I know it makes for a "better" headline, but don't be mislead by it thinking Astronomers actually thought dark energy was a cosmological constant, period. Read section 1.1 of the Euclid review paper from 2016 to get an impression straight from the horses mouth.
> Why then is the cosmological constant not the end of the story as far as cosmic acceleration is
concerned? There are at least three reasons.
Don't confuse accelerated expansion with the cosmological constant. Accelerated expansion's origin is most likely the cosmological constant aka dark energy, but they are not the same thing. The cosmological constant itself arises naturally (kind of how an integration constant emerges for indefinite integrals) when you derive the field equations of general relativity. It is basically a geometric degree of freedom inherent to the Einstein-Hilbert action. Any astronomer who fit any model of the universe to some dataset in the past 25 years was fitting a constant. And even this new paper acknowledges that it will take some tremendous effort to convince people that this constant really is changing over time (which introduces a slew of theoretical issues that need to be explained). It will be particularly interesting to see how (if at all) this can be reconciled with measurements of things like the CMB, because stellar and galactic astrophysics is much less well understood.
I wouldn't want to overstate it either, since I was just a lowly and IMHO mediocre PhD student at the time and my contribution to the paper was minor. Nevertheless, I believe I understand the relationship between the cosmological constant and dark energy just fine.
From the idea that the constant that equates to dark energy in the stress energy tensor is not commonly thought to be a constant. People didn't call it that way as some joke. This isn't even up to astronomers, this is something that mathematicians will tell you. And it could potentially cause some real trouble not just for our models of the universe but for the basic mathematics of relativity if it is changing. Motivating that alone will we hard, not to speak of the consequences down the line.
> it could potentially cause some real trouble not just for our models of the universe but for the basic mathematics of relativity if it is changing.
The Einstein Field Equation can accommodate models in which the density of dark energy changes with time just fine. The paper mr_mitm linked to discusses such models. There is no "real trouble" for the basic math of relativity involved here at all.
You can do all sorts of stuff with the Einstein field equations (or any differential equation for that matter). People do this all the time because it's an easy paper. Many people for example make the Ricci scalar into a function. Aka "what happens if I magically introduce this." That way you can sometimes explain some stuff on one end while breaking stuff at other ends. But whenever you introduce such new degrees of freedom, you need to accommodate them somewhere in the overall picture. f(R) gravity in that sense would be better motivated, because in the infrared it boils down to a dumb extra scalar field. And you could get tons of those easily from string theory. But if you want to use local field theories of gravity to explain horizons, then you need something like holography to provide a consistent picture. Even if you go for metastable degenerate vacua, it will be really hard to explain how this leads to an apparently continuously varying cosmological constant. Is it possible? Maybe. Is it more likely that we misunderstood stellar evolution? Absolutely.
No one is saying the cosmological constant can vary. The idea is simply that if dark energy is dynamical, it clearly cannot be explained by a cosmological constant, but perhaps a scalar field instead. And my point was that this idea is nothing new, unlike the headline seems to suggest.
As explained above this scalar field would manifest differently in the equations and basically be something that enters alongside a cosmological constant. Making dark energy or Λ itself vary at the level of the Einstein field equations means you're essentially making an integral constant vary which is not straightforward as people here purport.
> this scalar field would manifest differently in the equations
It shows up in the Einstein Field Equation as part of the stress-energy tensor. That is not a problem at all.
> and basically be something that enters alongside a cosmological constant.
The "cosmological constant" can just as easily be moved to the RHS of the Einstein Field Equation and also considered part of the stress-energy tensor, yes. There is no issue with that at all.
> Making dark energy or Λ itself vary
Is something nobody has claimed to be doing, as the GP said, so you are attacking a straw man.
"Varying dark energy density" does not mean trying to make the cosmological constant vary. It just means adding some other component of stress-energy whose density does not have to be constant, but which still produces accelerated expansion. Anything with pressure less than minus 1/3 of its energy density will do that. A scalar field is the simplest such thing, but not the only possibility. None of this poses the slightest problem for the Einstein Field Equation.
An interesting part from a different article mentioned here in the comments:
The results do not meet the so-called five-sigma threshold of statistical certainty that is the gold standard in physics for claiming a discovery. But many in the collaboration have shifted in recent months from a position of scepticism to confidently backing the finding.
