If space and time have no beginning or end, they create a mind-bending paradox: the universe should already be dead, and an exact replica of your life will repeat endlessly.
The most immediate problem an eternal universe creates is thermodynamic. The Second Law of Thermodynamics dictates that entropy—disorder—always increases. In a closed system with a finite amount of usable energy, an infinite amount of time guarantees that the system reaches "heat death." All stars would burn out, all black holes would evaporate, and space would settle into a uniform, freezing radiation bath. If the universe is infinitely old, it should already be in this dead state.
To avoid this paradox, an eternal universe requires a physical mechanism for renewal. This points toward cyclic models, where the universe undergoes endless sequences of expansion and contraction, resetting the entropy clock with each "Big Bounce." Alternatively, theories like eternal inflation propose that our visible universe is just one finite bubble in a constantly expanding multiverse where new regions of space continuously spark into existence.
An infinitely old and large universe also invokes the Poincaré recurrence theorem. In a bounded region of space, there is a finite number of ways matter and energy can be arranged. If time stretches infinitely into the past and future, every possible configuration of quantum states must eventually occur, and reoccur an infinite number of times. Every variation of history plays out endlessly across the expanse.
This infinite timeline eliminates the necessity of a "First Cause." Cosmological arguments for a creator rely on the premise that a finite chain of events must have a starting point. An eternal universe exists as a brute fact, requiring no genesis because there was never a "before" to trigger it.