New constraints on a theory that says dark matter was created just after the Big Bang  – rather than at the Big Bang – have been determined by Richard Casey and  Cosmin Ilie at Colgate University in the US. The duo calculated the full range of parameters in which a “Dark Big Bang” could fit into the observed history of the universe. They say that evidence of this delayed creation could be found in gravitational waves.

Dark matter is a hypothetical substance that is believed to play an important role in the structure and dynamics of the universe. It appears to account for about 27% of the mass–energy in the cosmos and is part of the Standard Model of cosmology. However, dark matter particles have never been observed directly.

The Standard Model also says that the entire contents of the universe emerged nearly 14 billion years ago in the Big Bang. Yet in 2023, Katherine Freese and Martin Winkler at the University of Texas at Austin introduced a captivating new theory, which suggests that the universe’s dark matter may have been created after the Big Bang.

Evidence comes later on

Freese and Winkler pointed out that presence of photons and normal matter (mostly protons and neutrons) can be inferred from almost immediately after the Big Bang. However, the earliest evidence for dark matter comes from later on, when it began to exert its gravitational influence on normal matter. As a result, the duo proposed that dark matter may have appeared in a second event called the Dark Big Bang.

“In Freese and Winkler’s model, dark matter particles can be produced as late as one month after the birth of our universe,” Ilie explains. “Moreover, dark matter particles produced via a Dark Big Bang do not interact with regular matter except via gravity. Thus, this model could explain why all attempts at detecting dark matter – either directly, indirectly, or via particle production – have failed.”

According to this theory, dark matter particles are generated by a certain type of scalar field. This is an energy field that has a single value at every point in space and time (a familiar example is the field describing gravitational potential energy). Initially, each point of this scalar field would have occupied a local minimum in its energy potential. However, these points could have then transitioned to lower-energy minima via quantum tunnelling. During this transition, the energy difference between the two minima would be released, producing particles of dark matter.

Consistent with observations

Building on this idea, Casey and Ilie looked at how predictions of the Dark Big Bang model could be consistent with astronomers’ observations of the early universe.

“By focusing on the tunnelling potentials that lead to the Dark Big Bang, we were able to exhaust the parameter space of possible cases while still allowing for many different types of dark matter candidates to be produced from this transition,” Casey explains. “Aside from some very generous mass limits, the only major constraint on dark matter in the Dark Big Bang model is that it interacts with everyday particles through gravity alone.” This is encouraging because this limited interaction is what physicists expect of dark matter.

For now, the duo’s results suggest that the Dark Big Bang is far less constrained by past observations than Freese and Winkler originally anticipated. As Ilie explains, their constraints could soon be put to the test.

“We examined two Dark Big Bang scenarios in this newly found parameter space that produce gravitational wave signals in the sensitivity ranges of existing and upcoming surveys,” he says. “In combination with those considered in Freese and Winkler’s paper, these cases could form a benchmark for gravitational wave researchers as they search for evidence of a Dark Big Bang in the early universe.”

Subtle imprint on space–time

If a Dark Big Bang happened, then the gravitational waves it produced would have left a subtle imprint on the fabric of space–time. With this clearer outline of the Dark Big Bang’s parameter space, several soon-to-be active observational programmes will be well equipped to search for these characteristic imprints.

“For certain benchmark scenarios, we show that those gravitational waves could be detected by ongoing or upcoming experiments such as the International Pulsar Timing Array (IPTA) or the Square Kilometre Array Observatory (SKAO). In fact, the evidence of background gravitational waves reported in 2023 by the NANOGrav experiment – part of the IPTA – could be attributed to a Dark Big Bang realization,” Casey says.

If these studies find conclusive evidence for Freese and Winkler’s original theory, Casey and Ilie’s analysis could ultimately bring us a step closer to a breakthrough in our understanding of the ever-elusive origins of dark matter.

The research is described in Physical Review D.

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Hypothetical particles called axions could form dense clouds around neutron stars – and if they do, they will give off signals that radio telescopes can detect, say researchers in the Netherlands, the UK and the US. Since axions are a possible candidate for the mysterious substance known as dark matter, this finding could bring us closer to understanding it.

Around 85% of the universe’s mass consists of matter that appears “dark” to us. We can observe its gravitational effect on structures such as galaxies, but we cannot observe it directly. This is because dark matter hardly interacts with anything as far as we know, making it very difficult to detect. So far, searches for dark matter on Earth and in space have found no evidence for any of the various dark matter candidates.

The new research raises hopes that axions could be different. These neutral, bosonic particles are extremely light and hardly interact with ordinary matter. They get their name from a brand of soap, having been first proposed in the 1970s as a way of “cleaning up” a problem in quantum chromodynamics (QCD). More recently, astronomers have suggested they could clean up cosmology, too, by playing a role in the formation of galaxies in the early universe. They would also be a clean start for particle physics, providing evidence for new physics beyond the Standard Model.

Signature signals

But how can we detect axions if they are almost invisible to us? In the latest work, researchers at the University of Amsterdam, Princeton University and the University of Oxford showed that axions, if they exist, will be produced in large quantities at the polar regions of neutron stars. (Axions may also be components of dark matter “halos” believed to be present in the universe, but this study investigated axions produced by neutron stars themselves.) While many axions produced in this way will escape, some will be captured by the stars’ strong gravitational field. Over millions of years, axions will therefore accumulate around neutron stars, forming a cloud dense enough to give off detectable signals.

