Alex Filippenko, a world-renowned research astrophysicist who helped discover the Nobel-worthy accelerating expansion of the Universe, is available to answer questions about astronomy and science in general, 3:00-6:14 pm EDT, 5/10/14. Named the 2006 US National Professor of the Year, he is well known for his ability to communicate complex subjects to the public. He has appeared in about 100 TV documentaries, including 7 seasons of "The Universe" series on H2, and he has produced several astronomy courses for The Great Courses.

(I'll be posting some of the top questions to Twitter!)

Edit: Folks, you have so many good questions, but I need to leave now, after 4 hours and 20 minutes (more than my scheduled 3.14!) on the AMA. But I will continue to answer questions occasionally on my new website, -- go to the "Ask Alex" tab. The questions and answers will be achived for future viewing.


Comments: 525 • Responses: 74  • Date: 

kremlinmirrors184 karma

Hi Alex! I took C10 with you along with one of your seminars (and a few trips to Lick Observatory) and I was wondering, for someone who has been out of school for a few years, what major new updates on our knowledge of the universe should I know? In other words, if I took your class again this year, what would be vibrantly new and different?

Thanks for doing this AMA!

notaknowngiverofucks303 karma

Additionally, as someone who is currently in your C10 class, can you give me a quick rundown of every single question that will be on the final on Wednesday?

Jk, thanks for the great class!

AlexFilippenko227 karma

Yeah, right -- you wish! But I'm really glad that you enjoyed my class. I love teaching it -- in fact, I would say that it's my favorite class, because it allows me to bring astronomy to a lot of students who didn't have any previous interest in the subject, or even in science. Go Bears!

AlexFilippenko168 karma

Cool that you took my class a few years ago! Yes, a LOT has happened since then. I think two of the most notable discoveries have been the huge number of exoplanets (planets orbiting other stars, as you know) found by the Kepler spacecraft, and the recent BICEP2 result that provides evidence for an early "inflationary" epoch that can be thought of as giving the "bang" to the "big bang." (Of course, the latter discovery still needs to be verified with other, independent measurements, so stay tuned!)

kremlinmirrors41 karma

Awesome, I'm going to have to read up on that! Thanks for the updates :)

AlexFilippenko90 karma

Yeah, it's great stuff. And last year the Planck satellite provided an amazing all-sky map of the cosmic microwave background radiation. It will be interesting to see whether further analysis of the Planck data confirms the amazing-looking BICEP2 result. There are other observations being made elsewhere, too, that should be able to check the result.

5Aces94 karma

How far out should I drive to get a good view of the milky way this summer? (east bay)

AlexFilippenko172 karma

Try to go out to the Sierra Nevada range around the time of new moon, and you will be treated to a truly spectacular sight. The skies there are really dark, and you're above the haze, so you can see a lot of faint stars. If you're not able to drive that far, then I suggest Pt. Reyes National Seashore, about 1-1.5 hours drive. It's the darkest place I know withing a short driving distance from the SF Bay Area.

dearastronomer88 karma


We collected a few questions in advance over at /r/astronomy, here's the first:

How common are "intermediate mass" black holes (between stellar and galactic-center sizes)? What about stellar mass black holes without a companion star? Is it likely that there are a few "naked" black holes (no companion or inflowing gas) within a 15 light year radius of Earth? 25 light year?

AlexFilippenko89 karma

We actually don't know how common "intermediate mass" black holes are, so that's a great question. There's a good candidate in G2, a large globular star cluster orbiting M31, the Andromeda Galaxy (the nearest big galaxy to our own, about 2.4 million light years away). But some of the other candidates have been explained away and are not necessarily IMBHs.

Stellar-mass BHs without a companion star almost certainly exist. Yes, you could call them "naked" BHs! One way to find them is through gravitational lensing: If they pass directly between us and a background star, they will cause that background star to brighten, owing to the focusing of light toward us.

But such black holes are sufficiently rare (coming from rare, very massive progenitor stars) that I don't think there are any within a radius of 15 or even 25 light years from us. Perhaps within 100-300 light years would be my guess.

Garbageman9968 karma

Hey Alex! What do you think, based on the evidence available, is the fate of the universe? I know the question is very broad, and so a broad answer would do:) Thanks!

AlexFilippenko104 karma

My guess right now is that the Universe will expand forever, because of the repulsive dark energy. If it really is Einstein's "cosmological constant" (which can be thought of as the vacuum energy of space), then the energy density will always remain the same, and the accelerating expansion of the Universe will eventually become an exponentiation, like the "inflation" that we think governed the Universe in its first trillionth of a trillionth of a trillionth of a second. Right now, the data are consistent with the dark energy being the cosmological constant, but we still don't know for sure.

SpaceStormy34 karma

What do you think the chances are that the constant could change?

AlexFilippenko69 karma

If it really is the "cosmological constant" then it won't change, by definition. However, the dark energy could be something else ("quintessence" or other stuff), in which case it could change in the future. In fact, it is even possible that the currently repulsive effect will someday become attractive. If that's the case, and if there's enough of the "stuff," then the Universe could ultimately end up in a Big Crunch.

