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Katie Mack Profile picture
Katie Mack
@AstroKatie
Cosmologist, pilot, author, connoisseur of cosmic catastrophes. @TEDFellow, CIFAR Azrieli Global Scholar. Cis. She/her. Dr. https://t.co/x5ErK7B0CB
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Jun 29, 2023 • 17 tweets • 4 min read
The news is out! The @NANOGrav pulsar timing array collaboration has detected a background of cosmic gravitational waves rippling through our Universe! Let’s talk about it!
(Full press conference tomorrow, )
1/n https://t.co/5jd4mp989Wnanograv.org/news/2023Annou…
@NANOGrav Q: What does this mean?
A: Whenever violent gravitational events (like colliding black holes) happen in the cosmos, it causes ripples in space itself: gravitational waves. This is the 1st detection of a “stochastic background” of these waves, coming from all over the cosmos.
2/n
Jun 18, 2023 • 4 tweets • 1 min read
Nothing exemplifies a complete failure to understand the point of science like demanding to settle a scientific issue through the medium of emotionally persuasive public shouting We actually do a lot of arguing and debating within science! But — and this is pretty important context — we do it with knowledge and expertise already in hand, having read and understood the relevant literature, and for the purpose of improving the work. Not to win via applause.
Apr 1, 2023 • 4 tweets • 1 min read
We don’t call Trump a liar and criminal because we don’t like him. We don’t like him because he’s a liar who does lots of crimes. He has explicitly bragged that his fans would happily excuse his crimes even if they were murders in the streets; let’s not pretend the righteous indignation and claims of innocence are genuine.
Apr 1, 2023 • 4 tweets • 2 min read
I applied to Twitter for verification in 2016, when I had about 40k followers and was starting to have a voice in the online astronomy/physics community. The checkmark suddenly appeared shortly after a snarky tweet of mine went super viral and my following doubled in a week. I don’t know how much of it has been due to going viral, or to my science, or just to how I tweet in general, and I don’t know how much verification helped, but I built up a wonderful little network of amazing people here whose work I admire and whose friendship I truly value.
Oct 31, 2022 • 4 tweets • 1 min read
The point of Twitter verification is that for certain individuals/organizations it’s useful to be able to verify their statements are coming from them. (This is why so many journalists/reporters are verified.) It’s supposed to help combat disinformation, not be a status symbol. People think of it as a status thing because a lot of people with status are verified but the causality is that if you’re well known, you’re more likely to be a target for impersonation and/or there’s more public interest in being able to verify that your statements are yours.
Aug 31, 2022 • 7 tweets • 3 min read
This is an image of the Cosmic Microwave Background (CMB). It's a projection of the whole sky onto an oval, aligned such that the center of the image is in the direction of the center of our Galaxy, and the edges are the opposite direction. A wide oval with small specks of blue and orange scattered a The CMB is often described as the afterglow of the Big Bang, but it's actually a DIRECT view of the Universe around us -- of parts so far away that the light has been travelling for about 13.8 billion years, so we see those regions as they were when the cosmos was STILL ON FIRE
Aug 16, 2022 • 4 tweets • 2 min read
This is neat: the @ehtelescope team has done some new processing of their black hole image data to tease out a ring of light that comes to us from BEHIND the black hole, curving all the way around to focus into a ring that we can use to measure the BH's mass & spin more precisely There's some helpful nitty-gritty about what this "photon ring" is really showing us in this blog post by @MattStrassler from May -- check out the figure here. Essentially what the @ehtelescope team did was remove the direct image to show us just the ring profmattstrassler.com/2022/05/10/bla… Two diagrams from Matt Stra...
Jul 24, 2022 • 4 tweets • 1 min read
This galaxy is so far away that the expansion of the universe is causing it to recede from us faster than the speed of light. Weirder: it has ALWAYS been receding faster than light. When the photons we are now seeing were first emitted, they were moving directly AWAY from us. How can we see light from something moving away so quickly? How does the light catch up?

The expansion has been slowing down (until recently), and, through a somewhat complicated process, that’s given the light a chance to catch up and start closing the distance.
Jul 13, 2022 • 5 tweets • 2 min read
Getting a spectrum of a very distant galaxy like this is huge because it tells us not only how far back in time we’re looking, but also what kinds of elements are in the galaxy, and therefore what its stars are doing. Hugely important for understanding how these galaxies formed. In general, when we see a star or galaxy in a telescope, the only pieces of info we have are its shape (for stars, that's usually one pixel), its surroundings, its brightness, and its spectrum. Astronomers get a LOT of data from the spectrum -- we don't have much else to go on!
Jul 12, 2022 • 15 tweets • 8 min read
Watch now! nasa.gov/nasalive #JWST first science images about to be presented! Some brief remarks now from Nobel Laureate John Mather (who is a lovely man, by the way) about the conception of the telescope, and more background about who put the project together. #JWST
Jul 11, 2022 • 5 tweets • 2 min read
You can watch the release of the special preview image from #JWST, presented by @POTUS, on NASA TV here: nasa.gov/live, in about 10 minutes. And refresh this page nasa.gov/webbfirstimages, where the images will be posted when they're released. UPDATE: Actually happening in about half an hour, apparently. There’s been a 30-minute delay.
Jun 5, 2022 • 4 tweets • 1 min read
Commentators online: Don’t worry about Covid! If you’re healthy & vaccinated it’s just a little flu!

