Ask a cosmology PhD student (almost) anything!

Great question! There's a few ways to answer it so I am going to give a few answers :)

Cosmology is finally (in the last ~2 decades) a precision science in the sense that many of the key parameters relevant to the field have been measured with only a few percent uncertainty in the measurement. What this means is that going forward huge amounts of data are going to need to be processed in order to a) eliminate remaining statistical uncertainties and b) simulate future sky surveys in order to get a better understanding of systematic uncertainties. Both of these things will be necessary for the field to continue advancing and for us to continue getting a better understanding of the universe. But both also require fully harnessing next-generation supercomputers and squeezing the most out of them for large scale data analysis and simulation. So one of the challenges I'm interested in is thinking of new and creative ways to harness computational resources to move cosmology forward (I'm also interested in quantum computing on this front, but that's one of my more 'crazy' interests).

As for what question I would want to solve: What is dark energy? We think that about 70% of the energy budget of the universe is in the form of something we call dark energy, because we know next to nothing about it. This is what is causing the accelerated expansion of the universe that was discovered in the 90s, and one of the big questions in cosmology today is what is it exactly. The most accepted theory is that it is vacuum energy of spacetime itself (the energy of 'empty space', so to speak), otherwise called the cosmological constant. However, there is a huge disagreement between the acceleration we observe and what we would expect from quantum field theory by calculating the actual vacuum energy - a disagreement on the order of 10^120. This has been called the worst theoretical prediction in the history of physics. So if I could solve one problem it would be that.

As for biggest discoveries: I think room-temperature (and ambient pressure) superconductivity would revolutionize technology in a way we haven't seen since the invention of the transistor. Additionally, fusion power that produces more energy than it takes would start a permanent transition away from fossil fuels and essentially solve humanity's energy problems for the near future (next few centuries at a minimum). These both would have a much bigger day-to-day impact on humanity than anything that could come out of cosmology

In terms of discoveries that give us a better understanding of our place in the universe but don't necessarily change day-to-day life, I would say solving the cosmological constant problem, and really figuring out the nature of dark matter. Both of these things represent huge gaps in our knowledge - collectively, we don't understand close to 95% of the stuff that makes up the universe! Going a bit further, I would say making progress towards a quantum theory of gravity would represent a similar shift as to what we saw in the 20s and 30s and the transition to quantum mechanics. This is a really good question though and if I thought more I might come up with a better answer.

Cosmos: I've seen it, and I wasn't a fan. I only saw the first season (of the NdGT version) and I wasn't super impressed - the science was pretty good, but the history was so, so bad to the point that it felt like they had an agenda. This isn't a novel criticism really, as others have pointed it out. Watching that show would leave you with the impression that science and religion are diametrically opposed, and in such a way that religion naturally opposes scientific progress. It was a bit unfortunate as I had looked forward to the reboot. The original version with Carl Sagan is great though, and avoids a lot of the ahistorical nonsense that the reboot has. I'd recommend that to anyone in a heartbeat, with the caveat that some stuff might be outdated. Not sure exactly what as it's been a hot minute since I've seen it.

I still can't imagine pre-big-bang, can you?

Nope! This is an area that is much more theoretical than what I have experience in. Nonetheless, the common response is that the Big Bang refers to the beginning of both space and time, so it doesn't make sense to talk about "before" the big bang any more than it makes sense to talk about what's south of the south pole. In reality, this is a bit inaccurate. Our best theories of fundamental physics break down at the big bang (actually, roughly 10^-43 seconds after the big bang, an era known as the Planck Epoch, so it is possible that as we gain a better understanding we will have more to say about this extreme era. There's enough to keep us occupied after the big bang in the mean time ;)

In that sense, how many concepts are still difficult to wrap your human brain around? Or, what took you a while?

Most actually. In undergrad I did a senior thesis on a super theory-heavy topic and kind of realized I wasn't cut out for that type of abstraction. Since then I have moved into stuff that has more concrete contact with observations so it's a bit easier to wrap your brain around. Nonetheless it's still not always easy - the day to day length scales that I think about are hundreds or thousands of light years, and I would be lying if I said I had an intuitive comprehension of what this meant.

Cavendish is interseting (imo) in that he laid the groundwork for many, many discoveries that came after him but doesn't get a lot of recognition in his own right. Part of the reason may be that he was known to be very asocial, to the point that he did not publish many of his findings while he was alive. They came to light after his death, but by then they had been rediscovered by others and credit gone to them. Genuinely a brilliant character that often gets shortchanged by history.

Cosmology is relatively well funded in terms of large survey telescopes, so there is a job market at least, unlike the situation for high energy particle theory for example. If you want to stay in the field, you pretty much either have to get a research role at a university or at a national lab. These jobs exist, but as with many academic disciplines there are far more candidates than there are positions. I know that a lot of people pivot out of the field once they get their PhD - quantitative finance is a big destination for these people, as is data science. The skills you learn doing cosmology are highly transferable since they essentially boil down to data analysis and modelling - the only issue being that you're entering the market in your late 20s after a PhD as opposed to early 20s after a BS degree.

There's a lot here! My answers for each may not be long, but don't hesitate to ask for elaboration.

