2.2 Cosmic Deceleration

summary

We know the universe is expanding, but what is its fate?

Isaac Newton thought about launching a cannonball from the surface of the Earth and wondered about its trajectory. He recognized that he could give it greater and greater velocity, increasing how far up it would travel. If he gave it a special velocity known as the escape velocity, it would have enough speed to overcome the gravitational pull from the Earth and eke its way out to infinity.
Cosmologists expected the universe to behave like this little cannonball after the Big Bang. After the initial kick, everything was in motion, but now the attractive gravity of all the matter in the universe was pulling back on this motion.
The exciting question to ask was: Does the universe have escape velocity from itself?
That is, if you take the combination of the velocity of expansion and the mass density of the universe, how do those equate? Will the universe expand forever like the cannonball that reached escape velocity, or will it fall back and recollapse some time in the future?

Einstein's viewpoint was entirely different.

About 100 years ago, Albert Einstein had developed a new theory of gravity called general relativity, which imagined gravity in a very different way. He decided one of the first things to test this theory upon was the whole universe, to see what general relativity had to say about the dynamics of the universe.
But Einstein was at a big disadvantage. At the time, astronomers were telling him that the universe was static, neither expanding nor contracting, but what they called the universe was just the Milky Way galaxy—they were unaware of the general expansion of space.
Einstein recognized that if the universe was static, the attractive gravity that tended to make things fall together must be counteracted in some way, and he made an amazing discovery. He realized that while the gravity of matter in the universe is attractive, the gravity of empty space itself could be repulsive, something he called the cosmological constant.
Einstein thought that these two forces were in perfect balance. But this is what we would call an unstable equilibrium. He later learned that the universe was indeed not static, and famously retracted his idea of the cosmological constant.

Can we measure how fast the expansion is slowing?

Just as in the cannonball analogy, when you measure the deceleration of the cannonball, you are in essence “weighing” the Earth. Combining that information with the current velocity of the cannonball, you can predict if it will escape the planet.
Similarly, if we can measure the deceleration of the expansion of the universe, we are essentially determining the mass of the universe. That, combined with our measurement of the current expansion rate, will tell us if we are on a trajectory to recollapse, or if the universe has escape velocity.
When we look out and measure the expansion rate of the universe, we can’t actually measure how fast it’s expanding right now. The information we get from these distant supernovae has a built-in delay, often millions or billions of years, so we are actually learning about the past expansion rate of the universe.
We use this to our advantage by comparing information from ~1 billion years ago to that of information from 2 billion, 3 billion, etc. years ago. This will tell us how quickly the expansion rate is slowing.
In the mid-1990s, the expectation was that the universe would do one of two things: either it would be very dense, rapidly decelerate and contract, ending in a “big crunch,” or it would slowly decelerate, becoming sparser and sparser. By measuring the deceleration of the universe, scientists hoped to “weigh” the universe and determine its ultimate fate.