As a former astronomer, one thing to highlight here is that statistical uncertainties aren't usually all that important in assessing how likely a result in astronomy is to be true. The main issue you grapple with in the field are your systematic uncertainties. When you report your uncertainty, it always is an uncertainty in the context of some model. However, if certain assumptions that your models have made are wrong, you will have underestimated your true uncertainty. Unfortunately, systematic uncertainties can't be reduced just by getting more data --- you have to somehow verify that the different assumptions your model has made are correct.
Astronomers spend a lot of time arguing with each other about whether they have properly incorporated all their systematic uncertainties. For measurements of dark energy this very quickly gets you into the weeds of Type Ia supernova physics (which is made more difficult because we don't know for certain what Type Ia supernovae are), stellar physics in Cepheids, the effect of metallicity, selection bias effects, and on and on.
interesting, thanks. I can't edit my old comment, but my original intention wasn't to discredit the news, or the study. I will word my comments better in the future.
I think it's worth noting that the five sigma standard traditionally comes from particle physics, though. Cosmology doesn't have the luxury of being able to generate arbitrary amounts of data, and in fact has been a precise science for only sixty years or so. Hubble's first measurement of the expansion of the universe was off by a order of magnitude.
That's basically a particle physics threshold, because in particle physics you tend to do many, many experiments (usually you're doing thousands a second). It's hard to get something that certain in astronomy, where you've only got one universe to observe.
It is also an informed threshold. FTL neutrinos where, IIRC, above 6 sigma and nobody believed it was true, while there were strong rumors that Higs had been found well before it hit 5 sigmas.
If you combine it with other data it's from 2.8 to 4.2 sigma. Pretty strong and it has gone up since last year with more data. Nothing definite yet, but certainly intriguing.
Agree that this is a much better summary, and generally gets to the point much better with relatively clear summaries of the issues. Once you get down to the mid-section, the NYT notably has a somewhat clearer description of the initial issues with supernovas and using them as references. The Dark Energy article [1] and the "Light Curve" portion of the Type Ia Supernova article [2] on Wikipedia offer further reading.
- Human's believe that Type Ia supernovas are the best way of measuring distance, because of an implied standard brightness and light curve that helps to avoid issues with only measuring redshift.
- Human's initially believed supernovas were supposed to show reduced brightness, further away, with a predictable reduction based on distance.
- Observations showed that supernovas were actually dimmer than expected.
- Implies that the universe is actually expanding, because supernovas were receding faster than expected.
- New results appear to show that the acceleration is decreasing. "there is something pushing galaxies away from each other, but it is not constant. It is declining.” "gently lifting its foot off the pedal"
> Dark energy, the new measurement suggests, may not resign our universe to a fate of being ripped apart across every scale, from galaxy clusters down to atomic nuclei.
Wasn’t ‘the universe won’t end in a “big rip” the widely held idea?
Yes, big rip was clearly disfavored by available data, and heat death ("big freeze") the most likely scenario. I haven't kept up in the past few years, abut AFAIK nothing so far has changed that.
> If dark energy really is Einstein’s constant, the standard model portends a bleak future: The universe will keep speeding up, forever, becoming darker and lonelier. Distant galaxies will eventually be too far away to see. All energy, life and thought will be sucked from the cosmos.
The laws of thermodynamics pretty much guarantees this anyways does it not?
Thermodynamics says there will be equilibrium, not that it will be all dark. But all the visible stars will run out at some point, and since nothing new can get into that sphere, each place in space will be "dark" at some point.
What about the CMBR, I've never heard anyone say that's decaying, but surely it's not an infinite source of energy?
The CMB is red-shifting due to the expansion of the universe. If the big bang was infinite then the CMB is also infinite, but at some point it becomes so redshifted that photons from it have a wavelength larger than the Hubble volume.
The collision probably won't do much more than trigger a period of star formation – although that will be accompanied by an increase in supernova frequency as the most massive of the newly-born stars quickly burn out in mere millions of years.
like two flammable gas clouds colliding perhaps. Literally one of the most energetic processes in the known universe, with 100x the rate of star formation.
Why would you be concerned? Earth and everything living on it will be long dead by then, and probably gone and swallowed by the sun. And maybe the Sun dies before that, too. So what would be particularly concerning about merging of our galaxy with another one?
