Expansion of the Universe

We’ve seen how we can measure the distance to galaxies, and how we can measure the speed of the galaxy. These measurements suggest that our universe is expanding. But more than that, we can use these measurements to determine the age of the entire universe. Watch the teacher video Measuring the Age of the Universe for an explanation of how Hubble’s Law can accurately tell us how old the universe really is.
Explore 29.1 The Age of the Universe to understand how modern cosmological theories use observations of the universe’s expansion to estimate its age, including the role of dark energy in accelerating the expansion.
The structure of the universe as a whole is largely determined by three “cosmological parameters” that astronomers use to describe the universe. These parameters are really just numbers - a constant value that describes some aspect of the universe. We will examine each of the three parameters individually.
Hubble’s Constant: The first cosmological parameter is Hubble’s Constant (the slope of the line in Hubble’s Law). We have already seen how Hubble’s Law is related to the age of the universe, but watch the teacher video Hubble’s Constant to see how it also relates to the future expansion of the universe.
Omega-M: The second cosmological parameter is “Omega-M” - the mass density of the universe. It is essentially a measure of how much “stuff” there is in the universe, kind of like the total weight of the universe. This value is important because it tells us about the overall shape of the universe. “The universe has a shape?” you ask… yes it does! Or at least, it could. Albert Einstein is famous for his theory of relativity, which (among other things) predicts that the fabric of space, known as spacetime, is curved by the presence of matter. The total mass in the universe, Omega-M, can cause the whole universe to be curved. Astronomers often discuss three possible curvatures: positive, negative, and flat. If Omega is greater than 1, the universe would be positively curved like the surface of a ball. If Omega is less than 1, it would be negatively curved like a saddle, and if it equals exactly 1, then the universe would be flat.
It is very strange to think about space being curved since we would have to ask “Where does space curve into?” For example, if you take a flat piece of paper, you can curve it up or down because the paper is only two-dimensional - you can curve it into the third dimension (up/down). But space is already three-dimensional, so if it curves, it must curve into some higher dimension that we cannot perceive or measure. Astronomers and other scientists frequently talk about higher dimensions of space that we cannot perceive.
The Cosmological Constant: The third and final cosmological parameter is the “cosmological constant.” Watch the teacher video Cosmological Constant for an explanation of how this value is related to “dark energy” and Einstein’s theory of general relativity.
Read 29.2 A Model of the Universe to explore the isotropic and homogeneous models of the universe, its expansion, and the influence of dark energy on the fate of the universe.