The correct option is 2 and 3 only.
Explanation
The Big Bang Theory is the prevailing cosmological model explaining the origin of the universe. It postulates that the universe began from a primordial state known as a "singularity."
Statement 1 is Incorrect: The singularity refers to the state of the universe at the beginning (time zero), prior to the expansion event. It does not refer to the state after the Big Bang; the period following the Big Bang is characterized by the expansion and cooling of the universe.
Statement 2 is Correct: According to the theory, all matter and energy in the universe were initially condensed into a single point of unimaginably small volume (often described in standard texts as a "tiny ball" or singular atom).
Statement 3 is Correct: Because the entire mass of the universe was compressed into this infinitesimal volume, the singularity is defined by infinite density and infinite temperature.
Key Takeaway: The "Singularity" in the Big Bang Theory represents the initial condition of the universe, characterized by infinitesimal volume, infinite density, and infinite temperature, before the onset of cosmic expansion.