Does dark energy's effect or strength vary over time?

[Cerenkov]

Hello.

It's my current understanding that dark energy is causing the universe's expansion to accelerate. It's also my current understanding that there is some mechanism that caused dark energy's effects to become more pronounced, several billion years ago. Which makes me wonder if the value or strength of dark energy is variable over time, starting off weaker than it was and then somehow gaining strength.

So, my first question is, am I mistaken here?

My second question hinges on something I believe to be the case, but which may have to be corrected. It's my current understanding that the physical constants of the universe are assumed to the same today, tomorrow and yesterday, allowing us to extrapolate either backwards or forwards in time, so that we can describe the physical conditions that held good in the distant past or in the distant future. For example, using this kind of extrapolation, we've been able to describe the first few minutes of the evolution of the universe with precision.

So, to my second question.

If the strength or effect of dark energy varies over time, doesn't this make such extrapolations either much more difficult, or even impossible? The whole basis of extrapolation being the constancy of the mechanisms at work in the universe. If dark energy introduces time-varying changes, how is the kind of extrapolation still viable?

Thank you.

Cerenkov.

 

[PeroK]

Hello.

It's my current understanding that dark energy is causing the universe's expansion to accelerate. It's also my current understanding that there is some mechanism that caused dark energy's effects to become more pronounced, several billion years ago. Which makes me wonder if the value or strength of dark energy is variable over time, starting off weaker than it was and then somehow gaining strength.

So, my first question is, am I mistaken here?

In the standard big bang model, the energy density of the vacuum is constant over time. But, as the universe expands the energy density of the matter and radiation reduces (same matter over a greater volume). And so the contribution from dark energy gradually dominates.

My second question hinges on something I believe to be the case, but which may have to be corrected. It's my current understanding that the physical constants of the universe are assumed to the same today, tomorrow and yesterday, allowing us to extrapolate either backwards or forwards in time, so that we can describe the physical conditions that held good in the distant past or in the distant future. For example, using this kind of extrapolation, we've been able to describe the first few minutes of the evolution of the universe with precision.

So, to my second question.

If the strength or effect of dark energy varies over time, doesn't this make such extrapolations either much more difficult, or even impossible? The whole basis of extrapolation being the constancy of the mechanisms at work in the universe. If dark energy introduces time-varying changes, how is the kind of extrapolation still viable?

Thank you.

Cerenkov.

There is a alternative idea that the vacuum energy density may be increasing over time. This is called quintessence and may lead to what's known as the big rip.

 

[PeterDonis]

It's my current understanding that dark energy is causing the universe's expansion to accelerate.

Yes.

It's also my current understanding that there is some mechanism that caused dark energy's effects to become more pronounced, several billion years ago.

No. All that happened is that the density of matter, which decreases as the universe expand, became smaller than the density of dark energy, which does not decrease as the universe expands. Nothing changed about dark energy at all. What changed was the effect of the matter.

am I mistaken here?

Yes. See above.

It's my current understanding that the physical constants of the universe are assumed to the same today, tomorrow and yesterday, allowing us to extrapolate either backwards or forwards in time, so that we can describe the physical conditions that held good in the distant past or in the distant future. For example, using this kind of extrapolation, we've been able to describe the first few minutes of the evolution of the universe with precision.

It's not the constants that are assumed to be the same, it's the laws. Certain constants (such as the fine structure constant) being the same is a consequence of the laws being the same, not an assumption in itself.

 

[phinds]

It's also my current understanding that there is some mechanism that caused dark energy's effects to become more pronounced, several billion years ago.

To expand on what @PeroK said, Dark Energy is believed to have been present since the beginning but matter dominated the expansion until about 6 or 7 billion years ago at which point the effects of DE started to be more than the effects of matter (including dark matter) because, as he pointed out, the density of DE doesn't change over time but the density of matter does (it decreases).

EDIT: I see fast-fingers Peter beat me to it

 

[Cerenkov]

Thank you very much for your replies.

I think I now see what you are describing.