But right now, the data are consistent with the cosmological constant (though not all other possibilities are ruled out).

iorgfeflkd6 karma

What data could separate a lambda vs non-lambda explanation of the accelerating expansion?

I have a physics background, so get technical if you want.

AlexFilippenko18 karma

We are trying to measure in detail the expansion history of the Universe, by more accurately measuring the distances (and redshifts) of supernovae. If that expansion history ends up being inconsistent with what is predicted from Lambda (the cosmological constant), then we will have ruled out Lambda.

SexxyScientist52 karma

Go Bears! Hi professor Filippenko I was wondering if you had any student in your class that made you think 'he/she will be the next Einstein'? - and what it was about them

Also, what is your favorite place on campus ?

AlexFilippenko79 karma

I've had a lot of brilliant students. But it's almost impossible to know who will be hugely creative and produce a true revolution of the Einsteinian sort. Those kinds of breakthroughs require insights that are not predictable through test scores, etc.

When I have time, I like to play tennis, so probably the tennis courts are my favorite. But lately I haven't had time... rats.

el_crunz48 karma

I know you often (always?) get voted best professor at Berkeley by the students - congrats! I wish I could take a class with you.

AlexFilippenko97 karma

I'm glad my students like my course! But there are many other great professors at UC Berkeley. This year, Robert Reich was voted "Best Professor."

Are you a UC Berkeley student? If so, I will next be teaching my Astronomy C10 class in Spring 2015, MWF 3-4 pm. If not, then consider this: I recorded an extensive, richly illustrated video course on astronomy (96 half-hour lectures) with "The Great Courses" that will teach you a lot and answer many of your questions: "Understanding the Universe: An Introduction to Astronomy." It happens to be on sale right now (probably for a very limited time) at a huge discount; see .

Astromike2340 karma

Hi Alex! When I was applying to astronomy grad programs almost a decade ago, I spoke to you on the phone about applying to Berkeley. In spite of having a very high physics GRE score, you told me that I would be wasting my time because your program is "academically rigorous, so we doesn't accept people with B.A.s in physics." I was so taken aback by your answer, I never got to ask: Why? I understand the importance of screening out folks who don't have the required physics background, but the criterion for applicants who earned a B.A. in undergrad seemed weirdly arbitrary.

(Side note: I ended up attending a different top-tier astronomy program, and now have a prestigious postdoc.)

AlexFilippenko68 karma

Hey, Mike. I'm sorry that I don't remember you personally. However, I don't recall ever saying to anyone that UC Berkeley doesn't accept people with B.A.s in physics. (Are you sure you talked to me rather than someone else?) It would be a rather odd thing for me to say, given that I myself have only a B.A. (rather than a B.S.) in physics. What degree is offered often depends on the individual institution, and on the particular course of study the student took.

If indeed I discouraged you from applying to UC Berkeley, it may have been because I thought you didn't have a good chance of getting in based on the courses you actually took (rather than the formal degree) and the grades you achieved in them. I didn't want you to waste your time. We get a large number of applicants. But this was not meant to imply that you couldn't get in elsewhere and that you wouldn't do well. And I'm glad you did!

Astromike2361 karma

Wow, getting closure on this after ten years is surprisingly cathartic. Thanks for replying!

dearastronomer35 karma

If it helps, I was also told (not by Alex) that I'd be wasting my time as an "adult" student pursuing astrophysics, and most likely wouldn't graduate.

I graduated with a 3.6 GPA with two publications, and less than a year after graduating, obtained a position with a university as a professional astronomer.

AlexFilippenko42 karma

Excellent! Don't let the naysayers stop you. If you have the brains, passion, and determination, you'll go a long way.

AlexFilippenko24 karma

You're welcome! And congratulations on following your dream and succeeding.

uberlad38 karma

What's your very best life advice?

AlexFilippenko224 karma

I urge you to find a profession that you really love, that you're passionate about, almost like a hobby... and it need not mean that you're making a lot of money. As I tell the students in my introductory astronomy class at UC Berkeley: If you spend every hour waiting for the day to end, and every day waiting for the weekend, and every week waiting for vacation, and every year waiting for retirement, what good is that? You're spending a major fraction of your life doing something that you don't like, that's not fun. Instead, to the degree possible. do something that brings you happiness and fulfillment. This worked really well for me, and I hope it will for you.

8236426 karma

Best way to keep up with the science on black holes?

AlexFilippenko76 karma

Well, use your favorite search engine, and go to Wikipedia, which is kept fairly well up to date.

flojin16 karma

Wikipedia is a self-correcting mechanism. At least that's what an astronomer told me at Chabot Space and Science Center.

AlexFilippenko57 karma

I may have been the person who told you this, since I go to Chabot quite a lot. Wikipedia has improved dramatically over the years. You can definitely use it as a first source. If you want to get into more details, you can look up the cited references. Yes, I recommend Wikipedia to my students.

potatoetomatoscrewyo24 karma

I have to ask, do you think we are alone in this universe? Do you think that there is a creator?

AlexFilippenko95 karma

I think that primitive life (bacteria and microbes) might be relatively common, though perhaps not. The odds for intelligent extraterrestrials are much, much lower -- perhaps just one example, or a few, per galaxy at any one time. However, there are 100 billion galaxies in the observable Universe, and I think intelligent life must exist somewhere out there.

