Cosmologists in 1998 were not surprised by the fact that distant galaxies are moving away from us, as that observation aligns with the Hubble law established in 1929. The Hubble law states that galaxies appear to be receding from us, and the velocity of their recession is proportional to their distance.
However, what did surprise cosmologists in 1998 was the discovery of the accelerating expansion of the universe. Prior to that, the prevailing belief was that the expansion of the universe was slowing down over time due to the gravitational pull of matter, which would eventually lead to a decelerating expansion or even a contraction. This expectation was based on the understanding that matter, including dark matter, would provide the necessary gravitational force to slow down the expansion.
The surprise came with the observations of distant supernovae conducted by two independent research teams, the Supernova Cosmology Project and the High-Z Supernova Search Team. They found that the distant supernovae appeared fainter, indicating that they were farther away than expected. This observation implied that the expansion of the universe was not decelerating as previously assumed but rather accelerating.
The acceleration of the universe's expansion suggests the presence of a mysterious energy component called dark energy. Dark energy is hypothesized to permeate space and exert a repulsive force, counteracting the attractive force of gravity. This discovery of the accelerating universe in 1998 led to a paradigm shift in cosmology and earned the researchers involved the Nobel Prize in Physics in 2011.
In summary, while the general recession of distant galaxies was already known from the Hubble law, the surprise in 1998 was the revelation that the expansion of the universe is accelerating rather than decelerating, challenging previous assumptions and introducing the concept of dark energy.