Expansion of the Universe: Theory & Rate

Instructor: Sergey Segal

Sergey has a Masters in Biomedical Engineering and has taught science and mathematics courses at the University of Wisconsin-Madison.

Scientists have observed that certain galaxies are moving away from the Earth at a speed faster than the speed of light. Learn more about why this happens, subsequently testing your knowledge with a quiz.

Introductory Concepts

Imagine looking up at the stars on a clear night. Have you ever wondered what will happen in the very distant future, billions of years from now? Will the night sky look the same? It turns out that not only objects within the universe such as stars and planets change over time, but also space itself. To avoid confusion, we define the term 'space' as the three-dimensional extent in which matter and energy exist. When we talk about the expansion of the universe, we're actually talking about the growth of the fabric of space on the scale of the entire universe.

It's pretty counter-intuitive and difficult to grasp how space can expand. Isn't it already infinite? How can something infinite become even larger? Well, that's what we'll be looking at here!

Measuring The Expansion of the Universe

For the purposes of our discussion, we'll assume that the space is flat and infinite in size on the global scale, as many scientists believe. The infinite part means that the universe has no boundaries, while the flat part means that if we took off on a spaceship in some direction, we would never come back around. But how can astronomers infer that the universe is expanding?

One way is to observe very distant galaxies and measure the speed at which they're moving away from us, using Earth as a reference point. As we'll see here, these recession velocities can be combined with one of the postulates of the Theory of Relativity to conclude that the universe is expanding.

Understanding the Expansion of the Universe

According to Einstein's Theory of Relativity, nothing can travel faster than the speed of light in space. Holding this claim to be true, we shouldn't find stars and galaxies that are moving away from us faster than light. However, this is inconsistent with observations. In the late 1920's, the astronomer Edwin Hubble observed that there are galaxies that are receding from us faster than the speed of light. In fact, we could draw a very large imaginary sphere with the Earth being a small point at its center, referred to as the Hubble Sphere, beyond which objects are moving away from us faster than light.

Do these observations contradict the Theory of Relativity? As it turns out, it's actually space itself that's expanding in between these galaxies. The postulate above only applies to entities that are moving through space. It doesn't limit the rate at which space expands.

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