I am a cosmologist, AMA
Ok ok disclaimer, I am a cosmology PhD candidate, don’t have the degree yet. However I do feel comfortable at this point calling myself a cosmologist (I think for the first time ever). In any case, with all the new people here, I think an AMA might be fun. I will try my best to answer all of the questions I get asked, but it may not happen quickly!
A bit about my research. I study the conditions in the early universe, specifically when the cosmic microwave background was forming, and I use CMB data to test our understanding of this era. The CMB formed roughly 300,000 years after the big bang, when the universe was 1/1000th its current size. The patterns that we see in the temperature fluctuations of the CMB can tell us a lot about the universe at this early time, and specifically we can try to use them to see if anything ‘unexpected’ happened at this time, like a hitherto undiscovered particle annihilating into ‘normal’ particles (for example).
Ask me anything about the early universe, or physics writ large, and I will do my best to answer!
The very early galaxies seen by JWST have been surprising to many I think, and if their age estimates hold up I think it will force us to rewrite the book on galaxy formation and evolution somewhat.
I've seen the MOND people claim they predicted the big, early galaxies. Do you think there's any merit in what they say, and could it force a rethink of the dark matter models (well, beyond the changes to Lambda-CDM that will be needed, I guess)?
MOND theories are diverse enough that you can get them to replicate a lot of observations, but very rarely (never as far as I know) can you get them to explain all of the observations that we have as well as dark matter. So I wouldn’t be surprised if there were some models out there which predict early massive galaxies, but without seeing them I would wager they make other predictions worse or in conflict with data. At this point, dark matter does such a good job of explaining that we see (a large variety of observations at that) that any alternative has a high bar to get over.
Very cool!
I literally just finished Katie Mack’s The End of Everything (Astrophysically Speaking). I loved it, and thought she did a great job of taking difficult concepts and making them comprehensible through metaphor, but that’s my outsider, lay opinion. What’s the insider opinion on the book?
I have not read it but have heard nothing but good things from friends who have! Katie Mack is very well respected and I've actually been meaning to read it just for kicks.
Did the results of the observations of gravitational waves from colliding black holes have any major effects on the scientific community at large?
Oh absolutely. The comparison has been made before, but the advent of gravitational wave astronomy has really given us another "sense" with which to gather information about the universe. Up until the first detection, in very little exception, all of the information we had about the universe was from gathering light. If you couldn't see it, you really couldn't learn too much about it. Gravitational wave astronomy gives us a tool to study things we can't see. I'm not super plugged in to the GW community but they can tell us quite a bit about things other than black holes even. For example, by detecting both the gravitational waves and light emitted from an event (say a neutron star merger), we can measure the expansion rate of the universe. Right now we can only do that directly using supernova, so it would be nice to have another way to do it to serve as a sanity check. In addition, the masses of some of the black hole collisions that have been detected are uncomfortably close to what we think is the cutoff for a black hole around the mass of the sun. If we detect more in this mass range we will have to rethink some mechanisms for their formation.
There is also a lot of effort going into detecting the "gravitational wave background". This is basically the sum of all the gravitational waves from events that happened in the early universe. Since gravitational waves can travel through materials, if we can detect this background we can learn about the universe at even younger times than we can now. This is because the youngest time we can actually observe now is the formation of the CMB: before this time the universe was so dense that light could not freely travel through it, so there is no freely travelling light from then for us to collect now. Gravitational waves could change this, but we will likely have to wait for next generation detectors such as LISA.
That’s awesome! I had no idea that gravitational waves could be observed to see things farther than we’ve seen before. Unrelated follow-up: is it possible to use the cmb as a reference point in 3-D space to begin to label where things (planets, stars, other space objects) are and how they are moving?
This is actually kind of a thing! We can use the rest frame of the CMB as a frame to judge other things by. You might find some of the discussion here interesting!
Is that a useful reference frame for anything or more of an interesting curiosity? (Certainly is an interesting concept to me)
The very early universe before would have been a very uniform place right? So what sort of processes would have been predicted to have been occurring that might have created gravitational waves 'beyond' the CMB that LISA might be able to detect?
You're exactly right that the early universe went through a stage of being extremely uniform. Currently, it is widely thought that this extreme uniformity at early times is a result of cosmic inflation, which one can think of as "stretching out" any irregularities into an extremely uniform state. These primordial gravitational waves would be a result of this inflationary process, essentially the rapid expansion of space. Inflation is also what gives rise to the initial inhomgeneities which act as seeds for structure formation at later times.