Regarding a creator: My own personal belief is that the Universe runs according to the laws of physics. What put them there, I don't know; you could call it God, if you wish, or you could call the laws themselves God.

Smashcrew22 karma

Hey Alex! What did you have for breakfast?

AlexFilippenko46 karma

I'm in Hawaii, so I actually had a papaya (yum!) and cereal with milk.

goobusmungus22 karma

Hi Alex, your my favorite Astrophysicist! Can you tell us what kind of new discoveries we can expect using gravitational lensing in conjunction with the Hubble Telescope?

AlexFilippenko31 karma

Studies of gravitational lensing with the Hubble Space Telescope can provide the mass distribution in clusters of galaxies, and thus give us new information on the presence and distribution of dark matter. This dark matter is quite puzzling... we know it's there, but we don't know what it is. It's one of the top mysteries in modern-day astrophysics!

fuck_they_found_me5 karma

What do you think of the MOND theory?

AlexFilippenko9 karma

I think it's probably incorrect. However, I do admit there's a slight chance that dark matter might possibly be the wrong explanation for the observations.

cccaceres18 karma

I just finished your astro c10 class (currently studying for the final!) and I you've probably heard it a million times already, but thank you for everything!

My question is, how much should professors be responsible for making their classes engaging and "fun" for their students? It seems that many of the classes I have taken, professors are only teaching it because they have to, taking all the enjoyment out of the topic and making students like myself quickly regret taking the class.

Also, lick observatory is fantastic, you should mention your efforts to keep it in the UC system, I'm sure you could get a few donations here!

AlexFilippenko28 karma

I'm glad you enjoyed my course! I devote a huge effort to make my class fun, and I think other professors should do likewise. After all, why should the student be interested in an introductory general-ed course if the professor doesn't seem passionate about the subject? Hey, thanks for the plug for Lick Observatory. I'll try to say more about it later during this Reddit AMA, but for those who are already interested, check out .

LegitCinnamon18 karma

What's your favorite food?

AlexFilippenko101 karma

I'm partial to astronomically themed candy, such as eclipse gum, orbit gum, starburst candies, Milky Way bars, and Mars bars (but this last item is now really hard to find in the US).

As my research group knows, I like pizza. I bring them free pizza at our weekly meeting and they justify their existence for the previous week!

Taleb24718 karma

Do you think the new horizons mission will discover anything new about Pluto ?

AlexFilippenko31 karma

Oh, I'm sure it will. Every new telescope, spacecraft, and window to the Universe provides new answers -- but also new questions (and that's part of the fun of science!). Even though Pluto is not a genuine "planet," it is still an important member of our Solar System and should be studied. And, after all, it's the first "dwarf planet"! (Check out the cool t-shirt on the web page my wife Noelle set up,

lookapandabear16 karma

Hi Alex, First I want to let you know that you are most likely the best professor I've had here at Cal. You're lucky to teach such an interesting topic, but I can't quite pinpoint what makes you so inspiring. What advice can you give other teachers and those who may aspire to be educators on the future?

AlexFilippenko25 karma

Thanks so much! I would say that they should wear their passion for their subject on the sleeves. Let it all hang out! That will help get students interested.

AFuddyDuddy16 karma


Do you ever not smile?

AlexFilippenko38 karma

Hah! Yes, of course. But when I'm discussing astronomy, it's hard not to smile!

seismicor15 karma

Hi, Alex. How long will it take until we have ships capable of taking us to the next solar system in a reasonable amount of time?

AlexFilippenko44 karma

Oh, gosh, I don't know. Probably not sooner than 1000 years from now, and perhaps much longer than that, even with our currently rapid growth in technology. The energy barriers are truly stupendous, especially if you want to take humans instead of computers/robots. I think it is far more likely that we will be sending computers/robots instead of humans to other worlds. Indeed, they may end up being our evolutionary descendants.

Fannybuns15 karma

Does the fact that the universe is "flat" suggest that we can put a minimum size on the entire universe that's at least as big as a hypothetical closed universe in which or local curvature is too small to detect with current techniques?

AlexFilippenko60 karma

Yes, that's right. I think current estimates place the volume of the entire Universe to be at least 80 times the volume of the observable part of our Universe.

But in fact, the entire Universe may well be far, far larger than that, according to inflation models. It could easily be the case that the ratio of the diameter of the entire Universe (all that there is) to the diameter of our observable Universe is as large as the ratio of the diameter of the observable Universe to the diameter of a proton: something like 1041 for the ratio. In other words, our observable Universe is like a proton relative to the entire Universe. Whoa!

AlexFilippenko14 karma

Folks, you have so many good questions, but I need to leave now, after 4 hours and 20 minutes (more than my scheduled 3.14!) on the AMA. But I will continue to answer questions occasionally on my new website, -- go to the "Ask Alex" tab. The questions and answers will be achived for future viewing.

Sunilsunil13 karma

I noticed you had a Reddit AMA last year on Pi Day. Is that why you decided to host today's AMA for 3 hours and 14 minutes (3.14)?

