When we think of heaven we instinctively look up at the sky and its starry appearance at night - we tend to think of it as a place. However, with the advent of space exploration we can no longer look up at the starry space and see it as heaven.
So, what do we mean when we pray "Our Father who art in heaven?"
Here 'heaven' is not a place but refers to the transcendent God in his majesty. 'Heaven' refers to the inner life of God, his joyful dynamic living communion with his Son in the Spirit, the eternal abiding reality of "He who Is" and whom we recognize as Father, since the Father is the Eternal Origin of the Trinity of persons.
The Father is also in heaven in that dimension of creation where the holy souls are already encountering the face to face vision of God. This is an enduring presence of God in the New Creation moving toward its final end. This is the Kingdom of Glory.
When we pray "Our Father who art in heaven" we are professing that we are the People of God in some sense already in a heavenly sphere of God's influence, yet aware of the pain and suffering we endure in our effort to achieve this final heavenly union with God.
Hollowed Be Thy Name