Oh it was incredible. I remember the day it was announced, my physics professor cancelled the entire day of class and just talked about how exciting this was non-stop.
It wasn't exactly unexpected, but it was a tremendous leap forward and EVERYONE celebrated it.
In other news, LIGO is back online after a long period out of service - can't wait to see what else they have to say!
What's your view on where all the antimatter is/went?
This is a really interesting question, but I will re-frame it a bit. First a bit of background. After the Big Bang the universe was very very hot, and there was so much thermal energy about that particle-antiparticle pairs were constantly popping into and out of existence. As the universe expanded, it cooled down, and eventually there wasn't enough thermal energy about to spontaneously create particle-antiparticle pairs. So there came a point where particle annihilation with their antiparticles could proceed, but the inverse reaction of particle creation no longer had this thermal bath of energy to draw from. Now, if there had been a symmetry between the amount of particles vs antiparticle, i.e. if there had been equal amounts, you would expect the end state of this process to be a universe filled with only radiation. All of the created pairs would annihilate with each other and leave behind the photons created in that process. We can do an experiment to see if this is actually what happened.
There, we did the experiment, that isn't what happened — the universe is not entirely radiation and instead is filled with particles (and not antiparticles). So we conclude that early on there was some asymmetry between particles and antiparticles, such that when they all finally annihilated there was some leftover fraction of normal matter. This is known as baryonic asymmetry and it is an open question still as to why this is the case! Whether or not the asymmetry was there from the beginning is also an open question, but the current view (as far as I understand) is that things started out symmetric and the asymmetry is the result of some dynamic process that resulted in an imbalance between matter and antimatter. This process of creating a baryon asymmetry from an initially symmetric state is known as baryogenesis and is a really interesting (to me) area of study. Unfortunately I don't know enough to provide an expert view on what the most likely mechanism for baryogenesis is, but hopefully this provides some context as to where the field is at.
I'm sorry you lost me super early on here: " particle-antiparticle pairs were constantly popping into and out of existence"
What a super dumb analogy for what an antiparticle is? And why would they stop existing and exist again?
Oooop no worries! An antiparticle is a partner for a particle that is identical in mass, but has the opposite charge. For an electron the anti particle is a positron, for a muon it is an anti-muon, etc. An analogy is this: if particles are little balls flying around, then an antiparticle is a hole (that can also move around) that is exactly the same size as the ball. If the two meet, the ball falls down the hole and lets out a sound, and both are gone since the ball has filled in the hole. The sound that it let out is a form of energy. Conversely, you can form a ball-hole pair by digging a hole and forming the dirt into a ball to fly around. This also takes some energy to do though.
In actuality, particles annihilate when they encounter an antiparticle, letting out light (energy). Conversely, if you add enough energy to a region of space, you can spontaneously form particle-antiparticle pairs (similar to the digging a whole analogy right above). This is called pair production. Sometimes these newly formed particles immediately collide into each other and annihilate again, but sometimes they fly away from each other until they meet some other particles.
Must they find their own unique pair, or if the antiparticle/particle is of the exact same mass it'll also annihilate? And if a pair gets annihilated but there's enough energy around to produce another pair, this new pair will have nothing to do with the other unique instance, right? Like, they don't somehow persist and come BACK into existence? (But how can we test that?)
Thank you for the links and the quick answer: sometimes getting into a new topic means I need to mentally set up a lot of scaffolding, and then reading new things might have a chance of sticking a tiny bit.
An interesting thing about fundamental particles is that all particles of a single type (say, electrons, muons, etc) are identical. So any electron colliding with any positron for example, will annihilate. And if a new pair gets created, then the new particles are identical to the ones that got annihilated.
Wow......
What's a good completely introductory book on....oh geez everything it sounds like.
For early universe cosmology (which covers many of these topics), you have to check out The First Three Minutes by Nobel laureate Steven Weinberg. Very approachable and mindblowing. For more general astronomy Cosmos by Carl Sagan is hard to beat. If you are interested specifically in the quantumness I was talking about (particle production), I would actually recommend the book Genius by James Gleick. It is a biography of physicist Richard Feynman, but includes approachable discussions of his science embedded in the scientific context of the time he was working in, which to me always makes things more understandable.
:D
Thanks for sharing your insight/time with us, it’s an interesting thread!
Here’s my question for you: in an unlimited budget scenario, is there any tool/technology could we build that stands to let us learn the most about our universe? Something new or scaling up something that exists already?
