Why did Harry's scar start to hurt again?

Question

When he was young, it hurt:

• when Voldemort was close to him
• when Voldemort felt strong emotions

It is my belief that your scar hurts both when Lord Voldemort is near you, and when he is feeling a particularly strong surge of hatred*.
(Goblet of fire, Chapter 30, The pensieve)
* or joy, as we discover later - but that is off topic

Also, he had "dreams" about Voldemort, that were in fact incursions in Voldemort's mind and let him see what Voldemort saw, heard, did.

The evidence suggests that at times, when your mind is most relaxed and vulnerable — when you are asleep, for instance — you are sharing the Dark Lord’s thoughts and emotions.
(Order of the Phoenix, Chapter 24, Occlumency)

But neither the scar nor the dreams were ever anything like divination features. He could not see the past, nor the future, not even what might be the future.

So after Voldemort is fully dead, he shouldn't feel any of these signs anymore.

In the Cursed child,

actions take place that induce a risk of Voldemort "coming back" or at least "be there".

And, as a "warning signal", Harry's scar hurts and he's having weird dreams. Why??

Answer 1

By all accounts Harry's scar should not be hurting...

As seen in the answers to this question on why Harry's scar hurts:

J.K. Rowling: Well, of course the pain he feels [in his scar] whenever Voldemort's particularly active is this piece of soul seeking to rejoin the master soul. When his scar is hurting him so much, that's not scar tissue hurting him. That's this piece of soul really wanting to get back out the way it entered. It really wants to- it entered this boy's body through a wound, and it wants to rejoin the master. So when Voldemort's near him, when he's particularly active, this connection, (JN: Oh, my gosh!) it was always there. That's what I always imagined this pain was. Yes, so there you go.

_{J.K. Rowling - [THE LEAKY CAULDRON - POTTERCAST'S INTERVIEW WITH J.K. ROWLING] - 12.23.2007}

However, we have time travel involved in this new story. So it is possible that with the creation of a new timeline where Voldemort is still alive, that the piece of soul is still inside Harry.

It is also possible the Cursed Child may have undone the previous reasoning, and that the scar is simply a warning of "bad things relating to Voldemort".

We do not have an official explanation from any of the contributors, and since "canon" in the Potterverse is fast and loose to begin with, there may not be a canon answer.

Answer 2

As Delphi is Voldemort’s daughter, it is possible that the connection they share will lead to Harry’s scar hurting when Delphi is particularly near or emotional.

As we have learned, family and love have very strong magical connections in Harry Potter so maybe the emotional connection between Delphi and Voldemort resulted in Harry’s scar reacting to Delphi’s emotions.

Answer 3

It has been addressed in Act Four, Scene Fifteen - A Beautiful Hill:

Harry: You know, I thought I'd lost him - Voldemort - I thought I'd lost him - and then my scar started hurting again and I had dreams of him and I could even speak Parseltongue again and I started to feel like I'd not changed at all - that he'd never let me go -

Albus: And had he?

Harry: The part of me that was Voldemort died a long time ago, but it wasn't enough to be physically rid of him - I had to be mentally rid of him. And that - is a lot to learn for a forty-year-old man.

So, it seems like a psychological issue, an after image of real trauma which may resurface after years. Why did it happen exactly during this time? Well, it can be coincidence or because of fights with Albus or because of The Force prophecy.