Is Dark Matter Made of Primordial Black Holes?

Astronomers studying the motions of galaxies and the character of the cosmic microwave background radiation came to realize in the last century that most of the matter in the universe was not visible. About 84% of the matter in the cosmos is dark matter, much of it located in halos around galaxies. It was dubbed dark matter because it does not emit light, but it is also mysterious: it is not composed of atoms or their usual constituents like electrons and protons.

Meanwhile, astronomers have observed the effects of black holes and recently even detected gravitational waves from a pair of merging black holes. Black holes usually are formed in the explosive death of massive stars, a process that can take many hundreds of millions of years as a star coalesces from ambient gas, evolves and finally dies. Some black holes are inferred to exist in the early universe, but there is probably is not enough time in the early universe for the normal formation process to occur. Some alternative methods have been proposed, like the direct collapse of primordial gas or processes associated with cosmic inflation, and many of these primordial black holes could have been made.

CfA astronomer Qirong Zhu led a group of four scientists investigating the possibility that today’s dark matter is composed of primordial black holes, following up on previously published suggestions. If galaxy halos are made of black holes, they should have a different density distribution than halos made of exotic particles. There are some other differences as well – black hole halos are expected to form earlier in a galaxy’s evolution than do some other kinds of halos. The scientists suggest that looking at the stars in the halos of faint dwarf galaxies can probe these effects because dwarf galaxies are small and faint (they shine with a mere few thousand solar luminosities) where slight effects can be more easily spotted. The team ran a set of computer simulations to test whether dwarf galaxy halos might reveal the presence of primordial black holes, and they find that they could: interactions between stars and primordial halo black holes should slightly alter the sizes of the stellar distributions. The astronomers also conclude that such black holes would need to have masses between about two and fourteen solar masses, right in the expected range for these exotic objects (although smaller than the black holes recently spotted by gravitational wave detectors) and comparable to the conclusions of other studies. The team emphasizes, however, that all the models are still inconclusive and the nature of dark matter remains elusive.

Reference(s):
“Primordial Black Holes as Dark Matter: Constraints from Compact Ultra-faint Dwarfs,” Qirong Zhu, Eugene Vasiliev, Yuexing Li, and Yipeng Jing, MNRAS 476, 2, 2018 (https://arxiv.org/abs/1710.05032)

https://www.cfa.harvard.edu/news/su201816

Advertisements
Aside

Exploring evidence of interaction between dark energy and dark matter

One of the most important problems of theoretical physics is to explain the fact that the universe is in a phase of accelerated expansion. Since 1998 the physical origin of cosmic acceleration remains a deep mystery. According to general relativity (GR), if the universe is filled with ordinary matter or radiation, the two known constituents of the universe, gravity should slow the expansion. Since the expansion is speeding up, we are faced with two possibilities, either of which would have profound implications for our understanding of the cosmos and of the laws of physics. The first is that 75% of the energy density of the universe exists in a new form with large negative pressure, called dark energy (DE). The other possibility is that GR breaks down on cosmological scales and must be replaced with a more complete theory of gravity. In this paper we consider the first option. The cosmological constant, the simplest explanation of accelerated expansion, has a checkered history having been invoked and subsequently withdrawn several times before. In quantum field theory, we estimate the value of the cosmological constant as the zero-point energy with a short-cut scale, for example the Planck scale, which results in an excessively greater value than the observational results….

read more at https://arxiv.org/pdf/1804.03296.pdf

Aside

CPT symmetric universe

Latham Boyle, Kieran Finn, Neil Turok
We propose that the state of the universe does it not spontaneously violate CPT. Instead, the universe before the Big Bang is the CPT reflection of the universe after the bang. Phrased another way, the universe before the bang and the universe after the bang may be re-interpreted as a universe/anti-universe pair, created from nothing. CPT selects a unique vacuum state for the QFT on such a spacetime, which leads to a new perspective on the cosmological baryon asymmetry, and a new explanation for the observed dark matter abundance. In particular, if we assume that the matter fields in the universe are described by the standard model of particle physics (including right-handed neutrinos), we predict that one of the heavy neutrinos is stable, and that its density automatically matches the observed dark matter density if its mass is 4.8×10^8 GeV. Among other predictions, we have: (i) that the three light neutrinos are majorana; (ii) that the lightest of these is exactly massless; and (iii) that there are no primordial long-wavelength gravitational waves. We mention connections to the strong CP problem and the arrow of time.
Read more at https://arxiv.org/pdf/1803.08928.pdf

Read also: “The Big Bang, CPT, and neutrino dark matter“, Latham Boyle, Kieran Finn, Neil Turok”

Beyond the WIMP: Unique Crystals Could Expand the Search for Dark Matter

Proposed detector technology would be sensitive to an unexplored range of particle masses

A computerized simulation of the large-scale distribution of dark matter in the universe. An overlay graph (in white) shows how a crystal sample intensely scintillates, or glows, when exposed to X-rays during a lab test. This and other properties could make it a good material for a dark matter detector. (Credit: Millennium Simulation, Berkeley Lab)

A new particle detector design proposed at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) could greatly broaden the search for dark matter – which makes up 85 percent of the total mass of the universe yet we don’t know what it’s made of – into an unexplored realm. Continue reading Beyond the WIMP: Unique Crystals Could Expand the Search for Dark Matter

Cosmic Instability Could Have Created Dark Matter

A proposed instability in the Higgs field could have seeded the Universe with primordial black holes that now serve as dark matter.

Our Universe may be sitting on the edge of destruction, as theory suggests that the Higgs field is in a metastable state. If this field tunneled to its “true” minimum energy state, the release of energy would be cataclysmic. The danger may be over-stated, as other physical mechanisms could have kept the Universe stable throughout its history. Nevertheless, the Higgs instability could have a major cosmological impact as the source of dark matter. According to this new scenario, dark matter consists of a large population of black holes that formed from fluctuations in the unstable Higgs field at the dawn of the Universe. These so-called primordial black holes have been proposed before, but this is the first hypothesis that doesn’t require physics beyond the standard model.

Physicists have long been aware that the Universe might rest in a “false vacuum.” This idea has recently taken on new urgency, as calculations based on the measured Higgs mass have shown that a lower energy state may exist for the Higgs field. Analyzing the implications of the Higgs instability, José Espinosa, from the Catalan Institution for Research and Advanced Studies (ICREA) in Spain, and David Racco and Antonio Riotto, both at the University of Geneva, have found that this menacing mechanism could—counterintuitively—be instrumental in creating the dark matter that makes galaxies and other life-accommodating structures possible. In their calculations, the team explored fluctuations in the Higgs field during an early expansion phase of the Universe, called inflation. Under certain assumptions, these fluctuations become seeds for microscopic black holes with masses around 1015 kg that could have a density consistent with cosmological predictions of dark matter.

Michael Schirber – https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.120.121301