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

What Is Dark Matter And Why Does It Matter?

First of all, let me explain something If you are not in an academic institution, or a research facility, etc., you may not know that for many of us, having regular, sometime weekly, colloquium or seminars is quite common. This is where we invite experts in various topics come to our institution or department and present a talk on a particular subject. I, myself, have given such seminars. This is how we learn about many things, often topics outside of our expertise or area of studies, and we learn about these things from authorities in these various fields. It is one of the unique privileges that we enjoy being in such an environment.

In other words, we do not learn about these topics from popular media, or even from 2nd or 3rd hand sources. And this usually takes time, i.e. it can't be done in short sound bites or in a few minutes.

I'm prefacing this video with such information because this is an example of a colloquium that we typically attend, and if you are not used to it, it may appear tedious to sit through an hour of such presentation. But there is usually no other way to learn about things, especially if you wish to learn about something beyond just a superficial level.

I've mentioned about dark matter many times on here, but here's another one. It is presented in a manner that even non-scientists may understand, even if you do not understand some of the intricate details.



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).

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.

The Dark Life Of The Higgs Boson

I decided to modify a bit the title of the Symmetry article that I'm linking to, because in that article, the possible link between the Higgs boson and dark matter is made. This allows for the study of the decay of the Higgs to be used to detect the presence of dark matter.

The Standard Model not only predicts all the different possible decays of Higgs bosons, but how favorable each decay is. For instance, it predicts that about 60 percent of Higgs bosons will transform into a pair of bottom quarks, whereas only 0.2 percent will transform into a pair of photons. If the experimental results show Higgs bosons decaying into certain particles more or less often than predicted, it could mean that a few Higgs bosons are sneaking off and transforming into dark matter.

Of course, these kinds of precision measurements cannot tell scientists if the Higgs is evolving into dark matter as part of its decay path—only that it is behaving strangely. To catch the Higgs in the act, scientists need irrefutable evidence of the Higgs schmoozing with dark matter.

So there you have it.

If you are not up to speed on the discovery of the Higgs (i.e. you've been living under a rock for the past few years), I've mentioned a link to a nice update here.

Zz.

Dark Matter Biggest Challenge

A very nice article on Forbes' website on the latest challenge in understanding Dark Matter.

It boils down to on why in some cases, Dark Matter dominates, while in others, it seems that everything can be satisfactorily explained without using it. It is why we continue to study this and why we look for possible Dark Matter candidates.  There is still a lot of physics to be done here.

Zz.

The Difference Between Ghosts And Dark Matter

A rather interesting piece that I stumbled upon on the NPR website. The author is trying to explain why, since both ghosts and dark matter can't be "seen", why wouldn't scientists believe in ghosts while a large percentage of physicists believe in the existence of dark matter.

So how do physicists and astronomers get away with claiming the existence of cosmic ghosts (dark matter and dark energy) when they would probably roll their eyes at descriptions of the more terrestrial haunted-house kind?

The answer is data, its prevalence and its stability.

There are literally thousands of studies now of those rotating-too-fast galaxies out there — and they all get the same, quite noticeable result. In other words, data for the existence of dark matter is prevalent. It's not like you see the effect once in a while but then it disappears. The magnitude of the result — meaning its strength — also stays pretty consistent from one study to the next. The same holds true for studies of dark energy.

We need to make something VERY CLEAR here, especially for non-scientists. While we do not know exactly what this dark matter is made of, or we don't know what it is, we have already a very clear set of parameters of its CHARACTERISTICS. Based on what we have already observed and measured, there are QUANTITATIVE properties of these so-called dark matter.

This is important because of two reasons: (i) there are no such definitive behavior, characteristics, and quantitative description of "ghosts", and (ii) these quantitative properties allow us to make measurements and rule out unsuitable candidates that to not fit into what we already know.

This article is similar to the public science event that I attended several years ago in which Dan Hooper of Fermilab/UofC described the science of ghosts. Back then, he too included the possibility on whether ghosts can be made up of dark matter, and based on what we know about dark matter and what people have described what ghosts can do, he concluded that ghosts cannot be made up of dark matter.

So no. Ghosts and Dark Matter are not in the same league.

Zz.

Complex Dark Matter

Don Lincoln has another video on Dark Matter, for those of you who can't enough of these things.



Zz.

Revealing the Nature of Dark Matter

This is a lecture given by Physicist Dan Hooper on Dark Matter at Fermilab. I haven't bump into Dan since we were both at Illinois Science Council event a few years ago at a bar in the Wrigleyville area in Chicago.



Zz.

Big Mystery in the Perseus Cluster

The news about the x-ray emission line seen in the Perseus cluster that can't be explained (yet) by current physics.



The preprint that this video is based on can be found here.

Zz.

Three US Dark Matter Projects Get Funding Approval

The US Dept. of Energy and National Science Foundation have jointly approved the funding of three dark matter search projects. These projects were selected based on the recommendation of the P5 panel, which released its report earlier this year.

Two key US federal funding agencies – the Department of Energy's Office of High Energy Physics and the National Science Foundation's Physics Division – have revealed the three "second generation" direct-detection dark-matter experiments that they will support. The agencies' programme will include the Super Cryogenic Dark Matter Search-SNOLAB (SuperCDMS), the LUX-ZEPLIN (LZ) experiment and the next iteration of the Axion Dark Matter eXperiment (ADMX-Gen2). 

Certainly, with High Energy Physics funding in the US being squeezed and shrinking each year, this is the best outcome on funding for the search of dark matter experiments.

