Is The Universe Infinite?

Why don't we know if the universe is infinite?

Because we can only see a small part of it. According to NASA, the observable universe is bounded by how far light has traveled since the Big Bang. Beyond that boundary, more universe almost certainly exists, but we can't observe it directly. Whether the total universe extends infinitely or is just very large is a question we can't directly test. We can only infer from what we see and from theoretical models. The honest answer is that we don't know.

What does 'flat universe' mean?

It means cosmic geometry doesn't curve back on itself. The universe could theoretically have different overall geometries: positively curved (closing back on itself like a sphere's surface), negatively curved (saddle-shaped), or flat (Euclidean). Measurements from the cosmic microwave background suggest the universe is flat to within about 0.4 percent. A perfectly flat universe could be infinite, though a very large flat universe could still be finite. Curvature measurements don't distinguish between these possibilities directly.

Does the universe have an edge?

Almost certainly not. Most cosmological models don't include a physical edge to the universe. The observable universe has a boundary (the cosmic light horizon), but that's just how far we can see, not a physical wall. Beyond it, space continues, with stars, galaxies, and other structures. If the universe is finite but unbounded (like Earth's surface), you could travel forever without hitting an edge. If infinite, you could also travel forever. Either way, no edge exists in the physical sense people sometimes imagine.

Will we ever know for sure?

Probably not directly. The fundamental problem is that we can never see beyond the observable universe. As cosmic expansion accelerates, distant regions actually become unobservable over time. Future cosmological observations will refine our measurements of geometry and other properties, possibly making infinite size more or less likely. But direct proof would require seeing beyond our cosmic horizon, which physics doesn't allow. The infinite vs finite question may remain unresolved indefinitely.