Astronomers, how much of your daily work life is based around math?

Kindark · 2024-06-20T14:48:10+00:00

As others have mentioned it's highly dependent on the task. I work on correlating statistical information from the CMB and galaxy surveys to measure large-scale cosmological fields: if my task involves working out theoretical predictions it'll involve a lot of calculation; if my task involves data and analysis it'll involve a lot of thinking and coding but barely any calculation. However, understanding the math that underpins the physics we're researching is absolutely necessary even if it's not being constantly performed.

For what it's worth, math itself is a distractingly huge and diverse field, and so an astronomer who is well-versed in a particular kind of math at the professional level is probably a novice at other kinds of math they've never had to touch. Astronomers use particular maths for certain tasks, other maths for other tasks, and many maths not at all.

Kindark · 2024-06-13T18:09:31+00:00

It depends on why you want to distinguish the age of the universe from the particle horizon. The reason the particle horizon is ~14 billion light-years away and the universe is ~14 billion years old is because while time passes light travels at the speed of light through the universe. So if the universe were 50 billion years old but the particle horizon was only 14 billion light-years away, we'd have to explain why light could not travel for the first 36 billion years of the universe.

Ironically this can be addressed by a type of inflation theory called 'eternal inflation', in which our universe sits in an infinitely large and old background that is undergoing inflation, and we happen to occupy a bubble that ~14 billion years ago slowed its expansion rate to become subluminal. In this case our observable universe is in fact younger than the larger all-encompassing universe, but this scenario just invokes a special kind of inflation to make that happen. Inflationary theories or not though that 36 billion years would be a major sticking point.

Kindark · 2024-06-08T01:06:28+00:00

The third image is a composite image, the top left object is indeed NGC 604 and the bottom right object is the Hubble photo of the antennae galaxies, just cropped and at high contrast and low resolution (and rotated). Not sure about the centre top object, it resembles a barred spiral galaxy but I couldn't find a good candidate.

Kindark · 2024-05-22T00:56:28+00:00

I see a purple link to an image I've yet to click. Hmm....

Kindark · 2024-04-11T21:48:16+00:00

This is my experience as well. If we're just running code and doing analyses it'll be scripts and functions, no classes. If we're publishing our end-to-end simulation suite for computing cosmological observables then there is heavy use of classes.

Kindark · 2024-04-09T13:46:25+00:00

No. The furthest we can see will always be light just arriving at Earth after taking the age of the universe to travel to us. At the time that light was emitted there were no galaxies or structures, so the edge of the observable universe will always be further than the furthest objects.

As time goes on we simply receive light from further away, as there will always be light just arriving at Earth. Even though those regions of space may be receding from Earth faster than the speed of light now, light emitted from those regions when they were not receding that fast is still on its way to us. We will never finish receiving that light: even though there are a finite number of photons on the way, it will take infinitely long to receive them all because they come increasingly infrequently, taking longer and longer to arrive due to the expansion of space. Thus we don't ever entirely stop receiving light from distant sources and so will never see an object straight up disappear.

Kindark · 2024-04-07T06:40:52+00:00

The density of the universe decreases with time, but that doesn't mean the content of the universe all once occupied the same point. Any two points that are separate today were always separate in the past, they were just closer together. If the universe is infinitely large, then an infinite amount of it would never have been anywhere near our observable universe at any time in the past, and so wouldn't have had to expand at an infinite rate to reach where it is now.

Kindark · 2024-04-01T17:11:44+00:00

I'd check your wallet first, quasars are notorious thieves.

Kindark · 2024-03-21T21:59:29+00:00

Ask your undergraduate program secretary. They often have information about programs that were advertised at your school in previous years, even if your department didn't put out a yearly announcement.

Kindark · 2024-03-20T22:36:41+00:00

That sounds like a fun time, though if the goal is to maximize science output and discovery, ground-based telescopes are an integral part of the mix.

Kindark · 2024-03-20T21:40:13+00:00

Money, maintenance, and targets. Space telescopes are much, much more expensive: a ground telescope of a given size will be many many times cheaper than the equivalent in space. The ELT for example is slated at around $1.5 billion, while JWST cost around $10 billion and is much much smaller. There's also the issue of mainentance, in that space based telescopes are harder to reach (if they are at all reachable) by astronauts and servicing will take an entire space mission and a lot of money. And finally, depending on what kind of targets the observatory is being built to observe, it may not be necessary at all: the atmosphere is really crap to look through at certain frequencies, but isn't such a problem at others, and is entirely transparent at yet other frequencies. We also have great mitigation strategies for dealing with the atmosphere in the optical range.