Well, I’m one of the sort that still believes a higher luminosity particle accelerator would pay dividends in terms of science output, but I think after the LHC not discovering supersymmetric particles and the earlier failure of the SSC the age of huge accelerators is over, or at the very least on pause.
One upcoming experiment I would want to accelerate the timeline on is LISA, which is a space based gravitational wave observatory which is shaping up to be extremely sensitive, sensitive enough to detect the stochastic gravitational wave background among other things. It will be to gravitational wave astronomy what Hubble was to astronomy.
Maybe I’m not thinking big enough? With a truly unlimited budget we could stick a massive observatory on the far side of the moon. Talk about prime viewing conditions! I’m salivating thinking about a multi wavelength, massive telescope up there.
All of this being said, I think in the short term this money would actually be better spent on non-fundamental projects. I personally think a massive investment in fusion technology is one of the most important steps we could take as a society, see as if we can figure it out it essentially would solve energy problems, period. Maybe some energy experts here can chime in and tell me why I’m wrong though!
Do any books or movies stand out as getting cosmology hilariously wrong?
Hm, honestly not many come to mind right now. I usually shut off my science side when I'm consuming media because 9 times out of 10 the inaccuracies don't really matter, but if I focus on them it can hinder the experience. But now I'm struggling to even think of any movies that involve cosmology to get wrong in the first place!
Has much change in landscape of programming and data analysis? It's been a decade since I left astrophysics after grad. Right around when things were moving towards Python and away from IDL and Fortran. Curious to know if the tools used in research has changed much in the interim.
I can say that now ~90% of day to day analysis is done in python. Some very important programs are written in C or Fortran still (I have in mind CAMB and CLASS to compute CMB power spectra, but I am sure the same is true in other fields), but they almost always have some python wrapper to allow the user to run them from within a jupyter notebook or something similar. I would say in my research I have used more C than the average PhD student, but that is because for a while I was working on large scale dark matter simulations that needed to be written in C.
What are the current ideas behind the role of dark energy in the big bang and initial stages of the universe?
What we refer to as "dark energy" is currently a catch-all term for this weird type of energy that is currently causing the accelerated expansion of the universe. The "weirdness" of it stems from the fact that its energy density remains constant as the universe expands. This is in contrast to normal matter, whose energy density dilutes like the inverse volume. That is, if you take a collection of particles and spread them out over twice the volume, their energy density drops by half. You've taken all of their energy and spread it out over a greater volume. Radiation is similar expect that its energy density drops like 1/V^(4/3), due to some relativistic effects. So if you take radiation and double the volume that it occupies, its new energy density will drop by a factor of 1/2^(4/3) ~ .4.
Dark energy is weird because this doesn't happen. If you take a space filled with dark energy and double the volume, you have the same energy density has before. This is really weird! I'm telling you that if you take this energy and spread it out over double the space, you have the same density as before! This is one reason why sometimes dark energy gets referred to as "vacuum energy", since it is as though the empty space itself has some energy density associated with it.
Now, we can run this argument in reverse. If we shrink the universe, the energy density of normal particles with go up, and the energy density of radiation will go up even faster, but the energy density in dark energy remains the same. In the early universe, the radiation contributed the most to the energy density of the universe, followed by normal matter, followed in a very very distance third by dark energy. So as far as we understand things now, dark energy had very little role to play in the initial stages of the big bang and the early universe, and has only become important relatively "late" in the universe. Whereas in the early universe radiation was the dominant form of energy, today it is dark energy. Somewhere in between matter was the dominant form, keeping in line with the relationships explained above. Dark energy only came to dominate things around 5 billion years ago - very recently cosmologically speaking!
Do you like science fiction? If so, what are your favorite books, short stories, and movies? And what do you love about them?
I loveeee science fiction. Listing my favorites would probably take too long. In general I really really love stories that do interesting things with time and how we perceive it. Arrival is one recent example that comes to mind (as well as the short story collection it originates from by Ted Chiang), and Slaughterhouse 5 is another if I am allowed to count that as sci fi.
Probably my favorite sci fi author of all time is Ray Bradbury, and my favorite of his collections is The Martian Chronicles. I think in general I prefer his style of scifi over "hard scifi" like that of Heinlein or Arthur C Clarke, but to be fair I have not read much from them. I love a lot of Asimov's short stories. The Last Question is one that always gets tossed out in discussions like this but its hard to argue with as a favorite. The only Philip K Dick I've read is Do Androids Dream of Electric Sheep? and I enjoyed it, but not as much as Bladerunner.
