Blackholes The Source Of Dark Energy?

Can blackholes at the center of galaxies be the source of the dark energy that we have been detecting?

That seems to be the conclusion based on two recently published papers [1,2]. Both of these are open access papers, so the full papers are available to everyone.

You may read an explanation and review of the papers at the AAS news website. The implication here is that if this is true, then dark energy is not something exotic or new since it can already be explained with General Relativity.

Now, if only we can find those pesky dark matter.... if they exist.

Zz.

[1] D. Farrah et al., Astrophy. J. Lett., v.944, p.L31 (2023).

[2] D. Farrah et al., Astrophy. J., v.943, p.133 (2023).

Total Amount of Matter in the Universe

We now have the most accurate measurement to date of the total amount of matter in our universe. A new paper published in The Astrophysical Journal[1] seems to indicate that our universe is composed of 31% matter, with the rest being dark energy.

And of that 31% of matter, 80% of that is dark matter, which we are still searching for. This means that the "ordinary matter" that is known within the Standard Model of elementary particle and that makes up you and I is only about 6.2% of the entire matter+energy of our universe. The remaining 93.8% are made up of "dark" stuff, i.e. dark energy and dark matter.

This means that we still do not know the nature of a huge portion of what makes up our universe. Would it be nice to be alive 50 or 100 years from now when we know more about these things then (hopefully!).

Z.

[1] https://iopscience.iop.org/article/10.3847/1538-4357/aba619

DESI Begins

A new eye on the sky is about to add to our knowledge of dark energy.



It's interesting that in the list of funding agencies, NASA is absent. This goes to show you that many of these research activities that seem to be "astronomy-related" are not the sole domain of NASA. In fact, the area of particle-astrophysics is more closely related to particle physics than astronomy.

The video didn't clarify explicitly that in looking at the "spectrum" of light from each of these celestial bodies, one gets the radial velocity of these bodies with respect to us (i.e. via the amount of redshift), not its distance from us. That last piece of information can only be "deduced" using the radial velocity and the Hubble equation, i.e. the Hubble constant, a number that is still being refined.

Still, this new telescope is going to be quite exciting in revealing more of the mysteries of dark energy.

Zz.

General Relativity Passes Its First Galactic Test

Ethan Siegel is reporting the latest result of a test of General Relativity at the galactic scale.[1]

This effect of gravitational lensing, which occurs in both strong and weak variants, represents the greatest hope we have of testing General Relativity on scales larger than the Solar System. For the first time, a team of scientists led by Tom Collett performed a precise extragalactic test of General Relativity, and Einstein's theory passed with flying colors.

This new result also puts a strong damper on alternative theories of gravity, such as MOND.

For the first time, we've been able to perform a direct test of General Relativity outside of our Solar System and get solid, informative results. The ratio of the Newtonian potential to the curvature potential, which relativity demands be equal to one but where alternatives differ, confirms what General Relativity predicts. Large deviations from Einstein's gravity, therefore, cannot happen on scales smaller than a few thousand light years, or for masses the scale of an individual galaxy. If you want to explain the accelerated expansion of the Universe, you can't simply say you don't like dark energy and throw Einstein's gravity away. For the first time, if we want to modify Einstein's gravity on galactic-or-larger scales, we have an important constraint to reckon with.

This is definitely a big deal of a result.

Zz.

[1] T.E. Collett et al., Science v.360, p.1342 (2018).

Dark Energy Levels Not Too Constrained For Star Formation

I've always had a bit of a problem with the anthropic scenario of our universe, i.e. the idea that we are living in a universe JUST fined-tuned to allow us to exist. My problem isn't with the observations so far, but rather how much people are already thinking that this must be true, the data are set, and that we can run away with it. Certainly many people outside of cosmology have tried to spin this into whatever directions that they want.

So when news like this comes along, I just want to yell "I told you so!". It is not that I agree or disagree with the conclusion, but it is to point out that in our attempt to understand all of this, our knowledge is still in its infancy, and that we really don't know enough yet to be able to say things one way or the other on many of the big issues. We do have a fuzzy idea on what direction it is going, but in a number of things and observations, more is required to understand things even better.

The new studies ran the simulation on the star formation of our universe against the amount of dark energy in our universe. They can, to put it crudely, dial in various level of dark energy in their simulations. They found that there is a wider range than initially expected for our present universe to form, i.e. it is not in a very narrow range that was thought of. So keeping everything relatively the same, we could see this present universe that we're in for a large range of dark energy.