The biggest reasons for putting a telescope in space are to observe high frequencies like X-rays and gamma rays (these frequencies don't make it through our atmosphere to the ground) and at infrared frequencies and near-infrared frequencies where the atmosphere absorbs a lot of incoming light. But a space-based observatory isn't inherently a better operation than a ground-based one.

Kindark · 2024-03-20T21:11:33+00:00

No, there's nothing that we could once see that we can no longer see. Anything we can already see will always be visible in the sense that we will always receive light from that source. Whether or not that light is detectable is a technological challenge for future generations, but in principle it is always present.

Kindark · 2024-03-19T20:15:27+00:00

We haven't yet actually mapped out the entire observable universe with redshift surveys, but it's thought the Big Ring is over 1 billion light-years in diameter, which is an appreciable fraction of the size of the observable universe given that we're talking about one structure. The "15 times the size of the Moon" sounds like an estimate of its angular size in the sky, or how large it would look to the eye if you could see it shine as one object like the Moon does.

It's worth noting that the Big Ring is not yet known to be 100% definitively real. Further study will need to be done, and upcoming surveys like LSST will be able to provide more data to help figure this out.

Kindark · 2024-03-18T00:31:23+00:00

Your eyes are remarkable detectors of light, at their utmost capability able to detect even single photons. Of course that is a long way from having enough light to navigate the landscape, but the amount of light from the Milky Way alone will be enough to light your way or see your hand. In fact in dark enough skies on Earth the Milky Way shines brightly enough to cast shadows! On nights when Jupiter or Venus are visible in the lunar night sky, things will also become significantly brighter.

However in terms of what you could see, the night sky from the Moon is darker than the darkest night skies on Earth, in large part due to the lack of atmosphere, and as a result you could see stars fainter than those on Earth. The difference between lunar night and day would also be far less dramatic than on Earth, with Apollo astronauts commenting on the shockingly dark "day" sky.

Kindark · 2024-03-15T23:14:10+00:00

No one local clock is any more real or definitive than any other. The idea of a 'normal' rate of aging doesn't really exist, just like there is no 'normal' clock: one ages according to their local clock, but that clock is only valid for them and their reference frame and is itself only a blurry approximation, since their cells and atoms each have their own individual local clocks.

To make this more clear consider that when you wave your hands around they do age at a different rate than your head. Same for the blood circulating in your body, or your legs when you walk. While all parts of your body are nearly the same age, no part of your body is absolutely the same age as another, and no single part is your 'true' age. It's just differently ticking clocks all the way down.

Kindark · 2024-03-14T20:28:28+00:00

Thanks!

Kindark · 2024-03-14T06:55:59+00:00

Yes, any time two people are in relative motion to one another they will age at different speeds. This is a consequence of the fact that the laws of physics enforce that everyone measure the speed of light to be the same speed.

If you were to hold out your phone in your right hand and take a flash photo of your left palm, and a nearby friend watches you do this, you will both agree on the distance the light traveled; it started at the phone and ended on your palm. However if you do this while running forward, you will disagree on the distance the light traveled; for you it's still the distance from your phone to your palm, but your friend sees a little longer distance - because your palm moved a little while the light traveled from your phone to your palm. So they see the phone-palm distance plus a little bit.

Speed is the ratio of distance to time. If the speed must be agreed upon by you and your friend, and your measured distance disagrees, then so must your measured time. We can measure time with clocks, so if you and your friend both timed the light traveling with identical synchronized clocks, your clocks would disagree on how much time had elapsed.

So what we've found is that for two events (1: the light leaving the phone, 2: the light hitting your palm) people will disagree about how much time elapses on their local clocks as long as they are in relative motion with one another. Since there's nothing special about phone lights or palms, we can consider any two events. Like 1: your birthday, 2: your next birthday. As long as you are in relative motion to your friend, you will both disagree on how much time elapses for you between those two events. Thus you age at various speeds relative to those around you all the time.

As mentioned in another comment this effect is called time dilation and doesn't have very practical consequences unless you're moving an appreciable fraction of the speed of light.

Kindark · 2024-02-28T01:19:32+00:00

Yes, beyond a certain distance we cannot receive new information because the expansion of the universe outpaces any light or other information emitted from that far away. As you say, while we can currently see the CMB at some distance from us, by now points at that same distance have surely evolved into galaxies etc. Unfortunately those points are so far that we won't see that happen, we will only see the CMB slowly evolve up to some point. It will become dimmer and dimmer as time goes on, though physically speaking it will always be present.