As for movies, I like basically any "good" scifi movie. 2001 A Space Odyssey is one of my favorites, Solaris, Interstellar, etc. Timecrimes is a good, underrated movie involving time travel. But I like just about any, these are just some of the first that came to mind.
I'm such a fan of Arrival and Ted Chiang. I'm not a big reader, but I've read some of his best stories and he's brilliant. I'm currently on a Greg Egan binge, but I'm also going through Exhalation from time to time and it's just genius. A lot of Ted Chiang is like science fiction Borges, and that's saying a lot!
Love Heinlein as well, top science fiction and great characters full of flaws. The guy knows his crooks!
I've recently been getting invested in the Space Weather conspiracy theorists, I'd love to hear the opinion of someone who's a little closer to cosmology than myself on this subject.
I can link videos and people if you like, but the short version goes as follows:
Electromagnetic fields in deep space shuttle around charged particles.
Large reactors like the sun and the galactic core are extremely sensitive to the particles entering them, the same way a gas engine would be sensitive to the fuel/air mixture entering it.
There is a periodic wave of high density particles headed straight for Earth that should usher in a new era of solar instability and ARE YOU READY? BUY A BUNKER KIT TODAY FROM MY PATREON
(Just kidding - but seriously these space weather people are very convincing, would you like to read up on them?)
Honestly, I've never heard of this and there's enough actual science out there for me to spend time reading up on the pseudo stuff! Sounds funny though.
To be honest, I don't think many scientists of any discipline really want to talk about "fringe" science, because often times the mental effort required to explain precisely why something is wrong is worth more than what is gained. Often times these fringe hypothesis are buttressed by an entire framework of pseudoscience that make them seem plausible to someone who a) is not an expert and b) has taken the time to delve in and read all of the "background" required. So its not as easy for the scientist to simply point out where some theory runs up against an accepted scientific principle, as often times within the framework presented there is some convoluted explanation for why that principle isn't being violated. Then the task becomes dismantling this entire framework and often times thats more effort than a random person wants to invest just to disprove something that they have good reason to think is wrong anyway.
In short, it can be hard to get scientists to answer if you insist on them not answering through the lens of "accepted science", since that is after all what they are experts in and understand. Countering fringe and pseudo science is often times a thankless and tiring job.
P.S. I didn't think your comment came across with any kind of tone! Hope mine doesn't either.
No, I don't think it's a problem of vocabulary or grammar structure, since you write fine and I imagine most of the people you have tried asking would have no trouble parsing you comments/questions. I don't think I have.
For one, if a scientist is telling you they're not well equipped to answer your question, I would take that at face value and not assume that your initial judgement is right (i.e. that what you need is a "straight answer from a scientist", even if it seems like that would be helpful). They are either unwilling to answer (for the reasons I mentioned above, whether or not it is justified in this particular case) or unable to (either due to a lack of expertise or knowledge about the subject, because they simply don't trust their reasoning). It's impossible for me to directly answer why you can't get answers to your questions without knowing what the questions are.
I understand what you are saying about the need to challenge the status quo. This happens all the time within the scientific community, and I think its a mistake to assume it is not happening because scientists aren't fielding challenges from non-experts though. Again, it's impossible for me to answer precisely in this case as I don't know what types of things you are asking.
As for Susskind and Penrose, I don't think any of them were ever "fringe" by the common understanding of the word, but you say here you just mean "theoretical" so perhaps I'm not understanding. Anyway, it is true that they have each at times held opinions that are to varying degrees not accepted or recognized by the larger community. That they nonetheless are well respected by the community is an indication that a) non status-quo views are tolerated by the larger community and b) despite having non-mainstream interests, they are able to communicate and defend their ideas well in a language understood by other experts (i.e. most of the time, with math).
I think most of the time that it is difficult to get a scientist to engage, it is not because they disagree about what the answer is, but that they disagree on what the problem even is. Susskind and Penrose have non-standard ideas about certain subjects, but all of these ideas relate to long standing problems or topics that everyone agrees one (e.g., for Susskind black holes). Not sure if that helps.
My assumption in the above is that you are a non-expert in relation to the scientists you have tried asking questions to. So my point was that I don't know that I'd claim they aren't accepting of challenges to the status quo because they have been dismissive of your questions or ideas, since that is a challenge coming from a non-expert. This isn't meant as a slight or anything, I'm just stating what I think would be a reason someone would act dismissive.
I think I understand fringe as you've defined it. You mean it as "theoretical", which I take to mean "plausible but not currently well supported: nothing glaringly wrong, but also not clear" or something along those lines. CCC is a good example of this, although I would argue that at this point there are enough concerns raised about its underpinnings that it makes sense to deprioritize it as an object of study. See my other comment for more about that I suppose.