Zz.

The Hunt For Axions

Axions, which in some circles is a Dark Matter candidate different from WIMPs, is the focus of this news story.

This is kind of a Cinderella story – the story of a favorite and an underdog, in what may be the world’s most esoteric sport. The leading candidate for what might make up dark matter is called a weakly interacting massive particle, or WIMP. It gets most of the research money and most of the ink.

But there’s another candidate, a bit of an also-ran, called the axion. Its profile is lower, but for Rosenberg, it just seems to fit in with how the universe works.

Of course, as with WIMPs, the search for axions has also been fruitless so far, as I've mentioned at least a couple of times earlier. We just have to sit back and wait to see how this develops in the next few years.

Zz.

Dark Matter

In case you are still clueless on what we call 'dark matter', this might help.



Zz.

Yes, We Have No WIMPS, So Far

I guess the big news of the week is the result out of LUX that they found no signature of dark matter candidate called WIMPS that was reported elsewhere earlier. The news was important enough that there was even a news piece on the CNN website.

After analysing three months of data taken by LUX, physicists found no evidence for dark-matter collisions. However, because the experiment is the most sensitive yet to a range of WIMP masses, LUX has provided important new information about what dark matter is not. In particular, LUX is more than 20 times more sensitive than previous experiments when it comes to detecting low-mass WIMPs – those with masses of about 5–10 GeV/c². 

Dark matter remains elusive, at least on earth and for this type. However, up in space, the Alpha Magnetic Spectrometer has a different story, obviously. We still need a better understanding of those excess positron.

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.

Possible Detection of WIMPs?

A preprint that was just released from the CDMS collaboration indicates a strong possibility of detection (finally!) of WIMPs. which is a candidate for dark matter.

You can also read a press coverage of this here.

Zz.

AMS Confirms Excess Positrons

Hope you guys were following the announcement today.

In any case, APS has released the paper on the latest result from AMS, reporting and confirming earlier reports on the excess positrons measurement. You can get acess to the PRL paper for free from the link. What does this all mean?

There has been no shortage of speculation about what might cause the positron “excess” above 10GeV. One idea is that relatively nearby cosmic bodies, such as pulsars, act as accelerators and colliders that produce antimatter, much like our terrestrial Large Hadron Collider. But a more exciting possibility is that the positrons come from the annihilation of dark matter particles, which may populate the Milky Way and its halo. Dark matter is, after all, a dominant form of the matter-energy budget of the Universe, but we don’t know its particle nature or how it interacts with itself and with normal matter (other than through gravitational interactions). It is no overstatement to say that identifying the dark matter is one of the greatest problems in modern science.

A lot of work remains to be done, but unlike the search for Supersymmetric particles, there are experimental indications that are consistent with Dark Matter models.

Zz.

Recent Results From The AMS Experiment

I don't know if the buzz surrounding this announcement will amount to anything, but there is certainly a buzz.

Samuel Ting will present the latest result from the Alpha Magnetic Spectrometer instrument that's orbiting earth. If you recall, the AMS went up with the Space Shuttle on one of its last missions. This could have some implications on the search for Dark Matter.

I probably won't have a chance to view the life feed. If you are planning on it, or if you have the chance to actually attend this event, I'd appreciate a brief report.

Zz.

No Dark Matter In Our Neighborhood?

{Don't miss our nomination period to nominate your most attractive physicists}

The most provocative report from this past week is the recent publication of a new result that tries to find any sign of dark matter in our part of the galaxy. Strangely enough, the researchers found none, or at least, there wasn't any need for the presence of dark matter to explain the observed celestial dynamics.

But that's not what Christian Moni Bidin, an astronomer at the University of Concepción in Chile, and colleagues find. Using data gathered with several telescopes, they studied old stars called red giants in a cylindrical region a couple of light-years wide and extending 13,000 light-years above the plane of the galaxy. Treating the stars a bit like atoms in a gas, researchers assumed that they were trapped in the gravitational "well" of the galaxy. So by studying distributions of the stars' speeds in three dimensions, they could deduce the well's shape and hence the total distribution of mass from both dark and ordinary matter along the cylinder. Subtracting the distribution of ordinary matter as determined from star counts would then reveal the distribution of dark matter.

When Moni Bidin and colleagues did the analysis, however, they found that no dark matter was needed to explain the stars' speeds. The researchers had expected to detect a complicated mass distribution with a contribution from the galaxy's disk of stars and gas and the presumably spherical "halo" of dark matter surrounding the disk. Instead, they found that the disk alone neatly explained their data, as they report in a paper in press at The Astrophysical Journal.

Of course, there are skepticism with this report, especially with regards to the type of analysis being done.

Or not. The new result may say more about the method than the distribution of dark matter, Navarro says. To get that distribution, at each position in space Moni Bidin and colleagues must subtract one large quantity (the amount of ordinary matter) from another large quantity (the amount of total mass) to get a small quantity. That process is likely to suffer from large uncertainties, Navarro says. "I applaud them for trying," he says. "I just don't think this method will ever give a conclusive answer." Moni Bidin says the method is robust and that larger surveys to come will pin down the dark matter distribution more precisely.

As with any report that are this controversial (re: superluminal neutrinos), we need to let the system works out on its own. A lot more studies need to be done with more analysis and observations. Things are seldom confirmed with just one observation using one technique, especially in a situation such as this where the methodology of analysis influence the result. So I certainly would not want to draw any kind of conclusion this soon.

Zz.