Final answer:
The expanding universe, described by Hubble's law, suggests that galaxies are moving apart with a recession velocity that is proportional to their distance. Extrapolating this expansion backward in time points to the Big Bang, which happened about 13.8 billion years ago. Lookback time and varying expansion rates due to gravitational and dark energy influences help to estimate the age of the universe.
Step-by-step explanation:
Understanding the Expanding Universe and the Age of the Universe
The concept of an expanding universe is central to modern cosmology and is grounded in the observation that galaxies are moving away from each other. This expansion is described by Hubble's law, which states that the recession velocity of a galaxy is proportional to its distance from us. The rate at which the universe is expanding is defined by the Hubble constant, and it has been estimated to be approximately 22 kilometers per second per million light-years.
When we extrapolate this expansion backwards in time, we approach the concept of the Big Bang, which took place around 13.8 billion years ago. All galaxies, currently observed to be receding from each other due to the expansion of space, would have been in approximately the same location at this point in time. The lookback time refers to the time it takes for light to travel from distant galaxies to us, allowing us to see them as they were in the past, essentially looking back in time.
Combining cosmological expansion with the lookback time gives us a way to understand the age of the universe. As the rate of expansion was not constant due to the interplay between the forces of gravity and dark energy, scientists have to account for these changes to estimate the age accurately. Observations and measurements of the cosmic microwave background (CMB), supernovae, and galaxy formation contribute to refining this model and our understanding of the temporal evolution of the universe.