I don't mean to make you frustrated. I'm not frustrated in this exchange, but of course if you don't feel like responding anymore that's totally alright.
I think (and most cosmologists I would wager) that at a fundamental level Lambda-CDM is wrong, or at the very least incomplete. For starters we don't know what dark matter is, and we know even less about dark energy, and any complete theory should be able to explain those. So that alone is reason to say that LCDM is not our "final theory" of cosmology. In addition there are many tensions arising, such as the H0 tension, which are stretching the model to its limits. This particular tension manifests as a >5 sigma disagreement between measurements of the expansion rate of the universe, depending on how you measure it. At first I thought this tension was becasue of systematic errors in the supernovae measurements used to measure the expansion rate one way, but since that article was published those measurements have been checked and rechecked. So I am beginning to think that this is actually pointing at LCDM being wrong in some way.
Even more "out there", I think Euclidean quantum gravity is a fruitful approach and could deliver more insights into quantum gravity than it already has. I am 100% not an expert here and my opinion should be taken with a lot of salt. I just like it on aesthetic and mathematical grounds.
If that's your point then I definitely agree. Most cosmologists and scientists I know and interact with on a day to day would agree: LCDM is certainly incomplete. Maybe some take objection to the characterization of "wrong" as that is often taken to me "is in contradiction to observation" which isn't really the case with LCDM. There's a difference between making a wrong prediction and being unable to predict something, and typically I would use "wrong" for the former and "incomplete" for the latter.
One somewhat unscientific part of the scientific process that doesn't often get talked about is how scientists choose what to spend their time on. One might assume that it makes more sense to study, formalize, and devote time to test a theory which explains something using fewer assumptions than some hypothetical other theory on Occam's razor grounds. I would agree from a practical perspective, but ultimately that's not a scientific reason to make that choice. In principle two falsifiable theories which explain some phenomenon, regardless of their complexity, are just as good targets of scientific study but in practice a choice must be made. Typically that takes the form of passing some judgement on a theory prior to extensive study of it, with decisions being informed on the theory's performance as well as the opinions of others in the community. Entropic gravity, for example, has not been shown to be in contradiction with any observation. But there has been doubt cast on its theoretical underpinning, and this informs a scientist's choice of whether or not to devote time to it, regardless of if there's been a nail in the coffin falsification of it. This is part of the reason that science is fundamentally a social phenomenon and activity.
What's it like being a cosmologist as a career? I'm curious because I'm an engineering student who just started an internship where I do engineer things for the first time. I had no idea what to expect going in, but over the past few weeks I'm starting to suspect that "technical jobs" like scientist and engineer are 95% email and 5% technical work. When you go to work, would you say you spend a majority of your time writing emails or collecting data / performing data analysis / surveying existing research?
Well, I definitely don't spend most of my time writing emails. We have Slack for that ;)
A decent portion of my time is spent doing what I would classify as "technical work". I'm counting under this umbrellas things like: writing code, making figures, reading papers, and writing up results. Overall >50% of my time is spent doing one of those things. It is highly variable, however. As I am still a grad student I am also teaching, and some quarters grading can take up a ton of time as well. Some days have more meetings than others, but most of them are "good" meetings in that they related to research I or others are doing. When I was taking classes that also ate into my time as well but I am doing with those.
My brother is studying astrophysics, studying the radioactive decay of black holes. He ultimately wanting to end up in Academia, and is trying his hardest to avoid going into engineering. So does it seem like the path you have taken got you where you wanted to go? Or are there off ramps that you could have taken and been happy with?
I think I've ended up in a spot that I enjoy and that is working for me right now. I don't know that I'll do this forever, mostly because the academic job market is pretty bad and I'm not willing to take a huge quality-of-life hit in order to stay in it. So, IDK if I would take any academic job just to stay in the field if it meant being unhappy with where I was living. But for me, for right now, I'm glad to be doing what I am.
There have been off ramps along the way. Physics as a degree teaches you decently transferable skills, especially if you are happy to go into data science or finance. And fortunately employers seem to have a favorable opinion of physics degrees. So I could have stopped after undergrad and had some career options to fall back on. I didn't get in to grad school the first time I applied, so that was another off ramp where I could have pretty easily pivoted and done something other than academia. I'm sure there will be other off ramps as well once I get my PhD and beyond. I'm not overly concerned about future employment beyond wanting to make sure I'm happy doing whatever it is that I end up doing.