The simulations allowed the researchers to adjust the amount of dark energy in the universe and watch what happened.

The results were a surprise. The research revealed that the amount of dark energy could be increased a couple of hundred times – or reduced equally drastically – without substantially affecting anything else.

So for dark energy, the parameter is not as "fine tuned" as one expected.

Zz.

https://academic.oup.com/mnras/advance-article-abstract/doi/10.1093/mnras/sty846/4963750?redirectedFrom=fulltext

https://academic.oup.com/mnras/advance-article-abstract/doi/10.1093/mnras/sty879/4966995?redirectedFrom=fulltext

Alternative Theories of Gravity In Deep Doo-Doo

This is a rather nice article on the troubled times facing many alternative theories to Einstein's General Relativity due to the recent results from the colliding neutron stars. It should be especially useful to laymen to read and understand the methodology and the scrutiny that every theory goes through in physics to be considered to be valid. The one "take-home-lesson" that you should see is that my often-repeated manta here is more true than ever:

"Physics just doesn't say what goes up, must come down. It must also say when and where it comes down" - Warren Siegel.

The quantitative aspect is what is able to separate theories from being wrong to being right. One can't just say "oh, gravity gets weaker as we go farther away". It must say how much weaker, how it behaves with distance, etc.. etc.. and make precise predictions (i.e. to what level of uncertainty). These are "numbers" that we can compare with experiments, if there are already results or if new ones are collected.

This was exactly what happened with the merging neutron stars result, where our verification of the speed of gravity matches that to such precision with the speed of light, that a number of alternative gravitational theories died instantly.

The moral of the story here is that you should not fall in such deep love with any theory yet to have substantial verification, and you should not jump too quickly when new theories appear. I still point out to the OPERA debacle a few years ago when the OPERA project thought they measured superluminal neutrinos. As soon as they published their results, a bunch of theoretical explanations appeared on the e-print arXiv website, proposing theories of superluminal neutrinos, without waiting for independent verification of the validity of the result.

Not surprisingly, they all died of a horrible death when the result was attributed to bad optical cable connection! The history of physics is littered with theories that died and disappeared into obscurity when they could not match the experimental results or observations. Any beliefs or ideology, no matter how beautiful, satisfying, or popular, will crumble at the hands of Mother Nature if she says so.

Zz.

What Astronomers Wish You Know About Dark Matter And Dark Energy

If you do a search of this blog, you will encounter numerous entries on both "dark matter" and "dark energy". It is something I've covered quite often, mainly because it is still an ongoing and active research area in astrophysics/astronomy/cosmology. Even high-energy physics/elementary particle physics is getting into the picture with particle astronomy.

In this article, Ethan Siegel gives you a condensed version of what "dark matter" and "dark energy" are, and what you need to know about them. But more importantly, if you think that you can discard them, you need to do more than just say that they are not needed.

It wasn't always apparent that this would be the solution, but this one solution works for literally all the observations. When someone puts forth the hypothesis that "dark matter and/or dark energy doesn't exist," the onus is on them to answer the implicit question, "okay, then what replaces General Relativity as your theory of gravity to explain the entire Universe?" As gravitational wave astronomy has further confirmed Einstein's greatest theory even more spectacularly, even many of the fringe alternatives to General Relativity have fallen away. The way it stands now, there are no theories that exist that successfully do away with dark matter and dark energy and still explain everything that we see. Until there are, there are no real alternatives to the modern picture that deserve to be taken seriously

It might not feel right to you, in your gut, that 95% of the Universe would be dark. It might not seem like it's a reasonable possibility when all you'd need to do, in principle, is to replace your underlying laws with new ones. But until those laws are found, and it hasn't even been shown that they could mathematically exist, you absolutely have to go with the description of the Universe that all the evidence points to. Anything else is simply an unscientific conclusion.

Zz.

Dark Energy Is Not An Illusion

This is a good intro to Dark Energy if you want to know more about it. Even if you don't buy into Ethan Siegel's argument, you at least have a good description of what we know of about Dark Energy at the moment, and why certain explanations for what have been observed have been ruled out.

Zz.

Dark Energy - What Is It?

I've posted many articles on Dark Energy. But here's another one aimed at the general public that actually is quite instructive. It describes not only why we think there is dark energy, but also the puzzling phenomenon of the apparent "switching" between one regime to another.

Please take note that, while it seems that this idea has been floating around for a while, the study of Dark Energy is very much still in its infancy. The general public may find it hard to understand, but we really do need a lot more experimental observations on this, and that is easier said than done. Detection of this is not easy and requires years of design and work, and not to mention, funding!