The expansion of the universe is the key. This expansion was responsible for the dilution of energy and matter across the increasing volume of the universe. This dilution has allowed dark energy to become dominant. No changes in dark energy are necessary for this to happen.

So, the acceleration we see is not due to a change in dark energy itself, so much as a changeover from one kind of dominance to another.

Thanks again,

Cerenkov.

 

[ohwilleke]

A major area of astronomy and cosmology research right now is testing the null hypothesis of a cosmological constant in which the density of dark energy is constant over time against alternative hypotheses in which it varies over time. I've seen more than half a dozen papers seriously trying to determine that by collaborations of physicists in the last year or so, but thus far, the results are inconclusive. See, e.g., Wang (2021).

Some varying dark energy hypotheses have been ruled out, but many are still in the running and are marginally preferred in terms of goodness of fit to the data statistics like a Chi-square test, or Bayesian statistical methods.

The renewed interest is driven, in part, by tensions in measurements of the Hubble constant and other cosmology parameters at different redshifts (basically early v. late), and in part, because the quality of the data with new telescopes or data processing tools has improved our ability to distinguish between different hypotheses with observational data that wasn't previously available. A new pre-print argues that if the Hubble tension is real, and not just a function of some sort of systemic error, that this necessarily means that a class of models (which includes essentially all mainstream models that have a constant dark energy density hypothesis) are incorrect:

We reverse the logic behind the apparent existence of H₀-tension, to design diagnostics for cosmological models. The basic idea is that the non-constancy of H₀ inferred from observations at different redshifts is a null hypothesis test for models within the FLRW paradigm -- if H₀ runs, the model is wrong[.]

From Chethan Krishnan, Ranjini Mondol, "H₀ as a Universal FLRW Diagnostic" arXiv:2201.13384 (January 31, 2022).

And there is certainly plenty of work arguing that the Hubble tension is due to a real physical discrepancy and not merely a result of measurement errors. See, e.g., Adam G. Riess, et al., "A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km/s/Mpc Uncertainty from the Hubble Space Telescope and the SH0ES Team" arXiv:2112.04510 (December 8, 2021) (submitted, ApJ) ("We find a 5-sigma difference with H₀ predicted by Planck+LCDM, with no indication this arises from measurement errors or analysis variations considered to date.").

There are, however, also plenty of papers attributing the Hubble tension to systemic error and/or suggesting that the systemic error in dark energy estimates is greatly understated, rather than reflecting "real" physical phenomena. See, e.g., Edvard Mortsell, et al., "The Hubble Tension Bites the Dust: Sensitivity of the Hubble Constant Determination to Cepheid Color Calibration" arXiv (May 24, 2021); S.L.Parnovsky "Bias of the Hubble constant value caused by errors in galactic distance indicators" arXiv:2109.09645 (September 20, 2021) (Accepted for publication at Ukr. J. Phys); Ritesh Singh, "Evidence for possible systematic underestimation of uncertainties in extragalactic distances and its cosmological implications" arXiv:2111.07872 (November 15, 2021) (published in 366 Astrophys Space Sci 99 (2021) DOI: 10.1007/s10509-021-04006-5); Young-Wook Lee, et al., "Discovery of strong progenitor age dependence of type Ia supernova luminosity standardization process and discordance in cosmology" arXiv:2017.06288 (July 13, 2021) (submitted to Apj); Roya Mohayaee, Mohamed Rameez, Subir Sarkar, "Do supernovae indicate an accelerating universe?" arXiv:2106.03119 (June 6, 2021).

See also, comparing the some of the papers arguing for systemic error and those arguing against this explanation, Eugene Oks, "Brief Review of Recent Advances in Understanding Dark Matter and Dark Energy" arXiv:2111.00363 (October 30, 2021) (93 New Astronomy Reviews 101632) DOI: 10.1016/j.newar.2021.101632.

 

[Cerenkov]

That sounds like interesting stuff ohwilleke, but w-a-y over my head.

Perok, PeterDonis and phinds said that dark energy has not varied over time. But, from what I can glean from your message, some scientists are considering the possibility that it does vary over time. If so, then what about extrapolating into the distant past? Surely that requires the laws of nature remain constant over time and do not vary?

