Motion in an expanding Universe

In 1925, following the relation between the period and the luminosity of Cepheid stars developed by Henrietta Swan Leavitt, Edwin Hubble provided the second observational proof for an unprecedented discovery: the Universe is expanding!

What does that mean and how can we tell this is happening? How does a static (or non expanding Universe) look different from an expanding one? To visualize this difference, let’s bake two cakes, one called the “static Universe cake” and the other the “expanding Universe cake”. The difference in the ingredients: baking soda. Let’s place various almonds in them both, all at the same distance. When you take the cakes out of the oven, you will  see that the almonds on the “static Universe cake” are still at the same distance from each other. From the Milky Way point of view nothing has moved, and everything has a zero velocity. In the “expanding Universe cake”, we can notice that the almonds are farther apart from each other. If I am standing on one of these almonds, I can see how almond 2 has moved further away from me than almond 2. So almond 2 is moving faster than almond 1!

Similarly, Hubble, using Cepheid stars, measured the velocity of galaxies farther than our Local Group, and found that their velocity increased the farther away they were! So our Universe was behaving just like an expanding cake (with baking soda)! This cake/universe comparison fails in one very central way though: the cake has edges, while our universe does not. If there are no edges to the universe, there is no center. And therefore no matter where we are in our universe, every “almond” sees the same thing from its perspective as we see it from our Milky Way. The universe is not expanding in a particular direction, but rather, space is created between any two points in the universe. In that sense our universe is more like an inflatable balloon. 

As time goes on, the distance between every galaxy increases. This means that if we go backward in time, galaxies and stars must be closer and closer together as our universe gets smaller. At the very beginning, everything was packed in a single tiny spot. That was the moment of the explosion, the Big Bang, which is estimated to have happened about 14 billion years ago. At the very very beginning, the universe was simply a hot dense mess. Light from the first 380’000 years after the Big Bang cannot reach us as the universe was too small and too hot for it to escape! The first light that we can see from our universe is known as the Cosmic Microwave background. As the Universe began expanding and cooling down, the lightest possible atoms started to form, and the universe began to fill with hydrogen and helium. Thanks to gravity,  elements started to attract and interact with each other, and the first stars were born.

As the expansion was happening, the force of gravity was pulling things together. Stars started to conglomerate into clusters, then galaxies, then groups of galaxies, and these groups into superclusters. These are organized in filaments that do not “expand” locally. Our Sun is not moving away from our galaxy, it is bound to it by gravity. So we move in the universe together with our galaxy. Even more, as a Galaxy we move together with our Local group, and that moves with Laniakea, our local supercluster. 

Move you say? So how come it all feels so still? Again, gravity is the key. It binds us to Earth, making our system of reference stationary, while everything appears to “move around us”. But in fact we are moving on a series of roller coasters in the Universe. First the Earth rotates counterclockwise around its axis once a day at a speed of more than 1,670 km/hr (~1000 miles/hr). Then, Earth revolves around the Sun once every 365 days, making our speed 100’000 km/hr (67’000 miles/hr). Our solar system goes around our galaxy at almost 800’000 km /hr (500’000 miles/hr). Further, the Milky Way moves in our Local Group. We are actually traveling towards Andromeda at 400’000 km/hr and Andromeda moves towards us at the same speed! And galaxies further than our Local group move even faster away from us in an expanding Universe! 

So how come stars appear fixed in the celestial sphere? The answer: they are just too far away for us to notice!

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