In this episode, we talk about what happens in the shadows around our planet, from the phases of the Moon to why eclipses can turn our satellite blood red!

Listen on Apple Podcast or on your favourite podcast player through Anchor.

Chris
Welcome back to a brand new episode of The Astroholic Explains!

Alfredo
Welcome back. Glad you can join us!

Chris
How are you feeling today? You excited for more questions?

Alfredo
Always excited to answer more questions.

Chris
We have a couple of interesting and potentially quite heavy questions for you today. And of course they’re quite heavy because they’re about gravity!

Alfredo
I knew it was going to be about gravity. Just because the first time you said ‘heavy’, you just made one of those faces like ‘I’m about to make a pun, I’m making a pun! Are you ready for the pun?’

Chris
Well, I’m glad you understand the ‘gravity’ of the situation. Anyway…

Chris
Ughhhhhh. (He sighs, loudly, for an extended period of time)

Chris
Don’t break the microphone! I’m gonna tell you a little story to get you in the mood for this question. So this requires us to go back to the beginning, quite literally, of the universe, when an otherwise very lovely day in the pre-universal swathes of nothingness was interrupted by a very, very noisy Big Bang.

Alfredo
It wasn’t noisy.

Chris
I’M TELLING THE STORY!

Alfredo
All right, sorry, I’ll shut up! I guess now the podcast is called ‘Chris Narrates Stories!’

Chris
We’d be better off for it! So the Big Bang. Existence occurs, for lack of a better phrase, and it has been expanding ever since. Everything in the universe has been expanding. And I’m sure in one of our previous podcasts, you have made the point of suggesting that, you know, the universe is pretty much the same everywhere.

Alfredo
Yeah. Good. On large scale, the universe is uniform, and isotropic. Cool, which means that it’s the same everywhere. And its also the same, whichever direction you look.

Chris
Okay, perfect! Because for the sake of this question I want to say “But it isn’t!”, or it can’t be, because at some points of the universe, little pockets of gravity or ‘gravity wells’ exist. Now, from what I understand about these, these are something to do with planet formation. We’ll get there in a bit. For now, how do gravity wells occur? Have they always been there? Do they suddenly grow? What causes that? Where did they come from?

Alfredo
Well, well, well.

Chris
(Laughs at the pun)

Alfredo
That was an incredibly well thought question. You see, this is the great thing of having a break throughout the year so that Chris can come up with all these convoluted stories to introduce questions. It’s great! So, as always, there is a lot to discuss. And you’re absolutely correct about that. We discuss a few things in the past, but never in such details. So on one hand, you’re perfectly right saying that the universe is not the same everywhere. There’s a lot of stuff here and a lot of outer space is more or less empty for our standards. The reason why I sort of interrupted and said that on a large scale, it’s that when you take the average density of the universe, you don’t have places which are wildly different. But when you’re looking into the details, and I’m not talking about planet size, I’m talking galaxies and clusters of galaxies size, massive, massive sizes, millions of light years across, you can see the changes in density. And, in fact, the distribution of matter in the universe is not spread out equally. Despite the universe being large scale uniform. It’s distributed in what we call the cosmic web.

Chris
Okay, I’ve heard of the cosmic web.

Alfredo
Tell me more about the cosmic web!

Chris
Okay, I might just be making this up. But I’m going to say that if you think of space, you have obviously infinite darkness intermingled with galaxies and stars and other such stuff. If you were to zoom back very very far, the space between them wouldn’t look that big if you zoom back far enough, and, you know, you would probably be able to draw lines over the top of them. And they would form lines and other such shapes that if you were to then look at that shape that you were drawn, it would kind of look like a web. …No? You look both perplexed and bemused.

Alfredo
I am perplexed bemused!

Chris
Is it like how all of the stars and everything I kind of joined together by like dark matter or something?

Alfredo
Closer. Also, not stars, but galaxies.

Chris
Ummm…Cosmic noodles!

Alfredo
Stunned silence

Chris
It’s like the invisible strings throughout space where there are like lines of dense material, where galaxies and stuff occur more than between the lines of the web.

Alfredo
Okay. I think that you have the concept in your head. It’s clear it’s a little bit tricky explaining it, but I think you’re getting there. It’s great. So, yes, you can imagine the distribution of galaxies in the universe. They’re not well spread out. We call it the cosmic web. To do a better description in 3D, I usually think of the fake spider webs for Halloween. When we think of a regular spiderweb it’s on a plane. You need to think of something 3D. So unless you’re thinking of those spiders in Australia, the covers entire trees…

Chris
Oh, gotcha. Okay, so it’s sort of like masses of web.

