What is at the Edge of the Universe?

Writing a blog about physics after a long time…. This blog might just be my best one or it might just be the most interesting blog you are going to read about physics. There are billions if not trillions of questions in astrophysics which astrophysicists have been trying to find the answer about. But, the hardest and the most mind-blowing question which has stumped physicists for thousands of years is that What is at the Edge of our universe? Is there even an edge to our universe or is it just infinite? 

 

Lemme ask you a question. Why is space dark? Well, we all know the night sky is dark because we are facing away from the sun and looking at the darker side of the universe. It is true that our space is not really dark, there are an infinite amount stars, galaxies just as luminous as our son or even more. But here is when it gets crazy! If the universe were infinite, you should have been looking in any direction and find a star or a galaxy. Well, if this were the case, then our night sky would have been as bright as the morning sky but this is not what we see. Then, maybe the universe is not infinite, and there really is a boundary to our universe, right? Well, not really. This Argument is known as the Olber’s Paradox. 

To answer this problem, we first need to understand if the universe is infinite or finite. 

We all know that the universe is expanding faster than the speed of light, but what is causing it? The mysterious energy because of which our universe is expanding faster than the speed of light has been named as dark energy by astronomers. When astronomers look far out into galaxies, they notice that the farther away a galaxy is, the faster it is moving away from us. But if we look out far away, deep beyond in space where galaxies are moving away from us faster than the speed of light, what would we see? The answer is we cannot. And that 

boundary is known as The Hubble Sphere. 

 

The Hubble Sphere is the boundary after which galaxies are moving away from us faster than the speed of light, so we cannot see them. The universe is expanding faster than the speed of light uniformly in all directions. This boundary is 14.4 Billion Light years.

 

We cannot see any galaxies beyond this point because the light coming from those galaxies is so slow that it can’t reach us. As a galaxy moves away from us, its light gets stretched more and more, and ultimately it becomes red shifted which means we cannot see it anymore.

 

When light becomes red shifted, it becomes infrared and our eyes cannot see infrared and that is why space is dark. Because all of the galaxies are so far away from us and their light becomes so red shifted, it becomes infrared and we can’t see it anymore.

 

94% of the galaxies we know of exist outside the hubble sphere, which means there is something more outside the hubble sphere.

 

This we call the Observable Universe.

 

One mind-boggling thing is that the Hubble sphere is also expanding. It has been expanding from billions of years. Distances increase linearly, the Hubble constant is 1/(age of the universe), which means the Hubble sphere is just c*(age of the universe), which grows over time. So, some of the galaxies which were outside the hubble sphere could get included into it because it is expanding. And by that we can see them, when their light reaches us. It is just so amazing that we can see galaxies which are moving away from us faster than the speed of light.

 

The observable universe contains quite literally everything we can ever possibly think about or find out about but that does not mean that there is nothing outside the observable universe. The sections of space born during or very close to the big bang, there light just hasn’t had enough time to reach us and they lie outside the observable universe. Remember, we use the word observable while saying an “Observable Universe” because that is all we can observe and there is much more out there beyond the observable universe. It is just like the horizon of a sea, you can see the horizon and imagine that the sea is finished but there is much more water there after the horizon.

 

Now to understand more about the universe, we need to know about the Shape of the Universe.

Einstein told us that space itself is a curved fabric of space-time. There are 3 options for the shape of the universe:-

1. The universe is positively curved which means it would be a sphere but in higher dimensions. 

 

It is not possible for our 3D brain to imagine this type of a shape, so let’s take a regular sphere for simpler explanation. 

2. The universe is negatively curved.

 

3. The universe is flat.

 

In the first one, it is quite easy to imagine how the universe would be both infinite and closed. If you flew your spaceship from Earth, you would eventually come back to where you started. It is not very easy to understand how this geometry works as it is a higher dimension shape.

 

It would seem like the universe is just a tiny section on a more massive curved surface. It would be like a hypersphere universe. The universe would seem flat at the largest scales. It is similar to like here on Earth where it feels like the Earth is flat but it actually isn’t flat, and is part of a massive curved surface. We are just so small that the observable universe seems flat although it is part of the broader universe, just like the Earth seems flat but isn’t. This version of the universe would be finite, unbounded, have no center, and is expanding. For ease, in the image below, the yellow section is the observable universe and the rest is the broader universe. 

 

Both of the negatively curved and flat universes would be infinite. In theory, in an infinite universe, things could repeat. Infinite amount of Earths could form with slight variations. Maybe you in that universe were reading the same blog but that version thought the blog was boring and closed it. All of it sounds like science fiction but in an infinite universe, this could be reality. If there were same versions of you, then they would 10 followed by billion trillion zeros away from you. You could theoretically reach the edge of the universe in this infinite universe if it stopped expanding or you could also not reach the end even if it stopped expanding. These were all the possibilities.

We, humas, even if we build a spaceship that can reach speeds really close to the speed of light, we still won’t be able to reach the galaxies beyond the hubble sphere itself, let alone the observable universe and that is because both the hubble sphere and the observable universe are expanding and the observable universe is expanding faster than the speed of light itself! 60,000 stars pass through the observable universe every second. That means in 2 trillion years, if humas survived, they would not be able to see anything in the sky. For them, it would be completely dark with no celestial bodies.

This just makes us realise how small we are. We would never be able to solve all of universe’s mysteries ever! Let’s end the blog with a Think About It! It is something new I am trying to make my blogs more interesting.

 Think About It!

Some Cosmologists believe that we are living inside a black hole. Yes! Black Holes may swallow up a lot of matter and energy and make infinite universes inside themselves. So, in theory, we might be living inside a black hole which is inside a black hole inside a black hole.

That is it for the Blog! Hope you enjoyed it and learnt something new. See you soon!

By,

Deeparsh Bhanot