The fact that our Universe’s expansion is accelerating implies that dark energy exists. But could it be even weirder than we’ve imagined?
Category: cosmology – Page 24
Superconductivity Inspires New Dark Matter Contender
As searches for the leading dark matter candidates—weakly interacting massive particles, axions, and primordial black holes—continue to deliver null results, the door opens on the exploration of more exotic alternatives. Guanming Liang and Robert Caldwell of Dartmouth College in New Hampshire have now proposed a dark matter candidate that is analogous with a superconducting state [1]. Their proposal involves interacting fermions that could exist in a condensate similar to that formed by Cooper pairs in the Bardeen-Cooper-Schrieffer theory of superconductivity.
The novel fermions considered by Liang and Caldwell emerge in the Nambu–Jona-Lasinio model, which can be regarded as a low-energy approximation of the quantum chromodynamics theory that describes the strong interaction. The duo considers a scenario where, in the early Universe, the fermions behave like radiation, reaching thermal equilibrium with standard photons. As the Universe expands and the temperature drops below a certain threshold, however, the fermions undergo a phase transition that leads them to pair up and form a massive condensate.
The proposed scenario has several appealing features, say Liang and Caldwell. The fermions’ behavior would be consistent with that of the cold dark matter considered by the current standard model of cosmology. Further, the scenario implies a slight imbalance between fermions with different chiralities (left-and right-handed). Such an imbalance might be related to the yet-to-be-explained matter–antimatter asymmetry seen in the Universe. What’s more, the model predicts that the fermions obey a time-dependent equation of state that would produce unique, potentially observable signatures in the cosmic microwave background (CMB) radiation. The researchers suggest that next-generation CMB measurements—by the Simons Observatory and by so-called stage 4 CMB telescopes—might reach sufficient precision to vet their idea.
Dark matter formed when fast particles slowed down and got heavy, new theory says
A study by Dartmouth researchers proposes a new theory about the origin of dark matter, the mysterious and invisible substance thought to give the universe its shape and structure. They say the hypothetical force shaping the universe sprang from particles that rapidly condensed, like steam into water.
The researchers report in Physical Review Letters that dark matter could have formed in the early life of the universe from the collision of high-energy massless particles that lost their zip and took on an incredible amount of mass immediately after pairing up, according to their mathematical models.
Hypothetical dark matter is believed to exist based on observed gravitational effects that cannot be explained by visible matter. Scientists estimate that 85% of the universe’s total mass is dark matter.
Cosmic afterglow revelation may “rewrite the history of the universe”
He told Newsweek that the unexpected result “upsets the usual interpretation of the nature of the CMB. It essentially means that we do not have solid evidence for a hot big bang. Taking the observed CMB and subtracting this foreground leaves too little for the hot big bang to be real.”
(The “hot big bang” refers to how the universe started in a hot, dense, state and has been cooling and expanding ever since.)
Kroupa added: “This shocking result means that we now need to revisit the very foundations of everything we know about cosmology, gravitation and the evolution of the Universe and how galaxies came to be.”
Could Dark Matter Be Evolving Over Time, and Not Dark Energy?
For a while now, there has been a problematic mystery at the heart of the standard cosmological model. Although all observations support the expanding Universe model, observations of the early period of the cosmos give a lower rate of acceleration than more local observations. We call it the Hubble tension problem, and we have no idea how to solve it. Naturally, there have been several proposed ideas: what if general relativity is wrong; what if dark matter doesn’t exist; what if the rate of time isn’t uniform; heck, what if the entire Universe rotates. So, let’s add a new idea to the pile: what if dark matter evolves?
While there have been several models proposing an evolving dark energy, the idea of evolving dark matter hasn’t been widely considered. The reason for this is twofold. First, the observations we have of dark matter are excellent. They point to the presence of some kind of material that doesn’t interact strongly with light. The only major weak point is that we haven’t observed dark matter particles directly. Second, the vast majority of folk opposed to dark matter focus on eliminating it altogether through things like modified gravity. They figure dark matter is fundamentally wrong, not something to be tweaked. That makes this new idea rather interesting.
In this work, the authors look at both evolving dark energy and evolving dark matter and argue that the latter is a much better fit to observational data. The first thing they note is that the two models are somewhat related. Since the evolution of the cosmos depends in part on the ratio of energy density to matter density, a model with constant dark matter and evolving dark energy will always appear similar to a model with evolving dark matter and constant dark energy.
30-year Mystery of Dissonance in The ‘Ringing’ of Black Holes Explained
A scientist from Tokyo Metropolitan University has solved the longstanding problem of a “dissonance” in gravitational waves emitted by a black hole.
Using high precision computing and a new theoretical physics framework, it was discovered that it was caused by a resonance between a pair of distinctive “modes” i.e. different ways in which a black hole can “ring.” The phenomenon offers new insights into the nascent field of black hole spectroscopy.
The research is published in the journal Physical Review Letters.
