Joestars & Zeppelis

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IS THE UNIVERSE INFINITE??

Blog# 187

Wednesday, April 27th, 2022

Welcome back,

It’s one of the most compelling questions you could possibly ask, one that humanity has been asking since basically the beginning of time: What’s beyond the known limits? What’s past the edge of our maps? The ultimate version of this question is, What lies outside the boundary of the universe? 

The answer is — well, it’s complicated. 

To answer the question of what’s outside the universe, we first need to define exactly what we mean by “universe.” If you take it to mean literally all the things that could possibly exist in all of space and time, then there can’t be anything outside the universe. Even if you imagine the universe to have some finite size, and you imagine something outside that volume, then whatever is outside also has to be included in the universe.

Even if the universe is a formless, shapeless, nameless void of absolutely nothing, that’s still a thing and is counted on the list of “all the things” — and, hence, is, by definition, a part of the universe.

If the universe is infinite in size, you don’t really need to worry about this conundrum. The universe, being all there is, is infinitely big and has no edge, so there’s no outside to even talk about.

Oh, sure, there’s an outside to our observable patch of the universe. The cosmos is only so old, and light only travels so fast. So, in the history of the universe, we haven’t received light from every single galaxy. The current width of the observable universe is about 90 billion light-years. And presumably, beyond that boundary, there’s a bunch of other random stars and galaxies.

But past that? It’s hard to tell.

Cosmologists aren’t sure if the universe is infinitely big or just extremely large. To measure the universe, astronomers instead look at its curvature. The geometric curve on large scales of the universe tells us about its overall shape. If the universe is perfectly geometrically flat, then it can be infinite. If it’s curved, like Earth's surface, then it has finite volume.

Current observations and measurements of the curvature of the universe indicate that it is almost perfectly flat. You might think this means the universe is infinite. But it’s not that simple. Even in the case of a flat universe, the cosmos doesn’t have to be infinitely big. Take, for example, the surface of a cylinder.

It is geometrically flat, because parallel lines drawn on the surface remain parallel (that’s one of the definitions of “flatness”), and yet it has a finite size. The same could be true of the universe: It could be completely flat yet closed in on itself.

But even if the universe is finite, it doesn’t necessarily mean there is an edge or an outside. It could be that our three-dimensional universe is embedded in some larger, multidimensional construct. That’s perfectly fine and is indeed a part of some exotic models of physics. But currently, we have no way of testing that, and it doesn’t really affect the day-to-day operations of the cosmos.

And I know this is extremely headache-inducing, but even if the universe has a finite volume, it doesn't have to be embedded.

When you imagine the universe, you might think of a giant ball that’s filled with stars, galaxies and all sorts of interesting astrophysical objects. You may imagine how it looks from the outside, like an astronaut views Earth from a serene orbit above. 

But the universe doesn’t need that outside perspective in order to exist. The universe simply is. It is entirely mathematically self-consistent to define a three-dimensional universe without requiring an outside to that universe. When you imagine the universe as a ball floating in the middle of nothing, you’re playing a mental trick on yourself that the mathematics does not require.

Granted, it sounds impossible for there to be a finite universe that has nothing outside it. And not even “nothing” in the sense of an empty void — completely and totally mathematically undefined. In fact, asking “What’s outside the universe?” is like asking “What sound does the color purple make?” It’s a nonsense question, because you’re trying to combine two unrelated concepts.

It could very well be that our universe does indeed have an “outside.” But again, this doesn’t have to be the case. There’s nothing in mathematics that describes the universe that demands an outside.

If all this sounds complicated and confusing, don’t worry. The entire point of developing sophisticated mathematics is to have tools that give us the ability to grapple with concepts beyond what we can imagine. And that’s one of the powers of modern cosmology: It allows us to study the unimaginable.

Originally published on https://www.space.com

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Black Holes: Seeing the Invisible!

Black holes are some of the most bizarre and fascinating objects in the cosmos. Astronomers want to study lots of them, but there’s one big problem – black holes are invisible! Since they don’t emit any light, it’s pretty tough to find them lurking in the inky void of space. Fortunately there are a few different ways we can “see” black holes indirectly by watching how they affect their surroundings.

Speedy stars

If you’ve spent some time stargazing, you know what a calm, peaceful place our universe can be. But did you know that a monster is hiding right in the heart of our Milky Way galaxy? Astronomers noticed stars zipping superfast around something we can’t see at the center of the galaxy, about 10 million miles per hour! The stars must be circling a supermassive black hole. No other object would have strong enough gravity to keep them from flying off into space.

Two astrophysicists won half of the Nobel Prize in Physics last year for revealing this dark secret. The black hole is truly monstrous, weighing about four million times as much as our Sun! And it seems our home galaxy is no exception – our Hubble Space Telescope has revealed that the hubs of most galaxies contain supermassive black holes.

Shadowy silhouettes

Technology has advanced enough that we’ve been able to spot one of these supermassive black holes in a nearby galaxy. In 2019, astronomers took the first-ever picture of a black hole in a galaxy called M87, which is about 55 million light-years away. They used an international network of radio telescopes called the Event Horizon Telescope.

In the image, we can see some light from hot gas surrounding a dark shape. While we still can’t see the black hole itself, we can see the “shadow” it casts on the bright backdrop.

Shattered stars

Black holes can come in a smaller variety, too. When a massive star runs out of the fuel it uses to shine, it collapses in on itself. These lightweight or “stellar-mass” black holes are only about 5-20 times as massive as the Sun. They’re scattered throughout the galaxy in the same places where we find stars, since that’s how they began their lives. Some of them started out with a companion star, and so far that’s been our best clue to find them.

Some black holes steal material from their companion star. As the material falls onto the black hole, it gets superhot and lights up in X-rays. The first confirmed black hole astronomers discovered, called Cygnus X-1, was found this way.

If a star comes too close to a supermassive black hole, the effect is even more dramatic! Instead of just siphoning material from the star like a smaller black hole would do, a supermassive black hole will completely tear the star apart into a stream of gas. This is called a tidal disruption event.

Making waves

But what if two companion stars both turn into black holes? They may eventually collide with each other to form a larger black hole, sending ripples through space-time – the fabric of the cosmos!

These ripples, called gravitational waves, travel across space at the speed of light. The waves that reach us are extremely weak because space-time is really stiff.

Three scientists received the 2017 Nobel Prize in Physics for using LIGO to observe gravitational waves that were sent out from colliding stellar-mass black holes. Though gravitational waves are hard to detect, they offer a way to find black holes without having to see any light.

We’re teaming up with the European Space Agency for a mission called LISA, which stands for Laser Interferometer Space Antenna. When it launches in the 2030s, it will detect gravitational waves from merging supermassive black holes – a likely sign of colliding galaxies!

Rogue black holes

So we have a few ways to find black holes by seeing stuff that’s close to them. But astronomers think there could be 100 million black holes roaming the galaxy solo. Fortunately, our Nancy Grace Roman Space Telescope will provide a way to “see” these isolated black holes, too.

Roman will find solitary black holes when they pass in front of more distant stars from our vantage point. The black hole’s gravity will warp the starlight in ways that reveal its presence. In some cases we can figure out a black hole’s mass and distance this way, and even estimate how fast it’s moving through the galaxy.

For more about black holes, check out these Tumblr posts!

⚫ Gobble Up These Black (Hole) Friday Deals!

⚫ Hubble’s 5 Weirdest Black Hole Discoveries

Make sure to follow us on Tumblr for your regular dose of space: http://nasa.tumblr.com.

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MBTI Booklist! (Credit :  @Indepth_mbti)

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