> Earth and everything living on it will be long dead by then
How do you know? It's not far fetched to think that, if humans don't go extinct in the meantime, they will continue to find ways to shape the world according to their needs. By the time the Sun goes red giant, we may well have found a way to alter the orbit of Earth. By the time the Sun goes supernova, we may be able to move to another star. Who knows.
But let's not get distracted. We need to tackle climate change first. That's our first self-made extinction challenge.
Hmm, yes, I don't. I just honestly don't get why that galaxy collision is concerning. What's the concern? For whom? Four billion years is very, very long compared to humankind's evolutional history. Homo may be 2 My old, that's 1% of the time it takes the sun to rotate a single time around the center of the galaxy. Sol completed already ~20 rotations since it exists. Homo's whole history is 1/2000th the time ntil Andromeda will arrive. So, what's the concern? Humankind will not exist for many, many hundreds of millions of years.
> It's not far fetched to think that, they will continue to find ways to shape the world according to their needs.
Humans fixing Earth's and Humanity's problems is totally out of character. I did not even consider it.
Earth's death is certain. Sun's death is certain. Galaxy collision might even go unnoticed for quite a while -- it's not like millions of stars would crash into each other -- very unlikely. It's more like a reshaping, but what's concerning about that? More supernovas around us? Yeah, maybe.
But why is it more concerning that any other demise? It is an interesting question, I think, about perceived danger.
Existence, universe, galactic processes and time scales are very fascinating. I do loose sleep over it, but because it is impossible to grasp, so marvellous and unlikely. And it is treated like shit by too many of my fellow earthlings. My worries are much closer and much more concrete than a galaxy collision, and I think much more urgent.
> How do you know? It's not far fetched to think that, if humans don't go extinct in the meantime, they will continue to find ways to shape the world according to their needs.
I'd say it's pretty far-fetched to imagine humans caring about things 7 generations into the future, let alone 3e7 generations.
> By the time the Sun goes red giant, we may well have found a way to alter the orbit of Earth. By the time the Sun goes supernova, we may be able to move to another star. Who knows.
We already know how to meaningfully alter Earth's orbit over such timescales[0], the sun won't go super-nova anyway (too small)[1], and we know what it would take to increase its lifetime by a few orders of magnitude even if organising ourselves on the numerical and time scales required is beyond us[2].
But even maximally extending the lifespan of Sol would take us to perhaps 100 trillion years if we're very lucky, and that's if we actually engage in the exact kind of long-term thinking that people currently criticise the Longtermism movement for even daring to consider.
Stellar Husbandry, wild! Obviously anyone in to sci-fi might have come across the idea of meddling with a star somehow, but I'd never thought to classify it all under the umbrella term stellar husbandry.
I don't think climate change, even if it lives up to the most alarmist of predictions will make humanity extinct. We'll all be gone regardless of that. A functioning civilization is composed of humans (at least in our case), and every generation of humans must have as its first priority the task of making more humans to replace the oldest (dying) generation. The fertility rate is sub-replacement, the effects of a demographic implosion cause more fertility rate issues, not less, and so that will accelerate. Too neurotic to have sex and make babies. Climate change wasn't what you needed to tackle after all.
Were I to have a religion, it would be following the poor, stressed programmer tasked with creating (on some insane deadline) the universe we inhabit. To fix various performance bugs, he (I like to think he's called Colin) had to implement all sorts of hacky workarounds: lazy loading so some information is only calculated when it's observed, a maximum speed of information propagation to enable sharding, a system to insert new nodes in the linked list of spacetime, pushing galaxies apart and ensuring the complex timelines of intelligent species never have to snap together and run on a single instance. The work is constant and unrewarding, but I am thankful to Colin for his work, however mysterious it might seem to us.
They say the deadline was quite literally 6 days. Some of the employees rage-quit and started their own business (they are doing fantastic but work conditions are reportedly hellish).
As far as we know, they were still tweaking the physical constants until not long ago.
Unlimited resource drain caused by conscious blobs of particles (a.k.a humans) invoking recursive thought patterns over their own existence.
Workaround: ponder function can return a feeling of existential dread accompanied by increased weights for headache "sensation", causing blob to terminate process.