AlexFilippenko51 karma

Yes, that's right, 3.14! (Of course, 14 minutes is 0.233 of an hour, so my AMA is actually lasting 3.233 hours, but that notation doesn't make it obviously related to pi.) I like Pi Day because it helps promote math and science education in primary and secondary schools. My wife, Noelle, who is quite interested in education and public outreach in math/science, recently created a new website, , that promotes Pi Day. It's still under development, getting ready for National Pi Day 2015 (the big one: 3.14.15 9:26:53 -- on this date and time, we get the first 10 digits of pi, and this happens only once per century). She welcomes your advice and support. There are fun pi-related t-shirts on sale on that website -- check them out!

Borian13 karma

I am always wondering ...
before scientist announce a big discovery or unexpected research result, how long do they sit on the "news" ?

at some point they have to be fairly certain that the calculations/experiments are going in a un-/predicted direction and "know" something before anyone else.

AlexFilippenko29 karma

So that's a tough call. It depends on the result, how "big" (important) it is, how likely someone else will "scoop" you (come up with the same result), etc. You want to be pretty certain that you're right, or at least that you've correctly evaluated the statistical uncertainties and the systematic biases that might be biting you in the butt. However, at some point you've got to decide to publish.

For example, in February 1998, the "High-z Supernova Search Team" (z = redshift) of which I was a member decided to go public with our discovery of the accelerating expansion of the Universe. We were pretty sure of the result, but not completely sure -- compelling confirmation took more than a decade! But we had to go public because another team, the Supernova Cosmology Project (of which I was previously a member), was in the process of reaching the same conclusion. In the end, the leaders of both teams were recognized with the Nobel Prize in Physics, 2011. (It's too bad that the Nobel Prize can go to at most 3 people, usually the formal leaders of the teams... of which I was not.)

flipapancake1713 karma

Some of my friends have taken your Introduction to Astronomy course at UC Berkeley and loved it. They say that your last lecture is especially inspirational. What do you tell them in that lecture?

AlexFilippenko65 karma

Well, one of the things I tell them is that each of them is special in a cosmic sense, not just an insignificant speck of dust in the Universe, because they have the intellectual capacity to understand what scientists are learning about the Universe. In a sense, it's as though the Universe has developed a way to know itself, through us. We are the observers of the Universe, the explorers of the Universe, the brains and conscience of the Universe. And it needn't have been like this! Alter the laws of physics ever so slightly, and our Universe may have been stillborn, devoid of such complexity. We were dealt a darn good poker hand, so to speak.

cccaceres22 karma

its the "supernova" of the class; he tears you apart by telling you how insignificant you are in the grand scheme of things. He then builds you back up by saying how important you actually are in the universe. By being the only thing in the universe that can study, look inward and understand how the universe works, he says how humanity is the "soul" of universe.

He also mentions that for those luck enough to have an education, not to squander it by doing a job you hate, even if it makes a lot of money. It would be better to be poor and happy over rich and miserable.

It sounds like what youve probably already heard a million times, but it becomes one of those "you had to be there" things to really get the full effect.

pandasgorawr17 karma

It was the standing ovation at the end that did it for me. Of all the classes I've taken so far I've never seen so much appreciation and respect for a teacher, and of course, Professor Filippenko is the only teacher I've had so far that deserves it all.

cccaceres7 karma

right? in all my time at Berkeley, I dont think I've seen that final clapping be more than a "thanks, its over!"

He definitely deserves it.

AlexFilippenko23 karma

Thanks so much for all the comments! I'm happy that I had such a positive effect on my students. It's very gratifying.

I'm just starting to do social media and more public outreach. You can keep in touch and up to date on these platforms:

@4AstroAlex (Note: In the Ask Alex section you can submit a question and I will be answering them and posting to twitter, facebook, and the site. I will also share upcoming Stellar Events. Of course, all of this will be as time permits. I can't guarantee rapid responses to most questions!)

M35Dude12 karma

What do you think about Fermi detecting ~GeV gamma-rays in novae?

Single degenerate or double degenerate for Type Ia?

AlexFilippenko15 karma

Such high-energy gamma rays require a powerful acceleration mechanism for the charged particles. I'm not sure the observations have been explained yet... I'll try to find time to look this up after the AMA is over (I'm typing really fast now, to answer as many questions as possible).

Regarding Type Ia supernovae: I think it's becoming pretty clear that some come from single-degenerate systems (a single white dwarf stealing material from a more normal companion star, for those who don't already know the terminology), while others come from double degenerates (i.e., two white dwarfs merging together).

sourmilksmell10 karma

I'm no scientist, so when it comes to the scale of numbers involved astrophysics, are the scientists themselves able to fully accept the size of everything?

Like the concept of things, like galaxies, being billions of light years away, or a million Earths could fill the Sun. I don't think I could imagine a million school buses lined up next to each other, let alone a million Earths.

Sorry, it's not much of a science question.

AlexFilippenko27 karma

These numbers are indeed hard to imagine, even for scientists. I try to help by using scale models. For example, suppose the Sun (which is actually 109 Earth diameters) were scaled down to the size of a period at the end of a sentence (about half a millimeter). Then, using the same scale factor, the nearest star (other than the Sun), 4.2 light years away in real life, would be about 14 km away. In between would be essentially empty space!

