Chris has a burning question about mass and why the universe doesn’t just collapse onto itself. Alfredo gives us the low down on what’s up with gravity!

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Hello and welcome to The Astroholic Explains! I’m Dr. Alfredo Carpineti

And I am producer Chris!

So producer Chris, since this was your idea, what are we doing here?

So the idea for this podcast came about after a number of belligerent questions that I would badger you with; eventually, you had enough and said “no, stop it, let’s just record these, we could share it with other people.

And here we are. I’m gonna be answering questions. I don’t know what the questions are!

No, I’m just gonna throw questions at you.

Hopefully I’ll know the answer at least.

I’m gonna throw one question at you per episode so hopefully, it won’t get too overwhelming and we’ll just be able to chat about these things!

Sounds great!

Ok, should we dive in?

Let’s jump in!

Okay, so today, I’ve actually got quite a hefty question with possibly quite a bit of explaining to do, so I hope you’ve got your helpful hat on today. Today’s question is about gravity and black holes. I can just imagine you’re gonna want to stop me through this entire question because I’m sure that so much of what I’m about to say it’s wrong, but black holes… they act in sort of 3D; like the typical explanation of the black hole is when you have a sheet stretched out and you place on it a weight and that pulls the sheet down, so it’s a good representation of gravity or space-time being bent and pulled down towards this weight…

Okay, I’ll stop you there because that’s usually the representation of just gravity, because with a black hole we need a step forward.

Well I think mostly what I’m gonna go for today is gravity and mass, so if this is how mass works. It’s black holes, forms orbits around them, they form galaxies around them. A black hole has mass and it forms these orbits. Galaxies surely also have some form of mass. What if multiple galaxies accumulate mass? Does the universe have like a collective mass? If that is the case, if all of these things weigh so heavily in three dimensions, why is the universe expanding not collapsing in on itself and why does everything appear to be moving away from everything else? if mass and gravity can be factored!

[record scratch]

[Alfredo Amazed] Whoa, okay!

I’m not going to apologize I quite pleased with that question!

No, no, it’s… it’s a very very interesting question..


multi-part indeed as you’re pretty much going full-on on pretty much the biggest mysteries of astronomy!

Come at me, Einstein!

Come at you, Einstein! I think it’s a very very strong start. Let’s start from the very beginning, in the words of Julie Andrews is a very good place to start. So, why we do that representation about gravity is because from general relativity, we know that gravity changes the geometry of space-time. So it alters how space-time works around any object obviously. Small objects, like us, but even the Earth, have only a tiny tiny effect. The bigger, the heavier the object, the bigger the effect.

If the object is spinning, it does this thing called frame-dragging because it’s sort of twisting the space-time around it. And we measure a frame-dragging, even around Earth I think, but obviously the heavier the object the easier is to witness.

That is why people don’t have orbits!

You are creating a gravitational effect; if you’re moving you’re creating gravitational waves but they’re so tiny that they just don’t matter.

The representation of a flat sheet that you’ve usually seen with a big spherical weight representing a star and then you have a smaller spherical weight that you send around..

and it spins…

And it spins until a certain point it falls in. The reasons it falls in; that doesn’t happen with gravity is because of the friction that slows down the object and so it loses energy and it goes towards the center. This luckily doesn’t happen with gravity, anything that it’s orbiting is simply constantly falling onto the object in question but missing.

It’s a great concept as stated by Newton. Newton wasn’t just thinking “oh the apple fell on my head there must be something that is pulling it down”. If you throw an apple it will fall after, I don’t know, if you are very strong, after ten meters; but could you be high enough on like a massive tower that you could throw..

The apple into orbit?

Yes, literally! That was his idea and a certain point they realize that oh if I can throw the apple around and keep missing Earth this, the force that pulls the apple down, must be the same force that keeps the Moon in orbit around the Earth.

So literally the astronauts on the International Space Station are constantly falling to Earth, they just luckily keep missing.

It is the same idea as to why when you’re sending a rocket into space you don’t to just send them up. If you just send them up they’ll be coming back down. You need to give them quite a high horizontal velocity so they can get into orbit.

So that was the first part. You have your standard sheet for the description [of gravity] and you’re changing space-time. If you had a black hole you would have to pull in the center of that sheet…

…way, way down…

…to infinity.


That’s what we think is the singularity of the black hole

But in 3D.

Yeah, in 3D.

So it’s not just being pulled downwards it’s being pulled in from the sides as well..

And on top of that, you need to consider that the black holes are spinning so you have the frame-dragging… it’s a lot of stuff! But we have good maths that help us do that!

It’s this kind of idea that’s make me then think why is everything moving apart not collapsing?

You are perfectly in line with the thoughts that went on until the late 1990s. So first we started with Einstein’s general relativity. General relativity is very complicated and there’s a lot of solutions that don’t represent the real universe. So you can put in stuff and you can create these weird and wonderful universes with weird mathematical properties and they don’t match the real universe.

