New Study Shows Rich Physics in Models of Hypothetical Boson Stars

Studying something you’re not sure exists may seem strange to a non-scientist. But when you’re dealing with things so large or so small or so weird that no one even knows what to look for, theoretical predictions can be more than informative, they can be essential.

Recently, scientists from the University of Delhi and Iowa State University uncovered new details on a hypothetical, exotic star made of a completely different kind of matter than the stars we are familiar with. These boson stars, if they exist, would probably live at the extremely bright centers of ancient galaxies and be comparable to experimentally well-known objects like neutron stars and black holes. The work indicates that these intriguing objects could have particularly interesting physics to teach us.

The Hubble eXtreme Deep Field is a composite image showing light from a small portion of the sky collected by the Hubble Space Telescope over ten years. Who knows what is out there that we don’t know about?
Image Credit: NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team. Public domain.

The Standard Model, a commonly accepted and experimentally well-tested theory of subatomic particles and their interactions, divides all known particles into either fermions or bosons. Matter– protons, neutrons, and electrons – is composed of fermions. Bosons, on the other hand, are particles that follow different rules and serve different functions. One distinguishing property is that while fermions can’t be in the same quantum state at the same time, bosons can be. For example, photons are bosons, and as such they can exist in superposition—with multiple photons occupying the same region of space at once. This means that they can act collectively in certain ways that fermions cannot.

The boson family also includes fundamental particles like the recently discovered (and appropriately named) Higgs boson. It also includes some composite particles – particles made up of other particles. Composite particles can be either fermions or bosons depending on their composition. The pi and K meson are examples of composite boson particles made of fermions and have been well-studied experimentally.

Scientists have yet to see matter composed of bosons, but it could exist. If it does exist, that means there must be a stable boson particle that has a little bit of mass likes the Higgs boson. We haven’t seen this yet, but it’s allowed by current theories.

Boson stars would be extremely dense and would be held together by their own gravity. They consist only of bosons and are the boson version of neutron stars, the densest stars in the universe. However, boson stars could be even denser. Their ability to be in the same quantum state means that the particles can be physically closer together.

Not only are boson stars allowed, they could explain the active galactic nuclei we see at the cores of some galaxies. The details behind these super bright centers are a mystery, with the leading theory suggesting they are the result of a supermassive black hole at the center of the galaxy that is pulling in mass. Boson stars could also be much bigger, spanning millions of light years or more. They are also possible candidates for the elusive dark matter in the universe.

Researchers Sanjeev Kumar, Usha Kulshreshtha, and Daya Shankar Kulshreshtha, study the properties of boson stars in the context of different theoretical models. These theoretical models are described by mathematical equations. To explore boson stars, the scientists solve the equations for the existence of boson stars and look at the properties of the result. In other words, for a given theory they look for the specific conditions under which boson stars could exist, what they would be like, and what they might have to teach us about the physics of compact objects.

In their most recent work, published in Physical Review D, the scientists looked at solutions for charged, compact boson stars in a model that explores the effect of something called the cosmological constant. The cosmological constant is a number that describes the energy per unit volume of the vacuum of space. We don’t know exactly what its value is, but it turns out to be important in determining the fate of the universe. The team’s results were a series of diagrams that provide information on the center of the star for different values of the cosmological constant.
They were surprised to see that the diagrams had several key features called bifurcation points, which suggest areas of rich physics. Although we don’t know exactly what that means yet, the bifurcation points occur for values of the cosmological constant that match observations and indicate that the universe is accelerating in its expansion. This means that the theory could offer a realistic description for compact stars in the universe, and that it contains a lot of interesting physics to explore.

Boson stars are just exciting possibilities at this point. However, as scientists learn more about the properties of boson stars, they are also learning how to detect them. That begs the question: If they do exist, what happens when you wish on a boson star?

—Kendra Redmond

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