The question of whether the universe will ultimately expand forever or contract depends on the balance between the energy and matter content of the universe, as well as the curvature of space-time. The current understanding, based on observational evidence and theoretical models, suggests that the universe is indeed expanding and will continue to do so. Here are the key reasons for this:
Initial Conditions: Observations indicate that the universe started with a hot, dense state known as the Big Bang. At that time, the expansion of the universe began, driven by an initial burst of rapid expansion called cosmic inflation. This expansion set the stage for the subsequent evolution of the universe.
Energy Density: The energy content of the universe, particularly the dominance of dark energy, plays a crucial role. Dark energy is a hypothetical form of energy that is thought to permeate space and is associated with the accelerating expansion of the universe. It exerts a repulsive gravitational effect, counteracting the attractive gravitational pull of matter and causing the expansion to accelerate over time.
Matter Content: While ordinary matter (atoms, particles, etc.) has a gravitational pull that tends to slow down the expansion, the presence of dark matter, which interacts only through gravity and does not emit light, has a significant influence. Dark matter contributes to the gravitational attraction, but it is not sufficient to halt the expansion. The combination of dark matter and dark energy leads to an overall expansion of the universe.
Curvature of Space: The geometry of the universe, as described by general relativity, also plays a role. If the universe had a positive curvature (closed geometry), it would eventually contract under the influence of gravity. If it had a negative curvature (open geometry), it would expand forever. Current observational data suggests that the universe is nearly flat, which allows for ongoing expansion.
Based on our current understanding, the interplay between dark energy, dark matter, and the overall geometry of the universe favors ongoing expansion. However, it's important to note that our understanding is still evolving, and future discoveries and refinements to our models may lead to revisions in our understanding of the ultimate fate of the universe.