Hi Reddit! As part of Dark Matter Day (https://www.darkmatterday.com/), Argonne National Laboratory is hosting an AMA with dark matter scientists Clarence Chang (Physicist at Argonne), Rupak Mahapatra (Professor of Physics and Astronomy at Texas A&M University) and Andrew Sonnenschein (Senior Scientist at Fermi National Accelerator Laboratory).

What is dark matter?

Well, we don’t really know. Less than 5 percent of the total mass and energy in the universe is the stuff we know about: like stars, planets, galaxies and gases.

Dark matter, which we have only detected indirectly, makes up about 85 percent of the total mass of the universe, and about a quarter of the universe’s total mass and energy.

We know it exerts influence on the matter we can see, but we can’t see or touch it.

Solving the mystery of dark matter is considered one of the most pressing pursuits in particle physics and could teach us much about the history and structure of our universe.

Two hypotheses posit that dark matter might interact with either particle- or wave- like properties. Mahapatra and Sonnenschein are searching for dark matter through these two respective avenues. Chang develops custom superconducting detector technologies to make these experiments possible.

Ask us anything!

Thank you so much for joining us. Lots of great questions. We're stepping away now, but we will come back to the remaining questions at a later time. Have a great day!

Argonne National Laboratory: https://www.anl.gov/

Proof: https://www.flickr.com/photos/argonne/51639359665/ https://www.flickr.com/photos/argonne/51639158539/in/photostream/ https://www.flickr.com/photos/argonne/51639359535/

Proof: Here's my proof!

Comments: 124 • Responses: 14  • Date: 

sts81612 karma

What do you think the odds are that dark matter and/or energy are simply just shortcomings in our current understanding of existing physics and aren't actually separate forms of matter and energy? Is it possible we just don't have an accurate understanding of gravity itself and that is causing a "false positive" in the belief of dark energy and matter?

ArgonneLab10 karma

Alternative theories to dark matter, such as modifications to the theory of gravity, have been around for a long time and are still a somewhat active area of theoretical research. However, the dark matter hypothesis is a simple theory that explains a large spectrum of observational evidence (such as the observed gravitational lensing of galaxies, the behavior of galaxy clusters, the large scale distribution of galaxies, and the distribution of matter in the early universe). These observations have been very challenging to explain with alternative theories of gravity.

DesiBail9 karma

What is the origin of this 85% value of mass of dark matter ? Is there a density to dark matter ? Does it vary ?

ArgonneLab11 karma

One of our precision measurements of the ratio of dark matter to ordinary matter comes from studying images of the early universe (about 380,000 years after the Big Bang) when it existed as a hot plasma. As a hot plasma, these images of the primordial universe can be readily interpreted, and the patterns give a precise measurement of the composition and energy densities of the dominant components of matter in our universe. From these measurements, we can deduce that, of the matter that gravitates normally, approximately 85% has to be something different from ordinary matter.

The distribution of dark matter has changed over time where dark matter began relatively uniformly distributed throughout the universe and clumps together into ever larger clumps as time progresses.

Today, there’s a huge variation from place to place of both the dark matter and normal matter densities. For example here on earth the density of ordinary matter is very high-- a few grams per cubic centimeter which is about 24 orders of magnitude larger than the local dark matter density (equivalent to only about 1 proton mass for every 3 cc of space). It’s only when averaged over very large volumes that the dark matter density becomes more than that of ordinary matter and is 85% of the total.

thecityandthecity8 karma

Do you expect dark matter to all be one type of thing, or a category including various things which might be discovered at different times?

ArgonneLab17 karma

It’s certainly a possibility that dark matter is more than one thing, although to keep things simple, physicists often only consider one possibility at a time (or else it would get very complicated to analyze our data!) Since ordinary matter consists of many things at the same time like electrons, protons, neutrons and so on, it seems reasonable that the “dark sector” would as well and there’s a large menu of exotic forms of matter to choose from-- there’s no reason we necessarily need to order just one thing from the menu.

stringjetg67 karma

I don’t even know what I don’t know and need to ask…. So…, what is the most exciting findings re dark matter (in your opinion)?

ArgonneLab8 karma

The field is going through a very creative phase right now-- the physics journals are bursting with new ideas for dark matter searches. Many of these are based on technologies developed for quantum computing research or so-called “quantum sensing”-- huge investments are being made in this area by governments and industry and there are spin offs that will be useful for dark matter searches. For example, recent searches for axion dark matter particles have been enabled by Josephson Parametric Amplifiers, a type of low noise amplifier developed for reading out qubits.

Sweaty-Ranger-13403 karma

Why do the detectors you make for the experiments have to be superconducting?

ArgonneLab6 karma

Most of our theories of dark matter predict detection signals that are very small, thus, detecting a signal from dark matter typically requires detectors that are extremely sensitive. The most modern techniques for detection of small electromagnetic signals tend to require low temperatures and exploit the physics of superconductivity. But there are also many types of dark matter detectors that do not require the use of superconductivity-- for example several of the most important experiments operating today look for flashes of light in large tanks of liquid xenon and argon and these do not need superconducting detectors.

aqlu3 karma

Can you break down the etymology for me? Why 'dark'? Why 'matter'?

ArgonneLab3 karma

“Dark matter” is actually a bit of a misnomer in our opinion. The things we are looking for are better described as “invisible” rather than dark, but “dark matter” is the name that has stuck for historical reasons and perhaps because it sounds more intriguing (we are not sure if this is why “dark matter day” falls on Halloween).

