One way to measure the universe's expansion is to look at galaxies that are very far away. These galaxies are called distant supernovae, and they are so far away that their light has been traveling for billions of years. By studying the light from these supernovae, astronomers can measure how fast the universe has expanded since the light was emitted.
Another way to measure the universe's expansion is to look at the cosmic microwave background (CMB). The CMB is the leftover radiation from the Big Bang, the event that created the universe. By studying the CMB, astronomers can measure how the universe has expanded since the Big Bang.
These two methods of measuring the universe's expansion give consistent results. The universe is expanding at a rate of about 70 kilometers per second per megaparsec. This means that for every million parsecs (3.26 million light-years) away a galaxy is, it is moving away from us at 70 kilometers per second.
The universe's expansion is accelerating, which means that the galaxies are moving away from us faster and faster. This acceleration is thought to be caused by a mysterious force called dark energy. Dark energy is one of the biggest mysteries in physics, and astronomers are still trying to figure out what it is and how it works.