Zz.

DES Sky Survey

I mentioned a while ago of the Dark Energy Survey project that is trying to map our universe and possibly produce a clearer picture of the dark energy phenomenon. This week we have one of the first mapping of our universe from DES, from just 3% of the DES data.

The DES dark matter map is not the first of its kind. Several pioneering analyses have come before it, most notably the Canada-France-Hawaii Telescope Lensing Survey, which used 4 times the number of distant galaxies that DES used to map an area of similar size but at higher resolution. The two teams have reached the same conclusions, though: The luminous matter that we can see is housed within the dark matter structures that we cannot see, and this dark matter forms a cosmic web of filaments, knots, and voids. As it continues collecting and analyzing data, DES will be able to map how these dark matter structures evolve over time.

There's a lot more to be discovered here as more of the data are analyzed.

Zz.

Dark Energy

In case you want an entertaining lesson or information on Dark Energy and why we think it is there, here's a nice video on it.



This video, in conjunction of the earlier video on Dark Matter, should give you some idea on what these "dark" entities are based on what we currently know.

Zz.

Secrets Of The Dark Universe: Simulating The Sky

I'll post the synopsis to this video, which you can also read on YouTube:

An astonishing 99.6% of our Universe is dark. Observations indicate that the Universe consists of 70% of a mysterious dark energy and 25% of a yet-unidentified dark matter component, and only 0.4% of the remaining ordinary matter is visible.

Understanding the physics of this dark sector is the foremost challenge in cosmology today. Sophisticated simulations of the evolution of the Universe play a crucial task in this endeavor.

This movie shows an intermediate stage in a large simulation of the distribution of matter in the Universe, the so-called cosmic web, accounting for the influence of dark energy. The simulation is evolving 1.1 trillion particles. The movie shows a snapshot of the Universe when it was 1.6 billion years old.

While this video may be obvious to people in the field, it would be nice if they had some narration to accompany each scene so that we know what we are looking at! After all, they went to all this trouble to make a visual representation of the simulation and posting it on YouTube for the public to see. Might as well put a little bit more effort in telling us what each of those different scenes are. Otherwise, all we see are cool images without learning anything much.

Of course, the physicist in me would like to know what kind of parameters were used, what are the assumptions, where was this/will this be published, etc. Y'know, the mundane stuff! :)

Zz.

Dark Energy Survey

Here's a short video on this project.



You may also visit their website to learn more.

Zz.

Nobel Prize Awarded For Dark Energy

The Nobel Prize in Physics this year was awarded to the 3 main figures in the discovery of Dark Energy. Half of the award is given to Saul Perlmutter of LBNL. The other half is given to Brian Schmidt of Australian National University and Adam Riess of Johns Hopkins.

This is a rather bold and brave move. While dark energy is considered to be "mainstream" in cosmology, its acceptance is still debatable in many circles, and certainly to the MOND folks. The Nobel prize tends to be more conservative, awarding it to people who has discovered something that is well-documented. Dark energy is still a phenomenon that we are still trying to pin down, as evident by many new efforts, such as DES, etc. that are about to go on line. So it is certainly a bold more (unprecedented?) to award for a discovery that has a degree of certainty that many considers to not be as high as other previous Nobel prize winning discovery..... YET.

And oh, Reuters-Thompson's prediction is wrong again! :)

Zz.

We Are Not Special

Sorry kids. Contrary to what your parents told you when you were young, you and I and everyone else are not special. Well, we are not special in terms of our place in the universe.

Y'see, once upon a time, we thought that we are at the center of the universe, and everything revolves around us. We believed that we are in a privileged location in the universe. Well, someone by the name of Copernicus, and later on, Galileo, practically destroyed that delusion.

Yet, there are still ideas (some call them theories) even today that still want to place us at this special location in the universe, and if they do that, they said that they can explain the universe accelerating expansion without any need to invoke dark energy. As always, Mother Universe can throw a wrench into the best-laid theory. This latest wrench comes in the form of the latest observational data that basically conclude that the universe is homogeneous at a length scale up to a gigaparsec.

In a paper appearing in Physical Review Letters, Pengjie Zhang at the Shanghai Astronomical Observatory and Albert Stebbins at Fermilab show that a popular void model, and many others aiming to replace dark energy, don’t stand up against telescope observation.

