The surprise in 1998 was not about the fact that distant galaxies are moving away from us, which was already known from the Hubble law. Instead, the surprise came from the discovery that the rate of expansion of the universe was accelerating over time.
The Hubble law, formulated in 1929 by Edwin Hubble, states that the recessional velocity of galaxies is proportional to their distance. This relationship implies that the universe is expanding, with galaxies moving away from each other as space itself expands.
However, cosmologists in the 1990s were expecting the expansion to be slowing down over time due to the gravitational pull of matter in the universe. This deceleration would occur because matter exerts an attractive force on other matter through gravity, and it was assumed that this gravitational attraction would gradually slow down the expansion.
To investigate the deceleration of the universe's expansion, two independent teams of astronomers, the Supernova Cosmology Project and the High-Z Supernova Search Team, set out to measure the brightness and distance of distant supernovae (exploding stars) in the late 1990s. These observations were crucial for understanding the expansion rate of the universe.
To their surprise, the data revealed that the universe was not only expanding but also accelerating in its expansion. The observed supernovae were fainter than expected for their distances, indicating that the expansion of the universe was accelerating rather than decelerating.
This discovery earned the Nobel Prize in Physics in 2011 and led to the realization that a mysterious force, now called dark energy, is driving the accelerated expansion of the universe. Dark energy is a hypothetical form of energy that permeates space and exerts a repulsive gravitational effect, causing the expansion to accelerate.
In summary, while the Hubble law provided evidence for the expansion of the universe, the surprising aspect of the 1998 discovery was the realization that the expansion is not slowing down but, in fact, accelerating due to the presence of dark energy.