Can you make a global reference to a rigid body?

I want to make a reference for the rigid body that I can manipulate in multiple functions. Right now I get this error

Assets/Scripts/CubeMovement.cs(11,32): error CS0236: A field initializer cannot reference the nonstatic field, method, or property `UnityEngine.Component.GetComponent()'

Here is my file

using System.Collections; using System.Collections.Generic; using UnityEngine;

public class CubeMovement : MonoBehaviour {

 public GameObject Cube; //The Cube
 public float speed;
 public float forwardSpeed;
 public float jumpSpeed;
  static Rigidbody rb = GetComponent<Rigidbody>();

 // Update is called once per frame
 void Update ()
 {

     //Cube.transform.position += new Vector3 (0f, 0f, .5f);



     //if you want to make it so it is whatever the script is attatched to, instead of "Cube.transform.position"; Do gameObject.transform.position
     //Also instead of 1f, 0f, 0f
     //You can customize it to whatever you want (x,y,z)
     //If you do decimals do "f" don't just do "5.4" do "5.4f"
     //This is in the update function, it will happen per frame. Feel free to have it in any other function :)

 }
 void FixedUpdate () 
 {
     //Rigidbody rb = GetComponent<Rigidbody>();
     if (Input.GetKey("a"))
     {
         rb.AddForce (-Vector3.right * speed * Time.deltaTime);
     }
     if (Input.GetKey("d"))
     {
         rb.AddForce (Vector3.right * speed * Time.deltaTime);
     }

     rb.AddForce( new Vector3(0,0,forwardSpeed*Time.deltaTime));
 }

 //function that will make the cube jump
 void Jump()
 {
     rb.AddForce(Vector3.up * jumpSpeed);
 }
 /*
 void LateUpdate() {
     Cube.transform.position += new Vector3 (0f, 0f, .5f);



 }*/

}