Alfredo
Yes. So it’s certainly not just a flat 2D line. You need to think of something that is has strands in all directions and at the node of every strand, there are clusters of galaxies. There are some galaxies also on the strands, but most of the strands are made of intergalactic gas. And as you said, dark matter.

Chris
This is what I was picturing. I was just rubbish at explaining it.

Alfredo
Yeah. I got that that was what you were visualizing. But it was a very good attempt at an explanation. So going back onto our cosmic web, how did it start? How can we have something from a universe that used to be very, very, very uniform and on large scale still is, how can we have such differences in smaller scales? And we believe the causing principle of that were quantum fluctuations a fraction of an instant after the Big Bang.

Chris
Wow. So these things have been there since the start.

Alfredo
Yeah, we know that there were differences in matter distribution from the cosmic microwave background, which is referred to as the echo of the Big Bang. It was the first time in the universe, about 380,000 years after the Big Bang, when the universe was large enough that the primordial energy, so the photons, the particle of light, for the first time didn’t have enough energy to excite the hydrogen and helium, and the primordial elements that make up the universe. And so suddenly, that light was free to move throughout the universe unimpeded. And that light is still here. Every time you see static on a television, probably now that’s not very often. But if you have an old TV set, about a few percentage of that static is actually photons from the Big Bang. And we can study it very well.

Chris
That’s so cool.

Alfredo
Yes, it’s super cool. Actually it is so ‘cool’. That is at minus 270 Kelvin.

Chris
Oh, yes, cheesy jokes.

Alfredo
I like them. But the cosmic microwave background has this sort of imprint of how the matter was distributed. And there were differences, tiny, tiny differences back then. But we believe that those are the differences that became the blueprint of the cosmic web. So those differences were primordial, potential wells. So what is a gravitational potential well, or a gravity well? It’s a way for us to describe the gravitational field around object, you have potential wells technically around anything that has a fundamental force. You can have an electromagnetic potential well around a magnet. So the usual description helps us visualize how the potential energy of an object that is orbiting, or it’s at the bottom of the potential well, or it’s going near the well, is affected. Is it clear what I mean by potential energy?

Chris
I have high school lessons echoing in the back of my mind, it is the potential energy that something possibly has? We can’t exactly measure it, but we know of its potential.

Alfredo
No.

Chris
In that case, I can’t remember. Educate us.

Alfredo
Well, I love that you’re just like, “Well, I’m just gonna get the meaning of each word separately. And hopefully, if I mash them together, it’s going to be correct.” So it’s usually in high school, you have potential energy and kinetic energy as being the two faces of the same coin. So, if you have…

Chris
Oh wait wait wait. Potential energy is when you push down on a spring and it holds that energy. Kinetic is when you remove your hand and it springs up.

Alfredo
Yes, correct. But that is elastic potential energy because the spring is elastic. And then you have a release of elastic energy, but that is kinetic energy.

Chris
The movement is kinetic.

Alfredo
Yeah. Potential is the energy stored by stored by our body. So you can have chemical potential energy if you have something that will release energy in a chemical reaction, gravitational potential energy when you’re not at the bottom of a potential well, etc. So with every kind of way to describe forces, you can have a potential energy associated with them. So what is this gravity well? It’s pretty much just a description. You shouldn’t confuse the gravity wells with the embedding diagram. But that is the way we visualize them. So an embedding diagram is what you probably have seen a million times. If I were to ask you, Chris, to describe a gravity well, tell me what it looks like.

Chris
In my mind, what I think of as a gravity well would kind of be like a black hole in that it would be a point in space time, which for some reason has a lot of mass and is drawing things towards it, except it doesn’t have an event horizon, but it is drawing things towards it in every direction.

Alfredo
That is very good. But I actually meant to describe if you were to draw a picture of the gravity well, how would you draw it? You probably seen a million pictures like that.

Chris
So, oh, gosh, is this really difficult to put into words. It would be like a funnel, so it starts off big and then it gets small in the middle and then it gets big again. And on the other side, if you can imagine that in sort of 3D, but then in every direction… like an anti-bowl?