Einstein put in its expectations for the real universe found the solution and then people started pointing out his solution was not stable. It means that the solution would be changing and the universe as we know it cannot be static. The idea at the time was the universe has always been.

Eventually, over time, people start bringing more evidence that the universe was expanding and it all started from a singularity the most a Big Bang. Consider that the term Big Bang was actually scientists mocking the whole idea that it all started in a single point, but it stuck.

Afterward, Vera Rubin discovered with others that there was a lot more matter in the universe that we could see, the so-called dark matter. We still have no evidence that it exists

[Chris laughs] We just go by the idea that it does!

Yeah, either our equations are wrong which they could easily be, or there is this dark matter that only interacts with gravity and not light. So at that point, we are like “oh you know what? There is a lot more matter in the universe!”

So even though the university at the moment is expanding simply because things started moving away from each other after the Big Bang, eventually the mass will pull everything back in: the Big Crunch.


And that was thought of as how the universe will end. And then we discovered that the universe is not only expanding is expanding at an accelerated rate so…

It’s getting faster and faster?

Yeah, so the estimation showed that the universe is not going to crunch back up. And the explanation for this accelerated expansion is what we call dark energy: a mysterious (because again we don’t know what it is) energy field that permeates all space and time pretty much push everything further apart.

So we have this gravity versus dark energy?

Yes, gravity versus dark energy! But dark energy seems that is going to win. We still have a lot of issues about it. We know that the measurement of what the expansion rate of the universe is using the first light in the universe the Cosmic Microwave Background, the light from the first time that photons were finally free to move around the universe about 380,000 after the Big Bang. This light was observed by the ESA Planck satellite. It made a very very accurate prediction for the expansion rate of the universe; it tells us what the sort of history of this expansion rate should be today based on everything we know and then we do measurements of the expansion rate using a nearby and also not so nearby galaxies and we got a different number.

The value that we measure nowadays is 9% higher than the value that we expect from the Big Bang, and there’s only one chance in a hundred thousand that the truth is actually like a value in the middle or that there is a mistake somewhere, so we are pretty confident that there is something very wrong with our theory of the universe.

[unsure] Yayyy…?

And we don’t know what it is!

Well, that’s exciting!

It is absolutely exciting! It’s fascinating and it keeps us humble. We know a lot about the universe but the only certainty we have is how much we don’t know. It’s a fantastic time… what a time to be alive!

It’s very very interesting to see how they’re gonna solve this conundrum. They’re going to look up. They’re gonna obviously strengthen the measuring methods to make sure that everything is correct but also going to expand searches for better understand dark energy or how this effect is produced if dark energy, again, doesn’t exist. Because we haven’t found evidence that dark energy exists, we’re only seeing the effect of what something that we call dark energy.

Okay, so understanding it correctly it’s basically the reason why everything is not falling in on itself is because of dark energy and acceleration.

Yeah, but things are also in motion. I know, the way we are used to think about gravity is all things fall down, but also gravity lets things spin around and I think there’s a tension between our experience of gravity and gravity in space.

Something that people always ask me about is Sagittarius A*, the supermassive black hole at the center of the Milky Way and they’re just like: “is it gonna it eat us?”

Although for material near a black hole, the black hole is a three-dimensional plug hole, if you’re far away… you just orbit it. And so if you don’t have enough matter things are not going to collapse.

And one thing about dark matter is that dark matter is believed to be diffused so expand further out of what we see of the visible galaxies. Being diffused sort of keep things in balance. So this is why the spiral galaxies have spiral arms because there is mass outside and mass inside and keeping the sort of balance. And the structure of the galaxies remains quite stable.

So yes, gravity can make you fall down and hold you back but it can also make you spin and fly and go even further than you could have ever thought!

I think that’s quite a poetic way of putting it but it fits, it seems to fit. But I think in that regard, I think the education system needs to overhaul the way it portrays gravity in particular to younger students, because there’s always that stereotypical image when you first learn about gravity it’s up and down, it’s things falling and that you don’t learn about the spin until I would guess later on in a scientific education.

This is a bias that we have before we even get into education.

I suppose it’s what we see.

Yes, it’s our senses. You are a child, you trip and you fall down.. maybe you don’t know that it’s gravity

But you don’t spin before you fall…

Maybe you don’t know that is gravity, but it is something that happens to you! Someone says “oh it’s gravity: something is bringing you down…” and we are the sum of our biases.

So we just keep going and we just need to tell people that gravity is not just falling down and it’s something that keeps us in orbit, it keeps us stable and then somehow it also keeps us safe!

Ok, cool well thank you for answering my question of today.

As always it’s my pleasure!

Do you have any burning questions for the Astroholic? If so, send them to me @illucifer on Twitter and I will spring it on him in an upcoming episode. See you next time!