Fundamental Nature Of Dark Matter Research Articles

Self-interactions of ULDM to the rescue?

One of the most important unanswered questions in cosmology is concerning the fundamental nature of dark matter (DM). DM could consist of spinless particles of very small mass i.e. m ∼ 10-22 eV. This kind of ultralight dark matter (ULDM) would form cored density profiles (called “solitons”) at the centres of galaxies. In this context, recently it has been argued that (a) there exists a power law relation between the mass of the soliton and mass of the surrounding halo called the Soliton-Halo (SH) relation, and, (b) the requirement of satisfying observed galactic rotation curves as well as SH relations is so stringent that ULDM is disfavoured from comprising 100% of the total cosmological dark matter. In this work, we revisit these constraints for ULDM particles with non-negligible quartic self-interactions. Using a recently obtained soliton-halo relation which takes into account the effect of self-interactions, we present evidence which suggests that, for m ∼ 10-22 eV, the requirement of satisfying both galactic rotation curves as well as SH relations can be fulfilled with repulsive self-coupling λ ∼ 𝒪(10-90).

Mirror neutron stars

The fundamental nature of dark matter is entirely unknown. A compelling candidate is Twin Higgs mirror matter, invisible hidden-sector cousins of the Standard Model particles and forces. This generically predicts mirror neutron stars, degenerate objects made entirely of mirror nuclear matter. We find their structure using realistic equations of state, robustly modified based on first-principle quantum chromodynamic calculations. We predict their detectability with gravitational waves and binary pulsars, suggesting an impressive discovery potential and ability to probe the dark sector.

Power of halometry

Astrometric weak gravitational lensing is a powerful probe of the distribution of matter on sub-Galactic scales, which harbor important information about the fundamental nature of dark matter. We propose a novel method that utilizes angular power spectra to search for the correlated pattern of apparent motions of celestial objects induced from time-dependent lensing by a population of Galactic subhalos. Application of this method to upcoming astrometric datasets will allow for the direct measurement of the properties of Galactic substructure, with implications for the underlying particle physics. We show that, with near-future astrometric observations, it may be possible to statistically detect populations of cold dark matter subhalos, compact objects, as well as density fluctuations sourced by scalar field dark matter. Currently-unconstrained parameter space will already be accessible using upcoming data from the ongoing Gaia mission.

Searching for light dark matter at fixed target experiments

Nowadays deciphering the fundamental nature of dark matter is one of the foremost open questions in fundamental science. In the last years the dark matter experimental program is primarily focused on weakly-interacting massive particles (WIMPs), but in the recent years many innovative dark matter candidates have emerged. One of the most viable models takes into account DM particles in MeV-GeV mass range that are completely neutral under Standard Model forces, but interact through a new force. Small-scale fixed-target experiments at accelerators have unique sensitivities to fully explore these models. Indeed, accelerator experiments can both produce and detect new particles, such as dark matter and the particles mediating new interactions. This article reviews the various experimental techniques to search for light dark matter, i.e. beam dump, missing mass/energy, and visible mediator search experiments.

Analysis of the Yukawa gravitational potential in f(R) gravity. I. Semiclassical periastron advance

The {\it concordance} cosmological model has been successfully tested throughout the last decades. Despite its successes, the fundamental nature of dark matter and dark energy is still unknown. Modifications of the gravitational action have been proposed as an alternative to these dark components. The straightforward modification of gravity is to generalize the action to a function, $f(R)$, of the scalar curvature. Thus one is able to describe the emergence and the evolution of the Large Scale Structure without any additional (unknown) dark component. In the weak field limit of the $f(R)$-gravity, a modified Newtonian gravitational potential arises. This gravitational potential accounts for an extra force, generally called fifth force, that produces a precession of the orbital motion even in the classic mechanical approach. We have shown that the orbits in the modified potential can be written as Keplerian orbits under some conditions on the strength and scale length of this extra force. Nevertheless, we have also shown that this extra term gives rise to the precession of the orbit. Thus, comparing our prediction with the measurements of the precession of some planetary motions, we have found that the strength of the fifth force must be in the range $[2.70-6.70]\times10^{-9}$ whit the characteristic scale length to fixed to the fiducial values of $\sim 5000$ AU.

A Potentially Useful Galactic Dark Matter Index

There have been a number of observational surprises with respect to galactic dark matter-to-visible matter ratios. These surprises confirm our continued lack of understanding of the fundamental nature of dark matter. Because of their apparent close ties with galactic gravitational entropy, at least four recent observations appear to provide the first evidence in support of Verlinde’s theory of gravity, dark energy and dark matter as emergent properties. They also appear to correlate with Roger Penrose’s gravitational entropy concept, as well as entropy defined in the Flat Space Cosmology (FSC) model. Given the observational support, two different testable versions of a “Dark Matter Index” (DMI) are introduced in this paper, and its utility is discussed in terms of potentially achieving a better understanding of the fundamental nature of dark matter.

Modified gravity and dark matter

The fundamental nature of Dark Matter (DM) has not been established. Indeed, beyond its gravitational effects, DM remains undetected by present experiments. In this situation, it is reasonable to wonder if other alternatives can effectively explain the observations usually associated with the existence of DM. The modification of the gravitational interaction has been studied in this context from many different approaches. However, the large amount of different astrophysical evidences makes difficult to think that modified gravity can account for all these observations. On the other hand, if such a modification introduces new degrees of freedom, they may work as DM candidates. We will summarize the phenomenology of these gravitational DM candidates by analyzing minimal models.

Fundamental physics from the sky: Cosmic rays, gamma rays and the hunt for dark matter

Can we learn about New Physics with astronomical and astro–particle data? Understanding how this is possible is key to unraveling one of the most pressing mysteries at the interface of cosmology and particle physics: the fundamental nature of dark matter. I will discuss some of the recent puzzling findings in cosmic–ray electron–positron data and in gamma–ray observations that might be related to dark matter. I will argue that recent cosmic–ray data, most notably from the Pamela and Fermi satellites, indicate that previously unaccounted–for powerful sources in the Galaxy inject high–energy electrons and positrons. Interestingly, this new source class might be related to new fundamental particle physics, and specifically to pair–annihilation or decay of galactic dark matter. This exciting scenario is directly constrained by Fermi gamma–ray observations, which also inform us on astrophysical source counterparts that could also be responsible for the high–energy electron–positron excess. Observations of gamma–ray emission from the central regions of the Galaxy as well as claims on a gamma–ray line at around 130 GeV also recently triggered a wide–spread interest: I will address the question of whether we are really observing signals from dark matter annihilation, how to test this hypothesis, and which astrophysical mechanisms constitute the relevant background.

Scalar field “mini-MACHOs”: A new explanation for galactic dark matter

We examine the possibility that galactic halos are collisionless ensembles of scalar field ``massive compact halo objects'' (MACHOs). Using mass constraints from MACHO microlensing and from theoretical arguments on halos made up of massive black holes, as well as demanding also that scalar MACHO ensambles of all scales do not exhibit gravothermal instability (as required by consistency with observations of LSB galaxies), we obtain the range: $m\alt 10^{-7} M_\odot$ or $30 M_\odot\alt m\alt 100 M_\odot$. The rather narrow mass range of large MACHOs seems to indicate that the ensambles we are suggesting should be probably made up of scalar MACHOs in the low mass range (``mini--MACHOs''). The proposed model allows one to consider a non--baryonic and non--thermal fundamental nature of dark matter, while at the same time keeping the same phenomenology of the CDM paradigm.