To reach these conclusions, the researchers examined various axion cloud interaction mechanisms, including self-interaction, absorption by neutron star nuclei and electromagnetic interactions. They concluded that for most axion masses, it is the last mechanism – specifically, a process called resonant axion-photon mixing – that dominates. Notably, this mechanism should produce a stream of low-energy photons in the radiofrequency range.

The team also found that these radio emissions would be connected to four distinct phases of axion cloud evolution. These are a growth phase after the neutron star forms; a saturation phase during normal life; a magnetorotational decay phase towards the later stages of the star’s existence; and finally a large burst of radio waves when the neutron star dies.

Turn on the radio

The researchers say that several large radio telescopes around the globe could play a role in detecting these radiofrequency signatures. Examples include the Low-Frequency Array (LOFAR) in the Netherlands; the Murchison Widefield Array in Australia; and the Green Bank Telescope in the US. To optimize the chances of picking up an axion signal, the collaboration recommends specific observation times, bandwidths and signal-to-noise ratios that these radio telescopes should adhere to. By following these guidelines, they say, the LOFAR setup alone could detect up to four events per year.

Dion Noordhuis, a PhD student at Amsterdam and first author of a Physical Review X paper on the research, acknowledges that there could be other observational signals beyond those explored in the paper. These will require further investigation, and he suggests that a full understanding will require complementary efforts from multiple branches of physics, including particle (astro)physics, plasma physics and observational radioastronomy. “This work thereby opens up a new, cross-disciplinary field with lots of opportunities for future research,” he tells Physics World.

Sankarshana Srinivasan, an astrophysicist from the Ludwig Maximilian University in Munich, Germany, who was not involved in the research, agrees that the QCD axion is a well-motivated candidate for dark matter. The Amsterdam-Princeton-Oxford team’s biggest achievement, he says, is to realize how axion clouds could enhance the signal, while the team’s “state-of-the-art” modelling makes the work stand out. However, he also urges caution because all theories of axion-photon mixing around neutron stars make assumptions about the stars’ magnetospheres, which are still poorly understood.

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Weakly interacting massive particles (WIMPs) are prime candidates for dark matter – but the hypothetical particles have never been observed directly. Now, an international group of physicists has proposed a connection between WIMPs and the higher-than-expected flux of antimatter cosmic rays  detected by NASA’s Alpha Magnetic Spectrometer (AMS-02) on the International Space Station.

Cosmic rays are high-energy charged particles that are created by a wide range of astrophysical processes including supernovae and the violent regions surrounding supermassive black holes. The origins of cosmic rays are not fully understood so they offer physicists opportunities to look for phenomena not described by the Standard Model of particle physics. This includes dark matter, a hypothetical substance that could account for about 85% of the mass in the universe.

If WIMPs exist, physicists believe that they would occasionally annihilate when they encounter one another to create matter and antimatter particles. Because WIMPs are very heavy, it is possible that these annihilations create antinuclei – the antimatter version of nuclei comprising antiprotons and antineutrons. Some of these antinuclei could make their way to Earth and be detected as cosmic rays

Now, a trio of researchers in Spain, Sweden, and the US has done new calculations that suggest that unexpected antinuclei detections made by AMS-02 could shed light on the nature of dark matter. The trio is led by Pedro De La Torre Luque at the Autonomous University of Madrid.

Heavy antiparticles

According to the Standard Model of particle physics, antinuclei should be an extremely small component of the cosmic rays measured by AMS-02. However, excesses of antideuterons (antihydrogen-2), antihelium-3  and antihelium-4 have been glimpsed in data gathered by AMS-02.

In previous work, De La Torre Luque and colleagues explored the possibility that these antinuclei emerged through the annihilation of WIMPs. Using AMS-02 data, the team put new constraints on the hypothetical properties of WIMPs.

Now, the trio has built on this work. “With this information, we calculated the fluxes of antideuterons and antihelium that AMS-02 could detect: both from dark matter, and from cosmic ray interactions with gas in the interstellar medium,” De La Torre Luque says. “In addition, we estimated the maximum possible flux of antinuclei from WIMP dark matter.”

This allowed the researchers to test whether AMS-02’s cosmic ray measurements are really compatible with standard WIMP models. According to De La Torre Luque, their analysis had mixed implications for WIMPs.

“We found that while the antideuteron events measured by AMS-02 are well compatible with WIMP dark matter annihilating in the galaxy, only in optimistic cases can WIMPs explain the detected events of antihelium-3,” he explains. “No standard WIMP scenario can explain the detection of antihelium-4.”

Altogether, the team’s results are promising for proponents of the idea that WIMPs are a component of dark matter. However, the research also suggest that the WIMP model in its current form is incomplete. To be consistent with the AMS-02 data, the researchers believe that a new WIMP model must further push the bounds of the Standard Model.

“If these measurements are robust, we may be opening the window for something very exotic going on in the galaxy, that could be related to dark matter, says De La Torre Luque. But it could also reveal some unexpected new phenomenon in the universe”. Ultimately, the researchers hope that the precision of their antinuclei measurements could bring us a small step closer to solving one of the deepest, most enduring mysteries in physics.

The research is described in the Journal of Cosmology and Astroparticle Physics.

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