of the universe was discovered).

If you just look at how to derive the Einstein Field Equation of General Relativity from first principles, the constant should be there; it isn't an add-on to General Relativity at all, it's part of it. It's just that there's no way to know from those first principles what its value is. That we had to figure out from observations.

[gpderetta]

Re: accellerating expansion, I was only commenting on how the expansion itself is a consequence of GE.

Thanks for your point about the constant being actually required. I do not understand the math, but is this similar on how integrals always have a constant as a free parameter that need to be determined by other means?

I assume that Einstein originally set the constant to exactly balance the expansion, but later set it to zero. In bot cases you are picking arbitrar values but the actual value need to determined by empirical observeations.

[pdonis]

> is this similar on how integrals always have a constant as a free parameter that need to be determined by other means?

Not really, no. It's a consequence of the assumptions made when the Einstein Field Equation (EFE) is derived from a Lagrangian using the principle of least action.

The assumptions are that the Lagrangian should be a Lorentz scalar (which is required of any Lagrangian) and that it should include no more than second derivatives of the metric. The Ricci scalar R meets this requirement and is the Lagrangian that was originally used by David Hilbert to derive the EFE (without a cosmological constant). But a simple constant (the cosmological constant) also meets the requirement, and therefore should be included in the Lagrangian; including it leads to the cosmological constant term in the EFE.

[gpderetta]

Thanks for the explanation in this and the sibling comment!

[pdonis]

> I assume that Einstein originally set the constant to exactly balance the expansion, but later set it to zero.

Einstein didn't include the constant at all in his original equation, published in 1915.

In (IIRC) 1917, he realized that his original equation did not allow a static solution for the universe as a whole. He also realized that including the cosmological constant term in his equation would be mathematically valid, and that if he picked just the right value for the constant, he could obtain a static solution for the universe. At that time, it was generally believed that the universe was static on large scales.

Then, later, when it was discovered that the universe is expanding, Einstein dropped the cosmological constant term. He later called including that term in 1917 "the greatest blunder of my life", because if he had just gone with his original field equation, without the constant, he could have predicted the expansion of the universe more than a decade before it was discovered.

> In bot cases you are picking arbitrar values

When the expansion of the universe was discovered, yes, it was already recognized that it is valid to include the constant in the Einstein Field Equation, so it couldn't just be un-included. Its value was just assumed to be zero since that was consistent with all observations that were known then.

In Einstein's original 1915 field equation, however, the constant wasn't "set to zero". It wasn't included at all; nobody even realized at that time that it was valid to include it.

[cygx]

> If you just look at how to derive the Einstein Field Equation of General Relativity from first principles, the constant should be there

I wouldn't go that far: Rather, adding it doesn't violate any of the heuristics used to come up with the field equations or action. So to avoid bias, one should keep it around. However, in the absence of observational evidence to constrain its value, it's also justified to start any investigation with its value assumed 0...

[pdonis]

> adding it doesn't violate any of the heuristics used to come up with the field equations or action. So to avoid bias, one should keep it around

Yes, that's what I was trying to say. I didn't mean that including it mathematically in the equations necessarily requires one to adopt a non-zero value for it; you are quite correct that one shouldn't do that unless one has observational evidence to back it up (and cosmologists in fact didn't adopt a non-zero value until observational evidence required it).

[grishka]

> There is no question about whether the universe is expanding.

Why?

> we have two different theoretical models that are giving different answers, and we don't understand (yet) how to reconcile them.

Maybe... just maybe... that's because it isn't actually expanding, but there's some other, unknown effects at play?

It's about time we think outside the box. To me, as a regular programmer kind of person, the very idea that the universe is expanding makes about as much sense as dark matter. I get it, both make the calculations work out, but they're basically "we have no clue what this actually is, here's our best guess" kind of variable.

[pdonis]

> Why?

Because all of the other theoretical models that attempt to explain the observed redshifts fail to match observations.

