Negative-Mass Bubbles

Exploring the idea of regions of space with "negative mass" in an expanding universe.

Negative mass! Bwaaa, so bizarre! But is it really? One way to think about mass is as a charge for the gravitational force. If electric charges can have both positive and negative signs, is it such a stretch to imagine positive and negative gravitational charges? Well kinda, yeah. Many years ago, it was shown [1] that taking account of general relativity, objects with negative mass would behave very strangely indeed! The intuitive description is that in GR, objects with positive mass always attract everything, and objects with negative mass always repel everything, so together, a pair of the two would just chase each other forever. Here is an illustration:

Positive and negative masses in Einstein's gravity. In ordinary flat space, two positive masses attract each other, two negative masses repel each other, and a negative mass and positive mass would chase each other forever.

So what more is there to say about these things, why revive the topic? Well recently [2,3] it was shown that in a universe with dark energy, it’s possible to construct an object that looks like it has negative mass, from far away. These “bubbles” are fundamentally very different from the simple point charges considered before, and their existence is intimately linked to the dark energy (and the exponential expansion of the universe that it induces). The point of this research project is to really explore these bubbles, to see how they behave mechanically, and very importantly, to see how they would have behaved in the early universe, and whether or not they would have left a detectable signal.

Questions to ask:

  • How does the classic calculation [1] change if space is permeated with a dark energy (and so is exponentially expanding)?
  • What are the scales involved? Is there a limit on how big or small the bubbles can be?
  • What about the very early universe, where there was a sort of dark energy due to a quantum field (the inflaton)? Could quantum fluctuations have formed bubbles like these, and if so, could there be a trace of them in the sky?

References

  1. H. Bondi, Rev.Mod.Phys. 29, 423–428 (1957) .
  2. Jonathan Belletête, M. B. Paranjape, Int.J.Mod.Phys. D22, 1341017 (2013) .
  3. Matthew C. Johnson, M. B. Paranjape, Antoine Savard, Natalia Tapia-Arellano, arXiv:1910.01774 (2020) .