Summary
The Borde–Guth–Vilenkin theorem shows that any universe which has, on average, been expanding cannot be eternal in the past.
Observation / Fact
The theorem applies to classical, quantum, inflationary, and multiverse cosmologies, independent of specific physical laws.
Why This Is Non-Trivial
It closes a major loophole by demonstrating that expansion itself implies a temporal boundary, regardless of speculative extensions.
What It Implies
Even hypothetical multiverses require a beginning. Physical reality cannot be past-eternal if expansion is present.
Related
- Past-Eternal Models
- Inflationary Cosmology