Actual people I know: It’s day 14, still testing positive, fever is worse, can barely move, feel like death… [2 months later] still fatigued with brain fog but can jog sorta… Anyway yeah I’m still wearing my mask thanks for asking
Apr 23, 2022 • 4 tweets • 2 min read
In case you need more #CosmologyFunFacts to break your brain, did you know that there are distant galaxies that are currently receding from us faster than the speed of light (due to cosmic expansion), and ALWAYS HAVE BEEN, and yet we have pictures of them? 🙃 I talked about this in the final lecture of my general relativity class and the students looked at me like 🤨 but I swear it’s true if you work through the math. The key is that while the GALAXIES are always receding faster than light, the LIGHT can (eventually) move toward us.
Apr 19, 2022 • 6 tweets • 2 min read
Today in my general relativity class I got to talk about my favorite SUPER WEIRD COSMOLOGY FACT which is that if you have galaxies of the same size at different distances, beyond a certain distance, the farther away the galaxy is the BIGGER it appears in the sky (!!!) If you want to dig into the math, you can look up "angular diameter distance", which is essentially a measure of how far away something appears to be based on how big it looks, and you'll find that ang. diam. distance only increases to a certain point, and then it goes down again
Apr 10, 2022 • 4 tweets • 2 min read
Commercial airplanes are reasonably safe, ventilation-wise, *when in flight*, due to high-quality filtration.

But during boarding, taxi, & after landing, the air isn’t flowing, & for up to an hour you’re sitting in a stagnant tube, breathing concentrated exhaled air. Masks help. How bad is the ventilation when the plane is on the ground? I carry a CO2 meter on flights to check air exchange, and I had to turn off the unhealthily-high-CO2 alarm entirely because I kept setting higher thresholds when it went off and eventually I gave up.
Mar 14, 2022 • 4 tweets • 1 min read
Pretty sure I have never in my professional life used more than the six digits of pi that I have memorized: 3.14159. In cosmology, we’re usually calculating stuff to within an order of magnitude; if we can get percent-level precision we’re THRILLED, but most of the time that precision is hiding a systematic error somewhere anyway.
Mar 7, 2022 • 4 tweets • 2 min read
Unfortunate developments in space diplomacy 😞 Meanwhile, some heated Twitter exchanges between retired US astronaut @StationCDRKelly and the current administrator of the Russian space agency Roscosmos
Mar 6, 2022 • 6 tweets • 2 min read
Every time I talk about #DarkMatter or #DarkEnergy several people jump in to say “ehhh sounds like a fudge factor/epicycles/aether!” And I get why — we’re talking about stuff we can’t directly see, it’s weird! — but, very sincerely, we did consider that. xkcd.com/1758/ There is a very small, very vocal group of astrophysicists who constantly work on alternative models in an attempt to do away with dark matter and/or dark energy. They write a lot of papers & get a LOT of press. But, they have yet to produce a model that compels the rest of us.
Mar 2, 2022 • 4 tweets • 2 min read
Having now handwritten half a semester’s worth of general relativity lecture notes, I have determined the following ranking of the most common Greek subscript/superscript indices in order of ease of tiny-writing and overall aesthetic value A row of small handwritten lowercase Greek letters: alpha, s IMPORTANT CORRECTION: previous version somehow tragically left out high-cuteness-value lambda. See below for definitive list. A set of small handwritten Greek lowercase letters: alpha, s
Feb 17, 2022 • 4 tweets • 1 min read
I understand why the confusion exists but "case rates are going down" is not the same as "case rates are low" and the former should never determine policy when the latter condition is not met. If you leap out of an airplane with no parachute, your speed will, in fact, go down at some point as you're approaching terminal velocity! That does not mean everything is going to be okay.
Feb 8, 2022 • 4 tweets • 1 min read
[teaching general relativity this morning]

STUDENT: Isn't there some standard, accepted notation for all this?

ME: 😭😭😭 “anyway here are three OTHER symbols that might be used for the covariant derivative, good luck”