1. Nothing, the distance between points is just getting larger ;) The real answer is hard to give without either a) simplifying things beyond the point of usefulness, or b) introducing a little mathematics. So I will resort to the latter. In normal geometry, the distance between two point is given by the Pythagorean theorem. Namely, if you have a point (x1, y1) and (x2, y2), the distance between them is given by.

d² = (x₂ - x₁)² + (y₂ - y₁)²
d² = dx² + dy²

Now, cosmology is not normal geometry in that the equation giving distances is different. In general, these distance equations for a specific type of space are called metrics, and the set of metrics that correspond to our universe all have the form:

d² = dt² + a(t)*dx²

You can see here that in the equation telling us how far apart things are, there is a dependence on time. This is interpretative as the expansion of space itself. As you might be able to see, there is no requirement for there to be some higher dimensional ambient space that our universe is expanding into.

2. That's a tough one, although I am slightly inclined to say invented rather than discovered. I go back and forth on this though so I'm not sure either way.

3. Another tough one, but I am inclined to say there are elements of mathematical reasoning that exist outside of formal logic (geometric reasoning being a big one). You can maybe prove all geometric theorems using formal logic, but I think the method of reasoning is certainly different.

4. Besides constants like e and pi, I would say maybe 6 as it is the first perfect number (the sum of its divisors equal the number itself).

5. No, probably not, or at least we don't have a great reason to think so outside of speculation.

6. [My answer above[(https://tildes.net/~science/odq/ask_a_cosmology_phd_student_almost_anything#comment-4z80) kind of answers this, but I would say answering what dark energy is would provide the most insight in the most ways.

7. I think quantum computing is experiencing a lot of hype right now and probably won't live up in key ways. I think it is almost certain that alien life exists. I also don't think science has the toolset to answer many fundamental questions that we want to answer, which leads me to

8. We don't know and likely will never know. This is a fundamental question that I do not believe science (at least as we understand the process now) will ever answer, even in principle. Claims to the contrary or to "vacuum fluctuations" (Lawrence Krauss' book for example) miss the point entirely. Sean Carroll has a great review of the question and I think his conclusions are generally valid. It's either the case that the question simply doesn't make sense, it is just a brute fact, or the result of a supernatural creator. I think all are believable.

Yup, not as much as I should like anymore. I'm currently reading The Fifth Season which I guess could be considered sci-fi? But is more likely Fantasy. Ray Bradbury is one of my favorite authors, as are the short stories of Isaac Asimov. I also really like sci-fi movies and video games of course - Star Trek, Star Wars, etc. Also love the more 'heady' stuff like Arrival and stuff (does that even count as "heady"?).

Not counting telescopes (which are observational but I suppose not 'experimental'), the big area in experimental cosmology are direct detection experiments to try and detect dark matter in the laboratory. What these experiments look like depend on which type of theorized dark matter particle they are trying to detect.

One of the leading candidates are WIMPs (weakly-interacting massive particles), which you can think of more or less as a standard particle that barely interacts with ordinary matter - they mostly pass right through. These theories do say that there should be some interaction with ordinary matter however, even if it is small, and that is what these direct detection experiments focus on. The individual designs vary, but they usually take the form of some large scintillator that is very heavily shielded from background effects. The idea is that the dark matter particle will recoil off the nucleus of the atoms in the detector (often either some type of crystal, or a noble gas) and the nucleus will release a bit of light that can be detected. The problem facing these experiments is that the background event detection has to be extremely low - I can't remember the exact details, but it's something insane like only allowing on the order of 10s of false positive over the liftetime of the detector. They are extremely precise experiments, but haven't found anything yet.

I'll briefly mention another type of detector which I think is pretty cool. Axions or axion-like particles are another proposal for dark matter, and these particles can have extremely light masses. Seriously, like up to 10^-22 eV (for reference, an electron has a mass of 511 eV). This is on the extreme end of models, but even the middle of the range models have masses in the range of a couple of eV. Since they are so light, the above type of experiment doesn't work as they would essentially just bounce off the nucleus without making it recoil (think a ping pong ball hitting a bowling ball). Instead, there are some experiments in progress to detect these things with super accurate atomic clocks. The atomic clocks developed at NIST for example are accurate to within 1 second per lifetime of the Universe. This is literally an insane amount of precision - they can detect the time dilation that comes from setting the clock on a table versus a meter down on the floor. Since they are so sensitive to changes in gravity, they can actually be used to detect when an object passes by them via it's local gravitational field. The idea is to set up an array of these clocks and look for correlated disturbances passing through the area that could correspond to an object passing through and causing the clocks to be off a bit. These experiments face similar issues in that background noise can very easily wash out whatever signal there may be, so there is ongoing work to understand the noise signature better.

Most of the time it’s just programming, and also trying to stay up to date with the literature.

Honestly the best thing you can probably do for your skin on a daily basis is to wear sunscreen when you go out, even if it’s just a normal day. Put a bit on your face and arms - the sun is a big radiation bomb! Other than that I could recommend getting one of those oil free face cleaners for use in the shower. Make sure you actually clean your face in the shower! It’s surprisingly an easy thing to forget.

For nonfiction: Cosmos, A Brief History of Time, The Very First Light, and Losing the Nobel Prize are all either very good books about cosmology or cosmologists.

For fiction: Not quite sure on this front. The Three Body Problem was pretty good and incorporated some stuff from cosmology. I suppose the Martian as well.