> Maybe... just maybe... that's because it isn't actually expanding, but there's some other, unknown effects at play?

Nope. Already been tried. Doesn't work.

> It's about time we think outside the box.

Cosmologists have been trying out of the box ideas for decades. All of them failed. That's why we have the models we have now: they're the only ones that survived that process.

> I get it, both make the calculations work out, but they're basically "we have no clue what this actually is, here's our best guess" kind of variable.

For dark matter, you are correct: we don't know what it is, and the term is basically just another way of saying "whatever we need to add to our model to make it match observations works basically like ordinary matter does in the equations".

But that is not true of expansion itself. All of the alternative models that have been proposed to eliminate the need for dark matter still have an expanding universe. The expansion itself is a much more solid conclusion than dark matter is.

[pdonis]

> It's about time we think outside the box

Another way of looking at it, to add to my previous post: dark matter and MOND are thinking outside the box. They cover both possibilities for how to expand on our current theories: either (a) there's more "stuff" out there than our current theories of particle physics know about; or (b) there's more aspects to gravity than our current theory of gravity knows about. Consider whatever "unknown effects" you like: they will end up coming down to one of those two possibilities.

Thinking "inside the box" would be something like: it's far more likely that either the data is wrong or we haven't calculated the predictions of our current theories correctly than that our current theories, which have tons of experimental confirmation, are wrong. Historically, most of the time in science, when there's been a discrepancy between theories and data, that is how the discrepancy has ended up being resolved: either we've figured out something was wrong with the data, or we've figured out that something was wrong with how we calculated the predictions of our current theories.

The reason why cosmologists are driven to consider models like dark matter and MOND is that they have checked and double checked and triple checked both the data and our predictions from our current theories, and the mismatch hasn't gone away. So they are driven to consider "out of the box" ideas, and, as above, dark matter and MOND cover the possibilities.

[mr_mitm]

There is much more evidence for the expansion of the universe than just the cosmological redshift.

Where do you get the idea that it "makes the calculations work out"? You got it backwards. The calculations showed that the universe must expand. Georges Lemaitre found that out. People were skeptical at first and said the calculations don't apply, but when the cosmic microwave background was discovered in the sixties, all but the most stubborn hardliners were convinced that the universe must expand and that there must have been a big bang, i.e. a singularity in the finite past.

The microwave background is basically a picture of the universe when it was a baby. There were no galaxies, only hot gas that just cooled down enough to turn from plasma to neutral gas which later clumped into galaxy clusters, galaxies and so forth. Because the speed of light is finite, we can literally see how the universe looked liked billions of years ago. And it looked hot, because it was compressed, and it looked young, because structures hadn't had the time to form yet. The gas composition also shows only light elements, because heavy elements need to be forged in supernovae. Young stars have lots of heavy elements, far away, old stars only consist of hydrogen and a bit of helium.

The tiny, tiny irregularities that we see in the cosmic microwave background match beautifully what we know about thermodynamics, statistical physics, quantum mechanics, general relativity and electrodynamics to a very high degree. It's marvelous, really.

> the very idea that the universe is expanding makes about as much sense as dark matter

The universe does not care about what makes sense to us. It just is. And the expansion is an observational fact.

On a side note, I find it a bit presumptuous to assume that a layperson knows better than legions of professional cosmologists. These are very smart people who work full time for the better parts of their lives on these problems while you clearly haven't put in the time to understand the fundamentals. They wouldn't come to the conclusions they come to if they didn't think they had merit. You can be curious about it, you can have questions about it (please do!), you don't have to understand it, but please trust experts on their opinion and show some humility and some respect. I'm sure you wouldn't appreciate a physicist who tried to tell you how to design your programs either. (We're known to write horrible software, just look at ROOT.) Imagine some amateur telling you "for-loops don't make sense to me, why don't you use GOTO". No offense, and sorry to say it like this, but it really bugs me a bit.