Ravusta9 karma

Who do like to collaborate with?

AlexFilippenko13 karma

Hard working, creative, inspired people who work pretty independently (i.e., they don't require a lot of hand holding on my part).

seismicor9 karma

Do you enjoy science fiction movies? Do you have a favourite one? Does it bother you when movies are scientifically incorrect?

AlexFilippenko16 karma

Occasionally, but I don't have time to watch many movies. I really liked "2001, A Space Odyssey" when I was a kid. And of course, the Star Trek TV series was great.

Regarding scientific accuracy: Well, you have to give them some creative license. And some of what we used to call "science fiction" is now known not to be fictional!

mcfuzzum8 karma

Prof. Filippenko! My sister was in one of your classes couple semesters back, I believe... She got me one of the Pi shirts that I believe your wife makes.

No questions really; just wanted to say you're awesome :)

AlexFilippenko17 karma

Thanks! The Pi shirts are really fun. When I walk around in them, I get so many comments, especially the one that says "My PIN in the last 4 digits of Pi." You can get them at -- and in so doing, you'll be supporting my efforts to save Lick Observatory.

Say "hi" to your sister for me!

dearastronomer7 karma

Question 2 from the r/astronomy question thread:

What is the ratio of black holes to currently-burning stars in a globular cluster? Does this differ from the ratio within the disk of the galaxy?

AlexFilippenko4 karma

I don't know the quantitative answer, but I think the ratio is very low. The reason for this is that very few stars have the ability to become a black hole. Black holes come from very massive stars, and such stars are very rare. By far the most common stars are the low-mass main-sequence stars, and they don't become black holes (they become white dwarfs instead).

The ratio might be higher in the disk of our Galaxy, because new very massive stars keep forming there (from gas and dust, which are now absent in globular clusters), and they have the potential to become black holes.

DoctorBritta7 karma

Hi Alex,

What can you tell us about dark matter?

AlexFilippenko8 karma

We don't know what it is, but it constitutes about 25% of the total matter + energy content of the Universe. Probably it's WIMPs (weakly interacting massive particles) left over from the Big Bang.

Arcitraz7 karma

Hi Mr. Filippenko, do you think there is a super massive black hole in the center of our galaxy? What is beyond the boundaries of the Universe? Can you better explain the idea of space having four dimensions? Thank you so much for doing this!

AlexFilippenko14 karma

Yes, there's a lot of evidence for supermassive black holes in the centers of galaxies, including our own, which has a roughly 4 million solar mass black hole. Indeed, stay tuned, because over the next year, that black hole will be tearing apart a star or a cloud of gas that's passing quite close to it.

For more information about black holes, take a look at the 12 half-hour lectures I recorded in a video course called "Black Holes Explained," produced by "The Great Courses." It goes on sale for $40 once in a while at the following website: .

Regarding the boundary of the Universe: we don't think it has a boundary. Either it's infinite, or it curves around like a balloon. But there could well be other dimensions (the "bulk") in which our Universe (a "brane" or membrane) exists.

If space has four dimensions, it's like our 3D Universe is like a slice of space. For example, a slice of an empty sphere is an empty circle. A one-lower-dimensional slice of a hypersphere (which is what our Universe might be) is an empty sphere. I hope that helps!

Eurofooty6 karma

Likely root cause of dark energy and dark matter?

AlexFilippenko9 karma

My own preference is that dark energy is vacuum energy (the cosmological constant) -- see a previous post. However, it might be some kind of new energy field (these often go by the name "quintessence"), like the inflaton that inflated the early universe. If that's the case, then some day the energy will decay into something else, as did the inflaton (it became the matter of which you are made -- so each of you actually contributed to inflation, the "big bang"!).

Most of the dark matter is probably a WIMP, or some combination of WIMPs (weakly interacting massive particles), though none has yet been definitively detected in a laboratory.

zarcusarquel6 karma

What is your favorite place to stargaze? Can you also mention some good locations to stargaze ? I watched your teaching series, its a long tie a go. There are two things I remember colorful shirts and OBAFGKM :P

AlexFilippenko11 karma

Well, relatively close to the SF Bay Area, Pt. Reyes National Park is really good and dark. The Sierras are excellent. Hawaii is great, and the Chilean Andes are fantastic because the center of our Milky Way Galaxy passes overhead during southern winter.

Great that you watched my video course! If it was the first edition, then you might want to examine the second edition, which is much better. Go to The Great Courses.

glaucon816 karma

Dumb, quickie reminder that Pi Day is also Einstein's birthday.

AlexFilippenko7 karma

It is indeed! One of the charms of Pi Day.

adotsquare6 karma

Hello, Alex! Queen's University Belfast student here. I did my MSc project in supernovae (SNe) and read plenty of your papers!

How does the field of SNe compare today to when you started your academic career, and where do you think its future efforts should be focused?

AlexFilippenko4 karma

Thanks for reading my research papers!

The field of supernovae has changed a lot over the years. We now use Type Ia supernovae as very accurate cosmological distance indicators, but we need to understand them even better in order to more precisely measure the expansion history of the Universe (thereby putting constraints on the nature of dark energy).