TexasPoonTappa69x3 karma

What's the difference between dark matter and dark energy?

ArgonneLab3 karma

Dark matter and dark energy address two different observations of our universe. Dark matter addresses the fact that we have many observations of material interacting gravitationally, but that the source of that gravity does not emit light. Dark Matter is in many respects just like ordinary matter except for the minor detail that it doesn’t interact with photons and is therefore invisible. It also needs to be electrically neutral and probably doesn’t have strong nuclear interactions as well (although in some models it can). Like ordinary matter, as the universe expands, the large scale density of dark matter becomes more dilute.

Dark Energy, on the other hand, addresses a very different set of observations, namely that the current expansion of space-time is accelerating, and seems to be a property of “empty space.” Unlike normal matter or dark matter, as the universe expands, the large scale density of dark energy does not change.

DesiBail3 karma

Edit:Reposting with question mark.

What evidence do we have that there is dark matter as part of all galaxies..and between galaxies..and not different things ?

ArgonneLab5 karma

There are many independent lines of evidence for the existence of dark matter based on astronomical evidence that goes back to the 1930s and has grown in significance over time. One particularly compelling example comes from measurements of galaxy clusters based on gravitational lensing which allowed maps of dark matter to be generated from the distortion of background galaxy images. For example, one cluster in particular has an especially striking image (see bullet cluster) where we can see the effects of gravitational lensing (which is dominated by the gravitational mass of the cluster) and notice that the lensing center is displaced relative to the normal matter in the cluster (which is primarily gas). This strongly indicates that the majority of the cluster’s gravitational mass comes from something other than the visible matter.

BraisLP3 karma

What mass range would you bet on and why? Please be brave and make it a narrow-ish mass range (-:

ArgonneLab4 karma

We would all prefer that the DM be something which can be discovered in our professional lifetimes. One of us would bet on the dark matter being the QCD axion with a mass of several 10s of micro electron volts. The theory of the QCD axion is extremely predictive and has essentially only one unknown parameter which determines both the mass and cosmological abundance-- in some very reasonable scenarios its mass should fall in this range and we will probably find it in the next decade.

jdtoast3 karma

Are neutrinos still a viable candidate for dark matter? And if so, can you explain why we can't do more experiments on the neutrinos we do detect here on earth that can explain more about it? Are they just too hard to predict to be able to do any useful experiments like that? I've just always been confused as to how neutrinos can be dark matter if they do interact with normal matter, albeit rarely.

ArgonneLab7 karma

The neutrinos that we know about (which we call Standard Model neutrinos) are part of the Dark Matter in the universe, but they cannot be the dominant component of the Dark Matter. One of the ways we know this comes from studying our images of the early universe, where we can readily compare our predictions to our measured images. From these images of our infant universe, we can infer that Dark Matter needs to be relatively slow moving and clumpy. On the other hand, because we know the Standard Model neutrinos are light and we know how they interact with normal matter, we can calculate their distribution, and we then find that the neutrinos aren’t sufficiently “clumpy” to explain the patterns that we observe.

Madoc_eu2 karma

Why is dark matter so shy?

I mean, there are many things that don't naturally occur on earth. Antimatter for example. But we can generate them artificially, because the theories tell us how. Even for stuff like exotic matter that we can't even produce artificially, we know about them by extension of the theories.

Why is dark matter so special that we don't have even the slightest clue as to what it is or how to produce it artificially? Antimatter and exotic matter aren't as "shy" as dark matter. We can at least predict that they have to exist. What makes dark matter so special that it's not even covered to the slightest tiny little bit by existing theories? This gives me the creeps, because it implies that most of the stuff that the universe is made out of is out of reach somehow.

I know this question can't be answered precisely. I would love to get a more playful or creative answer on this. Maybe you can relate and ruminate a bit, that would be great. Thank you!

ArgonneLab4 karma

For particle physicists, the possibility that dark matter is so weakly interacting with ordinary matter that we will never be able to study it is indeed a nightmare scenario. However, many theories that are extensions of known physics predict particles that have the right properties to be the dark matter but are merely weakly coupled to ordinary matter rather than completely non-interacting. Hopefully one of these extensions describes the universe we actually live in. Maybe we can take some comfort in the fact that so much of the universe has proven to be intelligible up to now-- amazing things about the universe have been figured out that just a century ago would have seemed forever beyond reach.

ZombieHitchens20122 karma

Do you think the longer we go without finding a dark matter particle the more sympathetic people will be to alternative explanations i.e. an alternative theory for how gravity works at such large scales?

ArgonneLab5 karma

Alternative theories to dark matter, such as modifications to the theory of gravity, have been around for a long time and are still a somewhat active area of theoretical research. However, the dark matter hypothesis is a simple theory that explains a large spectrum of observational evidence (such as the observed gravitational lensing of galaxies, the behavior of galaxy clusters, the large scale distribution of galaxies, and the distribution of matter in the early universe). These observations have been very challenging to explain with alternative theories of gravity.

Jekkelstein1 karma

Since you love mysteries so much, which Scooby-Doo character do you love most?

ArgonneLab5 karma

We love Scooby snacks! Does that count? :)

Piecesofbits1 karma

If you were to officially discover dark matter and how it works, how likely is it that your findings would be suppressed and kept secret from the masses?

ArgonneLab8 karma

This is one of the easier questions about dark matter! The results are all public, discussed at physics conferences and published. Sometimes if there’s a big new result it gets picked up by Reddit and mainstream media before more physicists working in the field ever heard about it, so we find out from our friends and family posting on Facebook.