Alfredo
That is the most complicated…. I was certain that you would give me the pretty much correct answer of what an embedding diagram is! So that was quite something. Dear listener, you’ve probably seen pictures of a galaxy or planet or the sun or star with a generic space background with a grid, and the grid that is formed with a depression around the object with mass.

Chris
Yeah, …that’s more easy.

Alfredo
Yes. So have you seen anything like that, Christopher?

Chris
…Yes.

Alfredo
Wonderful. That is how we often visualize a gravity well. But that is actually an embedding diagram. What a gravity well is is actually a mathematical description. And as you said correctly, it’s in three dimension. All those description with the embedded diagram are just approximation in a sort of two dimensional, like there was a flat sheet

Chris
I’ve seen the pictures of like flat planes drooping into a hole effectively. Okay, that popped to mind first, but because it’s something that isn’t just in one dimension, it’s in every direction is why I sort of tried to envision this sort of ball and it’s full of holes or whatever. I say a ball because of ‘all’ dimensions, three dimension and all directions. But inwards. I’ll stop talking.

Alfredo
No. It was quite fascinating that you were trying to approximate as best as any of us can approximate four dimensional space, but we cannot, unless we use maths.

Chris
Hurray(!).

Alfredo
I’m not asking you to do four dimensional maths. If you remember, like long ago I tried to explain to you what a tensor was.

Chris
The thing that people use to give birth?

Alfredo
Stunned silence

Chris
No?

Alfredo
I think that’s a forcep?

Chris
Oh, that’s not going in the episode.

Alfredo
That is 100% staying in the episode! No. Have you heard of a vector in maths?

Chris
Yeah, isn’t a vector kind of like a small sum that helps you work out other things like a formula?

Alfredo
No, but it’s okay. It’s alright.

Chris
Let’s not humiliate me any further

Alfredo
I’m not humiliating you! You’ve been absolutely great this episode. So this is pretty much what we need to know about gravity wells, so it’s how we explain gravity. The beginning and how they formed, is that gravity wells came from the quantum fluctuation of our very tiny universe just after Big Bang, and they were expanded when the universe went, we believe, into an inflation phase and became much much bigger very, very quickly. And so those tiny differences eventually became bigger and bigger, and gas and mass just fell into it. All the matter, all the gas, all fell into it. And even though we are now in quite a stable galaxy, there are subtle differences, and when stars go supernova and a lot of gas forms, eventually there are going to be regions that are a little bit more dense, and that gravitational well is a little bit larger, and that more things fell in three dimensionally and form star systems, etc.

Chris
That actually leads me quite nicely on to the final part of this question. The universe as it is now, we have this cosmic web and we have these gravity wells strewn through, that stream throughout the universe. Is expansion affecting them still? Are they now fixed in terms of ‘shape’ for lack of a better phrase, or is expansion pulling them apart still? Is more material falling into them and making them bigger? Or are they themselves getting bigger and more material is falling into them? Or are they just staying the same?

Alfredo
Actually, they are changing. Everything in the universe is about ‘becoming’ rather than ‘being’ but what is happening is that matter tends to concentrate. So for as much as the universe is expanding, thanks to dark energy, the influence of matter has still been, for most of the ages of the universe, dominant. So the cosmic web went from being quite hazy with strands and nodes that were roughly the same size to becoming more and more concentrated. So the strands are getting thinner and the nodes are getting chunkier. So the nodes are quite thick and you have the strands are spreading more and more thinly. And depending on how dark energy actually works, what you can have is eventually that these nodes and strands get separated from each other. It’s not exactly certain because there is a lot that we don’t know. In the last few years, we realized that what we don’t know is even more than what we thought we knew. So exciting. Yeah, it’s always exciting finding out how little we know, but we have a lot of uncertainty of what the universe could be. But yes, these potential wells are changing constantly.

Chris
That’s absolutely fascinating. And I am very glad that you put up with my big long story at the start to take us on this little journey.

Alfredo
No no, that was absolutely great. You mentioned something about planets, though?

Chris
I do actually have another question to do with gravitational wells and planets. But I think I would like to save that for a new episode.

Alfredo
So we’re having a brand new cliffhanger!

Chris
sort of cliffhanger. Okay, maybe we won’t immediately follow up this question on the next episode, but we will follow up in a future episode.

Alfredo
Okay, looking forward to that!

MAIN Image: Simulation of the universe revealing the Cosmic Web. Springel et al. (2005)