We still don't know which massive stars produce black holes vs. neutron stars at the end of their lives. Being part of a binary system can also influence the last stages of stellar evolution.

There are lots of interesting questions, but I'll move on to another topic now.

Good luck to you!

AstroEnthusiast6 karma

What is Astronomy Day, and why is it held?

AlexFilippenko14 karma

Astronomy Day is held twice per year on a Saturday, usually in mid-April through mid-May and October, around the time of first-quarter moon. It's a chance to raise public awareness of astronomy, and to get kids excited about space and technical fields. I like to say that astronomy is a "gateway science" -- kids get interested in science through astronomy, and many of them go on to pursue careers in other areas of science, technology, and engineering (i.e., not necessarily astronomy).

On Astronomy Day, people can learn more about astronomy through local activities at science centers and planetariums (e.g., in the San Francisco Bay Area where I live: the California Academy of Sciences, Chabot Space and Science Center, Lick Observatory), and to view celestial objects (including the Sun, properly filtered) through telescopes set up by amateur astronomers. (They love it when others look through their telescopes!) The first-quarter moon is an excellent sight through small telescopes, and it is visible in the afternoon and evening -- hence the scheduling of Astronomy Day on a Saturday close to first-quarter moon. Check your local listings for possible events.

yum_bacon5 karma

Hey Alex! What is the toughest thing about the universe for you to wrap your head around?

AlexFilippenko7 karma

Probably extra (tiny) dimensions in string theory, and the possibility that we are just a membrane in a higher-dimensional bulk. Black holes are pretty tough going, too.

Jabronista5 karma

I was roommates with Michael Ellison who used to do late night telescope work with you! Very cool. I see you on TV all the time.

AlexFilippenko3 karma

Say hi to Michael Ellison for me! Yes, he was one of my research students.

CalvinDehaze5 karma

Hey Alex!

I love you and all the scientists that lend their time to the myriad of TV shows about the Universe and such. Which leads me to this question; what's the best way to ask Amy Mainzer to marry me?

AlexFilippenko12 karma

Thanks! Those TV shows really do stimulate interest among the general public as well as science enthusiasts.

Regarding Amy: I don't know what advice to offer you. I'm surprised my own wife fell in love with a geek like me!

LandonCurtNoll5 karma

I heard that the University of California Office of the President plans to phase out funding for UC's Lick Observatory in 2016-2018. What do you think of that, and what can be done about it?

AlexFilippenko9 karma

Yes, that's what the people at UCOP have said, and I think it's a terrible idea. The core funding for Lick Observatory (about $1.4M per year keeps it running at a spartan level) should come from UCOP. They should at least provide funds that match external support. In any case, much of the future funding will probably have to come from private donations.

Lick Observatory serves all UC astronomers, and it is especially important for young astronomers (undergraduate students, graduate students, and postdoctoral scholars) because they can get telescope time there, unlike at many bigger, more expensive facilities. Lick is also excellent for long-term studies that require many nights each year, such as searches for exoplanets and monitoring exploding stars (supernovae). A lot of new types of instruments are developed or improved at Lick, such as laser-guide-star adaptive optics (which can make the Lick 3-m telescope get images as clear as those from the Hubble Space Telescope at near-infrared wavelengths). Lick also does much public outreach and education. I discuss all this more thoroughly in a recent 75-minute lecture: .

Everyone can help by writing letters to UCOP and providing donations. Even small donations help, if there are many of them! Please see the SaveLick website: , especially the "Help Save Lick" tab at the upper right.

You can see that I'm quite passionate about this subject! I'm trying to Save Lick largely for the next generation of astronomers and other budding young scientists/engineers.

Horta4 karma

What happened to the universe pre-inflation? Does the big-bang still hold up, even though, to this layman, most of the 'explosion' is thanks to the inflatron. What of the Ekpyrotic universe. And does inflation cover the creation of dark energy and dark matter?

AlexFilippenko16 karma

Inflation is what gave the "bang" to the "big bang." Inflation is an extremely important refinement to the big bang theory; it doesn't negate the big bang theory, which is on very solid observational ground. Yes, inflation is consistent with the creation of regular matter, antimatter, photons, and dark matter. The current "dark energy" (if that's what is causing the accelerating expansion of the Universe) can be thought of as another "inflaton" -- we appear to be entering a new inflationary era (though right now, we are only accelerating, not exponentiating).

I think the Ekpyrotic theory of the Universe isn't likely to survive if the recent BICEP2 results are confirmed by other, independent observations.

ContagiousCuriosity4 karma

As a current Cal student who has been studying for a week straight, I decided to reward my hard work with 10 minutes on Reddit. I was so surprised when I stumbled upon my university's most beloved professor doing an AMA! Makes me proud to think that we are really being taught by the experts.

I have heard so many great things about C10 and I hope to take it in the Spring. Also, I heard that you gave two wonderful talks at the Lair of the Bear!