Oh. I have a [PR#4519842] that implements a biased value system of labour which should reduce the time available for such patterns to occur, and limit the recursion to a brief few who can embed/summarise such artefacts in written record as a form of memoization.
I once saw the Heisenberg Uncertainty Principle described as the result of the simulation creators using an 8 bit variable to store position and momentum combinations which were each themselves 8 bit values.
Call it professional bias, but I like to think about it as a big VM. If for some reason, things go awry, the (said) programmer will hit the pause button, then diligently go to the last valid snapshot, fix the bug and then get things going again. If this isn't desired, fixing the current mess is also an option.
The time required for fixing bugs isn't really an issue in this case, he can take as long as he likes.
So the first lead coder was this Lucifer guy. Wrote the whole code for the physical universe layer in like days on Red Bull and maybe speed.
Unfortunately he was shady and found to be embezzling from the company so he was fired. Only then did the team actually look at that code. Absolutely incomprehensible mess, and it looked like he left back doors in so he and his pals could manifest weird stuff in the universe and fuck with the inhabitants.
Like get this... a human draws a five pointed star on the ground and says some weird stuff and it trips some code that looks like obfuscated malware that calls out to a dodgy server, but we can't remove it because the whole thing is a giant spaghetti monolith.
The Lord put together a proper team of angelic hosts to rewrite the whole thing using solid software engineering practices, but of course that was aeons ago. But when it finally does ship we will get a new heaven and a new Earth with a whole list of promised features.
Archangel Michael, who is in charge of the project, keeps moving the deadline back, but he says they're making progress. It's a microservice architecture.
And then there's the kid Jesus who promises us relief from all the skullduggery coded into our existence if we will only agree to subscribe to his Salvation as a Service (SaaS) subscription model.
I love this comment but I don't understand the "linked list" reference. What aspect of physics/the universe is that referring to? Also why does the maximum speed enable sharding? Struggling with that part too haha.
The standard model of cosmology is basically this but without making reference to something Prime Mover that caused it all to start, a Colin or God. The best science could do is show us that the universe just is, but even that wouldn't answer the question of why or where it came from even if the future and the past were infinitely long.
In the end, to be fair, we measure both of them with photons (quite literally – GWs with laser interferometry and neutrinos by looking for Cherenkov flashes).
A fair point, but it should be said that a scale also only measures a some sort of an EM-based proxy for gravity, typically the compression of a spring either mechanically moving a needle or being electronically detected with strain gauges[1].
I guess my point was that when we peer to the skies and detect EM radiation at various bands, that's also just a proxy for something else that we're actually interested in. The photons aren't interesting in themselves, it's what we can infer about them by following the causal chain backwards. In that sense, it doesn't matter whether the photons are produced on Earth or a billion parsecs away.
It is useful to point out that everything we do is electronic in some way. All normal forces are really electronic forces. It just isn't the case that we can't observe forces other than EM, just that in the end everything has to become EM for us to be able to observe it, and... that's not too remarkable considering that we're made of electronic matter and are of such scales that nuclear forces are not relevant except in so far as they make chemistry possible.
I don't that's the kind of gotcha you imply it is. Of course radiation is somehow involved in everything, it's what keeps matter together. We also read off the measurements with our eyeballs, and even if you used some sort of braille system, touching things boils down to EM interaction. The primary probe here is gravitational, any musings beyond that are of little value. I challenge you to even conceive an experiment where your own objection wouldn't apply.
Why do people still waste time and money on these post hoc theories of Dark Matter / Dark Energy, which were invented to explain anomalies in the conventional cosmological model like galaxy rotation curves?
We neither have direct evidence nor do we not have alternative explanations - to the contrary. They shouldn't even qualify as scientific theory, since they are neither falsifiable, have no predictive power or are supported by any independent experimental confirmation.
Oh,there are presents under the Christmas tree, it must've been Santa Claus!
They are patchwork "solutions" at best and by far not the only explanation for what's happening.
People have been looking everywhere for a better solution than Dark Matter and especially Dark Energy for decades now without success. They may hate the theories, for good reason, but nothing better has materialized yet. Modified gravity always seems intriguing until you slam in a counterexamples for example.
This is a lingering sense that we have missed something big.