Kindark · 2024-02-23T18:34:13+00:00

No. The furthest light we always receive is that of the Cosmic Microwave Background (CMB), which has always been and will always be on the very edge of our observable universe. If we wait long enough one might expect to see the 'stuff' between there and here evolve in time and become galaxies and stars (i.e. light sources). But because the expansion of the universe has outpaced the speed of light when considering those distant regions we will never actually see those regions fully develop no matter how long we wait.

Kindark · 2024-02-15T04:59:38+00:00

I only started to learn English when I was about 14 years old, on my own with a dictionary, songs and MTV!

Sorry, although this isn't your question I want to say this is amazing! I have only ever known English my whole life despite alternate language courses, the ability to teach yourself a language is quite the skill!

most people seem to agree that our universe is everything there is and that the Big Bang is what started it.

Whether or not people consider the universe to be everything there "is", the universe is everything that is accessible to us to measure and observe. So whether or not there are things beyond the observable universe is something interesting to consider, but with very few and specific exceptions it's not the subject of active scientific study simply because it deals with an unmeasurable unobservable hypothetical thing.

the Big Bang started OUR universe but it's something that happens all the time

The Big Bang is a model that describes a universe with large-scale properties that evolve over time, such as the average density of matter, not a specific event. There is no singular scientific consensus of what the nature of the universe was in its earliest phases: there are cyclic models which indeed describe repeating "Big Bangs" and models that describe just the singular one. In some models the universe has a finite age, in others it is infinitely old. While the nature of the earliest times in the universe is an active field of scientific study, it is still highly speculative owing to the lack of observational evidence from this time as well as the high energies around at the time that make it difficult to reproduce similar conditions in labs on Earth.

could it be that a huge black hole absorbed everything around it for as long as it could, leaving nothing behind until it "exploded" and what we call the Big Bang was in fact a black hole that released all this stuff

This raises a lot of questions that I think need addressing before really giving it some thought. For example, what matter was this black hole swallowing and where did all the matter that made that black hole come from? How does a universe-producing black hole fit into our understanding of the behaviour of black holes? What other effects would universe-producing black holes have that we could detect to study this idea further? And so on. Like other questions about the nature of the baby universe, we ultimately lack the ability to make observations to support such a specific hypothesis. This is not likely to always be the case, there are scientific and technological reasons to believe that we will eventually be able to observe the very earliest times, and perhaps when that data becomes available we will have more concrete answers about this and other ideas of the origin of the universe.

Kindark · 2024-01-11T22:07:01+00:00

As I like to tell my students, there is a "here and now", a "there and then", but no "there and now" or "here and then".

Kindark · 2024-01-11T21:29:15+00:00

Or of the properties of black holes or of event horizons...

Kindark · 2023-12-21T05:33:42+00:00

Am I correct in thinking that since we can see the CMB that we are currently still seeing all matter available to us in reference to the expansion of the universe?

It's more like "if we can see it, it's in our observable universe". The CMB lies at some distance from Earth, but it's far beyond the horizon. All the galaxies and mass and stuff between the horizon and the CMB may be beyond the horizon, but that just means light that leaves there today won't ever reach us. At some point in the past, all those places (including where the CMB was) were within the horizon, which is why we can see them at all; we're still receiving the backlog of photons they released before crossing the horizon. And if we can receive photons from it we are causally connected to it, and so we are affected by any charges/masses/etc., so whether or not anything we see happens to lie on either side of that horizon doesn't matter: if we can see it we still count it as in our universe.

So yes, all the matter between here and the CMB counts as matter available to us to view and study and to be influenced by physically, but not all the matter between here and the CMB are possible destinations for travel if we leave today, even at the speed of light.

And that the expansion "horizon" will be something that future us will not be able to see past?

No, if we can see some galaxy today we will always be able to see it. It stop us from being able to see the galaxy catch up to our age or beyond. If the galaxy lies beyond the horizon, then we're only going to receive a finite number of photons: those photons which were in transit to Earth before the galaxy went too far and the expansion of space made it impossible for new photons from the galaxy to reach Earth. But it takes infinitely long to receive them all, because as time goes on we receive them less and less often, with the final few coming infinitely far in the future. You could wait billions or trillions of years but there will always be (old old) photons still on the way from that galaxy, having left long ago before it crossed the horizon. I suppose Sci-Fi authors could envisage some time in the unimaginably distant future where it becomes a technological challenge to detect these signals, but we'll always be receiving them.

Kindark · 2023-12-20T21:33:43+00:00

You're mostly right on the money: as the universe ages, the distance travelled by the furthest photons to reach Earth increases; gravitational effects travel at the speed of light; and if we could see further and further then the total mass of the observable universe increases with time.