Can you help please?

Thank you.

Cerenkov.

 

[PeterDonis]

Perok, PeterDonis and phinds said that dark energy has not varied over time.

No, that's not what we said. Go back and read our posts again, carefully.

from what I can glean from your message, some scientists are considering the possibility that it does vary over time.

Yes. And that is perfectly consistent with what @PeroK, @phinds, and myself have said.

what about extrapolating into the distant past? Surely that requires the laws of nature remain constant over time and do not vary?

The density of dark energy varying over time does not require that the laws of nature vary over time.

 

[phinds]

The density of dark energy varying over time does not require that the laws of nature vary over time.

 

[Ibix]

some scientists are considering the possibility that it does vary over time. If so, then what about extrapolating into the distant past? Surely that requires the laws of nature remain constant over time and do not vary?

The point is that it varies according to a law of nature which doesn't vary. My (limited) understanding is that the different models are governed my a parameter $w$, and when this is -1/3 the dark energy density is constant, and if it's different from that the density either increases or decreases. Our current measurements are consistent with $w$ being -1/3, or a bit less or a bit more. So we can't rule out any of the options at this time.

I think that typically people will put in the $w=-1/3$ option (unless they are actively studying options here), because the maths is more straightforward than the others and we've no real reason to put in any other particular value.

 

[Cerenkov]

I'm sorry, but I will need some further help please.

I had thought that statements like...

No. All that happened is that the density of matter, which decreases as the universe expand, became smaller than the density of dark energy, which does not decrease as the universe expands. Nothing changed about dark energy at all. What changed was the effect of the matter.

and...

the density of DE doesn't change over time but the density of matter does (it decreases).

...were unequivocally saying that dark energy does not vary over time.

If nothing about dark energy changes (as Peter wrote) and the laws of nature do not change either, then I'm at a loss to reconcile this with ohwilleke's input and Ibix's statement that... 'it [dark energy] does vary according to a law of nature that doesn't vary.

To me these seem like mutually exclusive conditions. Nothing about it changes, yet it does vary according to an unvarying law of nature.

Please note that I'm just confused here and not arguing the point.

I'm away from my laptop for a few hours, but will be back to see how this thread has evolved.

Many thanks for any help given.

Cerenkov.

 

[PeterDonis]

My (limited) understanding is that the different models are governed my a parameter $w$

This is not just for dark energy, it is a general parameter governing equations of state for FRW-type models. The general equation of state is $p = w \rho$.

and when this is -1/3 the dark energy density is constant, and if it's different from that the density either increases or decreases.

This is not correct. As noted above, the parameter $w$ is not a parameter describing energy density; it's a parameter describing pressure and its relationship to energy density. The value of $w$ is separate from the question of whether energy density can change.

The term "dark energy" means $w = -1$, i.e., $p = - \rho$. A cosmological constant has this property, but so does a scalar field; the former corresponds to an energy density that is constant everywhere in spacetime, but the latter does not, and is in fact the simplest model of "dark energy" with an energy density that can change.

The significance of the value $w = - 1/3$ is that it is the value of $w$ that corresponds to an expansion that neither accelerates nor decelerates. Values of $w$ greater than $- 1/3$ correspond to decelerating expansion (the most common cases are $w = 0$, i.e., "matter", and $w = 1/3$, i.e., "radiation"). Values of $w$ less than $- 1/3$ correspond to accelerating expansion (the most common case, of course, being $w = -1$, i.e., "dark energy").

 

[PeterDonis]

I had thought that statements like...

No. All that happened is that the density of matter, which decreases as the universe expand, became smaller than the density of dark energy, which does not decrease as the universe expands. Nothing changed about dark energy at all. What changed was the effect of the matter.

and...

the density of DE doesn't change over time but the density of matter does (it decreases).

...were unequivocally saying that dark energy does not vary over time.

You are ignoring the context of those statements. Those statements were in response to your question in the OP, explaining why the fact that the expansion of the universe started accelerating several billion years ago does not require any "mechanism" to make the effects of dark energy more pronounced. All that is required is that the density of matter and radiation decrease over time as the universe expands, and we already know that happens.