Unfortunately, I have no question to ask, but I wanted to thank you for being such an awesome and enthusiastic professor. When some teachers care only about their own research and course content, it's amazing to see one who is so engaged in the community, genuinely passionate about the subject, and skilled at teaching and inspiring students. Faculty who go above and beyond (by doing things like speaking at the Lair) are the ones who make a good education priceless. Professors like you make Berkeley exceptional and we're lucky to have you!

(P.S. Camp Blue cheats! Sincerely, Camp Gold)

AlexFilippenko6 karma

Thanks for the kind words! If you're not a senior right now, your next chance to take my Astronomy C10 course is Spring 2015. Take it!

I'll be lecturing at the Lair again this summer -- Week 4, I think.

And don't both Camps Blue and Gold cheat?

Go Bears!

sunainaawasthi4 karma

hello Mr. Alex! my question is that how is there ice in space when ice is made up of hydrogen and oxygen and there is no oxygen in space (vacuum)?

AlexFilippenko11 karma

Ices and liquids can form on solid bodies. There's lots of molecules in comets, for example: water, carbon dioxide, ammonia, etc. These are in the frozen state, so they are called "ices." Comets probably brought a lot of water to Earth when it was very young, 4-4.5 billion years ago. Comets are left over from the early formation of the Solar System: they didn't get incorporated into planets, but rather were ejected (through gravitational encounters with planets) into the outer Solar System (e.g., the Oort cloud). Now, occasionally, they are perturbed and come back toward the inner Solar System.

shaunc3 karma

Hi Alex, I've seen you on a variety of TV shows like Nova and the Universe, and I'd just like to say I'm glad to see you out there. So many scientists, especially at advanced levels, are a complete bore. You have a great personality and an engaging cadence, plus you have a talent for making analogies to explain higher concepts. Seeing you on television is sort of like running into my way-more-intelligent uncle, I know I'm going to learn something and I won't feel like an idiot afterwards.

OK, the question. Do more TV, please?!

AlexFilippenko8 karma

Thanks! I enjoy being interviewed for "The Universe" show (which has already run for 7 seasons), but filming does take a lot of time. I do it as a form of public outreach and education. I've not been asked to host a show, but I would seriously consider it.

fostythesnowman3 karma

Hi, Mr. Filipenko, I just wanted to say thanks for doing this AMA! I watched your course from the Great Courses on Black Holes and found them incredibly helpful. In your opinion, what do you think are the biggest unexplained astronomical phenomena?

AlexFilippenko5 karma

Thanks so much! I'm glad you enjoyed my video courses. Tell others, because two of them ("Skywatching," and "Understanding the Universe") are currently on sale at The Great Courses website. Black Holes Explained will probably go on sale sometime in the next month or two.

The biggest unexplained astronomical phenomena are definitely gravitationally bound galaxies and clusters of galaxies (leading to the idea of "dark matter," but we don't really know what it is) and the accelerating expansion of the Universe (leading to the idea of "dark energy," but we don't really know what it is). Another possible explanation for the acceleration is that Einstein's General Theory of Relativity is wrong on large scales, but that would be really weird because it rests on such simple assumptions.

elsa_snow_queen3 karma

Hi Alex! I took your course last year, and I loved it. (Fun fact: I keep a list of funny things that professors say in each class, and you're the one with the most ever.)

Given the opportunity to travel anywhere in the known universe once, instantaneously, and with all proper/necessary protection for survival, where would you go?

AlexFilippenko7 karma


Well, if you guarantee my safety, I would want to travel through a wormhole into another Universe. But, unfortunately, we don't think that's possible, because wormholes are don't appear to be traversable.

So, instead, it would be awesome to travel to some other Earth-like exoplanet in a habitable zone (some are being found!) and see what kind of life (if any) has developed there.

jimmyx663 karma

Hi Alex, who was your science idol/role model when you were a burgeoning scientist?

I also took your class last year and had a wonderful time!

AlexFilippenko8 karma

I'm glad you enjoyed my course!

As an undergraduate at UC Santa Barbara, my mentor and role model was Professor Stanton J. Peale, who (among other things) predicted the rampant volcanism in Io (one of Jupiter's main moons).

As a graduate student at Caltech, my Ph.D thesis advisor was Professor Wallace L. W. Sargent. But I was also influenced a lot by the famous Richard Feynman -- he had an amazing ability to see the world in simple and new ways. He liked the fact that I (and one other student) asked questions in class, so he got to know us better and even invited us to lunch a few times, so he could tell us what he's working on and answer our questions.

erdoc143 karma

Hi! I'm a huge fan. I have a question of curiosity. I read that once a star has iron in it's core, further fusion cannot take place, and so the iron just builds in size as more hydrogen/helium/etc smashes into it. This is the death-call of a star.

If we somehow sent a huge mass (I know huge for us would be minuscule in the Sun, over 1 million times the size of Earth) of iron into the sun...would we accelerate our sun's death?


AlexFilippenko7 karma

Our Sun has too low a mass to ever fuse beyond carbon and oxygen. (It is now fusing hydrogen to helium.) Only massive stars (above 8 or even 10 solar masses) fuse up to iron at the end of their lives.

But yes, if you were to stick a giant amount of iron into the core of the Sun, replacing the hydrogen that's currently fusing there, it would not be good for our Sun's health.

seismicor3 karma

How often are scientists (astrophysicists) taken by a surprise? Do you remember the last time you were taken by a surprise?

