How do you modify gravity force in this script ?

I followed a tutorial to make the FirstPersonController script and it works quite well, but I'd like to make my character fall faster. I mean it looks like he's jumping on the moon. (I can modify the jump height but not its speed)

By the way if you find faulty things in the script, i'm all ears. Here it is :

using UnityEngine; using System.Collections;

[RequireComponent (typeof(CharacterController))] public class FirstPersonController : MonoBehaviour {

 public float movementSpeed = 5.0f;
 public float mouseSensitivity = 5.0f;
 public float jumpSpeed = 20.0f;

 float verticalRotation = 0;
 public float upDownRange = 60.0f;

 float verticalVelocity = 0;

 CharacterController characterController;

 // Use this for initialization
 void Start () {
     Screen.lockCursor = true;
     characterController = GetComponent<CharacterController> ();
 }
 
 // Update is called once per frame
 void Update () {
     //Rotation

     float rotLeftRight = Input.GetAxis ("Mouse X") * mouseSensitivity;
     transform.Rotate (0, rotLeftRight, 0);


     verticalRotation -= Input.GetAxis ("Mouse Y") * mouseSensitivity;
     verticalRotation = Mathf.Clamp (verticalRotation, -upDownRange, upDownRange);
     Camera.main.transform.localRotation = Quaternion.Euler (verticalRotation, 0, 0);


     //Movement

     float forwardSpeed = Input.GetAxis ("Vertical") * movementSpeed;
     float sideSpeed = Input.GetAxis ("Horizontal") * movementSpeed;

     verticalVelocity += Physics.gravity.y * Time.deltaTime;

     if ( characterController.isGrounded && Input.GetButtonDown ("Jump")) {
         verticalVelocity = jumpSpeed;
     }

     Vector3 speed = new Vector3 ( sideSpeed, verticalVelocity, forwardSpeed );

     speed = transform.rotation * speed;



     characterController.Move (speed * Time.deltaTime);
 }

}