That is not at all the same as claiming that we know for certain that the density of dark energy does not change. All it is saying is that the density of dark energy is not required to change in order to explain why the expansion of the universe started accelerating several billion years ago.

It is true that, as both @PeroK and @phinds said, our best current model says that the density of dark energy does not change over time. But our best current model is a model. It is not reality. Nobody has claimed that we know for certain that our best current model is exactly correct. Of course we don't know that; we can never know that about any model. So there is nothing inconsistent about, on the one hand, saying what our best current model says, that dark energy density does not vary, and on the other hand, considering the possibility that our best current model might be wrong and that dark energy density does vary.

Ibix's statement that... 'it [dark energy] does vary according to a law of nature that doesn't vary.

That is not what @Ibix said. He was referring to your statement (which is correct) that some scientists are considering the possibility that dark energy density does vary over time. He was explaining why that does not require the laws of nature to vary. He was not claiming that we know for certain that dark energy density does vary over time. We don't know that. We are investigating the possibility.

 

[Ibix]

This is not correct.

Ok - I think I need to do some reading then, because I thought that the equation of state did factor into the time evolution of the energy density. A topic for another thread, perhaps.

To me these seem like mutually exclusive conditions.

To be honest, I suspect that you are simply running into people making judgement calls about how much detail to put into answers and how to phrase what they do put in. And sometimes not being clear if we are talking about specific models or more generic ones.

For example, you quoted me as saying "[dark energy] does vary according to a law of nature that doesn't vary". I did say that, but in the context of rebutting your comment that changing dark energy meant changing laws of nature. I perhaps should have said "if it varies at all, dark energy varies according to a law of nature that doesn't vary". I think that if you can, as you did, point out where we appear to be contradicting each other then we can probably work out how we are failing to communicate and sort it.

 

[PeterDonis]

I thought that the equation of state did factor into the time evolution of the energy density.

It sort of does. The most common cases of "matter" ($w = 0$), "radiation" ($w = 1/3$), and "dark energy" in the form of a cosmological constant ($w = -1$) all have a particular time evolution of energy density, yes. (And the one that corresponds to constant energy density, for the case of a cosmological constant, is "dark energy", $w = -1$.)

However, as the example of the scalar field shows, these cases do not exhaust the possibilities; a scalar field has $p = - \rho$, i.e., $w = -1$, but it also has a possibly time varying energy density. (Unless I am misunderstanding something about the proposed scalar field models for dark energy.)

 

[ohwilleke]

That sounds like interesting stuff ohwilleke, but w-a-y over my head.

Perok, PeterDonis and phinds said that dark energy has not varied over time. But, from what I can glean from your message, some scientists are considering the possibility that it does vary over time. If so, then what about extrapolating into the distant past? Surely that requires the laws of nature remain constant over time and do not vary?

Can you help please?

Thank you.

Cerenkov.

Fair point.

"some scientists are considering the possibility that it does vary over time."

Correct. There is evidence pointing both ways and a lot of disagreement over how credible the evidence on both sides of the question is. Both side point out potential sources if error that were overlooked in the other side's papers.

Also, for perspective, there is a lot of consensus amidst these disagreements. Nobody is disputing the Big Bang theory. Nobody is disputing that general relativity with a cosmological constant predicts something reasonably close to what we observe. Nobody is disputing the the red shift in light observed from distant objects of particular kinds is related to the age of the source of the light, although some people argue that the formula relating age and redshift needs some minor technical tweaks that are big enough to matter in this case. Nobody is disputing that the universe is expanding. Nobody is claiming that the other side is trying to mislead people on purpose - everybody is acting in good faith trying to get to the truth and everybody agrees about that.

This is an intra-scientific community disagreement and discussion over the fine details of something about which the big picture is a matter of wide consensus.

"Surely that requires the laws of nature remain constant over time and do not vary?"

You can get varying amounts of dark energy over time even if the laws of nature remain constant and do not vary (although the cosmological constant of General Relativity would be wrong and only a good approximation of reality, if that were true; something other than the cosmological constant in the equation would explain dark energy if it varied in density over time).

