But notice the new heavens and new earth; there's no mention that it's merely a spiritual realm. It might be spiritual, or it might be physical (or it may be this dichotomy doesn't even make sense once we understand better).
7But by His word the present heavens and earth are being reserved for fire, kept for the day of judgment and destruction of ungodly men.
8But do not let this one fact escape your notice, beloved, that with the Lord one day is like a thousand years, and a thousand years like one day.
9The Lord is not slow about His promise, as some count slowness, but is patient toward you, not wishing for any to perish but for all to come to repentance.10But the day of the Lord will come like a thief, in which the heavens will pass away with a roar and the elements will be destroyed with intense heat, and the earth and its works will be burned up.
11Since all these things are to be destroyed in this way, what sort of people ought you to be in holy conduct and godliness,
12looking for and hastening the coming of the day of God, because of which the heavens will be destroyed by burning, and the elements will melt with intense heat!
13But according to His promise we are looking for new heavens and a new earth, in which righteousness dwells.
But I'm leaning ever more toward the idea that this earth and cosmos was originally intended to be permanent. When Adam and Eve ate the forbidden fruit, the cosmos became broken, and now can't exist forever in its current state. Death, not just for Adam and Eve, but for the entire cosmos, entered into the cosmos.
Rob Bell, in his book "Velvet Elvis", highlights some interesting passages that touch on this topic.
Matt 19:28 - Assuredly I say to you, that in the regeneration, when the Son of Man sits on the throne of His glory....
Acts 3:20-21 - ... He may send Jesus Christ, ... whom heaven must receive until the times of restoration of all things, which God has spoken by the mouth of all His holy prophets since the world began.
Col. 1:19-20 - For it pleased the Father that in Him all the fullness should dwell, and by Him to reconcile all things to Himself, by Him, whether things on earth or things in heaven, having made peace through the blood of His cross.If Adam had not sinned, the Curse of death and decay would not have entered into the cosmos, and presumably the cosmos would have existed eternally.
In speaking of regeneration and restoration of all things and reconciliation of all things, not just humans, to God, these passages indicate not an erasure of the physical cosmos, but a repairing of it like it was intended to be.
When Peter writes of the heavens and earth being melted and dissolved by fire, he does so just a few verses after he had written about the pre-Flood world having perished. The first destruction of the earth did not do away with the physicality of the earth; I'm unsure that the second destruction of the earth will either. It seems clear that the current cosmos will be melted down into nothingness, but is that a true nothingness, or is it just a nothing-but-bare-framework nothingness?
Like so many doctrines I grew up with, I find that the certainty with which these doctrines were taught is not quite so clear-cut when you approach the scriptures with a more open mind.
We know that the physical body of Jesus was raised from the dead, although it was raised incorruptible. Why should the physical universe be any different? It is destined to die, thoroughly, but perhaps it too, as Paul writes in Romans 8, will be freed from the bondage of corruption, under which it is currently groaning in travail.