Is the universe infinite?

Nowadays we know more about the universe than ever before. For example the fact that the universe’s expansion is acceleration. This is strange because as we’ve learned Einstein’s laws say that every object attracts every other object in the universe, so all this gravitational force should slow down the expansion. In spite of everything we do know, there are also subject we are not yet certain about, for example what is the shape of the universe? There are three mayor theories about this question, which don’t agree of the fact whether the universe is infinite or if there is a border at some point.

The first theory states the universe is positively curved. In this case the universe can be seen as a soccer ball. If we look at this  universe in 2D then we can state that this universe has no edges, you can go up or down and left or right without ever running into any sort of wall stopping you. In this case space would be finite because there is no way of escaping the surface of the ball. Think about it, it’s possible to go up, down, left or right but it would not make any sense to go through the surface to go inside the ball. This means that going in a straight line until we’ve gone ‘’around’’ the universe we would end up on the exact same spot making the universe finite. The last surprising fact about this universe is that when two parallel lines are drawn, they will always intersect at some point. For example, the longitude lines on Earth intersect at the north and the south Poles (just like the lines in the picture)

Contradicting the first theory, the universe could also be negatively curved. This second theory is basically the exact opposite of the first theory: the universe can be seen as a hollow saddle instead of a sphere. The fact that the universe is saddle shaped means that it is infinite. It is possible to move forward, backward, left or right but the object can keep going for as long as it wants because there will be no point at which it returns to the place it originally came from. One last thing we can conclude from this is the fact that two parallel lines will separate more and more, infinitely so that the distance between the two will become increasingly big.

Luckily I’ve left the most simple theory for the end. This theory states that the universe is flat and non-curved, it can be seen as a flat sheet of paper, but infinite. The sheet of paper would expand infinitely in all dimensions. Again, it is infinite because when moving in a straight line in any direction there will be no point at which you return to the starting point. To compare this universe with the others, two parallel lines will not separate nor will the intersect, they will stay parallel infinitely, meaning the distance between them will never change.

Up to this moment scientists are not sure about the shape and size of the universe, currently they think the most likely theory is…   the last theory in which the universe would be flat and infinite. However, they haven’t found enough evidence to confirm this theory is correct, and I don't think they ever will. If you don’t agree with scientists and you think that the universe is positively or negatively curved (or you just like it better), let me know your thoughts in the comments.

NOTE: The triangles in each of the images are to show the effect of the curves on lines once more: on a flat surface a triangle’s corners added up always equals 180°, on a positively curved surface a triangle’s corners will add up to more than 180°, and finally on a negatively curved surface a triangles corners, when added up will equal less than 180°

What happens at the centre of a black hole?

Many people see black holes as one of the most fascinating objects in space. However the name actually contains a mistake which many people never notice, black holes are no holes in space, rather they are 3D spheres in which a great amount of matter is packed into a small area. For example, a black hole can be the mass of ten suns packed into a sphere the same size as New York, to compare: 1.3 million Earths fit into the sun so 13 million Earth's should fit in New York. Because such a great amount of mass is packed into a small area, the density of a black hole is enormous and therefore it’s gravity is magnificent as well. In fact the gravitational force is so strong that even light can’t escape the gravitational pull of a black hole, which is why a black hole appears to be a dull hole in space even though it is actually a sphere. Light is radiation, weighs close to nothing, this shows, once again, that the gravity inside a black hole is so strong even the lightest imaginable things cannot escape.

The exact size of a black hole can be difficult to estimate, because a black hole pulls radiation towards itself therefore we aren’t able to catch light or any other form of electromagnetic radiation because none of it is able to escape from the black hole's pull. Because of this we aren’t able to see the black hole directly, nonetheless we can see the effects of a black hole. For example when a black hole passes through a cloud of dust and meteors, the black hole will suck all the objects towards itself and so leave an empty space.

The size of a black hole itself is difficult to estimate, their diameters can differ from literally a couple of metres to hundreds of miles. The most important thing to know is that all mass inside a black hole is trying to squeeze into a so called ‘’singularity’’. This gravitational singularity is an infinitely small ring which contains infinite mass and where gravity becomes infinite as well (there are some other things that become infinite but let’s stay with these three). Because all the mass in a black hole will want to go into that singularity which can be seen as an infinitely small space (because the centre can be set to be as big as one wants), the mass is infinite as well as there will always be more particles that want to go into that singularity. Since mass and gravitational force are connected, gravity in this case becomes infinite as well. When talking about the centre of a big hole. this is where the well-known funnel image comes in to play. Whenever an object enters a black hole and it nears the singularity, it will be stretched out (also known as spaghettified) because of the weird gravitational pull in the black hole. Following this the object will completely lose dimensions and eventually it will disappear into the singularity (making the infinite mass even greater).

