Lead
"It's strange that the universe is dark, there are countless glowing stars, and it's so dark."
This is the first reaction of many people when they see a picture of the universe or go out into the wild to observe the stars.
There is a strong contradiction between the countless glowing stars and this dim universe, and it is even more incomprehensible why these stars scattered in all directions of the universe can cause such a dark situation.
Although there are many people who are confused by this, there is also one thing that comes to mind that could be the reason, and that is ourselves.
Our bodies made of gas do not emit light, and the light that hits the star is partially blocked by these non-luminous materials, so the light we can see is much smaller than the light emitted by the star itself, so we can unexpectedly see the light more.
The night is not absolutely dark.
In fact, what we think of as nights is not absolutely dark.
Every star in the universe is actually a star in the night sky, and the celestial bodies that appear near their host star are illuminated by their light, but they are too far away to see the light.
And the distance between the Earth and the Sun is just enough to be illuminated without scorching.
People are not just celestial bodies that form a distance from stars like the sun, but our neighbor the moon can shine at night because the sun shines on it and it uses the reflected light as a light source.
Even if we can't see the light of the sun at night, we can still see the light of other stars in the night sky, and the color of the night sky will become brighter around the stars shining.
But in reality, most of the stars are too faint to be seen only in the dark night sky, and the other stars are extremely far away from each other.
Therefore, in the space between the stars, the universe is still dark, and only where individual stars can shine will be lit up.
And the stars don't move, so even in the bright part of the starry sky, the rest of the night sky is still pitch black.
Aren't there any other shining stars in these cosmic voids?
What went wrong?
We want to know if the universe can shine in a certain way, as if the gaps between the stars were decorated with neon lights.
Distance.
It's not that we don't have it, it's that we can't see it.
Because the void of the universe is very empty, there is nothing in between, and the light travels in a straight line, and if there is light in the universe, it will disappear quickly after propagating between such gaps.
In a space where there is no matter, light is not reflected and refracted and is only absorbed by the matter that comes into contact with it in the void where it does not exist.
Therefore, if there is light in the universe, then the light will spread in the universe at a speed of 300,000 kilometers per second, and it will disappear when it comes into contact with matter, and the brightness of the light will decay faster and faster as the distance is farther and farther away.
The end result is that the brightness of the light will become smaller and smaller as it gets too far, and when the light leaves the light source, its brightness will be strongest at a distance of several hundred kilometers, and then the brightness will decrease with the distance.
When the distance increases to tens of thousands of kilometers, the brightness will be greatly attenuated, and the remaining brightness is only about 1/10,000th of the initial brightness.
Even so, there is a very small space between the matter to glow, so when you get away, the most marginal place will be very little, very little, only about 1/100,000 of the original light.
If we imagine that there are many 1/10,000 light sources, and they are very evenly distributed, so that the light will always be diffused outward.
If we were very close to the light source, we might be able to see a tenth of the brightness, but the universe is really too big, and I think if we move away from many 1/10,000 light sources, the light source will be very dim, and we will even be dazzled by the number of light sources we can see.
Even so, there are still many gaps in the night sky where there are no light sources, and these light sources are already very faint when they come to us.
When there is no light source in the dark void, there is a very large difference between the proportion of light we can see and their brightness, so even though we have many light sources in the place where we can see the light, the dark place will still appear very dark.
Scatter.
But the universe is very, very big.
Within the observable universe, looking in all directions, the field of view can cover a very, very large range, as long as it is not obscured by certain objects, tens of thousands of stars will light up the night sky.
How bright should the night sky be then?
In fact, in the structure of the universe, there are celestial structures called "galaxies".
In galaxies, the stars are secluded and uniform bright lights, scattered like stars, and their concentration varies in different degrees.
For example, there is a lot of gas and dust in the universe after the Big Bang, and the denser parts of the universe will soon become galaxies, and the stars inside will affect each other because of their proximity, and eventually the larger star in the galaxy will form a constellation of planets.
The galaxies are so far apart that the gaps between galaxies are almost completely empty, and these voids are the darkest places in the universe, and that's why the universe looks the darkest.
If we can think that adding light sources and matter in these places would make the universe appear brighter, or even brighter, it is because we think that light travels in a straight line when it is material in one place and the light source is material in another place.
In fact, light has a direction in all directions, like other substances, and objects are not necessarily evenly distributed.
The matter between galaxies is present in all directions, so the light will also diffuse in all directions.
However, light is not refracted and reflected in an open space without matter, so its motion does not collide in an open space without matter.
In the open space where matter is not wanted, the distance between matter is a place where countless lights come out of contact with each other, so the universe is dark.
However, the distance between matter and light sources is not the same, meaning that light will reach the Earth at different times through different paths, and these lights are much weaker than the nearest light, but they are still individual light sources.
Therefore, the reason why there is no light in the universe is dark is because light travels in a straight line and the brightness decay of light rapidly with increasing distance.
At this time, we can't look at these stars and think that they can actually shimmer in some kind of material way, illuminating the universe like neon signs.
epilogue
What do you think our world is like?
Most of what we see every day is reflected by light, and this light is processed in the eyes by our brain to form sensations that we can perceive.
So, the world we see is actually what light looks like.
But is the appearance of light the structure of the universe?
Maybe not, because we can only see the light.
Could the universe be something else that we can't see?
If we could somehow help illuminate a planet farther away from the sun, would our stars become as bright as the sun when they were farther away from the sun? Or will the stars give us a brighter light?
The light travels in a straight line and can only go to the front.
So, what is the darkness of the universe?