Neutron star mergers chirp about vacuum energy

Abstract

Observations of gravitational waves from neutron star mergers open up novel directions for exploring fundamental physics: they offer the first access to the structure of objects with a non-negligible contribution from vacuum energy to their total mass. The presence of such vacuum energy in the inner cores of neutron stars occurs in new QCD phases at large densities, with the vacuum energy appearing in the equation of state for a new phase. This in turn leads to a change in the internal structure of neutron stars and influences their tidal deformabilities which are measurable in the chirp signals of merging neutron stars. By considering three commonly used neutron star models we show that for large chirp masses the effect of vacuum energy on the tidal deformabilities can be sizable. Measurements of this sort have the potential to provide a first test of the gravitational properties of vacuum energy independent from the acceleration of the Universe, and to determine the size of QCD contributions to the vacuum energy.

Pixelated Dark Energy

“…after all phase transitions have occurred. As noted in [], this leads to a modification for the equation of state in neutron stars, which could potentially be observed in gravitational waves emitted from binary merger events. It would be interesting to seek out other astrophysical phenomena sensitive to this feature of our model.…”

“…Since we have fixed the overall zero temperature constants appearing in the free energy of all phase transitions, we also expect that in regions of the Universe which are still at high temperature, the local vacuum energy density and thus the local value of the cosmological constant to be higher. This will occur, for example, in the interior of neutron stars, and it was proposed in references [179][180][181] that the effects of this could be detected via gravitational waves generated by a binary merger event.…”

Searches for other vacua. Part I. Bubbles in our universe

“…The critical difference between the vacuum transition and the condensed matter system are finite density effects. As shown in [17] and [18], new phases can exist in a very localized region inside compact objects, or a semi-local region (confined bubbles) if the landscape contains close by, degenerate or near degenerate vacua. These confined bubbles will be the main topic of this paper.…”

The QCD axion at finite density

“…However, under well defined assumptions, in particular that kaon condensation [26] takes place somewhat above n 0 , we show that spontaneous CP violation in the neutral Goldstone sector is triggered, resulting in axion condensation -see [27] for a first discussion of axion condensation in the context of a tuned non-standard axion. For a range of axion masses of order of the inverse radius of the NS, the axion gets sourced, with phenomenological consequences that go from a non-standard contribution to the mass of the star (closely resembling [28,29]) or the presence of an axionic brane [30], to an O(1) enhancement of the electric field in the star's magnetosphere. Other implications of such a sourcing, e.g.…”

“…A NS with a core of "vacuum energy" was considered as a generic scenario in [28,29], in the context of exotic QCD phases. It was found that the energy shift inside the NS leads to a significant change in the mass-radius relation of NSs, as well as to changes of the so-called chirp mass, one of main the gravitational wave observables of compact binary mergers.…”