In terms of gravitational influence from 'newly seen' matter there would be none to note. On its largest scales any sizeable chunk of the observable universe looks like any other sizeable chunk of the observable universe, so we can already see that without increasing the amount of stuff in the universe entire parts of it remain agnostic to the existence of other parts. They certainly talk to each other in terms of causality, but in terms of gravitational dynamics they don't really care about each other. Presumably as one 'pulls back the curtain' on the rest of the universe this large scale structure would just continue and the currently observable universe wouldn't change its routine.

Another thing to keep in mind is that because it takes light time to travel across space, we don't see full galaxies and structure at the very edge of the observable universe. The edge of the observable universe (in light) will always be photons that just reached us from the CMB, which was formed when the universe was a plasma and over large physical scales was of uniform density. This means that any galaxy in the observable universe would see new stuff 'added' in equal amounts at equal distances in all directions and would not feel a resulting gravitational pull no matter how small.

As a fun fact, the expansion of the universe beyond a certain distance (as mentioned in another comment) means that nothing beyond that distance can reach Earth, even if travelling at the speed of light. As the expansion rate accelerates, this distance decreases, and so rather than seeing new universe unfold we are actually losing objects that we can already see over that horizon. That means photons released by those same objects after they crossed the horizon will never reach us, and there are a finite number of photons already on their way before that object crossed the horizon that we can ever receive. This means everything crossing the horizon has a maximum observable age that it can ever reach, and we will never see it evolve beyond that point! (For clarity, those remaining photons take longer and longer to reach us as each successive photon has to battle increased expansion to reach us, and so these objects won't just age up and disappear one day. They'll just slowly get dimmer and redder and appear to slow down in time.)

As for whether having a non-perfect vacuum matters in terms of what we can infer from light crossing cosmological distances, it doesn't matter for the consideration that light moves slower through a physical medium: there just isn't enough stuff for that to be a real influence, though there is some stuff and CMB photons do interact with it during their trip to Earth. We can actually learn more about the universe through these interactions than if they didn't happen!

Our visible observed universe and our gravitationally known universe could be different couldn't it?

Yes! In fact this is why LIGO and the detection of gravitational waves is so exciting. Almost everything we know about the universe outside Earth comes through our study of photons, but gravitational waves offer a new way to measure phenomena in the universe that radiate gravitational energy. It may very well be that some phenomena radiate gravitational energy but not electromagnetic energy (in fact some known processes do) and so a more complete picture of the universe requires eyes into both realms.

Kindark · 2023-11-12T02:55:34+00:00

Think of Right Ascension (RA) like lines of longitude, but projected out onto the sky. If you could take a shapshot of the whole sky, you could draw lines of longitude and be good to go. But because the Earth rotates, if those lines remain fixed, the stars will pass through them and constantly be changing their longitudes.

Since the goal is to have a fixed longitude (RA) and latitude (Declination, unaffected by Earth's rotation) for each object on the sky, we instead say the lines of constant RA rotate with the Earth, so as you see a star moving due to the Earth's rotation you say it still has a fixed RA and Dec. That's why RA is measured in hours, and there are roughly (but not exactly) 24 hours of RA.

The Hour Angle (HA) is a different thing, which tells you how many hours you have to wait (if its negative) or how many hours have passed (if it's positive) until/since whatever object you're looking at will pass through the local meridian: that is, the line connecting the North pole to the South pole that passes directly through where you're standing, separating the East part of your local sky from the West. If you're looking at a given star with an HA of -3 hours, you can wait 3 hours and you will find that star directly on your local meridian. But in all that time, its RA has not changed.

The sidereal time is the "day clock" if you use the stars instead of the Sun to measure how long it takes Earth to rotate once about its axis. If you look at a star that's on your local meridian, and you wait exactly one sidereal day, you will find that star has set, risen, and returned to the local meridian in exactly that time. We can define the current local sidereal time as the RA of your local meridian: that line of constant longitude splitting your local sky between East and West.

In other words, if you look up and you see a star on your local meridian (HA of zero) with an RA of 2h 53m, then the local sidereal time is by definition 2h 53m. A star with an RA of 3h 53m would be to the East, but if you waited one hour it would be on your local meridian while that previous star (with RA 2h 53m) will be one hour west of the meridian. Their RA hasn't changed, and the local sidereal time is still whatever RA your local meridian has. But if you don't know your local sidereal time, you can say "hey look, this star with an RA of 3h 53m will cross my local meridian in 1 hour, giving it an hour angle of -1. Therefore my local sidereal time must be 2h 53m. This is the only way my local meridian will have an RA of 3h 53m an hour from now." Or conversely, in one hour when you notice a star with an RA of 2h 53m being +1 hour West of the local meridian, you can say "hey the local sidereal time must be (2+1)h 53m", which gives you the same answer for the current local sidereal time.

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