Scientists are also exploring the possibility that the laws of nature vary over time. So far, it looks like this isn't happening. But the results for the very deep past are never definitive or precise, because what was going on back then is hard to see.

The James Webb Space Telescope which is specifically designed to look at faint very old sources of light with unprecedented precision may help us answer both questions.

That is not at all the same as claiming that we know for certain that the density of dark energy does not change. All it is saying is that the density of dark energy is not required to change in order to explain why the expansion of the universe started accelerating several billion years ago.

It is true that, as both @PeroK and @phinds said, our best current model says that the density of dark energy does not change over time. But our best current model is a model. It is not reality. Nobody has claimed that we know for certain that our best current model is exactly correct. Of course we don't know that; we can never know that about any model. So there is nothing inconsistent about, on the one hand, saying what our best current model says, that dark energy density does not vary, and on the other hand, considering the possibility that our best current model might be wrong and that dark energy density does vary.

Just so.

 

[Cerenkov]

Peter, this is the second time this kind of thing has happened recently in a thread created by me.

In the edwin-hubble-and-the-evidence-for-universe-expansion thread, what Ibix wrote appeared to contradict what other members (including yourself) were saying. It took a while to sort that out but eventually it was resolved when Ibix revised his comments and eased the 100 versus 2 megaparsec tension.

In this thread it seemed to me that ohwilleke was taking a line about dark energy that appeared to contradict what other members (including yourself) were saying. And now it seems that there was no actual contradiction because A) I ignored the context of certain statements and B) I did not take take into account that what was being discussed are only models of reality and not reality itself.

For the record, I accept what you say in this thread and can now see where I erred. Thank you.

I have one final question.

How do I avoid being caught for a third time between the apparently contradictory statements taken by the very people I look to for clarity and not contradiction?

Thank you.

Cerenkov.

 

[PeterDonis]

How do I avoid being caught for a third time between the apparently contradictory statements taken by the very people I look to for clarity and not contradiction?

By paying attention to context. When you ask a question, people are going to answer it in the context of your question. If you ask a question about a particular phenomenon, such as the universe's expansion starting to accelerate a few billion years ago, you should expect people's answers to refer to that phenomenon. If you ask about a particular suggested explanation for that phenomenon, you should expect people's answers to be about that particular explanation. You should not assume that everything everybody says is always intended as a fully general claim that extends far beyond the particular context of your question.

 

[Cerenkov]

I will think on this and get back to you, Peter.

Thank you.

Cerenkov.

 

[Ibix]

Also, ask followup questions, as you've been doing. We do make mistakes (at least, I do...), and stuff we write generally won't have been read by anyone except ourselves, so even correct answers may not be perfectly clear. We all learn through conversation - that's why this place is set up as a discussion board, not a question/answer format.

 

[Cerenkov]

Ibix,

First off, thank you very much for (metaphorically) stepping into my shoes. Even if it wasn't your intention to do that, this it is how it feels to me.

In my life I have never achieved anything through insight, brilliance or smarts. Dogged persistence and mistake-making are how I've come to gain understanding of anything. That is my way. I have little doubt that this will continue to be the way that I will make progress in this forum.

So, your admission of mistake-making and also what you say about even correct answers not being entirely clear are both comforting and encouraging to me.

I will do as you suggest and continue to ask questions, even if they are the wrong ones. The mistakes I make with them I will try and turn into opportunities to learn.

Thanks again.

Cerenkov.

 

[Cerenkov]

Peter,

First off, I very much appreciate the help and guidance that you and the others have extended to me in this thread. I also appreciate the advice you've given me, which I quote here.

By paying attention to context. When you ask a question, people are going to answer it in the context of your question. If you ask a question about a particular phenomenon, such as the universe's expansion starting to accelerate a few billion years ago, you should expect people's answers to refer to that phenomenon. If you ask about a particular suggested explanation for that phenomenon, you should expect people's answers to be about that particular explanation. You should not assume that everything everybody says is always intended as a fully general claim that extends far beyond the particular context of your question.