AlexFilippenko13 karma

It's a rare and wonderful opportunity, when you are doing research -- especially if the discovery is big, like our (eventually Nobel-worthy) discovery of the accelerating expansion of the Universe.

In February 1985, I was observing with the Palomar 5-m Hale telescope. My former thesis advisor and I had one hour left at the end of a 5-night observing run, and I was feeling kind of low that entire observing run because of a breakup with my girlfriend. We had time to observe just two more galaxies, out of about 100 possibilities. I chose one, and lo and behold, there was an unexpected star in the field of view! We took a spectrum, and it turned out to be a supernova (an exploding star) of a new kind. This totally jazzed me, and it changed my career. A thrilling moment.

seismicor3 karma

How much do you think will the quantum computers revolutionize our understanding of the universe?

AlexFilippenko9 karma

They are definitely an exciting part of the future! Some types of computations are much less demanding of computer time when done with quantum computers.

MrFeexit2 karma

What in your opinion is the most significant thing the general population should know about the universe?

AlexFilippenko6 karma

Let me take an answer that I gave during my Pi Day 2013 AMA: "We are made of star stuff," as Carl Sagan used to say (but did not discover). The chemical elements (except hydrogen, helium, and a little bit of lithium) were produced long ago through nuclear reactions in stars, and explosions of some stars expelled these elements into interstellar space from which new stars, planets, and ultimately life formed. So the carbon in your atoms, the oxygen that you breathe, the calcium in your bones, and the iron in your red blood cells came from stars!"

StormyJMaster2 karma

Is it better to major or minor in Physics? I'm having a hard time deciding whether I want to major in Computer Science or major in Physics.

AlexFilippenko6 karma

It depends on what you want to do as a profession. But definitely it's a good thing to learn at least a couple of computer languages these days, both for a career in physics or in the high-tech world. Python and Javascript are good choices, but C++ and others are good as well.

uncommonclay2 karma

is the universe infinite ?

our universe anyway ...

seems like it goes on without end in every direction ...

AlexFilippenko6 karma

We don't know. It's at least very, very big -- see a previous post. It's either infinite or it wraps around like a huge hypersphere.

jcdill2 karma

What was it like flying with the Blue Angels?

AlexFilippenko4 karma

Totally awesome! I'll definitely never forget it.

AlexFilippenko5 karma

By the way, I actually documented the experience -- and I did some experiments and explained stuff….

I will be posting these interesting videos and photos soon to my new website

Being invited was an honor, and we took the time along the way to capture it and share with others.

I do already have some videos up at

Foonsaki2 karma

Do 1 and 2 dimensional beings exist? Or are 3 dimensions the absolute minimum for things to exist?

AlexFilippenko7 karma

You could imagine 2D creatures living in a 2D brane, or possibly even 1D creatures living in a 1D brane. Our Universe isn't like that, but there could be other universes of this sort. In modern string theory, there's a "landscape" of about 10500 universes, most of which differ from ours in all sorts of ways.

cv5cv62 karma


What's the highest atomic number element created by the r-process when a supernova occurs?

AlexFilippenko4 karma

Well, I think the r-process is responsible for essentially all of the elements heavier than bismuth (along with some isotopes of elements up to bismuth). So, that would be up to uranium, and possibly very short-lived heavier elements.

But note that we're not sure that supernovae are the main site of the r-process. Instead, it might be merging neutron stars, as in some of the short-duration gamma-ray bursts.

xrm42 karma

I usually watch The Universe while going to sleep. You voice is like Ambien for me.

AlexFilippenko4 karma

Hah! Am I that boring? (Okay, just kidding... I guess you mean soothing.)

crsf292 karma

Alex, quick question. Care to provide an opinion on the future of ISRU and Asteroid Mining?

AlexFilippenko10 karma

I'm not an expert on this, but my own feeling is that it's quite far off -- especially if you want to use the materials to actually build things on the asteroid itself (rather than bring them back to Earth). You have to first build the machines, factories, etc. on the asteroid. But of course, never say never.

TheOnlyOscar2 karma

What's a day in the life of an astrophysicist?

AlexFilippenko4 karma

It depends hugely on the day. Today I'm spending several hours on this AMA, I was interviewed for a TV morning show in Honolulu, and I'll be giving a public lecture this evening. (Maybe I'll have a chance to enjoy the beach for a few hours, but I've not yet finished preparing my talk... yikes!)

Other days I teach, or go observing, or meet with my research group, or write grant and observing proposals, or (generally) a combination of these activities. Always busy.

rush_n_attack1 karma

If you were a betting man, would you put money on the human race to overcome our social and environmental problems and not destroy ourselves in the long run? Or are we just too short-sighted, selfish, and immature?

AlexFilippenko5 karma

Well, I hate to be a pessimist, but right now I think our chances for long-term survival are pretty low. Maybe that's a problem faced by almost all "intelligent," mechanically able life forms... they destroy themselves before they have a chance to populate the galaxy. That could be part of the resolution to Fermi's paradox: "If they are so common, where are they?"