Galaxy surveys show the universe is homogeneous, at least on length scales up to a gigaparsec. Zhang and Stebbins argue that if larger scale inhomogeneities exist, they should be detectable as a temperature shift in the cosmic microwave background—relic photons from about 400,000 years after the big bang—that occurs because of electron-photon (inverse Compton) scattering. Focusing on the “Hubble bubble” void model, they show that in such a scenario, some regions of the universe would expand faster than others, causing this temperature shift to be greater than what is expected. But telescopes that study the microwave background, such as the Atacama telescope in Chile or the South Pole telescope, don’t see such a large shift.

 So for now, we are nothing special in our place in the Universe. But that's OK. I still like myself, and I still like you!

Edit: read another review of this work here.

Zz.

New Light On Dark Energy

If you have 2 hours (yowzah!) to spare, here's a video of a panel discussion on the latest development in our understanding of Dark Energy.



Zz.

"X" Marks The Spot

A lot of brouhaha has been going on this past week or so on the speculative particle called the "X" particle. The model for the existence of this particle could, in principle, explain the dark matter problem, and also the matter-antimatter imbalance of our universe.

Check out this MSNBC article, that gives you links to other articles, on this topic in case you missed it.

Zz.

The Accelerating Universe

This is a chapter of a book on the origin of dark energy. It is aimed at ".. general scientists..", which I think more to mean "physicists, astrophysicists, and astronomers". Still, it is a good review article on how the deduction of dark energy was arrived. For the general audience, it might be a bit too technical, but it provides a very necessary overview on the fact that this isn't something trivial to see.

Zz.

The Spontaneous Universe

It looks like Lawrence Krauss decided to join in the fun with regards to Hawking's claim that our universe does not need a creator. Krauss made an even more straightforward argument simply by using energy balance:

The existence of this energy, called dark energy, has another consequence: It changes the picture so that knowing the geometry of the universe is no longer enough to determine its future. While this may be a disappointment, the existence of dark energy and a flat universe has profound implications for those of us who suspected the universe might arise from nothing.

Why? Because if you add up the total energy of a flat universe, the result is precisely zero. How can this be? When you include the effects of gravity, energy comes in two forms. Mass corresponds to positive energy, but the gravitational attraction between massive objects can correspond to negative energy. If the positive energy and the negative gravitational energy of the universe cancel out, we end up in a flat universe.

Think about it: If our universe arose spontaneously from nothing at all, one might predict that its total energy should be zero. And when we measure the total energy of the universe, which could have been anything, the answer turns out to be the only one consistent with this possibility.

This, of course, has more verification than Hawking's argument of using M-theory. At the very least, the presence and nature of dark energy can be tested. So such an argument is a lot more compelling and more difficult to refute than using an unverified theory that has yet to gain wide acceptance among physicists.

Of course, Krauss got into all of of this because he has his own book that will come out in 2011.

Mr. Krauss, a cosmologist, is director of the Origins Project at Arizona State University. His newest book, "A Universe From Nothing" will be published by Free Press in 2011.

I bet you the subject matter is as controversial as Hawking, but will it get the same level of attention? Probably not.

Zz.

Dark Energy Particle Spotted?

No, not dark matter, but dark energy!

A new paper[1] has thrown a wrinkle into the dark energy puzzle. A "chameleon" particle has been proposed (link open only for a limited time) that can arise out of photon traveling from distant astronomical bodies while passing through magnetic fields, very much like the proposed axions, which have yet to be detected. However, the authors of this proposed chameleon particles claim that there is a "good evidence" that they may have been detected already.

In theory, photons that travel through magnetic fields can turn into chameleons, reducing the amount of light that reaches Earth from distant sources. The amount of dimming depends on the light's frequency. By comparing light emitted across a range of frequencies from the luminous centres of 77 active galaxies, Douglas Shaw at Queen Mary University of London and his colleagues have found what they call "good evidence" that some photons have gone missing in transit.

However, these chameleon particles cannot be distinguished from those axions.

By themselves, the observations of dimmed light by Shaw and his colleagues can't distinguish between models that rely on chameleons and models in which photons turn into other 'axion-like' particles. Either "would be an interesting discovery," says Shaw.

However, only the chameleon model predicts that the photons' polarizations should be aligned with the magnetic fields they traversed. So far, the team has studied data on light from three stars in the Milky Way galaxy and in each case found the required polarization3.

Looks like there are people who are already planning on testing this in the next couple of years, so it should be interesting to see how this turns out.

Zz.

[1] C. Burrage et al., Phys. Rev. Lett. v.102, p.201101 (2009).