Gravity won't apply after I hit jump, just floats up

Every time I hit the jump button it just keeps floating up. I can't quite figure out what to put to access the gravity and physics.

     void FixedUpdate()
 {
     Vector2 temp = new Vector2(); //create a temporary (temp) vector2 for holding our velocity changes

     temp.x = curMovement; //Assign our current movement information to the temp velocity variable

     grounded = Physics2D.OverlapCircle(groundCheck.position, groundCheckRadius, whatIsGround); //cast a circle around the ground check point to check for a ground object

     //jump logic
     if (jumpInit)
     {
         jumpInit = true; //jump has been initialized, we don't need this to be anymore.
         temp.y = jumpVelocity; //apply the jump velocity to the up and down (y) axis of our temp variable
     }
     else //if not jumping, then keep our current velocity 
         // access gravity and vector here?
    if (rb.velocity.y < 0)
     {
         rb.velocity -= Vector2.up * Physics2D.gravity.y * fallMultiplier * Time.fixedDeltaTime; 
         //Vector2.up * Physics2D.gravity.y = (0, -9.81) * 1.5 = (0, -14.715)
     }
     else if (rb.velocity.y > 0 && !jumpButton) //if no longer holding jump, fall sooner
     {
         rb.velocity -= Vector2.up * Physics2D.gravity.y * lowJumpMultiplier * Time.fixedDeltaTime; 
         //Vector2.up * Physics2D.gravity.y = (0, -9.81) * 1 = (0, -9.81)
     }

     rb.velocity = temp; //apply our velocity changes all at once
 }

}