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Message: <blockquote data-quote="WhiteWalker" data-source="post: 21057720" data-attributes="member: 548558">What is there left to discover or explain about gravity?Here are a few examples, some mysteries about gravity.The question of quantum gravity: Is gravity a quantized field? Or, unlike most other fundamental phenomena in Nature, is gravity "emergent" and fundamentally classical?Gravity on cosmic scales: Are the anomalous rotation curves of galaxies, the dynamics of galaxy clusters, and cosmic evolution overall really due to yet-to-be-discovered dark matter and dark energy, or is it perhaps our gravity theory that is incomplete?The cosmological constant problem: The zero-point energy of vacuum fluctuations shall gravitate. Indeed, it has the same equation of state as dark energy. Could it be dark energy? But then, quantum field theory tells us that its energy density is either infinite or many dozens of orders of magnitude too big compared to the observed value.The energy of the gravitational field: the gravitational field obviously carries energy (e.g., there is the gravitational potential energy that is released when two massive objects approach each other, or the energy carried by gravitational waves.) Yet general relativity tells us that in the immediate vicinity of an observer, spacetime is indistinguishable from empty spacetime, i.e., its local energy density must be zero. How can these two issues be convincingly reconciled?Event horizon firewalls: In a quantum mechanical universe, can an observer ever reach the event horizon? Or would the observer be destroyed by a "firewall"? Does the event horizon even exist, given the finite lifetime of an evaporating black hole?As these examples show, there is still plenty to do.</blockquote>

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