How can our universe be flat?

The big bang theory also suggests a spherical universe, as the universe is - according to the big bang theory - expanding in all directions at all times.

The concept of a flat universe comes from cosmology and refers to the geometry of space. A flat universe means that if you were to draw a triangle in space, the angles would add up to 180 degrees, and parallel lines would never meet, similar to how they behave in Euclidean geometry.

This concept is tied to the density of the universe, measured by the density parameter Omega (Ω). The critical density is the density at which the universe is perfectly balanced between expanding forever and eventually collapsing back on itself. If Ω is equal to 1, the universe is flat. Observations, particularly of the cosmic microwave background radiation and the distribution of galaxies, suggest that our universe is very close to flat, with Ω very close to 1.

The flatness of the universe has significant implications for its fate and suggests that it will continue to expand, but the rate of expansion may slow down over time. This geometry is consistent with the inflationary model of the universe, which proposes a rapid expansion of space shortly after the Big Bang, smoothing out any curvature the universe might have had initially to result in the flat geometry we observe today.