在codeigniter中提交表单并尝试一点ajax后保持模态打开

so this is my controller:

User_Authentication_Controller:

public function login_user(){
   $username = $this->input->post('txt_login_username');
   $password = md5($this->input->post('txt_login_password'));

   $result=$this->LogIn_Model->login($username,$password);
   redirect('Pages_Controller/homepage');
}

LogIn_Model:

public function LogIn($username,$password){
   $data = array(
   'username' => $username,
   'password' => $password
   );

   $query = $this->db->get_where('user_account',$data);
           return $query->result_array();   
}

LOGIN MODAL:

<section id="login_modal" class="modal fade" >
 <div class="modal-dialog">
  <div class="modal-content">
   <div class="modal-header"></div>
    <div class="modal-body" id="forms">
     <div class="loginmodal-container">
      <h1>Login to Your Account</h1><br>
       <form class="form-horizontal" id="form_login" action="" method="POST">   
        <div class='error_msg'></div>
         <input autofocus="autofocus" type="text" id="txt_login_username" name="txt_login_username" class="form-control" />
        <input type="password" id="txt_login_password" name="txt_login_password" class="form-control" />
        <button class="btn btn-primary" type="submit" name="btnLogin">Login</button>    
        <a href="<?php echo base_url();?>index.php/Pages_Controller/show_forgotpassword">Forgot Password?</a>                   
     </div>     
  </form>               
            </div>
        <div class="modal-footer">
                <center><button type = "submit" id="login_user" class="btn btn-danger" data-dismiss="modal" >CLOSE</button></center>
        </div>  
    </div>
  </div>
</section>

VIEW:

    <a href="#" onclick="$('#login_modal').modal({'backdrop': 'static'});" id="login">Login</a>

AJAX:

  $(function(){
    $('#login_user').submit(function(event){

        var user = $('#txt_login_username').val();
        var pass = $('#txt_login_password').val();

    $.ajax({
        type: 'POST',
        url: 'User_Authentication_Controller/login_user',
        data: {
        'user': user,
        'pass': pass
        },
        dataType: 'html',
        success: function(results){
            if(user != user){
                $('.error_msg').html('error msg');
                      return false;
                }
            }
        });
    });
});

ALL I WANT IS IF THERE IS AN ERROR! THE will work and the modal will not close! how will I do that with CI? Is there any possible way that just put the LogIn_Model->login(); in AJAX? please help! I need this thing badly! And in addition I want that what the user inputs and if he got it wrong when he submitted the username will still be at the textbox! how will I do that also? THank you so much for help!