We can also apply the image of a black hole to our own solar system:
The sun is necessary to maintain life on Earth like we know it nowadays. One day however, the sun will be ‘’burnt out’’. When that day comes and the Sun starts to collapse under its own gravity, the Earth’s climate will completely change: the temperatures are soon going to be dropping until after a couple hundreds of years the average temperature on Earth will near the absolute zero, 0 kelvin (-273.15°C). However; this might not actually ever happen because, before the Sun has even become big enough to be able to collapse under its own gravity to form a black hole, it must first expand until its size has almost tripled (the greater an object’s mass, the greater the object’s gravitational force). If the sun expands to such a level there is a good chance the Earth will be ‘’swallowed’’ before the sun even starts shrinking again. In addition to this planets like Jupiter and Saturn will start to close in towards the sun. Jupiter and Saturn’s gravity is fairly big as well, so they might pull Earth towards them, causing a collision between the planets.

Fun fact: for the Earth to become a black hole, all the mass of the entire Earth has to be mashed into a sphere the size of a marble. This shows how much mass is needed in an extremely small space for something to become a black hole.

Why is our sky blue?

To answer the question: Why is our sky blue? we must first find out what exactly light is and what makes it up. 

Usually people talk about the amount of light which is emitted by a lamp (just an example). The light they are referring to in this case is usually white or yellow, but it is in fact made up of literally billions of different wavelengths. The human eye can only see light of approximately 400 to 700 nanometres, this is called the visible light and it stretches from purple (<400 nm) all the way to red (700> nm). Just to compare, the entire light spectrum reaches from 10^-14 m, which equals 0.01 picometre, to 10⁴ m, which is equal to 10 kilometre.

We only need to have knowledge about visible light to answer the question, so let’s forget about the other spectra. From now on the term ‘’light’’ will be used, meaning the visible spectrum.

A white beam of light is made up of all colours. When this white beam of light is directed into a glass prism, the glass will slightly bend the light beam. However because blue light has a smaller wavelength than yellow and red, blue light will bend just a tiny bit more than the red and yellow light, thus separating the single beam into all the different colours. The effect: a rainbow is being formed. In natural rainbows, the glass prism is substituted by drops of water and the light source is the sun. That’s the reason why rainbows only appear when the sun is shining and it is raining at the same time.

Now that we know that light can be split into different colours, there is only one more thing applicable to the situation in our sky. It's generally known that we have an atmosphere around us which protects us from space, however the atmosphere has another very important function: in fact it has the same function as the prism in the previous example. The light source, the sun, sends its light beams towards the Earth and they hit the Earth at such an angle so that the atmosphere bends the light just enough to make sure the light blue light ( get it ;) ) is the light which hits the surface. The same magical event happens when the sun sets and the sky partly turns green, yellow and red; only this time the beams hit the atmosphere from another angle so the green, yellow and red colours hit the Earth's surface instead of the blue.

Fun fact: the (Dutch) handbook for natural sciences and mathematics describes a metre as the distance light travels in 1/299792458 second in a vacuum space. At first this seems like some random number but it actually corresponds with the speed of light, which is almost 300 million meters per second.

Why does the Moon stay in its orbit around the Earth?

What is a force? There is no easy answer to this question. We all know what forces do, for example make an object move or change the shape of an object. The force that has most influence on us is probably gravitational force, without it we wouldn’t be able to stay on the Earth’s surface. Another interesting force is the centripetal force, the force which causes an object to stay in the same orbit around another object. Without the centripetal force moving object would immediately move away from the central object.

An easy example of this is when someone has a piece of string with a heavy object attached to the end of the string. Whenever he holds on to the empty end of the string and he starts moving the heavy object in an orbit around his hand, the string makes sure the object doesn’t fly away. In this case, the string provides the centripetal force. If the string would suddenly snap or the person let go of it, the centripetal force would disappear and the object would move on in a straight line (as shown in the image by the green arrow).

This is the point where it gets a little more complicated. We can now conclude the fact that the heavy object has in fact been moving in a straight line all the time. However, the centripetal force pulled the object slightly towards the centre at every single point in the circle, making sure the object almost ‘’falls’’ towards the centre all the time. This way the object stays in its orbit.

We can now answer the main question in this post: Why does the Moon stay in its orbit around the Earth? The answer to this seems obvious, however, which force provides the centripetal force in the case of the Moon and the Earth? According to Newton’s second law of motion, every object in the universe attracts every other object in the universe. This happens because of a force called gravity. The combined gravity of the Earth and the Moon (centripetal force) cause the Moon to be in a constant ‘’freefall’’ towards the Earth, and thus making sure the Moon stays in its orbit around the Earth.