I will do as you suggest and try to do these things. But Peter, please do read what I've just said to Ibix.

When I've finally managed to understand something new, that's nearly always happened because I've made mistakes along the way. That and a large amount of guidance, mentoring and patience from others. I'm sorry, but given my track record, you are probably going to have to expect a lot of mistakes and slow progress from me. Perhaps the only positive that I bring to the table is dogged persistence.

With these things in mind I must now fess up to not really understanding how I should do what you advise, above. Nor do I really know what I should do next. All I can really promise is that I won't give up and that I will continue to learn by making mistakes along the way.

If you are happy to help me further, then you have my full commitment.

Thank you.

Cerenkov.

 

[PeterDonis]

given my track record, you are probably going to have to expect a lot of mistakes and slow progress from me

That's not an issue. You are asking questions in good faith, and that's the main thing. Everybody learns in their own way.

I must now fess up to not really understanding how I should do what you advise, above.

The only other advice I can give is that, if you see what looks like a contradiction, try considering other ways of reading things that would remove the contradiction.

In this particular case, @PeroK did say explicitly in post #2 that he was talking about our best current model. That is at least an indication that myself and @phinds, who were responding to the same question in your OP that he was, were also talking about our best current model.

Also, as a general rule, science can't prove anything with absolute certainty. So if it seems to you that anyone is saying something that seems to you to mean that scientists have shown something to be true with absolute certainty, it's highly likely that you are misinterpreting something somewhere.

 

[Cerenkov]

That's not an issue. You are asking questions in good faith, and that's the main thing. Everybody learns in their own way.The only other advice I can give is that, if you see what looks like a contradiction, try considering other ways of reading things that would remove the contradiction.

In this particular case, @PeroK did say explicitly in post #2 that he was talking about our best current model. That is at least an indication that myself and @phinds, who were responding to the same question in your OP that he was, were also talking about our best current model.

Also, as a general rule, science can't prove anything with absolute certainty. So if it seems to you that anyone is saying something that seems to you to mean that scientists have shown something to be true with absolute certainty, it's highly likely that you are misinterpreting something somewhere.

Hello Peter and thank you for the guidance.

Yes, that sounds like a good plan. Apparent contradictions might be resolved by stepping back and comparing what both parties are saying. I know that I can do this because in the Edwin Hubble thread I compared what Ibix was saying (100 megaparsecs) versus what the other contributors were saying about the diagram (2 megaparsecs). That's definitely a way forward and I will persist with it.

Now that you say, I did notice what peroK said at the time, but failed to assign any significance to how he qualified his statement. Now I know better. Thank you.

Strangely enough, I was actually aware of what you say, Peter. About absolute certainty and proofs in science. But I failed to bring that awareness to bear on the issues discussed in this thread.

I'd discovered these articles a good while ago.

http://ds-wordpress.haverford.edu/psych2015/projects/chapter/scientific-proof/
https://theconversation.com/wheres-the-proof-in-science-there-is-none-30570
https://www.forbes.com/sites/startswithabang/2017/11/22/scientific-proof-is-a-myth/?sh=3c642f392fb1
https://thelogicofscience.com/2016/04/19/science-doesnt-prove-anything-and-thats-a-good-thing/

This failure to connect what I was aware of with this discussion is a good worked example of how I learn through mistakes. I could have had a brilliant flash of insight and realized that the apparently contradictory statements about dark energy in this thread could be reconciled. If there are no absolutes in the empirical sciences, then it logically follows that the contradictory statements I read here cannot be absolutes either. Therefore, some way in which they might be reconciled must exist.

But no. It's not within me to creatively connect in this way. No matter. Even though I err, I persist. Mine is the Epimethian way of afterthought, not the Promethian way of forethought. Learning after the fact, not working it out beforehand.

You have my grateful thanks Peter and I wish I could shake your hand!

Cerenkov.

 

[PeterDonis]

You have my grateful thanks Peter and I wish I could shake your hand!

You're quite welcome! The smiley works fine as an online handshake. (Although someone has probably come up with a "handshake" emoji, it just hasn't spread widely enough yet.)