How do astronomers know that the universe is expanding?


I understand how they can tell how big a planet is or how hot a star is. The thing I don’t understand is how do they know the universe is expanding? We cant see the edge of the universe also things are moving away and towards us. So how do they know its expanding? Also, what is it expanding to? For example, at the edge of the universe, is there a wall or is there complete nothingness? Like what is it expanding towards?

Edit: hopefully I used the right flair. I wanted to use “Astronomy” but there isn’t a flair for that

In: Physics

A guy called Edwin Hubble noticed that the further away a galaxy is, the faster it is moving away from us. That means that on average everything we see is moving away from everything else. It follows that everything must have been closer together in the past.

As far as we know there is no edge to the universe, the stars and galaxies just keep going forever. There is no edge so the universe isn’t expanding into anything, it is just expanding.

When a car drives past you playing music, you know how the pitch changes? That’s called the Doppler Effect. The sound waves get scrunched up as the thing’s coming towards you, and stretched out as it’s moving away. A similar thing happens when light-emitters (like stars & galaxies) move away from us. This is known as Red Shift. Astronomers can use this phenomenon to determine that space itself is expanding. In all directions, the further away objects are from us, the redder their shift, the faster they are moving away.

They not to think of the space expanding into something. Draw a picture on a balloon. Then blow the balloon up. Every part of the picture is expanding away from every other part, but the only surface that exists for drawings to live on remains the balloon. The universe takes this concept one dimensions higher. 3d space is like the surface of a 4d balloon. The balloon is expanding in *time.* Yes, if you were somehow able to travel at arbitrarily high speeds forever, you would end up back where you started eventually.

The doppler effect is better understood visually so here a little video showing it.


All waves behave that way, and you propably experienced it in your life. Like the sound of a formular car passing by.


Or the sound of an emergency vehicle passing by.


In both case for the person inside the car, the sound doesn’t change. The frequency stay the same since they are not moving relatively to the source of the sound. But for people outside, the frequency increase or decrease depending if the source is going away from you or toward you.

Same happen to a star. If the star is going away from us or toward us the frequency of the light will shift. And as you can see in the [Light Spectrum](, the frequency change how the color look to us. For light we call that a red or blue shift, because that’s the extreme on the visible light spectrum. If the star is moving toward us, we see a blue shift, if the star is going away from us, we see a red shift.

If we look at all the stars we can in the universe, we see that the Galaxies very close to us are moving toward us, because of gravity. This is why Andremoda will collide with our Galaxy in the distant future. But if you look at ANY other stars, they are all moving away from us. Even more importantly, the further away the star is from us, the bigger the red shift we see, aka the faster they are moving away from us. This is the universe expanding.

The Red Shift – Astronomers measure the movement of objects relative to us using Doppler shift. When you hear a train coming, its whistle is heard at a higher frequency compared to when it is receding, right? In the same way, Light also has a Doppler shift, whereby its frequency is shifted depending on the motion of the emitting object!

Astronomers observed that light from distant objects in the universe is red-shifted (shift in the frequency of light towards red color), which tells us that the objects are all receding away from us. This is true in whatever direction you look at: all the distant galaxies are going away from us. This can only be due to the fact that the Universe is expanding.

In 1929 the astronomer Edwin Hubble measured the velocities of a large selection of galaxies. He expected that about equal numbers would be moving toward and away from us. After all, the Earth isn’t a particularly special place in the universe!

Instead, he discovered that almost all galaxies are moving away from us!

Since the time of Hubble, we have observed millions of galaxies with better equipment and verified his results. With the exception of a small handful of galaxies close to us, every galaxy is moving away from us.

And in fact, the farther away a galaxy is the faster it is moving away from us. The galaxies seem to be receding from us because the entire universe is getting larger. The space in between the galaxies is stretching! And the farther away a galaxy is the more space there is to stretch so the faster the galaxy appears to move away from us.

[Hubble’s Law](

Hubble constant = For most of the second half of the 20th century the value was between 50 and 90 (km/s)/Mpc. Although, even more recently, in February 2018, a higher value of 73 (km/s)/Mpc has been determined using an improved procedure involving the Hubble Telescope!

New Learnings – Mpc – Mega Parsecs (One parsec is equal to about 3.26 light-years)

Edwin Hubble discovered that there was a redshift of light coming from far distances. This redshift phenomenon is caused by space expanding stretching out the frequency of the light reaching us causing it to go from the higher energy blue spectrum to becoming red and having a lower energy frequency then when it was emitted.

The reason space is expanding is a bit more unsure, it is normally said to be caused by something called dark energy. Dark energy is as I know
antimatter and (positive) regular-matter particle collisions, the energy of these ‘quantum fluctuations’ creates some extra energy causing space to expand. The more empty space the more expanding, as you may have already heard space is expanding exponentially.

The thing we are expanding towards isn’t anything we aren’t really going towards anything else then the future.
It is only the distance between stuff that is increasing. It’s the nothingness that is growing. If you went in a straight line off earth and all the way around the universe you would end up where you started, like a globe, a lot like earth actually. Except you are not looping back over the curve of the earth. You are moving over the curve of the gravitational pull on your trip, over the grid of spacetime.

You can not get outside the universe, it is everything we can observe. Not everything, but the whole observable thing.

The destiny of The Universe is therefore a bit harder to predict. Can gravity win over the expanding space?
We aren’t sure. There are many funny predictions of how the universe will end.
It will not be soon atleast.
Because of the expansion we can predict what happened In the early universe, currently being pretty sure about the Big Bang. We measure disorder since the “before picture” of Big Bang and now in disorder, also called entropy. As The Universe expands entropy increases and the distance of the stars turn light years as the stars explode into black holes.

If you actually read this far you are swag..

It’s like droping a huge drop of ketchup on the floor. The only ketchup there is is the blob you let fall. But it would extend as the time passes.