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R. MacKenzie et al.
should be a gas of non-relativistic particles, while the issue of DE is closely tied
to that of the cosmological constant [1], since data [2] suggests that its energy
density is constant over the epochs scanned by the cosmic microwave background
(CMB). If treated as a dynamical phenomenon, DE demands an explanation for
the equation of state p = −ρ in terms of relativistic phenomena. From the point
of view of naturalness, explaining a value of a dynamically generated quantity
which is many orders of magnitude away from any of the scales of elementary
particle physics or gravity is a major challenge. There are explanations that obtain
such a sector as directly related to and derived from more powerful principles
applicable at high scales [3–5]. On the other hand, extended and space filling
objects, specifically domain walls as possible solutions to understanding Dark
Energy have been proposed earlier in a variety of scenarios [6–10]. In this paper we
pursue the latter approach, of invoking new species of particles and their interactions
at the new low mass scale, agnostic of their connection to the known physics other
than gravity. A more extensive discussion of the results reported here can be found
in [11].
We consider a new sector of particles with interaction mediated by an unbroken
abelian gauge symmetry denoted U(1) X . The core of our mechanism involves the
existence of a fermionic species that enters into a ferromagnetic state. As we will
show, it is required to have an extremely small mass and hence an extremely large
magnetic moment; we dub this species the magnino, 1 denoted M. We assume
that the medium remains neutral under the X-charge due to the presence of a
significantly heavier species Y of opposite charge which does not enter the collective
ferromagnetic state. The wall complex resulting from the formation of magnetic
domains then remains mutually bound, and due to interaction strength much larger
than cosmic gravity, remains frozen. The binding of the heavier species to this
complex due to the requirement of X-electrical neutrality then ensures that these
particles remain unevolving, and after averaging over the large scales of the cosmic
horizon act like a homogeneous space filling medium of constant density.
It is possible to explain DM within the same sector, including possible dark
atoms formed by such species [14–17]. This would also solve the concordance
problem, that is, the comparable energy densities carried in the cosmological
energy budget by the otherwise-unrelated components, DM and DE. Further, the
X-electromagnetism is expected to mix kinetically with the standard electromagnetism. The existence of cosmic magnetic fields at galactic and intergalactic scales
[18–20] is an outstanding puzzle of cosmology. Our mechanism relying as it does on
spontaneous formation of domains of X-ferromagnetism has the potential to provide
the seeds needed to generate the observed fields through such mixing.
In the following, in Sect. 2 we motivate the origin of negative pressure for
extended objects in cosmology. In Sect. 3 we discuss the calculation of the exchange
energy for a spin-polarised PAAI. Thus we motivate the possibility of occurrence of
an extended structure of domain walls, and their metastable yet long lived nature.
In Sect. 4 we discuss the main results of our proposal, obtaining suggestive values
for the masses and abundances for the scenario to successfully explain DE, and for
1 The term magnino was earlier introduced in a different connotation in [12, 13].
R. MacKenzie et al.
should be a gas of non-relativistic particles, while the issue of DE is closely tied
to that of the cosmological constant [1], since data [2] suggests that its energy
density is constant over the epochs scanned by the cosmic microwave background
(CMB). If treated as a dynamical phenomenon, DE demands an explanation for
the equation of state p = −ρ in terms of relativistic phenomena. From the point
of view of naturalness, explaining a value of a dynamically generated quantity
which is many orders of magnitude away from any of the scales of elementary
particle physics or gravity is a major challenge. There are explanations that obtain
such a sector as directly related to and derived from more powerful principles
applicable at high scales [3–5]. On the other hand, extended and space filling
objects, specifically domain walls as possible solutions to understanding Dark
Energy have been proposed earlier in a variety of scenarios [6–10]. In this paper we
pursue the latter approach, of invoking new species of particles and their interactions
at the new low mass scale, agnostic of their connection to the known physics other
than gravity. A more extensive discussion of the results reported here can be found
in [11].
We consider a new sector of particles with interaction mediated by an unbroken
abelian gauge symmetry denoted U(1) X . The core of our mechanism involves the
existence of a fermionic species that enters into a ferromagnetic state. As we will
show, it is required to have an extremely small mass and hence an extremely large
magnetic moment; we dub this species the magnino, 1 denoted M. We assume
that the medium remains neutral under the X-charge due to the presence of a
significantly heavier species Y of opposite charge which does not enter the collective
ferromagnetic state. The wall complex resulting from the formation of magnetic
domains then remains mutually bound, and due to interaction strength much larger
than cosmic gravity, remains frozen. The binding of the heavier species to this
complex due to the requirement of X-electrical neutrality then ensures that these
particles remain unevolving, and after averaging over the large scales of the cosmic
horizon act like a homogeneous space filling medium of constant density.
It is possible to explain DM within the same sector, including possible dark
atoms formed by such species [14–17]. This would also solve the concordance
problem, that is, the comparable energy densities carried in the cosmological
energy budget by the otherwise-unrelated components, DM and DE. Further, the
X-electromagnetism is expected to mix kinetically with the standard electromagnetism. The existence of cosmic magnetic fields at galactic and intergalactic scales
[18–20] is an outstanding puzzle of cosmology. Our mechanism relying as it does on
spontaneous formation of domains of X-ferromagnetism has the potential to provide
the seeds needed to generate the observed fields through such mixing.
In the following, in Sect. 2 we motivate the origin of negative pressure for
extended objects in cosmology. In Sect. 3 we discuss the calculation of the exchange
energy for a spin-polarised PAAI. Thus we motivate the possibility of occurrence of
an extended structure of domain walls, and their metastable yet long lived nature.
In Sect. 4 we discuss the main results of our proposal, obtaining suggestive values
for the masses and abundances for the scenario to successfully explain DE, and for
1 The term magnino was earlier introduced in a different connotation in [12, 13].