Today’s New Reason To Believe

by Jeff Zweerink

What was discovered over 75 years ago, has not been detected using electromagnetic radiation, dominates the mass budget of the universe, and is filled with concrete?

Give up?

This mysterious substance is dark matter (I threw the concrete in just to make it hard).

However, a massive new simulation showing how dark matter behaves in galaxies may shed light on ways astronomers can detect gamma rays from dark matter collisions in our Milky Way Galaxy (MWG).

Although scientists do not understand the nature of dark matter, most believe that it is some sort of undiscovered subatomic particle. One popular candidate theory argues that each particle we know (like electrons, quarks, neutrinos, etc.) also has a supersymmetric partner particle. During the early history of the universe, these supersymmetric particles would have been produced in abundance. As the universe cooled, production would have ceased and the heavier particles have decayed to lighter supersymmetric particles. According to the theory, the lightest of these particles should be stable and, therefore, is one of the leading dark matter candidates.

However, even the lightest supersymmetric particle will annihilate when it collides with its antiparticle, and this annihilation will produce a pair of gamma rays. Thus, where the density of dark matter is large enough, astronomers should see a gamma-ray signal. Previous studies argued that clumps of dark matter in galaxies will produce the strongest signal. A recent simulation reveals that the smooth dark matter halo of a galaxy should give off the most gamma rays.

These results are important because NASA recently launched the Fermi Gamma-ray Space Telescope. One of the primary motivations for this mission was to “search for signs of new laws of physics and what composes the mysterious dark matter.” Knowing where to look and what sort of signal to expect greatly facilitates this search.

As it currently stands, dark matter represents one of the many parameters of this universe that exhibit fine-tuning. RTB expects future discoveries about the nature of dark matter (both from space and in particle accelerators) to reveal this design in more detail.

by Jeff Zweerink

History disfavors any theory placing Earth in a geometrically special location. Early scientists, such as Ptolemy of ancient Greece, thought Earth resided at the center of the solar system. However, geocentric cosmology eventually gave way to heliocentrism most notably associated with Nicolas Copernicus. Since Copernicus’s time extensive observations have demonstrated that the Sun does not reside at the center of the Milky Way Galaxy (MWG). Nor does the MWG reside at the center of the Local Group of galaxies or the universe. Scientists refer to the fact that Earth is not in a central, specially favored position as the Copernican Principle.

Although this view provides a foundation of cosmological research, scientists don’t simply accept the Copernican Principle. They continue to test it.

One area of research particularly suited to testing is the universe’s mysterious dark energy. The first need to invoke dark energy to explain features of the universe arose as astronomers tried to understand observations of distant Type Ia supernovae. The supernovae appeared dimmer than expected and the simplest explanation was that dark energy was causing the expansion of the universe to accelerate. However, dark energy is not the only explanation.

The same supernovae data would arise if the MWG resides at the center of a large region (something similar in size to the observable universe) with a lower density than that of the surrounding regions. However, placing the solar system (located within the MWG) at the center of such a special region clearly violates the Copernican Principle. Nevertheless, scientists do not simply reject the low density region, also called the void model. They seek to test its validity.

In a previous TNRTB I highlighted one test of the void model that used the cosmic microwave background. Now scientists have developed another test using supernovae data. Reseachers started by characterizing void density profiles that could explain the supernovae data. Then they modeled in detail how the supernovae data would appear with a much larger sample than currently exists. They found that with a sufficient sample of supernovae data from a specified distance, the void model produced different results when compared to dark energy models. Observations over the next few years should definitively tell which model is correct.

If dark energy models prevail, cosmologists will continue to face the great challenge of trying to understand what it is and why it exhibits such extraordinary fine-tuning in order for this universe to support life. If the void models prevail, a guiding scientific principle will need revision. Either way, exciting times lay ahead.

If you would like to see a question about the multiverse addressed in this forum, send it to multiverse@reasons.org.

by Dr. Jeffrey Zweerink

“Test everything. Hold on to the good.” This biblical passage underscores a central principle of the scientific enterprise. Any successful model must undergo testing that will either affirm or falsify its validity. Many scientists work diligently to provide such tests for a popular (though virtually untested experimentally) model known as string theory.

Astrophysicists Rishi Khatri and Benjamin D. Wandelt of the University of Illinois at Urbana-Champaign seek to develop an observational test for the cosmic strings (not to be confused with the strings of string theory) that result from incorporating a popular form of inflation—brane inflation—into string theory. They outline the test in a recent Physical Review article (a more lay-accessible description appears in Science Daily).

The abundant neutral hydrogen that fills the universe emits electromagnetic radiation with a specific wavelength: 21 cm. Astronomers have mapped this radiation as a function of position in the sky as shown below (see the description at the Astronomy Picture of the Day). All the structure in the image arises from material within the Milky Way Galaxy.

The hydrogen in the early universe would have produced evenly distributed 21-cm radiation (similar to the cosmic microwave background radiation). According to the research of Khatri and Wandelt, the cosmic web of strings produced during inflation will leave a signature in the 21 cm wavelength radiation which would be detectable with future instruments. However, the expansion of the universe will have redshifted the radiation roughly one hundred times to a wavelength around 21 meters. To make measurements precise enough to detect the cosmic string signature would take a square array of radio telescopes more than 100 kilometers on a side!

This daunting technical challenge demonstrates the difficulty in testing string theory. However, the rewards are worth the effort because the detection of cosmic strings would reveal to scientists the energy where gravity and quantum mechanics unify. While these tests may lie far in the future, RTB anticipates that the outcome of such tests will further demonstrate the fine-tuning (necessary for life) in the fundamental laws of physics that govern our universe.